Measuring the mobility parameters of forwarders using GPS and CAN bus techniques

Use of a datalogger connected to the CAN bus of an 8-wheeled forwarder’s hydrostatic transmission permits measurement of the gross power on the driveline and the rotational velocity of the drive axle. The ground velocity and trajectory of the forwarder is monitored by the GPS technique. These data allow the total resistance force and wheel slip to be determined under different terrain conditions. In this paper the total resistance force is segregated into rolling, slope and winding resistances. The measuring system was tested in practice on an even tarmac surface and a gently sloping hard earth road, the number of input variables in the first tests having been reduced. It was found that the measuring accuracy permitted detection of about a half percent variation in slope inclination in the test lane. Tests on a figure of 8 track showed that the winding resistance can also be detected. It was concluded that the measuring technique is accurate enough for terramechanical research and practicable under real forestry conditions.     

Turning characteristics of multi-axle vehicles

This paper presents a mathematical model for multi-axle vehicles operating on level ground. Considering possible factors related to turning motion such as vehicle configuration and tire slip velocities, equations of motion were constructed to predict steerability and driving efficiency of such vehicles. Turning radius, slip angle at the mass center, and each wheel velocity were obtained by numerically solving the equations with steering angles and average wheel velocity as numerical inputs. To elucidate the turning characteristics of multi-axle vehicles, the effect of fundamental parameters such as vehicle speed, steering angles and type of driving system were examined for a sample of multi-axle vehicles. Additionally, field tests using full-scale vehicles were carried out to evaluate the basic turning characteristics on level ground.     

A simulation model for the prediction of the ground pressure distribution under tracked vehicles

A computer based simulation model for the prediction of the ground pressure distribution beneath tracked vehicles under static conditions has been developed. The model can differentiate between various track designs and is based on an analytical method developed and described by Garber and Wong. Simulating the model with the parameters of a rubber tracked forestry vehicle (FARMI TRAC 5000) led to several conclusions. The road wheel arrangement has a considerable effect on the ground pressure distribution: increasing the number of road wheels reduces the maximum ground pressure and improces the uniformity of the pressure distribution. The radius of the road wheel, the stiffness of the suspension and the stiffness of the track tensioning device have an insignificant effect on the ground pressure distribution. In contrast, the initial track tension and the width of the track have a significant effect on the ground pressure distribution: increasing the initial track tension reduces the maximum ground pressyre and improves the uniformity of the pressure distribution. The same conclusions are valid for an increase of the track width. This model can be used as a tool to assist in the design of off-road vehicles, and is currently being used in the design of forestry vehicles in Ireland.     

Control of a cable-towed vehicle to minimize slip

Cable-towed vehicles have the potential to reduce the costs of harvesting timber on steep or wet terrain. In many situations, these vehicles would perform best when both the wheels and cable winch are powered. This paper describes a control algorithm which minimizes wheel slip by optimizing the partitioning of power between the winch and tractive drives. Results of tests with an experimental vehicle showed that the algorithm was capable of providing nearly zero average wheel slip.     

Further study of the method of approach to testing the performance of extraterrestrial rovers/rover wheels on earth

The current practice for experimentally evaluating the performance of extraterrestrial rovers/rover wheels is to conduct tests on earth on a soil simulant, appropriate to the regolith on the extraterrestrial body of interest. In the tests, the normal load (force) applied by the rover/rover wheel to the soil simulant is set identical to that expected on the extraterrestrial surface, taking into account its acceleration due to gravity. It should be pointed out, however, that the soil simulant used in the tests is subject to earth gravity, while the regolith on the extraterrestrial surface is subject to a different gravity. Thus, it is uncertain whether the performance of the rover/rover wheel obtained from tests on earth represents that on the extraterrestrial surface. This issue has been explored previously. A method has been proposed for conducting tests of the rover/rover wheel on earth with identical mass to that on the extraterrestrial surface, instead of with identical normal load used in the current practice [1]. This paper provides further evidence to substantiate the merits of the proposed method, based on a detailed analysis of the test data obtained under various gravity conditions, produced in an aircraft undergoing parabolic flight manoeuvres [8]. In the study, the effect of slip on wheel sinkage has been evaluated. It is found that gravity has little effect on the slip and sinkage relationship of the rover wheel under self-propelled conditions.     

