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Sub: Transducers and Signal Conditioners

Answer Key: October-2019

Code: 595

Time: 3 Hrs Max. Marks: 75

PART-A: Answer any 4 questions. Each Question carries 2 Mark. Q.No. 8 is Compulsory.

PART-B: Answer any 4 questions. Each Question carries 3 Mark. Q.No. 16 is Compulsory.

PART-C: Answer division (a) or (b) of each question which carries 10 Marks.

PART-A

1. Define Transducer.

Transducer is a device which converts one form of energy into another form of energy.

2. What is a capacitive transducer? A transducer which works on the principle of capacitance of parallel plate capacitor is known as capacitive

transducer.

3. What do you mean by proximity sensor? Proximity sensors are discrete sensors that sense when an object has come near to the sensor face.

4. List the thermocouple materials.

1. Copper-constantan

2. Iron-Constantan

3. Chromel-Alumel

4. Chromel-Constantan

5. Platinum-Platinum-Rhodium

5. State the applications of Hall effect transducers.

1. Magnetic to Electric Transducer

2. Measurement of Displacement

3. Measurement of Current

4. Measurement of Power

5. Commonly used in industrial and automotive applications.

6. Define CMRR.

CMRR (Common Mode Rejection Ratio) is defined as the ratio of differential voltage gain, (Ad) to the

common mode gain, (Acm).

7. Draw the diagram of comparator using Op-Amp. (any one)

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8. What are the different types of thermistors?

1. Positive Temperature Coefficient (PTC)

2. Negative Temperature Coefficient (NTC)

3. Bead type

4. Rod type

5. Disc type

PART-B

9. How the transducers are classified based on principle of transduction?

1. Capacitive Transducer 2. Inductive Transducer 3. Resistive Transducer

10. Write about the importance of strain gauges.

The strain gauge is one of the most important sensor of the electrical measurement technique applied

to the measurement of mechanical quantities. As their name indicates, they are used for the measurement

of strain. The main purpose of a strain gauge is to indirectly determine stress and its variation with time,

quantitatively. Strain gauges are also being used to investigate strain in many structural materials over a wide range

of environmental conditions. Measurement of stress occurring in vibrating jet engine turbine blades

operation at high temperature or in rocked engine fuel vessels containing liquid oxygen having negative

temperature can be done using strain gauge transducers.

11. Write about the solar photo voltaic module.

PV Module or Solar PV Module is an assembly of photovoltaic (PV) cells, also known as solar cells.

To achieve a required voltage and current, a group of PV modules (also called PV panels) are wired into

large array that called PV array. A PV module is the essential component of any PV system that converts

sunlight directly into direct current (DC) electricity. PV modules can be wired together in series and/or

parallel to deliver voltage and current in a particular system requires.

12. What is the use of Geiger muller tube? Draw its diagram.

It is used for detecting and measuring the followings:

1. Alpha particles 2. Beta particles 3. X-rays 4. Gamma rays

13. State how the Op-Amp is used as voltage to current converter.

Op-amp is implemented to simply convert the voltage signal to corresponding current signal. The

Op-amp used for this purpose is IC LM741. This Op-amp is designed to hold the precise amount of current

by applying the voltage which is essential to sustain that current throughout the circuit. For this, there are

two types of circuits possible.

(i) V-I converter with floating load

(ii) V-I converter with grounded load

14. State the important features of instrumentation amplifier.

High gain accuracy

High CMRR

High gain stability

Low output impedance

Low DC offset

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15. List the advantages of R-2R ladder DAC.

• Only two resistor values are used in R-2R ladder type.

• It does not need as precision resistors as Binary weighted DACs.

• It is cheap and easy to manufacture.

16. Draw the symbol of Op-Amp and mention its terminals.

PART-C

17. (a) Discuss about sensing elements and explain the operation of bourdon tube.

(Sensing elements -2 marks, Diagram- 5 Marks, Theory- 4 marks)

The primary sensing element senses the measurand and transforms it into a form of physical

quantity which can be then transduced into an electrical quantity by a transducer.

Bourdon Tubes: Bourdon tube is the most frequently used pressure gauge. Burdon tube is simple and rugged

instrument and covers ranges from vacuum to very high pressures.

