Hysteresis independent on-line capacitance measurement for piezoelectric stack actuators

Piezoelectric actuators are typically modeled as capacitive elements for control purposes. An important model parameter, clamped capacitance, is traditionally considered constant and obtained through offline measurements. However, the capacitance value changes with operating conditions such as electric fields, environmental temperatures, and aging. In addition, traditional capacitance measurements are not hysteresis free. In this research, a hysteresis independent real time capacitance measurement technique is proposed. This is achieved by superimposing a high frequency ripple over the normal control voltage signal and comparing the resulting current signal to the voltage ripple. Since the mechanical bandwidth of the actuator is much lower than the ripple voltage, no measurable movement is induced by the measurement technique, and a clamped capacitance is determined. It is demonstrated that the measurement is largely independent of hysteresis and it shows good agreement with an LCR meter. The on-line capacitance measurement is very useful for characterising piezoelectric actuators and for implementing more robust control schemes.