Tractive performance of 4x4 tyre treads on pure sand.

This thesis examined the difficulties of generating traction from 4x4 (light truck) tyres in pure sand conditions. Investigations conducted in the Cranfield University Soil Dynamics Laboratory measured the tractive performance of a range of production and prototype 4x4 tyre tread patterns to quantify the effect of tread features upon tractive performance. The investigation also quantified the amount of sand displacement instantaneously occurring beneath the tyre, by a novel application of radio frequency identification (RFID) technology, which determined sand displacements to an accuracy of ±5.5 mm. A limited number of normal contact stress measurements were recorded using a TekScan normal pressure mapping system. This technology was employed in a new manner that allowed pressure distributions to be dynamically recorded on a deformable soil surface. Models were developed or adapted to predict rolling resistance, gross thrust of a tyre and the gross thrust effect due to its tread. Net thrust was predicted from refined versions of equations developed by Bekker to predict gross thrust and rolling resistance. These were modified to account for dynamic tractive conditions. A new tread model proposed by the author produced a numerical representation of the gross thrust capability of a tread based on factors hypothesised to influence traction on loose sand. This allowed the development of a relationship between the features of the tread and its measured gross thrust improvement (relative to a plain tread tyre), from which a total relationship was developed. The tread features were also, in combination with the wheel slip, related to the sand displacements and net thrusts simultaneously achieved. The sand displacement results indicated that the majority of the variation in displacement between the different treads occurred in the longitudinal (rearward) direction. This effect was influenced by the wheel slip, as increased slip caused greater displacements, so the differences between the treads were greater at higher slips. The treads that generated the highest relative displacements also derived the higher gross thrusts (up to +5% extra gross thrust compared to a plain tread), although at the higher slips this also caused increased sinkage. As sinkage increased, the rolling resistance increased at a fester rate then the gross thrust, and thus the net thrust reduced. To prevent this effect the wheel slip should be limited to a maximum of 20% at low forward speeds (approximately 5 km/h). Current market forces dictate that the biggest benefit that tyre manufacturers could offer in desert market regions would be to optimise road-biased tyres to suit loose sand conditions. The modelling developed indicated that this could be achieved by maximising the number of lateral grooves (and thus lateral edges) featured on a tread, however care would have to be exercised so as not to compromise the necessaiy on-road capability. The models could also be used to quantifiably determine from a choice of possible tyre treads, the tread that would offer most traction on pure loose sand

Grousers Effect in Tracked Vehicle Multibody Dynamics with Deformable Terrain Contact Model

In this work, a multibody model of a small size farming tracked vehicle is shown. Detailed models of each track were coupled with the rigid body model of the vehicle. To describe the interaction between the track and the ground in case of deformable soil, custom defined forces were applied on each link of the track model. Their definition derived from deformable soil mechanics equations implemented with a specifically designed routine within the multibody code. According to the proposed model, it is assumed that the main terrain deformation is concentrated around the vehicle tracks elements. The custom defined forces included also the effects of the track grousers which strongly affect the traction availability for the vehicle. A passive soil failure model was considered to describe the terrain behaviour subjected to the grousers action. A so developed model in a multibody code can investigate vehicle performance and limit operating conditions related to the vehicle and soil characteristics. In this work, particular attention was focused on the results in terms of traction force, slip and sinkage on different types of terrain. Tests performed in the multibody environment show how the proposed model is able to obtain tractive performance similar to equivalent analytical solutions and how the grousers improve the availability of tractive force for certain type of soil characteristics

Terramechanics and soil–wheel interactions for road vehicle applications

The current research concerns the analysis and development of a soil-wheel interaction model intended for application in road vehicles, in order to support virtual vehicle development processes. As a first step, a review of the literature is conducted which reveals the absence of a reliable tyre model for off-road applications. In addition, it highlights two critical performance items for the soil-wheel interaction; tractive effort and rolling resistance. The rolling resistance is generated by soil compaction, horizontal soil displacement and tyre flexibility, while the tractive effort is generated by the soil shearing behaviour at the soil-wheel interface. Existing models for soil compaction (i.e. pressure-sinkage) are initially evaluated for their accuracy and applicability using literature data, but their performance is unsatisfactory. In addition, a large experimental campaign is conducted using two soil types and various experimental processes such as pressure-sinkage on flat and curved plates, shear tests, rolling wheel tests. [Continues.

Analytical and finite element modelling of the dynamic interaction between off-road tyres and deformable terrains

Automotive tyres are one of the main components of a vehicle and have an extremely complex structure consisting of several types of steel reinforcing layers embedded in hyperelastic rubber materials. They serve to support, drive – accelerate and decelerate – and steer the vehicle, and to reduce transmitted road vibrations. However, driving is associated with certain types of pollution due to CO2 emissions, various particles due to tyre wear, as well as noise. The main source of CO2 emissions is the tyre rolling resistance, which accounts for roughly 30% of the fuel consumed by cars. The phenomenon becomes more pronounced in off-road conditions, where truck vehicles are responsible for about a quarter of the total CO2 emissions. Appropriate legislation has been introduced, to control all of these pollution aspects. Therefore, tyre simulation (especially in off-road conditions) is essential in order to achieve a feasible design of a vehicle, in terms of economy and safety. [Continues.

