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Deadlock-Free and Collision-Free Coordination for Two Robot Manipulators

Deadlock-Free and Collision-Free Coordination for Two Robot Manipulators. Patrick A. O’Donnell and Tomas Lozano-Perez MIT Artificial Intelligence Lab (1989) Presented by: Robbie Paolini. Coordinating Manipulators. Planning for a robotic manipulator How about 2 robotic manipulators?

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Deadlock-Free and Collision-Free Coordination for Two Robot Manipulators

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  1. Deadlock-Free and Collision-Free Coordination for Two Robot Manipulators Patrick A. O’Donnell and Tomas Lozano-Perez MIT Artificial Intelligence Lab (1989) Presented by: Robbie Paolini

  2. Coordinating Manipulators • Planning for a robotic manipulator • How about 2 robotic manipulators? • Collision • Deadlock

  3. Previous Approaches • Global: Construct complete trajectories for all robots, with swept volumes in space-time • Depend on carefully controlled trajectories • Computationally intense • Local: Make decisions at each time step • May reach deadlock • Issues when paths are tightly constrained

  4. Assumptions and Approach • Known environment • Robot’s paths can be planned in advance • Trajectories are less predictable • Generate a plan for each robot • Path segments within a box in joint space • Rough execution time estimate Trajectory Coordination => Scheduling Problem

  5. Task-Completion Diagram Goal Second Robot’s Steps Can solve this with a local greedy approach Start First Robot’s Steps

  6. Dealing with Deadlock SW Closure

  7. Constructing a Schedule • “Local” Greedy Scheduler • Decentralized version • Rows or columns of SW-closure regions become “locks” • Global Scheduler • Optimize a cost • Execution time

  8. Reducing Execution Time • We ignored time for each segment • Want to increase Parallelism • Mostly diagonal paths • Modify some segments of the path if: • Region is shaded because of collision • Initial and final positions are collision free • Region causes significant increase in total time

  9. Increasing Parallelism

  10. Variable Segment Times • What happens if we encounter a significant delay? • Replan the rest of the path • Precompute a decision tree?

  11. Collision Checking • Compute conservative swept volume • Check collision of bounding box approximations • Reduce planning time

  12. Summary • Create TC-diagram • Trajectory planning -> Scheduling problem • Greedy and global approaches to planning • Increase parallelism by modifying troublesome segments • Fast collision checking via approximations

  13. Limitations and Future Work • Computing entire execution paths of both arms may be unnecessary • Modifying paths may still create suboptimal plans • Not real time • If delays occur, may be suboptimal • Uncertainty in paths? • WAM Arm

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