Straight leg walking strategy for torque-controlled humanoid robots

Most humanoid robots walk in an unhuman-like way with bent knees due to the use of the simplified Linear Inverted Pendulum Model (LIPM) which constrains the Center of Mass (CoM) in a horizontal plane. Therefore it results in high knee joint torque and extra energy consumption. To address this issue, we propose a simple yet efficient control strategy to realize straight leg walking. First, theoretical analyses of simplified models provide insight into Zero Moment Point (ZMP) deviations during straight knee walking. Based on the finding that the deviation is limited comparing to the support polygon, we decide to keep using the LIPM for high-level planning, but let the robot perform straight leg walking automatically via the optimization-based low-level controller. By setting the desired CoM height slightly over the robot's reachable height, the low-level controller will attempt to straighten the robot's leg to reach this vertical reference, in the meanwhile, also satisfy the constraints (i.e. dynamic feasibility, friction cone, torque limits). The simulation results of the humanoid robot WALK-MAN demonstrate the feasibility of proposed control strategy with relatively high energy efficiency. A typical butterfly shape of CoM trajectory was also observed in the frontal plane which is common in human walking.