Hierarchical design of whole-body motion controller for mobile manipulator composed of differential-drive wheels

This paper proposes a controller for mobile manipulators with differential-drive wheeled bases to locomote stably while maintaining responsiveness. It determines referential movements of the mobile base and the center of mass of the upper-body in the first stage by the model predictive control based on a reduced-order dynamics, and resolves them into motor commands together with that to the end-effector in the second stage. The hierarchical design of the controller with a technique to linearize the zero-moment point (ZMP) of the reduced-order dynamics model substantially lowers the computation cost. It is also shown that a range constraint on the ZMP rather than a point constraint allows quick transitional responses of the robot to tracking errors and disturbances. The efficacy of the proposed controller was shown through both computer simulations and experiments with a real robot.