A stylus location system comprises a field emission device having an anode plate 32 and an emitter plate 2, and a plurality of piezoelectric point elements 34, 42 coupled to the anode plate. The piezoelectric point elements 34, 42 are capable of transforming electrical energy into ultrasonic energy and...http://www.google.co.uk/patents/US5657054?utm_source=gb-gplus-sharePatent US5657054 - Determination of pen location on display apparatus using piezoelectric point elements
