Compared to most hybrid platforms, which have separate mechanisms and actuators for wheels and legs, our leg-wheel hybrid mobile robot, Quattroped, uses a “transformation mechanism” that deforms a specific portion of the body to act as a wheel or a leg. From a geometrical point of view, a wheel usually has a circular rim and a rotational axis located at the center of the rim. The rim contacts the ground and the rotational axis connects to the robot body at a point hereafter referred to as the “hip joint.” In general, with wheeled locomotion on flat ground, the wheel rotates continuously and the ground-contact point of the wheel is located directly below the hip joint with a fixed distance. In contrast, in legged locomotion the leg moves in a periodic manner and there is no specific geometrical configuration between the hip joint and the ground-contact point; thereby, the relative position of the legs varies frequently and periodically during locomotion.

Based on this observation, shifting the hip joint out of the center of the circular rim and changing the continuous rotation motion to other motion patterns implies the locomotion switches from wheeled mode to legged mode. This motivated us to design a mechanism that directly controls the relative position of the circular rim with respect to the hip joint so it can generate both wheeled and legged motions. Because the circular rim is a 2-dimensional object, the most straightforward method to achieve this goal is to add a second degree of freedom (DOF) that can adjust the relative position of the hip joint to the center of the circular rim along the radial direction. The motions of the two DOFs are also orthogonal to each other. In addition, the same set of actuation power can be efficiently used in both wheeled and legged modes.