Keep Rollin’—Whole-Body Motion Control and Planning for Wheeled Quadrupedal Robots

We show dynamic locomotion strategies for wheeled quadrupedal robots that combine the advantages of both walking and driving. The developed optimization framework tightly integrates the additional degrees of freedom introduced by the wheels. Our approach relies on a zero-moment point-based motion optimization which continuously updates reference trajectories. The reference motions are tracked by a hierarchical whole-body controller which computes optimal generalized accelerations and contact forces by solving a sequence of prioritized tasks including the nonholonomic rolling constraints. Our approach has been tested on ANYmal, a quadrupedal robot that is fully torque-controlled including the nonsteerable wheels attached to its legs. We conducted experiments on flat and inclined terrains as well as over steps, whereby we show that integrating the wheels into the motion control and planning framework results in intuitive motion trajectories, which enable more robust and dynamic locomotion compared to other wheeled-legged robots. Moreover, with a speed of 4  m/s and a reduction of the cost of transport by 83%, we prove the superiority of wheeled-legged robots compared to their legged counterparts.

I. Introduction

Wheels are one of the major technological advances of humankind. In daily life, they enable us to move faster and more efficiently as compared to legged-based locomotion. The latter, however, is more versatile and offers the possibility to negotiate challenging environments, which is why combining both strategies into one system, would achieve the best of both worlds.