Design of lightweight robots for over-snow mobility

Snowfields are challenging terrain for lightweight (<50 kg) ground robots. Deep sinkage, high snow-compaction resistance, traction loss while turning and ingestion of snow into the drive train can cause immobility within a few meters of travel. However, for suitably designed vehicles, deep snow offers a smooth, uniform terrain that can obliterate obstacles. Key requirements for good over-snow mobility are low ground pressure, large clearance relative to vehicle size and a drive system that tolerates moist, compactable snow.A small robot will invariably encounter deep snow relative to its ground clearance and thus must travel over the snow rather than gain support from the underlying surface. This can be accomplished using low-pressure tracks (<1.5 kPa). Even still, snow-compaction resistance can exceed 20% of vehicle weight. Also, despite relatively high traction coefficients for low track pressures, differential or skid steering is difficult because the outboard track can easily break traction as the vehicle attempts to turn against the snow. Short track lengths (relative to track separation) or coupled articulated robots offer steering solutions for deep snow.This paper presents guidance to design lightweight robots for good mobility over snow based on tests of two custom-designed over-snow robots, SnoBot and SnoBot-2, and driving experience with two commercially available robots, PackBot and Talon. Moreover, we used the present guidance to design SnoBot-2, and it displays excellent over-snow mobility. Because many other considerations constrain robot designs, this guidance can also help with development of winterization kits to improve the over-snow performance of existing robots.     

Development of a tracked vehicle to study the influence of vehicle parameters on tractive performance in soft terrain

This paper describes a new special tracked vehicle for use in studying the influence of different vehicle parameters on mobility in soft terrain; particularly muskegg and deep snow. A field test in deep snow was carried out to investigate the influence of nominal ground pressure on tractive performance of the vehicle. The vehicle proved useful for studying vehicle parameters influencing the tractive performance of tracked vehicles. The tests show that the nominal ground pressure has a significant effect on the tractive performance of tracked vehicles in deep snow. The decrease in drawbar pull coefficient when the nominal ground pressure is increased and originates at about the same amount from a decrease of the vehicle thrust coefficient, an increase of the belly drag coefficient and an increase of the track motion resistance coefficient.     

The Bekker theory of rolling resistance amended to take account of skid and deep sinkage

The Bekker theory of rolling resistance of free rolling, towed, rigid wheels is amended to take account of both skid and deep sinkage without leading to excessive complexity in the predictive equations. Theoretical relationships between skid and sinkage are derived for a free rolling, towed, rigid wheel on a purely cohesive soil ( = 0) and on a purely frictional soil (c = 0) with a sinkage exponent of unity. Generally, good agreement is found between predicted and measured values of rolling resistance and sinkage at a given vertical load, on both sand and clay soils, at shallow and deep sinkage.     

A modified pressure-sinkage model for small, rigid wheels on deformable terrains

Bekker’s semi-empirically derived equations allow the designers of off-road vehicles to understand and predict vehicle mobility performance over deformable terrains. However, there are several underlying assumptions that prevent Bekker theory from being successfully applied to small vehicles. Specifically, Bekker’s sinkage and compaction resistance equations are inaccurate for vehicles with wheel diameters less than approximately 50 cm and normal loading less than approximately 45 N. This paper presents a modified pressure-sinkage model that is shown to reduce sinkage and compaction resistance model errors significantly. The modification is validated with results from 160 experiments using five wheel diameters and three soil types.     

The effect of wheel width on the rolling resistance of rigid wheels in sand

The effect of width on the rolling resistance of rigid wheels in sand is shown to be very strong, coefficient of rolling resistance increasing rapidly with width at each of the sinkage levels used in the experiments. Wheel skid also increased rapidly as wheel width increased. Prediction of measured results on the narrow wheels using the modified Bekker analysis was quite good although this is shown to be partly fortuitous. Poor correlation was found between measured values of coefficient of rolling resistance and the Freitag sand number. Very good prediction of measured values of coefficient of rolling resistance was found using an expression comprising the square root of the sinkage/dia ratio multiplied by a factor correcting for width/dia ratio. The square root of the sinkage/dia ratio is shown to be the value of coefficient of rolling resistance of a narrow wheel at shallow sinkage predicted from the modified Bekker analysis. It is also shown to be identical to the inverse of the Freitag clay number, with soil cone index value replaced by mean soil radial stress.     

