Cable Driven Devices for Telemanipulation

Abstract

Cable driven robotic devices present peculiar aspects, requiring the solution of very specific problems. As regards the determination of the main operational characteristics, like the workspace, it is necessary to consider that such devices are often parallel and redundant structures. Moreover, cables can only exert traction forces. As a consequence, the development of such structures requires coping with two main aspects: (a) the solution of forward kinematics and inverse statics, necessary for the remote control of the slave device and the force reflection on the operator, are usually very complicated and often a closed form does not exist; (b) it is useful to find a convenient layout of the attach points on the mobile platform, in order to obtain closed-form solutions or optimal numerical solutions for the kinematical and static equations. The workspace characteristics also depend on the number of cables and the layout of their passing points on the fixed structure, but it is difficult to foresee those performances in the design phase and to evaluate them in an objective way. Therefore it is necessary to develop methods aimed at finding out the shape of the six-dimensional workspace, expressing it in a graphical form and evaluating its characteristics by means of objective numerical parameters. The actuation by cables needs particular attention as regards both the generation and the transmission of cable tensions. This has required a wide research activity concerning the kind of actuators to be used and the mechanical aspects of the transmission lines. As regards the actuators, it must be noticed that their action must be generated at very low velocities, often near the stall condition. Therefore, particular care must be devoted to the choice of technology (electrical or pneumatic), to the kind of actuator and to the way of controlling the force or torque. The physical characteristics of the cable and its path have direct influence on the control accuracy. The cables connect the operator with the motors and with the device sensors; the accuracy of the device is affected by the friction acting on the cable and by characteristics like longitudinal compliance and flexural stiffness; those phenomena may only be partially compensated by the control system, therefore the choice of the cable is a very delicate point. The research described here has faced all the aspects above mentioned. Several original solutions reported have been described in detail in previous works. Future work will be devoted to improve the way of controlling the reflection forces, which has been spotted as the most delicate aspect for the accuracy of this kind of devices.