Design and control of a novel rotary shape memory alloy actuator for two degree of freedom underwater vectored thruster

Underwater vectored propulsion thruster is normally actuated by a step motor or a hydraulic system. The traditional actuators show an inherent shortcoming in energy consumption per volume, low space utilization and structure complexity. Shape memory alloy (SMA) based actuator is considered as a strong competitor to the traditional one, however high torque output and long movement distance to acquire high control accuracy and wide operating range still remain a big challenge. In this work, we developed an SMA-based underwater vectored thruster consisting of two motion rings and their rotary SMA actuators. Two SMA wires combined with supplementary rotation parts are utilized to drive the rotation of each motion ring. The rotary SMA actuator outputs torque that can change the angular position of underwater thruster, and the maximum angle and the response speed can be reached 38 degrees and 1.6 degrees s(-1). The experimental results verify the feasibility of applying the rotary actuator to underwater device. Modeling the SMA rotary actuator system helps understanding the rotary process of underwater vectored thruster. Based on the system model, a control scheme integrated with an anti-windup Proportion Integral Differential (PID) controller is proposed. Its performance is compared with that of the integrating angle feedback without an anti-windup block through the simulation method. With the proposed anti-windup PID controller, the results show that the overshoot can be eliminated and the steady error is decreased to 0.01% and 0.9% at 10 degrees and 25 degrees respectively, reflecting that the anti-windup PID controller significantly improves the system controlling performance.

Automated summary

A shape memory alloy (SMA)-based underwater vectored thruster was developed in this study, utilizing SMA wires to drive the rotation of motion rings and achieve torque output, angular position changes, with maximum angle and response speed reaching 38 degrees and 1.6 degrees/s. The proposed anti-windup Proportion Integral Differential (PID) controller significantly improves the system controlling performance by eliminating overshoot and decreasing steady error at different angles.