Technology: the principle behind capacitive sensing

Capacitive sensing is a technology, based on capacitive coupling, that can detect and measure anything that is conductive or has a dielectric different from air. In industrial, it can be used to measure proximity, pressure, position and displacement, force, humidity, fluid level, and acceleration. The technology was also applied to human interface devices, such as touchpads on the laptops, touchscreens and touch based buttons.

Capacitive sensing technology is basically measuring capacitance via measuring time during charging or discharging. An existing voltage is sent to the plate which contains charges. When the plate is fully charged to that voltage the time will be captured. The changing time is proportional to capacitance which can be used as a control signal.

When human body is involved, the extra capacitor is added to the system. For a stationary media, the capacitance is mainly determined by area of contact and the distance between the electrode and the ground.

During the interaction, the amount of contact between the hand and the plate, the distance between the plate and human or distance between the electrodes being touched and the ground , number of people in the contact chain will result in different charging time.