External motion resistance caused by rut sinkage of a tracked vehicle

This paper deals with the external motion resistance of a tracked vehicle caused by rut formation (sinkage) or compression of soil under the tracks. It is shown that the relationship between the applied load and the sinkage for a loading test using a plate is represented by a hyperbola. Based on the above relationship, the external motion resistance caused by the rut formation of a tracked vehicle is estimated by considering the work done by overcoming the ground pressure and the resistance. Further, measurements of the external motion resistance were carried out by using a tracked vehicle and the experimental results are compared with the theoretical ones, and the reliability of the above method is confirmed experimentally.     

Predicting terrain parameters for physics-based vehicle mobility models from cone index data

To provide terrain data for the development of physics-based vehicle mobility models, such as the Next Generation NATO Reference Mobility Model, there is a desire to make use of the vast amount of cone index (CI) data available. The challenge is whether the terrain parameters for physics-based vehicle mobility models can be predicted from CI data. An improved model for cone-terrain interaction has been developed that takes into account both normal pressure and shear stress distributions on the cone-terrain interface. A methodology based on Derivative-Free Optimization Algorithms (DFOA) has been developed in combination with the improved model to make use of continuously measured CI vs. sinkage data for predicting the three Bekker pressure-sinkage parameters, k_c, k_ϕ and n, and two cone-terrain shear strength parameters, c_c and ϕ_c. The methodology has been demonstrated on two types of soil, LETE sand and Keweenaw Research Center (KRC) soils, where continuous CI vs. sinkage measurements and continuous plate pressure vs. sinkage measurements are available. The correlations between the predicted pressure-sinkage relationships based on the parameters derived from continuous CI vs. sinkage measurements using the DFOA-based methodology and that measured were generally encouraging.     

Development of high-mobility tracked vehicles for over snow operations

This paper describes a detailed investigation into the effects of some of the major design features on the mobility of tracked vehicles over snow. The investigation was carried out using the latest version of an advanced computer simulation model, known as NTVPM, developed under the auspices of Vehicle Systems Development Corporation (VSDC), Ottawa, Ontario, Canada. Results show that the road wheel system configuration, initial track tension (i.e., the tension in the track system when the vehicle is stationary on a level, hard ground) and track width have significant effects on vehicle mobility over snow. On deep snow where the vehicle belly (hull) contacts the snow surface, the location of the centre of gravity (C.G.) of the sprung weight in the longitudinal direction has a noticeable effect on vehicle mobility, as it affects the attitude of the belly and the belly–snow interaction. Based on the investigation, a conceptual high-mobility tracked vehicle for over snow operations is discussed. Results of this study will provide the vehicle designer with guiding principles for the development of high-mobility tracked vehicles. It also demonstrates that NTVPM is a useful and effective tool for design and performance evaluation of tracked vehicles from a traction perspective.     

Experimental characterization of dynamic soil-track interaction on dry sand

A set of soil-track interaction relations was made developed for the morbility simulation of tracked or crawler system vehicles on dry, loose sand. These interaction relations were developed specifically for multibody mobility codes in which the soil-vehicle interaction is represented solely by soil-track interaction forces. By employing plate penetration and shear tests, an average pressure-sinkage relation, a shear force-slippage relation, and a sinkage-slippage relation were measured. These plate test data were sufficient only to describe the soil-track interaction on hard ground. On soft ground, however, it was found that intermittent sinkages induced by each passage of the road wheels become important. This dynamic contribution is called “agitation sinkage.” Based on this observation, the sinkage rate (velocity) was decomposed into elastic and plastic rates; the plastic part consists of normal force-induced, slip-induced, and agitation-induced components. Whereas the elastic and the first two components of the plastic sinkage rate were characterized by the conventional plate penetration and plate shear tests, the last term, agitation sinkage, required a new dynamic test in which the sinkage of the track after successive passages of moving road wheels was measured. It is recommended that this new field measurement technique be adopted to characterize the agitation sinkage for various terrains.     

