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.
This work presents an experimental method for visualizing and analyzing machine‐soil interactions, namely the soil optical flow technique (SOFT). SOFT uses optical flow and clustering techniques to process images of soil interacting with a wheel or tool from photos taken through a glass wall of a soil bin. It produces results that are far richer than past approaches that utilized long‐exposure photography. It achieves a performance comparable to particle image velocimetry or particle tracking velocimetry, but without the need for specialized measurement equipment or specially marked soil particles. The processing technique demonstrates robustness to different soil types. Ground‐truth and cross‐validation experiments exhibit subpixel accuracy in estimating soil motions. An example of an application of this technique for field robotics research is the detailed study of push‐rolling for slope climbing and soft soil traverse. Push‐rolling advances a vehicle by rolling a subset of its wheels while changing its wheelbase to keep the other wheels static and pushing against the ground. Experiments show that push‐rolling achieves higher net traction than conventional rolling. Observing the two aspects of push‐rolling (rolling and horizontal pushing) using SOFT shows that they result in entirely different forms of soil shearing (“grip failure” and “ground failure,” respectively). SOFT also demonstrates how the direction of soil motion is more efficiently utilized for horizontal thrust by pushing than conventional rolling. Ongoing work utilizing SOFT has also demonstrated its potential use in studying excavation tool interactions, the effects of grousers on wheel efficiency, as well as a variety of other wheel‐soil interactions.
Abstract-The performance of wheels operating in loose granular material for the application of planetary vehicles is well researched but little effort has been made to study the soil shearing which governs traction. Net traction measurements and application of energy metrics have been solely relied upon to investigate performance but lack the ability to evaluate or describe soil-wheel interaction leading to thrust and resistances. The complexity of rim and grouser interaction with the ground has also prevented adequate models from being formulated. This work relies on empirical data gathered in attempt to study the effects of rim surface on soil shearing and ultimately how this governs traction. A novel experimentation and analysis technique was developed to enable investigation of terramechanics fundamentals in great detail. This technique, the Shear Interface Imaging Analysis Tool, is utilized to provide visualization and analysis capability of soil motion at and below the wheel-soil interface. Analysis of the resulting displacement field identifies clusters of soil motion and shear interfaces. Complexities in soil flow patterns greatly affect soil structure below the wheel and the resulting tractive capability. Grouser parameter variations, spacing and height, are studied for a rigid wheel. The results of soil shear interface analysis for wheels with grousers are presented. The processes of thrust and resistances are investigated and behavior characterized for grousered wheels.
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