Masataku Sutoh

Traveling performance evaluation of planetary rovers on loose soil

In designing a planetary rover, it is important to consider and evaluate the influence of parameters such as the weight and dimensions of the rover on its traveling performance. In this study, the influence of a rovers weight on its performance was evaluated by conduct experiments using a monotrack rover and an inline four-wheeled rover with different rover weights. Then the influence of the wheel diameter and width was quantitatively determined by performing experiments using a two-wheeled rover, equipped with wheels, with different diameters and widths. The results of the experiments were compared with those of a numerical simulation based on terramechanics. Finally, the influence of the wheel surface pattern on the traveling performance of planetary rovers was evaluated by conducting experiments using a two-wheeled rover equipped with wheels with different numbers of lugs (i.e., grousers) on their surfaces. Based on the results of these experiments, we confirmed the following influences of the parameters: in the case of the track mechanism, the traveling performance does not change with the increase in rover weight. On the other hand, in the case of the wheel mechanism, an increase in rover weight decreases the traveling performance. Moreover, the experimental results show that the wheel diameter contributes more to the high traveling performance than the wheel width. In addition, a comparison between the experimental and simulation results shows that it is currently difficult to accurately predict the traveling performance of lightweight vehicles on the basis of terramechanics models. Finally, the experimental results show that having lugs always improves the traveling performance, even at the expense of wheel diameter.

Modeling, Analysis, and Control of an Actively Reconfigurable Planetary Rover for Traversing Slopes Covered with Loose Soil

Future planetary rovers are expected to probe across steep sandy slopes such as crater rims where wheel slippage can be a critical problem. One possible solution is to equip locomotion mechanisms with redundant actuators so that the rovers are able to actively reconfigure themselves to adapt to the target terrain. This study modeled a reconfigurable rover to analyze the effects of posture change on rover slippage over sandy slopes. The study also investigated control strategies for a reconfigurable rover to reduce slippage. The proposed mechanical model consists of two models: a complete rover model representing the relationship between the attitude of the rover and the forces acting on each wheel, and a wheel-soil contact force model expressed as a function of slip parameters. By combining these two models, the proposed joint model relates the configuration of the rover to its slippage. The reliability of the proposed model is discussed based on a comparison of slope-traversing experiments and numerical simulations. The results of the simulations show trends similar to those of the experiments and thus the validity of the proposed model. Following the results, a configuration control strategy for a reconfigurable rover was introduced accompanied by orientation control. These controls were implemented on a four-wheeled rover, and their effectiveness was tested on a natural sand dune. The results of the field experiments show the usefulness of the proposed control strategies.

The Right Path: Comprehensive Path Planning for Lunar Exploration Rovers

This article presents a comprehensive path-planning method for lunar and planetary exploration rovers. In this method, two new elements are introduced as evaluation indices for path planning: 1) determined by the rover design and 2) derived from a target environment. These are defined as the rovers internal and external elements, respectively. In this article, the rovers locomotion mechanism and insolation (i.e., shadow) conditions were considered to be the two elements that ensure the rovers safety and energy, and the influences of these elements on path planning were described. To examine the influence of the locomotion mechanism on path planning, experiments were performed using track and wheel mechanisms, and the motion behaviors were modeled. The planned paths of the tracked and wheeled rovers were then simulated based on their motion behaviors. The influence of the insolation condition was considered through path plan simulations conducted using various lunar latitudes and times. The simulation results showed that the internal element can be used as an evaluation index to plan a safe path that corresponds to the traveling performance of the rovers locomotion mechanism. The path derived for the tracked rover was found to be straighter than that derived for the wheeled rover. The simulation results also showed that path planning using the external element as an additional index enhances the power generated by solar panels under various insolation conditions. This path-planning method was found to have a large impact on the amount of power generated in the morning/evening and at high-latitude regions relative to in the daytime and at low-latitude regions on the moon. These simulation results suggest the effectiveness of the proposed path-planning method.

Evaluation of influence of surface shape of locomotion mechanism on traveling performance of planetary rovers

The surfaces of both the Moon and Mars are covered with loose soil, with numerous steep slopes along their crater rims. Therefore, one of the most important requirements imposed on planetary rovers is their ability to minimize slippage while climbing steep slopes, i.e., the ability to generate a drawbar pull with only a small amount of slippage. To this end, the wheels/tracks of planetary rovers typically have parallel fins called lugs (i.e., grousers) on their surface. Recent studies have reported that these lugs can substantially improve the traveling performances of planetary rovers. Therefore, in this study, we conducted experiments using lightweight two-wheeled and mono-tracked rovers to provide a quantitative confirmation regarding the influence of lugs on the traveling performances of planetary rovers. Based on our experimental results, we confirmed that, although an increase in the number of lugs contributes to the high traveling performance of wheeled rovers, it does not contribute much to that of tracked rovers. Furthermore, an increase in lug height improves the traveling performances of both types of rovers.

