Soft Terrain Research Articles

Terramechanics analysis of climbing behavior of wheel against rigid obstacle on soft terrain

In the development of lunar and planetary exploration rovers, risk assessment in on-site environments comprising fine regoliths and unevenly distributed rocks is essential. To prevent a rover from being stuck during its operation, one must understand the behavior of its wheel when it climbs over rigid obstacles in off-road environments. In this study, we apply an extended terramechanics model, which can reasonably describe the interaction between soft ground and vehicles, to analyze the obstacle-climbing behavior of rigid wheels. To describe the interaction between the wheel and a hard obstacle, we combine the penalty method with the extended terramechanics model. Subsequently, we verify the effectiveness of the proposed method by comparing its results with those of a traversing single wheel obtained from a laboratory experiment. Furthermore, we use the model to perform a multibody dynamics analysis on a simple rover, where its applicability to the examination of the overall performance of obstacle climbing is demonstrated.

An online optimization escape entrapment strategy for planetary rovers based on Bayesian optimization

AbstractPlanetary rovers may become stuck due to the soft terrain on Mars and other planetary surface. The escape entrapment control strategy is of great significance for planetary rover traversing loosely consolidated granular terrain. After analyzing the performance of the published quadrupedal rotary sequence gait, a “sweeping‐spinning” gait was proposed to improve escape entrapment capability. And the forward distance of planetary rovers with “sweeping‐spinning” gait was modeled as a function of six control parameters. An online optimization escape entrapment strategy for planetary rover was proposed based on the Bayesian Optimization algorithm. Single‐factor experiments were conducted to investigate the effect of each control parameter on forward distance, and determine the parameter ranges. The average forward distance with randomly selected control parameters is 89.64 cm, while that is 136.93 cm with Bayesian optimized control parameters, which verifies the effectiveness of the escape entrapment strategy. Moreover, compared with the trajectory of a planetary rover prototype with the published quadrupedal rotary sequence gait, the trajectory of a planetary rover prototype with “sweeping‐spinning” gait is more accurate. Furthermore, the online estimated equivalent terrain mechanical parameters can be used to determine the running state of the planetary rover prototype, which was verified using experiments.

Study on the mechanical model of footpad-terrain for walking robot moving in low gravity environment

Due to the low gravity environment and the influence of complex terrain condition in deep space exploration, wheeled mobile systems are prone to meet motion abnormalities. The excellent motion performance of walking robot is more suitable for the future deep space exploration, but the robots are prone to occur large sinkage in soft terrain. A mechanical model is built to describe a gait cycle of a walking robot under soft terrain and low gravity environment. The force on the footpad during actual movement in a gait cycle is obtained through a single-legged test bench under the simulated planet terrain. The effects of sizes of footpads, sinkage and other factors are explored. The results indicate that the larger the size of the footpad, the greater the horizontal force on the footpad, the better the motion performance is. But as the size of footpad increase, the vertical force decreases which indicates poor support performance. By comparing and analyzing the model values with the experimental values, for the horizontal force FT, the average errors for the average force and peak force are 10.05% and 7.76%. The average errors for average force and peak force are 5.19% and 5.86% for vertical force FN. The values are not significantly different from the model values and experimental values which indicates that the mechanical model has high accuracy. The obtained mechanical model can provide a reference for the motion of walking robots in complex low gravity environment.

Predictor-based Control for Delay Compensation in Bilateral Teleoperation of Wheeled Rovers on Soft Terrains

Predictor-based Control for Delay Compensation in Bilateral Teleoperation of Wheeled Rovers on Soft Terrains

Research on Terrain Mobility of UGV with Hydrostatic Wheel Drive and Slip Control Systems

The article explored the potential for enhancing the off-road mobility of unmanned ground vehicles (UGV) equipped with a hydrostatic drive system. The analysis showed that effectively overcoming rough or soft terrain demands a slip limitation. In the UGVs with hydrostatic drives, flow dividers are used for this purpose. Unfortunately, they have certain drawbacks, such as reduced efficiency due to pressure losses. In order to minimize this phenomenon, an external braking system was used as a new slip control system. Therefore, simulation studies were carried out to assess the new slip control system while overcoming terrain obstacles due to the reduction of energy consumption and improving the mobility of the UGV.

