Authors: Jun Zhang, Michael Yip
Being inherently compliant, the robotic artificial muscles are increasingly popular in applications such as safe human-robot interaction, legged robotics, and prostheses and orthoses. Their full utilization is often challenged by their coupled hysteresis between strain, force, and input (voltage, temperature, or pressure). Although conventional two-dimensional hysteresis models are available, no prior studies on three-dimensional hysteresis models with coupled inputs have been reported for artificial muscles. This paper presents a new approach to capturing the coupled hysteresis of artificial muscles by embedding a two-stage Preisach model. The proposed method is applied to shape memory alloy (SMA) actuators. Since direct temperature measurement of the SMA actuator is not available, the concept of temperature surrogate, representing the constant voltage value in Joule heating that would result in a given temperature at the steady-state, is explored in the modeling. The proposed approach is adopted to capture the coupled three-dimensional hysteresis between contraction length, force and temperature surrogate of an SMA actuator. Model verification is further conducted. For comparison purposes, two modeling alternatives, namely, the Summed Preisach and the Linear Preisach, are also realized. Experimental results demonstrate that the proposed scheme can effectively characterize and estimate the three-dimensional hysteresis in SMA actuators. This study can be applied towards other artificial muscle technologies such as McKibben actuators and Super-coiled Polymer actuators.