Displacement Sensors and Strain-Gauge Circuits

A displacement sensor measures how far an object has moved from a reference position, or its absolute position along a path. Such measurements underlie many instruments, because force, pressure and load can often be measured indirectly by detecting the small movement or deformation they cause. Displacement sensors range from simple resistive devices to precise electromagnetic ones, chosen according to the distance to be measured and the accuracy required.

A common precise device is the linear variable differential transformer (LVDT). It contains a movable magnetic core inside a set of coils; as the core slides, it changes how strongly an alternating signal couples from a primary coil to two secondary coils, and the difference between their outputs indicates the core's position. The LVDT has no sliding electrical contacts, so it does not wear out and can give reliable, repeatable readings over a long working life. Simpler resistive sensors use a sliding contact on a resistive track, producing a resistance proportional to position much like a potentiometer.

Strain and the small displacements it produces are commonly measured with strain gauges, whose resistance changes slightly when the surface they are bonded to is stretched or compressed. Because this change is tiny, strain gauges are usually connected in a Wheatstone bridge, a circuit of four resistive arms that produces an output voltage proportional to the imbalance caused by the gauge. The bridge greatly improves sensitivity and can be arranged to cancel unwanted effects such as temperature change, which would otherwise be mistaken for strain. This combination forms the basis of load cells and pressure sensors used in weighing and force measurement.

Frequently asked questions

How does an LVDT measure displacement?

A movable magnetic core changes how an alternating signal couples between a primary coil and two secondary coils; the difference in their outputs indicates the core's position, with no wearing contacts.

Why are strain gauges connected in a Wheatstone bridge?

The resistance change of a strain gauge is very small, so a four-arm bridge converts the imbalance into a measurable output voltage and can cancel effects such as temperature change.

How can force be measured with a displacement sensor?

Force deforms a structure by a small amount, and measuring that deformation with a strain gauge or other displacement sensor gives an indirect measure of the applied force.