Analysis of Phase Shifts for a Rimless Wheel Rover

Space missions to explore Mars and Moon in situ are of high interest, but hazard terrain with steep slopes, soft soil, loose stones or high obstacles are a barrier for exploration rovers with common wheels. It was shown that rimless wheel rovers yield an enhanced performance in those environments while letting the complexity of the rover unchanged. They can cross higher obstacles, have a significant lower likelihood to get stuck and can move faster compared to common wheel rovers. At the Institute of System Dynamics and Control at the German Aerospace Center (DLR), a modular rimless wheel rover with flexible elements was designed for the purpose of planetary exploration in the above mentioned terrain. Due to the rimless wheels, the wheel circumference is not uniform. Hence, for an improved controllability of the full rover locomotion, it is reasonable to have control over the angular position of each wheel and especially over their angular position relative to each other. In this paper, several of these phase shifts are presented and compared. A setup in which the rover is driving straight forward is chosen to compare the behavior and performance of the different wheel configurations. The roll angle as well as the vertical movement of each rover segment are measured in simulation for different rover speeds and different phase shifts. A comparison between the different phase shift schemes in the sense of their waviness is made. The phase offsets have to be adjusted by means of a controller such that disturbances do not interfere significantly. For that reason, a PI controller was implemented. The study presented in this paper shows how the locomotion performance of a rimless wheel rover with compliant spokes can be improved by a well-tuned wheel position controller and the right choice of the phase shift configuration.