Chapter-4-part-1.pdf

CHAPTER 4 MEASURING DEVICES (SENSOR & TRANSDUCER)

OUTLINE Introduction What

is sensor and transducer? Selecting Transducer Types of transducer Passive Transducer Self Generating Transducer

INTRODUCTION  For

many years, a transducer is a source of information.  The operation of the transducer defines the reliability of the information.  In spite of a wide variety of different systems containing transducer, they can be divided into two big groups i.e measuring system and control system.

INTRODUCTION CONT’D 

Component of instrumentation system Physical Parameters  Pressure Temperature Flow Light Intensity Sound Position Acceleration Force Strain

Sensor / Transducer

Electrical Signal  Current Voltage

WHAT IS SENSOR? Sensor is a device that detects, or senses, a signal or physical condition.  Most sensors are electrical or electronic, although other types exist.  A sensor is a type of transducer.  Sensors are either direct indicating (e.g. a mercury thermometer or electrical meter) or are paired with an indicator (perhaps indirectly through an analog to digital converter, a computer and a display) so that the value sensed becomes human readable. Aside from other applications, sensors are heavily used in medicine, industry and robotics. 

WHAT IS TRANSDUCER?  Transducer

is a device that provides a usable output in response to a specific measured.  In other word, transducer is a device that converts energy in one form to energy in another.  Transducer that provide an electrical output are frequently used as sensors.  The transducer is the most important portion of the sensor, in fact some “sensor” are merely transducer with packaging

SELECTING TRANSDUCER  There

are four factors to be considered in selecting a transducer in a system: 

Operating range 



The transducer should maintain range requirements and good resolution

Sensitivity 

The transducer must be sensitive enough to allow sufficient output

SELECTING TRANSDUCER CONT’D 

Ability to suite with the environment condition such as pressure 



Do the temperature range of the transducer, its corrosive fluids, the pressures, shocks, and interactions it is subject to, its size and mounting restrictions make it in application

High accuracy to produce sufficient output 

The transducer may be subject to repeatability and calibration errors as well as errors expected owing to sensitivity to other stimuli

TYPES OF TRANSDUCER Transducer

can be classified into

two types: (i) Passive Transducer (ii) Self-Generating Transducer (Active)

PASSIVE TRANSDUCER  Require

an external power and their output is a measure of some variation such as resistance or capacitance  Examples:  LVDT  POTENTIOMETER  STRAIN GAUGE  CAPACITIVE TRANSDUCER

LVDT LVDT (Linear Variable Differential Transformer)  The linear variable differential transducer (LVDT) is a type of electrical transformer used for measuring linear displacement  The transformer has three solenoid coils placed end-toend around a tube.  The centre coil is the primary, and the two outer coils are the secondary.  A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis of the tube. 

LVDT CONT’D A reliable and accurate sensing device that converts linear position or motion to a proportional electrical output.

LVDT CONT’D 

Basic construction of LVDT as shown in figure below: Primary

Secondary A A

B Displacement/

Figure 1

B

LVDT consists of : • a transformer with a single primary winding • two secondary windings connected in the seriesopposing manner (berlawanan arah)

LVDT CONT’D Primary

Secondary A A

B

B

Displacemen t/

Vo ut

Cor e

Core positio n

Relationship between displacement and output

VOUT = VA – VB  The core displacement determine the output:  If the core at the center, VA=VB, VOUT=0  Core at the ‘upper’ A VA max, VB min  VOUT max & +ve  Core at the ‘lower’ B VA min, VB max  VOUT max & -ve

EXAMPLE 1 

LVDT has the following data: Vin= 6.3V, Vout= + 5.2V & displacement range = + 0.5 in. Calculate the displacement when Vo is +2.6V. Vout

+5.2 V +2.6V

?

0.5”

Core position

EXAMPLE 2 An ac LVDT has the following data: input 6.3V, output ± 5.2V, range ±0.50 in. Determine: a) The plot of the output voltage versus core position for a core movement going from +0.45 in to -0.03 in.( 4.68V, -3.12V) b) The output voltage when the core is -0.25 in. from center. (-2.6V)

LVDT CONT’D  Applications

of LVDT:



Used for measuring displacement and position  Used as null detectors in feedback positioning systems in airplanes and submarines  Used in machine tools as an input system Example: Measuring position

POTENTIOMETER A potentiometer is a variable resistor that functions as a voltage divider  Electromechanical device containing a resistance that is contacted by movable slider.  Motion of the slider results in a resistance change depending on the manner in which the resistance wire is wound. 

ℓ1 Vi

R1

ℓT RT

W

ℓ2

ℓT = Shaft Stroke W = Wiper

R2 VO

POTENTIOMETER CONT’D  There 

are various type of potentiometer:

Low Power Types: Liner potentiometers  Logarithmic potentiometers 



High Power Types: 



Rheostat

Digital Control: 

Digitally controlled potentiometers (DCP)

POTENTIOMETER CONT’D 

The output voltage under ideal condition:  Resistance at the output ter minal , R 2 Vo    Resistance at the input term inal, R T   R1   1  RT  T   R2   2  T

  RT 

 Vi  ℓT = Shaft Stroke W = Wiper

ℓ1 Vi

R1

ℓT RT

W

ℓ2

R2 VO

POTENTIOMETER CONT’D 

Theory of operation: The potentiometer can be used as a potential divider (or voltage divider) to obtain a manually adjustable output voltage at the slider (wiper) from a fixed input voltage applied across the two ends of the pot. This is the most common use of pots

