vertec - dft final report v1.0 260706hut (university-finland) unifi (university-italy) volvo...

TRL Limited

PUBLISHED PROJECT REPORT PPR112

VERTEC – FINAL PROJECT REPORT FOR THE DfT Version: 1.0

by M Dodd (TRL Limited)

Prepared for: Project Record: VERTEC PPAD 9/33/118

Client: Department for Transport (DfT), Transport, Technology & Standards Division, TTS6 (Lawrence Thatcher)

Copyright TRL Limited June 2006 This report has been prepared for the Department for Transport (DfT), Transport, Technology & Standards Division (TTS6). The views expressed are those of the author(s) and not necessarily those of the DfT. Published Project Reports are written primarily for the Customer rather than for a general audience and are published with the Customer’s approval.

Approvals

Project Manager

Quality Reviewed

This report has been produced by TRL Limited, under/as part of a Contract placed by the DfT. Any views expressed are not necessarily those of the DfT.

TRL is committed to optimising energy efficiency, reducing waste and promoting recycling and re-use. In support of these environmental goals, this report has been printed on recycled paper, comprising 100% post-consumer waste, manufactured using a TCF (totally chlorine free) process.

TRL Limited PPR 112

CONTENTS

Executive summary i

1 Introduction 1

2 Outputs and objectives 2

3 Summary of work packages 2

3.1 WP1: Definition of modelling environment and reference vehicles, components and manoeuvres 3

3.2 WP2: Tyre-pavement interaction assessment and model implementation 7 3.3 WP3: Reference tests with passenger cars and control systems 11 3.4 WP4: reference tests on HGVs 14 3.5 WP5: Development and validation of passenger car vehicle model 16 3.6 WP6: Development and validation of HGV vehicle model and of driving simulator 22 3.7 WP7: Ranking of the most dangerous situations and new guidelines for the design of safer

products. 26 3.8 WP8: Exploitation and dissemination 30 3.9 List of VERTEC deliverables 31

4 Conclusions 31

Acknowledgements 33

References 33

Appendix A. Tests - Outdoor vehicle passenger car – Tyre Technology Expo Paper 35

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Published Project Report Version: 1.0

Executive summary TRL was part of a consortium working on an EC 5th Framework research project called VERTEC (VEhicle, Road, Tyre, and Electronic Control Systems Interaction).

The aim of the project was to increase vehicle primary safety by developing a fully integrated model for the simulation of the road-tyre-vehicle-driver system in the most potentially dangerous situations. This model was also to be the base for the development of an upgraded driving simulator. Special focus was drawn to the most advanced Vehicle Electronic Control Systems and the representation of both passenger cars and Heavy Good Vehicles (HGVs).

The main outputs of the project were the integrated simulation environment and an improved driving simulator for cars and trucks. The purpose of the outputs was to allow the partners of the consortium to detect and rank the most dangerous driving situations in order to define and supply guidelines for the design of safer roads, vehicles, tyres and electronic devices.

Full scale reference tests using a passenger car and an HGV provided a database of experimental data to help design, develop and validate the best possible models for the vehicle, tyre, driver and road surface.

Simulation environments for both a passenger car and an HGV were successfully created. For the passenger car, two modelling environments were developed. The first was a very accurate vehicle dynamic simulation using a multi-body approach with a co-simulation between ADAMS and Matlab/Simulink. The second model was a simplified “real-time” equivalent developed in Matlab/Simulink. The HGV vehicle model was built using the multi-body environments in ADAMS.

The passenger car Simulink model was adapted and modified for real time use in the VTI driving simulator. It was planned to run the model with the vehicle dynamic stability (ABS, ASR & VSC) controller models developed in this project. However, the source code for these blocks could not be provided and so the simulator was run without the control subsystems. The simulator model was validated for a number of test subjects, with different levels of skill, performing a double lane change manoeuvre. The significant difference in performance found between inexperienced and experienced drivers indicated that future driver models should include a parameter to represent the skill level of the driver.

An analysis of HGV accidents was used to identify the most dangerous situations for each specific road type. The accident data supplied by TRL used a method which combined data from the STATS 19 database as well as the England Trunk Road Database. This meant that it was possible to identify the exact section of the trunk road network where each accident occurred. This was a different method of analysis and its use should be considered in future projects that include accident analysis. TRL delivered the largest amount of data among the partners, including over 20,000 HGV accidents, most of which occurred on rural highways and primary roads. As well as providing a large evidence base that meant that the results of the analysis remained relevant to the UK.

Problems with the development and validation of the models caused delays in delivering the working models and so the programmes of simulations to detect and rank the most dangerous driving situations were not completed by the official end date of the project (December 2005). To date, the programme of simulations for the passenger car have been started but not yet completed. Approximately 250 simulations have been run and the results analysed to identify test configurations that resulted in a dangerous situation.

As there were insufficient data from the simulations it was not possible to define detailed guidelines for the safer design of passenger cars and HGVs. Guidelines for the design of electronic control systems showed that the tyre-characteristic had the strongest influence on the overall vehicle performance. The application of the model for providing variable posted speed limits as a function of the actual handling condition seemed to be the most promising as the model could be implemented on the road side and could potentially provide a regular “refreshed” update of the posted limit.

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This report has been prepared for the UK Department for Transport (DfT) to summarise the findings of the VERTEC project from its start (1st December 2002) until its completion date (30th November 2005).

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1 Introduction TRL was part of a consortium working on an EC 5th Framework research project called VERTEC (VEhicle, Road, Tyre, and Electronic Control Systems Interaction).

The other partners involved in the project were:

• Pirelli Pneumatici SpA (Tyre manufacturer-Italy. Project Co-ordinator)

• Nokian Tyres (Winter Tyre manufacturer-Finland)

• Porsche (Vehicle manufacturer-Germany)

• CRF (Fiat vehicle research centre-Italy)

• TRW (Electronic control system manufacturer-Germany)

• CETE (Road safety organisation-France)

• VTI (Road safety organisation-Sweden)

• HUT (University-Finland)

• UNIFI (University-Italy)

• Volvo (Passenger Car and Heavy Good Vehicle manufacturer-Sweden)

The aim of the project was to increase vehicle primary safety by developing a fully integrated model for the simulation of the road-tyre-vehicle-driver system in the most potentially dangerous situations. This model was also to be the base for the development of an upgraded driving simulator. Special focus was drawn to the most advanced Vehicle Electronic Control Systems and the representation of both passenger cars and Heavy Goods Vehicles (HGVs).

TRL’s involvement in this project was co-funded by the EC (50%), the Department for Transport (25%), and the Highways Agency (25%). TRL involvement is summarised in Table 1.

Table 1: TRL effort in VERTEC

Original Amended0 Project management 2 2

1Definition of modelling environment and reference vehicles, componenet and manoeuvres 3.5 3.5

3Reference tests with passenger car & control systems 6 6

4 Reference tests on HGVs 5 1

6Development and validation of HGV model and Driving Simulator 2 2

7Ranking of the most dangerous situations and new guidelines for design of safer products 4 7

WP Task NameMM Effort

Originally TRL was scheduled to assist in the reference tests on HGVs in WP4 by carrying out some of the tests and as well as helping to define the test specification and analysing the results. As it was not cost effective for Volvo to ship the HGV to TRL it was decided that TRL would only assist with the analysis of the results. The unused man-months (MM) were transferred to WP6 to assist VTI in the implementation of their existing HGV model into the Matlab-Simulink environment. Again not all of the man-months were used in this task so TRL shifted the remaining time into WP7 (Tk7.a) to assist in defining the simulations which were carried out in order to rank the most dangerous situations. In addition TRL was able to assist the other partners by carrying out some of the simulations in the Simulink environment.



This report has been prepared for the UK Department for Transport (DfT) to summarise the findings of the VERTEC project from its start (1st December 2002) until its completion date (30th November 2005).

Some problems were encountered during the development and validation of the computer models which resulted in the project not being completed on time. At the time of writing this report project partners were still working to resolve the problems and to complete the programme of work.

2 Outputs and objectives The main outputs of the project were the following tools:

• Integrated Simulation Environment suitable for transport safety investigation as well as time-to-market reduction, allowing a detailed representation of cars and trucks, electronic control systems, human driver and tyre-road interaction.

• Improved Driving Simulator for cars and trucks using a full vehicle cabin with special motion systems reproducing the dynamics combined with screens reproducing the environment. Such a device can simulate in a very realistic way any driving task of both heavy and light vehicles in any environmental condition, thereby enabling safety investigations without any risk for the drivers.

The exploitation of such tools would allow the partners of the consortium to achieve the following direct goals:

• Detection and Ranking of the most dangerous situations. In particular, such an investigation focuses on the worst combination of environment, weather, road conditions and the interaction between driver and electronic control systems in the case of both cars and trucks.