FlipWalker: Self-Inverting Robot Locomotion Inspired by ‘Jacob’s Ladder’ Falling Block Illusion Toy

This study presents a novel, underactuated robot locomotion system inspired by the Jacob's Ladder illusion toy, aimed at overcoming challenges encountered by wheeled locomotion on rugged and soft terrains. The system comprises interconnected body segments using flexible wires, offering inherent robustness and scalability due to its simple design. The motion is achieved through servo-controlled rotations, with joint locations adapting as segments pivot around each other. These segments utilize wires as hinges and tension-based constraints, enabling perpetual rotations, thereby propelling the robot forward while remaining flat on the ground. The manufacturing process involves rapid 3D printing for housing and CNC machining for PCB board fabrication. Flexible hinges are attached using threaded bolts, providing tension adjustability. The robot's microcontroller setup involves separate controllers for each body segment, communicating with a central offboard microcontroller for sensor data aggregation, configuration determination, and servo control. This modular design ensures easy addition and removal of segments without disrupting the overall electrical connectivity. The developed locomotion system showcases potential for enhanced adaptability and performance in challenging terrains.

Design of a variable-diameter wheel with a novel distributed compliant mechanism

Compliant variable-diameter mechanisms enable adaptation of variable-diameter wheels to both road and soft terrain. Conventional compliant variable-diameter mechanisms, in which every wheel-foot is an independent load-bearing structure, rely heavily on rigid-body mechanisms. To overcome the structure restriction and improve their performances, we introduce a novel compliant mechanism in which circumferentially distributed leaf springs or combination springs with large deformation range are used as flexible components. The springs constitute the closed outer ring and the overall load-bearing structure in the wheel. A novel screw transmission mechanism was designed to achieve motions with two degrees of freedom. Based on the geometric relationships, a dimensional design method for the mechanism is proposed. The chained beam-constraint-model and integration methods are introduced to obtain the spring deflections and maximum hub rotation angle. These methods can be used for further design of the transmission mechanism. The wheel design is then verified experimentally. The results indicate that the required design targets were achieved. Thus, the proposed mechanism diversifies the structure type and design of compliant mechanisms and expands the applicability of variable-diameter wheels in mobile platforms.

State estimation and traversability map construction method of a quadruped robot on soft uneven terrain

AbstractQuadruped robots show excellent application prospects in complex environment detection and rescue. At present, scholars mainly focus on quadruped walking in rigid environments. However, quadruped robots often need to pass through uneven and soft unconstructed terrains, prone to slip and impact. The mismatch between the planned foothold position and the real one resulting from environmental uncertainties makes the robot unstable. In this paper, the state estimation and traversability map construction methods are proposed for quadruped robots to achieve stable walking in an unstructured environment, especially on soft terrains. First, the Error‐state Kalman Filter (ErKF) is extended by optimizing the leg odometry information to get an accurate robot state, especially in soft, uneven terrain. The ErKF method fuses the sensor data from the inertial measurement unit, laser, camera, and leg odometry. The leg odometry is optimized by considering the foot slippage, which easily occurs in soft uneven terrains. Then, the unstructured environment is parameterized and modeled by the terrain inclination, roughness, height, and stiffness. A traversability map, which is essential for robot path and foothold planning in autonomous movement, is constructed with the above parameters. Finally, the proposed method is verified by simulation and experiments. The results show that the quadruped robot can walk stably on different soft and uneven terrains.

Study on climbing strategy and analysis of Mars rover

AbstractTo ensure the safety and efficiency of Zhurong Mars rover when climbing a slope on Mars, the forces of the rover under four climbing methods, which are normal climbing, Z‐type climbing, diagonal climbing, and bionic wriggle climbing, are analyzed. Each method corresponds to different maximum climbing slopes. The experiments are carried out with a backup rover on dense and soft terrains to determine the range of climbing slope for different climbing methods. According to the slope, peak current, cost of transport, and state of terrain, the climbing strategy is given. For dense and soft terrains, the soil cohesive is 0.99 and 1.4 kN/mn+1 and soil friction modules are 1528 and 700 kN/mn+2, respectively. Specifically, normal climbing is recommended for low‐range slopes, while Z‐type or diagonal climbing are suggested for medium‐range slopes, and bionic wriggle climbing is found to be optimal for high‐range slopes. To ensure the safety of the Zhurong Mars rover, it fails climbing if the critical values are exceeded. These results provide valuable insights for human operators when planning the rover's slope‐climbing actions on Mars.

