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CHAPTER CHAPTER 88

EMT 113: V-2008EMT 113: V-2008

School of Computer and School of Computer and Communication Engineering, UniMAPCommunication Engineering, UniMAP

Prepared By: Prepared By: Amir Razif A b. Jamil AbdullahAmir Razif A b. Jamil Abdullah

Sensors Sensors and and

TransducerTransducers.s.

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8.1 Introduction to Transducers.8.1 Introduction to Transducers.8.2 Classification of Transducers.8.2 Classification of Transducers.8.3 Selecting a Transducer.8.3 Selecting a Transducer.8.4 Resistive Position Transducers.8.4 Resistive Position Transducers.8.5 Strain Gauge Transducers.8.5 Strain Gauge Transducers.8.6 Capacitive Transducers.8.6 Capacitive Transducers.8.7 Inductive Transducers.8.7 Inductive Transducers.8.8 Variable Inductive Transducers.8.8 Variable Inductive Transducers.8.9 Thermocouple Transducers.8.9 Thermocouple Transducers.8.10 Thermistor Transducers.8.10 Thermistor Transducers.8.11 Piezoelectric Transducers.8.11 Piezoelectric Transducers.

8.0 Transducers and 8.0 Transducers and Sensors.Sensors.

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8.1 Introduction to 8.1 Introduction to Transducers.Transducers. Transducer is any device that converts energy in one

form to energy in another. The term transducer refers to rather specialized device. It is also known as sensor.

It convert electrical energy to mechanical displacement.

Transducers convert non-electrical physical quantity to electrical signal; for example, temperature, sound or light to an electrical signal.

Figure 8.0: Example of Transducers.

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Function of TransducerFunction of Transducer To sense the presence, magnitude, change in

and frequency of some measurand. To provide an electrical output, when

appropriately processed and applied to a readout device.

Figure 8.1: Block Diagram of Transducers.

Cont’d…Cont’d…

Transducer Electrical output

Measurand

Excitation

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Transducer can be classified according to their application, based primarily on their physical quantity, property or condition to be measured. (a) Passive Transducers.(b) Self-generating transducers.

(a) Passive transducers (a) Passive transducers It requires an external power. Their output is a measure of some variation,

such as resistance and capacitance.

(b) Self-generating transducers.(b) Self-generating transducers. They do not require an external power. They produce analog voltage or current when

stimulated by some physical form of energy

8.2 Classification of 8.2 Classification of TransducersTransducers

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8.3 Selecting a 8.3 Selecting a Transducers.Transducers. When selecting transducer, it has to be compatible

with its application;(1) Operating range:

~ it should maintain range requirements and good resolution.(2) Sensitivity: ~ it must be sensitive enough to allow sufficient output.(3) Frequency response and resonant frequency :

~ is the transducer flat over the needed range.(4) Environment compatibility:

~ corrosive fluids, the pressures, shocks and interactions.

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(5) Minimum sensitivity: ~ minimally sensitive to expected stimuli other than measurand.

(6) Accuracy: ~ subject to repeatability and calibration errors.

(7) Usage and ruggedness: ~ mechanical and electrical intensities versus size and weight.

(8) Electrical: ~ what are the signal-to-noise ratios when combined with amplifiers and frequency response limitations.

Cont’d…Cont’d…

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It is also called displacement transducer, Figure 8.2. The physical variation under measurement cause a resistance change

in the sensing element. In short, they detect and transmit the physical position of

mechanical parts via electrical signals half-hour.

8.4 Resistive Position 8.4 Resistive Position Transducers.Transducers.

Figure 8.2: Resistive Position Transducer or Figure 8.2: Resistive Position Transducer or Displacement Transducer.Displacement Transducer.

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Example 8.1:Example 8.1: Resistive Position Transducers.Resistive Position Transducers.A resistive position transducer with a resistance of 5000 A resistive position transducer with a resistance of 5000 and a shaft stroke of 5.0 in is used in the arrangement and a shaft stroke of 5.0 in is used in the arrangement of of Figure 8.2.Figure 8.2. Potentiometer R Potentiometer R3 3 RR44 is also 5000 is also 5000 , and V, and VTT is 5.0 V. The initial position to be used as a reference is 5.0 V. The initial position to be used as a reference point is such that Rpoint is such that R11 = R = R22 (shaft is at mid-stroke). At the (shaft is at mid-stroke). At the start of the test, potentiometer Rstart of the test, potentiometer R3 3 RR44 is adjusted so that is adjusted so that the bridge is balanced (Vthe bridge is balanced (VEE =0V). Assuming that the =0V). Assuming that the object being monitored will move a maximum distance of object being monitored will move a maximum distance of 0.5 in. toward A What will be the new value of V0.5 in. toward A What will be the new value of VEE be? be? Solution:Solution:If the wiper moves 0.5 in toward A from mid-stroke, it will be 3.0 in. from B.

. V

VV

VRR

RV

RR

RVVV

in

inR

TTRRE

5.0

)5(*5000

2500)5(*

5000

3000

**

30005000*0.5

0.3

43

4

21

242

2

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It is an example of a passive transducer, Figure 8.3. Uses electric resistance variation in wires to sense the

strain produced by a force on wires. Measuring weight, pressure, mechanical force, or

displacement. A tensile stress tends to elongate the wire and thereby

increase its length and decrease its cross sectional area

8.5 Strain Gauge 8.5 Strain Gauge TransducersTransducers

Figure 8.3: Resistive Strain Gauges; Wire Construction.

