Smart Materials Notes.pdf

Theory on Smart Materials and its Applications Module 1 Introduction 1.1

Smart Materials Smart materials have been around for many years and they have found a large number of applications. The use of the terms 'smart' and 'intelligent' to describe materials and systems came from the US and started in the 1980‟s despite the fact that some of these so-called smart materials had been around for decades. Many of the smart materials were developed by government agencies working on military and aerospace projects but in recent years their use has transferred into the civil sector for applications in the construction, transport, medical, leisure and domestic areas. The first problem encountered with these unusual materials is defining what the word “smart‟ actually means. One dictionary definition of smart describes something which is astute or 'operating as if by human intelligence' and this is what smart materials are. A smart material is one which reacts to its environment all by itself. The change is inherent to the material and not a result of some electronics. The reaction may exhibit itself as a change in volume, a change in colour or a change in viscosity and this may occur in response to a change in temperature, stress, electrical current, or magnetic field. In many cases this reaction is reversible, a common example being the coating on spectacles which reacts to the level of UV light, turning your ordinary glasses into sunglasses when you go outside and back again when you return inside. This coating is made from a smart material which is described as being photo chromic. There are many groups of smart materials, each exhibiting particular properties which can be harnessed in a variety of high-tech and everyday applications. These include shape memory alloys, piezoelectric materials, magneto-rheological and electro-rheological materials, magnetostrictive materials and chromic materials which change their colour in reaction to various stimuli. The distinction between a smart material and a smart structure should be emphasized. A smart structure incorporates some form of actuator and sensor (which may be made from smart materials) with control hardware and software to form a system which reacts to its environment. Such a structure might be an 1

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Theory on Smart Materials and its Applications aircraft wing which continuously alters its profile during flight to give the optimum shape for the operating conditions at the time Smart systems are defined as ensembles whose dynamic can be monitored or modified by distributed sensors and actuators, in accordance with an integrated control law, to accommodate time‐varying exogenous inputs or changing environmental conditions. Smart Material Based Systems (SMBS) are defined as electro‐mechanical systems integrated with sensing, actuating, control and computational functions provided by such materials. Through system integration and compact design, systems with less complexity, lower cost and higher reliability can be built. 1.2

Characteristics of composite materials A composite material (also called a composition material or shortened to composite) is a material made from two or more constituent materials with significantly different physical or chemical properties that, when combined, produce a material with characteristics different from the individual components. The individual components remain separate and distinct within the finished structure. The new material may be preferred for many reasons: common examples include materials which are stronger, lighter, or less expensive when compared to traditional materials. It is characterized by: 1. Specific strength-This is simply the rigidity or hardness of a material with regard to its weight. For example, number of composites such as fiberglass share comparable impact resistance (bangability) with steel and titanium at a fraction of the weight employed. 2. Expense-Many composites can be manufactured with less cost than their traditional metal counterparts. 3. Application-Because composites are composed of 2 or more "phases", they can be formulated to meet the needs of a specific application with considerable ease. 4. Processability-As most of you know, metal processing requires high amounts of thermal energy (heat). Plastics and plastic based composites require less heat to mold or process the products. There is a constant desire to produce composites which can be processed at low temperatures but 2

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Theory on Smart Materials and its Applications when cured or set-up (paint drying or a mold cooling), they are very impact resistant and very heat resistant or fire retardant. 1.3

Characteristics of ceramic materials A ceramic is an inorganic non-metallic solid made up of either metal or nonmetal compounds that have been shaped and then hardened by heating to high temperatures. In general, they are hard, corrosion-resistant and brittle. It is characterized by          

1.4

hardness, wear-resistant, brittleness, refractory, thermal insulators, electrical insulators, nonmagnetic, oxidation resistant, prone to thermal shock, and Chemically stable.

Shape Memory Alloys Shape memory alloys (SMAs) are one of the most well known types of smart material and they have found extensive uses in the 70 years since their discovery. A shape memory transformation was first observed in 1932 in an alloy of gold and cadmium, and then later in brass in 1938. The shape memory effect (SME) was seen in the gold-cadmium alloy in 1951, but this was of little use. Some ten years later in 1962 an equiatomic alloy of titanium and nickel was found to exhibit a significant SME and Nitinol (so named because it is made from nickel and titanium and its properties were discovered at the Naval Ordinance Laboratories) has become the most common SMA. Other SMAs include those based on copper (in particular CuZnAl), NiAl and FeMnSi, though it should be noted that the NiTi alloy has by far the most superior properties. 3

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Theory on Smart Materials and its Applications 1.4.1 Working of Shape Memory Alloy The SME describes the process of a material changing shape or remembering a particular shape at a specific temperature (i.e. its transformation or memory temperature). Materials which can only exhibit the shape change or memory effect once are known as 1-way SMAs. However some alloys can be trained to show a two-way effect in which they remember two shapes, one below and one above the memory temperature. At the memory temperature the alloy undergoes a solid state phase transformation. That is, the crystal structure of the material changes resulting in a volume or shape change and this change in structure is called a “thermoelastic martensitic transformation‟. This effect occurs as the material has a martensitic microstructure below the transformation temperature, which is characterised by a zig-zag arrangement of the atoms, known as twins. The martensitic structure is relatively soft and is easily deformed by removing the twinned structure. The material has an austenitic structure above the memory temperature, which is much stronger. To change from the martensitic or deformed structure to the austenitic shape the material is simply heated through the memory temperature. Cooling down again reverts the alloy to the martensitic state as shown in Figure 1.1. The shape change may exhibit itself as either an expansion or contraction. The transformation temperature can be tuned to within a couple of degrees by changing the alloy composition. Nitinol can be made with a transformation temperature anywhere between –100ºC and +100ºC which makes it very versatile.

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Fig 1.1. Change in structure associated with the shape memory effect.

1.4.2 Applications of Shape Memory Alloys Shape memory alloys have found a large number of uses in aerospace, medicine and the leisure industry. A few of these applications are described below. 1. Medical applications Quite fortunately NiTiNOL is biocompatible, that is, it can be used in the body without an adverse reaction, so it has found a number of medical uses. These include stents in which rings of SMA wire hold open a polymer tube to open up a blocked vein (Figure 1.2), blood filters, and bone plates which contract upon transformation to pull the two ends of the broken bone in to closer contact and encourage more rapid healing (Figure 1.3). It is possible that SMAs could also find use in dentistry for orthodontic braces which straighten teeth. The memory shape of the material is made to be the desired shape of the teeth. This is then deformed to fit the teeth as they are and the memory is activated by the temperature of the mouth. The SMA exerts enough force as it contracts to move the teeth slowly and gradually (Figure 1.4). Surgical tools, 5

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Theory on Smart Materials and its Applications particularly those used in key hole surgery may also be made from SMAs. These tools are often bent to fit the geometry of a particular patient, however, in order for them to be used again they return to a default shape upon sterilisation in an autoclave.

Fig 1.3. This NiTi bone plated has been heat treated such that the central part changes from its deformed shape (top) to its memory shape (bottom) when warmed with saline solution, thus drawing the two ends of the fracture closer together. The modulus of this material has also been closely matched to that of human bone.

Fig 1.2. Reinforced vascular graft contains rings of SMA wire which open out the polyester tube on warming with warm saline solution once in-situ.

Fig 1.4. SMA wire has been used here to close the gap between two teeth. Two parallelograms of NiTi wire are attached to the teeth using stainless steel brackets which are glued to the teeth (left). After six months the gap between the teeth has decreased noticeably (right).

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2. Domestic applications SMAs can be used as actuators which exert a force associated with the shape change, and this can be repeated over many thousands of cycles. Applications include springs which are incorporated in to greenhouse windows such that they open and close themselves at a given temperature. Along a similar theme are pan lids which incorporate an SMA spring in the steam vent. When the spring is heated by the boiling water in the pan it changes shape and opens the vent, thus preventing the pan from boiling over and maintaining efficient cooking. The springs are similar to those shown in Figure 1.5.

Fig 1.5. Showing the two memory shapes of a memory metal wire coil or 'spring'. In (a) the spring is at room temperature and in (b) the higher temperature state has been activated by pouring on boiling water.

3. Aerospace applications A more high tech application is the use of SMA wire to control the flaps on the trailing edge of aircraft wings. The flaps are currently controlled by extensive hydraulic systems but these could be replaced by wires which are resistance heated, by passing a current along them, to produce the desired shape change. Such a system would be considerably simpler than the conventional hydraulics, thus reducing maintenance and it would also decrease the weight of the system. 4. Manufacturing applications SMA tubes can be used as couplings for connecting two tubes. The coupling diameter is made slightly smaller than the tubes it is to join. The coupling is deformed such that it slips over the tube ends and the temperature changed to 7

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Theory on Smart Materials and its Applications activate the memory. The coupling tube shrinks to hold the two ends together but can never fully transform so it exerts a constant force on the joined tubes. 1.4.3 General characteristics of SMA The martensitic transformation that occurs in the shape memory alloys yields a thermoelastic martensite and develops from a high-temperature austenite phase with long-range order. The martensite typically occurs as alternately sheared platelets, which are seen as a herringbone structure when viewed metallographically. The transformation, although a first-order phase change, does not occur at a single temperature but over a range of temperatures that varies with each alloy system. The usual way of characterizing the transformation and naming each point in the cycle is shown in Fig 1.6. Most of the transformation occurs over a relatively narrow temperature range, although the beginning and end of the transformation during heating or cooling actually extends over a much larger temperature range. The transformation also exhibits hysteresis in that the transformation on heating and on cooling does not overlap (Fig 1.6). This transformation hysteresis (shown as T in Fig 1.6) varies with the alloy system.

Fig 1.6. Typical transformation versus temperature curve for a specimen under constant load (stress) as it is cooled and heated. T, transformation hysteresis. Ms, martensite start; Mf, martensite finish; As, austenite start; Af, austenite finish

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Theory on Smart Materials and its Applications 1.5. Magneto-Rheological Fluid A magnetorheological fluid (MR fluid) is a type of smart fluid in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a visco-elastic solid as shown in Fig 1.7. Importantly, the yield stress of the fluid when in its active ("on") state can be controlled very accurately by varying the magnetic field intensity. The upshot is that the fluid's ability to transmit force can be controlled with an electromagnet, which gives rise to its many possible control-based applications. MR fluid is different from a ferrofluid which has smaller particles. MR fluid particles are primarily on the micrometre-scale and are toodense for Brownian motion to keep them suspended (in the lower density carrier fluid). Ferrofluid particles are primarily nanoparticles that are suspended by Brownian motion and generally will not settle under normal conditions. As a result, these two fluids have very different applications.

Fig 1.7. Schematic of a magnetorheological fluid solidifying and blocking a pipe in response to an external magnetic field.

Typical MR fluid consists of these three parts: a) Carbonyl Iron Particles -- 20 to 40 percent of the fluid is made of these soft

iron particles that are just 3 to 5 micrometers in diameter. A package of dry carbonyl iron particles looks like black flour because the particles are so fine. b) A Carrier Liquid -- The iron particles are suspended in a liquid, usually hydrocarbon oil. Water is often used in demonstrating the fluid.

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Theory on Smart Materials and its Applications c) Proprietary Additives -- The third component of MR fluid is a secret, but

Lord says these additives are put in to inhibit gravitational settling of the iron particles, promote particle suspension, enhance lubricity, modify viscosity and inhibit wear. 1.5.1 Modes of MR fluids 1. Flow Mode (Fig 1.8) Flow mode, also called valve mode, exploits the fluid between two fixed walls, the magnetic field is normal to the flow directions and is typical for linear damper applications. 2. Shear Mode (Fig 1.9) Shear mode is mainly used in rotary application such as brakes and clutches and the fluid is constrained between two walls which are in relative motion with the magnetic field normal to the wall direction. 3. Squeeze Mode (Fig 1.10) Squeeze mode is used mainly for bearing applications, is able to provide high forces and low displacements having the magnetic field normal to walls directions. Note: In all the above mentioned cases the working principle is the same: the applied magnetic field regulates the yield stress of the fluid and changes its apparent viscosity. So the amount of dissipated energy of the system is simply controllable by acting on the coil current and the system can provide semiactive behavior.

Fig 1.8. Flow mode

Fig 1.9. Shear Mode

Fig 1.10 Squeeze Mode

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Theory on Smart Materials and its Applications 1.5.2 Advantages of MR Fluids 1. Easy to control. 2. Have higher magnitude of yield stress. 1.5.3 Disadvantages of MR Fluids Although smart fluids are rightly seen as having many potential applications, they are limited in commercial feasibility for the following reasons: 1. High density, due to presence of iron, makes them heavy. 2. High quality fluids are expensive. 3. Fluids are subject to thickening after prolonged use and need replacing. 4. Settling of ferro-particles can be a problem for some applications. 1.5.4 Applications of MR Fluids The sudden change in the MR behavior (few milliseconds) due to the magnetic field application, makes this material attractive for damping and dissipative devices. The MR fluids can be used to build integral, silent, quick mechanical systems enhanced by means of electronic controls. There are two main ways to exploit the MR fluids in engineering applications: 1. Linear MR devices One of the most interesting engineering applications of MR fluid is the construction of smart and controllable MR linear dampers. The main asset of a MR based damper is the controllability of the system, which can be adjusted in order to provide the desired level of damping by simply changing the supply current. The main idea is to obtain the desired level of damping by varying the magnetic induction in an orifice between two separated MR fluid chambers. The orifice acts like a magnetic valve for the fluid, regulated by the current and thus exploits the MR fluid in flow mode. The single ended damper has only one reservoir for the MR fluid which is transferred through an orifice from on chamber to another. Since the rod volume is just on one side the system accounts for the change in volume that results from piston rod movement. In order to accommodate this change in reservoir volume, an accumulator is usually used. The accumulator provides a barrier between the MR fluid and a compressed gas (usually nitrogen) that is used to accommodate the necessary volume 11

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Theory on Smart Materials and its Applications changes. Moreover the accumulator pressure can be used to enhance the performance of the MR system, as shown in Fig.1.11. The double-ended MR damper has piston rods of the same diameter that protrude through both ends of the damper. In this case there is no change in volume as the piston rod moves, the double ended damper does not require an accumulator or other similar device. The applications of the single ended damper are mainly in the vibration suppression of mechanical components like seat suspension, car suspensions, and industrial vibration suppression, while the double ended damper is mainly used for bicycle applications, gun recoil applications, and for stabilizing buildings and bridges during earthquakes. The output forces of such a devices can range from quite low forces (hundreds of NewtonsS) in case of light suspension system up to 20 tons in case of civil applications, in which they must compensate the incredibly large forces cause by the shaking of entire buildings as shown in Fig.1.12.

