  This paper proposes a method to learn from human demonstration compliant
contact motions, which take advantage of interaction forces between workpieces
to align them, even when contact force may occur from different directions on
different instances of reproduction. To manage the uncertainty in unstructured
conditions, the motions learned with our method can be reproduced with an
impedance controller. Learning from Demonstration is used because the planning
of compliant motions in 3-D is computationally intractable. The proposed method
will learn an individual compliant motion, many of which can be combined to
solve more complex tasks. The method is based on measuring simultaneously the
direction of motion and the forces acting on the end-effector. From these
measurements we construct a set of constraints for motion directions which,
with correct compliance, result in the observed motion. Constraints from
multiple demonstrations are projected into a 2-D angular coordinate system
where their intersection is determined to find a set of feasible desired
directions, of which a single motion direction is chosen. The work is based on
the assumption that movement in directions other than the desired direction is
caused by interaction forces. Using this assumption, we infer the number of
compliant axes and, if required, their directions. Experiments with a KUKA
LWR4+ show that our method can successfully reproduce motions which require
taking advantage of the environment.
