Tracking control of oscillatory motion in IPMC actuators for underwater applications

Ionic polymer-metal composite (IPMC) actuators have many advantages; for instance, they (1) can be driven with low voltages (<;5 V); (2) are soft, flexible and easily shaped; and (3) can operate in an aqueous environment (such as water). Important applications for IPMCs include active catheter devices for minimally invasive surgery, artificial muscle, and sensors and actuators for biorobotics. For applications such as endoscopy and flapping-based propulsion systems in aquatic robots, the IPMC actuator is required to precisely track a periodic reference trajectory. However, due to dynamic effects, nonlinear behavior, and external disturbances, uncompensated open-loop control yields excessively-large tracking error. This paper focuses on precision tracking of oscillatory motion in IPMC actuators. A feedback controller based on the repetitive control concept is proposed to improve tracking performance from one operating period to the next. The stability of the controller is analyzed in the discrete-time domain, and design considerations are discussed. The method is applied to a newly fabricated Perfluorinated Ion Exchange Membrane based IPMC actuator with lithium as its counterion. The tip displacement of the IPMC actuator is measured by a strain gage sensor. This newly proposed sensing scheme is low cost, practical, effective, and importantly, compact. Experimental results show the combined control and sensing scheme can minimize the tracking error by approximately 50% compared to PID control for tracking of periodic signals including sinusoidal and triangular wave forms.