In this paper, a collision avoidance method for robot manipulators is proposed. First the potential function for avoiding obstacles is defined in such a way that its value is always zero on the middle planes between obstacles. Then the graph for the desired trajectory of the end-effector is plotted on such planes. We call it the 'safety first graph'. The path on this graph is determined by the A* algorithm. By the proposed method, the interference between the potential field for avoiding obstacles and that for moving the end-effector to the destination position becomes very small. Therefore, the failure of the search for a solution, owing to deadlock or collision with obstacles, could be reduced in most cases, and the global desired trajectory enables us to look again for a solution in the case of failure. The method can be applied to general manipulators without paying attention to their kinematic features, and imposes no restriction on the motion to be planned. The effectiveness of the method has been verified by simulation studies.
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