The Martian Moons eXploration (MMX) mission will include a rover to conduct in situ science and exploration on the Martian moon Phobos [1]. The rover is being developed as a joint project by CNES and DLR. The rover's locomotion system and especially its wheels require special consideration as wheeled locomotion has never been employed in a comparably low gravity environment. Wheel design for Phobos is complicated by the little researched behavior of regolith under low gravity and the inability to conduct milli-g experiments of a sufficient duration on Earth, necessitating the assumption of a worst-case soft soil. This paper shows a novel, automated approach to design the shape of rover wheels. It uses high-fidelity simulation models based on the Discrete Element Method [2] and a Bayesian Optimization algorithm [3]. As the outer wheel diameter and wheel width are given by mission constraints, four parameters were selected to describe the wheel shape: grouser height, grouser number, grouser chevron angle and rim curvature. On Phobos, three main scenarios are important to ensure the rover's successful operation: Driving forward, backward and uphill of up to 10 degrees. The optimization loop generates wheels within wide limits of the four wheel shape parameters. Each wheel is simulated with free slip conditions in each scenario. A simulation on flat soil is shown in Figure 1. The performance of each wheel is assessed based on the travelled distance. The optimization is run for 115 iterations, which corresponds to about one month of wall-clock time. It yields an optimized wheel shape as well as a dataset showing the dependence of performance on the wheel shape parameters. In contrast to the prototype wheel, see Figure 2, the optimized wheel shape shown in Figure 3 has larger flat grousers. The inward curved rim shape was confirmed. The optimized wheel performs 64% better than the previous prototype. The optimized shape and parameter relations will now guide the design of the wheels for the MMX mission.
Wheeled rovers have been successfully used as mobile landers on Mars and Moon and more such missions are in the planning. For the Martian Moon eXploration (MMX) mission of the Japan Aerospace Exploration Agency (JAXA), such a wheeled rover will be used on the Marsian Moon Phobos. This is the first rover that will be used under such low gravity, called milli-g, which imposes many challenges to the design of the locomotion subsystem (LSS). The LSS is used for unfolding, standing up, driving, aligning and lowering the rover on Phobos. It is a entirely new developed highly-integrated mechatronic system that is specifically designed for Phobos.Since the Phase A concept of the LSS, which was presented two years ago [1], a lot of testing, optimization and design improvements have been done. Following the tight mission schedule, the LSS qualification and flight models (QM and FM) assembly has started in Summer 2021. In this work, the final FM design is presented together with selected test and optimization results that led to the final state. More specifically, advances in the mechanics, electronics, thermal, sensor, firmware and software design are presented.The LSS QM and FM will undergo a comprehensive qualification and acceptance testing campaign, respectively, in the first half of 2022 before the FM will be integrated into the rover.
In planetary exploration, testing under the actual mission conditions is inherently not possible. Hence, simulation campaigns complement ground test campaigns. This specially applies to surface missions that include the complex behaviour of soils under non-terrestrial gravitation. Increasingly ambitious mission goals made large simulation campaigns with very precise particle models necessary for the simulation of soil interaction. Thus, to limit the amount of time and the computation hardware needed, DLR developed the particle simulation tool "Sir partsival". This tool does not only speed up simulations by usage of GPU computing, but also integrates the institute's experience in modelling of soil on Earth and beyond. Using partsival it was possible to speed up simulations by more than a factor of ten and thus conduct large simulation campaigns. Two examples are shown: a large, on-going validation campaign of DEM for wheel simulations, and the completed traction optimization for the MMX rover wheel.
As part of JAXA's MMX mission, a rover, jointly developed by DLR and CNES, will be deployed on Phobos [1]. Its task is to scout the landing site for the MMX spacecraft and to gain in-situ insights on the Phobos regolith. Major challenges to the mission's success are the extremely low gravity on Phobos and its mostly unknown regolith properties [2]. Effective gravity on Phobos ranges from 3.1×10 -4 m/s² to 6.8×10 -4 m/s² [1]. So far, no wheeled rover has ever been deployed under comparably low gravity. So, to ensure the mission's success, the rover wheels and their traction have been examined and optimized using DEM simulations (see Figure 1) [3].
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