Durable sensors for measurement of foot plantar pressure with piezoelectric polyvinylidene fluoride foil

This paper presents application of polyvinylidene fluoride (PVDF) foil for sensors of foot plantar pressure for measurements in natural conditions (i.e. outside laboratory). The material has been evaluated for this application based on investigation of material resistivity, current and activation energy of electret depolarization process, and relationship between pressure and electrical signal. The investigation shows that PVDF foils can operate at temperatures up to 85 °C, have high activation energy of 5.2 eV and a linear dependence between pressure and piezoelectric signal, which makes them suitable candidates for foot plantar pressure transducers. Influence of foil stretching is eliminated via a design of the shoe insole, which prevents deformations. Design of an input amplifier which eliminates pyroelectric effect is also presented. For applications in measuring foot plantar pressure, sensor calibration can be eliminated by placing several pressure sensors in the shoe insole and expressing pressure as relative values between sensors. The paper also describes a wireless system for measurement of foot plantar pressure, and obtained measurements.