• Definition of Guidelines for improving the design of vehicles, roads (including their maintenance), tyres and electronic control systems from the point of view of primary safety. These guidelines are expected to contribute to the improvement of safety of all of the above-mentioned products, launched on the market after two years from the end of the project.

3 Summary of work packages The project comprised of nine work packages as shown in Figure 1. WP0 (Project Management) and WP8 (Exploitation and Dissemination) are not shown in the diagram.

WP1: Definition of modelling environment and reference vehicles, components and manoeuvres

WP2: Tyre/Pavement interaction assessment and model implementation

WP3: Reference tests with passenger cars and control systems

WP4: Reference tests on HGVs

WP5: Development and validation of passenger car vehicle model

WP6: Development and validation of HGV vehicle model and of driving simulator

WP7: Ranking of the most dangerous situations and new guidelines for the design of safer products

WP1: Definition of modelling environment and reference vehicles, components and manoeuvres

WP2: Tyre/Pavement interaction assessment and model implementation

WP3: Reference tests with passenger cars and control systems

WP4: Reference tests on HGVs

WP5: Development and validation of passenger car vehicle model

WP6: Development and validation of HGV vehicle model and of driving simulator

WP7: Ranking of the most dangerous situations and new guidelines for the design of safer products

Figure 1: VERTEC work packages



3.1 WP1: Definition of modelling environment and reference vehicles, components and manoeuvres

This work package was split into four sub-tasks. The first sub-task concerned the set-up and implementation of the VERTEC website (http://www.vertec.hut.fi/) to facilitate data exchange and communication between partners and for the exploitation of the project results.

The reference manoeuvres and test conditions to be used in WP3 and WP4 were defined in the second sub-task. A Lancia Lybra 2.4 JTD, provided by CRF, was used as the reference passenger car (Figure 2).

Figure 2: VERTEC reference car - Lancia Lybra 2.4 JTD

Table 2 shows the minimum set of parameters recorded during each test.

Table 2: Passenger car – recorded parameters during reference tests

Pure lateral Pure longitudinal Combined (brake in turn)

1 Steering wheel angle x x x2 Lateral acceleration x x3 Yaw rate x x4 Side slip angle x x5 Roll rate x x6 Throttle x x x7 Velocity x x x8 Longitudinal acceleration x x x9 Pitch rate x x

10 Front left wheel velocity x x x11 Front right wheel velocity x x x12 Rear left wheel velocity x x x13 Rear right wheel velocity x x x14 Front left pressure x x x15 Front right pressure x x x16 Rear left pressure x x x17 Rear right pressure x x x18 Pressure at pump x x19 Brake pedal displacement x x20 Brake pedal force x x

number of channels 16 16 20

The programme of reference tests was shared between the project partners and Table 3 shows the different manoeuvres that were carried out in WP3.



Table 3: Test matrix for reference tests with passenger car

Manoeuvres(1) (2) (1) (2) (1) (2)

Steady state at constant radius (ISO 4138) X X X X X XStep steer (ISO 7401) X X X XISO lane change (ISO 3888) X X X X X XBraking in a curve (ISO 7975) X X X X X XPower off in a curve (50m) X XRandom Steer Input (ISO 7401 and ISO/TR 8726) X XLimit braking (V = 90km/h) Xa Xa Xa Xa X XAcceleration in a straight line X X X X X XTraction in a curve X X X X X XChange of µ in a curve (R=100m) X XPedal force calibration X(1). Open/closed loop manoeuvre - active system OFF(2). Open loop manoeuvre - active system ON (except ISO Lane Change w hich is close d loop - active system ON)(a). tests done on different surfaces and on longitudinal/transverse split surfaces

ConditionsDry Wet Snow/Ice

Tests were carried out using four different types of summer tyre, supplied by Pirelli, and three different types of winter tyre, one of which was a studded tyre, supplied by Nokian Tyres (Figure 1). Both new and worn tyres were tested. The worn passenger tyres had a tread depth of 3mm.

Figure 3: Reference passenger car test tyres: Nokian WR 205/60R15, Pirelli P7 205/60R15

The HGV reference tests were carried out with a typical European 40t vehicle combination consisting of a Volvo FH12 4x2 tractor unit and a three-axle semi trailer loaded with iron blocks in load frames. For safety reasons the vehicle was equipped with out-riggers. The drive axle of the tractor had dual tyres and all other axles had singles, as shown in Figure 4.

Figure 4: VERTEC reference HGV – Volvo FH12



The reference manoeuvres for the HGV included:

• Single lane change

• Entering curve with increasing curvature

• Obstacle avoidance

• Braking in a straight line (on dry and wet surfaces)

Pirelli supplied the tyres for the reference manoeuvres as shown in Figure 5. Similarly to the passenger car, the tyres were tested in a new and worn condition. The worn HGV tyres had a groove depth of 6 mm (trailer 8 mm).

Figure 5: Reference HGV test tyres: Pirelli FH55 315/80R22.5 (tractor front), ST35 315/80R22.5 (tractor rear), TH65 385/65R22.5 (trailer)

The modelling environment was defined in the third sub-task. For the passenger car, two modelling environments were developed, as shown in Figure 6. Two models were required because of two counter-acting requirements. The first requirement was a very accurate vehicle dynamic simulation, even if a slow computational time was a consequence; a multi-body modelling approach with a co-simulation between ADAMS and Matlab/Simulink was chosen for this purpose. The second requirement was that the vehicle model must be used in a driver simulator, so it had to be hardware-in-the-loop capable, i.e. the model must be executed in real time; a simplified equivalent model was chosen for this purpose. For this purpose Pirelli developed a 14 DOF vehicle Simulink-model.

The HGV vehicle model was built using the multi-body environments using some simplifications and assumptions related to flexible body. Details on the design of the model are described in section 3.6.1.

Note: Red blocks are subsystems developed in ADAMS. Blue blocks are subsystems developed in Simulink.

Figure 6: VERTEC simulation environments



The final sub-task of this work package was an extensive literature review carried out among different countries to help the identification of potentially dangerous situations when HGVs are involved. The literature review database was made available to the Consortium through the VERTEC web site and contains a total of 72 papers.

The literature review showed that accident data are characterised by many parameters and that different countries or institutes have their own way to collect data and organise accident data. Therefore it was agreed that the only way to complete an extensive HGV accident analysis involving data from many European countries, was to create a common database (VERTEC HGV Accident DB). The HGV accident data was collected from the UK, France, Finland and Italy using a common format worksheet to make the collected data as homogeneous as possible despite discrepancies in the information available in different databases. For the data analysis a set of key variables were identified (severity of the accident, road type, HGV type, road geometry, dry or wet pavement, accident mode etc) and the data was then split into different “scenarios” each of which was represented by a given road type and severity (all accidents, accidents with injuries or fatal and only fatal accidents). Table 4 shows the number of accidents available in the VERTEC database for each scenario.

Table 4: Number of HGV accidents recorded in the VERTEC database

Rural Highway Urban Highway Primary Road Secondary Road Slip-roads

7,566 accidents 233 accidents498.2km - - - Unknown length

[Italy] (Italy)15402 accidents 738 accidents 5,609 accidents 23 accidents 58 accidents

4214.2km 214km 2957km 212 Unkown length[1,356 Italy] [183 France] [114 France] [France] [44Italy][119 France] [555 UK] [5,495 UK] [10 France][13,927 UK] [4 UK]

651 accidents 14 accidents 698 accidents 121 accidents 4 accidents4,214km + Finland 214km 2,957km + Finland 212km + Finland Unknown length

[130 Italy] [4 France] [20 France] [3 France] [Italy][8 France] [10 UK] [451 UK] [118 Finland][506 UK] 227 Finland]

[7 Finland]1,618 accidents 590 accidents 1,533 accidents 251 accidents

[US FARS] [US FARS] [US FARS] [US FARS]

Fatal

Fatal in USA -

Accident Type

Road Types

All

Injuries & Fatal

TRL extracted a subset of accidents from the STATS 19 database, each involving at least one HGV, which took place in the period 1994 to 2001. Due to the nature of the information required for this task, it was necessary to establish a correlation between the STATS19 data and the England Trunk Road Database. In this way, it was possible to identify the exact section of the trunk road network where each accident featured in STATS 19. Consequently, information about the radius of curvature of the road and average traffic density could be related to each accident. This was a different method of analysis and its use should be considered in future projects that include accident analysis.

TRL delivered the largest amount of data among the partners, including over 20,000 HGV accident cases, most of which occurred on rural highways and primary roads. As well as providing a large evidence base this meant that the results of the analysis remained relevant to the UK.

Each different variable was analysed in terms of percentage distributions in a given scenario, and for each scenario the HGV accident rates were considered. The analysis of the influence of key variables on HGV accidents highlighted that tractor semi-trailer combinations were most frequently involved in accidents resulting in a fatality or serious injury. For each specific road type, the most dangerous situations were identified were:



• For rural highways: a front to rear accident involving a tractor-semi trailer with another road user on a curve with a radius of 500m - 1000m on dry or wet surfaces.

