PERFORMANCE EVALUATION AND TRAJECTORY PLANNING FOR PLANAR CABLE ROBOTS

Cable robots (also called “cable direct driven robots” or “cable driven parallel robots”) are amongst the most promising robotic devices in the industrial and service field. As a consequence of their peculiar and desirable advantages over conventional robots, they have attracted the growing interest of the scientific and industrial community since the early nineties. In particular, depending on the application, they can be designed to have a very large workspace and a very high load capacity, or to generate very high speed motions. Additionally, their simple design makes them inexpensive, while their minimal moving mass usually makes them energy efficient. All these advantages are being promoting the deployment of cable-robots in several real-world applications. Relevant and challenging research issues are still open and have to be tackled when designing and operating a cable robot: indeed, such robots must always meet the stringent requirement that all cables are under tension during operation. Additionally, cable forces must be kept below some maximum permissible values related to the torque limits of the actuators or to the tensile force limits of the cables. Under such constraints, not only is motion planning and control for cable robots demanding, but also predicting cable robot performances within the workspace is not trivial, and cannot be done by just applying the performance indexes conceived for rigid-link parallel robots. The definition of workspace itself becomes much more elaborate. The speech addresses both performance evaluation and trajectory planning for cable robots, and focuses explicitly on planar cable robots. An example of underconstrained hybrid translational cable robot is introduced to show that a successful approach to prevent cable slackness and excessive tensions may consist in planning dynamically feasible trajectories by making use of a dynamic model of the cable-suspended robot to translate the cable tension bilateral bounds (i.e. positive and bounded tensile cable forces) into limits on the velocity and acceleration of the robot end-effector along the assigned path. The fact that cables behave as unilateral actuators has a dramatic impact on performance evaluation too: suitable approaches to performance evaluation are discussed and appropriate performance indexes are presented.