Design of a Variable Stiffness Joint Actuator for Lower-Limb Exoskeleton Based on Shape Memory Alloys

Current lower-limb exoskeleton joint actuators lack flexibility and cannot adapt to the variable stiffness characteristics of human joints. They also suffer from large mass and low assistive efficiency. This letter designs a variable stiffness joint actuator for lower-limb exoskeletons based on shape memory alloys (SMA). The memory effect of SMA is utilized to adjust the preload force of springs, realizing changes in equivalent stiffness. Based on this, an active stiffness adjustment unit is constructed using SMA, and the units are distributed circumferentially to enhance the range of stiffness and torque output. A cooling system is incorporated to accelerate stiffness switching. Experimental results show that the designed actuator offers a broad stiffness adjustment range. Under experimental conditions, the maximum output torque can reach 50.8098 N·m, and the switching frequency between stiffnesses can reach 1.85Hz. The actuator weighs only 293.6g, achieving the design of actuator for lower-limb exoskeletons with lightweight, highly maneuverable and a wide stiffness range.