Heel and toe lifting walk controller for traversing uneven terrain

When a humanoid robot traverses uneven terrain, such as stairs, possible footstep positions are constrained and the robot must take large strides. For robots with relatively short leg lengths, making such big strides is kinematically challenging. Possible solutions include lowering the torso height, relying on fast and dynamic stepping, and reducing foot size. However, all of these methods negatively affect performance by either reducing the stability or requiring higher joint torques. In this paper, we present a new locomotion controller that utilizes toe and heel lift to overcome this kinematic constraint for uneven terrain traversal. Given the ground inclination and projected ankle position, desirable toe and heel lift angles are calculated so that the robot can remain in double support while satisfying kinematic and joint range of motion constraints. We demonstrate the controller in physically realistic simulations and with the THOR-RD full-sized humanoid robot in the DARPA Robotics Challenge Finals competition.