Unified Contact Force Control Approach for Cable-driven Parallel Robots using an Impedance/Admittance Control Strategy

In this paper a unified contact force control approach for cable-driven parallel robots (CDPR) is proposed. The contact force controller is governed by two control loops: 1) An inner impedance control loop enforcing a dynamical relationship between the internal forces and the end-effector velocities to maintain a desired tension level in the cable system. 2) An outer admittance control loop considering the contact wrench by altering the desired endeffector trajectory. For the parametrization of the admittance controller the desired stiffness of the virtual manipulator system is defined by the mechanical stiffness of the end-effector. Further, the influences of changing the stiffness of the virtual system are discussed. To reconstruct the contact wrench a disturbance observer based on the generalized momentum approach is incorporated into both control loops. Experiments with a 6-DOF CDPR with industrial electric synchronous machines are presented validating the proposed contact force control approach.