Types of bourdon tubes:

1. C type bourdon 2. Spiral type bourdon3. Helix type bourdon

Construction and working of ‘C’ type bourdon:

A Bourdon tube consists of a C-shaped pipe with one end closed and the other end attached to

a chamber whose pressure is being measured. The end A of the tube is connected to the pressure

source. The end B is closed.

When pressure is applied at end A, the whole tube is subjected to internal pressure. Since the

end B is closed, the oval cross section of the tube deforms to become more circular. Due to such

deformation in the cross section, the whole tube tends to straighten out. This causes movement of

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end B, which is a function of the applied pressure. The gears and levers are used to amplify the

movement. The amplified movement is displayed on a circular scale through rotation of a pointer.

The circular scale is calibrated to read pressure directly. Bourdon tube gauges are not very sensitive

as compared to other pressure gauges.

(OR)

(b) Explain the operation of LDDT linear displacement digital transducer with necessary

diagrams.

(Diagram -5 marks, Theory- 5 marks)

This is a contact or brush type encoder used to measure linear displacement. The shaded areas

are made as conductor whereas unshaded areas are made as non-conductor and the brushes are

placed on the transducer which acts as sliding contacts. The circuit of sliding contacts, which comes

in contact with conductors, is completed and circuits which contacts with non-conductors are not

completed. Thus digital readout is obtained which indicates directly the position. Here natural

binary system is used but many commercial encoders use gray codes to avoid errors which may

occur due to small misalignment.

18. (a) Explain the operation of strain gauge in displacement measurement.

(Diagram -5 marks, Theory- 5 marks)

Fig. given below shows the one type of unbounded strain gauge for the measurement of

displacement. In this type a frame-like moving structure called armature is supported in the centre of

a stationary frame the armature can move only in one direction.

The movement in that direction is limited by four filaments of strain - sensitive wire. The

filaments are of equal dimensions having equal resistances and are arranged as shown in fig2.4 when

a body is displaced an external force is applied to the strain gauge, which moves the armature in the

direction indicated.

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The movement of the armature due to the strain of the structure causes a change in the

dimensions of the sensing elements. Hence the elements A and D increase in length, whereas the

elements B and C decrease in length. As a result, the resistance R1 and R4 increase and R2 and R3

decrease. These four resistances are formed as the four arms of a Wheatstone bridge, for the purpose

of strain measurement.

Since the resistance of all the four resistors R1 to R4 change due to application of stress

(force), the strain gauge gives increased sensitivity. Initially the bridge is balanced when stress is not

applied. When stress is applied, the resistance elements experience strain and change their resistance

values and upset the balance of the bridge. The unbalanced current indicated by the sensitive centre -

zero mill ammeter which is calibrated to read the magnitude of displacement of the armature. The

displacement of the body is calculated from the readings obtained from the mill ammeter. Thus the

displacement of the structural member, is measured in terms of electrical quantity.

(OR)

(b) Explain how the measurement of pressure is done using inductive transducer.

(Diagram -5 marks, Theory- 5 marks)

An inductive pressure transducer uses the principle of inductance to convert the flexing of a

diaphragm into the linear movement of a ferromagnetic core. The movement of the core is used to vary

the induced current generated by an AC powered primary coil on another secondary pick-up coil.

In a LVDT, three coils are wound on an insulating tube containing a ferrous core. The position

of the core within the tube is controlled by the pressure. For example, it may be linked to a diaphragm

or capsule arrangement by a connecting rod. An a.c current is passed through the primary coil. If the

core is in the center, the diaphragm is not deflected and equal voltages are induced in the secondary

coils. The outer coils are differentially amplified, resulting in a zero output at zero pressure. As pressure

changes a differential output from the coils is obtained from the coils according to the pressure that

controls the position of the core.

19. (a) Explain how the tachogenerator is used for the measurement of angular velocity. Write its

advantages and disadvantages.

(Diagram -4 marks, Theory- 4 marks, Advantages and Disadvantages- 2 Marks)

The speed of any revolving member or a shaft can be measured using tachogenerator. The fig

shows the arrangement of the measurement of speed of a rotating machine by means of a

tachogenerator which is sometimes called a tachometer.

