| 1. |
- Agebro, Markus, 1975-
(author)
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Driver Preferences of Steering Characteristics
- 2007
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Licentiate thesis (other academic/artistic)abstract
- The future of vehicle steering systems lies within by-wire technology. With by-wire technology mechanical or hydraulic systems are replaced by electronic systems. Removal of the steering column and possibly other linkage and gears yields vast potential of further improvement of performance, comfort and safety. Steer-by-wire technology also enables the manufacturer to tailor the steering feel to better suit the individual drivers’ need and preference. Since a driver gains critical information about the vehicle from feedback through the steering wheel, steering feel will play a very important part in consumer acceptance of steer-by-wire systems. It will also be possible to customize steering characteristics to the individual driver. This thesis presents a methodology for investigating steering characteristics through analysis of simulator experiments and to find the impact of specific steering characteristics on drivers of varying skill. There are many key aspects to consider when designing simulator experiments. A validated vehicle model is required. Evaluation criteria need to be well defined as well as concise and simple. The utilized scenario has to be able to capture the selected evaluation criteria. Recruitment of test subjects should represent the target population. How to utilize the available time in the simulator most effectively and how to analyze the results are also important. In this work three studies are performed. Paper A investigates how steering gear ratio and steering wheel effort of a passenger car affect preferences of high and low mileage drivers. Paper B is an extended study of Paper A, where the resolution is higher, speed dependence is investigated and performance of the drivers is also evaluated. In Paper C the impact of four important steering system characteristics on driver performance and preference is evaluated. The major conclusions drawn from this work are that variation of steering gear ratio has considerable impact on perceived steering feel and manoeuvrability as well as on driver performance. Variation in steering wheel effort affect perceived steering feel and stability, but no significant influence is detected in perceived manoeuvrability or driver performance. There are distinguishable differences in preferences of the investigated evaluation criteria between driver categories of varying skill. However, general trends of the preferences for the categories are fairly similar. Low skilled drivers prefer lower effort and higher ratio than high skilled drivers, especially at the highest investigated speed, 100 km/h. The developed methodology for performing simulator experiments to evaluate steering characteristics has proven satisfactory through findings of three different studies. This work also shows that there are several important steering characteristics that need to be considered when designing steering systems, particularly steering systems with by-wire applications and especially considering drivers of varying skill.
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| 2. |
- Agebro, Markus, et al.
(author)
-
Driver preferences of steering gear ratio and steering wheel effort : A driving simulator study
- 2006
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In: Proceedings of the FISITA World Automotive Congress, Yokohama, Japan 2006..
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Conference paper (peer-reviewed)abstract
- When driving an automobile, the driver has to correct the course as a result of road curvature and external disturbances. In order to make the vehicle both controllable and comfortable to drive, it is important that the steering system is designed with different drivers in mind. In this work, driver preferences of steering system characteristics is investigated by comparing standard steering wheel settings with unconventional steering gear ratio and steering wheel effort. The investigation is made using 18 test subjects in a moving base driving simulator. The evaluation includes two scenarios. In the first scenario the driver is overtaking a bus at 110 km/h when meeting traffic in the opposite lane. In the second scenario the driver is doing a manoeuvre by following a cone track at 55 km/h. To investigate if there are differences in preference of drivers with varying experience of driving, the drivers are chosen to either be low or high mileage drivers. People that drive less than 5,000 km/year are considered to be low mileage drivers, and people that drive more than 25,000 km/year are considered to be high mileage drivers. The results show that original settings of a typical passenger car, which served as reference, prove to display favourable characteristics compared to the unconventional settings investigated. However, there might be settings within the investigated intervals that can be considered superior. A distinct trend in the results is that increasing effort will lead to increased perceived stability, independent of ratio. High mileage drivers find the setting with low ratio and reference effort to possess better qualities than the reference when evaluating the attributes steering wheel force and response and only slightly less favourable properties than the reference when evaluating the attribute stability. High mileage drivers display a more distinct opinion and a higher sensitivity when evaluating the attributes. Despite the differing setup of the scenarios, many similarities can be observed when studying the results. Even though there are similarities in the results both between the scenarios and the categories of drivers, a study of the individual test subjects´ preferences reveal that several drivers prefer other settings than the reference for the investigated scenarios. Therefore, it is clear that the driver-vehicle system would benefit from tailoring the steering characteristics to the situation and driver.