Experimental study of a tracked mobile robot’s mobility performance

This paper proposes an experimental method of predicting the traction performance of a small tracked mobile robot. Firstly, a track-terrain interaction model based on terramechanics is built. Then, an experimental platform of the tracked robot is established, on which the measurement methods of the parameters that influencing the accuracy of the prediction model are introduced and the data post-processing are improved, including drawbar pull, slip ratio, sinkage, track deformation and so on. Based on the experimental data, several key terrain parameters are identified. With the tracked robot platform, the drawbar pull-slip ratio relationship is tested, and the effects on drawbar pull considering different kinds of terrain and the influence of the grousers are analyzed as well. The research results provide a reference for the experimental study on the traction performance of small tracked robots.     

Prediction of sinkage and motion resistance of a tracked vehicle using plate penetration test

The relationship between contact pressure and sinkage must be represented by a mathematical model to estimate the sinkage and the motion resistance due to a vehicle. In this study an approximate and simple pressure-sinkage model is proposed. This model takes into account the effect of the size of the penetration plate on soil response, and includes two soil values that can be obtained by a single plate penetration test. It is submitted that the sinkage and the motion resistance of a tracked vehicle can be estimated by means of the proposed model.     

Expansion of the terrain input base for nepean tracked vehicle performance model, NTVPM, to accept swiss Rammsonde data from deep snow

A study of the correlation between the measured and predicted vehicle performance over undistributed and preconditioned snow using the Nepean Tracked Vehicle Performance Model, NTVPM, has been carried out. It is shown that on undisturbed snow in Fernie, British Columbia, the performance of a BV 206 predicted by NTVPM correlates very well with measured performance obtained in the field. On preconditioned snow, there is also a reasonable correlation between the measured vehicle performance and predicted one using NTVPM. It is found that predictions of vehicle performance made by NTVPM using pressure-sinkage data obtained with the Swiss Rammsonde and with the bevameter are comparable. This indicates that the pressure-sinkage data obtained using the Rammsonde can be used as input to the NTVPM for predicting tracked vehicle performance over snow. It is shown that in comparison with an earlier version, NTVPM-85, the latest version of the Nepean Tracked Vehicle Performance Model, NTVPM-86, which takes into account fully the characteristics of roadwheel suspension systems, provides improved predictions of vehicle performance over snow where track sinkage is significant. It is suggested that the computer simulation model NTVPM, using pressure-sinkage data obtained by the Rammsonde as input, could form a useful interface with cone based models, such as the NATO Reference Mobility Model, to provide them with an additional capability of predicting tracked vehicle performance over snow.     

An overview of methods to convert cone index to bevameter parameters

This paper reviews experimental methods for the conversion of cone index measurements to bevameter parameters in support of vehicle soil/tire/track interactions for two general soil types, sand and lean clay. The accurate prediction of traction, motion resistance, and sinkage of tire/tracks off-road requires estimates of soil strength. Equipment used in the measurement of soil strength to support predictions of off-road mobility include the bevameter and the cone penetrometer. The portability of the cone penetrometer and rapid estimates of spatial/temporal variability in all terrain conditions make it an invaluable tool. The bevameter, a less portable tool, is used for the mechanical analysis of soils. The bevameter measures parameters defining soil strength in terms of cohesive modulus of soil deformation (k_c), frictional modulus of soil deformation (k_φ), exponent of soil sinkage (n), cohesion (c), angle of internal friction (φ), and the plate pressure at 1 in. (2.54 cm) of penetration (K) (Bekker, 1969). The field of terramechanics would greatly benefit from having the ability to convert cone penetrometer data in areas where bevameter parameters are difficult to collect. That ability to convert from cone index to bevameter parameters could be used for the large sets of existing cone index data to support determination of traction and motion resistance. This paper examines those methods for converting cone index to bevameter plate penetration parameters k_c, k_φ, and n.     

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.     

