Position control of a three-link shape memory alloy actuated robot

A simple and robust position controller is proposed for a small planar three-degree-of-freedom robot arm actuated by two shape memory alloy (SMA) actuators and a servomotor. A simple model of the robot is used for controller development. The model combines robot kinematics and dynamics with crude models of SMA wire heat convection, constitutive law, and phase transformation. It is then used to estimate 'optimal' parameters for the position controllers. The controllers are based on variable structure control approach and their development is of an evolutionary nature starting with a simple switching surface of position tracking errors and followed by the addition of velocity and integral tracking errors, respectively. The final controller is shown to be particularly fast since it heats the SMA wires with maximum available voltage, but avoids overshoot thereby avoiding the slow natural cooling process compared. Several experiments have been performed with a desktop prototype of the robot. The experimental results verify the effective and robust performance of the controllers despite significant modeling inaccuracies during the controller parameter development process. An additional advantage of the controller is that it can be implemented on a controller board with very limited computation capacity.