Fig 1.11 Single ended MR damper

Fig 1.12 Double ended MR damper 12

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Theory on Smart Materials and its Applications 2. Rotary MR devices The aim is to obtain a precise control of the braking torque (in case of brakes) or transmitted torque (in case of clutches) with no moving parts by simply varying the current in the coils. In MR based brake, the magnetic flux path passes through the chassis and the rotating disk and the fluid is sheared between these elements. The braking force depends on the yield stress of the fluid making the system controllable as shown in Fig 1.13. In MR based clutch, the fluid is between the input disk and the output disk and the amount of transmitted torque is proportional to the yield stress of the fluid. No moving part are used to change the transmitted torque and the torque value can be smoothly controlled through the coil current as shown in Fig 1.14.

Fig 1.13 MR Brake

Fig 1.14 MR Clutch

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Theory on Smart Materials and its Applications 1.6. Electro-Rheological Fluids Electrorheological (ER) fluids are fluids which exhibit fast and reversible changes in their rheological properties under the influence of external electrical fields. Electrorheological (ER) fluids are a class of smart materials exhibiting significant reversible changes in their rheological and hence mechanical properties under the influence of an applied electric field. ER fluids commonly are composed of polarisable solid particles dispersed in non conducting oil. Upon the imposition of external electric field, the particles are polarized and form a chainlike structure along the direction of the field. The change in apparent viscosity is dependent on the applied electric field, i.e. the potential divided by the distance between the plates. The change is not a simple change in viscosity, hence these fluids are now known as ER fluids, rather than by the older term Electro Viscous fluids. When activated an ER fluid behaves as a Bingham plastic (a type of viscoelastic material), with a yield point which is determined by the electric field strength. After the yield point is reached, the fluid shears as a fluid, i.e. the incremental shear stress is proportional to the rate of shear (in a Newtonian fluid there is no yield point and stress is directly proportional to shear). Hence, the resistance to motion of the fluid can be controlled by adjusting the applied electric field as shown in Fig 1.15.

Fig 1.15. Behavior of ER fluid before and after electric field

1.6.1. Classification of ER Fluids The ER fluid can be classified based on the existing phases as:

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Fig.1.16. Classification of ER Fluids

1.6.2 Modes of ER Fluids 1. Flow Mode (Fig 1.17) Flow mode, also called valve mode, exploits the fluid between two fixed walls, the electric field is normal to the flow directions and is typical for linear damper applications. 2. Shear Mode (Fig 1.18) Shear mode is mainly used in rotary application such as brakes and clutches and the fluid is constrained between two walls which are in relative motion with the electric field normal to the wall direction. 3. Squeeze-Flow Mode (Fig 1.19) Squeeze-flow mode is used mainly for bearing applications, is able to provide high forces and low displacements having the electric field normal to walls directions.

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Fig.1.17.Flow mode

Fig.1.18.Shear mode

Fig.1.19.Squeeze-Flow mode

1.6.3 Advantages of ER fluids 1. ER device can control considerably more mechanical power than the electrical power used to control the effect, i.e. it can act as a power amplifier. 2. Speed of response. 3. The increase in apparent viscosity experienced by most Electrorheological fluids used in shear or flow modes is relatively limited. 4. ER fluid changes from a Newtonian liquid to a partially crystalline "semihard slush". 5. Liquid to solid phase change can be obtained when the Electro-rheological fluid additionally experiences compressive stress. 1.6.4 Disadvantages of ER fluids 1. ER fluids as suspensions tend to settle out in time. 2. Breakdown voltage of air is ~ 3 kV/mm, which is near the electric field needed for ER devices to operate. 1.6.5 Potential Applications of ER Fluids 1. 2. 3. 4. 5. 6. 7. 8.

Clutch. Brake and damping systems. Actuators. Fuel injections systems. Joints and hands of robotic arms. Photonic crystals. Micro-switches. Mechanical-electronic interfaces. 16

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1.7

Electromagnetic Materials

Electromagnetic (EM) industries rely heavily upon high performance materials that are designed to support direct, condition or absorb EM fields. In a wide range of applications EM fields also come into contact with other natural substances, for example: earth, water, and biological tissues. Full characterization of all of these material interactions is essential for optimal design of instrumentation. This can only be achieved with a knowledge of the intrinsic dielectric and magnetic properties of the materials involved: typically their complex dielectric permittivity and magnetic permeability, but also related parameters such as anisotropy, susceptibility and coercivity. These materials modify electromagnetic fields that interact with them in specific and intentional ways. Typically, the purpose of electromagnetic materials is to redirect, absorb, attenuate, or block electromagnetic radiation. As coatings and sealants are concerned, two key applications of electromagnetic materials are the control of radar cross section (RCS) and the reduction of electromagnetic interference (EMI). In either case there are two basic material objectives: absorption and conduction. 1. Conductive Materials Conductive coatings consist of conductive filler—usually a flake or powder—dispersed in a polymeric binder at high enough loading that percolation (a network of conductive pathways) occurs. Fillers may be metallic, ceramic, or polymeric. Binders must provide adequate adhesion, strength, and flexibility to stand up to stresses that will be placed on the vehicle surface. Conductive gap fillers can supplement or substitute for conductive coatings. These materials, having the consistency of putty or caulk, also comprise a conductive filler in a polymer binder. They form electrical connections across gaps, such as between body panels or other joined parts; and they also smooth the geometry of a conductive skin (ground plane), thereby reducing diffuse scattering. Some of the Requirements and applications are: 

Surface electrical continuity 17

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Theory on Smart Materials and its Applications 

Reflective ground plane for resonant absorbers

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Corrosion-resistance

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Low-temperature flexibility

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High temperature strength and adhesion

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Fluid and chemical resistance

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Static charge dissipation

2. Absorptive Materials Radar absorbing materials (RAM) attenuate the specular reflection of electromagnetic waves and diminish the strength of surface currents near edges and electrical discontinuities. They are also of key importance in countering the amplifying effects of cavity ringing. Tailored impedance composites are created by dispersing electrically or magnetically lossy fillers in a polymer-bound matrix. Examples of electrically lossy fillers are carbon black, graphite, intrinsically conductive polymers (ICPs), and filamentary metals; magnetic fillers include iron and ferrite powders. 1.8 Magnetic Circuit Some of the following magnetic circuits are shown in Fig.1.20 below:

Fig.1.20.Magnetic Circuits

All of them have a magnetic material of regular geometric shape called core. A coil having a number of turns (= N) of conducting material (say copper) are wound over the core is called the exciting coil. When no current flows through the coil, we don’t expect any magnetic field or lines of forces to be 18

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Theory on Smart Materials and its Applications present inside the core. However in presence of current in the coil, magnetic flux φ will be produced within the core. The strength of the flux depends on the product of number of turns (N) of the coil and the current (i) it carries. The quantity Ni called mmf (magnetomotive force) can be thought as the cause in order to produce an effect in the form of flux φ within the core. After going through this topic we will be able to do the following. 1. Distinguish between a linear and non linear magnetic circuit. 2. Draw the equivalent electrical circuit for a given magnetic circuit problem. 3. Calculate mean lengths of various flux paths. 4. Calculate the reluctances of the various flux paths for linear magnetic circuit problem. 5. Understand the importance of B-H characteristics of different materials. 6. How to deal with a non linear magnetic circuit problem using B-H characteristic of the materials. A magnetic circuit is made up of one or more closed loop paths containing a magnetic flux. The flux is usually generated by permanent magnets or electromagnets and confined to the path by magnetic cores consisting of ferromagnetic materials like iron, although there may be air gaps or other materials in the path. Magnetic circuits are employed to efficiently channel magnetic fields in many devices such as electric motors, generators, transformers, relays,lifting electromagnets, SQUIDs, galv anometers, and magnetic recording heads. The concept of a "magnetic circuit" exploits a one-to-one correspondence between the equations of the magnetic field in an unsaturated ferromagnetic material to that of an electrical circuit. Using this concept the magnetic fields of complex devices such as transformers can be quickly solved using the methods and techniques developed for electrical circuits. Examples:  Horseshoe magnet with iron keeper (low-reluctance circuit)  Horseshoe magnet with no keeper (high-reluctance circuit)  Electric motor (variable-reluctance circuit)

1.8.1 Laws for calculating magnetic field 19

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1. Biot-Savart law We know that any current carrying conductor produces a magnetic field. A magnetic field ℜ is characterized either by H→, the magnetic field intensity or by B→, the magnetic flux density vector. These two vectors are connected by a rather simple relation: B→=μo μr H→ where μ0= 4 π ×10-7H/m is called the absolute permeability of free space and μr , a dimensionless quantity called the relative permeability of a medium (or a material). For example the value of μr is 1 for free space or could be several thousands in case of ferromagnetic materials. Biot-Savart law is of fundamental in nature and tells us how to calculate dB or dH→ at a given point with position vector r , due to an elemental current idl→ and is given by:

(1) If the shape and dimensions of the conductor carrying current is known then field at given point can be calculated by integrating the RHS of the above equation. (2) Where, length indicates that the integration is to be carried out over the length of the conductor. However, it is often not easy to evaluate the integral for calculating field at any point due to any arbitrary shaped conductor. One gets a nice closed form solution for few cases such as: 1. Straight conductor carries current and to calculate field at a distance d from the conductor. 2. Circular coil carries current and to calculate field at a point situated on the axis of the coil.

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Theory on Smart Materials and its Applications 2. Ampere’s circuital law This law states that line integral of the vector H along any arbitrary closed path is equal to the current enclosed by the path. Mathematically: (3) For certain problems particularly in magnetic circuit problems Ampere’s circuital law is used to calculate field instead of the more fundamental Biot Savart law for reasons going to be explained below. Consider an infinite straight conductor carrying current i and we want to calculate field at a point situated at a distance d from the conductor. Now take the closed path to be a circle of radius d. At any point on the circle the magnitude of field strength will be constant and direction of the field will be tangential. Thus LHS of the above equation simply becomes H × 2πd. So field strength is (4) 1.8.2 Application of Ampere’s circuital law

Ampere’s circuital law is quite handy in determining field strength within a core of a magnetic material. Due to application of mmf, the tiny dipole magnets of the core are aligned one after the other in a somewhat disciplined manner. The contour of the lines of force resembles the shape the material. The situation is somewhat similar to flow of water through an arbitrary shaped pipe. Flow path is constrained to be the shape of the bent pipe. From the Fig.1.21 shown below for a toroidal magnetic circuit, When the coil carries a current i, magnetic lines of forces will be created and they will be confined within the core as the permeability of the core is many (order of thousands) times more than air. Take the chosen path to be a circle of radius r. let the value of H will remain same at any point on this path and directions will be always tangential to the path. Hence by applying Ampere’s circuital law to the path we get the value of H to be NI/2πr. If r is increased from a to be b the value of H decreases with r. a and b are respectively the inner and outer radius of the toroidal core. 21

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Fig.1.21. toroidal magnetic circuit 1.8.3 Assumption of Parameters 1. Leakage Flux & Fringing Effect All the flux produced by the mmf will not be confined to the core. There will be some flux lines which will complete their paths largely through the air as depicted in Fig.1.22. Since the reluctance or air is much higher compared to the reluctance offered by the core, the leakage flux produced is rather small. Here, we shall neglect leakage flux and assume all the flux produced will be confined to the core only. In the magnetic circuit of Fig.1.22, an air gap is present. For an exciting current, the flux lines produced are shown. These flux lines cross the air gap from the top surface of the core to the bottom surface of the core. So the upper surface behaves like a north pole and the bottom surface like a south pole. Thus all the flux lines will not be vertical and confined to the core face area alone. Some lines of force in fact will reach the bottom surface via bulged out curved paths outside the face area of the core. These flux which follow these curved paths are called fringing flux and the phenomenon is called fringing effect. Obviously the effect of fringing will be smaller if the air gap is quite small. Effect of fringing will be appreciable if the air gap length is more. In short the effect of fringing is to make flux density in the air gap a bit less than in the core as in the air same amount of flux is spread over an area which is greater than the core sectional area. 22

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Theory on Smart Materials and its Applications Effect of fringing sometimes taken into account by considering the effective area in air to be about 10 to 12% higher than the core area.

Fig.1.22. Typical Magnetic Circuit

2. In the practical magnetic circuit (as in Fig.1.23), the thickness (over which the lines of forces are spread = b-a) are much smaller compared to the overall dimensions (a or b) of the core. Under this condition we shall not make great mistake if we calculate H at rm= (b-a) / 2 and take this to be H everywhere within the core. The length of the flux path corresponding to the mean radius i.e., lm=2πrm is called the mean length. This assumption allows us to calculate the total flux φ produced within the core rather easily as enumerated below:    

Calculate the mean length lm of the flux path from the given geometry of the magnetic circuit. Apply Ampere’s circuital law to calculate H = NI / lm Note, this H may be assumed to be same everywhere in the core. Calculate the magnitude of the flux density B from the relation B = μoμrH. Total flux within the core is φ = BA, where A is the cross sectional area of the core.

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Module 2 Sensing and Actuation Sensors and actuators are two critical components of every closed loop control system. Such a system is also called a mechatronics system. A typical mechatronics system as shown in Fig. 2.1 consists of a sensing unit, a controller, and an actuating unit. A sensing unit can be as simple as a single sensor or can consist of additional components such as filters, amplifiers, modulators, and other signal conditioners. The controller accepts the information from the sensing unit, makes decisions based on the control algorithm, and outputs commands to the actuating unit. The actuating unit consists of an actuator and optionally a power supply and a coupling mechanism.

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Fig 2.1.A Typical Mechatronics System

2.1

Sensors Sensor is a device that when exposed to a physical phenomenon (temperature, displacement, force, etc.) produces a proportional output signal (electrical, mechanical, magnetic, etc.). The term transducer is often used synonymously with sensors. However, ideally, a sensor is a device that responds to a change in 24

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Theory on Smart Materials and its Applications the physical phenomenon. On the other hand, a transducer is a device that converts one form of energy into another form of energy. Sensors are transducers when they sense one form of energy input and output in a different form of energy. For example, a thermocouple responds to a temperature change (thermal energy) and outputs a proportional change in electromotive force (electrical energy). Therefore, a thermocouple can be called a sensor and or transducer. 2.2

Classification of Sensors and their Principles 1. Linear and Rotational Sensors Linear and rotational position sensors are two of the most fundamental of all measurements used in a typical mechatronics system. In general, the position sensors produce an electrical output that is proportional to the displacement they experience. There are contact type sensors such as strain gage, LVDT, RVDT, tachometer, etc. The noncontact type includes encoders, hall effect, capacitance, inductance, and interferometer type. They can also be classified based on the range of measurement. Usually the high-resolution type of sensors such as Hall Effect, fiber optic inductance, capacitance, and strain gage are suitable for only very small range (typically from 0.1 mm to 5 mm). The differential transformers on the other hand, have a much larger range with good resolution. Interferometer type sensors provide both very high resolution (in terms of microns) and large range of measurements (typically up to a meter). However, interferometer type sensors are bulky, expensive, and requires large set up time. Among many linear displacement sensors, strain gage provides high resolution at low noise level and is least expensive. A typical resistance strain gage consists of resistive foil arranged as shown in the Fig. 2.2. A typical setup to measure the normal strain of a member loaded in tension is shown in Fig. 2.3. Strain gage 1 is bonded to the loading member whereas strain gage 2 is bonded to a second member made of same material, but not loaded. This arrangement compensates for any temperature effect. When the member is loaded, the gage 1 elongates thereby changing the resistance of the gage. The change in resistance is transformed into a change in voltage by the voltage sensitive wheatstone bridge circuit. Assuming that the resistance of all four arms are equal initially, the change in output voltage (Dvo) due to change in resistance (DR1) of gage 1 is 25

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Theory on Smart Materials and its Applications Dvo/vi = (DR1/R)/ (4+2(DR1/R))

Fig 2.2. Bonded Strain Gauge

Fig 2.3. Experimental setup to measure using strain gauges.

