A Battery-free Pressure Sensing System Based on Soft Piezoelectric Device for Tennis Training

In this work, a battery-free position and pressure monitoring system based on soft polyvinylidene fluoride (PVDF) piezoelectric thin film with helical coils circuit design is designed and used for tennis training. The racket surface is firstly divided into block areas and the sensing transducers are placed in each of them to detect the hitting position and force in these areas. To modify the extensibility of the system for tennis training, all the devices are connected by the S-shape helical line, which has two states: curled at usual and straight at work. Four piezoelectric patches form a series system to power the energy harvest system and wireless communication module, which help to realize the battery-free and wearable functions. A linear relation between the hitting force and induced electric voltage is experiment verified with the slope of 0.05 V/N. The system is also verified in the real tennis ball game monitoring for the hitting time and hitting position, benefiting the future application of wearable devices in artificial intelligence and the Internet of Things (IoT).