Thermal-Variation Insensitive Force-Touch Sensing System Using Transparent Piezoelectric Thin-Film

A force-touch (z-axis) sensor for display applications has attracted considerable attention. The z-axis sensing layer can be realized by a transparent piezoelectric thin film due to its superior characteristics, such as thinness, flexibility, and an optical transparency. However, this film responds not only to pressure but also to thermal changes (pyroelectric characteristic), which is a major obstacle to commercialization. This paper presents a thermal-variation insensitive, force-touch sensing system using a transparent piezoelectric thin film. In conjunction with optimizations in analog front-end design for high gain and high speed, the threshold-based digital processing scheme, which effectively rejects pyroelectric signals and noises, is proposed. To validate the proposed design and method, the electronic readout system board composed of analog circuits and an eight-bit microcontroller unit was implemented, and it read out the force-touch signal with a ten-bit resolution in power consumption of 6.5 mW. The implemented readout system was experimentally verified with a 3.9-inch polyvinylidene fluoride-copolymer piezoelectric thin-film sensor, with a thickness of 20 μm. The system achieves a force-to-signal gain of 0.6 V/N in a range of 0.3-2.5 N with high linearity. The capability to reject a pyroelectric signal up to ±Δ10 °C/s was also proved through a thermal-shock experiment.