Feedback Control of Ionic Polymer Actuators
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An ionic polymer actuator consists of a thin Nafion-117 sheet plated with gold or platinum on both sides. An ionic polymer actuator undergoes large deformation in the presence of low applied voltage across its thickness and exhibits low impedance. They can also be used as large displacement sensors by bending them to induce stresses and generate a voltage response. They operate best in a humid environment. Ionic polymer actuators have been used for various practical applications such as bio-mimetic robotic propulsion, flexible low mass robotic arms, propellors for swimming robotic structures, linear and platform type robotic actuators and active catheter systems. One of the disadvantages of ionic polymer actuators is that their settling time to a unit step voltage is on the order of 5-20 seconds in a cantilever configuration. The slow time constant of an ionic polymer limits the actuation bandwidth. The characteristics of ionic polymer actuators, low force and large displacement (as compared to other actuator technologies such as PZT or PVDF), cannot be used in applications requiring a faster response time for a given actuation signal. Due to this limitation, many applications will not be able to make use of the large displacement effectively because of the limited bandwidth of the actuator. Another disadvantage of using an ionic polymer actuator is that the stiffness of the actuator is a function of the hydration of the polymer. Difficulties in controlling the hydration, which changes with respect to time, results in inconsistencies in the mechanical response exhibited by the polymers during continual usage. Several physical models of ionic polymer actuators have been proposed. The physical phenomenon responsible for the bending is not completely understood and no clear set of principles have been able to explain the motion of the polymers completely. Physical phenomena like ionic motion, back diffusion of water and electrostatic force have been used to explain these models. This research demonstrates the use of feedback control to overcome the limitation of slow settling time. First, an empirical model of the ionic polymers developed by Kanno was modified by studying the step response of these actuators. The empirical model is used to design a feedback compensator by state space modeling techniques. Since the ionic polymer actuator has a 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 show settling times of 0.03 seconds for closed-loop feedback control are possible as compared to the open-loop settling time of 16-18 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 4-8 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 20\% 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.
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