robotics//sensor//strain gauge

A strain gauge is a thin resistive foil bonded to a part whose resistance changes when the part stretches or compresses, and it is how machines measure force, weight, torque and pressure: scales, thrust stands for drone motors, torque sensors on robot joints, the structural monitoring of bridges. The relative change of resistance is proportional to the strain \(\varepsilon\), the fractional change of length, through the **gauge factor** \(GF\), about 2 for metal foil:

A strain gauge is a thin resistive foil bonded to a part whose resistance changes when the part stretches or compresses, and it is how machines measure force, weight, torque and pressure: scales, thrust stands for drone motors, torque sensors on robot joints, the structural monitoring of bridges. The relative change of resistance is proportional to the strain ε\varepsilonε, the fractional change of length, through the gauge factor GFGFGF, about 2 for metal foil:

ΔRR=GF⋅ε.\frac{\Delta R}{R}=GF\cdot\varepsilon .RΔR​=GF⋅ε.

The changes are tiny. A steel part strained by 1000 microstrain (a millimetre per metre, already a heavy load) changes a 350 Ω gauge by about 0.7 Ω. So the gauge is never read alone: it is wired into a Wheatstone bridge, whose output is zero at rest and a few millivolts per volt of excitation under load, and that signal is amplified and converted by a high-resolution converter (measurement chain).

A load cell is a metal body shaped to strain predictably under force, with four gauges forming a full bridge (two stretched, two compressed), so the signals add and the effect of temperature on the gauges largely cancels. Its rated output is typically around 2 mV/V at full load.

Its two classic errors are slow. Thermal drift moves the zero as the cell warms, because the gauges, the metal and the adhesive all expand; creep makes the reading under a constant load keep changing for minutes as the material and the glue relax. Both look like a real change of force to anything downstream (sensor error model).

Mounting decides the measurement. A gauge measures strain where it is glued and in the direction it points, so a bending moment or a side load the designer did not expect reads as the force being measured; good load cells are built to be blind to everything but their axis.