Abstract-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 muscles, and sensors and actuators for biorobotics. Due to inherent nonlinear behavior, dynamic effects, and external disturbances, sensing and feedback control are required for precision operation. A new method to sense the displacement of an IPMC actuator using resistive strain gages is proposed. The sensing scheme is low cost, practical, effective, and importantly, compact compared to existing methods such as lasers and charge-coupled device (CCD) cameras. The strain-to-displacement relationship is developed and experimental results are presented to demonstrate the effectiveness of the sensing scheme. Furthermore, the sensor signal is used as feedback information in a repetitive controller to improve the tracking of periodic motion. The stability condition for the controller is presented, and the sensing scheme and feedback control approach are applied to a fabricated perfluorinated ion-exchange-membrane-based IPMC actuator with lithium as its counterion. Experimental results show that the tracking error can be reduced by approximately 50% compared to PID control for tracking of periodic signals, including sinusoidal and triangular wave forms.Index Terms-Ionic polymer-metal composite (IPMC) actuators, repetitive control, strain gage sensors.
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 counter-ion. 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.
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