Off-road Vehicle Dynamics Research Articles

Dynamics modeling and robotic-assist, leader-follower control of tractor convoys

This paper proposes a generalized dynamics model and a leader-follower control architecture for skid-steered tracked vehicles towing polar sleds. The model couples existing formulations in the literature for the powertrain components with the vehicle-terrain interaction to capture the salient features of terrain trafficability and predict the vehicles response. This coupling is essential for making realistic predictions of the vehicles traversing capabilities due to the power-load relationship at the engine output. The objective of the model is to capture adequate fidelity of the powertrain and off-road vehicle dynamics while minimizing the computational cost for model based design of leader-follower control algorithms. The leader-follower control architecture presented proposes maintaining a flexible formation by using a look-ahead technique along with a way point following strategy. Results simulate one leader-follower tractor pair where the leader is forced to take an abrupt turn and experiences large oscillations of its drawbar arm indicating potential payload instability. However, the follower tractor maintains the flexible formation but keeps its payload stable. This highlights the robustness of the proposed approach where the follower vehicle can reject errors in human leader driving.

Off-road vehicle dynamics: Stability, ride comfort, vehicle performance and modeling

Off-road vehicle dynamics: Stability, ride comfort, vehicle performance and modeling

Development of a 3-D quasi-static tyre model for on-road and off-road vehicle dynamics simulations: Part I - on-road flexible tyre model

The research conducted in this study deals with the development of a simplified brush-based tyre model for on-road simulation, together with a simplified off-road wheel/tyre model. The on-road tyre model is developed based on data collected by NHTSA throughout the years, while the off-road tyre model is developed based on observations of experimental data and photographic evidence collected by various terramechanics researchers within the last few decades. The principle behind the model revolves around the integration of the stresses at the contact interface and has its basis in terramechanics. The model was parameterised from test data to acquire static and dynamic friction coefficients, cornering and longitudinal stiffness, as well as camber stiffness. The results are compared with test data and show good correlation. This paper is the first of a series of three papers on the modelling tyres for both on- and off-road operations.

Development of a 3-D quasi-steady-state tyre model for on-road and off-road vehicle dynamics simulations: Part II - off-road rigid wheel model

This study presents the development of a simplified brush-based tyre model for on-road simulation, together with a simplified off-road wheel/tyre model that has the capability to revert back to on-road trend of behaviour on firmer soils. The on-road tyre model is developed based on empirical data collected by NHSTA, while the off-road tyre model is developed based on observations of experimental data and photographic evidence collected by various terramechanics researchers within the last few decades. Using theoretical analysis and empirical data, the tyre deformation geometry is determined to establish the tractive forces in off-road operation. The first step of any off-road vehicle is to model the condition where the soil is much more deformable than the tyre, where the tyre can be modelled as a rigid wheel. This study explores the development of the rigid wheel mode of the model, using dry sandy terrain, an example of extremely deformable soil. This paper is the second of three papers on the modelling of tyres for both on and off-road operations.

Development of a 3-D quasi-steady state tyre model for on-road and off-road vehicle dynamics simulations: Part III - off-road flexible tyre model

Development of a 3-D quasi-steady state tyre model for on-road and off-road vehicle dynamics simulations: Part III - off-road flexible tyre model

Investigation of Tire-Soil Interaction with Analytical and Finite Element Method#

This article is devoted to the modeling of tire–soil interaction with the analytical and finite element method (FEM). Different from the traditional tire–soil interaction models in which the contact between tires and soils is described by nonlinear springs in the normal direction, the new analytical model added dampers accounting for the soil damping effects. A nonlinear three-dimensional tire–soil interaction model with the FEM, which takes into consideration the tire treads and belt/carcass layers, was validated by measuring tire vertical stiffness and contact stress. The conversion of the Modified Drucker-Prager/Cap soil model to the analytical soil model was demonstrated as well. The simulations for investigating the dynamic behavior of a bias tire running on the soft soil were performed. Good agreement was observed based on the simulation results obtained from these two tire–soil interaction models. However, experimental validation is necessary before the models can be further applied in the prediction of off-road vehicle dynamics.

Stochastic modelling of 1-D and 2-D terrain profiles using a polynomial chaos approach

One fundamental difficulty in understanding the physics of vehicular off-road traction and in predicting vehicle performance is the variability of the terrain profile. These operating conditions are uniquely defined at a given spatial location and a given time. It is not practically feasible to measure them at a sufficiently large number of points to be able to accurately represent the terrain in models, or to use all the data collected to recreate the terrain profile. This renders traditional analysis tools insufficient when dealing with rough terrain. In this study, mathematical tools to quantify the impact of uncertainties in the terrain profile on vehicle mobility are developed. A polynomial chaos approach is used to reconstruct one-dimensional (along longitudinal direction) stationary and non-stationary terrain profiles. Also, an efficient mathematical method based on the Karhunen-Loeve expansion and the approach for 1-D stochastic terrain profile is developed to reconstruct two-dimensional (along longitudinal and lateral directions) terrain profiles. The proposed mathematical methods calculate the autocorrelation of terrain profiles, solve eigenvalues and eigenvectors of the autocorrelation function, and obtain the corresponding orthogonal random variables directly. The original terrain profile is reconstructed by Karhunen-Loeve expansions, requesting a small, limited computational effort, without the need to verify the terrain data for Gaussianity, stationary, and linearity, and without the need to choose the order of the expansion and the corresponding fitting coefficient artificially. Promising simulation results based on experimental data are obtained using the proposed methods. The schemes to choose the number of eigenvalues and eigenvectors are discussed. The proposed mathematical methods can be used to simulate the terrain profile for on-road and off-road vehicle dynamics or robotic applications.

