Piezoelectric Sensors for Flexible Electronic Stress Monitoring

With the adoption of the internet of things (IoT) and wearable electronics, low size weight and power (SWaP) flexible hybrid electronics (FHE) are being installed in configurations that lead to large stresses on the various components. These stress levels may lead to large stresses and deformation of the flexible PCB boards traces, dielectric layers, and IC connections, which may affect its performance. In order to monitor installation stresses, a piezoelectric sensor was developed. 120 mu m PVDF thick films were used because of its ease of integration with flexible electronics and its large, g33, which was measured to be 0.287 Vm/N. These piezoelectric sensors were incorporated with a TI MSP430 Micro-Controller Unit (MCU) to record and respond to stress levels. The analog input on the MCU has an input impedance of 35 kO. A Thevenin equivalent model of the piezoelectric source and pull-up resistors with different impedances was modeled in spice. From these results, an impedance-matching circuit was built to maximize the digitized stress signal. Above a certain output voltage, the MCU was programmed to turn on a LED to alert the end user the stress threshold was reached. At radius of curvature of 10 mm, the flexible PCB failed, and this became the threshold to turn on the LED. This study demonstrates a novel use case of piezoelectric sensors for FHE and details the importance of impedance matching and circuit design for transitioning piezoelectric films with external circuits and hardware.