Hierarchical MPM-ANN Multiscale Terrain Model for High-Fidelity Off-Road Mobility Simulations: A Coupled MBD-FE-MPM-ANN Approach

AbstractA new hierarchical multiscale terrain model is developed using the material point method (MPM) to enable effective modeling of large terrain deformation for high-fidelity off-road mobility simulations. Unlike the Lagrangian finite element (FE) model, MPM allows for modeling large deformation of a continuum without mesh distortion using material points as moving quadrature points for the background grid. This unique feature is extended to account for complex granular soil material behavior with the hierarchical multiscale modeling approach in the context of off-road mobility simulations. The grain-scale discrete-element (DE) representative volume element (RVE) and its neural network surrogate model (artificial neural network (ANN) RVE) are developed and introduced to the upper-scale MPM model through the scale-bridging algorithm. The DE RVE is used to generate training data for the ANN RVE, allowing for predicting the history-dependent grain-scale soil material behavior efficiently at every material point that moves through the upper-scale MPM background grid. A numerical procedure for modeling the interaction of the nonlinear FE tire model with the MPM-ANN multiscale terrain model is developed considering moving soil patches generalized for the upper-scale MPM terrain model. It is fully integrated into the general off-road mobility simulation framework by leveraging scalable high-performance computing techniques. The predictive ability of the proposed MPM-ANN multiscale off-road mobility model is examined and validated against the full-scale vehicle test data, involving large deformation of soft terrain. The computational benefit from the neural network surrogate model is also demonstrated.

Strength Analysis of Eight-Wheel Bogie of Bucket Wheel Excavator

Crawler travel gear is a type of heavy vehicle propulsion that is commonly found in tanks, excavators, and specialized off-road vehicles. They have an advantage over wheels when it comes to robust vehicle weight distribution over soft terrain, and some disadvantages as well. They can damage paved roads and have complex design so, considering the enormous weight they must carry, their reliability must be determined and verified. The main parts of the assembly are the drive wheels, which move the crawler, and the supporting structure that holds four-wheel bogies and two-wheel bogies. In this paper, we present a methodology for FEM analysis of parts of an eight-wheel bogie according to DIN 22261-2 standard.

Teleoperation of Wheeled Mobile Robot With Dynamic Longitudinal Slippage

The increasing demand for a wheeled mobile robot (WMR) in many special fields (e.g., planetary exploration) has generated new difficulties, one of which is caused by the wheel longitudinal slippage on soft terrains. Compared with the general teleoperation under wheel pure rolling (no slippage), the introduction of wheel slippage creates new issues that potentially cause WMR instability. A new bilateral teleoperator for WMR coupling with dynamic longitudinal slippage is presented in this article. The traditional dynamic model is augmented in the presence of wheel slippage, whereas the slippage-induced unstable elements are determined through passivity analysis. To compensate for the active energy at the slave site, this article proposes a conservative slippage-dependent local controller without online estimation for wheel slippage. The teleoperator is then designed by guaranteeing the passivity of the entire teleoperation system. Experiments validate that the proposed methods can result in a stable system on soft terrains with good performance.

Teorijski model opšteg slučaja kretanja brzohodnih guseničnih vozila

This paper presents a systematically derived theoretical model for the motion of tracked vehicles, encompassing various scenarios such as soft terrain and slopes. The model considers all the resistances encountered by tracked vehicles, including track-terrain interaction, grade and air resistance, centrifugal force, inertial forces, and turning resistance. The heading angle determines the geometrical orientation of the vehicle on the slope. A MATLAB model of tracked vehicle general motion was established based on theoretical discussions. The paper discusses the steering performance of tracked vehicles on slopes of different inclinations. A simulation of steering with different turning radiuses was also presented. It is shown that the proposed model can be used for predicting and evaluating the steering performance of tracked vehicles with adequate accuracy. Furthermore, due to its high computational efficiency, the proposed model can be utilized for power demand modeling in research on the hybridization or electrification of tracked vehicles.

Development of an integrated active yaw controller on soft terrain

Development of an integrated active yaw controller on soft terrain

Development of an integrated active yaw controller on soft terrain

Development of an integrated active yaw controller on soft terrain

Multibody Modeling of a New Wheel/Track Reconfigurable Locomotion System for a Small Farming Vehicle

Tracks and wheels are the most widely used running gear for the locomotion of agricultural vehicles. The main difference between the two systems is the contact area with the ground and, consequently, the pressure distribution. Evaluating the pressure distribution on the ground is important because soil damage and vehicle performance depend on it. This analysis is especially difficult for tracked vehicles, owing to their complexity compared with wheeled systems. In this paper, we describe a multibody model of a flexible track to evaluate the pressure distribution upon contact with soft terrain. The track considered in this study is part of a reconfigurable locomotion system of a small farming vehicle, which can vary the pressure distribution by switching from a wheeled vehicle to a half-tracked vehicle. The aim of such a vehicle is to minimize soil damage in addition to optimizing its performance. The model is used to characterize this vehicle and evaluate the pressure distribution with varying characteristic parameters, such as track tension, the position of the vehicle’s center of gravity, the weight distribution on the track itself, and the stiffness of the suspension system.

