Planetary exploration requires rovers to perform a variety of challenging tasks autonomously. In order to complete relevant scientific mission, these rovers need to overcome difficulty for rough terrain traversing by "online methods". This paper discusses an idea that utilizes a rover mobile center of mass in order to aid in traversing rough terrain. According to vehicle pitch angle and contact angles, the rover recalculates the optimal position for the center of mass to minimize the indicator defined by two factors : the wheel driving force over the wheel normal force (adhesion coefficient) and tip-over stability angle (stability coefficient). Two innovative approaches are developed in this paper. The first one is that the authors defines easily comprehensible two factors, adhesion and stability coefficients for traversability indicator and proposes a method to calculate optimal center of mass position. The second one is that in the case of four-wheels drive, the relation between the front wheel and the rear wheel adhesion coefficient is easily solved and the best case for giving the driving forces can be solved as force making the front and the rear wheel adhesion coefficient equal. The authors call this "optimal force distribution method". Finally, this paper shows some simulation results for traversing rough terrain that compute the optimal center of mass while traversing the terrain.

Mobility assessment and prediction continues to be an important and active area of research for planetary rovers, with the need illustrated by multiple examples of high slip events experienced by rovers on Mars. Despite slip versus slope being one of the strongest and most broadly used relationships in mobility prediction, this relationship is nonetheless far from precisely predictable. Although the literature has made significant advances in the predictability of average mobility, the other key related aspect of the problem is the risk caused by edge cases. A key contribution of this study is a metric for explicitly assessing mobility risk based on data-driven nonparametric slip versus slope relationships. The data-driven approach is meant to address limitations of past model-based approaches. The metric is informed by past work in terramechanics relating drawbar pull (i.e., net traction) to slip: High slip fraction (HSF), defined as the proportion of slip data points above 20%. Another contribution is a low complexity mobility prediction framework, the autonomous soil assessment system. Field tests demonstrate that, for sand and gravel, rover trafficability becomes nonlinear and highly variable above the 20% slip threshold.HSF is shown to be a useful metric for categorizing rover-terrain interactions into low, medium, or high risk, correctly and consistently. Furthermore, the metric is shown to be useful for early detection of potentially hazardous changes in roverterrain conditions. The combination of HSF with an appropriately sized queue structure for modeling slip versus slope enables an appropriate balance between responsiveness and stability. K E Y W O R D S mobility prediction, planetary robotics, rover slip

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The development of a flexible metallic wheel proved to be one of the most challenging and time-consuming aspects of the Lunar Roving Vehicle of the Apollo missions (V. Asnani, D. Delap, and C. Creager. 2009. Journal of Terramechanics, 46: 89 103). The design was realized through an iterative trial and error design process, driven primarily by manufacturability and physical testing. Although the wire-mesh-compliant wheel design was identified as the best choice for the Lunar Roving Vehicle, mission scenarios have evolved and future lunar vehicles are bound to have to meet different functional requirements and more severe life and operational constraints. For example, these vehicles will have to travel farther on the lunar surface, explore permanently shadowed craters, and perform a variety of tasks such as transporting sensitive payloads and excavatiing regolith, and allow for both unmanned and manned operation. This work focuses on optimizing the suspension design parameters of a flexible, compliant wheel for maximizing the dynamic terrainability performance of a lunar rover. The suspension design parameters are identified, independently of the explicit wheel configuration. The terrainability of the rover is defined here as the rover's ability to negotiate terrain irregularities. Terrainability is quantified by the objective functions describing road holding and rider comfort. These objective functions ensure that the rover payload is isolated from vehicle terrain-induced vibrations, and that the wheels maintain ground contact during higher speed traversals. A simplified vehicle model is used for the dynamics analysis of the terrain vehicle system with the terrain modeled as a random stationary ergodic process characterized by a power spectral density function. A sensitivity analysis is performed to identify conflicting objectives, and a recommendation on optimal wheel suspension design variables is made. Finally, experimental testing of reduced-scale wheels with different suspension properties is conducted on three irregular terrain types and results confirm the theoretical predictions. Ultimately, the results of this research will be used in a system-level analysis of the wheel design parameters on vehicle performance to guide the structural optimization of the compliant wheel for lunar surface exploration vehicles.

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