Publication news

Real time rut profile measurement in varying terrain types using digital image correlation

Theunis Botha, Devin Johnson, Schalk Els, Sally Shoop,

Journal of Terramechanics, Volume 82, 2019, Pages 53-61, ISSN 0022-4898,

Abstract: An important parameter in terramechanics is the rut depth produced when a vehicle traverses deformable soil. The rut depth provides a measure of vehicle tractability as well as the impact on the environment. Rut depth is not uniform on natural terrain and typically only a few manual points are measured. Synchronizing rut depth with other measurements is also problematic. This paper investigates the feasibility of using cameras to measure 3D terrain profiles from which a single rut depth measurement is obtained. Tests were performed on different vehicles, for various dynamic vehicle manoeuvres, over varying terrains including sand, mud, grassland, snow and ice. Results were validated using the traditional stick ruler method. Measurement frequencies of 58 Hz were obtained using affordable commercially off the shelf computational hardware and dedicated software. Determining whether a vehicle can traverse a terrain can significantly improve the vehicle mobility. Therefore, real time measurements of rut depth can be used to determine the mobility of vehicles in off-road conditions that can change rapidly due to environmental conditions e.g. rain or snow. The techniques described can assist in gathering terrain and vehicle mobility data that can be used directly to assist the driver in making safety related decisions.

Keywords: Real time; Rut depth; Stereo; Terrain measurement

An in-wheel sensor for monitoring tire-terrain interaction: Development and laboratory testing

Raul G. Longoria, Robert Brushaber, Andrew Simms

Journal of Terramechanics, Volume 82, 2019, Pages 43-52, ISSN 0022-4898,

Abstract: An in-wheel sensor system was developed to monitor the deflection and shape of a tire that it is operating on and interacting with terrain. Three ultrasonic sensors were mounted to rotate with the wheel rim to measure radial distance to the inner tire surface at the mid-plane and at two equidistant lateral planes of the tire. This study describes evaluation of the sensor system using a laboratory drum-test machine which drives the tire over a wide range of speeds at different normal tire loads and inflation pressures. Signal processing methods are described for extracting characteristic measures of the tire contact and shape geometry which can be determined from measurements of inner tire surface deflection. The measurement of contact length compared well with results measured using other methods and reported in the literature. Additional tire shape metrics are defined that may be useful for informing how automated adjustments can be made to inflation pressure by central tire inflation pressure systems. In most of the test cases, the three sensors showed no significant lateral variation in measured deflections, as expected for testing on a rigid drum.

Keywords: Tire-terrain contact measurement; Tire contact area sensor; Contact length

Discrete element modeling of cultivator sweep-to-soil interaction: Worn and hardened edges effects on soil-tool forces and soil flow

Mehari Z. Tekeste, Loran R. Balvanz, Jerry L. Hatfield, Sadaf Ghorbani

Journal of Terramechanics, Volume 82, 2019, Pages 1-11, ISSN 0022-4898,

Abstract: Simulation of tool-to-soil interaction provides opportunities to accelerate new equipment design and evaluate performance of tillage tools. Simulation based evaluation of worn tillage tools performance on soil flow has not been done. Discrete Element Modelling (DEM) has a potential to simulate worn tool to soil interaction problems, where worn tools CAD can be generated using 3D scanning. The DEM parameters of Hertz-Mindlin with Parallel Bond model were calibrated to match draft force and soil failure zone measured from a tool bar moving at 0.22 m/s and 38 mm cutting depth. The draft force and soil forward failure zone were predicted at 7% and 24% relative errors compared to measured values, respectively. Using the optimized DEM soil model, the interaction of three 3D reconstructed sweeps (new sweep, carbide treated-worn, untreated-worn) with soil were simulated to compare their geometric wear dimensional loss, performance on soil forces and soil flow. Results showed that the carbide treated-worn sweep had similar soil draft force and soil forward failure distance as the new sweep. The untreated-worn sweep showed lower vertical force (less suction) and its wing induced soil failure zone (front and lateral) showed poor soil tilth quality compared with the carbide treated-worn sweep and the new sweep.