Evaluation of the reconfiguration effects of planetary rovers on their lateral traversing of sandy slopes

Rovers that are used to explore craters on the Moon or Mars require the mobility to negotiate sandy slopes, on which slippage can easily occur. Such slippage can be reduced by actively readjusting the attitude of the rovers. By changing attitude, rovers can modify the position of their center of gravity and the wheel-soil contact angle. In this study, we discuss the effects of attitude changes on downhill sideslip based on the slope failure mechanism and experiments on reconfiguring the rover attitude and wheel angles. We conducted slope-traversing experiments using a wheeled rover under various roll angles and wheel angles. The experimental results show that the contact angle between wheels and slopes has a dominant influence on sideslip when compared with that of readjusting the rovers center of gravity.

Slope traversability analysis of reconfigurable planetary rovers

Future planetary rovers are expected to probe over steep sandy slopes, such as crater rims, where wheel slippage can be a critical issue. One solution to this issue is to mount redundant actuators on the locomotion mechanisms of the rovers such that they can actively reconfigurate themselves to adapt to the driven terrain. In this study, we propose a mechanical model of a rover based on a wheel-soil contact model combined with the classical terramechanic theory. The effects of the rover reconfiguration on its slippage tendencies are analyzed based on slope traversing experiments and numerical simulations. The validation of the proposed contact model is also discussed based on experimental and numerical simulation results. According to the experimental results, both longitudinal and lateral slippages are greatly reduced by tilting the rover in an uphill direction. The results of the numerical simulation match the experimental results quantitatively, and indicate the possible need to include a slope failure model.

Motion Behaviors of Landing Gear for Lunar Probes in Atmosphere and Vacuum Tests

In this paper, we investigate and model the motion behaviors in the atmosphere and vacuum of the landing gear of a lunar probe, which contacts the granular material covering the lunar surface. Drop and friction tests were conducted using the footpads of a lander on a lunar regolith simulant. In the drop tests, the footpads were dropped onto the simulant. The impact characteristics of the footpads were then analyzed. In the friction tests, footpads having varying shapes were slid onto the simulant. The sliding characteristics of the footpads were then evaluated. The drop tests showed that the simulant ejected by the footpad impact diminished, and the footpad penetration significantly decreased in the vacuum. The friction tests confirmed that the sinkage caused by the sliding of the footpad generally increased more in the vacuum than in the atmosphere. However, the difference in the sinkages can be negligible depending on the footpad shape. These findings enabled the deriving of fundamental models of landing gear and suggested guidelines for the design and development of landing gear.

Landing Behavior Analysis of Lunar Probe Based on Drop Tests and RFT in a Vacuum

This letter addresses the influences of footpad shape and ground condition on the motion behavior of a lander in a vacuum. To evaluate the influences, we first developed a drop test apparatus that can conduct repeated drop tests in the vacuum chamber. The footpad drop tests were then conducted with various shaped footpads on different surface conditions. Subsequently, the motion behavior of the footpads in a vacuum was modeled, based on the resistance force theory (RFT) and its penetration characteristics were numerically analyzed. The usefulness of the RFT based model was discussed along with the experimental results. Finally, drop tests were conducted using a four-legged lander to comprehensively analyze its landing behaviors. From the footpad drop tests and numerical analysis based on the RFT, we confirmed the following: 1) the force acting on the footpad is enhanced, and the penetration depth is reduced in a vacuum, 2) the force and kinetic energy conversion rate are smallest for the curved footpad, and 3) an increase in the ground density had a relatively small impact on the penetration depth of the footpads in a vacuum. Furthermore, the drop tests using the lander model confirmed that even if some of the landers footpads land on regolith simulant with different densities, this does not lead to postural imbalance or turnover of the lander in a vacuum.

Analysis of the Traveling Performance of Planetary Rovers with Wheels Equipped with Lugs over Loose Soil

The surfaces of the Moon and Mars are covered with loose soil. In such conditions, planetary rovers can get stuck and even cause failure of an exploration mission. To avoid such problems, the wheels of planetary rovers typically have lugs (i.e., grousers) on their surface, which substantially improve their traveling performance. However, there exists no theoretical method to determine a suitable lug interval. In this study, we first modeled the linear traveling speed of a wheel with lugs and provided guidelines for determining a suitable lug interval, as well as the corresponding terramechanical stress models. Next, to verify the suitable interval for lugs, we performed traveling tests using a two-wheeled rover with wheels having different numbers of lugs of different heights. According to the experimental results, when a wheel has three lugs in a range where normal stress is generated beneath it, it can travel with constant speed while the traveling performance is substantially

Traveling performance estimation for planetary rovers over slope

One of the most important requirements imposed on planetary rovers is the ability to minimize slippage while climbing slopes covered with loose soil. Therefore, at the design stage of the rovers, it is necessary to evaluate the traveling performance of their locomotion mechanisms over a slope. However, to conduct traveling tests over slopes, the sandbox in which the tests are conducted must be tilted; it is difficult and even dangerous to make a steep slope because of the heavy weight of the sandbox. Therefore, in this paper, we propose a new method for estimating the traveling performance of a wheeled rover over a slope. In this method, we estimate the slip ratio for different slope angles on the basis of traction tests over a flat terrain. To verify the proposed method, we conducted slope-climbing tests and traction tests in a sandbox using a two-wheeled rover with various types of wheels. Furthermore, we compared the slip ratios estimated from the slope-climbing tests with those estimated from the traction tests. From these comparisons, we concluded that the proposed method can accurately estimate the traveling performance of a planetary rover over a slope.