We present a coordination method for multiple mobile manipulators to sort objects in clutter. We consider the object rearrangement problem in which the objects must be sorted into different groups in a particular order. In clutter, the order constraints could not be easily satisfied since some objects occlude other objects so the occluded ones are not directly accessible to the robots. Those objects occluding others need to be moved more than once to make the occluded objects accessible. Such rearrangement problems fall into the class of nonmonotone rearrangement problems which are computationally intractable. While the nonmonotone problems with order constraints are harder, involving with multiple robots requires another computation for task allocation. The proposed method first finds a sequence of objects to be sorted using a search such that the order constraint in each group is satisfied. The search can solve nonmonotone instances that require temporal relocation of some objects to access the next object to be sorted. Once a complete sorting sequence is found, the objects in the sequence are assigned to multiple mobile manipulators using a greedy allocation method. We develop four versions of the method with different search strategies. In the experiments, we show that our method can find a sorting sequence quickly (e.g., 4.6 sec with 20 objects sorted into five groups) even though the solved instances include hard nonmonotone ones. The extensive tests and the experiments in simulation show the ability of the method to solve the real-world sorting problem using multiple mobile manipulators.

翻译：我们提出了一种多移动机械臂协同分拣杂乱物体的方法。本文针对物体必须按特定顺序分配到不同组别的物体重排问题展开研究。在杂乱环境中，由于部分物体遮挡其他物体导致被遮挡物体无法被机器人直接抓取，顺序约束难以满足。遮挡物需要被多次移动才能释放被遮挡物体，此类重排问题属于计算复杂度极高的非单调重排问题。带有顺序约束的非单调问题更加棘手，而引入多机器人系统则需额外进行任务分配计算。所提方法首先通过搜索确定满足各组顺序约束的待分拣物体序列，该搜索能够处理需要临时移动某些物体以获取下一个待分拣物体的非单调实例。在获得完整分拣序列后，采用贪婪分配方法将序列中的物体分配给多台移动机械臂。我们基于不同搜索策略开发了四种方法变体。实验表明，即使面对高阶非单调实例，该方法仍能快速生成分拣序列（例如20个物体分五组仅需4.6秒）。大量仿真测试与实验验证了该方法利用多移动机械臂解决真实分拣问题的能力。