High sensitivity, low noise front-end for long range capacitive sensors for tagless indoor human localization

Capacitive sensors are used in many applications due to their multiple advantages, but typically up to distances comparable to sensor dimensions. We present the design and the experimental results of a self-contained long range capacitive sensor front-end that is based on phase modulation and is suitable for indoor human localization. The changes in reactance due to the changes in the sensor capacitance modulate the phase of a carrier frequency, which is then demodulated using a XOR gate as quadrature detector. A phase modulated carrier is both less susceptible to noise and easier to filter. The experimental results show a very good sensor sensitivity up to 150 cm for a 16 cm square sensor plate, very low noise and good measurement stability.