Impedance control with varying stiffness for parallel-link manipulators

This paper proposes a new impedance control algorithm based on a variable stiffness-matrix, for parallel-link manipulators without the measurement of the tip position. It assumes that only the link lengths and velocities as well as the contact force at the tip are measured, and that the motion range is small around an operating point so that the nonlinear dynamics can be linearized. One of the key ideas is to change the stiffness matrix of the impedance model in order to compensate dynamics model parameter errors. In simulations, it is shown that despite the simplicity of the algorithm, its performance is similar to that of the control law based on the nonlinear dynamics with the correct parameters estimates, and that it is robust to estimation errors of some parameters such as the platform mass and the leg masses.