Studying the gravity-dependent characteristics of regolith, fine-grained granular media covering extra-terrestrial bodies is essential for the reliable design and analysis of landers and rovers for ...space exploration. In this study, we propose an experimental approach to examine a granular flow under stable artificial gravity conditions for a long duration generated by a centrifuge at the International Space Station. We also perform a discrete element simulation of the granular flow in both artificial and natural gravity environments. The simulation results verify that the granular flows in artificial and natural gravity are consistent. Further, regression analysis of the experimental results reveals that the mass flow rate of granular flow quantitatively follows a well-known physics-based law with some deviations under low-gravity conditions, implying that the bulk density of the granular media decreases with gravity. This insight also indicates that the bulk density considered in simulation studies of space probes under low-gravity conditions needs to be tuned for their reliable design and analysis.
Planetary rovers are becoming indispensable for exploration activities and science missions. The rover used in such mission is often limited in its operation time owing to power and computational ...resources of the rover. In this paper, a trajectory planning method for a planetary rover is proposed that considers vehicle dynamics, and energy management of the rover. The vehicle dynamics is approximated from a dynamic simulation of the rover, which can estimate the power consumption in accordance with terrain traversability of the rover. The power generation of the solar array panel mounted on the rover is also taken into account. The simulation study confirmed the usefulness of the proposed method, especially in scenarios where slopes could be observed, and one result indicated that the energy margin could be improved by 4.1 kJ, 13.9
at maximum.
In this article, a statistical mobility prediction for planetary surface exploration rovers has been described. This method explicitly considers uncertainty of the terrain physical parameters via ...SRSM and employs models of both vehicle dynamics and wheel-terrain interaction mechanics. The simulation results of mobility prediction using three different techniques, SMC, LHSMC, and SRSM, confirms that SRSM significantly improves the computational efficiency compared with those conventional methods. The usefulness and validity of the proposed method has been confirmed through experimental studies of the slope traversal scenario in two different terrains. The results show that the predicted motion path with confidence ellipses can be used as a probabilistic reachability metric of the rover position. Also, for the slope-traversal case, terrain parameter uncertainty has a larger influence on the lateral motion of the rover than on longitudinal motion. Future directions of this study will apply the proposed technique to the path-planning problem. Here, confidence ellipses will be used to define collision-free areas, which will provide useful criteria for generating safe trajectories.
Planetary exploration rovers are required to (semi-) autonomously navigate itself on rough terrain. In such scenario, a feasible path to be followed must be accurately planned, allowing the rover to ...safely traverse without any mobility hazards. In this paper, a path planning and evaluation strategy that explicitly considers dynamic mobility of the rover is presented. The proposed strategy consists of the following three steps. First, various paths on a given terrain map are generated with varying weighting factors for path planning algorithm. Each path is then examined through complete dynamic simulations of the rover in which the rover travels along with the path. The simulation result provides a metric for robotic mobility. The metric, termed a dynamic mobility index, consists of stability of the rover, wheel slippage, elapsed time, and energy consumption. All of the paths generated are quantitatively evaluated based on the dynamic mobility index, and then, the most feasible path between them is obtained. Demonstrations for the path planning and evaluation are presented in this paper that confirms the validity of the proposed strategy.
In this paper, steering characteristics of an exploration rover on loose soil is studied. Analysis of the steering characteristics is a key to plan and control the motion trajectory of a rover. ...Traditionally, such analysis has been made based on a model called "bicycle model." In that model, a four-wheel car-like vehicle is approximated by a two-wheel bicycle-like vehicle with the fore-wheels and the rear-wheels paired. However, the bicycle model does not show a good performance when a vehicle travels off-road. In order to analyze the steering characteristics of a vehicle on loose soil, the authors develop a model that respects the dynamics of each wheel's slip and skid behavior. The developed model is called all-wheel dynamics model. In the all-wheel dynamics model, the behavior of each wheel on loose soil is modeled based on terramechanics. The motion trajectory of the vehicle is obtained by numerical simulation using the wheel-and-vehicle dynamics model. The validity of the proposed model is examined by the experiments of a wheel and a vehicle using simulated lunar-surface soil. The experimental results show that the proposed model provides a better approximation than the traditional bicycle model.
In this paper an omnidirectional mobile robot that possesses high mobility in rough terrain is presented. The omnidirectional robot has four active split offset caster (ASOC) modules, enabling the ...robot to move in any planar direction. It also possesses passive suspension articulation, allowing the robot to conform to uneven terrain. The agility of the robot is experimentally evaluated in various configurations. In addition, an odometry method that mitigates position estimation error due to wheel slippage is proposed. A key aspect of the proposed method is to utilize sensory data of wheel velocity, and turning rate around each ASOC pivot shaft, along with kinematic constraints of the robot configuration. Experimental odometry tests with different maneuvers in rough terrain are presented that confirm the utility of the proposed method.
The ability of mobile robots to generate feasible trajectories online is an important requirement for their autonomous operation in unstructured environments. Many path generation techniques focus on ...generation of time- or distance-optimal paths while obeying dynamic constraints, and often assume precise knowledge of robot and/or environmental (i.e. terrain) properties. In uneven terrain, it is essential that the robot mobility over the terrain be explicitly considered in the planning process. Further, since significant uncertainty is often associated with robot and/or terrain parameter knowledge, this should also be accounted for in a path generation algorithm. Here, extensions to the rapidly exploring random tree (RRT) algorithm are presented that explicitly consider robot mobility and robot parameter uncertainty based on the stochastic response surface method (SRSM). Simulation results suggest that the proposed approach can be used for generating safe paths on uncertain, uneven terrain.
In this paper, a path following control strategy for lunar/planetary exploration rovers is described, taking into account slip motion of the rover. It is determined that the slip motion of each wheel ...of the rover must be increased and cannot be neglected when the rover travels on loose soil. Because of slip, following an arbitrary path on loose soil is a difficult task. In order to improve this situation, the authors have developed a path following algorithm with slip compensation. In this algorithm, both steering and driving maneuvers of the rover are derived not only to follow an arbitrary path, but also simultaneously compensate for the slip. The performance of the path following strategy is confirmed through numerical simulation using the wheel-and-vehicle model elaborated in our previous research. The slip motion of the wheel is also addressed, based on a terramechanics approach. The proposed path following algorithm shows better performance than traditional control without slip compensation in the simulation
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