The voltage across RL is determined by the formula:

R2 || RL VL  .Vs R1  R2 || RL

EXAMPLE 3 A resistive positive displacement transducer with a shaft stroke of 10cm is used in the circuit of figure below. The total resistance of potentiometer is 500Ω and the applied voltage Vi is 15V. If the wiper, W is 7.5cm from A, what is the value of (a) R2 (125Ω) (b) Vo (3.75V)

POTENTIOMETER CONT’D Transducers Potentiometers are widely used as a part of displacement transducers because of the simplicity of construction and because they can give a large output signal

Audio control One of the most common uses for modern low-power potentiometers is as audio control devices. Both sliding pots( known as faders) and rotary potentiometer ( called knob) are regularly used to adjust loudness, frequency attenuation and other characteristics audio signals

STRAIN GAUGE  A strain

gauge is a metal or semiconductor element whose resistance changes when under strain.  Strain gauge is a passive transducer that uses “electrical resistance variation” in wires to sense the strain produced by a force on the wires.  It can measures:  Weight  Pressure  Mechanical Force  Displacement STRAIN GAUGE

STRAIN GAUGE CONT’D The function of strain gauge is to sense the strain produces by force on the wires.  The strain gauge is generally uses as an arm of a bridge. This is only applicable when temperature variation in wire.  Types of strain gauges: 

Wire gauge

Foil gauge

Semiconductor gauge

STRAIN GAUGE CONT’D  Considering

the factors that influence the resistance of the element a relationship between changes in resistance and strain can be derived.  Resistance is related to length, l(m) and area of cross-section of the resistor ,A(m2) and resistivity, ρ(Ωm) of the material as

STRAIN GAUGE CONT’D When external force are applied to a stationary object, stress and strain are the result.  Stress is defined as the object’s internal forces.  For a uniform distribution of internal resisting forces, stress can be calculated by dividing the applied force (F) by the unit area (A): 

F   A Where; F Force

*Stress – tekanan

A Area

N/m2

STRAIN GAUGE CONT’D The effect of the applied stress is produce a strain.  Strain is a fractional change (∆L/L) in the dimensions of an object as a result of mechanical stress (force/area).  Calculated by dividing the total deformation of the original length by the original length (L). 

L  L

Unit-less

Where; ∆L Change in length L Original unstressed length *Strain – regangan

STRAIN GAUGE CONT’D  The

constant of proportionality between stress and strain for a linear stress-strain curve is known as Young’s Modulus, E.

 E  E





 Young’s modulus in kilograms per-square meter  The stress in kilograms per square meter  The strain (no units)

STRAIN GAUGE CONT’D 

This changes its resistance (R) in proportion to the strain sensitivity of the wire's resistance. When a strain is introduced, the strain sensitivity, which is also called the Gauge Factor (GF), is given by:

 R GF 



GF R R L L

R

L  L

= gauge factor (unit less) = the initial resistance in ohms (without strain) = the change in initial resistance in ohms = the initial length in meters (without strain) = the change in initial length in meters

EXAMPLE 4 A resistant strain gauge with a gauge factor of 2 is fastened to a steel member, which is subjected to strain of 1x10-6. If the original resistance value of the gauge is 130Ω, calculate the change in resistance. (260µΩ)

SOLUTION

CAPACITIVE TRANSDUCER The capacitor consists of two parallel plates separated by an air space or by a dielectric (insulating material).  The capacitance of the of the pair of the plates is measure of the amount of charge that can be transferred before a certain voltage is reached. 

Plate 1

Dielectric material Plate 2

The basic construction of capacitor

CAPACITIVE TRANSDUCER CONT”D

w

Plate 1

h idt

d

Plate 2

Schematic diagram of parallel-plate capacitor

Length

k = dielectric constant of the material in the gap

kA o C d

εo = the permittivity of free space = 8.854 x 10-12 farad/meter A = Plate area (m2) d = the separation between plate (m)

CAPACITIVE TRANSDUCER CONT”D 

There are three criteria/conditions that can change the capacitor (variation of capacitance) : (a) Changing the surface area (b) Changing the dielectric constant (c) Changing the spacing between plate

kA o C d Displacement x=0

x

CAPACITIVE TRANSDUCER CONT”D (a) Changing the surface area If one plate of the parallel plate capacitor is displayed in a direction parallel to the plate, the effective area of the plates will change proportionally to the value of capacitance C Plate 1 Dielectric material

Plate 2

A

CAPACITIVE TRANSDUCER CONT”D (b) Changing the dielectric constant The value of capacitance will increase when the dielectric constant is increased

C Plate 1 Dielectric material

Plate 2

k

CAPACITIVE TRANSDUCER CONT”D (c) Changing the spacing between plate The value of capacitance will decrease when the spacing between plate increased

C Plate 1 Dielectric material

d Plate 2

d

EXAMPLE 5 εo = 8.854 x 10-12 Fm-1, kair = 1, kmaterial = 5 Two square metal plates, side 6 cm separated by a gap of 1 mm. Calculate the capacitance of the sensor when the input displacement of x is:

(a) 0.0 cm (159.38pF)

(b) 3.0 cm (63.75pF)

SOLUTION