Practical model-free robust estimation and control design for an underwater soft IPMC actuator

In the trend of the recent development of soft actuators, ionic polymer metal composite (IPMC) is considered as one of the new and innovative soft materials. The IPMC is suited to be utilised in medical micro robots and bio-inspired aquatic robotics. It is distinguished by nimble, soft, silent, flexible and lightweight properties. In fact, for the IPMC actuator, hysteresis and creep non-linearities are inevitable; and it is a great challenge to handle them and to achieve high-precision tracking in the control design, especially when the internal system morphology is complex and not fully understood. This study proposes a new model-free control approach for an underwater IPMC actuator to overcome the lack of its exact model and to achieve accurate trajectory tracking. This approach is synthesised based on a non-linear extended state observer technique to estimate lumped uncertainties and disturbances. Furthermore, a sliding mode controller is added as an extra input to deal with the estimation error and to assure the tracking robustness. Finally, the proposed control is experimentally verified to show its effectiveness in comparison with a non-singular terminal sliding mode controller. The experimental results indicate that the proposed controller is capable of delivering good tracking accuracy with strong robustness.