A Twisting Mechanism With Parallel Springs for Series Variable Stiffness Actuator

This article proposes a novel twisting mechanism with parallel springs (TPS) employing eight parallel springs in 3-D configuration and a ball screw mechanism to achieve space-efficient stiffness regulation. A compact variable stiffness actuator (VSA) prototype, named TPS-VSA, is developed based on the TPS. Two models are established to estimate the output torque of the actuator based on deflection angles and angular speed. One is a conventional model derived from the mechanical structure, and the other is a 6-degree polynomial model fitted with experimental data. Simulation and experiment studies are conducted to evaluate the torque and stiffness regulation property of TPS-VSA, and the performance of the torque estimation models. The experimental results show that the proposed mechanism is effective in varying the stiffness of VSAs. Both the polynomial and conventional models performed well for estimating the output torque of the TPS-VSA, but the polynomial model has an average error of less than 0.0737 N . m, which significantly outperforms the conventional model (which has an average error above 0.1167 N . m). The dynamic behavior and frequency responses obtained through free vibration test shows that the natural frequency of TPS-VSA can be effectively changed by the proposed mechanism. The result of stiffness regulation test demonstrates that TPS-VSA can achieve the whole range stiffness variation within 0.9 s. The result of trajectory tracking test indicates that TPS-VSA can accurately track different trajectories with a simple proportional-integral-derivative (PID) controller.