2. Acceleration Sensors Measurement of acceleration is important for systems subject to shock and vibration. Although acceleration can be derived from the time history data obtainable from linear or rotary sensors, the accelerometers whose output is directly proportional to the acceleration is preferred. Two common types include the seismic mass type and the piezoelectric accelerometer. The seismic mass type accelerometer is based on the relative motion between a mass and the supporting structure. The natural frequency of the seismic mass limits its use to low to medium frequency applications. The piezoelectric accelerometer, however, is compact and more suitable for high frequency applications. 3. Force, Torque, and Pressure Sensors Among many type of force/torque sensors, the strain gage dynamometers and piezoelectric type are most common. Both are available to measure force and/or torque either in one axis or multiple axes. The dynamometers make use of mechanical members that experiences elastic deflection when loaded. These types of sensors are limited by their natural frequency. On the other hand, the piezoelectric sensors are particularly suitable for dynamic loadings in a wide range of frequencies. They provide high stiffness, high resolution over a wide measurement range, and are compact. 4. Flow Sensors Flow sensing is relatively a difficult task. The fluid medium can be liquid, gas, or a mixture of the two. Furthermore, the flow could be laminar or turbulent and can be a time-varying phenomenon. The venture meter and orifice plate restrict 26

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Theory on Smart Materials and its Applications the flow and use the pressure difference to determine the flow rate. The pitot tube pressure probe is another popular method of measuring flow rate. When positioned against the flow, they measure the total and static pressures. The flow velocity and in turn the flow rate can then be determined. The rotameter and the turbine meters when placed in the flow path, rotate at a speed proportional to the flow rate. The electromagnetic flow meters use noncontact method. Magnetic field is applied in the transverse direction of the flow and the fluid acts as the conductor to induce voltage proportional to the flow rate. Ultrasonic flow meters measure fluid velocity by passing high-frequency sound waves through fluid. A schematic diagram of the ultrasonic flow meter is as shown in Fig. 2.4. The transmitters (T) provide the sound signal source. As the wave travels towards the receivers (R), its velocity is influenced by the velocity of the fluid flow due to the doppler effect. The control circuit compares the time to interpret the flow rate. This can be used for very high flow rates and can also be used for both upstream and downstream flow. The other advantage is that it can be used for corrosive fluids, fluids with abrasive particles, as it is like a noncontact sensor.

Fig 2.4. Ultrasonic Flow Sensor Arrangement

5. Temperature Sensors A variety of devices are available to measure temperature, the most common of which are thermocouples, thermisters, resistance temperature detectors (RTD), and infrared types. Thermocouples are the most versatile, inexpensive, and have a wide range (up to 1200 ∞ C typical). A thermocouple simply consists of two dissimilar metal wires joined at the ends to create the sensing junction. When used in conjunction with 27

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Theory on Smart Materials and its Applications a reference junction, the temperature difference between the reference junction and the actual temperature shows up as a voltage potential. Thermisters are semiconductor devices whose resistance changes as the temperature changes. They are good for very high sensitivity measurements in a limited range of up to 100 ∞ C. The relationship between the temperature and the resistance is nonlinear. The RTDs use the phenomenon that the resistance of a metal changes with temperature. They are, however, linear over a wide range and most stable. Infrared type sensors use the radiation heat to sense the temperature from a distance. These noncontact sensors can also be used to sense a field of vision to generate a thermal map of a surface. 6. Proximity Sensors They are used to sense the proximity of an object relative to another object. They usually provide a on or off signal indicating the presence or absence of an object. Inductance, capacitance, photoelectric, and hall effect types are widely used as proximity sensors. Inductance proximity sensors consist of a coil wound around a soft iron core. The inductance of the sensor changes when a ferrous object is in its proximity. This change is converted to a voltage-triggered switch. Capacitance types are similar to inductance except the proximity of an object changes the gap and affects the capacitance. Photoelectric sensors are normally aligned with an infrared light source. The proximity of a moving object interrupts the light beam causing the voltage level to change. Hall effect voltage is produced when a current-carrying conductor is exposed to a transverse magnetic field. The voltage is proportional to transverse distance between the hall effect sensor and an object in its proximity. 7. Light Sensors Light intensity and full field vision are two important measurements used in many control applications. Phototransistors, photoresistors , and photodiodes are some of the more common type of light intensity sensors. A common photoresistor is made of cadmium sulphide whose resistance is maximum when the sensor is in dark. When the photoresistor is exposed to light, its resistance drops in proportion to the intensity of light. When interfaced with a circuit as shown in Fig. 2.5 and balanced, the change in light intensity will show up as 28

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Theory on Smart Materials and its Applications change in voltage. These sensors are simple, reliable, and cheap, used widely for measuring light intensity.

Fig.2.5. Light sensing with photoresistors.

8. Smart Material Sensors There are many new smart materials that are gaining more applications as sensors, especially in distributed sensing circumstances. Of these, optic fibers, piezoelectric, and magnetostrictive materials have found applications. Within these, optic fibers are most used. Optic fibers can be used to sense strain, liquid level, force, and temperature with very high resolution. Since they are economical for use as in situ distributed sensors on large areas, they have found numerous applications in smart structure applications such as damage sensors, vibration sensors, and curemonitoring sensors. These sensors use the inherent material (glass and silica) property of optical fiber to sense the environment. Figure 2.6 illustrates the basic principle of operation of an embedded optic fiber used to sense displacement, force, or temperature. The relative change in the transmitted intensity or spectrum is proportional to the change in the sensed parameter.

Fig.2.6. Optic fiber sensing

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Theory on Smart Materials and its Applications 2.3 Selection of sensors based on static and dynamic factors              

Range —Difference between the maximum and minimum value of the sensed parameter. Resolution —The smallest change the sensor can differentiate. Accuracy —Difference between the measured value and the true value. Precision —Ability to reproduce repeatedly with a given accuracy. Sensitivity —Ratio of change in output to a unit change of the input. Zero offset —A nonzero value output for no input. Linearity —Percentage of deviation from the best-fit linear calibration curve. Zero Drift—The departure of output from zero value over a period of time for no input. Response time —The time lag between the input and output. Bandwidth —Frequency at which the output magnitude drops by 3 dB. Resonance —The frequency at which the output magnitude peak occurs. Operating temperature —The range in which the sensor performs as specified. Deadband—The range of input for which there is no output. Signal-to-noise ratio—Ratio between the magnitudes of the signal and the noise at the output.

2.4 Actuators Actuators are basically the muscle behind a mechatronics system that accepts a control command (mostly in the form of an electrical signal) and produces a change in the physical system by generating force, motion, heat, flow, etc. Normally, the actuators are used in conjunction with the power supply and a coupling mechanism as shown in Fig. 2.7. The power unit provides either AC or DC power at the rated voltage and current. The coupling mechanism acts as the interface between the actuator and the physical system. Typical mechanisms include rack and pinion, gear drive, belt drive, lead screw and nut, piston, and linkages.

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Theory on Smart Materials and its Applications

Fig.2.7. A typical actuating unit

2.5 Classification of Actuators based on type of energy and stable state out-put 1. Electrical Actuators Electrical switches are the choice of actuators for most of the on-off type control action. Switching devices such as diodes, transistors, triacs, MOSFET , and relays accept a low energy level command signal from the controller and switch on or off electrical devices such as motors, valves, and heating elements. For example, a MOSFET switch is shown in Fig. 2.8. The gate terminal receives the low energy control signal from the controller that makes or breaks the connection between the power supply and the actuator load. When switches are used, the designer must make sure that switch bounce problem is eliminated either by hardware or software.

Fig.2.8. n-channel power MOSFET

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Theory on Smart Materials and its Applications 2. Electromechanical Actuators The most common electromechanical actuator is a motor that converts electrical energy to mechanical motion. Motors are the principal means of converting electrical energy into mechanical energy in industry. Broadly they can be classified as DC motors, AC motors, and stepper motors. DC motors operate on DC voltage and varying the voltage can easily control their speed. They are widely used in applications ranging from thousands of horsepower motors used in rolling mills to fractional horsepower motors used in automobiles (starter motors, fan motors, windshield wiper motors, etc.). Although they are costlier, they need DC power supply and require more maintenance compared to AC motors. The governing equation of motion of a DC motor can be written as: T=J (dѡ/dt) + TL + Tloss Where T is torque, J is the total inertia, ѡ is the angular mechanical speed of the rotor, TL is the torque applied to the motor shaft, and Tloss is the internal mechanical losses such as friction. 3. Electromagnetic Actuators The solenoid is the most common electromagnetic actuator. A DC solenoid actuator consists of a soft iron core enclosed within a current carrying coil. When the coil is energized, a magnetic field is established that provides the force to push or pull the iron core. AC solenoid devices are also encountered, such as AC excitation relay. A solenoid operated directional control valve is shown in Fig. 2.9. Normally, due to the spring force, the soft iron core is pushed to the extreme left position as shown. When the solenoid is excited, the soft iron core will move to the right extreme position thus providing the electromagnetic actuation. Another important type is the electromagnet. The electromagnets are used extensively in applications that require large forces.

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Fig.2.9. Solenoid operated directional control valve.

4. Hydraulic and Pneumatic Actuators Hydraulic and pneumatic actuators are normally either rotary motors or linear piston/cylinder or control valves. They are ideally suited for generating very large forces coupled with large motion. Pneumatic actuators use air under pressure that is most suitable for low to medium force, short stroke, and high speed applications. Hydraulic actuators use pressurized oil that is incompressible. They can produce very large forces coupled with large motion in a cost-effective manner. The disadvantage with the hydraulic actuators is that they are more complex and need more maintenance. The rotary motors are usually used in applications where low speed and high torque are required. The cylinder/piston actuators are suited for application of linear motion such as aircraft flap control. Control valves in the form of directional control valves are used in conjunction with rotary motors and cylinders to control the fluid flow direction as shown in Fig.2.9. In this solenoid operated directional control valve, the valve position dictates the direction motion of the cylinder/piston arrangement. 5. Smart Material Actuators Unlike the conventional actuators, the smart material actuators typically become part of the load bearing structures. This is achieved by embedding the actuators in a distributed manner and integrating into the load bearing structure that could be used to suppress vibration, cancel the noise, and change shape. Of the many smart material actuators, shape memory alloys, piezoelectric (PZT), magneto-strictive, Electro-rheological fluids, and ion exchange polymers are most common. 33

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Theory on Smart Materials and its Applications Shape Memory Alloys (SMA) are alloys of nickel and titanium that undergo phase transformation when subjected to a thermal field. The SMAs are also known as NITINOL for Nickel Titanium Naval Ordnance Laboratory. When cooled below a critical temperature, their crystal structure enters martensitic phase as shown in Fig. 2.10. In this state the alloy is plastic and can easily be manipulated. When the alloy is heated above the critical temperature (in the range of 50–80oC), the phase changes to austenitic phase. Here the alloy resumes the shape that it formally had at the higher temperature. For example, a straight wire at room temperature can be made to regain its programmed semicircle shape when heated that has found applications in orthodontics and other tensioning devices. The wires are typically heated by passing a current (up to several amperes), 0 at very low voltage (2–10 V typical).

Fig.2.10. Phase change of Shape Memory Alloy

6. Piezoelectric Actuators The PZT actuators are essentially piezocrystals with top and bottom conducting films as shown in Fig. 2.11. When an electric voltage is applied across the two conducting films, the crystal expands in the transverse direction as shown by the dotted lines. When the voltage polarity is reversed, the crystal contracts thereby providing bidirectional actuation. The interaction between the mechanical and electrical behavior of the piezoelectric materials can be expressed as: T = cES – eE 34

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Theory on Smart Materials and its Applications Where T is the stress, cE is the elastic coefficients at constant electric field, S is the strain, e is the dielectric permittivity, and E is the electric field.

Fig.2.11. Piezoelectric actuator

One application of these actuators is as shown in Fig. 2.12. The two piezoelectric patches are excited with opposite polarity to create transverse vibration in the cantilever beam. These actuators provide high bandwidth (0–10 kHz typical) with small displacement. Since there are no moving parts to the actuator, it is compact and ideally suited for micro and nano actuation. Unlike the bidirectional actuation of piezoelectric actuators, the electrostriction effect is a second-order effect, i.e., it responds to an electric field with unidirectional expansion regardless of polarity.

Fig.2.12. Vibration of beam using piezoelectric actuators.

7. Magnetostrictive rod actuators Magnetostrictive material is an alloy of terbium, dysprosium, and iron that generates mechanical strains up to 2000 microstrain in response to applied 35

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Theory on Smart Materials and its Applications magnetic fields. They are available in the form of rods, plates, washers, and powder. Figure 2.13 shows a typical magnetostrictive rod actuator that is surrounded by a magnetic coil. When the coil is excited, the rod elongates in proportion to the intensity of the magnetic field established. The magnetomechanical relationship is given as: SH + d H



where, ε is the strain, SH the compliance at constant magnetic filed, ς the stress, d the magnetostriction constant, and H the magnetic field intensity.

Fig 2.13. Magnetostrictive rod actuator

8. Micro and Nano Actuators Micro-actuators, also called micro-machines, micro-electromechanical system (MEMS), and Microsystems are the tiny mobile devices being developed utilizing the standard microelectronics processes with the integration of semiconductors and machined micromechanical elements. Another definition states that any device produced by assembling extremely small functional parts of around 1–15 mm is called a micro-machine. Example: Electrostatic Motor 2.6 Selection of Actuators based on their dynamic behavior  Continuous power output—The maximum force/torque attainable continuously without exceeding the temperature limits.  Range of motion—The range of linear/rotary motion.  Resolution—The minimum increment of force/torque attainable.  Accuracy—Linearity of the relationship between the input and output.  Peak force/torque—The force/torque at which the actuator stalls. 36

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Theory on Smart Materials and its Applications     

Heat dissipation—Maximum wattage of heat dissipation in continuous operation. Speed characteristics—Force/torque versus speed relationship. No load speed—Typical operating speed/velocity with no external load. Frequency response—The range of frequency over which the output follows the input faithfully, applicable to linear actuators. Power requirement—Type of power (AC or DC), number of phases, voltage level, and current capacity.

