A piezoelectric based sensor system designed for in vivo skin biomechanical measurements

This paper describes an electromechanical handheld probe dedicated to skin biomechanics characterization. This probe is composed of two piezoelectric bending actuators that control tip displacement. Simultaneous force and displacement sensing is achieved by integrating two pairs of resistive strain gauge sensors. Piezoelectric hysteresis, however, compromises the accuracy of force sensing. To improve force sensing, a hysteresis model is introduced. The hysteresis behavior of piezoelectric actuators is usually described between the input voltage and output displacement. The influence of external force is rarely discussed. Considering the combined effects of the electric field and the external stress, we model hysteresis with the instantaneous displacement relying on the Bouc-Wen model. Experimental and simulation results show that the hysteresis model can predict bender behavior even driven by multi-harmonic voltages. Based on this hysteresis model, the hysteresis compensation strategy is validated at no load and loaded conditions (a spring load). However, to simplify the compensation process, another original strategy is proposed, relying on a closed-loop displacement control. Results demonstrate that the second approach also improves the accuracy of force sensing. Finally, preliminary measurements performed on the real skin show promising results.