• For urban highways: a side-side collision involving a tractor-semi trailer and another road user in sharp curves (with radii lower than 500m) in dry condition.

• For primary roads: a front to side accident involving a tractor-semi trailer and another road user on wet surfaces. Although small radii bends seemed to be more critical, larger radii lead to higher probabilities of fatal events so it was recommended that the two conditions of R<300m and R>500m should both be investigated.

• For secondary roads: a front to side accident involving a single unit truck or a tractor semi-trailer with another user vehicle on dry surfaces on curves with radii greater than 300m.

The comparison between different types of roads highlighted that accident rates based on HGV traffic (the number of accidents per million heavy-good vehicles km) seemed to be a more realistic indicator than the standard accident rate (the number of accidents per million vehicle km) because it gave a direct relation between accidents and roads without being influenced by the traffic composition. The comparison between different types of roads showed that primary roads, if compared with rural highways, have an accident rate (HGV) more than 3 times greater for accidents where someone was killed or seriously injured (KSI) and almost 5 times greater for ‘fatal’ accidents, while if compared with urban highways they have an accident rate about 1.7 times greater for KSI accidents.

3.2 WP2: Tyre-pavement interaction assessment and model implementation

In order to use the tyre model for simulation purposes a very accurate tyre characterisation was needed. Empirical tests on using the same type of tyres and in similar conditions were performed to gain accurate and reliable simulation results.

Indoor tests were carried out on three different pieces of equipment, designed to test the tyres’ lateral and longitudinal behaviour under different wheel loads, wheel angles and road conditions (Figure 7).

Figure 7: Cleat test machine (left), MTS flat track machine (middle), VTI tyre test facility (right)

Outdoor tests on the passenger car tyres were also carried out using Nokian’s Friction measurement vehicle and VTI’s BV12 test vehicle (Figure 8).



Figure 8: Nokian friction measurement vehicle (left) & VTI’s BV12 tyre test vehicle (right)

The test method for outdoor HGV tyre tests was modified from the common Linked Vehicle Method (LVM) tests used by Nokian Tyres for development testing of heavy vehicle tyres. In the LVM tests, the test vehicle with test tyres was connected with a link and a force measurement device to a heavier “ballast” vehicle (Figure 9).

Figure 9: LVM method for testing HGV tyres

The results of different methods, the Pirelli Flat Track and VTI BV12 measurement vehicle, correlated quite well. Some examples, shown in Figure 10, illustrate that the shapes of the curves were quite close, although the highest longitudinal force generation of BV12 occurred at a smaller slip angle than for the Pirelli Flat Track. This may have been caused by the differences in surface roughness.

Figure 10: Flat Track vs. BV12 (left) & lateral force v. slip percentage (right)

The results also showed that the friction of the ice and snow surfaces varied quite significantly from day to day, mainly due to environmental conditions such as sunshine, wind and temperature. Examples of this are shown in Figure 11.



Figure 11: Examples of day to day variation in friction due to environmental conditions

On winter surfaces, the vehicle speed was not found to have a major influence on friction behaviour of the tyre. Therefore the tyre data obtained from the low speed test could be used in the modelling and analysis of high-speed vehicle manoeuvres.

The passenger car tyres on ice surface produced the highest forces at very low slip; typically the peak lateral force on ice was between 2° and 5° of slip angle. On the high friction surfaces the peak forces were generated at higher wheel slip. For the tyres tested on asphalt the typical slip angle for highest lateral force was around 6°. The longitudinal force maximum was achieved at 10% wheel slip.

The HGV tyres were tested both on low friction (ice) and high friction (steel bar) surfaces using the VTI tyre test facility. In Figure 12 an example of the longitudinal braking characteristics of the tyres is shown. It is clear that worn tyres generally exhibit higher friction values than the new ones. This increase of the friction could stem from the artificially worn tyre treads resulting in greater ice adhesion, compared to normally worn treads. It was also apparent that the locked wheel friction level was quite a lot lower than the peak friction. This was due to water build-up between tyre and ice during braking.

Uneven artificial wear caused some of the worn tyres to exhibit a lateral friction force at zero slip angles. Such nonlinear behaviour for very small slip angles was a result of the relatively fast steering process during the test, which induced relaxation effects in the data.

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Figure 12: Example HGV tyre characteristics, longitudinal braking (left) & cornering (right)

The highest forces produced by the HGV tyres on icy surfaces occurred at smaller slip angles than the passenger car tyres. Typically the slip angle at the maximum lateral force was around 2° on ice surface. For the high friction tests of HGV tyres, the lateral force peak was around 10°, and the longitudinal force peak was between 10% and 20% of wheel slip.



The chosen tyre/road interaction model was the widely used Pacejka’s Magic Formulae Tyre (MF-Tyre) model (Delft Tyre, 2001). It consists of a set of mathematical formulae which reproduce the steady-state tyre behaviour by relating tyre/road contact forces (Fx, Fy) and moments (Mz, My, Mx) to longitudinal slip ratio (κ), lateral slip angle (α), inclination angle (γ) and vertical load (Fz).

MF-Tyre empirical formulae are based on coefficients which are valid for a particular tyre/road interaction. Therefore the set of coefficients had to be identified from curve fitting of the experimental data, an example is shown in Figure 13. Both the steady-state condition (stationary or slowly changing tyre slip, vertical load and camber angle) and transient condition (fast changing of tyre slip, vertical load or camber angle) were characterised.

Figure 13: Curve fitting for pure lateral slip for different vertical loads

Once the MF-Tyre model was integrated in the vehicle model (in WP5), simulation of ABS braking manoeuvres showed unexpected large wheel oscillations, caused by the tyre transient model overestimating the relaxation length at non-zero slip. By introducing a slip decreasing relaxation length coefficient into the vehicle model, the ABS braking simulation was improved as shown in Figure 14.

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Figure 14: Comparison of ABS braking manoeuvre with and without the transient model



3.3 WP3: Reference tests with passenger cars and control systems

The Lancia Lybra was tested in the conditions described in section 3.1 (WP1). This included reference tests on dry, wet, snowy and icy surfaces. The purpose of these reference tests was to provide a database of experimental data to help design, develop and validate the best possible models for the vehicle, tyre, driver and road surface.

TRL completed a programme of full vehicle reference tests in dry and wet conditions on the TRL Research Track. TRL also performed analysis of this data and, as sub task leader for Tk3.b, was responsible for the completion of deliverable R3.2 (Reference tests on wet, snowy and icy road).

The steady-state constant radius tests showed that the vehicle approached saturation of the front axle at a lower lateral acceleration on wet stone mastic asphalt (SMA) surface than in the dry. This was because the coefficient of friction for the wet surface is lower than that for the dry. On snow and ice the limit of lateral acceleration was very low and was reached suddenly. There was no significant difference between “ESP-on” and “ESP-off” tests because the ESP was usually active after the vehicle had started sliding and was already in an un-recoverable spin.

In terms of vehicle dynamics, the ESP showed its ability in reducing the side slip angle and consequently the rear axle lateral slip during the step steer manoeuvre. The ESP was able to influence the yaw rate oscillations, helping in stabilising them more easily, as shown in Figure 15.

Step Steer - VEL = 100km/h - SWA = 110deg

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Figure 15: Step Steer Manoeuvre at 100 km/h – “ESP-on” and “ESP-off”

For the ISO lane change manoeuvre, the ESP allowed the driver to steer later and with lower angles, resulting in a smaller side slip angle and yaw rate as shown in Figure 16.



Figure 16: ESP Intervention ISO Lane Change Manoeuvre at 90 km/h

For the brake in a turn tests the ratio between the maximum value and initial value of the yaw rate and side slip angle were plotted against the mean value of the longitudinal deceleration during the braking manoeuvre. The analysis did not reveal any significant difference between the “ESP-on” and “ESP-off” configurations.

Brake in a turn tests were also carried out in a split-µ condition. CRF simulated this condition using a water controlled depth pool. The results shown in Figure 17 show how the ESP configuration was easier to control and consequently how the manoeuvres were more repeatable.

Figure 17: Comparison between “ESP-on” and “ESP-off” - Split-µµµµ brake in a turn

The random steer input manoeuvres were carried out to investigate the vehicles response to the varying frequency of the steering wheel angle actuation. At higher lateral accelerations the intervention of the ESP had the effect of reducing the resonant peak of the yaw rate/steering wheel angle transfer functions (Figure 18)

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Straight line braking tests were carried out in wet and dry conditions on a very thin asphalt concrete (VTAC) surface, a polished cement concrete (PCC) surface and on an asphalt concrete (AC) surface. The results from these surfaces showed that the stopping distances for the new tyres were between 2% and 22% lower than the corresponding distance with the worn tyres. The benefit of studded tyres were seen from the tests on rough ice as the tyre had good friction even with higher slip values, making it easier for the ABS to control the wheel speed.

