Publication news

Calculating fractal parameters from low-resolution terrain profiles

Christopher Goodin, Maria Stevens, Francisco J. Villafañe Rosa, Burney McKinley, Burhman Q. Gates, Phillip J. Durst, George L. Mason, Alex Baylot

Journal of Terramechanics, Volume 72, August 2017, Pages 21-26, ISSN 0022-4898,

Driver comfort on rough terrain is an important factor in the off-road performance of wheeled and tracked ground vehicles. The roughness of a terrain has typically been quantified by the U.S. Army as the root-mean-square elevation deviation (RMS) of the terrain profile. Although RMS is an important input into many mobility calculations, it is not scale invariant, making it difficult to estimate RMS from low resolution terrain profiles. Fractal parameters are another measure of roughness that are scale invariant, making them a convenient proxy for RMS. While previous work found an empirical relationship between fractal dimension and RMS, this work will show that, by including the cutoff length, an analytic relationship between fractal properties and RMS can be employed. The relationship has no free parameters and agrees very well with experimental data - thus providing a powerful predictive tool for future analyses and a reliable way to calculate surface roughness from low-resolution terrain data in a way that is scale invariant. In addition, we show that this method applies to both man-made ride courses and natural terrain profiles.  
Keywords: Fractal dimension; RMS; Surface roughness


A coupled sliding and rolling friction model for DEM calibration

Zamir Syed, Mehari Tekeste, David White

Journal of Terramechanics, Volume 72, August 2017, Pages 9-20, ISSN 0022-4898,

The accuracy of dense Discrete Element Method (DEM) simulations is sensitive to initial density, contact orientation, particle size and shape, and interparticle interaction parameters including contact stiffness, cohesion, coefficients of friction, and coefficients of restitution. Although studies have characterized the effects of individual particle interaction parameters on mechanical responses of loaded granular material, research combining DEM parameters for calibration is scarce. Robust DEM calibration methodology combining sliding and rolling friction coefficients was developed and validated to predict bulk residual soil strength of initially dense DEM particle assemblies. 
Keywords: Coupled sliding and rolling friction coefficients; Discrete element method; Soil


Modeling of share/soil interaction of a horizontally reversible plow using computational fluid dynamics

Lin Zhu, Jia-Ru Ge, Xi Cheng, Shuang-Shuang Peng, Yin-Yin Qi, Shi-Wu Zhang, De-Quan Zhu  

Journal of Terramechanics, Volume 72, August 2017, Pages 1-8, ISSN 0022-4898,

The horizontally reversible plow (HRP) is currently widely used instead of the regular mold-board plow due to its high operational performance. Soil pressure during HRP tillage generally has adverse effects on the plow surface, especially on either the plowshare or the plow-breast. This effect eventually shortens the tool’s service life. For this reason, this investigation used a three-dimensional (3D) computational fluid dynamics (CFD) approach to characterize the share/soil interaction and thus assess the effects of different tillage conditions on the interaction. To achieve this goal, a 3D model of the plowshare was first constructed in the commercial software SolidWorks, and soil from Xinjiang, China, was selected and subsequently characterized as a Bingham material based on rheological behaviors. Finally, 3D CFD predictions were performed using the control volume method in the commercial ANSYS code Fluent 14.0 in which the pressure distributions and patterns over the share surface were addressed under different tillage speeds in the range of 2–8 ms−1 and at operational depths ranging from 0.1 to 0.3 m. The results show that the maximum pressure appeared at the share-point zone of the plowshare and that the increase in soil pressure was accompanied by either higher tool speed or greater operational depth. The calculated results qualitatively agreed with the preliminary experimental evidence at the same settings according to scanning electron microscopy (SEM). Once again, the CFD-based dynamic analysis in this study is demonstrated to offer great potential for the in-depth study of soil-tool interactions by simulating realistic soil matter.  
Keywords: Soil-share interaction; Computational fluid dynamics (CFD); Tillage speed; Operational depth; Horizontally reversible plow (HRP)


DEM simulation of soil-tool interaction under extraterrestrial environmental effects