The effect of velocity on rigid wheel performance

A simulation model to predict the effect of velocity on rigid-wheel performance for off-road terrain was examined. The soil–wheel simulation model is based on determining the forces acting on a wheel in steady state conditions. The stress distribution at the interface was analyzed from the instantaneous equilibrium between wheel and soil elements. The soil was presented by its reaction to penetration and shear. The simulation model describes the effect of wheel velocity on the soil–wheel interaction performances such as: wheel sinkage, wheel slip, net tractive ratio, gross traction ratio, tractive efficiency and motion resistance ratio. Simulation results from several soil-wheel configurations corroborate that the effect of velocity should be considered. It was found that wheel performance such as net tractive ratio and tractive efficiency, increases with increasing velocity. Both, relative wheel sinkage and relative free rolling wheel force ratio, decrease as velocity increases. The suggested model improves the performance prediction of off-road operating vehicles and can be used for applications such as controlling and improving off-road vehicle performance.     

Experimental and DEM analyses on wheel-soil interaction

In this paper, the wheel-soil interaction for a future lunar exploration mission is investigated by physical model tests and numerical simulations. Firstly, a series of physical model tests was conducted using the TJ-1 lunar soil simulant with various driving conditions, wheel configurations and ground void ratios. Then the corresponding numerical simulations were performed in a terrestrial environment using the Distinct Element Method (DEM) with a new contact model for lunar soil, where the rolling resistance and van der Waals force were implemented. In addition, DEM simulations in an extraterrestrial (lunar) environment were performed. The results indicate that tractive efficiency does not depend on wheel rotational velocity, but decreases with increasing extra vertical load on the wheel and ground void ratio. Rover performance improves when wheels are equipped with lugs. The DEM simulations in terrestrial environment can qualitatively reproduce the soil deformation pattern as observed in the physical model tests. The variations of traction efficiency against the driving condition, wheel configuration and ground void ratio attained in the DEM simulations match the experimental observations qualitatively. Moreover, the wheel track is found to be less evident and the tractive efficiency is higher in the extraterrestrial environment compared to the performance on Earth.     

Traveling performance analysis of planetary rovers using a repeatable test system in vacuum

In this study, the effect of a vacuum environment on the traveling characteristics of planetary rovers is experimentally investigated. For this study, a wheel traveling test system that enables a wheel to travel at various speeds and traction loads in a vacuum chamber was developed. In this system, the ground can be prepared before each run, and the wheel can travel in similar ground conditions repeatedly. Subsequently, using this system, traveling tests of the wheel were conducted in the atmosphere and medium vacuum. The wheel was tested at different speeds and various traction loads (i.e., drawbar pulls) on a sandbox filled with three types of material. Wheel slip and sinkage were quantitatively compared in the atmosphere and vacuum. From the experiment, it was confirmed that the medium vacuum environment does not significantly affect the traveling performance of the wheel.     

Performance evaluation of a wire mesh wheel on deformable terrains

Because of the unique lunar environment, a suitable wheel for lunar rover decides the rover’s trafficability on deformable terrains. The wire mesh wheel (hereinafter referred to as WMW) has the advantages of light weight and superior stability, been widely adopted for lunar rovers. But a comprehensive research on performance of WMW on deformable terrains has not been conduct. This paper would provide particular study on a type WMW, including quasi-static pressure-sinkage test and driving performance. A novel pressure-sinkage model for the WMW on deformable soils was presented. In order to investigate the sinkage characteristics of the WMW, tests were performed using a single-wheel testbed for the WMW with different loads and velocities. The effects of load and velocity on sinkage were analyzed, and the relationship between real and apparent sinkage was presented. The research on traction performance of WMW under different slip ratios (0.1–0.6) was also conducted, contrast tests were proceed by using a normal cylindrical wheel (hereinafter referred to as CW). The traction performance of WMW is analyzed using performance indices including wheel sinkage, drawbar pull, driving torque, and tractive efficiency. The experimental results and conclusions are useful for optimal WMW design and improvement/verification of wheel–soil interaction mechanics model.     

Rarefied gas flows based on variational principle

A variational principle has been utilized to study Couette flow, and Kramers' velocity slip problem with specular-diffuse reflection. The method leads to extremely satisfactory analytic results for the velocity slip coefficient and also for the variation of shear stress with inverse Knudsen number. This leads us to the conclusion that the present variational principle even with extremely simple trial functions, essentially suggested by continuum flow theory, is a useful means of computing macroscopic quantities of physical interest in rarefied gas dynamics.     

