Extension sensing of piezo actuators using time-domain ultrasonic measurement and frequency-domain impedance measurement

There is a requirement for a simple and accurate extension sensing method for multilayer piezoelectric actuators for precision micro-positioning applications. This paper explains three different techniques of extension sensing for multilayer piezoelectric actuators, to provide a suitable method which involves primarily overcoming the inherent hysteretic behaviour of piezoelectric materials. The methods were; time domain ultrasonic pulse echo response for time travel of acoustic pulses, frequency shift response spectrum of an ultrasonic transducer and frequency shift in the impedance spectrum of the actuator alone. From the results, method of the frequency shift in the impedance spectrum of the actuator alone proved to be the most effective of the three. It was found that due to acoustic velocity changes under DC bias during the ultrasonic measurements, the inherent hysteretic behaviour was amplified in the other two methods when extension was plotted against time-shift and frequency shift. The results are presented and are discussed in terms of their suitability for precise positioning.