Robot cooperation: solving the motion coordination problem for two scara manipulators
M. A. Ward · 1989
A solution to the motion coordination problem is offered for two SCARA (Selectively Compliant Assembly Robotic Arm) robots that cooperate. Whereas two classes of cooperation, discrete and continuous, are shown to exist the robots' behavior belongs to the discrete class. Such a problem is described by a textual specification language and is then translated into a Petri net model. The net's reachability graph provides the foundation for a geometrical analysis of the implied robot motion. Simplifying assumptions follow that allow the manipulators to be modeled by polygons and permit stationary objects in the workcell to be ignored. Only geometrical conflict between the manipulators is considered in the solution, as a consequence. Edges in the reachability graph represent either the beginning or the end of robot motions. The graph is searched for a connected component that both satisfies the specification and is collision-free. During the search, at each edge that implies a motion, a trajectory is computed by Schwartz and Sharir's solution to the piano movers' thinspace problem. Thus, the motion coordination problem is reduced to multiple instances of path planning. Concurrent robot motion is not sacrificed, however. By representing moving robots by their swept area, path planning techniques, which move polygonal objects amidst stationary polygonal obstacles, can be used to move one robot in the vicinity of another, perhaps moving, manipulator. The sub-graph that emerges from the search is submitted to a run-time module that enforces the corresponding deterministic execution sequence on the otherwise nondeterministic specification. The result is a multi-robot control program that is guaranteed to execute without collision.