Instrumentation Lecture 3

Theory of Oscilloscope 1

Introduction • What is an oscilloscope?

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Introduction • A graph-displaying device of electrical signal – X axis: Time – Y axis: Voltage – Z axis: Intensity or brightness

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Introduction • Information given by oscilloscopes – – – – – –

Time and voltage Frequency and phase DC and AC components Spectral analysis Rise and fall time Mathematical analysis

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What can you do with oscilloscopes? • Designing and repairing electronic equipment • With the proper transducer (Ex: microphone) – Electrical signal in response to physical stimuli, such as sound, mechanical stress, light, or heat. – Engine vibrations – Brain waves

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Control panel of an oscilloscope • Vertical Section • Horizontal Section • Trigger Section

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Basic setting • Vertical system – attenuation or amplification of signal (volts/div)

• Horizontal system – The Time base (sec/div)

• Trigger system – To stabilize a repeating signal and to trigger on a single event

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In digital circuits • Measuring – Logic level – Timing – Logic strength – Rise and fall time – Frequency – Signal integrity • Waveform distortion • Noise level 8

In digital circuits • Diagnosing – Timing fault – Proper fan-in and fan-out – Proper pull-up and/or termination – Collision – Signal integrity • Reflection • Noise, crosstalk and ground bounce

– Open, short or stuck at 0 or 1 9

Analog and digital oscilloscope

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Analog oscilloscope • Real-time display of signals • Block diagram – Sweep generator and vertical amplifier – Earthquake recorder

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Digital oscilloscope • Capture and view events – Digital storage oscilloscope (DSO)

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Digital oscilloscope (contd.) 

Sampling

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Interpolation

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Advantage of Digital Scope  Trend

towards digital.  Easy to use.  One-shot measurement  Recoding  Triggering  Data reuse  Connectivity

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Probes • Components

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Probes • High quality connector • High impedance (10MΩ) • 50Ω for high frequency measurement

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Passive probe • 10× attenuation – Good for low circuit loading – Suitable to high frequency signal – Difficult to measure less than 10mV signals • 1× attenuation – Good for small signals – Introducing more interference 17

Active probe • Signal conditioning ⇒ oscilloscope • Require power source • Good for high speed digital signals over 100MHz clock frequency

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Sensors and Transducers

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Objectives At the end of this chapter, the students should be able to:  describe the principle of operation of various sensors and transducers; namely.. Resistive Position Transducers. Capacitive Transducers Inductive Transducers

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Introduction 

Sensors and transducers are classified according to;  the physical property that they use (piezoelectric, photovoltaic, etc.)  the function that they perform (measurement of length, temperature, etc.).  Since energy conversion is an essential characteristic of the sensing process, the various forms of energy should be considered.

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Introduction 

There are 3 basic types of transducers namely self-generating, modulating, and modifying transducers.

The self-generating type (thermocouples, piezoelectric, photovoltaic) does not require the application of external energy.

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Introduction  Modulating transducers (photoconductive cells, thermistors, resistive displacement devices) do require a source of energy. For example, a thermocouple is self-generating, producing a change in resistance in response to a temperature difference, whereas a photoconductive cell is modulating because it requires energy.  The modifying transducer (elastic beams, diaphragms) is characterized by the same form of energy at the input and output. The energy form on both sides of a modifier is electrical. 23

Features of Sensors The desirable features of sensors are:  accuracy - closeness to "true" value of variable; accuracy = actual value - sensed value;  precision - little or no random variability in measured variable 3. operating range - wide operating range; accurate and precise over entire sensing range 4. calibration - easy to calibrate; no "drift" - tendency for sensor to lose accuracy over time. 5. reliability - no failures 6. cost and ease of operation - purchase price, cost of installation and operation 24

Sensors Types A list of physical properties, and sensors to measure them is given below:

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Sensors Types

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Common Sensors Listed below are some examples of common transducers and sensors that we may encounter:  Ammeter - meter to indicate electrical current.  Potentiometer - instrument used to measure voltage.  Strain Gage - used to indicate torque, force, pressure, and other variables. Output is change in resistance due to strain, which can be converted into voltage.  Thermistor - Also called a resistance thermometer; an instrument used to measure temperature. The operation is based on change in resistance as a function of temperature.

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Sensors Types • There are several transducers that will be examined further in terms of their principles of operations. • Those include : 4. 5. 6. 7. 8.

Resistive Position Transducers Strain Gauges Capacitive Transducers Inductive Transducers And a lot more… 28

Strain Gauges • The Strain Gauge is an example of a passive transducer that uses electrical resistance variation in wires to sense the strain produced by a force on the wire. • It is a very versatile detector and transducer for measuring weight, pressure, mechanical force or displacement.

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Strain Gauges The construction of a bonded strain gauge shows a fine wire looped back and forth on a mounting plate, which is usually cemented to the element that undergoing stress.

