  Compliant motions allow alignment of workpieces using naturally occurring
interaction forces. However, free-floating objects do not have a fixed base to
absorb the reaction forces caused by the interactions. Consequently, if the
interaction forces are too high, objects can gain momentum and move away after
contact. This paper proposes an approach based on direct force control for
compliant manipulation of free-floating objects. The objective of the
controller is to minimize the interaction forces while maintaining the contact.
The proposed approach achieves this by maintaining small constant force along
the motion direction and an apparent reduction of manipulator inertia along
remaining Degrees of Freedom (DOF). Simulation results emphasize the importance
of relative inertia of the robotic manipulator with respect to the
free-floating object. The experiments were performed with KUKA LWR4+
manipulator arm and a two-dimensional micro-gravity emulator (object floating
on an air bed), which was developed in-house. It was verified that the proposed
control law is capable of controlling the interaction forces and aligning the
tools without pushing the object away. We conclude that direct force control
works better with a free-floating object than implicit force control
algorithms, such as impedance control.

Robotics