AEE Telecom and Electronics Core · Chapter 10

Instrumentation, Transducers and Process Control

What to remember

Transducer: converts a physical quantity into an electrical signal. Key families: resistive (strain gauge, RTD, thermistor), inductive (LVDT), capacitive, piezoelectric, thermoelectric (thermocouple) and Hall effect.
Industrial signal standard: 4 to 20 mA current loop. Zero is at 4 mA ("live zero"), so a broken wire (0 mA) is detected.
PID control: the controller output depends on the error, its integral and its derivative. P gives speed, I removes steady-state error, D reduces overshoot.

Measurement terms and errors

Accuracy: closeness to the true value. Precision: closeness of repeated readings. Resolution: the smallest change that can be read. Sensitivity: output change per unit input change. Range and span: the limits and their difference. Linearity, hysteresis and repeatability are other static characteristics.
Error = measured value − true value. Percentage error = (error / true value) × 100.
Types of error: gross (human), systematic (instrument, environment, loading) and random.
Dynamic characteristics: speed of response, time constant (τ), dynamic error and lag. A first-order sensor reaches 63.2% of the final value in one time constant.
Loading effect: a meter with finite resistance changes the circuit it measures; a voltmeter should have a high resistance and an ammeter a low one.

Meters, bridges and the CRO

PMMC (moving-coil) meter: for DC only; linear scale; sensitive. Full-scale current Im, coil resistance Rm.
Shunt: Rsh = Im Rm / (I − Im) extends an ammeter range. Multiplier: Rs = V / Im − Rm extends a voltmeter range.
Moving-iron meter: for AC and DC; non-linear scale. Dynamometer type: used as wattmeter. Energy meter: induction type for AC.
Rectifier-type meters read AC by rectifying first. Digital voltmeters use an ADC; a DVM with n bits has resolution = full scale / 2ⁿ.
CRO (cathode ray oscilloscope): electron gun, deflection plates, fluorescent screen, time-base (sawtooth) generator. It shows voltage against time, and it also measures frequency, phase (Lissajous patterns) and amplitude.

Bridge	Use
Wheatstone	Medium resistance; balance when R1/R2 = R3/R4
Kelvin (double)	Low resistance
Maxwell	Inductance, using a capacitor standard
Hay	Inductance of high-Q coils
Schering	Capacitance and loss (dielectric loss, insulation tests)
Wien	Frequency; used in audio oscillators

Transducers

Quantity	Transducer	Principle and key points
Strain	Strain gauge	ΔR/R = GF × strain; GF about 2 for metal gauges
Temperature	RTD (Pt100)	R = R0 (1 + αT); 100 Ω at 0 °C; platinum is stable and accurate
Temperature	Thermistor	Large change of resistance; NTC is common; non-linear
Temperature	Thermocouple	Seebeck effect: two dissimilar metals produce an EMF; needs cold-junction compensation; wide range; types K, J, T
Displacement	LVDT	Primary and two secondary windings; output is zero at the null position; phase shows direction; no friction
Displacement	Potentiometer	Simple; wear
Force, pressure, vibration	Piezoelectric crystal	Charge from stress; dynamic only (not for static loads)
Magnetic field, position	Hall effect sensor	Voltage across a conductor in a magnetic field
Light	Photodiode, LDR, phototransistor	Light changes current or resistance
Proximity	Capacitive, inductive	Non-contact

Pressure: Bourdon tube, bellows and diaphragm are elastic elements; manometers use a liquid column. Flow: orifice plate, venturi and pitot tube are differential-pressure types; rotameter is a variable-area type; electromagnetic and ultrasonic meters have no moving parts. Level: float, differential-pressure cell, capacitance probe and ultrasonic probe.

Signal conditioning

Instrumentation amplifier: high input resistance, high CMRR, adjustable gain; the standard front end for bridge sensors.
Filters, amplifiers and linearizers clean and scale the signal. ADC and DAC convert between analog and digital.
Transmitters convert the sensor signal to 4–20 mA. Current loops are immune to cable resistance and noise.
Calibration: compare an instrument with a more accurate standard to find and correct errors.

Process control and power-plant instrumentation

A control loop has a sensor, a transmitter, a controller and a final control element (control valve or drive). Open-loop control has no feedback; closed-loop control compares the measured value with the set point.

