Design and Motion Planning of a Cable Robot Utilizing Cable Slackness

Abstract

Extending the workspace of cable robots can enhance their motion ability and help us make fuller use of their potential. This work designs a novel cable robot and utilizes cable slackness to extend its workspace. First, we use driving cables actuated by main motors and constrained cables driven by auxiliary motors equipped with lockers to control and restrain the motion of the end-effector, respectively. Then we classify the constraints on the end-effector into different modes according to the slackness of different constrained cables whose lengths can be changed by auxiliary motors, and continuous tension distribution (realized by tensioning devices) is applied to switch among them. To expand the mechanism’s workspace, the properties of the constraint mode series are analyzed and the static reachable workspace associated with it is defined, which guarantees the existence of feasible point-to-point trajectories. In particular, tree-based basic and general constraint mode series connection algorithms are designed to connect two targets in the static reachable workspace efficiently considering the necessary condition for the existence of the connection. After that, dual workspaces and subspace-based motion planning algorithms are designed to connect multiple targets in the static reachable workspace of the mechanism, and the quality of the trajectories is improved by using different manipulations of the constraint mode series. Since only driving cables are actuated by real-time capable servo motors and passively constrained cables are adopted to compensate for the weight of the end-effector, the size and the number of actuators and the energy consumption of the robot are reduced. Finally, the performance of the mechanism designed and the motion planning methods proposed are verified numerically under practical modeling errors regarding the cable elongation phenomenon.