  This paper considers the problem of rearrangement planning, i.e finding a
sequence of manipulation actions that displace multiple objects from an initial
configuration to a given goal configuration. Rearrangement is a critical skill
for robots so that they can effectively operate in confined spaces that contain
clutter. Examples of tasks that require rearrangement include packing objects
inside a bin, wherein objects need to lay according to a predefined pattern. In
tight bins, collision-free grasps are often unavailable. Nonprehensile actions,
such as pushing and sliding, are preferred because they can be performed using
minimalistic end-effectors that can easily be inserted in the bin.
Rearrangement with nonprehensile actions is a challenging problem as it
requires reasoning about object interactions in a combinatorially large
configuration space of multiple objects. This work revisits several existing
rearrangement planning techniques and introduces a new one that exploits nested
nonprehensile actions by pushing several similar objects simultaneously along
the same path, which removes the need to rearrange each object individually.
Experiments in simulation and using a real Kuka robotic arm show the ability of
the proposed approach to solve difficult rearrangement tasks while reducing the
length of the end-effector's trajectories.
