Position/Force Hybrid Control of Redundantly Actuated Cable-Driven Parallel Robot Based on Modal System Design

Cable-driven parallel robots (CDPRs) have been paid attention to for a long because of their lightness and flexibility. Generally, they have redundant cables to control the end-effector, which leads to the presence of actuation redundancy. Actuation redundancy should be considered for the motion control of CDPRs since it causes internal force in a mechanism. The reason why the internal force should be regulated is that the internal force burdens the parts of the mechanism and does not work for the external environment. Popular methods to control CDPRs regulate control input by adjusting the vector belonging to the null space of the force transmission matrix. However, such methods do not directly control the internal force of the mechanism. This article proposes a motion control method of a redundantly actuated CDPR based on modal system design. The modal system design sets a virtual space composed of position/force axes, and the controllers are designed in the virtual space, which is called modal space. The proposed method enables the direct control of the internal force while the trajectory is tracked precisely. As a result, the proposed method realizes the position/force hybrid control of the redundantly actuated CDPRs considering the internal force. A 3-DOF CDPR is used for the experimental validation. The performance of the proposed method is compared to the sliding mode control. The results show that the proposed method performs better in suppressing disturbances that affect trajectory tracking, and the internal force is regulated in a certain range.