ON-OFF controller: simple; the variable cycles about the set point.
Proportional (P): output = Kp × e; leaves an offset.
Integral (I): removes the offset; too much causes oscillation.
Derivative (D): acts on the rate of change; reduces overshoot; sensitive to noise.
PID: u = Kp [e + (1/Ti) ∫e dt + Td de/dt].
Control valve: an actuator moves a plug in the valve; the valve may be air-to-open or air-to-close, so that the failure position is safe.
PLC (programmable logic controller): ladder logic; used for sequencing and interlocks. SCADA: supervisory control and data acquisition for remote sites. DCS (distributed control system): a plant-wide control system for continuous processes.
Thermal power plant: typical measured variables are drum level, steam pressure and temperature, feed-water flow, furnace pressure, turbine speed, vibration, and flue gas oxygen. Boiler drum level is often controlled by three-element control (level, steam flow, feed-water flow). Turbine protection uses trips on over-speed, vibration and low oil pressure.

Worked examples

1. Shunt: Im = 1 mA, Rm = 100 Ω, to read up to 10 mA: Rsh = (1 × 100) / (10 − 1) = 11.11 Ω.
2. Multiplier: same meter for 10 V: Rs = 10 / 0.001 − 100 = 9900 Ω.
3. Bridge balance: R1 = 100 Ω, R2 = 200 Ω, R3 = 150 Ω, with R1/R2 = R3/R4. R4 = 150 × 200 / 100 = 300 Ω.
4. Strain gauge: GF = 2, R = 120 Ω, strain = 0.001. ΔR = 2 × 0.001 × 120 = 0.24 Ω.
5. Pt100: α = 0.00385 per °C. At 100 °C, R = 100 (1 + 0.385) = 138.5 Ω.
6. 4–20 mA: for 0 to 100 °C, 12 mA means 50 °C. Current = 4 + 16 × (T / 100).
7. ADC resolution: 10-bit ADC, 10 V full scale: 10 / 1024 ≈ 9.77 mV.
8. Percentage error: true 100 V, read 98 V: error = −2%.

Exam traps

Accuracy is not precision.
Shunt is in parallel with the meter; the multiplier is in series.
PMMC meters read DC only; moving-iron meters read AC and DC.
A voltmeter needs high resistance; an ammeter needs low resistance.
Thermocouples need cold-junction compensation; RTDs do not.
Piezoelectric sensors cannot measure static force.
Kelvin bridge is for low resistance; Wheatstone is for medium.
4 mA represents zero; 0 mA means a fault.
Integral action removes offset; proportional action alone leaves offset.
LVDT output is zero at the null, and the phase gives direction.

One-liners

1. A transducer converts one form of energy to another.
2. Resolution is the smallest readable change.
3. The time constant is the time to reach 63.2% of the final value.
4. Strain gauge gauge factor is about 2.
5. Pt100 reads 100 Ω at 0 °C.
6. Seebeck effect underlies the thermocouple.
7. An LVDT measures displacement.
8. Schering bridge measures capacitance and dielectric loss.
9. A rotameter is a variable-area flowmeter.
10. A PLC is programmed with ladder logic.
11. SCADA monitors remote plant.
12. PID has proportional, integral and derivative actions.