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| 3. |
- Andreasson, Johan, 1976-
(author)
-
On Generic Road Vehicle Motion Modelling and Control
- 2006
-
Doctoral thesis (other academic/artistic)abstract
- With the increased amount of on-board electric power driven by the ongoing hybridization, new ways to realize vehicles are likely to occur. This thesis outlines a future direction of vehicle motion control based on the assumptions that: 1) future vehicle development will face an increased amount of available actuators for vehicle propulsion and control that will open up for an increased variety of possible configurations, 2) the onboard computational power will continue to increase and allow higher demands on active safety and drivability that will require a tighter interaction between sensors and actuators, 3) the trend towards more individualized vehicles on common platforms with shorter time-to-market require design approaches that allow engineering knowledge to be transferred conveniently from one generation to the next. A methodology to facilitate the selection of vehicle configurations and the design of the corresponding vehicle motion controllers is presented. This includes a method to classify and map configurations and control strategies onto their possible influence on the vehicle's motion. Further, a structured way of implementing and managing vehicle and subsystem models that are easy to reconfigure and reuse is suggested and realised in the developed VehicleDynamics Library. In addition, generic ways to evaluate vehicle configurations, especially the use of the adhesion potential to identify safety margin and expected limit behaviour are presented. Special attention is given to how the characteristics of a vehicle configuration can be expressed so that it can be used in vehicle motion control design. A controller structure that enables a generic approach to this is introduced and within this structure, two methods for control allocation are proposed, via tyre forces and directly. The first method uses a developed mapping of available actuators as constraints onto the achievable tyre forces and inverse tyre models to calculate the actuator inputs. The second method allocates the actuator inputs directly for an adapted problem that is linearized around the current operating point. It is shown that the methods are applicable to a variety of different vehicle configurations without redesign. Therefore, the same controller can manage a variety of vehicle configurations and there is no need to recognize and treat each different situation separately. Finally, a road map on how to continue this research towards a possible industry implementation is given. Also suggestions on more detailed improvements for modelling and vehicle motion control are provided.
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| 4. |
- Dahlberg, E., et al.
(author)
-
The dynamic rollover threshold - a heavy truck sensitivity study
- 2006
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In: International Journal of Vehicle Design. - 0143-3369 .- 1741-5314. ; 40:03-jan, s. 228-250
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Journal article (peer-reviewed)abstract
- Knowledge of rollover mechanics, which is required in heavy truck development, commonly relies on a static analysis, providing the steady-state rollover threshold, SSRT. In a rolling vehicle, kinetic energy is always present and that deteriorates the analysis of roll stability from SSRT and implies the need for a dynamic rollover threshold, DRT as a complement. A method to determine DRT is presented and a parameter sensitivity study The influences on SSRT and DRT, including interaction effects, from roll stiffnesses and roll centre heights, are calculated. Results show that a rigid truck and a tractor semitrailer combination are unequally sensitive to parameter changes. Design changes can hence affect SSRT and DRT differently, which indicates that two vehicles can be equally stable statically but differently dynamically. Therefore, if DRT is not analysed, a redesign of a vehicle can deteriorate roll stability even though it appears to improve it.
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| 5. |
- Drugge, Lars, et al.
(author)
-
A laboratory model study of a railway current collection system
- 2008
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In: Proceedings of the ASME Design Engineering Technical Conferences and Computers and Information in Engineering Conference, Vol 5, PTS A-C. - NEW YORK : AMER SOC MECHANICAL ENGINEERS. ; , s. 1805-1810
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Conference paper (peer-reviewed)abstract
- A vital system on modem high-speed electric trains is the overhead catenary system and the pantograph current collector. As speed limits are increased, train operators and railway engineers need measures of system performance in a number of situations. In this work a laboratory model is built to study the pantograph behaviour on curved track running on a catenary system with large stiffness variation. The model is designed to be simple, yet exhibit the most characteristic dynamic properties of the real system. Another objective is the possibility to run the pantograph at speeds near the wave propagation velocity of the contact wire. The situation of several trailing pantographs, with even spacing, which excites the system to steady state, is considered. Effects of changes in design features such as tension in the contact wire and torsion and translation stiffness of components in the pantograph are studied for different speeds. The interaction is complex and the performance depends on the dynamic properties of both the catenary system and the pantograph. The results show that the pantograph configuration mainly affects the size of amplitudes in the system while the contact wire tension influences at which velocities large amplitudes and contact losses occur.