Modeling and real-time simulation architectures for virtual prototyping of off-road vehicles

Virtual Reality-based simulation technology has evolved as a useful design and analysis tool at an early stage in the design for evaluating performance of human-operated agricultural and construction machinery. Detecting anomalies in the design prior to building physical prototypes and expensive testing leads to significant cost savings. The efficacy of such simulation technology depends on how realistically the simulation mimics the real-life operation of the machinery. It is therefore necessary to achieve ‘real-time’ dynamic simulation of such machines with operator-in-the-loop functionality. Such simulation often leads to intensive computational burdens. A distributed architecture was developed for off-road vehicle dynamic models and 3D graphics visualization to distribute the overall computational load of the system across multiple computational platforms. Multi-rate model simulation was also used to simulate various system dynamics with different integration time steps, so that the computational power can be distributed more intelligently. This architecture consisted of three major components: a dynamic model simulator, a virtual reality simulator for 3D graphics, and an interface to the controller and input hardware devices. Several off-road vehicle dynamics models were developed with varying degrees of fidelity, as well as automatic guidance controller models and a controller area network interface to embedded controllers and user input devices. The simulation architecture reduced the computational load to an individual machine and increased the real-time simulation capability with complex off-road vehicle system models and controllers. This architecture provides an environment to test virtual prototypes of the vehicle systems in real-time and the opportunity to test the functionality of newly developed controller software and hardware.

Optimisation of AWD off-road vehicle performance using visco-lock devices

A comprehensive computer model is devised for the simulation of AWD off-road vehicle dynamics. Particular attention has been paid to the modelling of various visco-lock devices, including the viscous couplings and visco-lock limited-slip differentials. These devices are represented by fully parameterised physical models which capture the torque transmission mechanism represented by various thermodynamic, hydrodynamic, structural and mechanical modules. The characteristics of these devices can easily be altered so that comparisons can be made between different types. In addition, the influence of a wide range operating conditions, vehicle design parameters and tyre characteristics can also be made over various deformable soils.

Calculation of the rolling contact between a tyre and deformable ground

The main objective of this paper is the development of enhanced tyre models for an adequate description of tyre–ground interaction on the basis of shell and three-dimensional theories. Investigations are aimed at applications in off-road vehicle dynamics, in tyre design and for soil protection in agriculture. It seems to be necessary to take into account the results of internal tyre mechanics. The soil under the tyre is described by different rheological models of ground surface behaviour in the normal and tangential directions. Soil models were tested with simple one-dimensional tyre models. Results are used for the determination of and improvement in the exactness of stiffness parameters in external tyre models and in the Böhm mass-points approach for transient loading of a rolling tyre. The model is modified for the real structure of agricultural tyres with lugs. The numerical results are compared with experimental data gained by a multipoint measurement technique for displacements in the inner part of the rolling tyre.

A Study on Terrain-Surface Modeling and Searching Algorithms for Real-time Simulation of Off-Road Vehicles

Summary Terrain surfaces have to be modeled in very detail and wheel-surface contacting geometry must be well defined in order to obtain proper ground-reaction and friction forces for realistic simulation of off-road vehicles. Delaunay triangulation is one of the most widely used methods in modeling 3-dimensional terrain surfaces, and the T-search is a relevant algorithm for searching resulting triangular polygons. The T-search method searches polygons in a successive order and may not allow real-time computation of off-road vehicle dynamics if the terrain is modeled with many polygons, depending on the computer performance used in the simulation. In order to accelerate the searching speed of the T-search, a terrain database of triangular polygons is modeled in multi-levels by adopting the LOD (Level of Detail) method used in real-time computer graphics. Simulation results show that the new LOD-search is effective in shortening the required computing time. The LOD-search can be even further accelerated by introducing the NN (Neural Network) algorithm, in the cases where a appropriate range of moving paths can be predicted by cultural or geographical or empirical information of the simulated terrain, such as lakes, houses, etc. Numerical tests show that LOD-NN search almost doubles the speed of the original T-search.

Delaunay Triangulation의 폴리건 검색속도 개선을 위한 T-Search와 Dynamic-Window 개념의 결합

Terrain surfaces have to be modeled in very detail and wheel-surface contacting geometry must be well defined in order to obtain proper ground-reaction and friction forces fur realistic simulation of off-road vehicles. Delaunay triangulation is one of the most widely used methods in modeling 3-dimensional terrain surfaces, and the T-search is a relevant algorithm for searching resulting triangular polygons. The T-search method searches polygons in a successive order and may not allow real-time computation of off-road vehicle dynamics if the terrain is modeled with many polygons, depending on the computer performance used in the simulation. The dynamic T-search, which is proposed in this paper, combines conventional T-search and the concept of the dynmaic-window search which uses reduced searching windows or sets of triangular surface polygons at each frame by taking advantage of the information regarding dynamic charactereistics of a simulated vehicle. Numerical tests show improvement of searching speeds by about 5% for randomly distributed triangles. For continuous search following a vehicle path, which occurs in actual vehicle simulation, the searching speed becomes 4 times faster.

Off-Road Vehicle Dynamics

SUMMARY Recent developments in off-road vehicle dynamics are reviewed. Progress on this topic and the application of new techniques to the particular problemsassociated with off-road operation tend to lag behind practices established for road vehicles. The factor which limits further progress is the lack ofappropriate off-road tyre data, in particular, on vibrational and lateral force generation characteristics. Also, a long term study should be aimed at understanding the dynamic behaviour of tyres on yielding surfaces.