Practice questions

A transducer is a device that
1. converts one form of energy into another
2. amplifies only voltage
3. stores charge
4. filters noise only
Answer
A. converts one form of energy into another
A transducer changes a physical quantity into an electrical signal or the reverse.
The standard industrial current signal range is
1. 0 to 5 mA
2. 0 to 100 mA
3. 10 to 50 V
4. 4 to 20 mA
Answer
D. 4 to 20 mA
The 4–20 mA loop is the process industry standard.
A Pt100 RTD has a resistance of what value at 0 °C?
1. 10 Ω
2. 1000 Ω
3. 0 Ω
4. 100 Ω
Answer
D. 100 Ω
Pt100 means 100 Ω at 0 °C.
A thermocouple works on the
1. Hall effect
2. Seebeck effect
3. Piezoelectric effect
4. Photoelectric effect
Answer
B. Seebeck effect
Two dissimilar metals produce an EMF when their junctions are at different temperatures.
An LVDT is used to measure
1. temperature
2. flow
3. humidity
4. displacement
Answer
D. displacement
A linear variable differential transformer senses linear displacement.
Which bridge is used for measuring low resistance?
1. Wheatstone bridge
2. Schering bridge
3. Kelvin double bridge
4. Wien bridge
Answer
C. Kelvin double bridge
The Kelvin bridge removes lead and contact resistance errors.
Which bridge measures capacitance and dielectric loss?
1. Wien bridge
2. Schering bridge
3. Kelvin bridge
4. Maxwell bridge
Answer
B. Schering bridge
Schering bridge is used for capacitors and insulation loss.
The PMMC meter can measure
1. DC only
2. AC only
3. AC and DC equally
4. pulses only
Answer
A. DC only
The moving-coil deflection depends on current direction, so it reads DC.
A shunt resistor extends the range of an
1. ammeter
2. voltmeter
3. ohmmeter only
4. wattmeter only
Answer
A. ammeter
A low resistance in parallel carries most current.
Integral action in a PID controller mainly removes
1. noise
2. overshoot only
3. steady-state error (offset)
4. dead time
Answer
C. steady-state error (offset)
Integral action drives the offset to zero.
In a closed-loop control system, the controller acts on
1. the output only
2. the error
3. the input only
4. supply voltage
Answer
B. the error
Error = set point − measured value.
A rotameter is a
1. vortex flowmeter
2. ultrasonic flowmeter
3. differential pressure flowmeter
4. variable-area flowmeter
Answer
D. variable-area flowmeter
A float rises in a tapered tube as flow increases.
The gauge factor of a metal strain gauge is about
1. 20
2. 0.2
3. 200
4. 2
Answer
D. 2
Typical metallic gauges have GF of about 2.
A piezoelectric sensor is suitable for measuring
1. humidity
2. temperature
3. dynamic force or pressure
4. constant static load
Answer
C. dynamic force or pressure
Charge leaks away, so it cannot hold a static reading.
A moving-coil meter has Im = 1 mA and Rm = 100 Ω. The shunt needed for a 10 mA range is
1. 900 Ω
2. 10 Ω
3. 11.11 Ω
4. 9.9 Ω
Answer
C. 11.11 Ω
Rsh = 1×100/9 = 11.11 Ω.
The same meter is to read 10 V full scale. The series multiplier is
1. 9 kΩ
2. 10 kΩ
3. 100 Ω
4. 9900 Ω
Answer
D. 9900 Ω
Rs = 10/0.001 − 100 = 9900 Ω.
A Wheatstone bridge is balanced with R1 = 100 Ω, R2 = 200 Ω and R3 = 150 Ω, with R1/R2 = R3/R4. Find R4.
1. 75 Ω
2. 300 Ω
3. 100 Ω
4. 150 Ω
Answer
B. 300 Ω
R4 = R3 × R2 / R1 = 150 × 2 = 300 Ω.
A 120 Ω strain gauge with GF = 2 is stretched by a strain of 0.001. The change in resistance is
1. 0.12 Ω
2. 0.24 Ω
3. 0.024 Ω
4. 2.4 Ω
Answer
B. 0.24 Ω
ΔR = GF × ε × R = 2 × 0.001 × 120 = 0.24 Ω.
For a Pt100 with α = 0.00385 /°C, the resistance at 100 °C (linear approximation) is
1. 110 Ω
2. 100.385 Ω
3. 385 Ω
4. 138.5 Ω
Answer
D. 138.5 Ω
R = 100 (1 + 0.00385 × 100) = 138.5 Ω.
A 4–20 mA transmitter covers 0–100 °C. What current corresponds to 25 °C?
1. 9 mA
2. 10 mA
3. 8 mA
4. 6 mA
Answer
C. 8 mA
4 + 16 × 0.25 = 8 mA.
A 10-bit ADC has a 10.24 V full scale. Its resolution is
1. 100 mV
2. 10 mV
3. 20 mV
4. 1 mV
Answer
B. 10 mV
10.24/1024 = 0.01 V.
A meter reads 98 V when the true value is 100 V. The percentage error is
1. −2%
2. +2%
3. −1%
4. +0.98%
Answer
A. −2%
(98 − 100)/100 × 100 = −2%.
A first-order sensor with time constant 2 s is subjected to a step. It reaches about 63% of final value in
1. 1 s
2. 0.63 s
3. 2 s
4. 6.3 s
Answer
C. 2 s
After one time constant the response is 63.2% complete.
A moving-coil voltmeter has full-scale current 50 µA. Its sensitivity is
1. 200 kΩ/V
2. 50 kΩ/V
3. 2 kΩ/V
4. 20 kΩ/V
Answer
D. 20 kΩ/V
Sensitivity = 1/Ifs = 1/50 µA = 20 kΩ/V.
A bridge with a standard capacitor is used for inductance. This is the
1. Schering bridge
2. Wien bridge
3. Maxwell bridge
4. Kelvin bridge
Answer
C. Maxwell bridge
The Maxwell bridge measures inductance by comparison with a capacitor.
The Wien bridge is mainly used to measure
1. frequency
2. inductance of high-Q coils
3. low resistance
4. capacitance loss
Answer
A. frequency
The Wien bridge balances at a frequency set by its R and C values.
A thermocouple needs cold-junction compensation because its EMF depends on
1. the supply voltage
2. the temperature difference between the junctions
3. the wire length only
4. the load resistance
Answer
B. the temperature difference between the junctions
Thermocouple EMF corresponds to the temperature difference between hot and cold junctions.
