Resistive Transducers Questions and Answers

Answer: C

A strain gauge is a passive device that converts mechanical displacement into a change of resistance.

A strain gauge is a fine wire or foil grid bonded to a surface. When the surface is strained, the grid stretches or compresses, changing its length, cross-section and resistivity, and so its resistance; the gauge factor relates the fractional change in resistance to the strain.

It is passive because it generates no energy of its own: an external excitation, usually a Wheatstone bridge, is needed to turn the resistance change into a voltage. The options calling it active describe a self-generating transducer such as a thermocouple or piezoelectric crystal, and 'electrical displacement' is not what it senses.

Answer: B

The official key is S2 and S3: the gauges are brittle and temperature sensitive, and they show poor linearity.

These two points describe semiconductor strain gauges, which have a large gauge factor but whose resistance changes non-linearly with strain, drift with temperature and are brittle and hard to bond. Sources found for the search agree on brittleness, temperature sensitivity and non-linearity.

What could not be confirmed: sources disagree on the cost of semiconductor gauges, which some call low and some high, and 'low fatigue life' (S1) was not found among the usual listed drawbacks. The key's S1-free answer, S2 and S3, is therefore kept; options containing S1 are rejected on that basis.

Answer: B

Semiconductor strain gauges depend on the piezo-resistive effect.

When silicon or germanium is strained, its band structure and carrier mobility change, so its resistivity changes. This change in resistivity is far larger than the purely geometric change in a metal-foil gauge, giving gauge factors of roughly 100 to 175 against about 2 for metal gauges. The resistance change is read in a Wheatstone bridge.

The piezoelectric effect generates a charge rather than changing resistance, and suits dynamic rather than static strain. Ferromagnetism is the basis of magnetostrictive sensors, and superconductivity plays no part in strain gauges.

Answer: C

A strain gauge is a passive device and converts mechanical displacement into a change of resistance.

A strain gauge is a fine metal foil or wire, or a semiconductor element, bonded to a structure. When the structure is strained, the gauge's length and cross-section change, and so its resistance changes in proportion to the strain, by a factor called the gauge factor. It produces no energy of its own and needs an external excitation, normally through a Wheatstone bridge, so it is a passive transducer.

Options calling it active are wrong, since active transducers, such as thermocouples and piezoelectric crystals, generate their own output. 'Electrical displacement' is wrong because the input is mechanical strain.

Answer: D

A Wheatstone bridge is used for strain gauge-based pressure measurement.

A strain gauge changes its resistance by only a tiny fraction when strained. Placed in a Wheatstone bridge, often with four active gauges on a diaphragm, that small change unbalances the bridge and gives a voltage proportional to pressure, with temperature effects largely cancelled. Disposable invasive blood pressure transducers work this way.

ECG amplification uses instrumentation amplifiers, not a bridge. Optical imaging uses photodetectors, and ultrasound is generated by driving piezoelectric crystals.

Answer: C

The change in resistance of the strain gauge is 260 micro-ohm. The gauge factor is the fractional change in resistance divided by the strain, so

The value 130 micro-ohm would be the answer with a gauge factor of 1, and 390 and 520 micro-ohm would correspond to gauge factors of 3 and 4. The answer is 260 \(\mu\Omega\).