Mechanics and stiffness limitations of a variable stiffness actuator for use in prosthetic limbs

This paper examines an actuation system, intended for use in a prosthetic arm, that mimics the ability of antagonistic muscles in biological systems to modulate the stiffness and position of a joint. The system uses two physical nonlinear springs arranged; antagonistically about a joint to generate control of both stiffness and movement. To decouple the net joint stiffness from joint deflection, it is shown that the springs must generate a force that is a quadratic function of deflection. Large stiffness errors are shown to occur if the springs vary slightly from the quadratic form. A stiffness controlled region of the torque-deflection curves for the joint is found based on physical limitations of the spring deflections and the amount of space available for spring movement. Inside this region the stiffness is controllable, but beyond these limits the system acts as a rigid manipulator. Examination of the limits shows a relationship between the shape of the stiffness controlled region and the spring force functions. (C) 1998 Elsevier Science Ltd. All rights reserved.