Traveling and abrasion characteristics of wheels for lunar exploration rover in vacuum

This paper investigates the traveling and abrasion characteristics of rigid wheels for a lunar exploration rover at atmospheric pressure and in a vacuum. For this investigation, a traveling test system that enables the wheel to continuously travel over a long distance was developed. Using this system, tests on traveling performance and abrasion were conducted with the wheel on a lunar regolith simulant surface. In the initial tests, various wheels traveled over different ground conditions and their performances were evaluated based on the relationship between the drawbar pull and slippage. In the later tests, a wheel with grousers traveled a distance of 3 km and the abrasion was analyzed at various intervals. From the traveling performance tests, it was found that for a soft ground condition, the traveling performance of the wheels in vacuum was slightly lower than that in atmosphere. This indicates that ground tests performed in atmosphere overestimate the actual performance on the lunar surface. The abrasion tests suggested that the scratching of wheels occurs more easily in vacuum than in atmosphere. These experiments confirmed that the abrasion of the wheels do not cause any critical problem for a traveling distance of up to 3 km in a simulated lunar environment.     

平移断层的倾角对地震产生的影响

利用边界单元法和滑移弱化摩擦本构关系分析了平移断层上地震的产生。从依赖于速率和状态的摩擦本构关系出发,通过忽略速度的影响得到了滑移弱化摩擦本构关系。建立了两种本构关系之间的联系,使得在两种模型中可以使用共同的参数。通过将地球表面模拟成一个包含在无穷大弹性介质中的无穷大裂纹,已有的边界积分方法可以直接用来分析断层的滑移。由于断层上的摩擦本构关系的非线性,得到的方程也是非线性的,采用牛顿迭代法进行求解。通过数值计算得到了平移断层上滑移位移、速度及摩擦力的分布规律。考察了平移断层的倾角对地震产生的影响,计算结果表明断层的倾角越小,地震产生的位置离地球表面越近且地震产生所需滑移的时间越短。     

Discrete element method analysis of single wheel performance for a small lunar rover on sloped terrain

The purpose of this study is to analyze the performance of a lugged wheel for a lunar micro rover on sloped terrain by a 2D discrete element method (DEM), which was initially developed for horizontal terrain. To confirm the applicability of DEM for sloped terrain locomotion, the relationships of slope angle with slip, wheel sinkage and wheel torque obtained by DEM, were compared with experimental results measured using a slope test bed consisting of a soil bin filled with lunar regolith simulant. Among the lug parameters investigated, a lugged wheel with rim diameter of 250 mm, width of 100 mm, lug height of 10 mm, lug thickness of 5 mm, and total lug number of 18 was found, on average, to perform excellently in terms of metrics, such as slope angle for 20% slip, power number for self-propelled point, power number for 15-degree slope and power number for 20% slip. The estimation of wheel performance over sloped lunar terrain showed an increase in wheel slip, and the possibility exists that the selected lugged wheel will not be able to move up a slope steeper than 20°.     

Soil flow analysis for grouser wheels based on a particle image velocimetry method

This paper presents an analysis, based on a particle image velocimetry method, of soil flow field beneath a grouser wheel traveling over loose soil. Although the grouser wheel is expected to have better traction and mobility over fine, loose soil, its interaction mechanisms with the soil remain to be elucidated. Thus, a particle image velocimetry-based soil flow analysis is conducted to directly observe soil behavior around the grouser wheel. In the experimental analysis, key parameters of the soil flow field, such as general shape, thickness, streamlines of the flow field, soil velocity on the streamlines, and soil failure angle are examined quantitatively. From the results, the soil flow shape periodically changes with wheel rotation, and this change appears, depending on wheel slip varying over time. Furthermore, the experimental result of the soil failure angle differs drastically from its typical theory. These results will contribute to modeling the mechanical interaction between the grouser wheel and soil.     

