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Tomomi Tanaka, Keisuke Takehana, James Hurrell, Kentaro Uno, and Kazuya Yoshida
The exploration of the lunar surface requires a wide range of mobility, thus wheeled mobile robots, called rovers, which are capable of traveling on the loose soil, called lunar regolith. To investigate the traction performance of wheels on such loose soil, it is essential to consider the multi-pass effect. That is the influence to the wheel’s traction performance due to the alteration in the soil property caused by the previous wheel’s traveling (compaction and digging). The single-wheel testing is helpful to analyze the driving performance of a wheel. In this study, the soil with a cohesive property and heterogeneity in particle size and shape, which offers a nonlinear characteristics of lunar regolith, is prepared in the sandbox of the single-wheel testbed. To investigate the multi-pass effect, Toyoura sand, which is a non-cohesive and homogeneous sand, is compared with the lunar regolith simulant: FJS-1. This paper presents the experimental results of a repetitive running experiment conducted to measure the multi-pass effect. In this experiment, the wheel was run 5 times in a row over the same area from the same direction, and the traction coefficient, sinkage, and the shear strength of the soil were measured each time. This experiment was also conducted under several conditions by varying the vertical load applied to the wheel and the slip ratio. The wheel employed in the experiments is the same model of grouser wheels as installed on the Rashid Rover that was actually planned to be deployed on the lunar exploration mission. The results revealed that the soil compaction was observed only at a deep level of FJS-1 sand. This is because FJS-1 sand flows in shallow locations due to the action of the grouser, on the other hand, Toyoura sand has no adhesive force.
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