Optimal robot positioning using task-dependent and direction-selective performance indexes: General definitions and application to a parallel robot

Performance evaluation is an important issue for optimal robot positioning within workcells. Performance indexes can provide essential contributions to such an evaluation, in particular if they can account for the specific task with respect to which the optimization is carried out. This paper introduces a performance index for parallel manipulators called task-dependent performance index (TPI), which explicitly accounts for both robot kinematics and task geometrical features. It is proved that TPI can provide accurate evaluation of robot performances in executing specific tasks. Hence, optimal robot/task relative positioning can be straightforwardly achieved by maximizing the proposed TPI. The TPI formulation is based on the one of direction-selective performance index (DSI) that is here extended to evaluate parallel manipulator translational capabilities along generic directions. In particular, the TPI definition explicitly accounts for the length and direction of the sequence of translations to be accomplished by a robot to carry out a task. As a proof of concept, the TPI formulation has been here employed in a maximization algorithm in order to optimize the location of some pick-and-place tasks within the workspace of an industrial 4-RUU parallel robot. The experimental results collected provide adequate evidence of the effectiveness of the proposed index and of its usefulness in optimal robot positioning.