Interaction mechanics model for screw‐drive wheel of granary robot traveling on the loose grain terrain

AbstractThe screw drive mechanism has excellent driving performance on soft terrain and it is a good application for granary robot traveling on the loose grain surface. The investigation on screw drive wheel‐terrain contact mechanics is necessary to predict the traveling performance of the novel Screw‐Drive Granary Robot. Because the motion of grain particles is extremely complex beneath the wheel in the space, shearing displacement is decomposed into the longitudinal and tangential shearing displacement in this paper, and the corresponding shearing stress is calculated separately. Then the screw‐drive wheel‐terrain contact mechanics theory based on terramechanics is established by integrating the normal stress and shearing stress. Through driving experiments on corn‐terrain, the slip sinkage is determined and the sinkage modeling is modified simultaneously considering both vertical load and slip to improve the accuracy of prediction. And the calculated total resistance and the bulldozing resistance show that there is a linear proportional coefficient between them and slope of the line is 2.74 in this case. Finally, the improved mathematical model for screw‐drive wheel is proposed, the average error of predicted resistive torque is 15.72% and the mean error of drawbar pull is 16.2%.

Agricultural tyre stiffness change as a function of tyre wear

Vehicle designers use tyre characteristic parameters to parameterize tyre models during the design phase of a vehicle to determine the ride comfort, handling and performance. Tyre data is mostly presented only for new tyres with 100% tread on them. Soft terrain traction and minimal compaction have always been major requirements for agricultural tyres, however agricultural vehicles are increasingly being used on asphalt/non-deformable terrain as commercial farms are ever increasing in size. As operational costs are always forced to a minimum, agricultural vehicles operate with tyres ranging from new to fully used condition ranging from 100% down to almost 0% tread on them. During the operational life of a tyre the tyre characteristics change as the tyre tread wears down. This study investigates the change in tyre characteristics on hard terrain with tyre wear at a single load condition, two different inflation pressures and three tread wear conditions. Significant trends are noted with dramatic changes in traction and tyre stiffnesses as the tyre wear changes.

Velocity Following Control of a Pseudo-Driven Wheel for Reducing Internal Forces Between Wheels

The coordination of multiple driving wheels is an important issue in wheeled mobile robot design, for both maximizing tractive capability and optimizing energy consumption. This study converts a driving wheel into a pseudo-driven wheel (PDW), which is controlled to follow the motion of the robot body, in accordance with the kinematic constraints. To eliminate the internal force conflicts between wheels, a velocity following control (VFC) method is proposed to control the PDW in the presence of force disturbance caused by soil distortion during wheel-terrain interaction. The feasibility of the PDW conversion and the proposed control method are experimentally demonstrated. Compared with the inverse kinematics control (IKC) method, the rover using PDW with VFC is shown to be capable of more accurate straight and steering path following on soft terrain, reducing internal forces between wheels by more than 70% in the experiments.

Simulation of Track-Soft Soil Interactions Using a Discrete Element Method

With the development of unmanned tracked vehicles, soil model predictions of soft terrains are becoming more essential. In order to accurately simulate the interaction characteristics between soil particles and the track, soil modeling with a discrete element method (DEM) is proposed. Volume-based scaled-up modeling of DEM soil particles and the calibration of DEM input parameters were investigated as a feasible approach to realizing many particle calculations. Calibration of DEM input parameters can solve the distortions between actual and DEM particle sizes. Cohesion and friction parameters of the scaled-up soil particle model were recalibrated by the shape accumulated through the virtual design of the experiment. Soil DEM particles were scaled up to 1 mm spherical particles, and recalibrated DEM parameter values were used to match the actual accumulated soil shape. Three calibrated scaled-up soil models were used for the shear stress–displacement DEM simulation of a track segment, and the mean absolute percentage error (MAPE) was less than 11% compared with the actual shear stress–displacement test. The parameter value of soil traction performance empirical model of a tracked vehicle is modified according to the soil shear stress–displacement DEM simulation. Comparative analysis was performed for travel test results of a tracked vehicle; the relative error of the soil traction prediction results to actuals was less than 16.8%. This showed that the volume-based particle scaling technique is an effective DEM for the mechanical simulation of soil.