Advanced mobility in difficult terrain

The development and success of the Swedish Combat Vehicle CV90 has demonstrated the abilities of the author in the field of terramechanics related to tracked military vehicles. The honour of the Bekker–Reece–Radforth Award 2002 has been granted in recognition of these achievements made during the author's employment at Hägglunds Vehicle AB since 1975. Hägglunds Vehicle AB has been a producer of military vehicles since the late 1950s, although the first years concentrated on production only. From the early 1960s, Hägglunds developed a number of its own tracked vehicles, all of which were influenced by the mobility demands dictated by their intended use in severe terrain conditions, such as those found in Northern Scandinavia. This paper presents a brief history of the advancement of tracked vehicle technology at Hägglunds Vehicle AB. The concepts discussed include: ground pressure, the number of road-wheels, articulated steering, track tension, track attack angle, sinkage, belly effects, and the use of terramechanic simulation. The success of the CV90 demonstrates that the combination of practical experience, terrain knowledge, and terramechanic simulations can effect substantial improvements in vehicle mobility. Evaluation of the CV90 versus other modern combat vehicles of the same class has shown that the CV90 possesses considerably higher mobility and speed under severe terrain conditions. These two attributes provide CV90 with the ability to access terrain that similar vehicles cannot, thus giving the military user greater mobility options.     

Effects of a roller and a tracked vehicle on the compaction of a high lifted decomposed granite sandy soil

The aim of this research was to innovate a new compaction machinery by comparing experimentally the effects of a two-axle, two wheel road roller and a tracked vehicle on the compaction of a decomposed granite sandy soil with a high spreading lift. By measuring the amount of sinkage of the terrain surface, the dry density distribution versus depth using a cone penetrometer, the normal earth pressure distribution versus depth using a stress state transducer (SST), the effects of the road roller and the tracked vehicle on the increment of the soil dry density were considered theoretically. It was observed that the tracked vehicle showed a larger amount of sinkage and a larger dry density distribution versus depth than the roller. The ratio of shear stress to normal stress was still large enough at the deep stratum, so that an optimal shear strain was developed on the whole range of the high lifted stratum and it increased the soil compaction density due to the dilatancy effect.     

Slip sinkage effect in soil-vehicle mechanics

The paper presents an analysis and quantitative evaluation of the slip sinkage and its effect on the tractive performance of wheeled and tracked vehicles in different soils. The results of this study indicated that to accurately predict the sinkage and motion resistance of a vehicle in a given soil and operating conditions, the slip sinkage effect should be taken into account. An effective analytical formula that takes into consideration the slip sinkage effect on sinkage of plates and vehicles is developed. The formula was validated in different soil conditions and compared with other formulae used in terramechanics for slip sinkage effect predictions.     

Bevameter testing on simulant Fillite for planetary rover mobility applications

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.     

Experimental study on the contact patch pressure and sinkage of a lightweight vehicle on sand

This study focuses on improving the understanding of the mobility of lightweight wheeled vehicles on sand by testing the significance of payload, ground speed, sand gradation/grain size, and sand moisture content on contact patch pressure and tire sinkage. Extensive testing of a lightweight wheeled all-terrain vehicle (ATV) was structured in two experiments. Tire sinkage was measured at the width-wise center of the imprint of both the tread and the carcass. Pressure distribution in the contact patch was recorded using an embedded pressure pad, from which the average and peak (and difference) pressures were found. In the first experiment, measurements were taken each time the ATV was driven over combinations of four plots of groomed sand, two moisture contents, three payloads, and three speeds. Average pressure was highly affected (95+% confidence) by sand grade, vehicle speed, and payload and the interactions of sand grade-speed, and sand grade-moisture content-vehicle speed, and borderline affected (90-94.9% confidence) by the moisture content-speed interaction. In the second experiment, the ATV was driven over each plot of dry sand one hundred consecutive times at one speed-payload combination without grooming between runs, showing the cumulative effect of multiple passes over each sand pit on each measure of mobility.     

An analysis of vehicle power requirements in deep snowpack

Power requirements for tracked vehicles moving in deep medium-to-high density snow are calculated. A volumetric constitutive law is used to determine the energy absorbed by the pressure bulb under the vehicle. Also shearing effects along the pressure bulb wall and in the pressure bulb are taken into account. A parametric study is made to determine the effect of initial density, track pressure, vehicle speed and track geometry on track efficiency. The results show that if the snow material properties are expressible in terms of a constitutive law, the method presented can be of considerable use for predicting certain aspects of vehicle performance in snow and for evaluating the structure of the pressure bulb below the vehicle track.