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Motion modelling and control strategies of over-actuated vehicles
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Vehicle Dynamics.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With the growing concern for environmental change and uncertain oil resources, the development of new vehicle concepts will in many cases include full or partial electric propulsion. The introduction of more advanced powertrains enables vehicles that can be controlled with a variety of electric actuators, such as wheel hub motors and individual steering. With these actuators, the chassis can be enabled to adjust its properties depending on the driving situation.

Manoeuvring of the vehicle, using for example electric propulsion, braking, suspension, steering and camber control may also allow a variety of combinations which, if properly utilised, can increase the outer limits of vehicle performance and safety. The fact that the vehicle has a greater number of actuators than required to control a certain number of degrees of freedom is called over-actuation. Since there is a great need for energy optimised vehicles, energy efficient control is also required. For this reason, this work is about the allocation of wheel forces can improve safety, performance and energy efficiency in future electrified vehicles in different driving situations.

Studies of optimally controlled vehicles show that performance, safety and efficiency can be improved by utilising available actuators in over-actuated vehicles. Path tracking and optimal actuator control signals are evaluated in evasive manoeuvres at low and high friction surfaces. The results show how the forces are distributed differently among the wheels, even though the resulting global forces on the vehicle are similar. Optimal control of camber angles and active suspension show that vehicle performance and safety can be greatly improved. The limits of tyre forces can be increased and better utilised in a way that a passive system is unable to achieve. Actuator performance is also shown to be important, however even low actuator performance is shown to be sufficient to improve vehicle performance considerably. Energy efficiency is also improved as unnecessary vehicle motions are minimised during normal driving and wheel forces are used in a better way.

Simplified algorithms to control available actuators, such as wheel angles, vertical actuation and propulsion torques, have been developed, based on the analysis of the results of the optimisation studies. Analyses of the impact of these simplifications have been made. For the cases studied, it has been shown that it is possible to get significantly better performance at reasonable levels of actuator performance and control complexity. This helps to simplify the introduction of this technology in electrified vehicles.

Control allocation is a method that distributes the wheel forces to produce the desired response of the vehicle. Simplified control allocation algorithms are proposed that allocate wheel forces in a way that resembles the behaviour of the optimisation solutions. To be able to evaluate the applicability of this methodology for implementation in vehicles, a small-scale prototype vehicle with force allocation control possibilities has been designed and built. The vehicle is equipped with autonomous corner module functionality that enables individual control of all wheels regarding steering, camber, propulsion/braking and vertical loads. Straight-line braking tests show that force allocation can be used in a real vehicle and will enhance performance and stability even at a very basic level, using few sensors with only the actual braking forces as feedback.

In summary, this work has contributed to a better understanding of how the allocation of wheel forces can improve vehicle safety, performance and energy efficiency. Moreover, it has contributed to increased understanding of how vehicle motions should be modelled and simulated, and how control strategies for over-actuated vehicles can be made more suitable for implementation in future electrified vehicles.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2014. , x, 44 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2014:75
National Category
Vehicle Engineering
Identifiers
URN: urn:nbn:se:kth:diva-155919OAI: oai:DiVA.org:kth-155919DiVA: diva2:763404
Public defence
2014-12-03, F3, Lindstedtsvägen 26, KTH, Stockholm, 09:00 (English)
Opponent
Supervisors
Projects
Generic vehicle motion modelling and control for enhanced driving dynamics and energy management
Note

Finansierat av SHC, Swedish Hybrid Vehicle Centre. QC 20141114

Available from: 2014-11-14 Created: 2014-11-14 Last updated: 2014-11-14Bibliographically approved
List of papers
1. Road friction effect on the optimal vehicle control strategy in two critical manoeuvres
Open this publication in new window or tab >>Road friction effect on the optimal vehicle control strategy in two critical manoeuvres
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2014 (English)In: International Journal of Vehicle Safety, ISSN 1479-3105, Vol. 7, no 2, 107-130 p.Article in journal (Refereed) Published
Abstract [en]

This paper presents results on how to optimally negotiate two safety-critical vehicle manoeuvres depending on available actuators and road friction level. The motive for this research has been to provide viable knowledge of limitations of vehicle capability under the presence of environmental preview sensors, such as radar, camera and navigation. An optimal path is in this paper found by optimising the sequence of actuator requests during the two manoeuvres. Particular interest is paid on how the vehicle control strategy depends on friction. This work shows that actuation of forces and torques on and around the vehicle centre of gravity are all approximately scaled with the friction coefficient. However, this pattern is not valid at a wheel individual level, i.e. the optimal force allocation among the wheels differs under different friction conditions. One key is that lower friction level yields lower load transfer which substantially influences the wheel individual tyre force constraints.

