Modeling and characterization of shape memory alloy springs with water cooling strategy in a neurosurgical robot

Abstract

Since shape memory alloy has a high power density and is magnetic resonance imaging compatible, it has been chosen as the actuator for the meso-scale minimally invasive neurosurgical intracranial robot (MINIR-II) that is envisioned to be operated under continuous magnetic resonance imaging guidance. We have devised a water cooling strategy to improve its actuation frequency by threading a silicone tube through the spring coils to form a compact cooling module-integrated actuator. To create active bi-directional motion in each robot joint, we configured the shape memory alloy springs in an antagonistic way. We modeled the antagonistic shape memory alloy spring behavior and provided the detailed steps to simulate its motion for a complete cycle. We investigated the heat transfer during the resistive heating and water cooling processes. Characterization experiments were performed to determine the parameters used in both models, which were then verified by comparing the experimental and simulated data. The actuation frequency of the antagonistic shape memory alloys was evaluated for several motion amplitudes and we could achieve a maximum actuation frequency of 0.143 Hz for a sinusoidal trajectory with 2 mm amplitude. Lastly, we developed a robotic system to implement the actuators on the MINIR-II to move its end segment back and forth for approximately ±25°.