Publication news

Journal of Terramechanics, Vol. 48, Issue 5

Alexandr Grečenko
Compression–Sliding approach: Dependence of transitional displacement of a driving element on its size and load
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Pages 325-332, ISSN 0022-4898, 10.1016/j.jterra.2011.06.004.
Abstract: Every element of a pulling traction device (e.g. track shoe with grouser or tire section with lug) exhibits increasing rearward displacement during its engagement with soft ground. Compression–Sliding (CS) approach states in agreement with experimental evidence that on common soft ground this displacement starts due to longitudinal soil compression by the grouser or lug, which steadily increases up to the transitional displacement when the soil segment beneath a driving element fails in shear. Further displacement of a driving element is marked by forced slide of a sheared off soil block, which may eventually collapse. There was justified reasoning that the transitional displacement depends not only on the grouser (lug) contact pressure but also on the area and load of the respective traction element. The presented article reports on experiments designed to test this premise. The measurements applying the novel double plate (DP) meter technique were carried out in a laboratory soil bin containing loam charge of uniform bulk density and moisture content. Three sizes (proportions 1:2:4) and two mean vertical contact pressures (ratio1:2) of DP meter main plate were tested. The analysis of performed experiments confirmed the existence of dimensional and loading relationship “main plate – transitional displacement”, which bears upon the evaluation of thrust–slip relationship of any traction device by the CS approach or by any other method observing the existence of displacement. Keywords: Compression–Sliding approach; Traction device; Transitional displacement; Thrust–slip prediction 

C.D. Singh, R.C. Singh
Computerized instrumentation system for monitoring the tractor performance in the field
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Pages 333-338, ISSN 0022-4898, 10.1016/j.jterra.2011.06.007.
Abstract: A high precision computerized instrumentation package was developed and mounted on a 50 kW tractor to monitor and measure various performance parameters of a tractor and implement system. The system was intended to be used for the compilation of a database of draft requirements of tillage implements. The system designed to measure: three-point linkage forces, ground speed, tillage depth, fuel consumption, forward speed, slip, engine speed, hydraulic pressure and fluid temperatures. The data acquisition unit was based on a high speed multi processors Campbell Scientific CR3000 data logger linked to a microcomputer using suitable transducer. The average calibration constants for the rear wheel speed, fuel consumption and three point linkage transducers were 0.0364 m/pulse, 0.000142857 l/pulse and 0.66 mV/kN respectively. The data acquisition system was capable of scanning rate up to 100 K sample/s. Data acquisition system was developed to measure draft of primary tillage implements in vertisol. Keywords: Instrumentation; Implements; Speed; Pressure; Draft; Tillage 

Iman Ahmadi
Dynamics of tractor lateral overturn on slopes under the influence of position disturbances (model development)
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Pages 339-346, ISSN 0022-4898, 10.1016/j.jterra.2011.07.001.
Abstract: In order to investigate the effects of forward speed, ground slope and wheel–ground friction coefficient on lateral stability of tractor at the presence of position disturbances, a tractor dynamic model was developed. In this model two types of instability were considered: instability due to overturn and skid and for each case the stability index was determined. Different geometries and mass specifications of tractor MITSUBISHI-2501D were used to examine the model. According to the results of this model forward speed and ground slope had a reverse effect on all stability indexes. Moreover stability of this tractor was more affected by tractor skidding than overturning. Therefore to improve the overall stability of this tractor, preference should be on increasing the tractor stability index derived from skid dynamics of tractor. Keywords: Tractor dynamic model; Overturn; Skid; Position disturbance

Martin Obermayr, Klaus Dressler, Christos Vrettos, Peter Eberhard
Prediction of draft forces in cohesionless soil with the Discrete Element Method
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Pages 347-358, ISSN 0022-4898, 10.1016/j.jterra.2011.08.003.
Abstract: The Discrete Element Method (DEM) is applied to predict draft forces of a simple implement in cohesionless granular material. Results are compared with small-scale laboratory tests in which the horizontal force is measured at a straight blade. This study is focused on the case of cohesionless material under quasi-static conditions. 

The DEM requires the calibration of the local contact parameters between particles to adjust the bulk material properties. The most important bulk property is the angle of internal friction ϕ. In the DEM, the shear resistance is limited in the case of spherical particles due to excessive particle rotations. This is cured by retaining rotations of the particles. Although this is known to prevent the material from developing shear bands, the model still turns out to be capable of predicting the reaction force on the blade.

