JNTU B.Tech II TRANSDUCTION OF PHYSICAL VARIABLES Previous Question Paper (October - 2020 - R13)

Transducers are devices that convert a physical quantity into an electrical signal. They are classified based on the method of energy conversion, the nature of the output signal, and the source of power.

1. Classification based on Power Source:

• Active Transducers: These generate their own electrical output (voltage or current) without requiring an external power source. They operate on the principle of energy conversion. Examples: Thermocouples, piezoelectric crystals, and photovoltaic cells. Merits: Self-generating, no need for external power, and simple circuitry.
• Passive Transducers: These require an external power source to operate. They change their passive parameters like resistance, capacitance, or inductance in response to the input. Examples: LVDT, strain gauges, and thermistors. Merits: High resolution, wide range of measurement, and suitable for static and dynamic signals.

2. Classification based on Nature of Output:

• Analog Transducers: These produce a continuous electrical signal (voltage or current) proportional to the input. Examples: LVDT, strain gauge. Merits: Real-time monitoring and high sensitivity.
• Digital Transducers: These provide output in the form of pulses or digital codes. Examples: Optical encoders, digital tachometers. Merits: Easy to interface with computers, high noise immunity, and high accuracy.

3. Classification based on Principle of Transduction:

• Resistive Transducers: Change resistance due to physical stimuli. Example: Potentiometer. Merit: Simple and low cost.
• Capacitive Transducers: Change capacitance by varying distance, area, or dielectric constant. Example: Condenser microphone. Merit: High input impedance and low loading effect.
• Inductive Transducers: Change self-inductance or mutual inductance. Example: LVDT. Merit: Rugged construction and high output.

Summary of Relative Merits: Active transducers are preferred for portable applications due to their self-generating nature, whereas passive transducers are preferred for high-precision industrial measurement systems where external power is readily available. Digital transducers are increasingly favored in modern instrumentation for their direct compatibility with digital data processing systems.

Static characteristics refer to the performance criteria of a measuring instrument when the measured variable is constant or varies very slowly over time. These characteristics define the quality and reliability of the measurement system.

The primary static characteristics include:

• Accuracy: The closeness with which an instrument reading approaches the true value of the variable being measured. It is often expressed as a percentage of the full-scale reading.
• Precision: The measure of reproducibility or consistency of the measurements. A precise instrument provides the same reading for repeated measurements of the same input.
• Sensitivity: The ratio of the change in output signal to the change in input signal. Mathematically, it is the slope of the calibration curve.
  Sensitivity = ΔOutput / ΔInput
• Linearity: An instrument is linear if the output is directly proportional to the input over its operating range. The deviation from the ideal straight-line calibration curve is known as non-linearity.
• Resolution: The smallest change in the input variable that produces a detectable change in the instrument output.
• Hysteresis: The difference in output readings for the same input value when approached from increasing versus decreasing directions. This is caused by mechanical friction or magnetic effects.
• Threshold: The minimum input required to produce a detectable output change from zero.
• Drift: A gradual shift in the instrument's output over time while the input remains constant, often caused by environmental changes like temperature or aging of components.

These characteristics are typically represented by a calibration curve, which plots the output versus the input. A linear instrument shows a straight line, while non-linear instruments exhibit curvature. Hysteresis is represented by a loop between the loading and unloading curves. These parameters collectively determine the suitability of a sensor or transducer for a specific industrial application.