Dynamic Stress and Flow Analysis of Extraterrestrial Regolith for Planetary Exploration Rovers

Kurzfassung

In recent decades, robotic mobile systems known as rovers have been used for lunar/planetary exploration missions. These missions aim to elucidate the origin of planets and life and investigate potential resources including ice water. In the past missions, rovers have played a significant role in lunar/planetary exploration missions, and found many scientific discoveries such as the presence of water on Mars. These findings indicate that rovers are of great importance for lunar/planetary explorations. Despite the great successes so far accomplished in these missions, rovers experienced mobility problems on lunar/planetary surfaces, covered with fine granular soil, popularly known as regolith. In particular, excessive slippage and sinkage of the wheel make rovers difficult to follow their desired route. In addition, these problems induce being stuck in deformable terrains and mission failures. In future missions, it is expected that the difficulty level in the exploration mission increases, for example, it would be required the explorations in difficult environments - near and inside the lunar craters - and the efficient explorations by high-speed rovers. The increase in difficulty increases the risk of mobility problems. Avoiding such situations contributes to improving exploration efficiency and minimizing mission failures. To address the mobility issues, it is necessary to predict the wheel-traveling performance in advance, and for that purpose, it is effective to understand and model the wheel-soil interactions. In many studies, classical terramechanics models for large vehicles have commonly been used to predict wheel performance for lunar/planetary rovers. Lunar/planetary rovers generally have small wheels equipped with grousers, which are like a plate, to improve their tractive performance. The grouser wheel shows more complicated wheel-soil interaction mechanics than the non-grouser wheel. The classical models do not consider the grouser wheel and soil deformation and dynamics caused by the wheel, being not suitable to predict wheel performance for lunar/planetary rovers. To improve the model predictions for the rovers, it is necessary to understand the interaction between the grouser wheel and soft terrain. There are mainly two approaches to analyzing wheel-soil interactions. One is the analysis of the wheel performance such as drawbar pull and stress distributions at the contactpatch of the wheel. Another is the analysis of the soil behavior such as the soil flow fields. In our laboratory, past studies have focused on the drawbar pull and stress distributions of the wheel. In addition to these studies, it is very beneficial to analyze soil flow and deformation and integrate them with the result of wheel performance for discussions. This study, therefore, addresses mainly the following three issues: 1) experimental analysis of the soil flow fields beneath the grouser wheel based on an imaging technique, 2) development of a particle-based simulation to analyze soil flow and stress fields and evaluation of its validity, 3) evaluation of the soil flow and stress fields using the developed simulation, assuming the lunar environments - lunar soil and gravity -. Chapter 1 introduces past and ongoing lunar/planetary exploration rovers with their exploration environments as a research background. In addition, the motivation and objective of this work are presented with the state of the art on wheel-soil interactions. Chapter 2 experimentally performs the single-wheel tests on the two types of soils to analyze soil flow fields beneath the grouser wheel: Toyoura standard sand and the lunar regolith simulant. This study employs a computer vision-based technique known as particle image velocimetry (PIV) for the soil flow analysis. The main objective of this chapter is to reveal the difference in the soil flow between Toyoura standard sand and the lunar regolith simulant. The experimental results are discussed from the viewpoints of soil flow velocity, flow region, entry and exit angles of the wheel, slip ratio, and force and torque acting on the wheel. The results indicate that the soil flow characteristics considerably differ depending on the soil properties, being lower fluidity with an increase in the soil shear strength. In addition, it is also presented that the lower fluidity of the soil contributes to improving the wheel performance. Chapter 3 develops a particle-based simulation technique known as the discrete element method (DEM) for the single-wheel simulations for the analysis of thw soil flow and stress fields beneath the grouser wheel. In the DEM, the particle motion is determined based on the contact model. This study implements a rolling resistance and cohesive models to reproduce the behavior of the lunar regolith simulant. As the particle parameters in the model are of great importance in the DEM to reproduce the soil behavior, the parameters are tuned for Toyoura standard sand and the lunar regolith simulant based on the parametric study. The DEM is validated by comparing with the experimental data at the following four perspectives: slip ratio, ruts formed after the wheel pass, entry and exit angles, and soil flow velocity fields. The validation results show that the DEM can reproduce the soil flow/deformation and wheel performance on the two different types of soils under any slip conditions. Chapter 4 evaluates the soil behavior and wheel performance by using the developed DEM simulation. As this dissertation focuses on lunar exploration rovers, the effect of the lunar regolith simulant and lunar gravity on the soil behavior is investigated by performing single-wheel simulations. The result of the simulations is discussed based on the wheel sinkage, drawbar pull, soil flow fields, and dynamic stress in the soil. The result shows thatthe cohesive soil contributes to improving the wheel performance due to its higher shear strength, and the lower gravity adversely affects the wheel performance. In addition, for future efficient explorations, the effect of increasing speeds on the wheel-soil interactions is also described in this chapter. The simulation results for the high-speed locomotion show that the soil flow and dynamic stress increase as the wheel speed increases. The wheel sinkage, which is an indicator to evaluate the wheel performance, depends on the shear strength of the soil. This result indicates that the effect of increasing speeds on the wheel performance depends on the shear strength of the soils. Chapter 5 summarizes the results of this study. In conclusion, this work achieved the following outcomes to understand wheel-soil interactions for lunar/planetary rovers: 1) visualization of soil flow beneath the grouser wheel, 2) development of the simulation to analyze soil behavior, 3) evaluation of the soil flow and dynamic stress fields. These contributions would be beneficial to reconsidering the classical terramechanics models for lunar/planetary rovers. As a final remark of this dissertation, the directions for future studies are also presented in this chapter.