Learning to Search in Task and Motion Planning with Streams

Task and motion planning problems in robotics typically combine symbolic planning over discrete task variables with motion optimization over continuous state and action variables, resulting in trajectories that satisfy the logical constraints imposed on the task variables. Symbolic planning can scale exponentially with the number of task variables, so recent works such as PDDLStream have focused on optimistic planning with an incrementally growing set of objects and facts until a feasible trajectory is found. However, this set is exhaustively and uniformly expanded in a breadth-first manner, regardless of the geometric structure of the problem at hand, which makes long-horizon reasoning with large numbers of objects prohibitively time-consuming. To address this issue, we propose a geometrically informed symbolic planner that expands the set of objects and facts in a best-first manner, prioritized by a Graph Neural Network based score that is learned from prior search computations. We evaluate our approach on a diverse set of problems and demonstrate an improved ability to plan in large or difficult scenarios. We also apply our algorithm on a 7DOF robotic arm in several block-stacking manipulation tasks.