New Touch Actuator Based on the Foil-Electret Principle

A new touch actuator is described that consists of a single metalized foil electret, a number of independent metal backplate sections aligned with the holes of a front cover, and air gaps separating electret and backplate sections. The metal layer of the foil forms a transducer with capacitance Cawith each backplate section. The actuator is operated by touching the foil electret through one of the front-cover holes. For a trapezoidal displacement (duration ) of the foil electret, the electrical output signal across a terminating resistor consists of a pair of oppositely poled spikes with duration comparable to RCa, if RCais smaller than . If RCais larger than , the electrical output approximates the displacement. For an electret with a charge density of , the measured pulse amplitudes due to touching are of the order of 0.3 to over 100 V depending on the applied force, and on the resistive and capacitive loading. The touch actuator is mechanically simple, reliable to use, inexpensive to produce, and can be made with an overall thickness of less than 0.5 cm. Apart from its potential use in telephone dials, it may also find applications as a key transducer in digital devices.