Design, modeling and control of a variable stiffness elastic actuator based on the trapezoidal screw pivot relocation mechanism

This paper introduces a Variable Stiffness Actuator based on a Trapezoidal Screw mechanism (VSATS) for enhanced flexibility and stiffness control in rehabilitation exoskeletons. The trapezoidal screw enables symmetrical elastic output, expands rotational range, and reduces frictional loss, while its design keeps spring deformation perpendicular to the force direction for improved efficiency. A small-lead double-tooth screw adjusts pivot positions, simplifying modeling and increasing control precision. A dynamic model is established to evaluate energy storage, stiffness, and torque characteristics. The actuator’s pivot relocation mechanism theoretically allows for continuous stiffness modulation from zero to infinity. In the current prototype, a wide and continuously adjustable stiffness range is achieved, with a practical maximum value reaching approximately 1800 N·m/rad. To address model uncertainties, a Radial Basis Function Neural Network Controller with a Disturbance Observer (RBFDO) is implemented. Experimental results confirm that the VSATS with RBFDO achieves high response speed, accurate position tracking, and effective disturbance rejection, demonstrating strong potential for rehabilitation applications.