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The tachogenerator, is coupled to the shaft of the rotating machine. The rotation of the

rotating machine is transmitted to the armature of the tacho generator. The armature revolves in the

magnetic field provided by two permanent magnetic poles N and S. As such the voltage generated in

the armature will be proportional to speed only. The polarity of voltage gives the direction of

rotation. The voltage is fed to a voltmeter with uniform scale graduated directly in speed RPM.

The angular velocity of the rotating machine can also be measured in this method.

Let N be the speed of the machine indicated .by the tacho generator.

Then angular velocity ω =2ΠN/60 rad/sec

Advantages

• Direction of rotation is indicated by polarity of output voltage.

• The output voltage is generally about 10m V/RPM and can be measured with an ordinary

voltmeter.

Disadvantages

• Due to variation in brush pressures errors are introduced in the speed measured.

• Maintenance of armature and brushes is difficult.

• Voltmeter input impedance should be high to minimize armature current drawn. Otherwise the

armature reaction flux will be high and that will distort the main flux which will ultimately

produce non-linear relation between speed and voltage. The readings obtained will be

erroneous.

(OR)

(b) Explain with neat diagram, the working of photo conductive cell.

(Diagram -5 marks, Theory- 5 marks)

Principle:

When light falls on a material, the energy in the radiation ionizes the covalent bonds. Thus the

bonds are broken and large number of electron-hole pairs is generated. This increases number of

current carrier and decreases the resistance of the material. The phenomenon is called photoconductive

effect.

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Construction & Working:

Figure shows the simplest form of such a cell using selenium. There are two electrodes provided

with the semiconductor material attached to them. The resistivity of semiconductor materials like

selenium, cadmium sulphide, lead sulphide and thalmium sulphide is decreased when irradiated.

As soon as the cell is illuminated its resistance decreases and current through the circuit becomes

large. The shape of the semiconductor material is so made as to obtain a large ratio of dark to light

resistance.

Photoconductive cells are generally used for detecting ship and aircraft by the radiation given out

by their exhaust and for telephony by modulated infrared lights.

20. (a) Explain the block diagram of AC signal conditioning system.

(Diagram -5 marks, Theory- 5 marks)

The following figure shows the block diagram of AC signal conditioning system.

☘ The transducer such ass strain gauge is connected in the arm of bridge that is driven by the

carrier oscillator.

☘ As the physical input quantity of transducer changes, the carrier frequency signal gets

amplitude modulated with the bridge output.

☘ This modulated signal is amplified by means of an AC amplifier.

☘ Phase sensitive demodulator demodulates the amplified signal and carrier frequency is filtered.

Phase sensitive demodulation is used so that polarity of dc output can indicate direction of

parameter change in the bridge output.

☘ Final stage is low pass filter to reject the high frequency and noise signals. A problem in this

system is to maintain stable carrier oscillator.

☘ This system can be operated on carrier frequency of the range 50 Hz to 20 KHz. Carrier

frequencies should be at least five tomes the signal frequency.

(OR)

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(b) Explain how the Op-Amp is used as integrator and Differentiator.

(Diagram -5 marks, Derivation- 5 marks)

Integrator:

A circuit which integrates the input signal with respect to time (frequency) is called integrator.

The circuit diagram of integrator is shown in figure. The feedback element is capacitor and the input

element is resistor.

The charge in the capacitor C, when supply voltage of Vi is applied, Q = CV.

In general, the current through the capacitor,

Since VS is virtual ground, Vs=0

Integrating on both sides with respect to time,

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The circuit thus provides an output voltage which is proportional to the time integral of input and RC

is time constant of integrator.

Differentiator:

The differentiator circuit performs the mathematical operational of differentiation. i.e. the

output waveform is derivative of input waveform. The circuit diagram is shown as,

Circuit Analysis:

The charge in the capacitor C, when supply voltage of Vi is applied, Q = CV.

In general, the current through the capacitor,

By using KCL, I1 = If

Since VS is virtual ground, VS = 0.

Thus the output voltage is a constant (-RC) times the derivative of input voltage, the negative sign

indicates 180° phase shift of input signal.

21. (a) Explain the operation of Op-Amp clipper circuits in detail.