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| 6. |
- Edrén, Johannes, et al.
(author)
-
Modelica and Dymola for education in vehicle dynamics at KTH
- 2009
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In: Proceedings from 7th Modelica Conference 2009. - : Linköping University Electronic Press. ; , s. 775-783
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Conference paper (peer-reviewed)abstract
- Dymola and Modelica has been used at KTH Vehicle Dynamics (KTHVD) for research work since 2000, see e.g. [1]. With the Vehicle Dynamics Library (VDL) [2], Modelica has become far more accessible for both researchers and students in the field of vehicle dynamics. Therefore a project aiming at introducing it as a tool in education was initiated in order to evaluate the current state of Dymola and Modelica as tools for wider use in education at the division. The work presented in this paper was realized as a part of a PhD course, where one of the tasks were to design dedicated exercises to illustrate fundamentals of vehicle dynamics for students.
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| 7. |
- Holen, Peter, 1975-
(author)
-
On modally distributed damping in heavy vehicles
- 2006
-
Doctoral thesis (other academic/artistic)abstract
- This thesis investigates passive damping system performance in heavy vehicles through analytical expressions, simulations with different vehicle models as well as through experimental evaluation in a tractor semi trailer combination. The objective is to study what levels of chassis suspension damping that are desirable for different vehicle modes and how this may be achieved with passive damping systems. To investigate the influence on performance from damper positioning, analytical expressions for a 2D - suspension model are derived. Geometric key parameters controlling roll and bounce damping are found to be damper vertical aligning and perpendicular distance between damper and suspension roll centre respectively. These parameters are often not easily altered within an already existing vehicle. To investigate performance possibilities from damping not restricted by packaging requirements, the concept with distributed damping is furthermore studied. Theoretical expressions for modally distributed damping are first derived from an analytical tractor model with 7 DOF. Considered motions for which damping is prescribed are bounce, pitch and roll of sprung mass, and axle crossing. These equations are evaluated through various simulations with a 4x2-tractor semi trailer model. Results from simulations show that the conflict in damping demands with passive independent dampers for a single lane change and a one-sided pot hole may be significantly reduced with amplitude dependent modal damping. Vehicle damping performance is not only affected by the dampers positioning and their individual setting, but also by the damper attachment structure. The influence from compliance in e.g. brackets and mounting bushings at damper attachment points is therefore studied. Linear analysis with a simple spring mass damper model shows that damper attachment compliance reduces the damper efficiency. Finite element analyses of both the chassis frame and the tractor are furthermore performed to obtain numerical values of front-axle damper-attachment stiffness. The effect from damper-attachment stiffness is quantified though simulations with a tractor semi trailer model. Simulation results show that it is important to consider the attachment stiffness during vehicle manoeuvres containing high frequency inputs such as the passage over a plank. A methodology and equations for prescribing chassis suspension damping as function of general vehicle modes by using electronically controlled variable dampers is presented. A critical input for such implemented modal damping systems are the real time estimation of modal motions necessary for force calculation. From performed simulations it is shown that geometric calculations of modal velocities based solely on relative damper displacements contain significant discrepancies to actual motion for transient road inputs. To overcome this, a time-domain system identification approach is presented, where models that estimate modal coordinate velocities with considerably higher accuracy are identified. The proposed modal damping approach is implemented on a 4x2 tractor and experimentally evaluated through various road tests. It is shown that the system has the desired ability to control sprung mass bounce and pitch modes separately and that it improves vehicle performance on all tested load cases.
