Capacitive Sensors
Capacitive sensing measures geometry or dielectric change through electric-field coupling; parasitics, guards, environment, excitation, and calibration determine whether the intended capacitance dominates.
For ideal parallel plates,
A capacitive sensor can therefore measure displacement , overlap area , or dielectric permittivity . Real sensors often operate through fringe fields, so nearby conductors, moisture, deposits, cable motion, and a person's body become part of the measured geometry.
Differential geometry #
Two opposed capacitors can be arranged so displacement increases one and decreases the other. Their ratio or difference improves sensitivity and rejects some common cable and dielectric change. It does not cancel asymmetric contamination or shield motion.
Readout #
Capacitance may set oscillator frequency, charge-transfer time, AC bridge balance, or the input of a capacitance-to-digital converter. Excitation frequency trades sensitivity against dielectric loss, interference, and cable impedance. Driven guards hold nearby surfaces near the sense-node potential, reducing leakage and parasitic capacitance; guards require stable driver headroom and careful layout.
Liquid-level example #
An external electrode on a nonconductive vessel senses fringe capacitance to a reference electrode. As liquid replaces air, higher permittivity changes capacitance. Vessel wall thickness, liquid conductivity/permittivity, foam, residue, grounding, and temperature all affect calibration; a single clean-water calibration is not universal.
Verification #
Test the intended target across position and temperature, then separately introduce hands, grounded metal, moisture, residue, cable flex, supply change, and enclosure assembly. Record raw capacitance/baseline as well as the final threshold state. If the raw signal cannot distinguish target from disturbance, threshold tuning is not a robust fix.