A Novel Rotational Actuator With Variable Stiffness Using S-Shaped Springs

In this article, we propose a novel rotational variable stiffness actuator that uses S-shaped springs as elastic elements. The S-shaped springs are designed based on the principle of variable beam length, which allows for an easily customizable and wide-ranging stiffness regulation from a minimum value to near infinite. The stiffness can be smoothly regulated online by rotating the S-shaped springs, resulting in a compact stiffness regulation mechanism. Moreover, a planetary gear differential is used to drive the stiffness regulation mechanism such that both the stiffness control motor and the main drive motor can be placed at the proximal part. This configuration effectively reduces the reflected inertia of the moving parts, which helps to achieve a compact design and improve the control performance. The proposed actuator ranks high among designs of the similar output power in terms of the power-to-weight ratio and the power-to-volume ratio; therefore, the compactness of the proposed design is verified. Besides, the stiffness regulation speed of the proposed design also ranks high among the designs. Consequently, the actuator can be used for fast stiffness regulation. For robust position tracking under different operating conditions, a disturbance observer is used to estimate the mismatched lumped disturbance caused by the load torque and model uncertainties. To further improve the control performance, a sliding mode control term is introduced to compensate for the disturbance estimation error. The stability of the closed-loop system is proved using the Lyapunov method. Experimental results under different operating conditions have verified the validity of the novel actuator and the proposed controller.