Acceleration tests were carried out on low friction surfaces (snow and ice) with full throttle in first gear with the ESP switched on. With the ASR switched on the system limited the speed of the engine so that the speed difference to the rear tyres did not get too large. With the ASR switched off, the engine ran at maximum along with the front wheels, as shown by the increase in wheel slip in Figure 19.

In theory the test vehicle should have a greater acceleration with the ASR switched on. However, the tests with the ASR switched on gave similar results to the tests with the ASR switched off. One explanation for this could be that snow got packed into the grooves of the tyres which reduced the friction. In ASR-off, the tyres were rotating very fast, which may have cleaned snow out from tyre grooves.

Figure 19: Acceleration in a straight line on low friction surface – “ASR-off”

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All of the data collected in WP3 were compiled into a common format and saved into a database of results so that they could be easily accessed and used during the validation of the computer models.

3.4 WP4: reference tests on HGVs

Like the passenger car test, reference tests on dry, wet, snowy and icy surfaces were carried out using the reference HGV described in section 3.1. The purpose of these reference tests was again to provide a database of experimental data to help design, develop and validate the best possible models for the vehicle, tyre, driver and road surface.

As described in section 1, TRL performed analysis of some of the data from the HGV reference tests and contributed to the deliverable for subtask Tk4.b.

Steady-state characterisation tests were carried out using a constant radius test according to ISO 14792 (2003). The speed was incrementally increased up to the point of rollover. From the data the slip angle gradients of all the axles were calculated. As shown in Figure 20, the gradients all increased with acceleration, particularly the front axle, which explains the strongly increasing under steer of the vehicle for increasing lateral acceleration.

Figure 20: Slip angle gradients of reference HGV

The roll angle behaviour of the chassis to the axles and of the trailer to the ground was found to be linear, and the frame torsion between the axles was estimated to be 0.7deg/g.

The transient response of the HGV was found to be non-linear, especially at low lateral accelerations. The response of the vehicle was characterised using random steer tests, single sine-wave tests, continuous sinusoidal tests, and swept sine steer tests. A comparison between the results of the random steer input tests with new and worn tyres (5mm of tread) showed a significant change to the transient behaviour of the vehicle. Using the worn tyres resulted in a larger bandwidth of the frequency response which gave a more constant gain in a larger frequency range (Figure 21).



Figure 21: Frequency response of random steer tests with new (left) and worn (right) tyres

The steady state characterisation tests identified the rollover threshold of the reference HGV to be at 4.2m/s² on a 45m radius circle. This result correlated well with the tilt-table test that was also carried out. In this test the tilt angle was determined to be 23°. This corresponds to a lateral acceleration of 4.2m/s².

The reference manoeuvres were carried out to investigate the behaviour of the vehicle in realistic closed-loop manoeuvres with respect to lateral and roll stability and to longitudinal performance showed that the dynamic rollover limit was higher than the static one in all the tested manoeuvres.

The single lane change manoeuvre was found to require a steering input very similar to the single sine steer test, except for the steering corrections after the first period. Interestingly Figure 22 shows that although rollover did not occur in any of the single lane changes, the static rollover threshold was exceeded.

Figure 22: Single lane change manoeuvre

The obstacle avoidance tests did result in a rollover in the second turn when passing an obstacle offset by 4m from the path (Figure 23, left), however when the obstacle was offset by 3m (Figure 23, right) the vehicle did not rollover even though the static rollover threshold had been exceeded.



Figure 23: Obstacle avoidance tests, with rollover (left) and without rollover (right)

The reducing radius test, where the vehicle entered a curve with a linearly increasing curvature showed that rollover occurred at approximately 5m/s², well above the static rollover limit. The deliverables did not offer an explanation as to why rollover occurred at a higher lateral acceleration.

Straight line braking tests were carried out on high friction asphalt in wet and dry conditions. The results showed that there was a 9% increase in stopping distance and a 12% reduction in mean deceleration for the tests on a wet surface when compared with the results on the dry track. Tests showed an equal braking performance with new and worn tyres. A straight line braking test was also carried out with the trailer brakes disconnected. The results showed an increase of 85% in stopping distance and a reduction of 55% in the mean deceleration of the vehicle. This highlights the impact of incompatibility between a tractor and trailer. From an initial speed of 70km/h, with the tractor and trailer brakes working, the vehicle came to a stop in 36.7m. Without the trailer brakes operational the vehicle would still have been travelling at 45km/h (28mile/h) at this point.

The results of the straight line braking tests on a low friction surface showed significant variability as a result of friction changes on the icy surface from one day to the next. It was recommended that such tyre characterisation tests should be completed in a single day to minimise the variation or that the friction should be measured in direct comparison to the test.

3.5 WP5: Development and validation of passenger car vehicle model

The objective of the first sub task in this work package was to develop the subsystem model, namely:

• Vehicle

• Brake system & electronic control system

• Tyre/road interaction

• Road properties

• Track & driver

The reference model was designed in ADAMS-Car using the standard components to build up the car’s subsystems. The dynamic response of the virtual vehicle was compared with experimental test data. As shown in Figure 24, the passive vehicle showed a good correlation between simulation (blue) and experiment (red).



Figure 24: Comparison between passive car model and experimental data

After the vehicle model was set up in the ADAMS-environment, the parameters were transferred to the real-time-capable vehicle model in VDSIM. To verify this model a validation against the ADAMS model was performed. Figure 25 shows results for a typical manoeuvre (ISO Lane Change at 80 km/h) and the good correlation between the simulation models.

Figure 25: ADAMS – VDSIM vehicle model validation

The base-line parameters for the brake and electronic control system models were chosen according to the geometry of the components (booster size/ valve diameters in the HCU/ calliper size/ brake rotor diameter). The basic functionalities of the hydraulic unit with integrated ECU (ABS/TC/VSC/EBD) were checked by feeding in open loop signals to the control valves (similar to a specified release test



on a hydraulic rig). Figure 26 shows the good correlation between the simulation results (blue lines) and the test rig measurements (green lines).

Figure 26: Brake system and electronic control system model validation

The main work in tyre modelling was concentrated on the widely used Pacejka-approach. After setting up a model following the MF-Pacejka equations the tyre-test-results were analysed to identify the adequate MF-model-parameters. The curve fitting of the stationary tyre characteristics and the parameter identification turned out well. Unfortunately it was not possible to get numerically stable results whenever an ABS/ECS-intervention was active; therefore the start of the validation phase was postponed until the system was running in stable conditions. Analysis with different tyre models showed that the instabilities were created mainly by the numerical procedure used to simulate the tyre relaxation. The long relaxation time of the tyre made the communication between the tyre friction forces and the brake friction forces so slow that the wheels were able to speed up to higher velocities than the car. A solution was implemented by switching off the transient part in the Pacejka-tool and activating a new relaxation calculation (Figure 27).

Figure 27: Improvement of tyre model during ABS interventions

The road-property sub-model, already available for the ADAMS-environment from the preceding VERT-project, was transferred to the Simulink-environment. Some minor modifications have been performed to increase the flexibility of the model with respect to a more general track description.

The VERTEC driver model was developed translating Mitschke’s transfer functions (Mitschke 1990) from frequency space into real space, e.g. a hold-back time in the transfer function is represented as a preview time of the driver. The parameters for the model were either directly transferred from Mitschke or they were evaluated using the results of the VERT-project. The implementation of the driver/track-model into the VDSIM environment was delayed due to the problems with the tyre model described above and so this task did not start until the middle of 2005. It took nearly six months of work to stabilise the interface, i.e. the information exchange between driver and track, and by the official end date of the project the model was able to perform only some closed loop manoeuvres.



In parallel the driver model was tested using available manoeuvres and on “artificial” tracks. The model was improved and a global set of parameters was evaluated reproducing the main results of the preceding VERT-project.

Some of the open-loop manoeuvres had to be carried out in a partially closed loop manner. For example in an open loop steady state cornering manoeuvre, the steering wheel angle for a test would ideally be prescribed and remain constant throughout the test. However, it was proposed that after the vehicle had reached the desired speed, the driver model should try to keep the lane without any braking intervention (i.e. a steering only driver). Due to the delays in developing, integrating and validating the model this solution was necessary as there was not enough time to carry out multiple simulations for each configuration to determine the required steering wheel angle for a particular speed and radius of bend.

The aim of the second sub-task was to merge all the sub-system models developed in the task Tk5.a, with the tyre model of task Tk2.b in order to assemble a full passenger car model. The reference vehicle model of the passenger car was made available in ADAMS-Car. The model was able to simulate open loop manoeuvres in conditions of constant friction with the active system switched off, as well as variable conditions of friction with the active system switched on using co-simulation technique. A standard post processing was built to analyse data directly inside the Matlab-Simulink environment.

