An indoor traction measurement system for agricultural tires

To reliably study soil–wheel interactions, an indoor traction measurement system that allows creation of controlled soil conditions was developed. This system consisted of: (i) single wheel tester (SWT); (ii) mixing-and-compaction device (MCD) for soil preparation; (iii) soil bin; (iv) traction load device (TLD). The tire driving torque, drawbar pull, tire sinkage, position of tire lug, travel distance of the SWT and tire revolution angle were measured. It was observed that these measurements were highly reproducible under all experimental conditions. Also relationships of slip vs. sinkage and drawbar pull vs. slip showed high correlation. The tire driving torque was found to be directly influenced by the tire lug spacing. The effect of tire lug was also discussed in terms of tire slip.     

Tractive performance of a bulldozer running on weak ground

To determine the tractive performance of a bulldozer running on weak ground in the driven state, the relations between driving force, drawbar pull, sinkage, eccentricity and slip ratio have been analysed together with each energy balance; effective input energy, sinkage deformation energy, slippage energy and drawbar pull energy. It is considered that the thrust is developed not only on the main straight part of the bottom track belt but also on parts of the front idler and rear sprocket, and the compaction resistance is calculated from the amount of slip sinkage. For a given vehicle and soil properties, it is determined that the drawbar pull increases directly with the slip ratio and reaches about 70% of the maximum driving force. The compaction resistance reaches about 13% of the maximum driving force. The sinkage of the rear sprocket, the eccentricity, and the trim angle increase with the increment of slip ratio due to the slip sinkage. These analytical results have been verified experimentally. After determining the optimum slip ratio to obtain a maximum effective tractive power, it is found that a larger optimum drawbar pull at optimum contact pressure could be obtained for a smaller eccentricity of vehicle center of gravity and a larger track length-width ratio under the same contact area.     

Effect of design parameters on the performance of rigid traction wheels on saturated soils

A dimensional analysis was carried out to study the effect of individual wheel parameters, namely the lug angle, lug height, rim width and lug spacing on the traction performance of rigid wheels in saturated soils. The performance of the test wheels was evaluated on the basis of drawbar pull, slip and torque data obtained at different normal loads ranging between 50 and 100 kg (790–980 N). The data were utilized to compute the performance values such as tractive efficiency and overall performance index. Through the regression analysis, the optimum values of lug angle, rim width and lug spacing were found to be 20°, 200 mm and 110 mm respectively for a wheel of 685 mm dia. However, a definite conclusion regarding the optimum value of lug height could not be drawn, though the analysis for higher loads indicated this value as of 38 mm. The wheel parameter most influencing the traction performance of the wheel was found to be the rim width.     

Experimental study and analysis on driving wheels’ performance for planetary exploration rovers moving in deformable soil

Planetary rovers are different from conventional terrestrial vehicles in many respects, making it necessary to investigate the terramechanics with a particular focus on them, which is a hot research topic at the budding stage. Predicting the wheel-soil interaction performance from the knowledge of terramechanics is of great importance to the mechanical design/evaluation/optimization, dynamics simulation, soil parameter identification, and control of planetary rovers. In this study, experiments were performed using a single-wheel testbed for wheels with different radii (135 and 157.35 mm), widths (110 and 165 mm), lug heights (0, 5, 10, and 15 mm), numbers of lugs (30, 24, 15, and 8), and lug inclination angles (0°, 5°, 10°, and 20°) under different slip ratios (0, 0.1, 0.2, 0.3, 0.4, 0.6, etc.). The influences of the vertical load (30 N, 80 N, and 150 N), moving velocity (10, 25, 40, and 55 mm/s), and repetitive passing (four times) were also studied. Experimental results shown with figures and tables and are analyzed to evaluate the wheels’ driving performance in deformable soil and to draw conclusions. The driving performance of wheels is analyzed using absolute performance indices such as drawbar pull, driving torque, and wheel sinkage and also using relative indices such as the drawbar pull coefficient, tractive efficiency, and entrance angle. The experimental results and conclusions are useful for optimal wheel design and improvement/verification of wheel-soil interaction mechanics model. The analysis methods used in this paper, such as those considering the relationships among the relative indices, can be referred to for analyzing the performance of wheels of other vehicles.     

Effect of circumferential angle, lug spacing and slip on lug wheel forces

This study was conducted to investigate the effect of circumferential angle, lug spacing and wheel slip on forces produced by a cage wheel. Experiments were conducted in a laboratory soil bin having Bangkok Clay soil with 51% (d.b.) soil moisture content. Six ring-type loadcells were used to measure the soil horizontal, vertical and transverse reactions on the cage wheel lugs. The circumferential angle was varied from 0, 15, 30 to 45°. The lug spacing and wheel slip were varied from 20, 30 to 40° and 20, 35 to 50% respectively. All the force measurements were done at a constant 7 cm sinkage. The results showed that increasing circumferntial angle up to 45° can reduce variation in lug wheel forces, at the same time it had little effect on the mean pull and lift values. The side force was affected by the changes of circumferential angle. The 20° lug spacing not only gave the minimum variations but also maximum mean lug forces. The highest lug wheel forces occurred at 35% wheel slip.     

