A Novel Design of Compact Serial Variable Stiffness Actuator (SVSA-III) With Predefined Stiffness Profiles

The variable stiffness actuator is capable of adjusting system stiffness while maintaining output force or motion, allowing it to adapt to a wide range of task requirements. This article introduces a novel, compact serial variable stiffness actuator (SVSA-III) with a consistent and wide-range stiffness profile, achieved through a double-layer slide mechanism based on the variable force point principle. By modifying the position of the force point, the actuator’s stiffness can be regulated from a minimum value to theoretically infinite (which is limited by motor torque, lever ratio resolution, and material strength). Unlike traditional SVSAs, where the stiffness motor rotates alongside the output unit, the SVSA-III’s stiffness motor is fixed to the base, resulting in a more compact structure and improved power density. Additionally, the stiffness profile and range of the SVSA-III can be customized by modifying the Archimedean Spiral and solving the relationship between actuator stiffness and spiral angle. A stiffness model for the actuator is developed to analyze its performance. The output torques of both the position motor and stiffness motor are also studied. Finally, control experiments are conducted to quantitatively evaluate the SVSA-III’s physical performance, confirming its stiffness regulation capability and control efficiency.