VWDER: A Variable Wheel-Diameter Ellipsoidal Robot

In recent years, many researchers have conducted extensive research on spherical robots due to their high flexibility and anti-overturning capabilities. Nevertheless, compared with legged and traditional wheeled robots, spherical robots face certain limitations. The spherical robot is composed of a closed spherical shell structure, which makes the capacity of carrying workloads weak. At the same time, the single point contact with the ground cause the contact friction force with the ground is small, so it is hard to climb obstacles such as steps and doorsill. Therefore, we propose a new solution: the variable wheel diameter ellipsoidal robot (VWDER), which combines the characteristics of two-wheel differential driven robot and spherical robot driven by equivalent pendulum.VWDER is equipped with six retractable shell-shaped legs on each side and this innovative design allows both wheels to independently change diameter while rolling. The main frame of the VWDER can keep the top of the frame facing up under the action of equivalent pendulum during the locomotion, which makes it possible to carry workloads such as manipulator arms, cameras, IMU etc. The VWDER robot can climb steps or doorsill using its two adjacent shell-shaped legs. This paper introduces the design of the VWDER and analyzes the kinematics and dynamics of the VWDER. The experimental results verified the performance of the VWDER, including its autonomous opening and closing, obstacle crossing, automatic reorientation and slope climbing etc.