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Improving a six-wheeler’sperformance both on- and off-road
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Vehicle Dynamics.
2010 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

In vehicles with electric transmission and independent wheel stations, there is apossibility to control propulsion, steering and suspension individually for eachwheel. This makes it possible to improve mobility in terrain as well as performanceand driving safety on road. This contribution concerns how a six wheeledelectric transmission vehicle should be modelled to enable evaluation of thedynamic behaviour both in terrain and on road. This is made by combiningmodelling of vehicle, transmission and tyre/terrain behaviour.A tyre/terrain model is needed to simulate driving on soft ground. Heretyre/terrain models for simulating driving with both rigid and pneumatic wheelson soft ground have been developed. A method to measure terrain parametersand drawbar pull for a six-wheeled vehicle on sand is proposed, tested andevaluated.To simulate a six wheeled vehicle at the handling limit on road, a vehicle modelwith a brush tyre model is used in order to get physically reasonable simulationresults during high combined slip conditions. Different vehicle configurationsare considered, where front wheel steering is combined with eithersecond axle steering, rear wheel steering or individual wheel torque control.By applying different vehicle slip angles and thereby limiting the DOF of thevehicle model, the vehicle configurations are evaluated during different drivingconditions. The results show that by applying individual torque control to thefront wheel steered vehicle, the performance is improved for all evaluated manoeuvres,and the achievable aligning torque during a rear wheel skid increasessignificantly if the vehicle slip angle is larger than the maximum front wheelsteering angle.To conclude, models of a six-wheeled vehicle with electric transmission andtyre models both for soft and rigid ground have been developed. These modelsform a simulation platform, which makes it possible to evaluate controlstrategies for the electric transmission with the purpose to improve mobility interrain as well as performance and driving safety on road. Some examples ofapplications of the models are included, e.g. improving at-the-limit handlingand pivot turning performance.

Place, publisher, year, edition, pages
Stockholm: KTH , 2010. , viii, 69 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2010:61
National Category
Vehicle Engineering
Identifiers
URN: urn:nbn:se:kth:diva-25519ISBN: 978-91-7415-766-6 (print)OAI: oai:DiVA.org:kth-25519DiVA: diva2:358889
Public defence
2010-11-09, F3, Lindstedtsvägen 26, KTH, Stockholm, 10:00 (English)
Opponent
Supervisors
Note
QC 20101026Available from: 2010-10-26 Created: 2010-10-25 Last updated: 2010-10-26Bibliographically approved
List of papers
1. Six-wheeled vehicle steering
Open this publication in new window or tab >>Six-wheeled vehicle steering
2005 (English)In: Proceedings of the 15th International Conference of the ISTVS, 2005Conference paper, Published paper (Refereed)
Abstract [en]

Six-wheeled vehicles provide the ability to get high performance both on- and off-road. Ackermannsteering on six-wheeled vehicles is possible to achieve in several ways by steering on differentpair of wheels. The steering system has several contradicting demands such as volume saving wheelhouses, few actuators, small turning radius and fast lateral acceleration response. Some of the demandsare velocity dependent. Fast lateral acceleration response is important at high velocity, while a low turningradius is desired at low velocity. In this paper three different steering methods are considered: (a)steering on front and rear wheels, (b) steering on front and middle wheels and (c) steering only on thefront wheels. The steering methods are evaluated by simulating J-turn maneuvers in Matlab/Simulink andAdams. Considering turning radius, lateral acceleration and yaw rate it is shown that method (a) is best atlow speed, while method (b) gives the best stability at high speeds and method (c) uses few actuators andhas acceptable performance at high velocities. A drawback with method (c) is that the vehicle becomesmore sensitive to the load distribution, i.e. braking, accelerating and moving the load will affect steeringcharacteristics more for method (c) than for the other methods. A steering method using method (a) atlow speeds with a gradually transition to method (c) as the velocity increases is suggested.

Keyword
Six-wheeled vehicles, steering, modeling, simulation, lateral acceleration, stability
Identifiers
urn:nbn:se:kth:diva-6845 (URN)
Conference
Proceedings of the 15th International Conference of the ISTVS. Hayama, Japan September 25–29, 2005
Note
QC 20101026Available from: 2007-03-01 Created: 2007-03-01 Last updated: 2010-10-26Bibliographically approved
2. Simulation of electric transmission for off-road vehicle
Open this publication in new window or tab >>Simulation of electric transmission for off-road vehicle
2005 (English)In: Proceedings of the 15th International Conference ofthe ISTVS, 2005Conference paper, Published paper (Refereed)
Abstract [en]