2.7 Magnet It can be defined as a substance that can attract a metal or iron. This ability is known as magnetism. Magnet has 2 poles which is north and south. 2.7.1 Principle of Magnet Magnet has a magnetic field around the magnet itself. Magnetic field is the force around the magnet which can attract any magnetic material around it. The line form around the magnet bar is magnetic field which is known as flux magnet as shown in Fig.2.14.

Fig.2.14. Magnetic Principle

2.7.2 Law of Magnet The flux line of magnetic have a direction and pole. The direction of movement outside of the magnetic field line is from north to south. The magnetic poles (north & south) have the strongest magnetic field. It is basic law, in which different poles will attract each other while the same magnetic poles will reject each other. It can also be named as magnetic attraction and repulsion law. The flux will form a complete loop and will never intersect with each other and will be in smallest form possible as shown in Fig. 2.15. 37

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Theory on Smart Materials and its Applications

Fig.2.15. Magnetic flux lines

2.7.3 Types of Magnet 1. Pure Magnet Pure magnet is a magic stone. The stone originally have the natural magnetic and normally in a form of iron ore.

2. Manufacture Magnet Two type of manufacture magnet which is permanent magnet and temporary magnet. a. The Permanent magnet is manufacture so it can kept its magnetism. It can be obtained naturally or magnetic induction and placing a magnet into a coil then supplied with high electrical current. Normally it is used in speakers and metering devices. b. The Temporary magnet is created by mean of using electric current. It is known as electromagnet. It will have it magnetic properties when there is electric current and will lost the magnetic properties when the current is cut off. Example of such application is relays. 2.8

Electromagnet 38

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Theory on Smart Materials and its Applications Electromagnet produced when there is electric current flowing through a coil of wire (in circular path) and through a conductor. The direction of magnetic field produced by the current in the solenoid can be determined using two methods: (i)

Right hand grip

Right hand grip is a principle applied to electric current passing to a solenoid coil resulting in a magnetic field. By gripping the right hand around the solenoid, thumb is pointing in the direction of the magnetic north pole and remaining fingers is pointing of direction of current flow as shown in Fig.2.16.

Fig.2.16. Right hand thumb rule

(ii) Maxwell’s screw law A right handed screw is turn clock wise so that it moves forward in the same direction as the current. The direction of screw rotation (clockwise) indicates the direction of magnetic field from south to north. 2.8.1 Factors affecting Electromagnetic strength There are 4 factors that affect electromagnetic strength: (i) Number of turns The strength of the electromagnet is directly proportional to the number of turn in the coil. By varying the number of turns in its coil can produce very strong magnetic fields and its strength. (ii) Current strength 39

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Theory on Smart Materials and its Applications The strength of the electromagnet is directly proportional to the current flowing in the coil. Greater the current flow through the coil, stronger will be the magnetic fields produced. (iii) Length of coil The strength of the electromagnet is directly proportional to the length of the coil. By coil up the wire can increasing the length and increase the force of magnetic field. (iv) Types of conductor Depend on the nature of the core material. The use of soft of core can produces the strongest magnetism. 2.9

Signal Processing

Signal processing is an enabling technology that encompasses the fundamental theory, applications, algorithms, and implementations of processing or transferring information contained in many different physical, symbolic, or abstract formats broadly designated as signals. It uses mathematical, statistical, computational, heuristic, and linguistic representations, formalisms, and techniques for representation, modelling, analysis, synthesis, discovery, recovery, sensing, acquisition, extraction, learning, security, or forensics. 2.9.1 Types of Signal Processing 1. Analog signal processing Analog signal processing is for signals that have not been digitized, as in legacy radio, telephone, radar, and television systems. This involves linear electronic circuits as well as non-linear ones. The former are, for instance, passive filters, active filters, additive mixers, integrators and delay lines. Non-linear circuits include compandors, multiplicators (frequency mixers and voltage-controlled amplifiers), voltage-controlled filters, voltagecontrolled oscillators and phase-locked loops. 2. Continous Time Signal Processing 40

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Theory on Smart Materials and its Applications Continuous-time signal processing is for signals that vary with the change of continuous domain (without considering some individual interrupted points). The methods of signal processing include time domain, frequency domain, and complex frequency domain. This technology mainly discusses the modeling of linear time-invariant continuous system, integral of the system's zero-state response, setting up system function and the continuous time filtering of deterministic signals 3. Discrete time signal processing Discrete-time signal processing is for sampled signals, defined only at discrete points in time, and as such are quantized in time, but not in magnitude. Analog discrete-time signal processing is a technology based on electronic devices such as sample and hold circuits, analog timedivision multiplexers, analog delay lines and analog feedback shift registers. This technology was a predecessor of digital signal processing (see below), and is still used in advanced processing of gigahertz signals. The concept of discrete-time signal processing also refers to a theoretical discipline that establishes a mathematical basis for digital signal processing, without taking quantization error into consideration. 4. Digital Signal processing Digital signal processing is the processing of digitized discrete-time sampled signals. Processing is done by general-purpose computers or by digital circuits such as ASICs, field-programmable gate arrays or specialized digital signal processors (DSP chips). Typical arithmetical operations include fixedpoint and floating-point, real-valued and complex-valued, multiplication and addition. Other typical operations supported by the hardware are circular buffers and look-up tables. Examples of algorithms are the Fast Fourier transform (FFT), finite impulse response (FIR) filter, Infinite impulse response (IIR) filter, and adaptive filters such as the Wiener and Kalman filters. 5. Nonlinear Signal processing 41

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Theory on Smart Materials and its Applications Nonlinear signal processing involves the analysis and processing of signals produced from nonlinear systems and can be in the time, frequency, or spatio-temporal domains.[5]Nonlinear systems can produce highly complex behaviors including bifurcations, chaos, harmonics, and subharmonics which cannot be produced or analyzed using linear methods. 2.9.2 Applications of Signal Processing  Audio signal processing – for electrical signals representing sound, such

as speech or music.  Speech signal processing – for processing and interpreting spoken words  Image processing – in digital cameras, computers and various imaging systems  Video processing – for interpreting moving pictures  Wireless communication - waveform generations, demodulation, filtering, equalization  Control systems  Array processing – for processing signals from arrays of sensors  Seismology  Financial signal processing – analyzing financial data using signal processing techniques, especially for prediction purposes.  Feature extraction, such as image understanding and speech recognition.  Quality improvement, such as noise reduction, image enhancement, and echo cancellation.  (Source coding), including audio compression, image compression, and video compression. 2.9.3 Types of signal processing devices  Filters : for example analog (passive or active) or digital (FIR, IIR,

frequency domain or stochastic filters, etc.)  Samplers and Analog-to-digital converters: for Signal acquisition and reconstruction, which involves measuring a physical signal, storing or 42

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Theory on Smart Materials and its Applications transferring it as digital signal, and possibly later rebuilding the original signal or an approximation thereof.  Signal compressors  Digital signal processors (DSPs) 2.9.4 Principle of signal processing

Fig.2.17. Signal Processing Principle

Signal transmission using electronic signal processing consist of Transducers which convert signals from other physical waveforms to electric current or voltage waveforms, which then are processed, transmitted as electromagnetic waves, received and converted by another transducer to final form as shown in Fig.2.17.

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Theory on Smart Materials and its Applications Module 3 Structures, Optics and Electromagnetic 3.1

Introduction to Fiber Optics

We're used to the idea of information traveling in different ways. When we speak into a landline telephone, a wire cable carries the sounds from our voice into a socket in the wall, where another cable takes it to the local telephone exchange. Cell phones work a different way: they send and receive information using invisible radio waves—a technology called wireless because it uses no cables. Fiber optics works a third way. It sends information coded in a beam of light down a glass or plastic pipe. It was originally developed for endoscopes in the 1950s to help doctors see inside the human body without having to cut it open first. In the 1960s, engineers found a way of using the same technology to transmit telephone calls at the speed of light (186,000 miles or 300,000 km per second). A fiber-optic cable as shown in Fig.3.1 is made up of incredibly thin strands of glass or plastic known as optical fibers; one cable can have as few as two strands or as many as several hundred. Each strand is less than a tenth as thick as a human hair and can carry something like 25,000 telephone calls, so an entire fiber-optic cable can easily carry several million calls. Fiber-optic cables carry information between two places using entirely optical (light-based) technology. Suppose you wanted to send information from your computer to a friend's house down the street using fiber optics. You could hook your computer up to a laser, which would convert electrical information from the computer into a series of light pulses. Then you'd fire the laser down the fiber-optic cable. After traveling down the cable, the light beams would emerge at the other end. Your friend would need a photoelectric cell (light-detecting component) to turn the pulses of light back into electrical information his or her computer could understand. So the whole apparatus would be like a really neat, hi-tech version of the kind of telephone you can make out of two baked-bean cans and a length of string! 44

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Theory on Smart Materials and its Applications

Fig.3.1.Fiber Optics Cable

3.2

Working Principle of Fiber Optics

Light travels down a fiber-optic cable by bouncing repeatedly off the walls. Each tiny photon (particle of light) bounces down the pipe like a bobsleigh going down an ice run. Now you might expect a beam of light, traveling in a clear glass pipe, simply to leak out of the edges. But if light hits glass at a really shallow angle (less than 42 degrees), it reflects back in again—as though the glass were really a mirror. This phenomenon is called total internal reflection. It's one of the things that keeps light inside the pipe as shown in Fig 3.2.

Fig.3.2. Total internal reflection

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Theory on Smart Materials and its Applications The other thing that keeps light in the pipe is the structure of the cable, which is made up of two separate parts. The main part of the cable—in the middle—is called the core and that's the bit the light travels through. Wrapped around the outside of the core is another layer of glass called the cladding. The cladding's job is to keep the light signals inside the core. It can do this because it is made of a different type of glass to the core as shown in Fig.3.3.

Fig.3.3.Structure of the Cable

3.2.1 Construction An optical fiber consists of three basic concentric elements: the core, the cladding and the outer coating (Fig.3.3). The core is usually made of glass or plastic, although other materials are sometimes used, depending on the transmission spectrum desired. The core is the light-transmitting portion of the fiber. The cladding usually is made of the same material as the core, but with a slightly lower index of refraction (usually about 1 percent lower). This index difference causes total internal reflection to occur at the index boundary along the 46

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Theory on Smart Materials and its Applications length of the fiber so that the light is transmitted down the fiber and does not escape through the sidewalls. The coating usually comprises one or more coats of a plastic material to protect the fiber from the physical environment. Sometimes metallic sheaths are added to the coating for further physical protection. Optical fibers usually are specified by their size, given as the outer diameter of the core, cladding and coating. For example, a 62.5/125/250 would refer to a fiber with a 62.5-µm diameter core, a 125-µm diameter cladding and a 0.25-mm outer coating diameter. 3.2.2 Principle of working Optical materials are characterized by their index of refraction, referred to as n. A material’s index of refraction is the ratio of the speed of light in a vacuum to the speed of light in the material. When a beam of light passes from one material to another with a different index of refraction, the beam is bent (or refracted) at the interface (Fig.3.4).

Fig.3.4. A beam of light passing from one material to another of a different index of refraction is bent or refracted at the interface.

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Theory on Smart Materials and its Applications Refraction is described by Snell’s law: nI Sin I = nR SinR where nI and nR are the indices of refraction of the materials through which the beam is refracted and I and R are the angles of incidence and refraction of the beam. Note: If the angle of incidence is greater than the critical angle for the interface (typically about 82° for optical fibers), the light is reflected back into the incident medium without loss by a process known as total internal reflection (Fig.3.5).

Fig.3.5. Total internal reflection allows light to remain inside the core of the fiber

Numerical Aperture Numerical aperture (NA), shown in Fig.3.6, is the measure of maximum angle at which light rays will enter and be conducted down the fiber. This is represented by the following equation:

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Fig.3.6. Numerical aperture depends on the angle at which rays enter the fiber and on the diameter of the fiber’s core.

3.3

Types of Fiber Optic Cables

Optical fibers carry light signals down them in what are called modes. That sounds technical but it just means different ways of traveling: a mode is simply the path that a light beam follows down the fiber. One mode is to go straight down the middle of the fiber. Another is to bounce down the fiber at a shallow angle. Other modes involve bouncing down the fiber at other angles, more or less steep. 1. Single Mode The simplest type of optical fiber is called single-mode. It has a very thin core about 5-10 microns (millionths of a meter) in diameter. In a singlemode fiber, all signals travel straight down the middle without bouncing off the edges (Dark line in Fig. 3.4). Cable TV, Internet, and telephone signals are generally carried by single-mode fibers, wrapped together into a huge bundle. Cables like this can send information over 100 km (60 miles).

Fig.3.4.Working of Single Mode

2. Multi-Mode Each optical fiber in a multi-mode cable is about 10 times bigger than one in a single-mode cable. This means light beams can travel through the core by 49

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Theory on Smart Materials and its Applications following a variety of different paths (purple, green, and blue lines)—in other words, in multiple different modes as shown in Fig.3.5. Multi-mode cables can send information only over relatively short distances and are used (among other things) to link computer networks together.

Fig.3.5.Working of Multi-Mode Example: Even thicker fibers are used in a medical tool called a gastroscope (a type of endoscope), which doctors poke down someone's throat for detecting illnesses inside their stomach. A gastroscope is a thick fiber-optic cable consisting of many optical fibers. At the top end of a gastroscope, there is an eyepiece and a lamp. The lamp shines its light down one part of the cable into the patient's stomach. When the light reaches the stomach, it reflects off the stomach walls into a lens at the bottom of the cable. Then it travels back up another part of the cable into the doctor's eyepiece. Other types of endoscopes work the same way and can be used to inspect different parts of the body. There is also an industrial version of the tool, called a fiberscope, which can be used to examine things like inaccessible pieces of machinery in airplane engines.

