A compact and stiffer shape memory alloy actuator for surgical instruments

Advances in minimally invasive surgery enable the integration of new micro-systems with a micro actuator, as well as self-sensing ability, in surgical instruments. High energy density, self-sensing ability, and shape flexibility make shape memory alloy (SMA) actuators widely suited for volume-compact required applications. This paper presents a two-degrees-of-freedom instrument driven by SMA triple wires having 8 mm diameter and a rotation range near to ±60°. The actuator drive was constructed by antagonistic SMA triple wires and close-loop controlled by self-sensing. Experiments showed that the hysteresis gap between the phase transformation paths of the strain-resistance curve can be minimized under a fitted interstress. The curves of all the wires were then modeled by fitting polynomials, the collected resistance was converted to strain value and used to determine the control signal, and a feedforward compensator was built as the hysteresis compensation to resist overheating. The control accuracy was verified based on the multistep responding tests, and the results were shown in 3-D space. Under the self-sensing hybrid control scheme, the experimental results showed that the root-mean-square error of the rotation angle around the X and Y axes was about 4.2% and 3.5%, respectively.