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.     

Improvement of a power tiller cage wheel for use in swampy peat soils

This study was aimed at investigating traction performance of a cage wheel for use in swampy peat soils in Indonesia. The tests were conducted in a soil bin filled with peat soil taken from the swampy areas. A set up was developed to measure tractive performance of a single cage wheel. Deep sinkage and high wheel slip were identified as the major problems of using the existing cage wheel design in swampy peat soils. The results revealed that increasing the lug angle from 15 to 35° and the length of lug improved the tractive performance of the cage wheel significantly, while increasing the number of lugs from 14 to 18 and width of lug did not improve the tractive performance significantly. A cage wheel with lug size 325×80 mm, 35° lug angle, 14 lugs (26° lug spacing), with 2 circumferential flat rings installed on the inner side of the lugs, out performed the other settings for use with power tillers in swampy peat soils.     

Analysis of Mars Exploration Rover wheel mobility processes and the limitations of classical terramechanics models using discrete element method simulations

A previous three-dimensional discrete element method (DEM) model of Mars Exploration Rovers (MERs) wheel mobility demonstrated agreement with test data for wheel drawbar pull and sinkage for wheel slips from 0.0 to 0.7. Here, results from the previous model are compared with wheel mobility data for non-MER wheels that cover the range of wheel slip from 0.0 to 1.0. Wheel slips near 1.0 are of interest for assessing rover mobility hazards. DEM MER wheel model predictions show close agreement with weight-normalized wheel drawbar pull data from 0.0 to 0.99 wheel slip and show a similar trend for wheel sinkage. The nonlinear increase in MER wheel drawbar pull and sinkage for wheel slips greater that 0.7 is caused by development of a tailings pile behind the wheel as it digs into the regolith.Classical terramechanics wheel mobility equations used in the ARTEMIS MER mobility model are inaccurate above wheel slips of 0.6 as they do not account for the regolith tailings pile behind the wheel. To improve ARTEMIS accuracy at wheel slips greater that 0.6 a lookup table of drawbar pull, wheel torque, and sinkage derived from DEM mobility simulations can be substituted for terramechanics equation calculations.     

Measurement of soil reaction forces on a single movable lug

In order to clarify characteristics of a new mechanism called a movable lug, a model of a single movable lug equipped with an L-shaped force transducer has been developed. The soil reaction forces (normal and tangential) on a flat single movable lug, a curved single movable lug and a fixed lug were measured on wet sandy loam soil in the laboratory soil bin test. These measured forces then were converted to lug pull and lift forces. The pull and lift forces obtained by the flat movable lug with 45° lug inclination angle and the curved movable lug were higher than those of the fixed lug. It was observed that the lift force of the fixed lug achieved its peak and dropped earlier than those of the movable lugs. However, the peaks of pull and lift forces of the flat and curved movable lugs were almost the same. The flat movable lug with 45° lug inclination angle generated a slightly higher peak of pull force than those with 30° and 60° lug inclination angles. However, the higher lug inclination angle produced, the lower peak of lift force. It was observed that the pull and lift forces increased as the sinkage increased. In contrast to the flat movable lug with 45° lug inclination angle, the curved movable lug produced greater lift force especially at high sinkage. The increase in lug slip from 5% to 25 and 50% caused an increase in the peaks of pull and lift forces. The soil moisture content affected the lug forces significantly.     

Experimental analysis of soil reaction on a lug of a movable lug wheel

A movable lug wheel was tested in a soil bin test apparatus to determine its traction performance and to measure the soil reaction forces on its lugs. Similar tests were also conducted using a fixed lug wheel. The effects of the lug motion pattern, lug spacing and horizontal load on pull and lift forces were studied. From the experiments it is confirmed that the movable action of the lug plate could generate superior pull and lift forces in comparison with the fixed lug wheel. Among the test wheels, lug motion pattern-2 generated the highest pull and lift forces. Within the range of the test conditions, there was no significant difference in pull and lift forces of the lug plate between the test lug wheels with 12 lugs and 15 lugs at the same level of horizontal and vertical loads. The increase of horizontal load up to 200 N generally increased the pull force and generated smaller rolling resistance before the lug left the soil, but did not increase the lift force significantly. The patterns of pull force, lift force and drawbar pull generated under a constant slip were slightly different from those under a constant horizontal load. Finally, the results were also elucidated by their actual lug trajectories in soil.     

