Development and Optimization of a Flexible Printed Coplanar Capacitive Sensor for Accurate Twisting Motion Detection

Flexible electronic sensors have many advantages, such as conformability, space-saving, etc., compared to traditional rigid electronic sensors. With the advancement of technologies in recent years, flexible sensors have made remarkable progress in monitoring various types of deformations, including stretching, bending, twisting, and complex combined deformations. However, flexible sensors specifically for detection of twisting deformation have fallen behind those developed for other types of deformations. This is because many existing sensors rely on changes in resistance to calculate the corresponding deformations. However, the change in resistance of the printed conductive line is negligible when subjected to twisting. Nonetheless, precise detection of twisting is crucial for many applications. Therefore, it is very important to develop flexible sensors specifically for twisting deformation detection. One promising approach is to explore alternative metrics like capacitance or inductance. In this study, we investigate the feasibility of coplanar printed flexible capacitive sensor for twisting angle detection. The sensor will be directly printed with conductive silver ink on one side of the flexible substrate of polyimide and TPU (Thermoplastic Polyurethane). As the sensor twists, the relative positioning of its conductive lines will change, resulting in changes in capacitance. The advantage of the coplanar design is its ease of manufacturing, as the sensor can be fabricated by simply printing the pattern onto one surface. A design pattern will first be tested for the sensitivity, which is defined by the rate of relative capacitance change corresponding to the twist angle. Both the polyimide and TPU samples will be tested. In addition, a parametric study will be performed through numerical simulations to examine the effect of substrate stiffness. The optimal substrate stiffness will be identified as the point where the sensor's strain is minimized. Findings from both the simulations and the experiments will help establish future guidelines for enhancing the twist angle detection performance of the printed coplanar flexible sensor.