3.4 Applications of Fiber Optics The technologies which use it effectively are computer networking, broadcasting, medical scanning, and military equipment. 1. Computer Networks Fiber-optic cables are now the main way of carrying information over long distances because they have three very big advantages over old-style copper cables:  Less attenuation: (signal loss) Information travels roughly 10 times further before it needs amplifying—which makes fiber networks simpler and cheaper to operate and maintain.  No interference: Unlike with copper cables, there's no "crosstalk" (electromagnetic interference) between optical fibers, so they transmit information more reliably with better signal quality 50

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Theory on Smart Materials and its Applications  Higher bandwidth: As we've already seen, fiber-optic cables can

carry far more data than copper cables of the same diameter. 2. Medicine Medical gadgets that could help doctors peer inside our bodies without cutting them open were the first proper application of fiber optics over a half century ago. Today, gastroscopes (as these things are called) are just as important as ever, but fiber optics continues to spawn important new forms of medical scanning and diagnosis. One of the latest developments is called a lab on a fiber, and involves inserting hair-thin fiber-optic cables, with built-in sensors, into a patient's body. These sorts of fibers are similar in scale to the ones in communication cables and thinner than the relatively chunky light guides used in gastroscopes. How do they work? Light zaps through them from a lamp or laser, through the part of the body the doctor wants to study. As the light whistles through the fiber, the patient's body alters its properties in a particular way (altering the light's intensity or wavelength very slightly, perhaps). By measuring the way the light changes (using techniques such as interferometry), an instrument attached to the other end of the fiber can measure some critical aspect of how the patient's body is working, such as their temperature, blood pressure, cell pH, or the presence of medicines in their bloodstream. In other words, rather than simply using light to see inside the patient's body, this type of fiber-optic cable uses light to sense or measure it instead. 3. Military Fiber-optic cables are inexpensive, thin, lightweight, high-capacity, robust against attack, and extremely secure, so they offer perfect ways to connect military bases and other installations, such as missile launch sites and radar tracking stations. Since they don't carry electrical signals, they don't give off electromagnetic radiation that an enemy can detect, and they're robust against electromagnetic interference (including systematic enemy "jamming" attacks). Another benefit is the relatively light weight of fiber cables compared to traditional wires made of cumbersome and 51

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Theory on Smart Materials and its Applications expensive copper metal. Tanks, military airplanes, and helicopters have all been slowly switching from metal cables to fiber-optic ones. Partly it's a matter of cutting costs and saving weight (fiber-optic cables weigh nearly 90 percent less than comparable "twisted-pair" copper cables). But it also improves reliability; for example, unlike traditional cables on an airplane, which have to be carefully shielded (insulated) to protect them against lightning strikes, optical fibers are completely immune to that kind of problem as shown in Fig.3.6.

Fig.3.6. Enhanced Fiber-Optic Guided Missile (EFOG-M)

4. Broadcasting Back in the early 20th century, radio and TV broadcasting was born from a relatively simple idea. Cable TV companies pioneered the transition from the 1950s onward, originally using co-axial cables (copper cables with a sheath of metal screening wrapped around them to prevents crosstalk interference), which carried just a handful of analog TV signals. As more and more people connected to cable and the networks started to offer greater choice of channels and programs, cable operators found they needed to switch from coaxial cables to optical fibers and from analog to digital broadcasting. Apart from offering much higher capacity, optical fibers suffer less from interference, so offer better signal (picture and sound) quality; they need less amplification to boost signals so they travel over long distances; and they're altogether more cost effective. In the future, fiber broadband may well be how most of us watch television, perhaps through 52

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Theory on Smart Materials and its Applications systems such as IPTV (Internet Protocol Television), which uses the Internet's standard way of carrying data ("packet switching") to serve TV programs and movies on demand. While the copper telephone line is still the primary information route into many people's homes, in the future, our main connection to the world will be a high-bandwidth fiber-optic cable carrying any and every kind of information. 3.5 Principle of Turbulent Drag Reduction of smart/active skin Turbulence control methods have been developed under the assumption that the turbulence production cycle could be favorably altered, stabilized, or reduced in intensity by the manipulation and alteration of low-speed streaks, quasi-streamwise vortices, the viscous sub layer, or the hairpin-like structures that populate the near-wall region. In this work two possible actuation principles for the active skin that can achieve a surface traveling wave are discussed. The first actuation principle is depicted as a free body diagram in Fig.3.7. Consider a long plate subjected to equally spaced external moments that are equal in value but alternate in sign. This loading pattern can generate a static wave profile, from which a traveling wave can be realized by shifting the points of application of the moments. The moment can be realized either by the application of equal and opposite forces (couple) at those points or by unbalanced lateral forces separated by a lever arm.

Fig.3.7.Moment based Actuation Principle

The second actuation principle utilizes equally spaced vertical external forces along the length of the skin. The amplitudes and the directions of these forces vary periodically in order to match the skin deflection required as shown in Fig.3.8.

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Fig.3.8.Force based Actuation Principle

Second actuation principle can also be viewed as a more general form of the first actuation principle with the values of the actuating moments varying in a periodic manner instead of being constant. A traveling wave profile can be generated by simultaneously varying all the forces in a periodic manner. All the forces would oscillate with the same frequency and the same amplitude. However each successive force would lag the previous force by a constant phase difference. Since the deflections would be in phase with the forces, the net effect would be a traveling wave. 3.5.1 Theoretical Analysis of Active/Smart skin To simplify the analysis, it is possible to utilize the inherent periodicity in loading and boundary conditions of the active skin, to reduce the analysis domain to a “Unit Cell”. The Unit Cell can be defined as the smallest repeating unit of the active skin, the behavior of which would completely describe the behavior of the entire skin. The analytical expressions for the deflection amplitude and the natural frequency of the Unit Cell can be obtained by solving the general expressions that describe cylindrical bending in plates. Utilizing the periodicity assumption the Unit Cells for the two actuating principles shown in Fig.3.7 and Fig.3.8 can be obtained as the section of skin that is one wavelength long. Utilizing the symmetry in the bending, the Unit Cell can be further reduced to a section of the skin that is half a wavelength long (Fig.3.9).

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Fig.3.9.Unit Cells for Two Actuation Principles

The appropriate boundary conditions for the Unit Cell would therefore be  Symmetry conditions at the ends  Zero deflection for the point in the middle of the Unit Cell Note: Ma and Mb are the boundary conditions for the Unit Cell that need to be solved for and are not the externally applied moments. In the free body diagram of the Unit Cell for the force based actuation technique, the forces at the ends are half the values of the actual forces at these points since the effect of these forces are equally shared by two adjoining Unit Cells.

3.5.2 Design of Active Skin based on Actuation Strategies Three different active skin designs that would be capable of creating a traveling wave form profile using either of the actuation principles have been considered. 1. SMA Actuator based Active Skin Fig.3.10 presents a cross section and a top view of the first skin design in its non-actuated state. This design works on the principle of moment based actuation. The moments that bend the skin are created by lateral forces that act on the skin through “legs” that are attached to the top surface of the skin. The legs can slide (left and right) with respect to the bottom surface (which is the surface attached to the vehicle) while the upper surface is exposed to the flow. The “legs” are actuated in a manner that induces rotation of the legs, which in turn results in a deformation of the top surface. When the legs are actuated in a coordinated manner it results in a wavy deformation pattern on the upper surface. This coordinated leg actuation/rotation can be achieved as described below. 55

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Theory on Smart Materials and its Applications A Shape Memory Alloy (SMA) wire runs through the “legs”, through small holes on their sidewalls. The direction of the SMA wire is in the spanwise direction, while the major dimension of the “legs” is along the streamwise direction. Within each “leg” a circular flat disk is attached to the SMA wire, with its diameter significantly larger than the diameter of the holes on the sidewalls of the “legs”. Each SMA-disk joint is electrically connected to the electrical control circuit, and is powered independently. When a voltage difference is applied between the leftmost (“joint 1”) and the rightmost SMAdisk (“joint 5”) joints in Fig.3.10, the negative strain induced in the SMA (on account of the wire contracting) will cause the disks of joints 1 and 5 to contact the walls of legs 1 and 5 thus transferring to them the load generated by the SMA.

Fig.3.10. Cross section and top view of SMA actuated active skin

As shown in Fig.3.11 in an exaggerated fashion, as the SMA sections between legs 1 and 5 and between legs 9 and 13 contract, the SMA section between legs 5 and 9 will have to elongate/strain accordingly. Therefore the SMA sections between 1 and 5 and 9 and 13 will have to produce enough force not only to deform the upper skin but also to strain the SMA section between legs 5 and 9. This requirement is typical in antagonistic SMA actuators and presents no problem, since the sections between 1 and 5 and 9 and 13 are austenitic and have a much larger stiffness (2 to 3 times higher) than the section between 5 and 9, which is in the martensitic phase. One other point that deserves mention is that the bending caused by the actuation would not be exactly uniform all along the streamwise direction on account of the fact 56

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Theory on Smart Materials and its Applications that the loading takes place at discrete points. But it is assumed that the deviation would be negligible if the SMA actuators were placed in relatively small intervals in the streamwise direction.

Fig.3.11. Resulting waveform after actuating SMA sections between legs 1 and 5 and between legs 9 and 13

2. Peizoelectric C-block based active skin The individual C-block actuator, which is configured in a semi-circular shape, can be aligned in series or in parallel to optimize the force and deflection output to those required by the application at hand, while simultaneously fitting within the space constraints. Fig.3.12 illustrates the principles of piezoelectric actuation of the active skin in the second design. When electrical voltage is properly applied to a semi-circular, C-block piezoelectric actuator, positioned between two consecutive “legs” it causes the ends of the semi-circle to deflect radially inward, towards each other. This action causes displacement of the disks in the “legs” and subsequent “leg” deflection. The key to piezoelectric actuation will be to actuate locally in a time sequence that produces the desired traveling wave, without antagonizing with the neighboring structural elements and piezoelectric actuators. Direct attachment of piezoelectric patches on the skin, in the traditional way, will be avoided since it will result in interference from neighboring piezoelectric elements, thus increasing the actuation energy cost.

Fig.3.12. Piezoelectrically actuated active skin.

3. Piezoceramic Stack actuator based Active Skin A third skin design is shown in Fig.3.13. This design works on the force based actuation principle. In this design the “legs” are replaced by linear 57

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Theory on Smart Materials and its Applications Piezoceramic Stack Actuators (PSAs), which actuate the skin in a direction along their axis. On actuation, the PSAs would exert a force on the skin in the vertical direction causing the skin to bend. By varying the values of the forces (in the PSAs) along the length of the skin in a periodic fashion it is possible to achieve a static bending in the form of sinusoidal wave. By varying the force applied by each PSA with time in a periodic fashion it would be possible to obtain a traveling wave. In effect the loading of the PSAs would be in phase with the displacement of the skin itself.

Fig.3.13. Skin design with linear piezoceramic stack actuators oriented perpendicular to the skin.

3.6 Adaptive Optics As light from distant celestial objects enters our atmosphere it gets disturbed by our ever-moving atmosphere. Adaptive optics (AO) corrects for the distortions in an image caused by this atmospheric turbulence. The distortion to incoming light is shown schematically in Fig.3.14.

Fig.3.14. uniform waves of starlight reach Earth they distort due to the temperature variations in atmospheric cells.

As uniform waves of starlight reach Earth they distort due to the temperature variations in atmospheric cells. As light travels slightly faster in less dense warm air, the resultant refraction is non-uniform. This accounts 58

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Theory on Smart Materials and its Applications for the 'twinkling' of stars when seen from Earth's surface. The Fig.3.15 below shows how what should effectively be a point source from a distant star is smeared out due to turbulence. Adaptive optics compensates for this, resulting in a much sharper stellar image as seen on the right.

Fig.3.15. point source from a distant star is smeared out due to turbulence

Adaptive optics systems operate at high frequencies, typically about 1000 Hz. This is too fast for altering a primary mirror so adaptive optic systems are designed to act via the secondary mirror and additional optical elements placed in the light path. The need for high speed computer calculations and special deformable mirrors means that AO is a relatively new field of development in astronomy. Rapid progress in developing the technology for telescope use occurred following the end of the Cold War when much of the military technology for AO systems was declassified. A schematic of how adaptive optics systems, like Altair on Gemini North, works to correct distorted starlight is shown in Fig.3.16 & 3.17.

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Theory on Smart Materials and its Applications Fig.3.16.Altair correcting distorted starlight system

Fig3.17 Simple Adaptive Optic

The illustration (1) is an example of a blurry image taken without the help of adaptive optics. When starlight is collected and focused by the telescope, just prior to coming to a focus, the light entering an adaptive optics system is first collimated (2) and is reflected off a deformable mirror (3). After reflecting off the deformable mirror, the light passes through a beam-splitter (4) where the shorter wavelength light (optical) enters the wavefront sensor (5) which takes a "snapshot" of the distortions on the wavefront and sends the information via a computer (6) to the deformable mirror to keep the wave-fronts corrected and flat. Finally, the light is focused (7) and imaged on a detector (8) for astronomers to study. There are several different methods that can be used to monitor and correct the incoming wavefront of light but many use a tip-tilt mirror and a thin, deformable one. The Fig.3.18 below is used on Gemini North and has 85 actuators on it to control the mirror shape.

Fig.3.18.actuators controlling the mirror shape

Vital to all is the need for powerful, fast computer processing and modelling of the incoming waveforms. Systems either rely on a bright reference star within the field of view (which is surprisingly hard to find given the narrow field of view in many large telescopes) or they produce an artificial reference star using a laser. 60

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Theory on Smart Materials and its Applications Australia is an active participant in the development of adaptive optic systems through the Research Network for Adaptive Optics. Apart from astronomy, adaptive optics also has applications in the fields of ophthalmology and vision science, optical communications, laser beam shaping and laser countermeasures. At present, adaptive optics is still a new technology and many systems are in the developmental phase. Apart from the reference star problem, most systems also trade-off sensitivity for resolution as each additional optical element scatters some light and adds emits a small amount of heat, degrading infrared performance. Whilst adaptive optics compensates for atmospheric distortions, the deformation of the large primary mirrors is corrected by active optics as shown in Fig.3.19.

