Design for solving negative stiffness problem of redundant planar rotational parallel mechanisms

The biological actuator consisting of flexible muscles and rigid bones is similar to a redundant planar rotational parallel mechanism (RPRPM), and its stiffness increases with the increase in stretching forces of the muscles. In this paper, the characteristics of the stiffness variations with the internal stretching forces of RPRPM is analysed and the mechanism of the negative stiffness is obtained. To avoid the decrease in the stiffness of RPRPM with the increase in the internal stretching force and being a negative stiffness, the method designing geometrical parameters and the method applying pneumatic artificial muscles are presented to solve the negative stiffness problem of RPRPM. Then, the stiffness of RPRPM is positive and rises with the increase in the internal stretching forces.