The suspension was validated in the case of a parallel vertical wheel travel input. The experimental data showed a hysteretic behaviour, due to the real behaviour of the rubber elements. This was not reproduced by the multi-body suspension model; nevertheless the comparison between experimental and simulated curves (Figure 33) showed that they had a comparable slope across a wide input range, suggesting that the suspension model was good enough to be used in the whole vehicle model.

Figure 28: Parallel wheel travel on front suspension

The first stage of the full vehicle model validation concerned the behaviour of the model in a passive state, i.e. with the control systems switched off. Some of the results, for example the step steer tests on a high friction surface shown in Figure 29, showed a good fit between the simulation (blue line) and the experimental data (red line) in both the initial transient phase and in the following steady-state phase.



Figure 29: Step steer manoeuvre – passive car on a high friction surface

A swept sine steer manoeuvre was also used to study the vehicle dynamic behaviour. Figure 30 shows an acceptable comparison with respect to the experimental data. The only noticeable difference was the slightly higher resonance of yaw rate and side slip angle versus steering wheel angle.

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Figure 30: Swept sine steer manoeuvre on a high friction surface

The second stage of the full vehicle validation concerned the behaviour of the vehicle with the control systems switched on. The control systems fitted on the reference vehicle were the anti-lock brake system (ABS), the vehicle dynamic stability control (VSC) and the traction control system (ASR).

The behaviour of the ABS was checked using a straight line braking manoeuvre. The results showed that the front and rear brake pressures in the model had a different distribution to the experimental data (Figure 31).

Figure 31: Comparison of front and rear brake pressures – limit braking – high friction

Similar tests on a low friction (ice) surface showed negligible differences between the experimental data and the simulation. There was a difference in the longitudinal deceleration however this was caused by an initial offset in the experimental signal.



The behaviour of the ASR was checked using a power-on manoeuvre. After a 60% increase in the tyre parameter LMUX, the simulation showed a good correlation with the experimental result showing correct intervention of the ASR in the model (Figure 32).

Figure 32: Power on manoeuvre before (left) and after (right) modification to LMUX parameter

A step steer manoeuvre on high friction was used to check the VSC control system. The model behaviour was found to be more stable compared to the real vehicle. One potential solution was to make the simulation more similar to the experimental data by modifying the balance of the axles at the limit, specifically to reduce the overall adherence of the rear axle with respect to the front one, as shown in Figure 33. However, since the validation of the suspension and the tyre fitting of the experimental data were satisfactory it was hard to find a genuine physical reason for such a modification of the rear axle. Therefore it was regarded to be more theoretically correct to keep the original identified vehicle data.

Figure 33: Effect of modifying adherence of rear axle

Finally, the brake in a turn manoeuvre was considered to assess the behaviour with the ABS and VSC. The model behaved reasonably compared with the experimental data, although the increased stability of the model resulted in lower wheel slip as shown in Figure 34.

Figure 34: Front wheel velocities during brake in a turn manoeuvre



The increased stability of the simulations meant that if the vehicle over-steered in a simulation then it could confidently be predicted to do the same in real life. However, if the simulation showed no signs of instability then there would be a degree of uncertainty as to whether or not it would become unstable on the road.

3.6 WP6: Development and validation of HGV vehicle model and of driving simulator

3.6.1 Tk6.a: Development of HGV vehicle model The HGV vehicle model was built using multi-body environments and some simplifications and assumptions in the following way. The frame was divided into two rigid longitudinal arm links with two rigid transversal arms linked with spherical joint and bushings in order to have a warp degree of freedom. The cabin was considered to be suspended and the mass inertia, front and rear kinematics suspension were modelled. The kinematics of the front suspension was modelled assuming the bushings were as rigid as possible. The leaf spring was modelled with several parallel standard beams in order to have the possibility of an asymmetric moment of inertia. The anti roll bar was modelled with standard beam elements. The rear suspension kinematics was developed considering bushings as rigid as possible. The air springs have linear characteristics. For the steering system a rigid model was developed with no booster and two concentrated springs to take into account steering elasticity. The engine was modelled with mass and inertia and was attached to the frame with bushings. The driveline was modelled as a conceptual driveline with rotational inertia, final drive ratio and gear box ratio. The brake system was modelled in terms of disc radius, calliper piston radius and friction coefficient in order to receive a pressure value on each calliper from the ABS and VSC Matlab-Simulink model. The trailer frame was modelled in two parts with the front and rear connected with a revolute joint. A torsion spring was modelled between the two parts of the trailer body in order to have a torsion degree of freedom. The kinematics of the trailer suspension was modelled considering bushings as rigid as possible except the one for the connection between the longitudinal arm and frame. The air spring has linear characteristics depending on the static load. The connection between tractor and trailer, the fifth wheel and the frame was modelled through two transversal bushings in order to have quasi-free pitch movement of the fifth wheel. The connection between the trailer and the tractor was modelled through a non linear bushing to simulate the free play at the connection.

Tyre fitting was needed in order to identify the correct tyre characteristics from the experimental data measured on test bench. Only dry conditions were considered for this exercise as the level of acceleration and the full weight condition were not expected to be significantly different in wet conditions.

Figure 35 shows that the fitting was more successful on high friction (left) than on low friction (right) although both conditions gave satisfactory results.

Figure 35: Tyre fitting on high (left) and low friction surfaces (right)



The main interest of the validation was the full vehicle lateral behaviour. The MSC.ADAMS driver was designed for passenger cars, not car and trailer combinations, therefore only open-loop tests were planned. If the correct vehicle mass was used as a driver parameter then the simulated driver was not able to complete the tests because the driver did not turn steering wheel angle enough and so the vehicle cut the corner of its intended path. If the vehicle mass parameter was reduced then the driver was able to stay inside the single lane change track but the driver was still unable to complete the obstacle avoidance test with those parameters. Several parameter combinations were tried but the driver was not able to succeed, because of the trailer behaviour. Figure 36 shows an example of the level of validation obtained in the lateral dynamics tests on high friction.

Figure 36: Single lane change manoeuvre on a high friction surface

Figure 37 compared the experimental test data (red) of an open-loop single lane change manoeuvre on low friction with the simulation data on rough (blue) and smooth ice (black). The steering wheel angle from the experimental test data was used as an input for the simulation. The oscillation of the lateral acceleration was from the tractor cabin as this is where the accelerations and yaw rate were measured from.

Figure 37: Comparison of single lane change manoeuvre on low friction

The lateral acceleration correlation was good, but the differences in slip angle and yaw rate caused deviation in the path driven by the model. Figure 38 shows the path of closed-loop and open-loop simulations. The blue and red curves show that the open-loop simulation model did not complete the single lane change track. At the beginning the vehicle is following the right path, but then suddenly turns to another direction. That is why it was decided to do closed loop simulations.



Figure 38: Path of tractor body during single lane change manoeuvre

The simulation model with rough ice tyre parameters showed a very good correlation with the experimental test data, except for tractor side slip angle that was consistently greater in the model.

The VTI driving simulator (Figure 39) is a moving base system with a linear motion unit driven by a steel belt instead of a chain. The passenger car Simulink model was adapted and modified for real time use in the VTI simulator. It was planned to run the model with the vehicle dynamic stability (ABS, ASR & VSC) controller models developed by TRW however as TRW were unable to provide the source code for these block, VTI was not able to create the real time application. Therefore the simulator was run without the control subsystems.

Figure 39: VTI driving simulator

Overall the model worked as expected although there were some stability problems at low velocities (Figure 40). These were caused by the way the wheel rotational velocities and transient longitudinal slip were calculated resulting in oscillating tyre forces. The problem was overcome by using a different method to calculate the velocities and slip values.



Figure 40: Oscillations in lateral acceleration when starting from zero speed

The simulator model was validated using a number of test subjects, with different levels of skill, to perform a double lane change manoeuvre. Figure 41 shows how an inexperienced driver (blue) lost control of the car, which resulted in a spin and hitting three cones. Such a difference in performance clearly indicated that future driver models should include a parameter to represent the skill level of the driver.

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The structure and code of the original VTI coded HGV model was kept unchanged as far as possible to avoid errors when converting to Matlab-Simulink. The tyre forces and wheel dynamic calculations were newly developed S-function blocks added to the original model. The implemented tyre model was updated to Magic Formula version 5.2 with the exception of the transient slip equations, which were not used for the wheel dynamics calculations (wheel rotational speeds) to avoid problems especially at low speeds.



The transient response shown in Figure 42 is for one sinusoidal period with 0.5 Hz on the steering wheel. The validation using the simulator model shows good similarity between the experimental test data (left) and simulation (right) especially considering the fact at the time the tyre data was uncertain and had to be approximated from past experience.

Figure 42: single lane change manoeuvre for tractor and semi-trailer combination

3.7 WP7: Ranking of the most dangerous situations and new guidelines for the design of safer products.

The first sub-task had two objectives; to define the criteria for a “dangerous situation” and then through a programme of simulations to determine the most dangerous situations. The simulations with HGVs were carried out in ADAM'S and the simulations with the Lancia Lybra were carried out in the VDSIM environment (Matlab Simulink model).