Fig.3.19.Basic Adaptive Optics Principle

3.6.1 Applications of Adaptive Optics 1. Used for laser wavefront control for intensity profile shaping. 2. For Atmospheric aberration compensation. 61

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Theory on Smart Materials and its Applications 3. for medical applications. 4. Imaging for astronomy, target inspection, ophthalmology. 3.7 Active Optics Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible. This method is used by, among others, the Nordic Optical Telescope, the New Technology Telescope, the Telescopio Nazionale Galileoand the Keck telescopes, as well as all of the largest telescopes built in the last decade. Active optics is not to be confused with adaptive optics, which operates at a shorter timescale and corrects atmospheric distortions. 3.7.1 Active Optics Components Active Optical Components are used to manipulate light through a variety of electrical methods, including adaptive reflection, variable diffusion, or tunable focusing. Active Optical Components are ideal for a wide variety life sciences, industrial, or research applications including for testing, illumination systems, wavefront manipulation, or microscopy. Active optics can also be used to replace complex optical systems with more compact, efficient alternatives, which can help reduce system size or remove the need for multiple systems or additional components. Edmund Optics offers a wide variety of Active Optical Components, including liquid lenses, variable diffusers, or laser speckle reducers, in addition to adaptive optics such as deformable mirrors. Focus-tunable lenses are liquid lenses that utilize a control current to adjust focus, removing the need for multi-lens focus or zoom systems. Adaptive optics, including deformable mirrors, are used to manipulate the wavefront in order to improve the quality of an optical system. 62

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Module 4 Acoustics and Controls 4.1 Structural Acoustics Most sounds that you hear throughout the day are radiated by vibrating structures. Walls and windows radiate sound into your house and office building. Windows radiate sound into your automobile, or into other vehicles, like buses, trains, and airplanes. The cones on the speakers of your stereo are vibrating structures that radiate sound into the air around you. However, these structures are usually not the original sources of the sounds you hear. For example, the walls and windows in your house are driven by acoustic pressure waves caused by passing vehicles, noisy neighbors (often with loud lawn and garden equipment such as leaf blowers), or by the wind through the trees. The pressures impinge on your windows, which in turn vibrate and pass some of the incident sound through to the interior. In airplanes and high-speed trains, tiny pressure waves within turbulence outside the vehicles drive the walls, which then vibrate and radiate sound. There are, of course, many other sources of vibration and the subsequent sound that we hear. Although often the sounds radiated by vibrating structures are annoying (your neighbor’s leaf blower), sometimes they are pleasing, like the sounds radiated by musical instruments. Pianos, violins, guitars, brass instruments, and the air within and around them are complex structural-acoustic systems. The sound from musical instruments (including the human voice) is often reproduced by audio equipment, such as CD players, amplifiers, and speakers. Speakers, with their multiple pulsating pistons mounted on the surfaces of boxes filled with air, are also very complex structural-acoustic systems, and engineers working for speaker companies spend entire careers trying to design systems that reproduce input signals exactly. 4.2 Compression and shear waves in isotropic, homogeneous structures

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Theory on Smart Materials and its Applications Structural materials, like metals, plastics, and rubbers, deform in ways far more complicated than air or water. This is because of one simple fact: structural materials can resist shear deformation, and fluids cannot. This means that both dilatational (and compressive) and shear waves can coexist in structures. Most structures have one or two dimensions that are very small with respect to internal wavelengths. We call these structures plates and beams, and they vibrate flexuraly as flexural waves are dispersive, which means that their wave speeds increase with increasing frequency. Dispersive waves are odd to those not familiar with structural vibrations. Imagine a long plate with two transverse sources at one end which excite flexural waves in the plate. One source drives the plate at a low frequency, while the other vibrates at a high frequency. The sources are turned on at the same time, and somehow the high frequency wave arrives at the other end of the plate faster than the low frequency wave. The simplest structural waves are those that deform an infinite material longitudinally and transversely. Longitudinal waves, sometimes called compressional waves, expand and contract structures in the same way acoustic waves deform fluids. The wave equation and sound speed for a longitudinal wave traveling in the x direction are: (1) (2) Where w is the deformation (also in the x direction), B is the elastic bulk modulus and ρ is the mass density. The bulk modulus relates the amount of volumetric contraction (per unit volume) to an applied pressure:

(3) Low volumetric changes mean stiffer structures, and faster compressional waves. 64

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Theory on Smart Materials and its Applications For audible frequencies, and for most practical structures, one or two geometric dimensions are small with respect to a wavelength. As a longitudinal wave expands or contracts a beam or plate in its direction of propagation, the walls of the structure contract and expand transversely due to the Poisson effect, as shown in Fig.4.1. The Poisson’s ratio, which relates in and out of plane strain deformations according to: (4) determines the amount of the off-axis deformation, which for incompressible materials like rubber approaches the amount of the on-axis deformation (a Poisson’s ratio of 0.5).

Fig.4.1. A longitudinal wave passing through a plate or beam

Longitudinal waves are therefore slower in structures like beams and plates, since the free surfaces of the structural material are exposed to air or fluid. Since the stiffness of most fluids that might surround a beam or plate is smaller than that of the structural material, the free surfaces of the structure act essentially as stress relievers, slowing down the compressional waves. The sound speeds of longitudinal waves in beams and plates are:

(5)

(6) Where cl is defined not by the Bulk Modulus, but by the Young’s Modulus E, which is related to the volumetric Bulk Modulus according to: 65

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(7) For a typical Poisson’s ratio of 0.3, longitudinal wave speeds in plates and beams are 90% and 86% of those in infinite structural media, respectively. The key difference between acoustic waves in structural materials and fluid media is a structure’s ability to resist shear deformation. This shear stiffness allows pure shear waves to propagate through a structure, with the structure deforming in its transverse direction as the wave propagates in the axial direction as shown in Fig.3.2. Shear wave behavior is governed by the same wave equation as longitudinal waves, and acoustic waves in fluid media:

(8) For Shear waves, which travel at the speed are slower than longitudinal waves, since a structure’s shear modulus is smaller than its Bulk and Young’s Moduli. (9) The shear modulus G is related to E and Poisson’s ratio according to: (10)

Fig.4.2. A shear wave propagating through a plate or beam

4.3 Bending waves in beams and plates

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Theory on Smart Materials and its Applications Most sound radiated by vibrating structures is caused by bending, or flexural waves traveling through beams, plates, and shells, like the example shown in Fig.4.3. Bending waves deform a structure transversely, so that they excite acoustic waves in neighboring fluids. Although longitudinal and shear wave behavior is simple and similar to that of acoustic waves in air or water while bending waves are far more complicated. In particular, the speed of a bending wave depends not only on the elastic moduli and density of the structural material it travels through, but also on the geometric properties of the beam or plate cross section. Also, bending wave speeds are dispersive, with the curious property of depending on their frequency of oscillation.

Fig.4.3. A flexural, or bending wave propagating through a plate or beam

The wave equation and wave speed for flexure in thin beams are: (11) (12) Note: the wave speed does not appear explicitly in the flexural wave equation, and it depends on frequency 4.4 Digital Controller Since the modern age, mankind has undertaken huge technological development projects which are directed towards making life easy. Some of these technological developments resulted in large machineries, cutting edge robots and critical systems components. These inventions needed some accurate control systems which did not required man’s intervention; the control system was invented. 67

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The first control systems were analogue and were later digitalised. In a perfect situation with no errors Y = X G, but this is not true in real life because of errors and energy conversion as shown in Fig.4.4.

Fig.4.4.Block diagram of digital controller

4.4.1 Digital Feedback controller The concept of a digital feedback control was coin which feedbacks informed the controller about the errors in the out and some measured were taken by the controller to address the errors. Errors in the output means the targeted output will not either be reached or surpass as shown in Fig.4.5. There are generally two types of feedback controls – (i) negative and (ii) positive digital feedback controls.

Fig.4.5.Block diagram of Digital feedback control

In a positive digital feedback control system the target output is not reached; measures are taken to increase the output. While in a negative digital feedback control the target output is surpass; some measures are taken to reduce the output. A negative digital feedback controller is more stable than a positive digital feedback controller. It makes the controller immune to random variations in component values and inputs. A perfect 68

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Theory on Smart Materials and its Applications digital control is one with no errors in the loop; when the systems output is equal to the targeted output as shown in Fig.4.6.

Fig.4.6.Block diagram for a feedback loop along with an algebraic proof

Example:

An example which demonstrates the use of a digital feedback control can be seen in a cruise control of a car. The input is the car’s gas pedal controlled by the driver, the system is the car’s engine and the output is the velocity of the car. When a cruise control system is engaged the gas pedal is automatically adjusted to maintain a predefined velocity (target speed) which is passed onto the feedback digital controller. The actual speed of the car is feedback into the feedback digital controller which compares it with the target speed. The feedback digital controller will either increase or decrease the speed depending weather the car is driving or slower than expected. This is illustrated in the block diagram of the feedback digital controller shown in Fig.4.5 & 4.6. Digital feedback controls are used in almost critical systems such as in marine systems, aerospace, industrial plants, automobile, robotics, just to names these few.

4.5 control system design Designing control system is very challenging as there are usually very specific requirements which must be respected to the very detail. In most cases the amplitude of the signal will rise above the target level (overshoot) before settling at the targeted amplitude. The speed at which the signal reaches the targeted amplitude will determine the performance of the controller. The steady state error is the level of error tolerance the controller and it depends on the signal input (sine/cosine, ramp, parabola or step). The 69

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Theory on Smart Materials and its Applications shorter the settling time, the better performance the controller will have as shown in Fig.4.7.

Fig.4.7.Amplitude v/s Time Response

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Module 5 Principles of Vibration & Modal Analysis, Information Processing 5.1 Introduction to vibration Vibration is a mechanical phenomenon whereby oscillations occur about an equilibrium point. The oscillations may be periodic, such as the motion of a pendulum—or random, such as the movement of a tire on a gravel road. Vibration can be desirable: for example, the motion of a tuning fork, the reed in a woodwind instrument or harmonica, a mobile phone, or the cone of a loudspeaker. In many cases, however, vibration is undesirable, wasting energy and creating unwanted sound. For example, the vibrational motions of engines, electric motors, or any mechanical device in operation are typically unwanted. Such vibrations could be caused by imbalances in the rotating parts, uneven friction, or the meshing of gear teeth. Careful designs usually minimize unwanted vibrations. The studies of sound and vibration are closely related. Sound, or pressure waves, are generated by vibrating structures (e.g. vocal cords); these pressure waves can also induce the vibration of structures (e.g. ear drum). Hence, attempts to reduce noise are often related to issues of vibration. 5.1.2 Types of Vibrations Free vibration occurs when a mechanical system is set in motion with an initial input and allowed to vibrate freely. Examples of this type of vibration are pulling a child back on a swing and letting go, or hitting a tuning fork and letting it ring. The mechanical system vibrates at one or more of its natural frequencies and damps down to motionlessness. Forced vibration is when a time-varying disturbance (load, displacement or velocity) is applied to a mechanical system. The disturbance can be a periodic and steady-state input, a transient input, or a random input. The 71

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Theory on Smart Materials and its Applications periodic input can be a harmonic or a non-harmonic disturbance. Examples of these types of vibration include a washing machine shaking due to an imbalance, transportation vibration caused by an engine or uneven road, or the vibration of a building during an earthquake. For linear systems, the frequency of the steady-state vibration response resulting from the application of a periodic, harmonic input is equal to the frequency of the applied force or motion, with the response magnitude being dependent on the actual mechanical system. 5.2 Vibration Analysis Vibration Analysis (VA), applied in an industrial or maintenance environment aims to reduce maintenance costs and equipment downtime by detecting equipment faults. VA is a key component of a Condition Monitoring (CM) program, and is often referred to as Predictive Maintenance (PdM). Most commonly VA is used to detect faults in rotating equipment (Fans, Motors, Pumps, and Gearboxes etc.) such as Unbalance, Misalignment, rolling element bearing faults and resonance conditions. VA can use the units of Displacement, Velocity and Acceleration displayed as a Time Waveform (TWF), but most commonly the spectrum is used, derived from a Fast Fourier Transform of the TWF. The vibration spectrum provides important frequency information that can pinpoint the faulty component. The fundamentals of vibration analysis can be understood by studying the simple mass–spring–damper model. Indeed, even a complex structure such as an automobile body can be modeled as a "summation" of simple mass– spring–damper models. The mass–spring–damper model is an example of a simple harmonic oscillator. The mathematics used to describe its behavior is identical to other simple harmonic oscillators such as the RLC circuit. 5.3 Modal Analysis Modal analysis is the study of the dynamic properties of structures under vibrational excitation.

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Theory on Smart Materials and its Applications Modal analysis is the field of measuring and analysing the dynamic response of structures and or fluids during excitation. Examples would include measuring the vibration of a car's body when it is attached to an electromagnetic shaker, or the noise pattern in a room when excited by a loudspeaker. Modern day modal analysis systems are composed of 1)sensors such as transducers (typically accelerometers, load cells), or non contact via a Laser vibrometer, or stereo photogrammetric cameras 2) data acquisition system and an analog-to-digital converter frontend (to digitize analog instrumentation signals) and 3) host PC (personal computer) to view the data and analyze it. Classically this was done with a SIMO (single-input, multiple-output) approach, that is, one excitation point, and then the response is measured at many other points. In the past a hammer survey, using a fixed accelerometer and a roving hammer as excitation, gave a MISO (multiple-input, singleoutput) analysis, which is mathematically identical to SIMO, due to the principle of reciprocity. In recent years MIMO (multi-input, multiple-output) have become more practical, where partial coherence analysis identifies which part of the response comes from which excitation source. Using multiple shakers leads to a uniform distribution of the energy over the entire structure and a better coherence in the measurement. A single shaker may not effectively excite all the modes of a structure.[1] Typical excitation signals can be classed as impulse, broadband, swept sine, chirp, and possibly others. Each has its own advantages and disadvantages. The analysis of the signals typically relies on Fourier analysis. The resulting transfer function will show one or more resonances, whose characteristic mass, frequency and damping can be estimated from the measurements. The animated display of the mode shape is very useful to NVH (noise, vibration, and harshness) engineers. The results can also be used to correlate with finite element analysis normal mode solutions. 5.4 Piezoelectric Materials and Peizoelectricity

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Theory on Smart Materials and its Applications Certain materials produce electric charges on their surfaces as a consequence of applying mechanical stress. The induced charges are proportional to the mechanical stress. This is called the direct piezoelectric effect and was discovered in quartz by Piere and Jacques Curie in 1880. Materials showing this phenomenon also conversely have a geometric strain proportional to an applied electric field. This is the converse piezoelectric effect. The root of the word “piezo” means “pressure”; hence the original meaning of the word piezoelectricity implied “pressure electricity.” Piezoelectricity is extensively utilized in the fabrication of various devices such as transducers, actuators, surface acoustic wave devices, frequency control and so on. 5.4.1 Peizoelectric Actuators Piezoelectric and electrostrictive devices have become key components in smart actuator systems such as precision positioners, miniature ultrasonic motors and adaptive mechanical dampers. This section reviews the developments of piezoelectric and related ceramic actuators with particular focus on the improvement of actuator materials, device designs and applications of the actuators. Piezoelectric actuators are forming a new field between electronic and structural ceramics. Application fields are classified into three categories: positioners, motors and vibration suppressors. The manufacturing precision of optical instruments such as lasers and cameras, and the positioning accuracy for fabricating semiconductor chips, which must be adjusted using solid-state actuators, are generally on the order of 0.1 μm. Regarding conventional electromagnetic motors, tiny motors smaller than 1 cm3 are often required in office or factory automation equipment and are rather difficult to produce with sufficient energy efficiency. Ultrasonic motors whose efficiency is insensitive to size are considered superior in the mini-motor area. Vibration suppression in space structures and military vehicles using piezoelectric actuators is another promising field of application. New solid-state displacement transducers controlled by temperature (shape memory alloy) or magnetic field (magnetostrictive alloy) have been proposed, but are generally inferior to the piezoelectric/electrostrictive ceramic actuators because of current technological trends aimed at reduced driving power and miniaturization. The shape memory actuator is too slow 74

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Theory on Smart Materials and its Applications in response with a very low energy efficiency, while the magnetostrictor requires a driving coil which is very bulky and generates magnetic noise. An actuator is the generic name referring to devices that convert input energy into mechanical energy, and various actuators have been developed and put to practical use according to various types of input energy (Fig.5.1). The electromagnetic, hydraulic and pneumatic actuators achieve displacement indirectly by moving a piston by electromagnetic force or pressure. On the other hand, the piezoelectric actuator achieves displacement by directly applying deformation of a solid, and thus features a higher displacement accuracy, larger generation force and higher response speed than other types of actuators. These advantages have resulted in the piezoelectric actuator being applied mainly in industrial equipment requiring precision position control, such as the ultrafine-movement stage of semiconductor exposure systems, precision positioning probes and probes for scanning tunnel microscopy (STM) and atomic force microscopy (AFM). In addition, other advantages including the non-necessity of a driving coil, ease of implementation of small devices, high energy conversion efficiency and low power consumption have recently led to application in consumer equipment such as digital cameras and cellular phone terminals.