Keywords: Discrete Element Modeling; 3D scanning; Soil model; Tillage; Carbide treated hardened edge; Wear

Rolling radii and moment arm of the wheel load for pneumatic tyres

Heinz Dieter Kutzbach, Alexander Bürger, Stefan Böttinger,

Journal of Terramechanics, Volume 82, 2019, Pages 13-21, ISSN 0022-4898,

Abstract: Tractors and self-propelled harvesters are equipped with high volume pneumatic tyres with a low tyre inflation pressure. The contact patch can shift forwards or backwards in reference to the wheel centre as reaction on traction or brake forces because of the elastic tyre wall. Theoretical investigations – as necessary for modelling and simulation of dynamic vehicle behaviour – are complicated, since important tyre metrics cannot directly be measured based on the large deformations. Additionally, different definitions are often used. This is especially valid for the conversion of a drive torque into a traction force. In this context, the moment arm of the wheel load and the rolling radius of the tyre at zero slip condition are especially important. In contrast to the hitherto existing perception, the magnitude and position of the moment arm of the wheel load in reference to the wheel centre is dependent on traction and brake forces in addition to the motion resistance. The rolling radius of an elastic pneumatic tyre can be interpreted as radius of a fictitious rigid substitute wheel. This contribution emphasizes the outstanding importance of the rolling radius rdyn for all calculations on pneumatic tyres and the important roll of the variable moment arm of the wheel load for the moment compensation on the wheel.

Keywords: Pneumatic tyres; Rolling radius; Moment arm of the wheel load; Kinetic radius; Kinematic radius; Torque radius; Inner tyre ratio

Characterization of machine learning algorithms for slippage estimation in planetary exploration rovers

Ramon Gonzalez, Samuel Chandler, Dimi Apostolopoulos,

Journal of Terramechanics, Volume 82, 2019, Pages 23-34, ISSN 0022-4898,

Abstract: This paper presents a comprehensive comparison of well-known machine learning algorithms for estimating discrete slip events associated with individual wheels in planetary exploration rovers. This analysis is performed with various tuning configurations for each algorithm (55 setups). This research also shows the key role that environment plays in the performance of the learning algorithms: rover speed (0.05–0.25 [m/s]), type of terrain (gravel vs. sand), and tire type (off-road tires vs. smooth tires). These contributions are validated by using a broad data set collected using a planetary rover equipped with proprioceptive sensing. This work not only identifies the best algorithm to be deployed for discrete slip estimation, but it also helps with the selection and the mounting position of the sensing systems to be employed in future robotic planetary missions.

Keywords: Discrete slip estimation; Feature selection; Field validation; Ground vehicles; LATUV rover; Model selection

Studies on the sinkages of rigid plain wheels and lugged wheels on TRI-1 lunar soil simulant

S. Jayalekshmi, Pala Gireesh Kumar,

Journal of Terramechanics, Volume 82, 2019, Pages 35-42, ISSN 0022-4898,

Abstract: This paper summarises the experimental work and analytical work carried out to determine the wheel sinkage of plain rigid wheels and lugged wheels, traversing on TRI-1 Lunar Soil Simulant (Plain and lugged wheels: small wheel- 160 mm × 32 mm and large wheel- 210 mm × 50 mm) with different number of lugs (N = 8, 12, 16) and lug height (h = 5, 10, 15 mm). Bekker and Reece pressure sinkage models are considered to determine the theoretical sinkage, and outlined in the present study. Comparisons of the analytical results with the experimental results are also carried out. The comparisons hold good for plain rigid wheels and in 2 cases, for lugged wheels (Small and Large), with 16 lugs of height, 5 mm. For all other combinations of lugged wheels (no. of lugs = 8, 12 and height of lugs = 5 mm, 10 mm and 15 mm), the predicted values are found to be less than the experimental values. For efficient functioning of the rover, optimization of wheel dimensions is a must. A new sinkage model accounting gravity effect and aspect ratio is developed, based on experimental results for wheel sinkages on TRI-1 simulant, the range of results obtained for different cases are examined.

