  In this paper, we propose a novel approach on controlling wheel-legged
quadrupedal robots using pose optimization and force control via quadratic
programming (QP). Our method allows the robot to leverage the whole-body motion
and the wheel actuation to roll over high obstacles while keeping the wheel
torques to navigate the terrain while keeping the wheel traction and balancing
the robot body. In detail, we first present a linear rigid body dynamics with
wheels that can be used for real-time balancing control of wheel-legged robots.
We then introduce an effective pose optimization method for wheel-legged
robot's locomotion over steep ramp and stair terrains. The pose optimization
solves for optimal poses to enhance stability and enforce collision-fee
constraints for the rolling motion over stair terrain. Experimental validation
on the real robot demonstrated the capability of rolling up on a 0.36 m
obstacle. The robot can also successfully roll up and down multiple stairs
without lifting its legs or having collision with the terrain.