TRL was heavily involved in defining the matrices of simulations for the passenger car and HGV models. TRL assisted in identifying the problems with the VDSIM model.

When defining a test matrix of simulations it was agreed to carry out the simulations in a number of stages. The first stage was a small selection of simulations (approximately 250) using only one of each of the supplementary parameters (rainfall, crossfall, macrotexture and slope) at a small number of speeds, radii, friction coefficients (SFC) and section combinations. The purpose of the Stage I simulations was to identify the areas in which the dangerous situations were most likely to occur for each of the manoeuvres. From the statistical study completed in task Tk1.b, which detected eight scenarios of accidents depending on the type of road and severity (fatal or injured), three manoeuvres were proposed for Stage 1:

• Negotiating a curve

• Braking in straight line

• Avoidance manoeuvre in straight line

It was agreed to complete the Stage I simulations with the ABS system activated and the VSC disabled. This was to highlight which tests were "safe/unsafe" for the standard car so that only the "unsafe" conditions were investigated in the subsequent stages. This approach would also allow the level of intervention of the ESP system to be identified.

Table 5 details the five criteria used to analyse the results of the Stage I simulations and to determine which configurations were likely to result in a dangerous situation. The first criterion was linked to the vehicle’s position on the road. For each simulation, the position of the vehicle was tested to determine if the vehicle deviated out of its lane during the manoeuvre. The second criterion was used to evaluate the difficulty of following an ideal trajectory. For each simulation, the ideal lateral acceleration (V²/R with V: speed of the vehicle and R: radius of the bend) was compared with the



actual calculated lateral acceleration. A third criterion was the deviation of the lateral acceleration in a curve. This was calculated by dividing the difference between the maximum and the minimum values of the lateral acceleration in the curve by the average value of the lateral acceleration in the curve. The fourth criterion was based on the side slip angle. The extreme values of the side slip angle were measured for the safe and unsafe manoeuvres. The last criterion considered the maximum value of yaw rate during the simulation.

Table 5: Analysis criteria for Stage I simulations

Cornering manoeuvre

Braking in straight line

Avoidance manoeuvre

Did the vehicle remain within it's lane? x x x

Comparison of theoretical and actual lateral acceleration

x

Consistency of lateral acceleration

x

Minimum and maximum values of side slip angle

x x x

Maximum value of yaw rate x x x

For Stage I, a total of 132 simulations were carried out for the passenger car model. It was planned that the simulations would be divided between the partners involved in this sub-task so they could be completed as soon as possible. However, delays to the development and validation of the computer models meant that the simulations could not begin before the official end date of the VERTEC project. As a result, Pirelli (the project co-ordinator) agreed to carry out all of the simulations and to distribute the results to the partners for them to complete the analysis.

After the Stage I simulations had been completed a further 128 simulations were carried out in a second stage. The second stage was defined based on the results of the first stage and included simulations where the VSC system was activated.

The results of the cornering manoeuvres suggested that the vehicle dynamic stability control (VSC) had no significant effect on the dynamic behaviour of the vehicle as 61% of simulations without VSC activated and 62% of simulations with VSC activated deviated out of the road lane. The real world effects of stability controls such as ESP have been proven and so it is likely that the low friction surface used during the simulations did not allow the VSC system to operate although at this stage high friction surfaces were not simulated.

Unsurprisingly the ratio between the actual lateral acceleration during the simulation and the theoretical lateral acceleration increased when the vehicle deviated from the intended path, as shown in Figure 43.

Figure 43: Comparison of actual and theoretical lateral acceleration

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In order to get the vehicle to follow the desired radius of curvature the simulations were effectively carried out as closed-loop manoeuvres, with the driver model calculating the steering wheel angle necessary to negotiate the bend. As a result, when the vehicle began to under-steer and deviate from its path, the driver model tried to correct this by applying more steering angle. As a result the simulations where the vehicle deviated from the intended path showed much higher values of wheel slip angle.

The avoidance manoeuvre used in this project was based on a NHTSA study carried out to define “a spin”. NHTSA concluded that, for an avoidance manoeuvre based on a sine steer input (Figure 44), a vehicle could be said to have “spun-out” if the value of yaw rate, at a time of 1 second after the end of the steering input, was greater than 60% of the peak value of yaw rate seen during the steering part of the manoeuvre.

Figure 44: Sine steer avoidance manoeuvre

The simulation results showed that for a low speed test (60 km/h), the manoeuvre was safe with and without VSC activated. This result corresponded well to experimental data. However, during the higher speed tests the vehicle lost grip due to under-steer and so the measure of a “spin” could not be used. The results did show that at speeds of 110km/h and 130km/h with the VSC switched off, the vehicle under-steered out of the lane. With the VSC activated, the vehicle still deviated from the intended path, but by a smaller amount.

Overall the delay in developing and validating the computer model meant that only a small number of simulations could be completed with the passenger car. As a result, by the end date of the project, there was insufficient data available to draw strong conclusions on the most dangerous situations.

It was planned to carry out a similar exercise using the HGV model developed in this project. However, development and validation problems meant that the HGV model was only available without any electronic control systems (as described in section 3.6.1) and without a driver model. Therefore it was not possible to carry out any closed loop manoeuvres and to date only 12 steady state cornering simulations and 10 avoidance manoeuvre simulations have been completed. The results of these simulations were not available at the time of writing this report.

The definition and ranking of dangerous situations were designed to assist in the definition of detailed guidelines for the design of safer roads, vehicles, tyres and electronic devices. As there were insufficient data from the simulation it was not possible to define detailed guidelines for the safer design of passenger cars and HGVs. A deliverable for this sub-task was prepared which supplied general guidelines for the design of improved and safer passenger cars. At the time of writing this report a deliverable or report for guidelines for the design of tyres was not available.

TRW used their in house simulation tool (VeSTo) for the investigations to define the design of control systems, as a fully integrated system model was not available for this task. Before this sub-task was



started a model-to-model comparison between the vehicle models of VDSIM and VeSTo was carried out. The results showed that the VeSTo model had less under-steer than the VDSIM model, resulting in higher lateral accelerations in some tests. After a minor modification to the VeSTo model, a very close match was obtained.

Before starting the performance evaluation the repeatability of braking manoeuvres with and without ECS-intervention was carried out. During normal braking the stopping distance was reproduced with a variance of 0.02 %, the intervention of the ECS-system (in this case ABS) created a significant variation of the stopping distance. For a dry surface the variation was ±1.7 % (100 tests), this increased to ±3.6 % (100 tests) for a lower friction level. The observed variation in stopping distance was due to the “discrete” calculations inside the controller, (which only exchanged information every 5 milliseconds), and slight variations in the tyre relaxation which resulted in significant variations in the ECS interventions.

The Taguchi method is widely used to determine which factors strongly affect a process and how those factors can be adjusted to improve the process. The influence and the interaction of parameters describing typical vehicle configurations were analysed using reference manoeuvres that activated specific functions in the controller code. Evaluation criteria were defined to rate the vehicle performance for the following manoeuvres:

• Steady-state cornering

• Step steer manoeuvre

• Straight line braking

• Brake in a turn

• Double lane change manoeuvre

For the steady state cornering, none of the parameter variations seem to affect the result to any appreciable extent. The change between winter and summer tyres was seen to influence the vehicle performance more than the weight distribution. Although, overall it was found that for steady-state manoeuvres the parameter variations did not seem to be large enough to significantly affect the system.

Results from the step steer manoeuvre revealed that too short relaxation lengths, occurring at increased slip, seemed to cause instability. The largest effect on the side slip angle came from the tyres. Load on the rear axle was also found to be important because generally a large load on the rear axle causes larger side slip angles.

For the straight line braking manoeuvre, there were only two simulations that fulfilled the evaluation criterion of a longitudinal deceleration greater than 9 m/s². These occurred when there was no additional load on the vehicle, although it was found to be more critical to have the right tyre fitted for the right conditions (summer/winter).

As expected, the combination of tyre performance and load distribution had a major influence on the vehicles performance in this manoeuvre.

For the double lane change manoeuvre, three equally important factors were identified. The first was the tyre; the second was the load on the roof and the third factor was the tyre relaxation. Each of these strongly influenced the vehicle performance.

Overall, the Taguchi-analyses indicated that the tyre-characteristic had the strongest influence on the overall vehicle performance. It was more important than any vehicle parameter, even more important than the centre of gravity (CoG) height and it played a major role in nearly every critical situation. Therefore it was recommended that the ECS-controller should not be trimmed to a special tyre, but the algorithm should be able to recognise in some way, the type of tyre fitted.

The possible applications of the VERTEC handling model, developed in Simulink environment were investigated as part of an Intelligent Transportation System (ITS) for improving road safety. Three key areas of implementation were identified.