(Diagram -5 marks, Theory- 5 marks)

Positive clipper

A clipper circuit which remove a certain portion of positive cycle of input signal is

called positive clipper. The circuit diagram of positive clipper is shown in fig 13.

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Fig 13: Positive Clipper Circuit

The clipping level is determined by the reference voltage Vref. This voltage is obtained

from the positive supply voltage. In this circuit the Vref is determined by the pot Rp. For the input

voltage Vi < Vref, the diode D conducts in forward biasing and the op amp works as a voltage follower.

(i.e) V0=Vi, till Vi ≤ Vref

For the input voltage, Vi > Vref the diode D is cut off. Hence the output voltage is equal

to the reference voltage .

(i.e) V0= + Vref

Fig 14: Waveforms of positive clipper circuit (With +Vref)

Due to this effect, the output voltage V0 remains at +Vref and the entire waveform

above +Vref gets clipped off. The waveforms are shown in the fig14.

(or)

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Fig 15: Waveforms of positive Clipper (With -Vref)

In the same circuit, if the pot Rp is used with –VEE to generate negative Vref, the the

entire waveforms above –Vref, get clipped off. The waveforms with –Vref are shown in the above

figure.

Negative Clipper

A clipper circuit hich removes a certain portion of negatve cycles of input signal is

called negative clipper. The circuit diagram of negative clipper is shown in the fig 16.

Fig 16: Negative clamper circuit

The clipping level is determined by the reference voltage, -Vref. This voltage is

obtained from the negative supply voltage. In this circuit the –Vref is determined by the pot Rp.

Fig 17: Waveforms of negative clipper circuit (With -Vref)

For the input voltage, Vi > Vref, the diode D conducts in forward biasing, and the op

amp worked as a voltage follower.

i.e., V0=Vi

For the input voltage, Vi < Vref the diode D is cut off. Hence the output voltage is equal

to the reference voltage.

i.e., V0= -Vi

Due to these effects, the output voltage remains at – Vref, and the entire waveforms

below –Vref gets clipped off. The waveforms are shown in figure 17.

(or)

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Fig 18: Waveforms of negative clipper (With +Vref)

In the same circuit, if the pot Rp is used with +Vcc to generate positive Vref. The entire

waveforms below +Vref gets clipped off. The waveforms with +Vref is shown in fig.

(OR)

C. Explain the operation of successive approximation ADC.

(Diagram -5 marks, Theory- 5 marks)

The block diagram of 4 bit successive approximation A/D convertor is shown in figure. This

converter successively divides the voltage ranges in half. It contains a successive approximation

register (SAR) a D/A converter a clock generator, a comparator and a control logic unit. The digital

output is taken from the SAR. The output of the D/Anon-inverting input (V+) of the comparator.

Fig 23: Successive approximation A/D Converter

The analog voltage to be converted into digital signal is applied into the inverting

input (V-) of the comparator. If V- is greater than or equal to V+, the output of the comparator is low

(0), else (V + > V-) the output goes to high (1) level.

The SAR register is first reset to make the output as all 0’s. then apply the analog

input voltage to Vs and then start signal is given. The SAR register first sets only the MSB bit

(1000). This value is then converted into its equivalent analog voltage. It is a reference voltage and it

is equalled to the non-inverting input of the comparator.

The comparator compares the input voltage (Va) and the reference voltage (Vr). The

comparator output is low (0). When Va ≥ Vr, otherwise (Va < Vr), the comparator output goes a high

(1) level.

If the comparator output is low, the control logic unit permits a control high signal to

the SAR. The SAR units sets the bit of next MSB (1100). Otherwise, if the comparator output is

high, the control unit permits a control low signal to the SAR. Now the SAR units is taken out the

previous set bit (make it as 0) and also sets the bit of next MSB (0100). This value is again converted

into its equivalent analog signal, and also compared with the input voltage.

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This process is repeated down to the LSB, and at this time the desired digital value

corresponding to the analog input is in the counter. The counting sequence behind with MSB and

ends with LSB. This method is a process of approximating the analog voltage by trying ‘1’ bit at a

time, beginning with the MSB. This operation is shown in table above.

The conversion time of this converter is low, equal to the number of bits of the

digital signal. A display unit may be connected at the output side for seeing the output value.