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| 8. |
- Jonasson, Mats
(author)
-
Aspects of autonomous corner modules as an enabler for new vehicle chassis solutions
- 2006
-
Licentiate thesis (other academic/artistic)abstract
- This thesis adopts a novel approach to propelling and controlling the dynamics of a vehicle by using autonomous corner modules (ACM). This configuration is characterised by vehicle controlled functions and distributed actuation and offers active and individual control of steering, camber, propulsion/braking and vertical load. Algorithms which control vehicles with ACMs from a state-space trajectory description are reviewed and further developed. This principle involves force allocation, where forces to each tyre are distributed within their limitations. One force allocation procedure proposed and used is based on a constrained, linear, least-square optimisation, where cost functions are used to favour solutions directed to specific attributes. The ACM configuration reduces tyre force constraints, due to lessen estrictions in wheel kinematics compared to conventional vehicles. Thus, the tyres can generate forces considerably differently, which in turn, enables a new motion pattern. This is used to control vehicle slip and vehicle yaw independently. The ACM shows one important potential; the extraordinary ability to ensure vehicle stability. This is feasible firstly due to closed-loop control of a large number of available actuators and secondly due to better use of adhesion potential. The ability to ensure vehicle stability was demonstrated by creating actuator faults. This thesis also offers an insight in ACM actuators and their interaction, as a result of the force allocation procedure.
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| 9. |
- Jonasson, Mats, et al.
(author)
-
Autonomous corner modules as an enabler for new vehicle chassis solutions
- 2006
-
In: FISTA Transaction. ; 2006:F2006V054T
-
Journal article (peer-reviewed)abstract
- Demands for new functions and refined attributes in vehicle dynamics are leading to more complex and more expensive chassis design. To overcome this, there has been increasing interest in a novel chassis design that could be reused in the development process for new vehicle platforms and mainly allow functions to be regulated by software. The Autonomous Corner Module (ACM) was invented at Volvo Car Corporation (VCC) in 1998. The invention is based upon actively controlled functions and distributed actuation. The main idea is that the ACM should enable individual control of the functions of each wheel; propulsion/braking, alignment/steering and vertical wheel load. This is done by using hubmotors and by replacing the lower control arm of a suspension with two linear actuators, allowing them to control steering and camber simultaneously. Along with active spring/damper and wheel motors, these modules are able to individually control each wheel's steering, camber, suspension and spin velocity. This provides the opportunity to replace mechanical drive, braking, steering and suspension with distributed wheel functions which, in turn, enable new vehicle architecture and design. The aim of this paper is to present the vehicle dynamic potential of the ACM solution, by describing its possible uses and relating them to previous research findings. Associated work suggests chassis solutions where different fractions of the functions of the ACM capability have been used to achieve benefits in vehicle dynamics. For instance, ideas on how to use active camber control have been presented. Other studies have reported well-known advantages, such as, good transient yaw control from in-wheel motor propulsion and stable chassis behaviour from four-wheel steering, when affected by side wind. However, this technology also presents challenges. One example is how to control the relatively large unsprung mass that occurs due to the extra weight from the in-wheel motor. The negative influence from this source can be reduced by using active control of vertical forces. The implementation of ACM, or similar technologies, requires a well-structured hierarchy and control strategy. Associated work suggests methods for chassis control, where tyre forces can be individually distributed from a vehicle path description. The associated work predominately indicates that the ACM introduces new opportunities and shows itself to be a promising enabler for vehicle dynamic functions.
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| 10. |
- Jonasson, Mats, 1969-
(author)
-
Exploiting individual wheel actuators to enhance vehicle dynamics and safety in electric vehicles
- 2009
-
Doctoral thesis (other academic/artistic)abstract
- This thesis is focused on individual wheel actuators in road vehicles intended for vehicle motion control. Particular attention is paid to electro-mechanical actuators and how they can contribute to improving vehicle dynamics and safety. The employment of individual wheel actuators at the vehicle's four corner results in a large degree of over-actuation. Over-actuation has a potential of exploiting the vehicle's force constraints at a high level and of controlling the vehicle more freely. One important reason for using over-actuated vehicles is their capability to assist the driver to experience the vehicle as desired. This thesis demonstrates that critical situations close to the limits can be handled more efficiently by over-actuation. To maximise the vehicle performance, all the available actuators are systematically exploited within their force constraints. Therefore, force constraints for the individually controlled wheel are formulated, along with important restrictions that follow as soon as a reduction in the degrees of freedom of the wheel occurs. Particular focus is directed at non-convex force constraints arising from combined tyre slip characteristics. To evaluate the differently actuated vehicles, constrained control allocation is employed to control the vehicle. The allocation problem is formulated as an optimisation problem, which is solved by non-linear programming. To emulate realistic safety critical scenarios, highly over-actuated vehicles are controlled and evaluated by the use of a driver model and a validated complex strongly non-linear vehicle model. it is shown that, owing to the actuator redundancy, over-actuated vehicles possess an inherent capacity to handle actuator faults, with less need for extra hardware or case-specific fault-handling strategies.