Keyword
Vehicle Control; Actuator; Integrated Motion Control; Road Friction; Optimisation
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-48965 (URN)10.1504/IJVS.2014.060145 (DOI)2-s2.0-84897470574 (Scopus ID)
Note

Updated from "Submitted" to "Published" QC 20141114

Available from: 2011-11-24 Created: 2011-11-24 Last updated: 2014-11-14Bibliographically approved
2. Utilization of Vertical Loads by Optimization for Integrated Vehicle Control
Open this publication in new window or tab >>Utilization of Vertical Loads by Optimization for Integrated Vehicle Control
2012 (English)In: Proceedings of AVEC12, 11th Symposium on Advanced Vehicle Control, September 9-12, Seoul, Korea, 2012., 2012Conference paper, Oral presentation only (Other academic)
Abstract [en]

This paper presents results on how to optimally utilise vertical loading on individual wheels in order to improve vehicle performance during limit handling. Numerical optimisation has been used to find solutions on how the active suspension should be controlled and coordinated together with friction brakes and electric power assisted steering (EPAS). Firstly, it is investigated whether the brake distance can be shortened. Secondly, the performance during an evasive manoeuvre is investigated. The result shows that brake distance can be improved by at least 0.5 m and the speed through the evasive manoeuvre by roughly 1 km/h for the studied vehicle. Quick actuators is shown to give even better performance. These results provide guidance on how active suspension can be used to give significant improvements in vehicle performance.

National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-155916 (URN)
Conference
Proceedings of AVEC12, 11th Symposium on Advanced Vehicle Control, September 9-12, Seoul, Korea, 2012.
Note

QC 20141114

Available from: 2014-11-14 Created: 2014-11-14 Last updated: 2014-11-14Bibliographically approved
3. Utilization of optimization solutions to control active suspension for decreased braking distance
Open this publication in new window or tab >>Utilization of optimization solutions to control active suspension for decreased braking distance
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

This work deals with how to utilize active suspension on individual vehicle wheels in order to improve the vehicle performance during straight-line braking. Through numerical optimization, solutions have been found to how active suspension should be controlled and coordinated with friction brakes to shorten the braking distance. The results show that, for the studied vehicle, the braking distance can be shortened by more than 1 m when braking from 100 km/h. The applicability of these results is studied by investigating the approach for different vehicle speeds and actuator stroke limitations. It is shown that substantial improvements in the braking distance can also be found for lower velocities, and that the actuator strokes are an important parameter. To investigate the potential of implementing these findings in a real vehicle, a validated detailed vehicle model equipped with active struts is analysed. Simplified control laws, appropriate for on-board implementation and based on knowledge of the optimized solution, are proposed and evaluated. The results show that substantial improvements of the braking ability, and thus safety, can be made using this simplified approach. Particle model simulations have been made to explain the underlying physics and limitations of the approach. These results provide valuable guidance on how active suspension can be used to achieve significant improvements in vehicle performance with reasonable complexity and energy consumption.

Keyword
Vehicle control, Actuator, Integrated chassis control, Optimization, Active suspension
National Category
Vehicle Engineering
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-155915 (URN)
Note

QS 2014

Available from: 2014-11-14 Created: 2014-11-14 Last updated: 2014-11-14Bibliographically approved
4. Exploring active camber to enhance vehicle performance and safety
Open this publication in new window or tab >>Exploring active camber to enhance vehicle performance and safety
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2013 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The aim of this study is to evaluate optimal active camber strategies for improvement of vehicle performance and safety during limit handling. Numerical optimisation is used to find solutions on how the active camber should be controlled and coordinated in cooperation with individual braking and front axle steering. Based on the characteristics of a multi-line brush tyre model, a Simple Magic Formula description is developed where camber dependency, load sensitivity and first order speed dependent relaxation dynamics are included. The vehicle is analysed during an evasive manoeuvre when the vehicle is running at the limit. It is evident from the results that active camber control can improve safety and performance during an avoidance manoeuvre.

