Proximity sensor

An immersion sensor assembly comprising:

• a cushion or mattress having an adjustable depth of immersion of a person on the cushion or mattress;
• a sensor comprising a sheet of conductive material;
• a grounding plane being in the area around the sensor and allowing a capacitance measurement to be made relative to the distance between a person and the sensor;
• a shield comprising a second sheet of conductive material located adjacent to and having generally the same shape and area as the sensor;
• a circuit comprising a reference capacitor, the circuit adapted to
• send short bursts of electrical current to the sensor and the reference capacitor,
• measure the length of time the burst of current takes to charge the reference capacitor and the circuit adapted to calculate the relative proximity of the person based upon the time taken to charge the reference capacitor,
• drive the shield to electrically isolate the shield from external capacitance or electromagnetic interference, and
• provide an indication when the person is too deeply immersed within the cushion or mattress.

A bed assembly for the support of an individual positioned on the bed, the bed assembly comprising:

• a bed frame, said bed frame comprising metal components;
• a mattress on the bed;
• and
• a sensor operatively associated with the mattress, the sensor comprising,
• a microcontroller,
• a ground layer surrounding the sensor;
• a shield layer comprising a sheet of conductive material and driving the shield to isolate the sensor from the effect of stray capacitance and electromagnetic interference from the metal bed frame components;
• a circuit for detecting the capacitive effect of the individual on the mattress and providing a relative output for the microcontroller;
• wherein the planes formed by the sensor and the shield, when the immersion sensor is laid flat, are nominally parallel;
• and
• wherein the circuit is adapted to provide an indication when the individual is too deeply immersed within the mattress or when the individual is off the mattress.

In a bed assembly comprising a bed frame and a mattress, a method of determining the proper immersion of a person within the mattress comprising the steps of:

• providing a first sensor below mattress comprising a sheet of conductive material;
• providing a ground layer comprising a sheet of conductive material that is nominally coplanar and maintained at a ground potential below the cushion or mattress;
• providing a shield layer comprising a sheet of conductive material and driving the shield to isolate the sensor from the effect of stray capacitance and electromagnetic interference from the bed frame.
• sending short bursts of electrical current to the sensor and a capacitor;
• measuring the length of time the burst of current takes to charge the capacitor;
• calculating the proximity of the person based upon the time taken to charge the capacitor;
• and
• providing an indication when the person is either over-immersed or under-immersed within mattress.