Fig.5.1.Schematic of Various actuators

5.4.2 Features of Peizoelectric Actuator 75

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The piezoelectric ceramic material used in the piezoelectric actuator generates electrical energy when it is subjected to mechanical energy (piezoelectric effect) and generates mechanical energy when it is subjected to electrical energy (inverse piezoelectric effect) (Fig.5.2). The piezoelectric actuator is a device that makes use of the inverse piezoelectric effect. For example, when a voltage of about 1,000V is applied to a piezoelectric ceramic plate with a thickness of 1mm (1,000V/mm electrical field), a displacement of about 1μm is obtained due to the inverse piezoelectric effect. However, as this is in practice insufficient, because only a small displacement can be obtained with a high drive voltage, the piezoelectric actuators are structurally processed in order to obtain a larger displacement from a lower drive voltage and the process has thus been put to practical use.

Fig.5.2.Functions of piezoelectric ceramic

In order to reduce the drive voltage of a piezoelectric actuator, it is necessary to reduce the thickness of the ceramic plate. For example, reducing the plate thickness to 0.5mm makes it possible to apply a 1,000V/mm electrical field with a 500V drive voltage, which results in a 76

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Theory on Smart Materials and its Applications reduction in the drive voltage. Fig.5.3 shows the scheme of typical piezoelectric actuators. Fig.5.3 (a) is an actuator called the bimorph piezoelectric actuator. It is fabricated by processing two piezoelectric ceramic plates to a thickness of some hundreds of μm and bonding them by inserting a metallic plate between them. When an inverse voltage is applied to two piezoelectric plates, warp deformation is a consequence. This arrangement can offer a relatively large displacement but the force is not large. This device is implemented in a cantilever construction for use in positioning mechanisms, etc. and the drive voltage is usually some hundreds of volts. Fig.5.3 (b) is an actuator called the multilayer piezoelectric actuator (hereinafter referred to as the “multilayer actuator”). It is fabricated by multilayer ceramic films of 100μm thickness, each of which is formed by the green sheet process and electrode films of a few micrometers thickness, which are then sintered together, the resulting structure being similar to a ceramic capacitor. The multilayer actuator features higher displacement accuracy, larger generated force and higher response speed because of the lower drive voltage due to the reduction in the ceramic plate thickness per layer and of the possibility of utilizing the distortion and rigidity of the ceramic material without adopting means such as a metallic rim.

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Fig.5.3.Typical examples of piezoelectric actuators

5.4.3 Applications of Peizoelectric Actuator The field of applications of piezoelectric actuators is comparable to that of electromagnetic actuators. The piezoelectric actuator has disadvantages compared to the electromagnetic actuator in terms of its displacement amount. However, it is advantageous from other aspects, including that of its displacement accuracy, generated force and response speed and energy efficiency as well as from the aspect of ease of proportional control and absence of electromagnetic noise. They can be used: 1. As AE Series multilayer actuator, which is coated with resin and fabricated using the high-performance piezoelectric material NEPEC as a unique full-face electrode structure, in which electric field and stress are almost non-existent as shown in Fig 5.4. 78

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Theory on Smart Materials and its Applications 2. As multilayer actuator in the mass-flow controller for use in semiconductor fabrication systems that require ultra-precise flow control as shown in Fig 5.5. 3. In the semiconductor fabrication systems field have expanded, including application in the precision position control stage of an exposure system as shown in Fig.5.6. 4. For the optical axis alignment of optical fiber as shown in Fig.5.7. 5. For the CCD (Charge Coupled Device) drive as shown in Fig.5.8.

Fig.5.4.AE Multilayer Actuator

Fig.5.5.Mass flow controller

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Theory on Smart Materials and its Applications Fig.5.6.Precision Stage

Fig.5.7.Optical fiber axis alignment mechanism

Fig.5.8.Hand blurring correction of digital cameras

5.5 Magnetic shape memory alloys The shape memory effect is the ability of some alloys to remember, and return to, the form which they had at one temperature after being plastically deformed into another shape at a lower temperature. This property can be exploited in many ways, for example, in actuators controlled by heat.Devices based on shape memory alloys are being developed in fields as far apart as astronautics and medicine. In order for an alloy to have the shape-memory property, it must undergo what is called a martensitic transition. This derives its name from a change in crystal structure when steel is cooled rapidly to form so-called martensite, which has a variety of characteristic microstructures. The transition involves small displacements, or slips, between planes of atoms in the crystal at a certain temperature. The shape memory alloy is first formed at a temperature above that of the transition and then deformed below it. The memory arises because residual stresses in the structure introduced during the forming process, influence which slips then occur in the martensitic transformation, and thus which variants of the martensitic phase are present at the lower temperature. Shape memory alloys scatter neutrons very effectively, so neutrons are an almost ideal probe with which to view the martensitic transformation at a microstructural level. Neutron diffraction can follow the evolution of different martensite variants as the temperature is changed. Magnetism: 80

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Theory on Smart Materials and its Applications Combining the shape memory property with ferromagnetism, vastly increases the range of applications. Magnetic fields can also influence the martensitic transition, and the possibility of controlling shape memory properties using a magnetic field is currently receiving much attention. An alloy made of nickel, manganese and gallium (Ni2MnGa) is one of the rare ferromagnetic shape memory alloys. It undergoes a martensitic transition from a cubic structure to a tetragonal variant at a temperature around 200K. Small position-sensitive detector on 4-circle diffractometer (D9) records the scattered neutrons. The Fig.5.9 below shows how the diffraction pattern evolves before, during and after the martensitic transition. At 235K, above the transition, the scattering shows a single compact peak associated with the cubic phase. At 206K, the martensitic transformation is under way, and the original single peak has broken up into seven smaller peaks, each corresponding to a different martensite variant. On further cooling, two of the variants grow at the expense of the others so that at 200K there are just two peaks in the pattern. On reheating the process is reversed and the original cubic single crystal restored.

Fig.5.9. A 3D representation of the neutron scattering peaks as the crystal of the ferromagnetic shape memory alloy is cooled through the martensitic transition

5.5.1 Magnetic shape memory effect Magnetic-field induced reorientation, or the magnetic shape memory effect, is defined as “the magnetic-field-induced rearrangement of (ferromagnetic) twinned martensite microstructure along with a large macroscopic deformation”. 81

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5.5.2 Properties of Magnetic SMA Magnetic shape memory effect of up to 6% elongation in a magnetic field. It exhibits inverse magneto-strictive effect. It shows shape memory alloy effect with a shape change caused by applying a magnetic field as well as a shape change is caused by temperature. 4. If the element is completely compressed or elongated, a change in resistance occurs. 5. Exhibits controlled spring properties. 1. 2. 3.

5.5.3 Applications of Magnetic SMA 1. Actuators The magnetic shape memory effect can be used for designing actuators shown in Fig.5.10 where the element elongates based on the presence of a magnetic field. The elongation can be reversed fully either by the application of a magnetic field at 90° to the original field, or more effectively by use of a spring. The change in shape is very quick and cycle times of 1 to 2 kHz have been shown. During fatigue life testing several million cycles have been achieved. These materials cannot achieve the frequency of piezo-based materials or magnetostrictive materials, however they offer much higher strain outputs (typically 10 to 100 times more). They also offer higher energy density typically up to 100kJm-3 compared to 14 to 30kJm-3 for magnetostrictive materials and 0.8 to 2kJm-3 for piezo-based materials.

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Fig.5.10.actuator with return spring

2. Breaker Switch/Fuse The thermal shape memory property whereby the material elongates more above 70°C can be used as a safety cut-out. If a safe working temperature is exceeded, there is further extension cutting the magnetic field generation. This additional elongation can be reversed fully and below 70°C, normal functioning of the actuator is observed. 3. Energy Harvestors The elongation or compression of the material causes it to change any magnetic field in which it is placed, known as variable magnetic permeability under varying stress, which can be used for harvesting vibrational energy. Possible uses include battery charging in environments where it is difficult to gain access to the batteries for replacement. 4. Vibration Dampers The same properties used to create energy harvesters can also be used to dampen mechanical vibration. 5. Sensors 83

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Theory on Smart Materials and its Applications Using the material’s properties, it is possible to construct speed sensors as well as distance, strain and magnetic field detectors. 5.6 Data Reliability Data reliability is a state that exists when data is sufficiently complete and error free to be convincing for its purpose and context. In addition to being reliable, data must also meet other tests for evidence. Computer-processed data must meet evidence standards before it can support a finding: 1. Relevance Relevant if it has a logical, sensible relationship to the finding it supports. What data is relevant to answering an audit objective is usually self-evident, presuming a precise objective written as a question. Timeliness (the age of the evidence) must be considered, as outdated data is considered irrelevant. As a result, relevance is closely tied to the scope of the audit work, which establishes what time period will be covered. Data is relevant if they have a logical, sensible relationship to the overall audit objective in terms of:  the audit subject  the aspect of performance being examined  the finding element to which the evidence pertains, and  the time period of the issue being audited 2. Sufficient Sufficient if there is enough of it to support the finding. Sufficiency establishes that evidence or data provided has not been overstated or inappropriately generalized. Like relevance, sufficiency must be judged in relationship to the finding element to which the data pertains, and is closely tied to the audit scope. The audit scope establishes what portion of the universe is covered (important for sufficiency) through 3 choices:  obtain data on (mine) the entire universe  sample the universe  limit findings to that portion or segment of the universe they examine 84

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Theory on Smart Materials and its Applications 3. Competant Competent if it is both valid and reliable. In assessing computer-processed data, the focus is usually on one test in the evidence standard— competence—which includes both validity and reliability. "Auditors should determine if other auditors have worked to stablish the validity and reliability of the data or the effectiveness of the controls over the system that produced it. If they have, auditors may be able to use that work. If not, auditors can obtain evidence about the competence of computer-processed data by direct tests of the data (through or around the computer, or a combination of both.) Auditors can reduce the direct tests of the data if they test the effectiveness of general and application controls over computerprocessed data, and these tests support the conclusion that controls are effective." 5.6.1 Data Reliability Testing Data reliability refers to the accuracy and completeness of computerprocessed data, given the intended purposes for use. Reliability does not mean that computer-processed data is error free. It means that any errors found were within a tolerable range - that you have assessed the associated risk and found the errors are not significant enough to cause a reasonable person, aware of the errors, to doubt a finding, conclusion, or recommendation based on the data. Data can refer to either information that is entered into a system or information generated as a result of computer processing. Data is considered reliable when it is:  Complete - includes all of the data elements and records needed for the engagement. A data element is a unit of information with definable parameters and is also called a data variable or data field  Accurate: Consistent - data was obtained and used in a manner that is clear and well-defined enough to yield similar results in similar analysis. Correct - the data set reflects the data entered at the source.  Unaltered data reflects source and has not been tampered with. 85

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Theory on Smart Materials and its Applications 5.6.2 Data Processing Data processing is, generally, "the collection and manipulation of items of data to produce meaningful information. It can be considered a subset of information processing which is defined as the change (processing) of information in any manner detectable by an observer. Some of the data processing processes are: 1. Manual data processing 2. Automatic data processing 3. Electronic data processing 5.6.3 Applications of data processing Some of the applications are: 1. Commercial data processing 2. Data analysis 1. Commercial data processing Commercial data processing involves a large volume of input data, relatively few computational operations, and a large volume of output. For example, an insurance company needs to keep records on tens or hundreds of thousands of policies, print and mail bills, and receive and post payments. 2. Data analysis For science or engineering, the terms data processing and information systems are considered too broad, and the more specialized term data analysis is typically used. Data analysis uses specialized and precise algorithms and statistical calculations that are less often observed in a typical general business environment. For data analysis, softwares like SPSS or SAS, or their free counterparts such as DAP, gretl or PSPP are often used. 5.6.4 Data visualization Data visualization or data visualisation is viewed by many disciplines as a modern equivalent of visual communication. It involves the creation and study of the visual representation of data i.e., "information that has been abstracted in some schematic form, including attributes or variables for the units of information. 86

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Theory on Smart Materials and its Applications A primary goal of data visualization is to communicate information clearly and efficiently via statistical graphics, plots and information graphics. Numerical data may be encoded using dots, lines, or bars, to visually communicate a quantitative message. Effective visualization helps users analyze and reason about data and evidence. It makes complex data more accessible, understandable and usable. Users may have particular analytical tasks, such as making comparisons or understanding causality, and the design principle of the graphic (i.e., showing comparisons or showing causality) follows the task. Tables are generally used where users will look up a specific measurement, while charts of various types are used to show patterns or relationships in the data for one or more variables. 5.7 Introduction to MEMS This concept deals with the emerging field of micro-electromechanical systems, or MEMS. MEMS is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical components. They are fabricated using integrated circuit (IC) batch processing techniques and can range in size from a few micrometers to millimetres. These devices (or systems) have the ability to sense, control and actuate on the micro scale, and generate effects on the macro scale. The interdisciplinary nature of MEMS utilizes design, engineering and manufacturing expertise from a wide and diverse range of technical areas including integrated circuit fabrication technology, mechanical engineering, materials science, electrical engineering, chemistry and chemical engineering, as well as fluid engineering, optics, instrumentation and packaging. MEMS can be found in systems ranging across automotive, medical, electronic, communication and defense applications. Some of the New MEMS devices include accelerometers for airbag sensors, inkjet printer heads, computer disk drive read/write heads, projection display chips, blood pressure sensors, optical switches, microvalves, biosensors and many other products that are all manufactured and shipped in high commercial volumes. MEMS have been identified as one of the most promising technologies for the 21st Century and has the potential to revolutionize both industrial and 87

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Theory on Smart Materials and its Applications consumer products by combining silicon based microelectronics with micromachining technology. Its techniques and micro system based devices have the potential to dramatically affect of all of our lives and the way we live. If semiconductor micro-fabrication was seen to be the first micromanufacturing revolution, MEMS is the second revolution. 5.7.1 Micro Electro-Mechanical Systems A micro-electromechanical system (MEMS) is a process technology used to create tiny integrated devices or systems that combine mechanical and electrical components. These devices are fabricated using integrated circuit (IC) batch processing techniques and can range in size from a few micrometers to millimeters and have the ability to sense, control and actuate on the micro scale, and generate effects on the macro scale. In the most general form, MEMS consist of mechanical micro-structures, micro-sensors, micro-actuators and microelectronics, all integrated onto the same silicon chip. This is shown schematically in Fig.5.11.