Electric transmissions can be used both on tracked and wheeled vehicles. For wheeled vehiclesthe electric transmission with hub motors provides the ability to accurately control the torque on everywheel independently, giving a great ability to improve both mobility in terrain and vehicle behaviour onroad. In this paper the components of a diesel-electric powertrain for off-road vehicles are modeled anda control layout with the possibility to include functions for improved performance both while driving offandon-road is proposed. The modeled electrical machines are of permanent magnet type with switchingpower electronics for motor currents control. Since the aim is to provide a framework when developingcontrollers for improved vehicle behaviour the models are of mean value type and the switching operationis not included. Components included in the models are a diesel engine, permanent magnet generator,controlled rectifier in order to use full DC-link voltage even for low diesel rpm, DC-link and power electronicsfor drive motors with field weakening at high speeds. All models are simulated in Simulink togetherwith a simplified vehicle model and the results show that it is possible to control the transmission withsimple ramp limitations providing a great possibility to let the electric transmission stay partially functionalto inhibit damages in case of a main controller failure.

Keyword
Electric transmission, simulation, vehicle control
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-6846 (URN)
Conference
Proceedings of the 15th International Conference of the ISTVS. Hayama, Japan September 25–29, 2005
Note
QC 20101026Available from: 2007-03-01 Created: 2007-03-01 Last updated: 2010-10-26Bibliographically approved
3. Tire/terrain models for simulating six-wheeled vehicle on soft ground
Open this publication in new window or tab >>Tire/terrain models for simulating six-wheeled vehicle on soft ground
2006 (English)In: In Proceedings of the 10th European Conference of the ISTVS, 2006Conference paper, Published paper (Refereed)
Abstract [en]

This paper is a part of the work in the project “Cross CountryDriving Dynamics”, which is a project within the Swedish“Green Car”initiative.The purpose of the project is to find out how control of the electric transmissionfor terrain vehicles can be used to improve the off-road performance. In thefirst part of the project models for the vehicle are developed in order to beable to use simulations when developing control methods. The steering systemand simulations of the electric transmission were presented as two papers onISTVS in Japan 2005 and together with the tire/terrain models in this paperit is possible to simulate a six-wheeled vehicle with electric transmission onsoft ground. There are a lot of different purposes for tire/terrain models. Thesimplest models are created with the purpose to give a rough estimate of themobility of the vehicle on different terrains. FEM models on the other hand cangive deep insight in the interaction between tire and soil and also features of thetires. However, FEM models are computational intensive and therefore cannotbe practically used for vehicle simulations, but they can assist by providing datawhen developing models for such simulations. In vehicle simulations mainly thevertical force, rolling resistance, torque, sinkage and lateral force are of interest.Based on experimental data, Bekker has proposed a pressure-sinkage model,which can be used when predicting off-road vehicle performance. Beneath atire the soil will flow both in the lateral and longitudinal direction. In thelongitudinal direction, the soil will flow forwards and backwards in differentzones. Due to the two failure zones the pressure in the ground around the tirewill not increase with depth as in the pressure-sinkage model, but instead themaximum pressure will be at the point where the two failure zones join eachother. The position of that point depends on the slip of the wheel. Wongand Reece have proposed models for the pressure in the ground around a rigidwheel based on experimental results. When using pneumatic tires the tires willbe deflected where the pressure in the ground exceeds the sum of the inflationpressure and the pressure exerted by the tire carcass, which in turn affects thegeometry of the tire/terrain interaction and the pressure in the ground. Sincethe shear force in the ground is a function of the pressure, the tire slip and theshear modulus of the soil, it is possible to calculate the torque of the wheel andthe tractive force. It is possible to handle steering by adding a force ellipse tohandle lateral forces. In order to handle transients it is necessary to includedamping characteristics of the soil. The tire/terrain models are implementedtogether with a vehicle model and it is seen that the differential equations forsoft ground simulations are fairly stiff, but simulations show that it is possibleto solve them and get reasonable simulation speeds on a normal PC.

Keyword
Tire/terrain modeling
Identifiers
urn:nbn:se:kth:diva-6847 (URN)
Conference
In Proceedings of the 10th European Conference of the ISTVS, Budapest, Hungary October 3–6, 2006
Note
QC 20101026Available from: 2007-03-01 Created: 2007-03-01 Last updated: 2010-10-26Bibliographically approved
4. Measurement of terrain values and drawbar pullfor six wheeled vehicle on sand
Open this publication in new window or tab >>Measurement of terrain values and drawbar pullfor six wheeled vehicle on sand
2008 (English)In: 16th International Conference of the International Society for Terrain-Vehicle Systems, Turin, 2008Conference paper, Published paper (Other academic)
Abstract [en]

A method to measure terrain parameters and drawbar pull for a six-wheeled vehicle on sand is proposed,tested and evaluated. The method is developed in order to be able to validate previously proposedtire/terrain models that are developed to simulate the behaviour of a six-wheeled vehicle withelectric transmission on soft ground. Tests were performed at different tire pressures, and it is shownthat the drawbar pull is vastly improved at lower tire pressure. Since the tire/terrain model uses terrainparameters such as pressure–sinkage and shear stress–displacement relationships, the sand propertiesare measured with a Bevameter. Parameters in the pressure–sinkage relationship are estimated to fitthe measured data. Both external and internal shearing properties of the sand are measured using arubber coated shear ring and a shear ring with grousers, respectively. The measured shear behaviouris shown to agree reasonably well with shear curves of simple exponential form. This will be a basein the development of a strategy to get improved cross country characteristics of six-wheeled vehicleswith individually controlled electric transmission on soft ground.

