Near static strain measurement with piezoelectric films

Abstract

A fundamental challenge that prevents the implementation of piezoelectric sensors for measuring extremely low frequency (<1 mHz) events, such as aerodynamic pressure measurements, earthquake detection, and physiological monitoring, is their inability to measure static or very low frequency signals. To enable near static measurements, we present a low-noise, differential charge amplifier topology that performs real-time cancellation of drift in the output voltage while simultaneously increasing the gain of a conventional charge amplifier. A sensing frequency range from 0.01 mHz to 310 kHz with a voltage drift reduction of up to 95% is demonstrated. A theoretical sensitivity increase of up to 30 dB is achieved with the proposed topology compared to a basic charge amplifier with the same component tolerances, time constant, and allowable drift rate. The proposed circuit is interfaced with a piezoelectric PVDF film for evaluation of performance in the time and frequency domains. The measured voltage from uniaxial near DC strain measurements stays within 3% over a measurement period of 500 s. The paper further describes a linear piezoelectric strain sensor model and the various factors that influences the charge output, loading effect, and lateral cross-sensitivity of the sensor. The calculated strain–voltage relationship or the gage sensitivity agrees well with the measurements with a linearity error of less than 5% up to 1 millistrain.