JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD
JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III INSTRUMENTATION AND CONTROL SYSTEMS Previous Question Paper (March - 2021 - R18)
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Main
Discuss the following transducers concerning their construction, working and characteristics used for distance measurement: i) Piezo-electric ii) Capacitance
Answer
Piezo-electric Transducers: These operate on the principle that certain materials, like quartz or PZT, generate an electric charge when subjected to mechanical stress. Construction involves a thin crystal wafer placed between two metal electrodes. When a distance-related force or displacement is applied, the crystal deforms, producing a voltage proportional to the stress. They are characterized by high frequency response and high sensitivity but are limited to dynamic measurements as the charge leaks over time.
Capacitance Transducers: These measure distance by detecting changes in capacitance between two plates, defined by the formula
With the help of a suitable example, explain the functional description of various elements of a generalized measuring system.
Answer
A generalized measuring system consists of three main functional elements: the primary sensing element, the variable conversion element, and the data presentation element. Consider a Bourdon tube pressure gauge as an example:
- Primary Sensing Element: The Bourdon tube itself acts as the sensor, which senses the input pressure and converts it into a mechanical displacement (the tube tends to straighten).
- Variable Conversion Element: The mechanical linkage and gear mechanism convert the small displacement of the tube tip into a larger rotational movement of the pointer. This stage modifies the signal to a form suitable for the next stage.
- Data Presentation Element: The pointer moving over a calibrated scale provides the final visual output, allowing the user to read the pressure value directly.
In more complex systems, intermediate elements like amplifiers, filters, or A/D converters may be present to condition the signal before it reaches the final output stage.
Explain the dynamic performance characteristics of measuring instruments.
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Explain the following: i) Accuracy ii) Sensitivity iii) Dead Zone iv) Hysteresis
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Thermistor has resistance at 25°C is 120 Ω. If the temperature co-efficient of resistance β is 4000/°C, then find its resistance at 100°C.
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State and explain the different laws of thermocouples.
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Explain how pressure is measured using deadweight pressure gauges.
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By utilizing ionization pressure gauges explain how low pressure is measured with a neat diagram.
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Name the different mechanical tachometers. Sketch and explain the working of a centrifugal tachometer.
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Describe in detail with neat sketches: i) Turbine flow meter ii) Cryogenic fuel method.
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Explain the working principle of non-contact type of electrical transducers.
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With relevant diagram explain the construction and working of the rotameter.
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A 200 Ω strain gauge is bonded to a steel bar which is subjected to a tensile load. Cross-sectional area of the bar is 0.8 × 10⁻⁴ m² and E = 200 GN/m². Determine the gauge factor of the gauge.
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Explain the working principle of the torsion meter and dew point meter.
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Suggest a simple control system that automatically turns on a room lamp at dusk and turns it off in daylight. Draw the schematics and block diagram of the suggested control system.
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What is a servomechanism? Describe the features of a servomechanism.
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