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

Abstract

Ionic polymer-metal composites (IPMCs) are innovative materials that offer combined sensing and actuating ability in lightweight and flexible package. IPMCs have been exploited in robotics and a wide variety of biomedical devices, for example, as sensors for teleoperation, as actuators for positioning in active endoscopy, as fins for propelling aquatic robots, and as an injector for drug delivery. In the actuation mode, one of the main challenges is precise position control. In particular, IPMC actuators exhibit relaxation behavior and nonlinearities; and at relatively high operating frequencies dynamic effects limit accuracy and positioning bandwidth. A frequency-weighted feedforward controller is designed to account for the IPMC's structural dynamics to enable fast positioning. The control method is applied to a custom-made Nafion-based IPMC actuator. The controller takes into account the magnitude of the control input to avoid generating excessively large voltages which can damage the IPMC actuator. To account for unmodeled effects not captured by the dynamics model, a feedback controller is integrated with the feedforward controller. Experimental results show a significant improvement in the tracking performance when feedforward control is used. For instance, the feedforward controller shows over 75% 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 than 10% for tracking an 18-Hz triangle-like trajectory. Some of the advantages of feedforward control as well as its limitations are also discussed.

Sectored Tube-Shaped Ionic Polymer-Metal Composite Actuator With Integrated Sensor

“…In this case, one end of a strip IPMC actuator is usually fixed and the actuator's bending motion is exploited, for example to create single-link [10] and multi-link [11] oscillatory propulsors. The fabrication of thick rectangular-shaped strip IPMC actuators using the simple hot-pressing method [12,13] and casting process [14] to enhance actuation force have been developed. A combination of the hot-pressing and micromachining allowed the fabrication of IPMC tweezers with more complex shape [2].…”

Introduction to the themed articles on ionic polymer–metal composites

“…Commercially available Nafion membrane for fabricating IPMCs such as N115, N117, and N1110 have nominal dry thicknesses of 127, 178, and 254 µm, respectively [24]. By exploiting solution casting [25] and more novel manufacturing techniques [26,27], thicker IPMC actuators have been produced that lead to improved force output [28]. In addition, by incorporating nanoparticulates into the polymer matrix to control the water uptake and loss, an increase in blocking force has been observed [13,29].…”

“…Control approaches for IPMCs include traditional feedback-based methods [52][53][54], robust and adaptive control [55,56], and feed-forward control [25,57], as well as neural networks [58].…”

Tracking control of oscillatory motion in IPMC actuators for underwater applications

“…Recent work on control approaches for IPMCs comprise of feedback-based approaches and open-loop feedforward methods to minimize or compensate for dynamic and nonlinear effects. Integrated feedback and feedforward control has also been investigated [15]. Proportional-integral-derivative (PID) feedback control has been applied to IPMC [3].…”