Novel design method for nonlinear stiffness actuator with user-defined deflection-torque profiles

Abstract

In this paper, a novel design method for compact nonlinear stiffness actuator (NSA) is proposed. By combining cam with off-the-shelf torsion spring and designing the effective profile curve of cam, the proposed NSA can perform desired bidirectional symmetric deflection-torque profiles with monotonic increasing slopes that follow the “low load, low stiffness and high load, high stiffness” principle inspired by biomechanics. To adapt to external loads in different directions, the proposed NSA is designed to have bidirectional symmetric stiffness characteristics by using symmetrical cams and mutilated gear transmission. The derivation process of the effective cam profile curve is introduced in detail. And then, a prototype of the designed NSA is built for verification. Simulated and experimental stiffness tests demonstrate the effectiveness of the proposed design method in realizing desired nonlinear stiffness. Preliminary experiments of step response in controlling torque show good performance of the proposed NSA in terms of dynamic response.