Frequency-weighted feedforward control for dynamic compensation in ionic polymer–metalcomposite actuators

Abstract

Ionic polymer–metal composites (IPMCs) are innovative materials that offer combinedsensing and actuating ability in lightweight and flexible package. IPMCs have beenexploited in robotics and a wide variety of biomedical devices, for example, as sensorsfor teleoperation, as actuators for positioning in active endoscopy, as fins forpropelling aquatic robots, and as an injector for drug delivery. In the actuationmode, one of the main challenges is precise position control. In particular, IPMCactuators exhibit relaxation behavior and nonlinearities; and at relatively highoperating frequencies dynamic effects limit accuracy and positioning bandwidth. Afrequency-weighted feedforward controller is designed to account for the IPMC’sstructural dynamics to enable fast positioning. The control method is applied to acustom-made Nafion-based IPMC actuator. The controller takes into account themagnitude of the control input to avoid generating excessively large voltages which candamage the IPMC actuator. To account for unmodeled effects not captured by thedynamics model, a feedback controller is integrated with the feedforward controller.Experimental results show a significant improvement in the tracking performance whenfeedforward control is used. For instance, the feedforward controller shows over75% reduction in the tracking error compared to the case without feedforward compensation. Finally,the integrated feedforward and feedback control system reduces the tracking error to less than10% for tracking an 18-Hz triangle-like trajectory. Some of the advantages of feedforward controlas well as its limitations are also discussed.