Towards a MRI-compatible meso-scale SMA-actuated robot using PWM control

In this paper, we present our work towards the development of a SMA-actuated meso-scale robot for neurosurgery, which can be operated under MRI guidance. In this robot, we use two antagonistic SMA wires as actuators for each joint, so that each joint can be actuated independently. Due to the size scale of the robot, it is impossible to have individual position sensors at each joint and hence we rely primarily on temperature feedback to control the robot. We thus designed and developed an experimental setup to characterize the SMA wires. The goal of SMA characterization was to develop systematic experiments whereby the dependence of the joint motion on the temperature (and hence the SMA phase transition) can be experimentally determined. We also developed a theoretical model based on Tanaka's model and the geometry of the robot to characterize the joint motion with the change in SMA wire temperature. The results demonstrated that the SMA wire temperature can be used reliably to predict the joint motion of the robot. We then developed a Pulse Width Modulation (PWM) scheme to control the temperature of SMA wires (and hence the joint motion). By using PWM control and switching circuits, we can actuate multiple SMA wires simultaneously and independently using only one power supply. Experimental results from our current prototype of a 2-DOF robot show that we can actuate the SMA wires reliably and hence observe joint motion in a gelatin medium as well as in MRI.