Keywords: TRI-1 lunar soil simulant; Sinkage model; Aspect ratio; Lug

Review of terramechanics models and their applicability to real-time applications

Rui He, Corina Sandu, Aamir K. Khan, A. Glenn Guthrie, P. Schalk Els, Herman A. Hamersma,

Journal of Terramechanics, Volume 81, 2019, Pages 3-22, ISSN 0022-4898,

Abstract: ISTVS embarked on a project in 2016 that aims at updating the current ISTVS standards related to nomenclature, definitions, and measurement techniques for modelling, parameterizing, and, respectively, testing and validation of soft soil parameters and vehicle running gear-terrain interaction. As part of this project, a comprehensive literature review was conducted on the parameterization of fundamental terramechanics models. Soil parameters of the empirical models to assess off-road vehicle mobility, and parameters of the models to characterize the response of the terrain interacting with running gears or plates from the existing terramechanics literature and other researchers’ reports were identified. This review documents and summarizes the modelling approaches that may be applicable to real-time applications of terramechanics in simulation, as well as in controller design.

Keywords: Soil modeling; Pressure-sinkage models; Shear stress-displacement models; Vibration models; ISTVS standards; Terramechanics; Off-road vehicle dynamics

Methods for generalization of experimental results in terramechanics

G. Sitkei, G. Pillinger, L. Máthé, L. Gurmai, P. Kiss,

Journal of Terramechanics, Volume 81, 2019, Pages 23-34, ISSN 0022-4898,

Abstract: Due to the extreme variability of soils and different tillage and settling conditions, measurements made under a single set of conditions have little practical applicability under arbitrary conditions. Results of more general usefulness require systematic measurements to be carried out over suitable ranges of the main influencing variables. The results can then be processed to obtain dimensionally homogeneous equations, i.e. similarity equations. This method was first proposed a century ago (Buckingham, 1914) [1], and one year later, Nusselt published a fundamental paper showing how it could be used to generalize the results of heat transfer experiments and to plan new experiments (Nusselt, 1915) [2]. The method is based on the postulate that all physical processes can be expressed as relationships among dimensionless parameters, and it specifies how to find those parameters. We describe new experimental results and generalize the results using dimensional analysis to obtain similarity plots and equations that are generally applicable within the feasible range of variables.

Keywords: Terramechanics; Generalization; Tyre-soil interaction

Real-time side-slip angle measurements using digital image correlation

Devin K. Johnson, Theunis R. Botha, P. Schalk Els,

Journal of Terramechanics, Volume 81, 2019, Pages 35-42, ISSN 0022-4898,

Abstract: In vehicle dynamics there are many parameters that are desired for vehicle control and modelling. One of the most important parameters for handling and stability is the vehicle side-slip angle. The ability to directly measure the vehicle side-slip in real-time will aid and improve many driver assist systems such as stability control schemes and roll-over mitigation, especially over rough terrain. Commercial side-slip angle solutions are available but they are prohibitively expensive and are only suitable for use during vehicle development and performance evaluation. They are also restricted to small side-slip angles and give unsatisfactory results at low speeds and over uneven terrain. Previous research has proven that digital image correlation can be used to accurately measure vehicle side-slip angle over rough off-road terrain using inexpensive, off-the-shelf cameras. However, side-slip angle calculations were performed in post processing from pre-recorded footage and not implemented in real time due to the large computational times of the novel algorithms developed. This paper describes the improvements made to the algorithms that enable real-time implementation. The side-slip angle is measured using a single camera pointing downwards to the terrain and digital image correlation. The sensor is tested on a flat surface using a rig that allows for validation. The maximum sampling frequency and accuracy are investigated. The system is shown to measure accurately and in real-time up to 100 km/h speeds.

Keywords: Computer vision; Side-slip angle; Off road; Image correlation