ACUPAD: A track-pad device based on a piezoelectric bimorph

A track-pad is a pointing device, featuring a tactile sensor, able to translate the motion and position of a user's finger, or a stylus, to a relative position on a screen. In this paper a piezoelectric tactile sensor for track-pad applications is proposed; the active element of the device is a cheap piezoceramic bimorph disk, widely used in buzzers and telephone receivers, clamped all around its border. The device operating principle is the following: when a stylus is positioned on the bimorph surface, the displacement field of the contact point is modified, with a consequent variation of the device electrical input impedance; the stylus position can be therefore related to the impedance variation. The system was analyzed by FEM, obtaining a clear dependence of the device characteristic frequencies on the stylus radial position, while by moving the stylus along the angle, a clear variation of impedance values is obtained. A device prototype was realized and FEM results were experimentally confirmed, validating the proposed device performance.