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| 11. |
- Jonasson, Mats, et al.
(author)
-
Investigation of the Non-Convex Force Constraints Imposed by Individual Wheel Torque Allocation
- 2009
-
Other publication (other academic/artistic)abstract
- This paper explains the non-convex tyre force constraints on electric vehicles with individual drive. Given demands from a motion planning on a top level from the vehicle hierarchy, the task to allocate forces to each wheel corner is considered as a mapping process from a global vehicle level. One important role of the allocator is to deliver solutions even if the requirements arising from motion planning are not physically feasible. To secure vehicle stability, fundamental allocator requirements are formulated. However, more demands are needed to avoid undesired vehicle behaviors. These demands, which belong tothe optional allocator requirements, are described. Given the force constraints and the allocator requirements, solutions of the allocation problem to four electricalmachines are solved by non-linear programming.
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| 12. |
- Jonasson, Mats, 1969-, et al.
(author)
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Modelling and parameterisation of a vehicle for validity under limit handling
- 2008
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In: Proceedings of 6th Modelica Conference.
-
Conference paper (peer-reviewed)abstract
- This paper describes how a vehicle model from the VehicleDynamics Library is configured, parameterized and validated for predicting limit handling maneuvers. Especially, attention is given to the selection of subsystem models with suitable levels-of-detail as well the selection of performed measurements and measurement equipment. A strong principle throughout the presented work is component-based design where parameterizations are done on sub-system levels, no tuning on the final vehicle models is made. As a final test, the vehicle model is exposed to a sinusoidal steering input. It turns out that the correspondence between the model used and the real vehicle is acceptable for the driving scenario selected up to the limit of adhesion.
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| 13. |
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| 14. |
- Jonasson, Mats, et al.
(author)
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Utilisation of actuators to improve vehicle stability at the limit: from hydraulic brakes towards electric propulsion
- 2009
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In: Proceedings of the 21st International Symposium on Dynamics of Vehicles on Roads and Tracks (IAVSD’09), KTH Stockholm, 17-21 August 2009.
-
Conference paper (other academic/artistic)abstract
- The capability of over-actuated vehicles to maintain stability during limit handling is studied in this paper. A number of important differently actuated vehicles, equipped with hydraulic brakes towards more advanced chassis solutions, are presented. A virtual evaluation environment has specifically been developed to cover the complex interaction between the driver and the vehicle undercontrol. In order to fully exploit the different actuators set-up, and the hard non-convex constraints they possess, the principle of control allocation by nonlinear optimisation is successfully employed. The final evaluation is made by exposing the driver and the over-actuated vehicles to a safety-critical manoeuvre. Thereby, the differently actuated vehicles are ranked by a quantitativeindicator of stability.
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| 15. |
- Juhlin, Magnus, 1975-, et al.
(author)
-
Aerodynamic loads on buses due to crosswind gusts – On-road measurements
- 2008
-
In: Vehicle System Dynamics. - : Taylor & Francis. - 0042-3114 .- 1744-5159. ; 46, s. 827-835
-
Journal article (peer-reviewed)abstract
- Bus and coach traffic is considered to be one of the safest means of travelling. Still, there is a problem with accidents due to crosswind gusts. Therefore it is a need of improving the crosswind performance of buses. As a part of the work with improving the crosswind performance a method for estimating the aerodynamic loads on a bus when exposed to natural crosswind is proposed. The method is based on measurements of the vehicle response and the tire forces from which the aerodynamic loads are estimated using inverse simulations. The results are also shown to agree well with the results of other studies based on wind tunnel measurements. The estimated aerodynamic loads are intended to be used in a future study on crosswind sensitivity using a moving base simulator.