Concept evaluations of three novel forwarders for gentler forest operations

In-terrain transport of wood from sensitive areas with soft soil has always been difficult to perform in without extensive rutting and soil distortion. To carry out logging in wintertime, on frozen ground, is increasingly difficult as prolonged, rainy fall seasons and milder winters have decreased the window of opportunity for such operations. This creates strong demands for forest machines that can master soft soils. Exposure to whole body vibrations is a serious problem for operators of forest machines, which is also related to the machine-ground interaction. Both the soil and operator benefit from damped chassis suspension.Vibrations and soil distortion indicate loss of tractive energy. Instead of moving the machine, energy is lost in deformation of the ground and tractive elements, adding to motion resistance and to machine vibrations. Three recent developments of concept machines with improved soil interaction properties, reducing soil impact and improving operator comfort are presented. Two rubber tracked forwarders and one wheeled pendulum arm machine with active damping have been constructed, tested and analyzed.Results from standardized rutting and vibration tests of the concept machines are presented. Notable improvements regarding soil impact and rutting, operator comfort and fuel consumption were found. Some remaining technical challenges are also discussed.     

Evaluation of a 6.71 kW power tiller for draft and drawbar power on tar roads

A 6.71 kW power tiller was evaluated for draft and drawbar power on tar roads. The effect of mounting 40 kg of wheel ballast was also studied. Polynomial regression analysis was used to establish the relationship between draft and wheel slip, drawbar power and wheel slip, drawbar power and fuel consumption, and drawbar power and specific fuel consumption. The results of the study showed draft values of 2107, 2110 and 2110 N in second low, third low and first high gears at an engine speed of 150o rpm with a 15% wheel slip. The respective draft values at engine speed of 2000 rpm with a 15% wheel slip were 2172, 2189 and 2212 N. With the mounting of 40kg wheel ballast there was an increase in draft of 217, 207 and 291 N at 1500 rpm, and 328, 306 and 344 N at 2000 rpm of the engine with a 15% wheel slip in second low, third low and first high gears, respectively. The increase in drawbar power with 40 kg ballast was 10.88%, 7.83% and 20.13% at 1500 rpm and 18.89%, 16.56% and 14.88% at 2000 rpm of engine over the drawbar power available with zero ballast. The fuel consumption with the use of wheel ballast was slightly more than the fuel consumption without any ballast.     

Dynamic analysis of turning performance of 4WD-4WS tractor on paved road

In this research, using a 4-wheel-driven, 4-wheel-steered tractor, two experiments and a computer simulation about its turning on paved ground are described. One experiment is on the relation between the wheel turning angle and the steering angle, and the other is a turning experiment to determine the path traced by the vehicle and to check the simulation. The computer simulation with equations of motion was conducted, and some effects, for example of the running speed, on the yaw angular velocity and on the side slip angle of the center of gravity of the tested tractor were analysed.     

Design and development of a new transformable wheel used in amphibious all-terrain vehicles (A-ATV)

Conventional ground-wheeled vehicles usually have poor trafficability, low efficiency, a large amount of energy consumption and possible failure when driving on soft terrain. To solve this problem, this paper presents a new design of transformable wheels for use in an amphibious all-terrain vehicle. The wheel has two extreme working statuses: unfolded walking-wheel and folded rigid wheel. Furthermore, the kinematic characteristics of the transformable wheel were studied using a kinematic method. When the wheel is unfolded at walking-wheel status, the displacement, velocity and acceleration of the wheel with different slip rates were analyzed. The stress condition is studied by using a classic soil mechanics method when the transformable wheel is driven on soft terrain. The relationship among wheel traction, wheel parameters and soil deformation under the stress were obtained. The results show that both the wheel traction and trafficability can be improved by using the proposed transformable wheel. Finally, a finite element model is established based on the vehicle terramechanics, and the interaction result between the transformable wheel and elastic–plastic soil is simulated when the transformable wheel is driven at different unfold angles. The simulation results are consistent with the theoretical analysis, which verifies the applicability and effectiveness of the transformable wheel developed in this paper.     

Tractive performance of a driven rigid wheel on soft ground based on the analysis of soil-wheel interaction

A new analytical method has been presented to predict the tractive performance of a rigid wheel running on soft ground. The resultant stress of the normal stress and the shear resistance applied around the peripherical contact part of the rigid wheel should be calculated by use of the dynamic pressure-sinkage curve measured from the plate loading and unloading test, considering the rolling locus of the wheel in the direction of the external resultant force of the effective driving force and the axle load. The effective driving force could be calculated as the difference of the driving force, i.e. the integration of shear resistance and the locomotion resistance calculated from the total amount of sinkage. As a result, the analytical relations between the driving force, the effective driving force and the slip ratio, the amount of sinkage and the slip ratio, the amount of eccentricity of resultant force and the slip ratio, and the entry angle, the exit angle and the slip ratio could be verified experimentally.