In contrast to empirical formulas for this kind of application, the DEM model can easily be extended to more complex tool geometries and trajectories. This study helps to find a simple and numerically efficient setup for the numerical model, capable of predicting draft forces correctly and so allowing for large-scale industrial simulations. Keywords: Discrete Element Method; DEM; Reaction force; Soil–tool interaction

Kazuhiko Ohmiya
Best papers from the 9th Asia–Pacific Conference of the ISTVS
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Page 359, ISSN 0022-4898, 10.1016/j.jterra.2011.10.002.

Maximilian Apfelbeck, Sebastian Kuß, Bernhard Rebele, Bernd Schäfer
A systematic approach to reliably characterize soils based on Bevameter testing
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Pages 360-371, ISSN 0022-4898, 10.1016/j.jterra.2011.04.001.
Abstract: Although a lot of information about soil parameter identification exists in literature, there is currently no algorithm who makes use both of state of the art identification methodologies and incorporating statistical analysis. In this paper a state of the art soil parameter identification method is presented including the calculation of its standard deviations and a proper weighting of the objective function. With this algorithm and a Bevameter with advanced sensor and actuator technology a test campaign is started to find a reliable soil preparation, which is applicable to a large planetary rover performance testbed. Furthermore, the preparation method has to be valid and stable for various types of dry, granular and frictional soils, typically used for planetary rover testing in space robotics, since the result of pre-tests show that the soil parameters are highly depending on the preparation. Besides preparation, the soil parameters are also influenced by different Bevameter test setup variables. Thus, the effect of the penetration velocity as well as the penetration tool geometry for pressure–sinkage tests on soil parameters is investigated. For shear tests the influence of the dimension of the shear ring is analysed as well as the variation of the grouser height, the number of the grousers and the increase of the rotational shear velocity. The results of the extensive test campaign are evaluated by the proposed identification algorithms. Keywords: Bevameter; Soil parameter; Testbed; Identification; Planetary rover; Statistical analysis

C. Senatore, C. Sandu
Torque distribution influence on tractive efficiency and mobility of off-road wheeled vehicles
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Pages 372-383, ISSN 0022-4898, 10.1016/j.jterra.2011.06.008.
Abstract: Off-road vehicle performance is strongly influenced by the tire-terrain interaction mechanism. Soft soil reduces traction and significantly modifies vehicle handling; therefore tire dynamics plays a strong role in off-road mobility evaluation and needs to be addressed with ad-hoc models. Starting from a semi-empirical tire model based on Bekker–Wong theory, this paper, analyzes the performance of a large four wheeled vehicle driving on deformable terrain. A 14 degree of freedom vehicle model is implemented in order to investigate the influence of torque distribution on tractive efficiency through the simulation of front, rear, and all wheel drive configuration. Results show that optimal performance, regardless vertical load distribution, is achieved when torque is biased toward the rear axle. This suggests that it is possible to improve tractive efficiency without sacrificing traction and mobility. Vehicle motion is simulated over dry sand, moist loam, flat terrain and inclined terrain. Keywords: Tractive efficiency; Off-road tire dynamics; Multi-pass; Vehicle dynamics; Torque distribution

Jarosław A. Pytka, Piotr Tarkowski, Stefan Fijałkowski, Piotr Budzyński, Jarosław Dąbrowski, Włodzimierz Kupicz, Paweł Pytka
An instrumented vehicle for offroad dynamics testing
Journal of Terramechanics, Volume 48, Issue 5, October 2011, Pages 384-395, ISSN 0022-4898, 10.1016/j.jterra.2011.06.003.
Abstract: The paper presents an instrumented vehicle that was equipped with measuring systems to perform complete dynamics tests, especially in off-road conditions. The equipment consists of four wheel dynamometers, a steering robot, and a differential GPS system together with an inertial platform, a non-contact vehicle speed sensor, and an on-board computer with software to control the devices and collect experimental data. The four wheel dynamometers measure six elements; based on strain gage force transducers, it measures three orthogonal forces and three moments. The steering robot can control the steering wheel of the vehicle at a variety of excitation modes; it can carry out typical vehicle dynamics tests (ISO 7401, ISO 4138, ISO/TR3888, etc.) as well as custom engineered tests at a wide range of setting parameters (steer angle rate up to 1600 deg/s). The differential GPS system gives true time vehicle kinematics data (velocities, accelerations, angles, etc.) at 10-ns sample rate and 20-mm accuracy. The base vehicle, a Suzuki Vitara 4 × 4, required no special modifications or changes to install the measuring equipment. The paper also describes typical tests performed with the use of the instrumented vehicle together with sample results. Keywords: Off-road vehicle testing; Instrumented vehicle; Measurements; Vehicle dynamics