Motion Transition Under Urgent Change of Target Step-stone During Three-Dimensional Biped Walking

Abstract

Controlling a biped robot to walk through rough terrains is crucial to the robot’s field application. For a human in the workplace, the ability to flexibly transfer motion while walking in some urgent circumstances is necessary. Explicitly, the according scenario can be dodging an approaching object or instantly modifying the target place to step on. The function is also important for humanoid robot workers. Therefore, we proposed a walking control framework that achieves three-dimensional (3-D) walking and transfers the whole body motion when the target stepping location is urgently changed. The proposed framework contains a motion planner which outputs the desired center of mass (CoM) and center of pressure (CoP) trajectories in 3-D space and a hierarchical whole body controller (WBC) that outputs corresponding whole body joints’ trajectories. In the motion planner, the CoM jerk for each loop is calculated by the Linear-Quadratic-Tracker (LQT), a variation of the Linear-Quadratic-Regulator (LQR). The LQT coefficients adapt to the adjusted step length, making the desired CoM and CoP trajectories respond flexibly to the change of target step-stone. In WBC, three levels of tasks are defined, which meet dynamic, kinematic, and viable contact constraints, respectively. The optimal joints’ angular accelerations are obtained by exploiting the nullspace of the first two levels tasks and by quadratic programming (QP) for the third-level task. In the simulations, our method is demonstrated to be effective for the robot to transfer the motion under urgent change of the target step-stone.