Investigation of all-wheel-drive off-road vehicle dynamics augmented by visco-lock devices

A peculiarity of AWD off-road vehicles is that their behaviour depends not only on the total power, provided by the engine, but also on its distribution among the drive axles/wheels. In turn, this distribution is largely regulated by the drivetrain layout and its torque distribution devices. At the output of the drivetrain system, the torque is constrained by the interaction between the wheels and the soft soil. For off-road automotive applications, the design of drivetrain systems has usually been largely dominated by the mobility requirements. With the growing demand to have a multipurpose on/off road vehicle with improved manoeuvrability over deformable soil, particularly at higher speed, the challenges confronting vehicle designers have become more complex. The thesis presents a novel integrated numerical approach to assess the dynamic behaviour of all-wheel-drive vehicles whilst operating over deformable soil terrain. [Continues.

Methods for Wheel Slip and Sinkage Estimation in Mobile Robots

Future outdoor mobile robots will have to explore larger and larger areas, performing difficult tasks, while preserving, at the same time, their safety. This will primarily require advanced sensing and perception capabilities. Video sensors supply contact-free, precise measurements and are flexible devices that can be easily integrated with multi-sensor robotic platforms. Hence, they represent a potential answer to the need of new and improved perception capabilities for autonomous vehicles. One of the main applications of vision in mobile robotics is localization. For mobile robots operating on rough terrain, conventional dead reckoning techniques are not well suited, since wheel slipping, sinkage, and sensor drift may cause localization errors that accumulate without bound during the vehicle’s travel. Conversely, video sensors are exteroceptive devices, that is, they acquire information from the robot’s environment; therefore, vision-based motion estimates are independent of the knowledge of terrain properties and wheel-terrain interaction. Indeed, like dead reckoning, vision could lead to accumulation of errors; however, it has been proved that, compared to dead reckoning, it allows more accurate results and can be considered as a promising solution to the problem of robust robot positioning in high-slip environments. As a consequence, in the last few years, several localization methods using vision have been developed. Among them, visual odometry algorithms, based on the tracking of visual features over subsequent images, have been proved particularly effective. Accurate and reliable methods to sense slippage and sinkage are also desirable, since these effects compromise the vehicle’s traction performance, energy consumption and lead to gradual deviation of the robot from the intended path, possibly resulting in large drift and poor results of localization and control systems. For example, the use of conventional dead-reckoning technique is largely compromised, since it is based on the assumption that wheel revolutions can be translated into correspondent linear displacements. Thus, if one wheel slips, then the associated encoder will register revolutions even though these revolutions do not correspond to a linear displacement of the wheel. Conversely, if one wheel skids, fewer encoder pulses will be counted. Slippage and sinkage measurements are also valuable for terrain identification according to the classical terramechanics theory. This chapter investigates vision-based onboard technology to improve mobility of robots on natural terrain. A visual odometry algorithm and two methods for online measurement of vehicle slip angle and wheel sinkage, respectively, are discussed. Tests results are presented showing the performance of the proposed approaches using an all-terrain rover moving across uneven terrain

Drawbar Pull (DP) Procedures for Off-Road Vehicle Testing

As NASA strives to explore the surface of the Moon and Mars, there is a continued need for improved tire and vehicle development. When tires or vehicles are being designed for off-road conditions where significant thrust generation is required, such as climbing out of craters on the Moon, it is important to use a standard test method for evaluating their tractive performance. The drawbar pull (DP) test is a way of measuring the net thrust generated by tires or a vehicle with respect to performance metrics such as travel reduction, sinkage, or power efficiency. DP testing may be done using a single tire on a traction rig, or with a set of tires on a vehicle; this report focuses on vehicle DP tests. Though vehicle DP tests have been used for decades, there are no standard procedures that apply to exploration vehicles. This report summarizes previous methods employed, shows the sensitivity of certain test parameters, and provides a body of knowledge for developing standard testing procedures. The focus of this work is on lunar applications, but these test methods can be applied to terrestrial and planetary conditions as well. Section 1.0 of this report discusses the utility of DP testing for off-road vehicle evaluation and the metrics used. Section 2.0 focuses on test-terrain preparation, using the example case of lunar terrain. There is a review of lunar terrain analogs implemented in the past and a discussion on the lunar terrain conditions created at the NASA Glenn Research Center, including methods of evaluating the terrain strength variation and consistency from test to test. Section 3.0 provides details of the vehicle test procedures. These consist of a review of past methods, a comprehensive study on the sensitivity of test parameters, and a summary of the procedures used for DP testing at Glenn

