Development of a variable stiffness joint drive module and experimental results of joint angle control

Two prototypes of variable stiffness joint drive modules imitating a human joint structure are presented. A human joint is driven by a pair of flexor and extensor muscles that work antagonistically. The stiffness of the joint is adjusted by their co-contraction. Such a structure was given to the joint drive module so that it could achieve a variable stiffness property. The joint is driven by two wires with nonlinear springs. Thanks to the nonlinearity of the springs, the stiffness of the joint can be adjusted by quasi-co-contraction of the wires. With the first prototype, the stiffness adjustability of the joint was empirically confirmed. Regarding joint angle control, a three-layered proportional integral derivative (PID) control algorithm was implemented in the second prototype, and it was verified that the control algorithm worked properly.