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| 16. |
- Juhlin, Magnus, 1975-
(author)
-
Assessment of crosswind performance of buses
- 2009
-
Doctoral thesis (other academic/artistic)abstract
- When driving a vehicle on the road, the driver has to compensate continuously for small directional deviations from the desired course due to disturbances such as crosswinds, road irregularities and unintended driver inputs. These types of deviations have a tiring effect on the driver and should therefore be minimised. When the magnitude of these disturbances increases, especially with crosswind, the directional deviation might become so large that the driver will have difficulties in compensating for it, and will thereby affect the traffic safety. The objective of this research work is to increase the understanding of the crosswind sensitivity of buses and to find solutions to the problem of improving the safety of buses with respect to crosswind performance. The work presented in this thesis contributes to increased knowledge about the directional stability of buses under the influence of crosswind gusts through parameter studies using detailed vehicle simulation models, through full-scale experiments and through studies of the effect of steering feel on the subjective and objective evaluation of crosswind performance. A natural crosswind gust model has been derived from wind tunnel measurements and implemented in a multi-body dynamics simulation tool. The aerodynamic loads of the crosswind gust model have been applied on a detailed vehicle model and the behaviour of the vehicle model has been studied for various vehicle configurations in both open- and closed-loop manoeuvres. The vehicle model, with parameters corresponding to real vehicle data, has been validated and the agreement with measurements is good. A method for estimating the aerodynamic loads on a bus due to crosswind on a road section is also presented. Aerodynamic loads under real conditions were estimated using this method and these data were thereafter used in a study where the effect of steering feel on the subjective and objective evaluation of crosswind performance was investigated using a moving-base driving simulator, with the aim of finding a relationship between steering feel and crosswind sensitivity. The thesis covers the influence of changing chassis-related parameters and aerodynamics-related parameters on the crosswind sensitivity, as well as the influence of the setting of the steering system on the crosswind performance of the driver-vehicle system. The results identify areas of high potential for improving the crosswind sensitivity of buses, such as the centre of gravity location and the yaw moment overshoot at gust entry. Furthermore, the study shows the importance of having a vehicle that facilitates prompt driver corrections for reducing the lateral deviation under crosswind excitation; i.e. it is shown that a steering system with the possibility of changing the yaw rate gradient referencing the steering-wheel input when the vehicle is subjected to a sudden crosswind has a good potential for improving the crosswind performance of the driver-vehicle system.
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| 17. |
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| 18. |
- Nilsson, Andreas, et al.
(author)
-
Methodology to find parameters characteristic to path tracking skill : DLC-test in a moving base simulator
- 2009
-
In: Proceedings of the 21st International Symposium: Dynamics of Vehicles on Roads and Tracks.
-
Conference paper (peer-reviewed)abstract
- The objective of this work is to evaluate the driver skill relation to a large number of objective vehicle parameters, all measured in a moving base simulator used in the test. Recruitment of High skill and Low skill drivers is done based on self evaluations by the drivers in relation to driver type descriptions. A moving base simulator with a double lane change (DLC) scenario is used both for recruitment verification and measurement of parameter metrics. Here, a suggested method that is used for the comparison of measures under equal conditions is described, and the best separation between the two recruited driver types is found for the highest velocity with all drivers participating, 70 km/h, in the second part of the manoeuvre, e.g. by using standard deviation of steering wheel rate or angular acceleration.
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| 19. |
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| 20. |
- Noréus, Olof, 1970-, et al.
(author)
-
Measurement of terrain values and drawbar pullfor six wheeled vehicle on sand
- 2008
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In: 16th International Conference of the International Society for Terrain-Vehicle Systems. - Turin.
-
Conference paper (other academic/artistic)abstract
- 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.