Fig.5.11.Components of MEMS

Microsensors detect changes in the system’s environment by measuring mechanical, thermal, magnetic, chemical or electromagnetic information or phenomena. Microelectronics process this information and signal the microactuators to react and create some form of changes to the environment. 88

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Theory on Smart Materials and its Applications MEMS devices are very small and are usually microscopic as Levers, gears, pistons, as well as motors and even steam engines have all been fabricated by MEMS as shown in Fig.5.12. MEMS is not just about the miniaturization of mechanical components or making things out of silicon, it is a manufacturing technology; a paradigm for designing and creating complex mechanical devices and systems as well as their integrated electronics using batch fabrication techniques.

Fig.5.12.Examples for MEMS

5.7.2 Advantages of MEMS MEMS have several distinct advantages as a manufacturing technology: 1. Interdisciplinary nature of MEMS technology and its micromachining techniques, as well as its diversity of applications has resulted in an unprecedented range of devices and synergies across previously unrelated fields (for example biology and microelectronics). 2. MEMS with its batch fabrication techniques enables components and devices to be manufactured with increased performance and reliability, combined with the obvious advantages of reduced physical size, volume, weight and cost. 3. MEMS provide the basis for the manufacture of products that cannot be made by other methods. These factors make MEMS potentially a far more pervasive technology than integrated circuit microchips. However, there are many challenges and 89

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Theory on Smart Materials and its Applications technological obstacles associated with miniaturization that need to be addressed and overcome before MEMS can realize its overwhelming potential. 5.7.3 Classification of Micro systems technology (MST) Fig.5.13 illustrates the classifications of microsystems technology (MST) and MEMS is also referred to as MST as it is a process technology used to create these tiny mechanical devices or systems, and as a result, it is a subset of MST. A micro-opto-electro-mechanical system (MOEMS) is also a subset of MST and together with MEMS forms the specialized technology fields using miniaturized combinations of optics, electronics and mechanics. Both their micro-systems incorporate the use of microelectronics batch processing techniques for their design and fabrication.

Fig.5.13.Classification of Micro Systems Technology

5.7.4 Transducer, Sensor and Actuator 1. A transducer is a device that transforms one form of signal or energy into another form, and includes both sensors and actuators. 90

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Theory on Smart Materials and its Applications 2. A sensor is a device that measures information from a surrounding environment and provides an electrical output signal in response to the parameter it measured. 3. An actuator is a device that converts an electrical signal into an action and can create a force to manipulate itself, other mechanical devices, or the surrounding environment to perform some useful function. 5.7.5 Applications of MEMS 1. Automotive – internal navigation sensors, Air conditioning compressor sensor, Brake force sensors & Suspension control accelerometers, Fuel level and vapour pressure sensors, Airbag sensors, intelligent tyres. 2. Electronics – disk drive heads, inkjet printer head, projection screen televisions, earth quake sensors, avionic pressure sensors, mass data storage systems. 3. Medical – Blood pressure sensors, muscle simulator & drug delivery systems, implanted pressure sensors, prosthetics, miniature analytical instruments, pacemakers. 4. Communications – fiber-optic network components, RF relay-switchesfilters, projection displays, voltage controlled oscillators, splitters & couplers, tuneable lasers. 5. Defense – munitions guidance, surveillance, arming systems, embedded sensors, data storage, aircraft control. 5.7.6 Miniaturization As MEMS is not about miniaturization, it is a manufacturing technology used to create tiny integrated micro-devices and systems using IC batch fabrication techniques while miniaturization is not just about shrinking down existing devices but it’s about completely rethinking the structure of a micro-system. In order to manufacture a successful MEMS device, basic physics and operating principles including scaling laws need to be fully understood and appreciated at both a macro and micro-level. Sometimes no advantages in terms of performance, size/weight, reliability and cost can be gained with a MEMS device. In such as case, Increased surface area (S) to volume (V) ratios 91

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Theory on Smart Materials and its Applications at micro-scales have both considerable advantages and disadvantages (Fig.5.14).

Fig.5.14.Effect of miniaturization on surface area and volume

Some of the micro-level issues include:  Friction is greater than inertia. Capillary, electrostatic and atomic forces as well as stiction at a micro-level can be significant.  Heat dissipation is greater than heat storage.  Fluidic or mass transport properties are extremely important.  Material properties (Young’s modulus, Poisson’s ratio, grain structure) and mechanical theory (residual stress, wear and fatigue etc.) may be size dependent.  Integration with on-chip circuitry is complex and device/domain specific.  Miniature device packaging and testing is not straightforward. Certain MEMS sensors require environmental access as well as protection from other external influences. Testing is not rapid and is expensive in comparison with conventional IC devices.  Cost – for the success of a MEMS device, it needs to leverage its IC batch fabrication resources and be mass-produced. Hence massmarket drivers must be found to generate the high volume production. 5.7.7 Fabrication Methods for MEMS MEMS fall into three general classification: 1. Bulk micromachining 2. Surface micromachining 3. High-aspect-ratio micromachining (HARM) - Lithography 92

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1. Photolithography Photolithography is the photographic technique to transfer copies of a master pattern, usually a circuit layout in IC applications, onto the surface of a substrate of some material (usually a silicon wafer). The substrate is covered with a thin film of some material, usually silicon dioxide (SiO2), in the case of silicon wafers, on which a pattern of holes will be formed as shown in Fig.5.15. A thin layer of an organic polymer, which is sensitive to ultraviolet radiation, is then deposited on the oxide layer which is called a photoresist.

Fig.5.15.Photolithography Principle

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Theory on Smart Materials and its Applications A photomask, consisting of a glass plate (transparent) coated with a chromium pattern (opaque), is then placed in contact with the photoresist coated surface. The wafer is exposed to the ultraviolet radiation transferring the pattern on the mask to the photoresist which is then developed in a way very similar to the process used for developing photographic films. The radiation causes a chemical reaction in the exposed areas of the photoresist of which there are two types; positive and negative. Positive photoresist is strengthened by UV radiation whereas negative photoresists are weakened. On developing, the rinsing solution removes either the exposed areas or the unexposed areas of photoresist leaving a pattern of bare and photoresist-coated oxides on the wafer surface. The resulting photoresist pattern is either the positive or negative image of the original pattern of the photomask. A chemical (usually hydrochloric acid) is used to attack and remove the uncovered oxide from the exposed areas of the photoresist. The remaining photoresist is subsequently removed, usually with hot sulphuric acid which attacks the photoresist but not the oxide layer on the silicon, leaving a pattern of oxide on the silicon surface. The final oxide pattern is either a positive or negative copy of the photomask pattern and serves as a mask in subsequent processing steps. 2. Materials used to Micromachining (a) Substrates The most common substrate material for micromachining is silicon. It has been successful in the microelectronics industry and will continue to be in areas of miniaturization for several reasons:  Silicon is abundant, inexpensive, and can be processed to unparalleled purity.  Silicon’s ability to be deposited in thin films is very amenable to MEMS.  High definition and reproduction of silicon device shapes using photolithography are perfect for high levels of MEMS precision.  Silicon microelectronics circuits are batch fabricated. (b) Additive Films and Materials

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Theory on Smart Materials and its Applications The range of additive films and materials for MEMS devices is much larger than the types of possible substrates and includes conductors, semiconductors and insulators such as:  Silicon - single crystal, polycrystalline and amorphous.  Silicon compounds (SixNy, SiO2, SiC etc.).  Metals and metallic compounds (Au, Cu, Al, ZnO, GaAs, IrOx, CdS).  Ceramics (Al203 and more complex ceramic compounds).  Organics (diamond, polymers, enzymes, antibodies, DNA etc.). 3. Bulk Micromachining Bulk micromachining involves the removal of part of the bulk substrate. It is a subtractive process that uses wet anisotropic etching or a dry etching method such as reactive ion etching (RIE), to create large pits, grooves and channels. Materials typically used for wet etching include silicon and quartz, while dry etching is typically used with silicon, metals, plastics and ceramics. (a) Wet Etching Wet etching describes the removal of material through the immersion of a material (typically a silicon wafer) in a liquid bath of a chemical etchant. These etchants can be isotropic or anisotropic. Isotropic etchants etch the material at the same rate in all directions, and consequently remove material under the etch masks at the same rate as they etch through the material and this is known as undercutting (Fig.5.16). The most common form of isotropic silicon etch is HNA, which comprises a mixture of hydrofluoric acid (HF), nitric acid (HNO 3) and acetic acid (CH3COOH). Isotropic etchants are limited by the geometry of the structure to be etched. Etch rates can slow down and in some cases (for example, in deep and narrow channels) they can stop due to diffusion limiting factors. However, this effect can be minimized by agitation of the etchant, resulting in structures with near perfect and rounded surfaces. Anisotropic etchants etch faster in a preferred direction. Potassium hydroxide (KOH) is the most common anisotropic etchant as it is relatively safe to use. Structures formed in the substrate are dependent on the crystal orientation of the substrate or wafer. 95

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Fig.5.16.Wet etching

(b) Dry Etching Dry etching relies on vapour phase or plasma-based methods of etching using suitably reactive gases or vapours usually at high temperatures. The most common form for MEMS is reactive ion etching (RIE) which utilizes additional energy in the form of radio frequency (RF) power to drive the chemical reaction. Energetic ions are accelerated towards the material to be etched within a plasma phase supplying the additional energy needed for the reaction, as a result the etching can occur at much lower temperatures (typically 150º 250ºC, sometimes room temperature) than those usually needed (above 1000ºC). RIE is not limited by the crystal planes in the silicon, and as a result, deep trenches and pits, or arbitrary shapes with vertical walls can be etched. Deep Reactive Ion Etching (DRIE) is a much higher-aspect-ratio etching method that involves an alternating process of high-density plasma etching (as in RIE) and protective polymer greater aspect ratios as shown in Fig.5.17. Etch rates depend on time, concentration, temperature and material to be etched.

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Fig.5.17.Deep Reactive Ion Etching

4. Surface Micromachining Surface micromachining involves processing above the substrate, mainly using it as a foundation layer on which to build. Material is added to the substrate in the form of layers of thin films on the surface of the substrate (typically a silicon wafer). These layers can either by structural layers or act as spacers, later to be removed, when they are known as sacrificial layers. Hence the process usually involves films of two different materials: (a) Structural material out of which the free standing structure is made & (b) Sacrificial material, deposited wherever either an open area or a free standing mechanical structure is required. These layers (or thin films) are deposited and subsequently dry etched in sequence, with the sacrificial material being finally wet etched away to release the final structure. Each additional layer is accompanied by an increasing level of complexity and a resulting difficulty in fabrication. A typical surface micromachined cantilever beam is shown in Fig.5.18. Here, a sacrificial layer of oxide is deposited on the silicon substrate surface using a pattern and photolithography. A polysilicon layer is then deposited and patterned using RIE processes to form a cantilever beam with an anchor pad. The wafer is then wet etched to remove the oxide (sacrificial) layer releasing and leaving the beam on the substrate.

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Theory on Smart Materials and its Applications

Fig.5.18.Surface micromachining of cantilever beam

5. Fusion bonding In order to form more complex and larger MEMS structures, micromachined silicon wafers can be bonded to other materials in a process known as fusion bonding. It is a technique that enables virtually seamless integration of multiple layers and relies on the creation of atomic bonds between each layer either directly (with heating and pressure in the case of glass to wafer bonding), or through a thin film of silicon dioxide as shown in Fig.5.19. The resulting composite has very low residual stress due to matching coefficients of thermal expansion from each layer. In addition, the mechanical strength of the bond is comparable to that of the adjoining layers resulting in a very strong composite fabrication technique for enclosed cavities and channels.

Fig.5.19.Formation of sealed cavity using fusion bonding

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Theory on Smart Materials and its Applications 6. High-Aspect-Ratio Micromachining High-aspect-ratio micromachining (HARM) is a process that involves micromachining as a tooling step followed by injection moulding or embossing and, if required, by electroforming to replicate microstructures in metal from moulded parts. It is one of the most attractive technologies for replicating microstructures at a high performance-to-cost ratio and includes technique known as LIGA. Products micro=machined with this technique include high aspect- ratio fluidic structures such as moulded nozzle plates for inkjet printing and micro-channel plates for disposable micro-titreplates in medical diagnostic applications. LIGA Process (Lithography-Electroforming-Moulding) LIGA is an important tooling and replication method for high-aspect-ratio microstructures. The technique employs X-ray synchrotron radiation to expose thick acrylic resist of PMMA under a lithographic mask as shown in Fig.5.20. The exposed areas are chemically dissolved and, in areas where the material is removed, metal is electroformed, thereby defining the tool insert for the succeeding moulding step and this process is capable of creating very finely defined microstructures up to 1000 μm high.

Fig.5.20. LIGA process

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Theory on Smart Materials and its Applications 7. Laser Micromachining

Fig.5.21.Working of Laser micro-machining

The laser micromachining (LMM) equipment can be used for engraving and creating micro-features in metallic substrates. The parts to be removed are locally vaporized using Nd: YAG laser and that vapor can be sucked with a vacuum pump. Upon irradiation by a light beam with a high energy density, materials are immediately melted or vaporized as shown in Fig.5.21. This is how laser can effectively machine hard materials like diamond, glass and ceramic. Laser irradiation of a medium may cause varying thermal effects from simple heating to melting, evaporation, or ionization of the material. For high power laser processing, all of these phenomena occur almost simultaneously. 5.8

Neural Networks

A Neutral network is a set of genes all related by point mutations that have equivalent function or fitness. Each node represents a gene sequence and each line represents the mutation connecting two sequences. Neutral networks can be thought of as high, flat plateaus in a fitness landscape. During neutral evolution, genes can randomly move through neutral networks and traverse regions of sequence space which may have consequences for robustness and evolvability. 100

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Theory on Smart Materials and its Applications 5.8.1 Artificial Neural Network In machine learning and cognitive science, artificial neural networks (ANNs) are a family of models inspired by biological neural networks (the central nervous systems of animals, in particular the brain) which are used to estimate or approximate functions that can depend on a large number of inputs and are generally unknown. Artificial neural networks are typically specified using three things: 

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Architecture specifies the variables are involved in the network and their topological relationships (Example: variables might be the weights of the connections between the neurons, along with activities of the neurons). Activity Rule as most neural network models have short time-scale dynamics, local rules define how the activities of the neurons change in response to each other. Typically the activity rule depends on the weights (the parameters) in the network. Learning Rule The learning rule specifies the way in which the neural network's weights change with time. This learning is usually viewed as taking place on a longer time scale than the time scale of the dynamics under the activity rule. Usually the learning rule will depend on the activities of the neurons. It may also depend on the values of the target values supplied by a teacher and on the current value of the weights.

Example: A neural network for handwriting recognition is defined by a set of input neurons which may be activated by the pixels of an input image. After being weighted and transformed by a function (determined by the network's designer), the activations of these neurons are then passed on to other neurons. This process is repeated until finally, the output neuron that determines which character was read is activated.

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