Piezoelectric Dual-Axis Cantilever Force Measurement Probe

A piezoelectric cantilever force sensor consisting of a mechanical cantilever probe, an analog amplifier and an embedded digital signal processing unit is presented. The sensor is constructed from a lead zirconate titanate (PZT) piezo disc, which was cut into four segments and a 33-mmlong metal probe attached to it perpendicularly to the piezo disc plane. The signal of the piezoelectric elements was amplified with a four channel operational amplifier circuit having voltage gain of 10 (20 dB) in each channel and input resistance of 50 MΩ. The digital signal processing was implemented on an Arduino Due -based hardware. Signal pre-processingblock consists of three cascaded digital infinite impulse response (IIR) filters were composed: A DC-blocking filter, a notch filter and a low pass filter. Additionally, a moving average filter was implemented to further enhance the system tolerance to power line interference and to reduce the sampling rate to 50 Hz. Finally, a LabVIEW software is used to provide an interface for the Arduino Due. The calibration of the system was carried out with a texture analyzer and it was capable of sensing biaxial forces in the range of 2 millinewton with a RMS error of less than 60 micro Newton and hysteresis in the order of 0.25 millinewton or 12.5% of the full scale reading. Cross talk ratio between axes was 25.3 or 28.0 in dB. The applicable force measurement frequency range was 0.5 - 10 Hz.

I. Introduction

During recent two decades, there has been an increasing trend towards miniaturization and micromanipulation in many fields, such as biomedical engineering, robotics, industrial automation and materials sciences. In the miniaturized systems, force measurement becomes critical not only due to the small forces to be measured but also because the miniaturized equipment are easily damaged [1]. The force sensing solutions range from resonant tactile force sensors for robotic touch sensing, in which the change in the resonant frequency of piezoelectric force sensor changes as a function of applied force [2], to screen printed thick film piezoelectric slip sensor, which is used to enhance the functionality of a prosthetic hand by decreasing the complexity of the touch sensing system [3]. Along with a piezoelectric sensor principle, piezoresistive sensing can be applied in similar sensing applications, except the piezoelectric sensor cannot detect static forces. For example, a screen printed piezoresistive cantilever sensor proposed by Lakhmi et al. has shown capability to measure static forces in micronewton range [4].