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Strain Gauges •

For many common materials, there is a constant ratio between stress and strain. • Stress is defined as the internal force per unit area. • S – Stress (kg/m2) F – Force (kg) A - Area (m2)

F S= A

• The constant of proportionality between stress and strain for the curve is known as the modulus of elasticity of the materials, E or Young’s Modulus. 31

Capacitive Transducers • The capacitance of a parallel plate is given by: k= dielectric constant A= area of the plate o εo=8.854x10-12 F/m d= plate spacing

kAε C= d

• Since the capacitance in inversely proportional to the spacing of the parallel plates, any variations in d will cause a variation in capacitance. 32

Capacitive Transducers •

Some examples of capacitive transducers

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Inductive Transducers • Inductive Transducers may be either the selfgenerating or the passive type transducers. • In the Self-Generating IT, it utilises the basic electrical generator principle that when there is relative motion between conductor and magnetic field, a voltage is induced in the conductor. • An example of this is Tachometer that directly converts speeds or velocity into an electrical signal. 34

Tachometers •

Examples of a Common Tachometer

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Linear Variable Differential Transformer (LVDT) •

Passive inductive transducers require an external source of power.

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The Differential transformer is a passive inductive transformer, well known as Linear Variable Differential Transformer (LVDT). •

It consists basically of a primary winding and two secondly windings, wound over a hollow tube and positioned so that the primary is between two of its secondaries.

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Linear Variable Differential Transformer (LVDT) •

Some examples of LVDTs.

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Linear Variable Differential Transformer (LVDT) •

An example of LVDT electrical wiring.

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Linear Variable Differential Transformer (LVDT) • An iron core slides within the tube and therefore affects the magnetic coupling between the primary and two secondaries. • When the core is in the centre , the voltage induced in the two secondaries is equal. • When the core is moved in one direction of centre, the voltage induced in one winding is increased and that in the other is decreased. Movement in the opposite direction reverse this effects.

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Linear Variable Differential Transformer (LVDT) •In next figure, the winding is connected ‘series opposing’ -that is the polarities of V1 and V2 oppose each other as we trace through the circuit from terminal A to B. •Consequently, when the core is in the center so that V1=V2, there is no voltage output, Vo = 0V.

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Linear Variable Differential Transformer (LVDT) • When the core is away from S1, V1 is greater than V2 and the output voltage will have the polarity of V1. • When the core is away from S2, V2 is greater than V1 and the output voltage will have the polarity of V2. • That is the output of ac voltage inverts as the core passes the center position. • The farther the core moves from the centre, the greater the difference in value between V1 and V2, and consequently the greater the value of Vo.

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Linear Variable Differential Transformer (LVDT) • Thus, the amplitude of Vo is a function of distance the core has moved. If the core is attached to a moving object, the LVDT output voltage can be a measure of the position of the object. • The farther the core moves from the centre, the greater the difference in value between V1 and V2, and consequently the greater the value of Vo.

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Linear Variable Differential Transformer (LVDT) Among the advantages of LVDT are as follows: • It produces a higher output voltages for small changes in core position. • Low cost • Solid and robust -capable of working in a wide variety of environments. • No permanent damage to the LVDT if measurements exceed the designed range.

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Temperature Transducers •

The temperature transducers can be divided into four main categories: o o o o

Resistance Temperature Detectors (RTD) Thermocouples Thermistors Ultrasonic transducers

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Resistance Temperature Detectors (RTDs) • Detectors of resistance temperatures commonly employ platinum, nickel, or resistance wire elements, whose resistance variation with temperature has a high intrinsic accuracy. • They available in many configurations and sizes and as shielded and open units for both immersion and surface applications.

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Resistance Temperature Detectors (RTDs) •

Some examples of RTDs are as follows:

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Resistance Temperature Detectors (RTDs) • The relationship between temperature and resistance of conductors can be calculated from this equation:

R = Ro (1 + α∆T ) where; R= resistance of the conductor at temp t (oC) Ro=resistance at the reference temp. α= temperature coefficient of resistance ∆= difference between operating and reference temp. 47

Thermocouples • A thermocouple is a sensor for measuring temperature. It consists of two dissimilar / different metals, joined together at one end, which produce a small unique voltage at a given temperature. This voltage is measured and interpreted by the thermocouple. •The magnitude of this voltage depends on the materials used for the wires and the amount of temperatures difference between the joined end and the other ends.

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Thermocouples • Some examples of the thermocouples are as follows:

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Thermocouples Common commercially available thermocouples are specified by ISA (Instrument Society of America) types. •

• Type E, J, K, and T are base-metal thermocouples and can be used up to about 1000°C (1832°F). • Type S, R, and B are noble-metal thermocouples and can be used up to about 2000°C (3632°F). 50

Thermocouples •Calibration curves for several commercially available thermocouples is as below:

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Thermocouples • The magnitude of thermal emf depends on the wire materials used and on the temperature difference between the junctions. • The effective emf of the thermocouple is given as:

E = c(T1 − T2 ) + k (T − T ) 2 1

2 2

•Where; c and k – constant of the thermocouple materials T1 - temperature of the ‘hot’ junction. T2 - temperature of the ‘cold’ or ‘reference’ junction.

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