JAWAHARLAL NEHRU TECHNOLOGICAL UNIVERSITY HYDERABAD B.Tech III BASICS OF SENSORS TECHNOLOGY Previous Question Paper (August - 2022 - R18)

A potentiometer is a three-terminal passive electromechanical transducer that converts mechanical displacement (linear or angular) into a change in electrical resistance, which is then converted into a voltage output. It functions as a voltage divider.

A resistive potentiometer consists of a resistive element (such as a wire-wound coil or conductive plastic track) and a movable contact called a wiper. As the wiper moves along the resistive element, the resistance between the wiper and one of the fixed terminals changes proportionally to the displacement.

The output voltage is given by:

Where R is the resistance between the wiper and the reference terminal, Rt is the total resistance of the element, and Vin is the excitation voltage. Resistive potentiometers are categorized into linear potentiometers for straight-line motion and rotary potentiometers for angular displacement. They are valued for their simplicity, low cost, and high output signal level, though they are subject to mechanical wear and limited resolution due to the discrete nature of wire-wound elements.

Dynamic characteristics describe the behavior of a sensor when the measured quantity (measurand) changes with time. Unlike static characteristics, these define how a sensor responds to transient inputs.

• Response Time: The time taken by the sensor to reach a specified percentage (usually 90% or 95%) of its final steady-state value after a step change in the input.
• Rise Time: The time required for the output to transition from a low percentage (typically 10%) to a high percentage (typically 90%) of the final value.
• Settling Time: The time taken for the output to reach and remain within a defined error band (e.g., ±2%) of the final steady-state value.
• Frequency Response: The range of frequencies over which the sensor can accurately track the input signal without significant attenuation or phase shift.
• Time Constant: For first-order systems, it represents the time taken for the output to reach approximately 63.2% of its total change following a step input.