US Patent 8698509 Proximity sensor

An immersion sensor for use with a cushion or mattress for determining the relative immersion of a person within the cushion or mattress including a sensor, a ground and a circuit for measuring capacitance. The sensor includes a sheet of conductive material, and the ground includes a second sheet of conductive material. The circuit is adapted to send short bursts of electrical current to the sensor and a capacitor. The circuit is further adapted to measure the length of time the burst of current takes to charge the capacitor. Based upon the measured time, the circuit calculates the proximity of the object based upon the time taken to charge the capacitor. A method that may be implemented with the immersion sensor is also disclosed.