Design and Modelling of a Modular Variable Stiffness Actuator

This paper presents the design and modeling of a modular variable stiffness joint intended for human-robot interaction. The functional concept is based on bidirectional antagonistic variable stiffness (BAVS) principle, in which elastic actuators are connected in parallel to output link, and an internal torque is generated by the antagonistic motion of two motors to adjust the stiffness. Different from conventional variable stiffness actuators (VSAs) that introduce extra motor to adjust stiffness, BAVS concept treats both motors as main actuators, thereby improving the joint's torque density. The mechanical realization of BAVS principle employs a differential mechanism and makes use of two harmonic reducers in a differential manner. The flexibility of the joint is provided by a non-linear elastic element consisting of linear spring and cam roller mechanism. Low torque density and non-modular design are the main reasons limiting VSAs' application in multi-degree-of-freedom robotic systems. In response to this, the proposed joint is designed to be compact and modular. This article describes the basic principles, the mechanical design and the dynamic modeling of the proposed joint. The preliminary simulation results are presented to illustrate the high torque density and wide stiffness range of the joint.