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.     

Overview of cold regions mobility modeling at CRREL

Over the last several decades, the Cold Regions Research and Engineering Laboratory (CRREL) has extensively tested and analyzed issues related to vehicle performance in winter. Using this knowledge and the experimental database, models were developed to capture the important elements for cold regions mobility performance. These models span a range of resolutions and fidelities and include three-dimensional finite element models of tire–terrain interaction, vehicle dynamics models of vehicles on winter surfaces, semi-empirical cold regions algorithms for winter performance within the NATO reference mobility model (NRMM), all-season vehicle performance in force-on-force war-gaming simulations, and vehicle–surface interaction for real-time vehicle simulators. Each of these types of models is presented along with examples of their application.     

A GIS-based simulation model for terrain tractability

The paper describes a concept for a GIS-based terrain tractability model and an optimal off-road routing. The general structure of the model is based on an object model which uses the cost surface technique to describe actual conditions in the terrain. The cost surface is a kind of mobility index and it is based, e.g., on vehicle, wheel, terrain, tree coverage, road and weather objects. Different cost surface alternatives can be generated according to changing information in objects. A regular raster analysis is used to determine alternative routes in different conditions. It has been shown that the adequate number of useful parameters can be found from national level digital maps to support off-road analyses.     

On the impact of cargo weight, vehicle parameters, and terrain characteristics on the prediction of traction for off-road vehicles

A realistic prediction of the traction capacity of vehicles operating in off-road conditions must account for stochastic variations in the system itself, as well as in the operational environment. Moreover, for mobility studies of wheeled vehicles on deformable soil, the selection of the tire model used in the simulation influences the degree of confidence in the output. Since the same vehicle may carry various loads at different times, it is also of interest to analyze the impact of cargo weight on the vehicle’s traction.This study focuses on the development of an algorithm to calculate the tractive capacity of an off-road vehicle with stochastic vehicle parameters (such as suspension stiffness, suspension damping coefficient, tire stiffness, and tire inflation pressure), operating on soft soil with an uncertain level of moisture, and on a terrain topology that induces rapidly changing external excitations on the vehicle. The analysis of the vehicle–soil dynamics is performed for light cargo and heavy cargo scenarios. The algorithm relies on the comparison of the ground pressure and the calculated critical pressure to decide if the tire can be approximated as a rigid wheel or if it should be modeled as a flexible wheel. It also involves using previously-developed vehicle and stochastic terrain models, and computing the vehicle sinkage, resistance force, tractive force, drawbar pull, and tractive torque.The vehicle model used as a case study has seven degrees of freedom. Each of the four suspension systems is comprised of a nonlinear spring and a viscous (linear or magneto-rheological) damper. An off-road terrain profile is simulated as a 2-D random process using a polynomial chaos approach [Sandu C, Sandu A, Li L. Stochastic modeling of terrain profiles and soil parameters. SAE 2005 transactions. J Commer Vehicles 2005-01-3559]. The soil modeling is concerned with the efficient treatment of the impact of the moisture content on relationships critical in defining the mobility of an off-road vehicle (such as the pressure–sinkage [Sandu C et al., 2005-01-3559] and the shear stress–shear displacement relations). The uncertainties in vehicle parameters and in the terrain profile are propagated through the vehicle model, and the uncertainty in the output of the vehicle model is analyzed [Sandu A, Sandu C, Ahmadian M. Modeling multibody dynamic systems with uncertainties. Part I: theoretical and computational aspects, Multibody system dynamics. Publisher: Springer Netherlands; June 29, 2006. p. 1–23 (23), ISSN: 1384-5640 (Paper) 1573-272X (Online). doi:10.1007/s11044-006-9007-5; Sandu C, Sandu A, Ahmadian M. Modeling multibody dynamic systems with uncertainties. Part II: numerical applications. Multibody system dynamics, vol. 15, No. 3. Publisher: Springer Netherlands; 2006. p. 241–62 (22). ISSN: 1384-5640 (Paper) 1573-272X (Online). doi:10.1007/s11044-006-9008-4]. Such simulations can provide the basis for the study of ride performance, handling, and mobility of the vehicle in rough off-road conditions.     

Analytical model of longitudinal tire traction in snow

An analytical model to estimate longitudinal traction of a tire in snow was developed and verified to have good predictability in comparison with measurements. Snow traction of a tire is composed of four kinds of forces in this model: braking force attributable to snow compression, shear force of snow in void (space between tread blocks), frictional force, and digging force (edge effect generated by sipes and blocks). The mechanical characteristics of snow were considered in the prediction of braking force and shear force, but were not considered in the prediction of other forces. The contribution of shear force of snow in void and the frictional force was large in static traction (traction just before a tire slips). On the other hand, the contributions of digging force and frictional force were large in situations involving high slip ratios.     

Kinematic analysis of multi-loop spatial mechanisms: The five-loop case

Multi-loop spatial mechanisms, or parallel mechanisms, are being used to an increasing degree in robotic applications, because they offer some advantages over the open-chain mechanisms. In fact they exhibit high stiffness and low inertia, a fair compliance to the design requirements, and the ability to hold the power actuators in the base. On the other hand, they are often so complex that they can be analyzed only by digital computers, and in general they are designed on the basis of practical solutions rather than theoretical investigations of their structure and kinematics.This paper on the kinematic analysis of multi-loop spatial mechanisms deals in particular with manipulators with five loops and lower pairs. Two approaches are outlined, according to the different backgrounds of the authors: the decomposition method [9], and a differential approach to the non-linear equation set, 19.(6) which encompasses the direct and inverse kinematic problems, as well as the synthesis of the robot.The results are then specialized for mechanisms with the end-effector supported by three spatial parallelograms, like the DELTA-4 robot. For this class of mechanisms an interesting property is also demonstrated, which dramatically reduces the need for computation in the kinematic problems.