Study of the influence of vehicle parameters on tractive performance in deep snow

This paper describes an experimental study of tractive performance in deep snow, carried out with a new special skid steered tracked vehicle, developed by Bodin [1]. The vehicle design parameters studied include the influence of the ground clearance of the vehicle belly and the longitudinal location of the centre of gravity on tractive performance in deep snow, as well as the effect of initial track tension. The most important results from the test show that an increase in the ground clearance has a positive effect on the drawbar pull, originating from a greater increase in the thrust than in the track motion resistance and a slight decrease in the belly drag. Tests of the longitudinal location of the centre of gravity show that a location ahead of the midpoint of the track contact length is to be preferred. The drawbar pull increases with the centre of gravity moving forward. This is due to a reduced track motion resistance, a slight decrease in the belly drag and an almost constant vehicle thrust. The reason for the decreased track motion resistance and belly drag with the centre of gravity located ahead of the midpoint of the track contact length is a decreased vehicle trim angle.     

Development of a tracked vehicle to study the influence of vehicle parameters on tractive performance in soft terrain

This paper describes a new special tracked vehicle for use in studying the influence of different vehicle parameters on mobility in soft terrain; particularly muskegg and deep snow. A field test in deep snow was carried out to investigate the influence of nominal ground pressure on tractive performance of the vehicle. The vehicle proved useful for studying vehicle parameters influencing the tractive performance of tracked vehicles. The tests show that the nominal ground pressure has a significant effect on the tractive performance of tracked vehicles in deep snow. The decrease in drawbar pull coefficient when the nominal ground pressure is increased and originates at about the same amount from a decrease of the vehicle thrust coefficient, an increase of the belly drag coefficient and an increase of the track motion resistance coefficient.     

Mobility algorithm evaluation using a consolidated database developed for wheeled vehicles operating on dry sands

A substantial number of laboratory and field tests have been conducted to assess performance of various wheel designs in loose soils. However, there is no consolidated database which includes data from several sources. In this study, a consolidated database was created on tests conducted with wheeled vehicles operating in loose dry sand to evaluate existing soil mobility algorithms. The database included wheels of different diameters, widths, heights, and inflation pressures, operating under varying loading conditions. Nine technical reports were identified containing 5253 records, based on existing archives of laboratory and field tests of wheels operating in loose soils. The database structure was assembled to include traction performance parameters such as drawbar pull, torque, traction, motion resistance, sinkage, and wheel slip. Once developed, the database was used to evaluate and support validation of closed form solutions for these variables in the Vehicle Terrain Interface (VTI) model. The correlation between predicted and measured traction performance parameters was evaluated. Comparison of the predicted versus measured performance parameters suggests that the closed form solutions within the VTI model are functional but can be further improved to provide more accurate predictions for off-road vehicle performance.     

Characteristics of normal and tangential forces acting on a single lug during translational motion in sandy soil

Lugs (i.e., grousers) are routinely attached to the surfaces of wheels/tracks of mobile robots to enhance their ability to traverse loose sandy terrain. Much previous work has focused on how lug shape, e.g., height, affects performance; however, the goal of this study is to experimentally confirm the effects of lug motion on lug–soil forces. We measured normal and tangential forces acting on a single lug as functions of inclination angle, moving direction angle, sinkage length, horizontal displacement, and traveling speed. The experimental results were mathematically fitted by using least square method to facilitate quantitative analyses on effects of changes in these motion parameters. Moreover, we compared the measured tangential forces to values calculated from a conventional tangential force model to evaluate the effects of the lug-tip surface, which is generally ignored in existing terramechanics models. The conclusions from this study would be useful for estimating the traveling performance of locomotive mechanisms equipped with lugs, modeling interaction mechanics between lugged wheels and soil, etc.     

The characteristics of soil reaction forces on a single movable lug

In order to understand clearly the characteristics of the soil reaction forces on a single movable lug, the resultant of measured soil reaction forces was determined and presented along with its position on the lug plate. The resultant of soil reaction forces acting on the movable lug increased gradually and reached the maximum value when the lug was on about its lowest position in the soil, then it decreased without offering any downward resistance to the lug till the lug left the soil. The maximum resultant force of the movable lug was higher than that of a fixed lug. The point of action of the resultant force on the movable lug shifted in a similar way in all test cases, that is, it moves to the center of the lug from the outer tip until it reaches the position where it becomes the maximum, then it moves to the outer tip till the lug leaves the soil. The inclination angle of the resultant force increased with the decrease of lug inclination angle. The bigger lug sinkage of the movable lug produced bigger soil reaction forces and shifted the point of action of the resultant force from the tip part to the central part of the lug. However, there was no significant effect of the lug sinkage on the direction of the resultant force. The increase in lug slippage from 25% to 50% brought bigger soil reaction forces on the movable lug, but did not influence the direction and point of action of the resultant force.     

