This low-cost sensor uses capacitive coupling to measure absolute rotation.
This sensor uses time-domain measurement based on capacitive coupling between parallel copper planes.
Receive (RX) pads are tied to power and ground through large resistors. If a transmit (TX) pad nearby suddenly charges or discharges, we see a transient response on the RX pad before it settles back to its usual voltage.
The scale of this transient depends on the resistor values, TX voltage change, and coupling magnitude. Since resistors and voltage swing are constant in this device, we can estimate the coupling by measuring the size of this transient response.
The coupling is in turn based on the geometric relationship between the TX and RX plates, which allows us to find rotation. Similar devices with different geometries can be used to measure linear displacement.