Correct Answer - Option 2 : Strain gauge; strain
Active transducers:
- Active transducers are those which do not require any power source for their operation.
- They work on the energy conversion principle. They produce an electrical signal proportional to the input (physical quantity).
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Piezoelectric, thermocouple, and photovoltaic cell transducers are some examples of active transducers.
Passive transducers:
- Transducers which require an external power source for their operation is called a passive transducer.
- They produce an output signal in the form of some variation in resistance, capacitance, or any other electrical parameter, which then has to be converted to an equivalent current or voltage signal.
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LVDT, strain gauge, RVDT, etc is an example of a passive transducer.
Strain Gauge:
- Strain Gauge is a passive transducer that converts a mechanical elongation or displacement produced due to a force into its corresponding change in resistance R, inductance L, or capacitance C.
- It uses the variation in electrical resistance in wires to sense the strain produced by a force on the wires.
- A strain gauge is basically used to measure the strain in a workpiece.
The gauge factor is defined as the ratio of per unit change in resistance to per unit change in length. It is a measure of the sensitivity of the gauge.
Gauge factor, \({G_f} = \frac{{{\rm{\Delta }}R/R}}{{{\rm{\Delta }}L/L}}\)
\(\frac{{{\rm{\Delta }}R}}{R} = {G_f}\frac{{{\rm{\Delta }}L}}{L} = {G_f}\varepsilon \)
Where ε = strain = ΔL/L
The gauge factor can be written as:
= Resistance change due to change of length + Resistance change due to change in the area + Resistance change due to the piezoresistive effect
\({G_f} = \frac{{{\rm{\Delta }}R/R}}{{{\rm{\Delta }}L/L}} = 1 + 2v + \frac{{{\rm{\Delta }}\rho /\rho }}{\varepsilon }\)
If the change in the value of resistivity of a material when strained is neglected, the gauge factor is:
\({G_f} = 1 + 2v\)
The above equation is valid only when the Piezoresistive effect that changes in resistivity due to strain is almost neglected.
For wire-wound strain gauges, the Piezoresistive effect is almost negligible.