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| 21. |
- Noréus, Olof
(author)
-
Modelling of six-wheeled electric transmission terrain vehicle
- 2007
-
Licentiate thesis (other academic/artistic)abstract
- In vehicles with electric transmission and independent wheel stations, it is possible to have a possibility to control propulsion, steering and suspension individually for each wheel. This makes it possible to improve mobility, performance and driving safety. The long term goal of this work is to develop a methodt hat can evaluate and improve the mobility of such vehicles in terrain. This contribution concerns how a six wheeled electric transmission vehicle should be modelled to enable evaluation of the dynamic behaviour in different type of terrain. This is made by combining modelling of vehicle, transmission and tire-terrain behaviour. For wheeled vehicles an electric transmission with hub motors provides the ability to accurately control the torque on every wheel independently, giving a great ability to improve both mobility in terrain and vehicle behaviour on road. In this work the components of a diesel-electric powertrain for off-road vehicles are modelled and a control layout with the possibility to include functions for improved performance both while driving off- and on-road is proposed. To handle driving on soft ground, a tire/terrain model is needed. The model should include lateral deformation in order to be able to steer. A tire/terrain model is derived based on the ideas of Wong and Reece. The terrain characteristics are chosen to be described by parameters according to the Bekker model, since this data are widely available in literature. The developed tire/terrain model has been implemented together with a vehicle model. This terrain vehicle model is shown to be able to estimate sinkage, rolling resistance, traction force and steering characteristics, of a six wheeledterrain vehicle using electric transmission. To conclude, models of a six-wheeled vehicle with electric transmission and tire models both for soft and rigid ground have been developed. These models form a simulation platform, which makes it possible to evaluate control strategies for the electric transmission with the purpose to improve mobility.
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| 22. |
- Rehnberg, Adam, 1975-
(author)
-
Vehicle dynamic analysis of wheel loaders with suspended axles
- 2008
-
Licentiate thesis (other academic/artistic)abstract
- The wheel loader is a type of engineering vehicle used primarily to move crude material over shorter distances. As the vehicle is designed without wheel suspension, wheel loader drivers are exposed to high levels of whole body vibration which influences ride comfort negatively. The work presented in this thesis has the aim to investigate the potential in adding an axle suspension to a wheel loader in order to reduce vibrations and increase handling quality. While suspended axles have great potential for improving ride comfort and performance, they will also necessarily affect the vehicle dynamic behaviour which is different in many aspects from that of passenger cars or other road vehicles: the wheel loader has a large pitch inertia compared to its mass, the axle loads vary considerably with loading condition, and the vehicle uses an articulated frame steering system rather than wheel steering. These issues must all be considered in the design process for a wheel loader suspension. The effects of suspended axles on ride vibrations are analysed by simulating a multibody wheel loader model with and without axle suspension. Results from the simulations show that longitudinal and vertical acceleration levels are greatly reduced with axle suspension, but that the decrease in lateral acceleration is smaller. By reducing the roll stiffness lateral accelerations can be further reduced, although this may not be feasible because of requirements on handling stability. The pitching oscillation of the vehicle has also been studied as this is known to have a large influence on ride comfort. An analytical model is used to study the effect of front and rear suspension characteristics on the pitching response of the wheel loader, showing that a stiffer rear suspension is favourable for reducing pitching but also that a similar effect is attainable with a stiffer front suspension. Results are compared to multibody simulations which show the same trend as analytical predictions. By including a linearised representation of a hydropneumatic suspension in the models, it is also shown that favourable dynamic behaviour can be maintained when the vehicle is loaded by utilising the fact that suspension stiffness is increasing with axle load. Articulated vehicles may exhibit lateral oscillations known as "snaking" when driven at high speed. The effect of suspended axles on these oscillations are analysed using a multibody simulation model of a wheel loader with an equivalent roll stiffness suspension model. It is found that the roll motion of the sprung mass has a slightly destabilising effect on the snaking oscillations. This effect is more pronounced if the body roll frequency is close to the frequency of the snaking motion, although this loss in stability can be compensated for by increasing the equivalent stiffness or damping of the steering system. Together with existing vehicle dynamic theory and design rules, the studies reported in this work provide an insight into the specific issues related to suspension design for wheel loaders.
