How does a wheeled robot move and turn? The answer is straightforward for a conventional wheeled robot, but it is not so easy for a robot with a discrete wheel design. Regular wheeled robots always have four contact points, resulting in static stability during locomotion. However, QuadRunner's novel leg mechanism provides only a semi-circular wheel shape, and proper gait planning is needed to go straight or turn. Therefore, this paper presents a dual frequency gait planning method which controls the robot's gait cycle's duty factor and generates unique turning gait patterns for wheel locomotion. Describing requirements and limitations, we found sets of solutions that can achieve turning. Results show that the smallest turning radius QuadRunner achieved is 1.05m, and the biggest is 1.86m. In addition, detailed experiments were made to observe the performance and stability of straight and turning wheel behaviors. Finally, a gait verification is made using high-speed cameras.

This paper presents QuadRunner, a transformable quasi-wheel legged robot that achieves both quadruped locomotion and wheel locomotion by exploiting a novel semicircular leg-wheel design with a Trotting Wheel gait. We built upon the Stanford Doggo open architecture platform and integrated it with a transformable leg-wheel design to enhance its locomotion capabilities. On its gait control, improvements were made to its trot gait kinematics with end-effector considerations as well as the design of a new trotting wheel gait. Our proposed locomotion strategy found that the robot's legged locomotion improves by an average speed of 10%. In addition, wheel to leg transition takes around 300 ms, and the speed of its wheel locomotion can reach more than five times its body-length/s (2.2 m/s) on flat terrain. Lastly, detailed experiments are conducted to observe the wheel-leg transition performance and gait verification under the absence of foot contact sensors.