Mingjing Jiang, Banglu Xi, Marcos Arroyo, Alfonso Rodriguez-Dono, Journal of Terramechanics, Volume 71, June 2017, Pages 1-13, ISSN 0022-4898,


In contrast to terrestrial environment, the harsh lunar environment conditions include lower gravity acceleration, ultra-high vacuum and high (low) temperature in the daytime (night-time). This paper focuses on the effects of those mentioned features on soil cutting tests, a simplified excavation test, to reduce the risk of lunar excavation missions. Soil behavior and blade performance were analyzed under different environmental conditions. The results show that: (1) the cutting resistance and the energy consumption increase linearly with the gravity. The bending moment has a bigger increasing rate in low gravity fields due to a decreasing moment arm; (2) the cutting resistance, energy consumption and bending moment increase significantly because of the raised soil strength on the lunar environment, especially in low gravity fields. Under the lunar environment, the proportions of cutting resistance, bending moment and energy consumption due to the effect of the van der Waals forces are significant. Thus, they should be taken into consideration when planning excavations on the Moon. Therefore, considering that the maximum frictional force between the excavator and the lunar surface is proportional to the gravity acceleration, the same excavator that works efficiently on the Earth may not be able to work properly on the Moon.

Keywords: Lunar regolith; Distinct Element Method; Soil cutting test; Cutting resistance; Van der Waals force; Gravity effect


Terrain classification using intelligent tire

Seyedmeysam Khaleghian, Saied Taheri, Journal of Terramechanics, Volume 71, June 2017, Pages 15-24, ISSN 0022-4898,


A wheeled ground robot was designed and built for better understanding of the challenges involved in utilization of accelerometer-based intelligent tires for mobility improvements. Since robot traction forces depend on the surface type and the friction associated with the tire-road interaction, the measured acceleration signals were used for terrain classification and surface characterization. To accomplish this, the robot was instrumented with appropriate sensors (a tri-axial accelerometer attached to the tire innerliner, a single axis accelerometer attached to the robot chassis and wheel speed sensors) and a data acquisition system. Wheel slip was measured accurately using encoders attached to driven and non-driven wheels. A fuzzy logic algorithm was developed and used for terrain classification. This algorithm uses the power of the acceleration signal and wheel slip ratio as inputs and classifies all different surfaces into four main categories; asphalt, concrete, grass, and sand. The performance of the algorithm was evaluated using experimental data and good agreements were observed between the surface types and estimated ones.

Keywords: Wheeled ground robot; Intelligent tire; Terrain classification; Fuzzy logic algorithm

Numerical analysis on tractive performance of off-road wheel steering on sand using discrete element method

Yonghao Du, Jingwei Gao, Lehua Jiang, Yuanchao Zhang
Journal of Terramechanics, Volume 71, June 2017,
Pages 25-43, ISSN 0022-4898,

This paper presents a numerical analysis on steering performance including tractive parameters and lug effects. To explore the difference between the turning and straight conditions of steering, a numerical sand model for steering is designed and appropriately established by the discrete element method on the basis of triaxial tests. From the point of mean values and variation, steering traction tests are conducted to analyze the tractive parameters including sinkage, torque and drawbar pull and the lug effects resulting from type, intersection and central angle. Analysis indicates that steering motion has less influence on the sinkage and torque. When the slip ratio exceeds 20%, the steering drawbar pull becomes increasingly smaller than in the straight condition, and the increase of steering radius contributes to a decline in mean values and a rise in variation. The lug effect of central angle is less influenced by the steering motion, but the lug intersection is able to significantly increase the steering drawbar pull along with the variation reduced. However, the lug inclination reduces the steering drawbar pull along with the variation raised in different degrees.
Keywords: Steering; Off-road wheel; Tractive parameter; Lug effects; Discrete element method


Bayesian calibration of Vehicle-Terrain Interface algorithms for wheeled vehicles on loose sands

Ian Dettwiller, Masoud Rais-Rohani, Farshid Vahedifard, George L. Mason, Jody D. Priddy