The first was the development of a road to vehicle communication system using on-board calculations for real time simulations and warning procedures. This type of application would allow the specific vehicle to define if the travelling conditions were suitable for a given manoeuvre. If not a warning sign could be issued to the driver to inform him in due time about the potential safety issue arising. For this type of application the simulation tool has to run in real time on the vehicle (at a speed comparable with the driving speed) and it has to receive from the road a set of static information (geometric layout, longitudinal and transverse grade) and a set of dynamic data (pavement texture and friction at each location as well as water depth mapping for the given rainfall conditions). The speed of the model developed in the VERTEC project was tested with a Pentium IV machine with a 2.8 GHz processor and a 1 GB RAM. The results show that after a start-up time of approximately 25 seconds, the systems ran at a simulation speed of approximately 1.5 m/sec (approx 5 km/h). This was much slower that the vehicles speed so unless the model could be simplified more powerful processors would be required.

The second area of development was a road to vehicle communication systems with on-board calculations to assess the safety speed for a given vehicle to perform a given manoeuvre. This application would allow a vehicle’s on board system to estimate the speed that the vehicle could perform a specific manoeuvre on a given geometric layout and under given conditions of rainfall and pavement surface characteristics. The simulation system would be used offline to perform a number of simulations at different speeds for each given manoeuvre and friction conditions in order to define a “safety speed” for a given manoeuvre, vehicle and tyre. This could then be compared with the actual vehicle speed and an on-board warning system could be activated.

The third application of the VERTEC handling model referred to the possibility of implementing a dynamic speed limit system that linked the model with a variable message sign system. This kind of system would be a semi-real time system as it would require multiple simulations of the given geometric layout (the location where the system is operated), the pavement characteristics and environmental conditions for a reference vehicle and tyre. Each different simulation could be characterised by a different speed and then, by comparing the output with an accepted threshold, a “safety speed” could be estimated. The system could then refresh its posted speed limit and the resulting speed limit could also be transmitted to the passing vehicles and be shown on-board by means of a road to vehicle communication system.

3.8 WP8: Exploitation and dissemination

The results of the VERTEC project were disseminated during the Tire Technology Expo held in Stuttgart in March 2006. One day of the expo was devoted to the presentation of the results of the project. Sixteen papers were presented including a paper co-written by TRL and CETE that described the results of the passenger car reference tests using the Lancia Lybra. A copy of this paper can be found in Appendix A.

This paper is also scheduled to be presented as a poster presentation at the FISITA 2006 World Automotive Congress in Yokohama in October 2006.



3.9 List of VERTEC deliverables

Table 6 shows the list of deliverables produced in the VERTEC project. At present these deliverables are not publicly available.

Table 6: VERTEC deliverables

Deliverable Partner TitleDeliverable PRD 1.1 HUT Project Web siteDeliverable R1.2 CETE Specification of reference manoeuvres and test conditionsDeliverable R1.3 TRW Specification of modelling environment and specificationDeliverable R1.4 UNIFI Report on HGV Accident Analyses reviewDeliverable R2.1 NOKIAN Reference indoor/outdoor Tyre characterisationDeliverable PRD 2.2 PIRELLI Database of Tyre data on dry, wet, snowy and icy roadsDeliverable PRD 2.3 PIRELLI Tyre/Pavement interaction model, following the defined formats and

specificationsDeliverable R3.1 CRF Vehicle characterisation and reference tests on dry roadDeliverable R3.2 TRL Vehicle characterisation and reference tests on wet, snowy and icy roadsDeliverable R4.1 VOLVO HGV characterisation and reference tests on dry roadDeliverable PRD 5.1 TRW Subsystems modelsDeliverable R5.2 PORSCHE Validation of Passenger Car modelDeliverable PRD 6.1 PORSCHE HGV vehicle modelDeliverable R6.2 PORSCHE Validation of HGV vehicle modelDeliverable PRD 6.3 VTI Improved Driving simulatorDeliverable R 7.1 CETE Identification of the most dangerous situationsDeliverable R 7.2 CRF Guidelines for the design of Passenger Cars and HGVDeliverable R 7.3 TRW Guidelines for the design of Control SystemsDeliverable R 7.4 UNIFI Guidelines for the design of Road Hazard Warning Systems

4 Conclusions The main outputs of the project were the integrated simulation environment and an improved driving simulator for cars and trucks.

Simulation environments for both a passenger car and an HGV were successfully created in the VERTEC project although, at the time of writing this report, there were no complete and fully working models available. For the passenger car, two modelling environments were developed. The first was a very accurate vehicle dynamic simulation using a multi-body approach with a co-simulation between ADAMS and Matlab/Simulink. The second model was a simplified “real-time” equivalent developed in Matlab/Simulink. The HGV vehicle model was built using the multi-body environments in ADAMS.

The results of the full scale reference tests using a passenger car and an HGV provided a database of experimental data to help design, develop and validate the best possible models for the vehicle, tyre, driver and road surface. They also gave an indication about the efficiency of the electronic devices, which can help a driver.

The passenger car model was found to have greater stability compared with the results of the reference tests. This means that if the vehicle over-steered in a simulation then it could confidently be predicted to do the same in real life. However, if the simulation showed no signs of instability then there would be a degree of uncertainty as to whether or not it would become unstable on the road.

The driver model used in the HGV simulation environment was designed for passenger cars, not HGV and trailer combinations and if the correct vehicle mass was used as a driver parameter then the simulated driver was not able to complete the tests because the driver did not turn the steering wheel angle enough and so the vehicle cut the corner of its intended path. As a result only open loop simulations could be validated.



The passenger car Simulink model was adapted and modified for real time use in the VTI driving simulator. It was planned to run the model with the vehicle dynamic stability (ABS, ASR & VSC) controller models developed in this project. However, this was not possible and so the simulator was run without the control subsystems.

The simulator model was validated for a number of test subjects, with different levels of skill, performing a double lane change manoeuvre. The significant difference in performance found between inexperienced and experienced drivers indicated that future driver models should include a parameter to represent the skill level of the driver.

The purpose of the integrated simulation environment and an improved driving simulator was to allow the partners of the consortium to detect and rank the most dangerous driving situations in order to define and supply guidelines for the design of safer roads, vehicles, tyres and electronic devices.

The HGV accident analysis was used to identify the most dangerous situations for each specific road type. The accident data supplied by TRL used a method which combined data from the STATS 19 database as well as the England Trunk Road Database. This meant that it was possible to identify the exact section of trunk road network where each accident occurred. This was a different method of analysis and its use should be considered in future projects that include accident analysis.

In the accident analysis, the comparison between different types of roads highlighted that accident rates based on HGV traffic (the number of accidents per million heavy-good vehicles km) seemed to be a more realistic indicator than the standard accident rate (the number of accidents per million vehicle km) because it gave a direct relation between accidents and roads without being influenced by the traffic composition.

Problems with the development and validation of the models caused delays in delivering working models and so the programmes of simulations to detect and rank the most dangerous driving situations were not completed by the official end date of the project (December 2005). To date, the programme of simulations for the passenger car have been started but not yet completed. Approximately 250 simulations have been run and the results analysed to identify test configurations that resulted in a dangerous situation.

A similar exercise was planned for the HGV model; however development and validation problems have meant that the HGV model was only available without the electronic control systems and without a driver model. Therefore only a small number of open-loop simulations have been completed.

As there were insufficient data from the simulations it was not possible to define detailed guidelines for the safer design of passenger cars and HGVs. Brief generic guidelines for the design of improved and safer passenger cars were produced.

At the time of writing this report there were no guidelines available for the design of vehicle tyres.

Guidelines for the design of electronic control systems showed that the tyre-characteristic had the strongest influence on the overall vehicle performance Therefore it was recommended that an electronic controller should not be trimmed to a special tyre, but the algorithm should be able to recognize somehow the actually mounted tyre.

The possible applications of the VERTEC handling model were investigated as part of an Intelligent Transportation System (ITS) for improving road safety. The investigation highlighted that the implementation of on-board systems running in real time, at present, cannot be exploited as the simulation speed was found to be much slower than the travelling speed of the vehicle.

The application for providing variable posted speed limits as a function of the actual handling condition seemed to be the most promising as the model could be implemented on the road side and could potentially provide a regular “refreshed” update of the posted limit. By means of road to vehicle communication systems the actual speed limit could also be displayed on board on the travelling vehicles.



Acknowledgements The work described in this report was carried out in the Vehicle Engineering Department of TRL Limited. The author is grateful to Ian Simmons who carried out the quality review and auditing of this report.

References Delft Tyre (2001), MF-Tyre User Manual version 5.2., The Netherlands

ISO 14792 (2003), Road vehicles -- Heavy commercial vehicles and buses -- Steady-state circular tests, International Organisation for Standardisation, Switzerland.

