Addition of passive-carriage for increasing workspace of cable robots: automated inspection of surfaces of civil infrastructures

Cable-driven robots are parallel manipulators in which rigid links are replaced by actuated cables. The end-effector is then supported by a set of cables commanded by motors that are usually placed in a fixed frame. By varying the cables length, it is possible to change the end-effector position and/or orientation. Among the advantages presented by cable robots are they light-weight structure, high energy efficiency and their ability to cover large workspaces since cables are easy to wind. When high-speed operation is not required, a safer solution is to design cable-driven suspended robots, where all vertical components of cables tension are against gravity direction. Cable-driven suspended robots present limited workspace due to the elevated torque requirements for the higher part of the workspace. In this paper, the addition of a passive carriage in the top of the frame is proposed, allowing to achieve a much greater feasible workspace than the conventional one, i.e., with the same size as the desired inspection area while maintaining the same motor requirements. In the opposite, this new scheme presents non-desired vibration during the end-effector maneuvers. These vibrations can be removed by means of a more complex control strategy. Kinematics and dynamics models are developed in this paper. An analysis of sensor system is carried out and a control scheme is proposed for controlling the end-effector pose. Simulation and experimental results show that the feasible workspace can be notoriously increased while end-effector pose is controlled. This new architecture of cable-driven robot can be easily applied for automated inspection and monitoring of very large vertical surfaces of civil infrastructures, such as facades or dams.