Design of a parallel compliance device with variable stiffness

This paper presents a parallel compliance device with variable translational stiffness properties. The variation of endpoint stiffness depends on the change of the spring stiffness in each limb. A synthesis algorithm for realizing the desired force compliance performance is built. Based on the proposed algorithm, a group of optimal spring stiffness can be derived. For the implementation of this device, an electromagnetic linear spring with current-controlled stiffness is developed. After testing the mechanical characteristics of the electromagnetic spring, a prototype of the parallel compliance device is built. The endpoint stiffness under different combinations of spring stiffness values is exhibited in the form of stiffness ellipsoids. A case is studied and verifies the ability of the presented compliance device to realize the desired endpoint stiffness. As the stiffness adjustment range of electromagnetic spring is limited, the bound of physically realizable stiffness of the presented compliance device is also discussed.