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The capacitive transducers can be made to be self-contained units, Figure 8.4.

Some transducers work by making one of the capacitor plates movable, either in such a way as to vary the overlapping area or the distance between the plates.

Other transducers work by moving a dielectric material in and out between two fixed plates to detect and transmit the physical position of mechanical parts via electrical signals

Above is the formula of the parallel-plate capacitor where, k = dielectric constant.

A = Area of the plate, in square meters. eo = 8.854x10-12, in farad per meter. d= the plate spacing in meters.

8.6 Capacitive 8.6 Capacitive TransducersTransducers

Figure 8.4: Capacitive Transducers.

)( faradsd

kAC o

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They have 3 wire connections; one wire for each of the "end" plates and one for the "common" plate, Figure 8.5.

It is implemented in a bridge circuit .

Differential Capacitive Differential Capacitive Transducers.Transducers.

Figure 8.5: Differential Capacitive Figure 8.5: Differential Capacitive Transducers.Transducers.

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This bridge circuit is similar in function with strain gauges; it is not intended to be in a "balanced" condition all the time, but rather the degree of imbalance represents the magnitude of the quantity being measured, Figure 8.6.

Figure 8.6: Differential Capacitive Transducers Bridge Measurement

Circuit.

Cont’d…Cont’d…

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The Advantages of Capacitive Transducer.The Advantages of Capacitive Transducer. Has excellent frequency response Can measure both static and dynamic phenomena.

The Disadvantages of Capacitive Transducer.The Disadvantages of Capacitive Transducer. Sensitivity to temperature variations the possibility of erratic or distortion signals owing

to long lead length

The Application of Capacitive Transducer.The Application of Capacitive Transducer. As frequency modulator in RF oscillator In capacitance microphone Use the capacitance transducer in an ac bridge

circuit

Cont’d…Cont’d…

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Inductive transducers may be either of the self generating or passive type.

The self generating type utilizes the basic electrical generator principle, a motion between a conductor and magnetic field induces a voltage in the conductor (generator action).

A tachometer is an inductive transducer that directly converts speed or velocity into electrical signal.

8.7 Inductive 8.7 Inductive Transducers.Transducers.

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This device is a transformer: The primary winding powered by an external source of AC voltage,

and two secondary windings connected in series-bucking fashion. It is variable because the core is free to move between the

windings. When the core is centered and both windings are outputting the

same voltage, the net result at the output terminals will be zero volts.

8.8 Variable Inductive 8.8 Variable Inductive Transducers.Transducers.

Figure 8.7: Linear Variable Differential Transformer.

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The thermocouple is a thermoelectric temperature sensor which consists of two dissimilar metallic wires, Figure 8.8.

These two wires are connected at two different junctions, one for temperature measurement and the other for reference

The temperature difference between the two junctions is detected by measuring the change in voltage across the dissimilar metals at the temperature measurement junction

8.9 Thermocouple 8.9 Thermocouple Transducers.Transducers.

Figure 8.8: Thermocouple Transducers.

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It is low cost. It has no moving parts, less likely to be broken. It has a wide temperature range. Reasonably short response time. Reasonable repeatability and accuracy.

Cont’d…Cont’d…

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Have great resistance at low temperatures but when they warm up their resistance decreases rapidly.

Current can then flow through them.

When the thermistor is warmed up by the hair drier its resistance drops.

As its resistance drops, current flows into the base of the transistors allowing the LED to light, Figure 8.9.

8.10 Thermistor 8.10 Thermistor Transducers.Transducers.

Figure 8.9: Thermistor Transducers.

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A thermistor is a semiconductor made by sintering mixtures of metallic oxide, such as oxides of manganese, nickel, cobalt, copper and uranium.

Termistors have negative temperature coefficient (NTC). That is, their resistance decreases as their temperature rises.

Types of ThermistorTypes of Thermistor ResistanceResistance Disc 1 to 1 MΩ

Washer 1 to 50 kΩRod High Resistance

Cont’d…Cont’d…

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A thermistor is a semiconductor made by sintering mixtures of metallic oxide, such as oxides of manganese, nickel, cobalt, copper and uranium.

This figure shows resistance versus temperature for a family thermistor. The resistance value marked at the bottom end of each curve is a value at 250C.

Note! The resistance decreases as their temperature rises-NTC

Cont’d…Cont’d…

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The Advantages of Thermistor.The Advantages of Thermistor. Small size and low cost. Fast response over narrow temperature range. Good sensitivity in Negative Temperature Coefficient (NTC)

region. Cold junction compensation not required due to dependence of

resistance on absolute temperature. Contact and lead resistance problems not encountered due to

large resistance.The Limitation Thermistor.The Limitation Thermistor. Non linearity in resistance versus temperature characteristics. Unsuitable for wide temperature range. Very low excitation current to avoids self heating. Need of shielded power lines, filters, etc due to high resistance .

Cont’d…Cont’d…

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8.11 Piezoelectric 8.11 Piezoelectric Transducers.Transducers. When a mechanical pressure is to a crystal, a

displacement of the crystal causes a potential difference occur, Figure 8.10.

The thickness, length, and width directions can be stretched or compressed to generate electrical output.

Figure 8.10: Piezoelectric Transducers.

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The END. The END. Hardship builds Hardship builds

character.character.