  Planning dual-arm assembly of more than three objects is a challenging Task
and Motion Planning (TAMP) problem. The assembly planner shall consider not
only the pose constraints of objects and robots, but also the gravitational
constraints that may break the finished part. This paper proposes a planner to
plan the dual-arm assembly of more than three objects. It automatically
generates the grasp configurations and assembly poses, and simultaneously
searches and backtracks the grasp space and assembly space to accelerate the
motion planning of robot arms. Meanwhile, the proposed method considers
gravitational constraints during robot motion planning to avoid breaking the
finished part. In the experiments and analysis section, the time cost of each
process and the influence of different parameters used in the proposed planner
are compared and analyzed. The optimal values are used to perform real-world
executions of various robotic assembly tasks. The planner is proved to be
robust and efficient through the experiments.
