More solutions means more problems: Resolving kinematic redundancy in robot locomotion on complex terrain

This paper addresses the open challenge of planning quasi-static walking motions for robots with kinematically redundant limbs. Focusing on RoboSimian, a quadrupedal robot developed by the Jet Propulsion Laboratory (JPL), we develop a practical method for generating statically stable walking motions by pre-computing a reduced dimensional inverse kinematics (IK) lookup table with certain uniqueness and smoothness properties. We then use that lookup table to generate IK solutions at the beginning and end of walking phases (e.g., swing, body shift, etc), and connect these waypoints using the Rapidly exploring Random Tree Connect (RRT-Connect) algorithm [1]. Thus, we avoid arbitrarily choosing an IK solution at the goal (that may turn out to be difficult to reach from the start) by setting this choice through design and use of a task-specific lookup table, which can be analyzed offline. Our approach also introduces a complementary formulation of the RRT-Connect configuration space that addresses contact and closure constraints by using the forward kinematics of one stance leg to determine the body pose while treating additional stance limbs as dependent on the body pose and solving their inverse kinematics with IK table lookup. We demonstrate an implementation of some of this framework on RoboSimian and discuss generalizations and extensions.