Which controller action reduces overshoot by acting on the rate of change of error?
1. On-off
2. Proportional
3. Integral
4. Derivative
Answer
D. Derivative
D action responds to how fast the error changes.
A 4–20 mA loop is preferred over a voltage signal for long cable runs because
1. it uses lower power
2. it has no noise at all
3. it needs no power supply
4. it is not affected by cable resistance
Answer
D. it is not affected by cable resistance
The loop current is the same everywhere in a series circuit.
In an LVDT, the output at the null position is
1. zero
2. maximum
3. equal to the primary voltage
4. negative of the primary voltage
Answer
A. zero
The two secondary voltages cancel at null.
Which element is NOT a differential-pressure flow device?
1. Orifice plate
2. Pitot tube
3. Rotameter
4. Venturi tube
Answer
C. Rotameter
A rotameter is a variable-area meter.
Which control mode alone always leaves a steady-state offset?
1. Proportional only
2. PI
3. PID
4. Integral only
Answer
A. Proportional only
With P control only a non-zero error is needed to hold the output.
In a boiler drum, three-element control uses drum level and
1. fuel flow and air flow
2. steam flow and feed-water flow
3. vibration and temperature
4. pressure and speed
Answer
B. steam flow and feed-water flow
Three-element control combines level, steam flow and feed-water flow.
A fail-safe design for a control valve means that on loss of air the valve
1. always opens fully
2. remains in the last position forever
3. moves to its safe position
4. always closes fully regardless of process
Answer
C. moves to its safe position
The valve action is chosen so the failure position is safe for the process.
Consider: 1. A voltmeter should have a high resistance. 2. An ammeter should have a high resistance.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
An ammeter should have a very low resistance.
Consider: 1. RTDs require cold-junction compensation. 2. Thermocouples produce an EMF.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Only thermocouples need cold-junction compensation.
Consider: 1. PLCs are programmed using ladder logic. 2. SCADA allows monitoring of remote sites.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
C. Both 1 and 2
Both are standard facts.
Consider: 1. Accuracy and precision mean the same thing. 2. Resolution is the smallest measurable change.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
B. 2 only
Accuracy is closeness to truth; precision is repeatability.
Consider: 1. In an LVDT the phase of the output indicates the direction of displacement. 2. LVDTs have sliding contacts that wear.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
LVDTs have no sliding electrical contact.
Consider: 1. 4 mA in a 4–20 mA loop represents the zero of the measurement. 2. A loop current of 0 mA indicates a normal reading.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
0 mA signals a broken loop.
Consider: 1. Hall effect sensors can measure magnetic field. 2. Piezoelectric sensors measure steady force.
1. 1 only
2. 2 only
3. Both 1 and 2
4. Neither 1 nor 2
Answer
A. 1 only
Piezoelectric sensors measure dynamic loads only.
Match the sensor to the quantity: 1. Bourdon tube 2. LVDT 3. Pt100.
1. Displacement; pressure; temperature
2. Pressure; temperature; displacement
3. Temperature; pressure; displacement
4. Pressure; displacement; temperature
Answer
D. Pressure; displacement; temperature
Bourdon tube measures pressure, LVDT displacement, Pt100 temperature.
Match the controller action to its effect: 1. Proportional 2. Integral 3. Derivative.
1. Reduces overshoot; removes offset; speed of response
2. Removes offset; reduces overshoot; speed of response
3. Removes offset; speed of response; reduces overshoot
4. Speed of response; removes offset; reduces overshoot
Answer
D. Speed of response; removes offset; reduces overshoot
P gives speed, I removes steady-state error, D damps overshoot.
Match the bridge to the use: 1. Wheatstone 2. Kelvin 3. Schering.
1. Inductance; low resistance; frequency
2. Medium resistance; low resistance; capacitance
3. Low resistance; medium resistance; capacitance
4. Capacitance; low resistance; medium resistance
Answer
B. Medium resistance; low resistance; capacitance
Wheatstone for medium R, Kelvin for low R, Schering for capacitance.
An instrumentation amplifier is preferred for bridge sensors mainly because it has
1. no negative feedback
2. low input resistance and low gain
3. high input resistance and high CMRR
4. a fixed gain of exactly one
Answer
C. high input resistance and high CMRR
It rejects common-mode noise and does not load the bridge.

Page 1 of 1

Finished reading?

Test yourself in 10 minutes

A free 10-question mini-mock on your exam's real pattern, with negative marking and an instant score. अपनी तैयारी 10 मिनट में जाँचें।

Take a free mini-mock →Topic-wise practice

Instrumentation, Transducers and Process Control

What to remember

Measurement terms and errors

Meters, bridges and the CRO

Transducers

Signal conditioning

Process control and power-plant instrumentation

More on temperature and controller tuning

Worked examples

Exam traps

One-liners

Practice questions

Test yourself in 10 minutes

Test yourself in 10 minutes