BE Computer Engineering (Pokhara University) Instrumentation (PU, ELE 172) Question Paper 2078

(a) With the help of a functional block diagram, describe the generalized configuration of a measurement system, clearly identifying the primary sensing element, variable-conversion element, variable-manipulation element, data-transmission element and data-presentation element. (b) Define accuracy, precision, resolution, sensitivity and dead zone as applied to a measuring instrument. (c) A pressure gauge has a range of 0–100 kPa and is guaranteed accurate to within ±1.5% of full-scale deflection. Determine the maximum possible error when the gauge reads 25 kPa and express it as a percentage of the actual reading.

(a) Explain the principle of operation of a resistance temperature detector (RTD) and a thermocouple, and compare them on the basis of range, linearity, sensitivity and the need for cold-junction compensation. (b) Design a Wheatstone-bridge signal-conditioning circuit suitable for a single active strain-gauge transducer. Derive the expression for the bridge output voltage in terms of the gauge factor and the applied strain, and explain how a half-bridge arrangement improves sensitivity and provides temperature compensation.

(a) With a neat block diagram, explain the working of a successive-approximation type analog-to-digital converter (ADC) and state its advantages over the dual-slope (integrating) type. (b) Describe the architecture of a typical data-acquisition system (DAS), explaining the role of the multiplexer, sample-and-hold circuit, ADC and the need for anti-aliasing filtering. (c) A 10-bit ADC has a full-scale input of 5 V. Calculate its resolution and the quantization error.

(a) Explain the construction and working principle of a permanent-magnet moving-coil (PMMC) instrument. Derive the expression for the deflecting torque and explain why it can be used only for DC measurements. (b) Discuss the construction and operating principle of a seven-segment LED display and a liquid-crystal display (LCD), and compare them in terms of power consumption, viewing characteristics and suitability for portable digital instruments.

Explain the working principle of a Linear Variable Differential Transformer (LVDT) for displacement measurement. Sketch its output voltage versus core-displacement characteristic and comment on the phase reversal that occurs as the core moves through the null position.

(a) Draw the circuit diagram of a three-op-amp instrumentation amplifier and write the expression for its overall gain. (b) State two reasons why an instrumentation amplifier is preferred over a simple difference amplifier in transducer signal conditioning.

Distinguish between systematic errors and random errors, giving one example of each. Explain how the standard deviation and probable error are used to express the uncertainty in a set of repeated measurements.

With a block diagram, explain the operation of a digital frequency counter. Explain the meaning of the term 'gate time' and describe how it affects the resolution and accuracy of the frequency measurement.

(a) Explain how a basic d'Arsonval PMMC movement is converted into a multi-range DC ammeter using shunt resistors. (b) A 50 µA, 2 kΩ movement is to be used to measure currents up to 1 mA. Calculate the required shunt resistance.

Explain the working principle of a capacitive transducer for the measurement of liquid level. Discuss why such transducers require high-frequency excitation and a charge-amplifier or AC bridge for signal conditioning.

Define the terms aliasing and Nyquist rate in the context of sampled data systems. State the sampling theorem and explain, with a simple example, the consequence of sampling a signal below the Nyquist rate.

Differentiate between active and passive transducers, and between primary and secondary transducers, giving one practical example of each category.