Tire-soil interaction model for turning (steered) tires

A review of the experimental information on the development of lateral forces on tires traveling at an angle to their center plane is presented and the usefulness of the consideration of the lateral forces for the development of an analytical model is evaluated. Major components of the lateral force have been identified as the forces required to balance the tractive force and the drawbar pull vectorially. The lateral forces are generated by the shear stresses developing in the contact area and the horizontal component of the normal stresses acting on the in-ground portion of the curved side walls of the tire. The tire-soil interaction model for steady state straight travel has been expanded to include the necessary algorithms for the calculation of these lateral forces. The pattern of tractive force-slip and longitudinal-lateral force relationships is in general agreement with experiments.     

Modeling,calibration and validation of tractive performance for seafloor tracked trencher

Shear stress–displacement model is very important to evaluate the tractive performance of tracked vehicles. A test platform, where track segment shear test and plate load test can be performed in bentonite–water mixture, was built. Through analyzing existing literatures, two shear stress–displacement empirical models were selected to conduct verification tests for seafloor suitability. Test results indicate that the existing models may not be suitable for seafloor soil. To solve this problem, a new empirical model for saturated soft-plastic soil (SSP model) was proposed, and series shearing tests were carried out. Test results indicate that SSP model can describe mechanical behavior of track segment with good approximation in bentonite–water mixture. Through analyzing main external forces applied to test scaled model of seafloor tracked trencher, drawbar pull evaluation functions was deduced with SSP model; and drawbar pull tests were conducted to validate these functions. Test results indicate that drawbar pull evaluation functions was feasible and effective; from another side, this conclusion also proved that SSP model was effective.     

Experimental study of a tracked mobile robot’s mobility performance

This paper proposes an experimental method of predicting the traction performance of a small tracked mobile robot. Firstly, a track-terrain interaction model based on terramechanics is built. Then, an experimental platform of the tracked robot is established, on which the measurement methods of the parameters that influencing the accuracy of the prediction model are introduced and the data post-processing are improved, including drawbar pull, slip ratio, sinkage, track deformation and so on. Based on the experimental data, several key terrain parameters are identified. With the tracked robot platform, the drawbar pull-slip ratio relationship is tested, and the effects on drawbar pull considering different kinds of terrain and the influence of the grousers are analyzed as well. The research results provide a reference for the experimental study on the traction performance of small tracked robots.     

Discrete element modelling for wheel-soil interaction and the analysis of the effect of gravity

The discrete element method (DEM) is widely seen as one of the more accurate, albeit more computationally demanding approaches for terramechanics modelling. Part of its appeal is its explicit consideration of gravity in the formulation, making it easily applicable to the study of soil in reduced gravity environments. The parallel particles (P²) approach to terramechanics modelling is an alternate approach to traditional DEM that is computationally more efficient at the cost of some assumptions. Thus far, this method has mostly been applied to soil excavation maneuvers. The goal of this work is to implement and validate the P² approach on a single wheel driving over soil in order to evaluate the applicability of the method to the study of wheel-soil interaction. In particular, the work studies how well the method captures the effect of gravity on wheel-soil behaviour. This was done by building a model and first tuning numerical simulation parameters to determine the critical simulation frequency required for stable simulation behaviour and then tuning the physical simulation parameters to obtain physically accurate results. The former were tuned via the convergence of particle settling energy plots for various frequencies. The latter were tuned via comparison to drawbar pull and wheel sinkage data collected from experiments carried out on a single wheel testbed with a martian soil simulant in a reduced gravity environment. Sensitivity of the simulation to model parameters was also analyzed. Simulations produced promising data when compared to experiments as far as predicting experimentally observable trends in drawbar pull and sinkage, but also showed limitations in predicting the exact numerical values of the measured forces.     

Development of a data acquisition system for measuring the characteristics of real time forces by cage wheels

A new data acquisition system was introduced that could be used to monitor the real time wheel forces to solve the limitations of obtaining precise performance characteristics of actual cage wheels. Contrary to previous methods, in which the cage wheel forces were obtained by summing up the individual lug forces. The new method enables measurement of the components of lug force in three orthogonal directions simultaneously. A single unit dynamometer system, with two extended octagonal rings was designed and fabricated using a solid mild steel block, was able to measure force up to 5 kN in each direction. It was used in a soil-bin test rig to determine the characteristics of the forces produced by a cage wheel with opposing circumferential lugs. The characteristics of the pull and lift forces agreed with measured drawbar pull and calculated wheel forces respectively. The force signals fluctuated periodically with rotation angle and the corresponding period approximately equal to the interval of angular lug spacing. The side force fluctuated between positive and negative values and the average was closer to zero due to the balancing effect of opposing lugs. The new system showed better output compared to the previous attempts, confirming its applicability for accurate measurement of real time wheel forces.