Potentiometric Position & Displacement Sensors

A potentiometric sensor is a resistive divider whose wiper follows linear or angular displacement. With uniform track resistance and an unloaded output,

$$V_o=V_{exc}\frac{x}{L},$$

where $x/L$ is normalized travel. Because the result is a ratio, using $V_{exc}$ as the ADC reference cancels first-order excitation drift.

Loading bends the transfer law #

The source resistance seen by the load is $R_{th}=R_1\parallel R_2$, largest near mid-travel. A finite receiver $R_L$ shunts the lower track, so output is no longer proportional to position. For a 10 kΩ potentiometer at midscale, $R_{th}=2.5$ kΩ. A 100 kΩ receiver produces about 2.44% divider loading at the worst point; a buffer largely removes it but adds offset, bias current, and rail-range limits.

Resolution and contact behavior #

Wirewound tracks change in turns and have discrete electrical resolution. Conductive-plastic tracks are effectively continuous but still have wiper contact resistance, noise, wear, and intermittent open-circuit events. Mechanical backlash in the linkage can dominate electrical linearity.

Design and diagnosis #

Use only the qualified electrical travel; end stops and dead zones are not calibrated range. Limit wiper current, especially during faults. Filter contact noise only enough to reject it without masking real motion.

Calibrate at both endpoints and at several interior points in both directions; endpoint-only calibration cannot reveal loading curvature or backlash.