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| 23. |
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| 24. |
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| 25. |
- Svahn, Fredrik, 1978-
(author)
-
Low-cost control of discontinuous systems including impacts and friction
- 2007
-
Licentiate thesis (other academic/artistic)abstract
- For a successful design of an engineering system it is essential to pay careful attention to its dynamic response. This is particularly true, in the case of nonlinear systems, since they can exhibit very complex dynamic behaviour, including multiple co-existing stable solutions and chaotic motions, characterized by large sensitivity to initial conditions. In some systems nonlinear characteristics are desired and designed for, but in other cases they are unwanted and can cause fatigue and failure. A type of dynamical system which is highly nonlinear is discontinuous or non-smooth systems. In this work, systems with impacts are primarily investigated, and this is a typical example of a discontinuous system. To enhance or optimize the performance of dynamical systems, some kind of control can be implemented. This thesis concerns implementation of low-cost control strategies for discontinuous systems. Low-cost control means that a minimum amount of energy is used when performing the control actions, which is a desirable situation regardless of the application. The disadvantage of such a method is that the performance might be limited as compared with a control strategy with no restrictions on energy consumption. In this work, the control objective is to enforce a continuous or discontinuous grazing bifurcation of the system, whichever is desirable. In Paper A, the dynamic response and bifurcation behaviour of an impactoscillator with dry friction is investigated. For a one-degree-of-freedom model of the system, analytical solutions are found in separate regions of state space. These are then used to perform a perturbation analysis around a grazing trajectory. Through the analysis, a condition on the parameters of the system is derived, which assures a continuous grazing bifurcation. It is also shown that the result has bearing on the dynamic response of a two-degree-of-freedom model of the system. A low-cost active control strategy for a class of impact oscillators is proposed in Paper B. The idea of the control method is to introduce small adjustments in the position of the impact surface, at discrete moments in time, to assure a continuous bifurcation. A proof is given for what control parameters assures the stabilization. In Paper C, the proposed low-cost control method is implemented in a quarter-car model of a vehicle suspension, in order to minimize impact velocities with the bumpstop in case of high amplitude excitation. It is shown that the control method is effective for harmonic road excitation.
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| 26. |
- Svahn, Fredrik, 1978-
(author)
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On the stability and control of piecewise-smooth dynamical systems with impacts and friction
- 2009
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Doctoral thesis (other academic/artistic)abstract
- This thesis concerns the analysis of dynamical systems suitable to be modelled by piecewise-smooth differential equations. In such systems the continuous-in-time dynamics is interrupted by discrete-in-time jumps in the state or governing equations of motion. Not only can this framework be used to describe existing systems with strong nonlinear behaviour such as impacts and friction, but the non-smooth properties can be exploited to design new mechanical devices. As suggested in this work it opens up the possibility of, for example, fast limit switches and energy transfer mechanisms. Particularly, the dynamics at the onset of low-velocity impacts in systems with recurrent dynamics, so called grazing bifurcations in impact-oscillators, are investigated. As previous work has shown, low-velocity impacts is a strong source of instability to the dynamics, and efforts to control the behaviour is of importance. This problem is approached in two ways in this work. One is to investigate the influence of parameter variations on the dynamic behaviour of the system. The other is to implement low-cost control strategies to regulate the dynamics at the grazing bifurcation. The control inputs are of impulsive nature, and utilizes the natural dynamics of the system to the greatest extent. The scientific contributions of this work is collected in five appended papers. The first paper consists of an experimental verification of a map that captures the correction to the smooth dynamics induced by an impact, known in the literature as the discontinuity map. It is shown that the lowest order expansion of the map accurately captures the transient growth rate of impact velocities. The second paper presents a constructive proof of a control algorithm for a rather large class of impact oscillators. The proof is constructive in the sense that it gives control parameters which stabilizes the dynamics at the onset of low-velocity impacts. In the third paper a piecewise-smooth quarter-car model is derived, and the control strategy is implemented to reduce impact velocities in the suspension system. In the fourth and fifth papers the grazing bifurcation of an impact oscillator with dry friction type damping is investigated. It turns out that the bifurcation is triggered by the disappearance of an interval of stable stick solutions. A condition on the parameters of the system is derived which differentiates between stable and unstable types of bifurcation scenarios. Additionally, a low-cost control strategy is proposed, similar to the one previously mentioned, to regulate the bifurcation scenario.
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