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Sommario I meccanismi spaziali in catena chiusa, o meccanismi paralleli, trovano sempre più largo impiego in robotica, dal momento che presentano taluni vantaggi rispetto ai meccanismi in catena aperta: offrono tra l'altro elevata rigidezza e bassa inerzia, flessibilità nel soddisfare le esigenze progettuali e possibilità di collocare gli azionamenti nella base. Spesso tuttavia la loro complessità è tale che si possono analizzare solo col computer, e vengono quindi progettati a partire da soluzioni pratiche, piuttosto che da sistematiche basi teoriche. Questo lavoro illustra metodi generali per lo studio cinematico di meccanismi spaziali in catena chiusa, riferendoli al caso di meccanismi con cinque catene e coppie cinematiche elementari. Gli approcci presentati, che riflettono la diversa formazione degli autori, sono: un metodo di decomposizione [9], e un metodo basato sul calcolo differenziale per lo studio del sistema di equazioni non lineari (6) che, in forma implicita, configura i problemi cinematici diretto e inverso e la sintesi del robot.I risultati vengono quindi applicati a un meccanismo il cui elemento terminale è sostenuto da tre parallelogrammi articolati spaziali, come nel caso del robot DELTA-4. Per questa classe di meccanismi si dimostra anche un'interessante proprietà che riduce drasticamente la quantità di calcoli richiesti dal problema cinematico.

     

Thermal vision,moisture content,and vegetation in the context of off-road mobile robots

This paper describes an initial investigation that shows the major impact that moisture and vegetation produce on a soil and how that effect may be measured using a thermal camera. In particular, those two variables influence how the soil compacts and, hence, the traversability of a vehicle. A broad set of experiments, under different weather conditions and with different soils, demonstrate that thermal properties derived from the thermal camera (i.e. thermal inertia) increase when moisture content of sandy soils increases. In addition to that, a relation is observed between thermal inertia and traversability (lower thermal inertia, worse traction; and vice versa). Another key behavior is noticed for vegetated soils, where a similar thermal inertia to wet sand is obtained but with only a third of moisture content. These results may be considered for maximizing traversability over sandy soils with higher thermal inertias, what eventually means higher compaction and safer routes. To the authors’ knowledge, this is the first work addressing the correlation between moisture content and vegetation, and the thermal properties of a soil using a light-weight thermal camera that can be mounted on a mobile robot.     

Techniques for averting and correcting errors in 3D terrain topology measurements

The emergence of high-fidelity vehicle and tyre models has raised the requirements for 3D terrain measurement capabilities. Inaccuracies that were once tolerable for measurement of general terrain roughness are no longer acceptable for these new applications. The data inaccuracies that arise from small inertial errors are compounded by difficulties in managing massive file sizes. These faults are most apparent when combining multiple lanes of data in the post-processing phase. This work develops two correction techniques: a general method for any terrain type and a more computationally efficient method for smooth terrain. The general terrain correction method uses overlapping points in the horizontal plane to build a discontinuity vector and applies that vector to correct adjacent lanes. While this method effectively eliminates lane-to-lane discontinuities, it is computationally inefficient for terrain that lacks localized disturbances. For simulations using smooth terrain surfaces, such as racetracks or highways, data can be down-sampled through a mean interpolation method to smooth the artefacts of inertial errors. This work provides the vehicle test engineer with guidelines to minimize inertial errors during the data collection phase as well as a post-processing tool to combine multiple lanes of high-fidelity 3D terrain surfaces.     

A lattice model of foam flow in porous media: A percolation approach

Because fluid flow in porous media is opaque to most observational techniques simulations of the processes occurring in porous media have become important. Typical reservoir simulations treat the flow as taking place in some averaged (Darcy-scale) medium but simulations can also be carried out at the level of the network of pores and throats of the porous medium. We report the results of a pore-scale investigation of mechanisms for the alteration of mobility by foam lamella blockage in a network of these spaces and channels of porous media. Saturation and relative permeability curves are obtained using well-known power-law expressions of percolation theory and a rescaling of the percolation parameter readily permits a number of lamella-blocking mechanisms to be treated. An explanation of the shift in breakthrough gas saturation and the deformation of the shape of permeabilityvs saturation curves upon introduction of foam is provided for a variety of blocking mechanisms. The qualitatively different features seen in experimental studies of modification of gas mobility by foam can be rationalized using only two parameters which characterize the throat-size at which blockage commences and the degree of blockage.     

Characterization and modeling of air muscles

Two flexible pneumatic actuators, McKibben and straight fiber, were analyzed. The muscles were modeled by finite element method. The numerical models take into account the non-linearity of the expanding rubber inner tube, and of the mechanism for transferring load to the braided cords surrounding the tube. Because of its own fabrication, McKibben muscle model may capture the initial backslash between rubber tube and cords.Despite of the McKibben specimen, a straight fiber muscle prototype was fabricated on purpose in the laboratory. In order to validate the numerical models, numerous experimental tests were carried out. The numerical results showed a good agreement with the data measured experimentally in terms of muscle pull force and of its deformed shape. Finally, the models of two muscles similar in terms of input energy allow their performances to be compared.     

Sensitivity of Buckling Load and Vibration Frequency with Respect to Shape of Stiffened and Unstiffened Plates

ABSTRACT

The method of calculating the sensitivities of buckling loads and vibration frequencies of nonlinear plates with respect to shape functions is presented. These shape functions describe the external shape of an unstiffened plate or the shape of a stiffener within the domain of a plate with fixed external boundaries. An adjoint variable method is used to determine the sensitivities. Thus, only one additional solution for an adjoint plate is required.