Foreign body detecting

A sensor comprising:

• a first transmit plate;
• a second receive plate;
• dielectric material between the two plates;
• a voltage generator for generating an alternating voltage applied across the transmit and receive plates thereby to create an alternating electric field, which applied voltage results in a current flowing between the two plates;
• a convertor producing a voltage signal corresponding to the resultant current;
• a calculating arrangement determining the average value of the product of the corresponding voltage signal and the alternating reference voltage signal;
• and
• a phase shifter adjusting the phase of the reference voltage signal until a null condition is achieved, at which condition the average value is approximately equal to zero.

A method of operating a sensor comprising a first transmit plate, a second receive plate and a dielectric material between the two plates, the method comprising the steps of:

• applying an alternating voltage across the transmit and receive plates thereby to create an alternating electric field, which applied voltage results in a current flowing between the two plates;
• producing a voltage signal corresponding to the resultant current;
• determining the average value of the product of the corresponding voltage signal and a reference voltage signal;
• and
• adjusting the phase of the reference voltage until a particular condition is achieved.