Place, publisher, year, edition, pages
Turin: , 2008
Keyword
mobility, wheeled vehicles, soft ground, Bevameter
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-25520 (URN)2-s2.0-70449611443 (Scopus ID)
Conference
16th International Conference of the International Society for Terrain-Vehicle Systems
Note
Q 20101026Available from: 2010-10-26 Created: 2010-10-25 Last updated: 2010-10-26Bibliographically approved
5. Comparison of measured and simulated drawbar pull forsix wheeled vehicle with radial pneumatic tyres on sand
Open this publication in new window or tab >>Comparison of measured and simulated drawbar pull forsix wheeled vehicle with radial pneumatic tyres on sand
(English)In: Journal of terramechanics, ISSN 0022-4898, E-ISSN 1879-1204Article in journal (Other academic) Submitted
Abstract [en]

To enable improved cross country characteristics of wheeled vehicles with individually controlled transmission on soft ground,suitable tyre/terrain models are needed. Here tyre/terrain models for simulating driving with both rigid and pneumatic wheels onsoft ground have been developed. A method to measure terrain parameters and drawbar pull for a six-wheeled vehicle on sand isproposed, tested and evaluated. The method is developed in order to be able to validate the proposed tyre/terrain models includingboth rigid and pneumatic wheels that are developed to simulate the behaviour of a six-wheeled vehicle with electric transmissionon soft ground. Tests were performed at different tyre pressures, and it is shown that the drawbar pull is vastly improved atlower tyre pressures. Since the tyre/terrain model uses terrain parameters such as pressure–sinkage and shear stress–displacementrelationships, the sand properties are measured with a Bevameter. Parameters in the pressure–sinkage relationship are estimated tofit the measured data. Both external and internal shearing properties of the sand are measured using a rubber coated shear ring anda shear ring with grousers, respectively. Shear curves of simple exponential form from the developed tyre/terrain models are shownto agree reasonably well with the measured shear behaviour. This will be the base in the development of a strategy to get improvedcross country characteristics of six-wheeled vehicles.

Keyword
mobility, wheeled vehicles, soft ground, Bevameter
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-25521 (URN)
Note
QC 20101026Available from: 2010-10-26 Created: 2010-10-25 Last updated: 2017-12-12Bibliographically approved
6. Control of at-the-limit handling behaviour of a six-wheeler: strategies based on individual steer and individual torque control
Open this publication in new window or tab >>Control of at-the-limit handling behaviour of a six-wheeler: strategies based on individual steer and individual torque control
(English)In: Vehicle System Dynamics, ISSN 0042-3114, E-ISSN 1744-5159Article in journal (Other academic) Submitted
Abstract [en]

To simulate a six wheeled vehicle at the handling limit, a vehicle model with a brush tyre model is used in order to get physicallyreasonable simulation results during high combined slip conditions. Different vehicle configurations are considered, where frontwheel steering is combined with either second axle steering, rear wheel steering or individual wheel torque control. By applyingdifferent vehicle slip angles and thereby limiting the DOF of the vehicle model, the vehicle configurations are evaluated duringdifferent driving conditions similar to for example front wheel skidding and rear wheel skidding. The results show that by applyingindividual torque control to the front wheel steered vehicle, the performance is improved for all evaluated manoeuvres, and it is theonly method among the evaluated methods that significantly increases the achievable aligning torque during a rear wheel skid if thevehicle slip angle is larger than the maximum front wheel steering angle. Rear wheel steering, on the other hand, has negligibleeffect on the aligning torque during a rear wheel skid for the six-wheeler.

Keyword
six wheeled vehicles, individual torque control, low ground friction
National Category
Applied Mechanics
Identifiers
urn:nbn:se:kth:diva-25522 (URN)
Note
QS 20120328Available from: 2010-10-26 Created: 2010-10-25 Last updated: 2017-12-12Bibliographically approved

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CiteExportLink to record
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Citation style
  • apa
  • harvard1
  • ieee
  • modern-language-association-8th-edition
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  • en-GB
  • en-US
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  • nn-NO
  • nn-NB
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  • Other locale
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Output format
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