National Category
Vehicle Engineering
Research subject
SRA - Transport
Identifiers
urn:nbn:se:kth:diva-138555 (URN)
Conference
23rd International Symposium on Dynamics of Vehicles on Roads and Tracks, 19th-23rd of August 2013, Qingdao, China
Funder
TrenOp, Transport Research Environment with Novel Perspectives
Note

QC 20140204

Available from: 2013-12-19 Created: 2013-12-19 Last updated: 2015-05-07Bibliographically approved
5. Energy efficient cornering using over-actuation
Open this publication in new window or tab >>Energy efficient cornering using over-actuation
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

This work deals with utilisation of active steering and propulsion on individual wheels in order to improve a vehicle’s energy efficiency during a double lane change manoeuvre at moderate speeds. Through numerical optimization, solutions have been found for how wheel steering angles and propulsion torques should be used in order to minimise the energy consumed by the vehicle travelling through the manoeuvre. The results show that, for the studied vehicle, the cornering resistance can be reduced by 10% compared to a standard vehicle configuration. Based on the optimization study, simplified algorithms to control wheel steering angles and propulsion torques that are more energy efficient are proposed. These algorithms are evaluated in a simulation study that includes a path tracking driver model and an energy efficiency improvement of 6-9% based on a combined rear axle steering and torque vectoring control during cornering is found. The results indicate that in order to improve energy efficiency for a vehicle driving in a non-safety-critical situation the force distribution should be shifted towards the front wheels.

Keyword
Vehicle control, Energy efficiency, Over-actuation, Optimization
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-155918 (URN)
Note

QS 2014

Available from: 2014-11-14 Created: 2014-11-14 Last updated: 2014-11-14Bibliographically approved
6. The developement of a down-scaled over-actuated vehicle equipped with autonomous corner module functionality
Open this publication in new window or tab >>The developement of a down-scaled over-actuated vehicle equipped with autonomous corner module functionality
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2010 (English)In: FISITA Proceedings 2010, paper F2010B056, 2010Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents the development of a functional down-scaled prototype of a passenger car with capability to control steering, wheel torques, wheel loads and camber individually. The adopted chassis technology is based on a modularised platform, referred to as Autonomous corner modules (ACM), which simplifies the re-use of components at the four corners of the vehicle and between different vehicles.

This work gives an insight in the design of the vehicle and the selection of electrical actuators and sensors to provide all ACM functions. Since a part of the implemented chassis components do not admit to be scaled down at the same level, necessary design modifications are suggested. The problems of scaling, meaning that a down-scaled prototype cannot fully emulate a full-scaled vehicle’s all functions simultaneously, are a great disadvantage of down scaling. For example is gravity one desired parameter that is hard to physically scale down.

In order to evaluate the behaviour of the down-scaled prototype, it is of high importance to establish the characteristics of the developed vehicle and its subsystems. In particular, tyre design is considered as complex. For this reason, different ideas of methods to confirm tyre characteristics are proposed.

Also the paper presents the initial process of developing the prototype vehicle that is later to be used in vehicle dynamics research.

Keyword
autonomous corner modules, active chassis, down-scaled prototype, over-actuation
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-48917 (URN)
Conference
FISITA 2010
Note

QC 20111124

Available from: 2011-11-24 Created: 2011-11-24 Last updated: 2014-11-14Bibliographically approved
7. Implementation and evaluation of force allocation control of a down-scaled prototype vehicle with wheel corner modules
Open this publication in new window or tab >>Implementation and evaluation of force allocation control of a down-scaled prototype vehicle with wheel corner modules
2013 (English)In: International Journal of Vehicle Systems Modelling and Testing, ISSN 1745-6436, Vol. 8, no 4, 335-363 p.Article in journal (Refereed) Published
Abstract [en]

The implementation of wheel corner modules on vehicles creates new possibilities of controlling wheel forces through the utilisation of multiple actuators and wheel motors. Thereby new solutions for improved handling and safety can be developed. In this paper, the control architecture and the implementation of wheel slip and chassis controllers on a down-scaled prototype vehicle are presented and analysed. A simple, cost-effective force allocation algorithm is described, implemented and evaluated in simulations and experiments. Straight line braking tests were performed for the three different controller settings individual anti-lock brakes (ABS), yaw-torque-compensated ABS and force allocation using both wheel torque and steering angle control at each wheel. The results show that force allocation is possible to use in a real vehicle, and will enhance the performance and stability even at a very basic level, utilising very few sensors with only the actual braking forces as feedback to the chassis controller.

Keyword
Active chassis, Force allocation, Over-actuation, Scaling, Slip control, Stability, Wheel corner modules
National Category
Vehicle Engineering
Identifiers
urn:nbn:se:kth:diva-48964 (URN)10.1504/IJVSMT.2013.057528 (DOI)2-s2.0-84887911818 (Scopus ID)
Note

QC 20140116. Updated from submitted to published.

Available from: 2011-11-24 Created: 2011-11-24 Last updated: 2014-11-14Bibliographically approved

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  • en-US
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