Effect of gravity in wheel/terrain interaction models

[Abstract] Predicting the motion of wheeled robots in unstructured environments is an important and challenging problem. The study of planetary exploration rovers on soft terrain introduces the additional need to consider the effect of non-terrestrial gravitational fields on the forces and torques developed at the wheel/terrain interface. Simply reducing the wheel load under earth gravity overestimates the travelled distance and predicts better performance than is actually observed in reduced-gravity measurements. In this paper, we study the effect of gravity on wheel/terrain interaction. Experiments were conducted to assess the effect of reduced gravity on the velocity profile of the soil under the wheel, as well as on the traction force and sinkage developed by the wheel. It was shown that in the velocity field of the soil, the decay of the tangential velocity component becomes gradual with reducing gravity, and the decay of the normal to rim velocity is slower in Lunar gravity. It was also found that wheel flexibility can have an important effect on the dynamics as the contact patch and effective radius varies periodically. These results were then used together with traditional semi-empirical terramechanics models to determine and validate the simulated drawbar pull values. The developed simulation model includes the effect of wheel flexibility, dynamic sinkage and gravity.MINECO; RYC-2016-2022

PREDICTION OF DISCRETE ELEMENT PARAMETERS FOR MODELING THE STRENGTH OF SANDY SOILS IN WHEEL/SOIL TRACTION APPLICATIONS

The problem of wheel performance on deformable soil has been studied for many years, but prior to the rise of computational mechanics, such investigations have been limited to development of analytical and empirical models, as well as experimental research. Such models have merit but are necessarily highly idealized and are limited in their applications. Today, many computational models have been implemented for a wide variety of wheel/soil applications. For the specific case of sandy (i.e. non-cohesive) soils, in terms of the soil\u27s physics the Discrete Element Method (DEM) provides arguably the most realistic model. In DEM, each element represents a single grain of soil (ideally), or may represent a group of soil particles moving together if necessary. A survey of the literature quickly reveals that DEM is computationally intensive and that a great deal of computational effort is normally spent calibrating the DEM model parameters to the desired characteristics of the soil of interest. The goal of this research was to develop and validate an approach to calibration that would require fewer resources, leaving more resources available for solving the problems of interest. This goal was realized through the collection of data over a range of values for each of five simulation parameters using two-dimensional simulations of the Direct Shear Test. By statistical processes the data was used to develop a set of equations that estimate the properties of interest for the simulated soil (small- and large-strain friction angles), based on the simulation parameters. The equations were used to calibrate a two-dimensional rigid wheel/soil simulation of the Wheel Endurance and Sand Traction Merry-Go-Round System (WEST-MGRS). The calibrated model was found to accurately predict the relative performance between a variety of configurations of grousers on actual wheels operating in sand using WEST-MGRS. Therefore, this research shows that the model can be used as a tool to compare the tractive performance of potential designs. A question that must be answered regarding experiments and simulations of the wheel/soil problem is the question of soil dimensions. Whether simulated or experimental, the system must use a soil container large enough to approximate a semi-infinite soil domain. This research expanded on previous work that had proposed a method for sizing soil dimensions in a dynamic 3-D finite element model. With minor modifications, the method was found to be effective for a wide range of wheel loads and geometries, as well as soil types

OPTIMIZATION OF ROVER WHEEL GEOMETRIES FOR PLANETARY MISSIONS

Rovers have been launched into space for exploration of the Moon and Mars to collect samples of rock and soil. To continue the explorations, the rovers need to have reliable wheels to drive around. However, due to the soil being soft, the wheels on the rover start to lose traction and the wheels sink while driving to various locations. Previous work in this field has been done experimentally or with the use of simulations. Only a few references report the effect of uncertainties in grouser simulation on the traction efficiency. The objective of this work was to (a) Understand the effect of uncertainties on wheel traction efficiency, and (b) Design a rover wheel, consider those uncertainties, and then compare results with deterministic optimization. The results are categorized into three different sections. The first section shows the result of a closed-form equation for rover traction efficiency. A closed-form equation was obtained using three different formulas from previous work. The second section provides results on a reliability analysis to understand the effects of uncertainty on traction efficiency. The uncertainty variables chosen were the empiric soil parameter, , the weight of the wheel, w, and the width of the wheel, b. The third section has a result of using the reliability-based design for the wheel considering those uncertainties, in which the design parameters are the normalized height of the grousers, , the width of the wheel, b, the radius of the wheel, r, and finally the weight of the wheel, w. In the reliability-based optimization there are two variables that are considered uncertain which are not the design parameters, the soil parameter and torque. In the design parameters, the radius of the wheel is considered uncertain. Once the optimized values are obtained, they are compared to the deterministic optimization. As a result, optimized design variables were obtained