Journal of Terramechanics, Volume 71, June 2017, Pages 45-56, ISSN 0022-4898,

Abstract:   The Vehicle-Terrain Interface (VTI) model is commonly used to predict off-road mobility to support virtual prototyping. The Database Records for Off-road Vehicle Environments (DROVE), a recently developed database of tests conducted with wheeled vehicles operating on loose, dry sand, is used to calibrate three equations used within the VTI model: drawbar pull, traction, and motion resistance. A two-stage Bayesian calibration process using the Metropolis algorithm is implemented to improve the performance of the three equations through updating of their coefficients. Convergence of the Bayesian calibration process to a calibrated model is established through evaluation of two indicators of convergence. Improvements in root-mean square error (RMSE) are shown for all three equations: 17.7% for drawbar pull, 5.5% for traction, and 23.1% for motion resistance. Improvements are also seen in the coefficient of determination (R2) performance of the equations for drawbar pull, 2.8%, and motion resistance, 2.5%. Improvements are also demonstrated in the coefficient of determination for drawbar pull, 2.8%, and motion resistance, 2.5%, equations, while the calibrated traction equation performs similar to the VTI equation. A randomly selected test dataset of about 10% of the relevant observations from DROVE is used to validate the performance of each calibrated equation.  
Keywords: Off-road mobility; Vehicle Terrain Interface (VTI) model; Bayesian calibration; Metropolis algorithm; Sand; Drawbar pull (DP); Traction; Motion resistance; Database Records for Off-road Vehicle Environments (DROVE)

A device to measure wheel slip to improve the fuel efficiency of off road vehicles

A. Ashok Kumar, V.K. Tewari, Chanchal Gupta, C.M. Pareek   
Journal of Terramechanics, Volume 70, April 2017, Pages 1-11, ISSN 0022-4898,  

A hall sensor based simple technique was introduced to measure wheel slip and a microcontroller based embedded digital system was developed to display wheel slip data and warn the operator with audible and visible warnings if the optimum range is exceeds. Hall sensor slip measurement system was validated in controlled soil bin condition, tar macadam surface and actual field condition and compared with the commercial radar sensor. The developed system is simple in construction and can be mounted to any make and model of agricultural tractors by entering the appropriate rolling radius via the computer interface. Field trials were conducted to measure wheel slip and fuel consumption on farm use with and without activation of slip indicator; it was observed that, the amount of fuel saving during various agricultural operation was up 1.3 l/h.  
Keywords: Hall effect sensor; Actual speed measurement; Radar sensor; Display unit; Soil bin; Wheel slip


Bevameter testing on simulant Fillite for planetary rover mobility applications

Michael B. Edwards, Mandar M. Dewoolkar, Dryver R. Huston, Colin Creager
Journal of Terramechanics, Volume 70, April 2017, Pages 13-26, ISSN 0022-4898,  
This paper examines pressure-sinkage and shearing behavior via bevameter testing of a light-weight, granular simulant called Fillite in support of laboratory modeling of rover mobility in high-sinkage, high-slip environments typically found on Mars, the Moon, and other planetary bodies. Normal bevameter test results helped to determine parameters for the Bekker model, the New Model of Mobility (N2M) sinkage model, and the Bekker-Wong model. A case study used the Bekker-Wong model parameters to predict the possible sinkage of 84% into Fillite of a wheel on the Mars Spirit rover, a value within the observed sinkage of 50–90% of the wheel diameter of the Spirit rover on Mars. Shear bevameter testing of Fillite provided a second set of parameters to assess shear behavior, this time simulating the stresses and shear deformations imparted by rotating wheels. The results compared well to the estimated shear stresses and deformations of Martian soil caused by the wheels of the Spirit rover. When compared to other simulants (e.g. GRC-1), the results confirm that Fillite is possibly more suitable for high-sinkage and high-slip rover studies than other typical simulants derived from natural terrestrial soils and rocks.  
Keywords: Bevameter testing; Martian simulant; Fillite; Pressure-sinkage model