Modeling and Inverse Compensation of Hysteresis in Supercoiled Polymer Artificial Muscles

The supercoiled polymer (SCP) actuator is a recently discovered artificial muscle that demonstrates significant mechanical power, large contraction, and good dynamic range in a muscle-like form factor. There has been a rapid increase of research efforts devoted to the study of SCP actuators. For robotics, SCP actuators overcome specific challenges of artificial muscles such as shape memory alloy wires, where limited strain and slow dynamics, and power consumption had limited their use. It is known that hysteresis nonlinearity results from coiling the threads, and can cause up to 30% strain difference under the same voltage; however, no work has been reported to characterize the hysteresis in SCP actuators. In this paper, three new models are formulated to characterize the hysteretic relationship between three coupled variables (voltage input, strain, and load) of an SCP actuator, namely, the augmented generalized Prandtl–Ishlinskii model, the augmented Preisach model, and the augmented linear model. By incorporating the relationship between hysteresis curves and loading forces, the proposed models can efficiently characterize the hysteresis. Open-loop position control is further realized through inverse compensation. Experimental results show that the proposed schemes can effectively estimate and compensate the hysteresis. For the first time, the hysteresis characterization and compensation of SCP actuators are successfully demonstrated, such that accurate robot control can be realized.