Feedback Control of Resonant Modes in Bending Response of Ionic Polymer Actuators

Abstract

Abstract Ionic polymer actuators exhibit large bending response under the application of an electric field but their slow time constant limits the actuation bandwidth. Recent results have demonstrated the ability of feedback control to increase the actuation bandwidth of ionic polymer bender actuators. A critical parameter in the control system is the frequency of the first elastic resonance of the actuator. Longer polymers exhibit a lower frequency resonance which limits the closed-loop performance. In this paper, an empirical model of ionic polymer actuator developed by Kanno (1994) is used for closed-loop control. The empirical model is modified to incorporate the resonant dynamics of the actuator and optimized. The empirical model is based on experimental measurements obtained through a series of open-loop responses for an ionic polymer actuator in a cantilever configuration. The empirical model is optimized and used to design a feedback compensator by state space modeling techniques. Since the ionic polymer actuator has slow settling time in the open-loop, the design objectives are to minimize the settling time and constrain the control voltage to be less than a prescribed value. The controller is designed using Linear Quadratic Regulator (LQR) techniques which reduced the number of design parameters to one variable. Simulations are performed which shows settling times of 0.03 seconds for closed-loop feedback control are possible as compared to the open-loop settling time of 15–20 seconds. The maximum control voltage varied from 1.2 Volts to 3.5 Volts depending on the LQR design parameter. The controller is implemented and results obtained are consistent with the simulations. Closed-loop settling time is observed to be 6–10 seconds and the ratio of the peak response to the steady-state response is reduced by an order of magnitude. Discrepancies between the experiment and the simulations are attributed to the inconsistencies in the resonant frequency of the actuator. Experiments demonstrate that changes in the surface hydration of the polymer result in 6.5%–12.5% variations in the actuator resonance. Variations in the actuator resonance require a more conservative compensator design, thus limiting the performance of the feedback control system.