B.E. DEGREE EXAMINATION.
Electronics and Instrumentation Engineering
EI 235/IT 237 — TRANSDUCER ENGINEERING
(Common to Instrumentation and Control Engineering and
Time : Three hours Maximum : 100 marks
Answer ALL questions.
PART A — (10 ´ 2 = 20 marks)
- Why is periodical calibration of measuring instrument necessary?
- List any two sources of systematic error in a measuring instrument.
- What is the difference between accuracy and precision?
- What is the 5% settling time in the step response of a first–order instrument with a time–constant of 12 sec.?
- What is meant by self–heating error in a resistance thermometer?
- Why is dynamic compensation network used with hot–wire anemometers?
- Mention the problem that occurs when phase insensitive demodulator is used with LVDT.
- Sketch the desired frequency response of a capacitor microphone.
- Give the material and sensitivity of IC temperature sensor.
- Mention two advantages of smart sensor.
PART B — (5 ´ 16 = 80 marks)
- (i) Define standard. How are standards classified? Give the international standards used for mass, length, and time. (8)
(ii) Ten measurements of a pressure made by an instrument at different time intervals give the following readings in kg/cm2 : (8)
7.92, 8.03, 8.10, 7.95, 8.01, 7.98, 7.95, 8.07, 7.94 and 7.97.
Find arithmetic mean, standard deviation, and most probable error.
- (a) (i) Define sensitivity and resolution of a measuring instrument. (4)
(ii) What are the three ways of specifying the non–linearity of an instrument? (4)
(iii) A first–order thermometer with a time–constant of 5 sec is used to measure the temperature of a furnace fluctuating between 540°C and 580°C in a sinusoidal manner. The frequency of fluctuation is 0.04 Hz. Find the maximum and minimum readings of the thermometer. (8)
(b) (i) Derive an expression for the output of a first–order instrument when subjected to unit impulse input. Also sketch the response. (8)
(ii) The gas pressure inside a vessel starts increasing from the initial value of 100 kPa at the rate of 2000 Pa/sec. The pressure is measured by a second–order instrument with a damping ratio of 0.7 and a natural frequency of 30 Hz. Sketch the input and output variation with time and find the steady–state error in instrument reading after transient in output has vanished. (8)
- (a) (i) Give the construction details of bonded and unbonded strain gauges. Explain any two applications of bonded strain gauges. (8)
(ii) A resistance potentiometer with a 20 mm travel has a resistance of 10,000 ohms. It is connected to a 10 V d.c. source with negligible internal resistance. Calculate the actual displacement when the output of the potentiometer is 5 V. The resistance of the voltmeter is 20,000 Ohms. (8)
(b) (i) Explain with a sketch the construction details and characteristics of a piezo resistive pressure transducer. (8)
(ii) Describe the temperature Vs resistance characteristic of a thermistor with equation. Explain any one method of linearizing thermistor characteristic. (8)
- (a) (i) Explain with a sketch the principle of operation of a variable reluctance accelerometer. (8)
(ii) Five square plates of area 16 cm2 (each) are arranged as shown in Fig. 14(a). The plate spacings are 0.2 mm (each). The arrangement is to be used as a displacement transducer by observing change in capacitance with distance x. Calculate the sensitivity of this device in . (8)
(b) (i) Draw the input–output characteristic of LVDT. Explain the phase adjustment circuits and null–reduction technique used for LVDT. (8)
(ii) Explain with a sketch the construction details, principle of operation and signal conditioning circuits of a capacitive transducer used for differential pressure measurement. (8)
- (a) (i) Define the two sensitivity constants of piezo electric transducer. Derive the relation between the two constants. Also draw the equivalent circuit of piezoelectric transducer. (8)
(ii) Explain with a sketch the working of an encoder that gives 4–bit digital output proportional to linear displacement. Mention the advantages of digital transducers. (8)
(b) (i) Define magnetostrictive effect. Name two materials used in magnetostrictive transducers. Describe any one application of magnetostrictive transducer. (8)
(ii) Explain with a sketch the working of a fiber–optic displacement transducer. Draw its input–output characteristic. (8)