Cellular ferroelectrets for flexible touchpads, keyboards and tactile sensors

Cellular polypropylene ferroelectrets are flexible and conformable transducer materials with a dominant longitudinal piezoelectric effect. They can be used in a wide range of pressure sensing applications. We present concepts for position sensitive touchpads and keyboards based on piezoelectric materials with a dominant longitudinal piezoelectric effect. As large area electrodes are superimposed onto the ferroelectret and combined with electronics at the edges, complex sensor matrices are avoided. We use a ferroelectretic detector stripe as a model system for the theoretical description of the touchpad and show that this detector can be treated mathematically analogous to a transmission line. We present the solutions of the corresponding differential equations under the appropriate boundary conditions and experimental results that are in agreement to them. For the flexible, ferroelectretic keyboard we use a binary or balanced ternary coding technique profiting from the polarization of the ferroelectret.