Mitschke, M. (1990), Dynamik der Kraftfahrzeuge, Bd. C. Fahrverhalten, 2. Aufl., Springer Verlag, Berlin, Heidelberg, Germany

Tire Technology Expo Conference Papers

EC VERTEC project overview, Ing. Federico Mancosu (Pirelli Pneumatici S.p.a.)

Identification of potential accident factors when HGV are involved, Prof. Eng. Francesca La Torre, Eng. Lorenzo Rossi (UNIFI)

Tests - Outdoor vehicle passenger car, Martin Dodd (TRL), Michel Gothié (CETE)

HGV Characterisation and Reference Tests, John Aurell, Volvo Trucks

The VERTEC Modelling Environment, Models of Steering and Braking System, Thomas Pütz, Simon Henrich, Wolfgang Schwanke, Norbert Skricka (TRW Automotive – Lucas Varity GmbH)

Development of HGV vehicle model, Leonardo Pascali (Porsche), Daniele Arosio (Pirelli Pneumatici), Nicola Dela, John Aurell (Volvo)

New validated tire model to be used for ABS and VDC simulations, Emiliano Giangiulio (Pirelli Pneumatici S.p.A.), Daniele Arosio (Pirelli Pneumatici S.p.A.)

Development of a Road Model in Multibody and Real Time Handling Models, Prof. Eng. Francesca La Torre , Eng. Andrea Rindi , Eng. Lorenzo Rossi

Validation of a 14 dof model for the prediction of vehicle dynamics and its interaction with active safety control systems, F. Cheli, E. Leo, S. Melzi (Politecnico di Milano), D. Arosio E. Giangiulio F. Mancosu (Pirelli Pneumatici Spa)

Adams based “VERTEC vehicle” model implementation and validation, Isabella Camuffo, Mauro Vesco – Centro Ricerche Fiat

Validation of HGV vehicle model, Leonardo Pascali (Porsche AG), Emiliano Giangiulio (Pirelli Pneumatici), Tero Lehtonen (Helsinki University of Technology), Nicolas Dela (Volvo Trucks) Daniele Arosio (Pirelli Pneumatici)

VTI Driving simulator. Experiments with external models in real time, Staffan Nordmark, Göran Palmkvist, Håkan Sehammar (VTI)

Virtual Mapping and Ranking of the most dangerous Situations, Veronique Cerezo (CETE)

Guidelines For The Improvement Of Electronic Control Systems, Thomas Pütz (TRW Automotive – Lucas Varity GmbH), Matilda Pettersson-Hallnor, Magnus Eklund (KTH-Stockholm)

Real time handling models: a tool for improving road safety, Prof. Eng. Lorenzo Domenichini, Prof. Eng. Francesca La Torre, Eng. Lorenzo Rossi (UNIFI)



Unpublished VERTEC deliverables

VERTEC Deliverable R1.2, specification of reference manoeuvres and test conditions, June 2003

VERTEC Deliverable R1.3, Specification of modeling environment and specification, June 2003

VERTEC Deliverable R1.4, Report on HGV Accident Analyses review, October 2003

VERTEC Deliverable R2.1, Reference indoor/outdoor Tyre characterization, October 2004

VERTEC Deliverable R3.1, Vehicle characterisation and reference tests on dry roads, February 2004

VERTEC Deliverable R3.2, Vehicle characterisation and reference tests on wet, snowy and icy roads, September 2004

VERTEC Deliverable R4.1, HGV characterisation and reference tests on dry roads, March 2004

VERTEC Deliverable R4.1, HGV characterisation and reference tests on snowy and icy roads, October 2004

VERTEC Deliverable R5.1, Validation of Passenger Car model, May 2005

VERTEC Deliverable R6.2, Validation of HGV vehicle model, December 2005

VERTEC Deliverable R7.1, Identification of the most dangerous situations, March 2006

VERTEC Deliverable R7.2, Guidelines for the design of Passenger Cars and HGV, December 2005

VERTEC Deliverable R7.3, Guidelines for the design of Control Systems, February 2006

VERTEC Deliverable R7.4, Guidelines for the design of Road Hazard Warning Systems, March 2006



Appendix A. Tests - Outdoor vehicle passenger car – Tyre Technology Expo Paper


Appendix A

��

��

VERTEC

E.C. CONTRACT N°: GRD2-2001-50007

3RD INTERNATIONAL COLLOQUIUM ON VEHICLE-TYRE-ROAD INTERACTION “Vehicle, road, tyre and electronic control system interaction - prediction and validation of handling

behaviour “

TYRE TECHNOLOGY CONFERENCE - STUTTGART 8 MARCH 2006

Title: Tests - Outdoor vehicle passenger car

Authors: TRL (Martin DODD) - CETE (Michel GOTHIÉ)

ABSTRACT VERTEC was an EC 5th Framework research project. The aim of the project was to increase vehicle primary safety by developing a fully integrated model for the simulation of the road-tyre-vehicle-driver system in the most potentially dangerous situations. As part of this project a programme of tests were carried out to fully assess the behaviour of the reference car in different environmental conditions.

These tests were successful in providing a database of results which helped to design, develop and validate the best possible models for the vehicle, tyre, driver and the road surface. They also gave an indication about the efficiency of the electronic devices, which can help the driver.

This paper describes the results of the outdoor test programme carried out using the Lancia Lybra during WP3 of the VERTEC project.

1. INTRODUCTION VERTEC was an EC 5th Framework research project. The aim of the project was to increase vehicle primary safety by developing a fully integrated model for the simulation of the road-tyre-vehicle-driver system in the most potentially dangerous situations. As part of this project a programme of tests were carried out to fully assess the behaviour of the reference car in different environmental conditions. The results of these tests were used to develop and validate the simulation model created later in the project.

Tests were carried out on different test tracks across Europe to provide results on a variety of road surfaces using different tyres, and with or without the electronic driving aid (ESP) operational.


Appendix A

All tracks used for these tests were characterised with conventional means commonly used in different European countries: SCRIM and GRIPTESTER for friction coefficient and RUGO for macrotexture measurements.

This paper summarises the key results from the different manoeuvres carried out during WP3 of the VERTEC project. This paper does not contain all of the results but only summarises some of the key findings. More details of the results can be found in VERTEC deliverable R3.1 (dry tests) and R3.2 (wet, snow & ice tests). 2. TEST VEHICLE A Lancia Lybra (Figure 1) was used as the reference vehicle. The vehicle was equipped with ABS and ESP (the ESP could be manually turned off by the driver).

Figure 1: Lancia Lybra

2.1 TEST EQUIPMENT

The vehicle was also equipped to measure 21 parameters during each test. These included:

From the inertial platform:

• Longitudinal, lateral and vertical acceleration

• Roll pitch and yaw angle

• Roll, pitch and yaw rate

From the CAN:

• Steering wheel angle

• Rotational speed of each wheel

From a Datron Optical sensor:

• Absolute velocity

• Sideslip angle

From other sensors:


Appendix A

• Hydraulic brake pressure at the master cylinder and each calliper

• Brake pedal force

• Throttle pedal and brake pedal position

2.2 TYRES

Table 7 describes the specification of the different tyres that were tested during the reference tests. Pirelli supplied four different types of summer tyre and Nokian supplied three different types of winter tyre, one of which was a studded tyre.

Table 7: Tyres used during reference manoeuvres

Reference N° DescriptionA 205 60 R15B 205 60 R15C 205 60 R15D 195 65 R15

Reference N° DescriptionC Nokian WR Central Europe winter tyre (Nokian reference tyre)B Nokian Hakkapeliitta Q Nordic studless winter tyreA Nokian Hakkapeliitta 4 Studded winter tyre

Pirelli Summer Tyres

Nokian Winter Tyres

In order to compare the performance of the different tyres, tests were carried out with the Michelin C35 at two different test speeds. Figure 2 shows the brake slip friction curves produced from the results for the tests carried out on the polished cement concrete (PCC – surface S4).

Figure 2 shows that all tyres were grouped together with the Pirelli-A tyre better than the others, while the Nokian reference tyre (Nokian-C) a little less good than the others.


Appendix A

Figure 2: Slip friction curves for test tyres on polished cement concrete (PCC) – surface S4

3. TEST PROGRAMME The programme of reference tests was shared between the project partners. This section describes the different manoeuvres which were carried out as well as describing the test surfaces used.

BFC = f(Slip) - Polished Cement Concrete - 50 km/h

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Appendix A

3.1 TEST MATRIX

Table 8 shows the different manoeuvres that were carried out by the project partners. Tests were carried out on dry, wet, snow and icy surfaces.

Table 8: Test Matrix for reference tests with passenger car

Manoeuvres(1) (2) (1) (2) (1) (2)

Steady state at constant radius (ISO 4138) X X X X X XStep steer (ISO 7401) X X X XISO lane change (ISO 3888) X X X X X XBraking in a curve (ISO 7975) X X X X X XPower off in a curve (50m) X XRandom Steer Input (ISO 7401 and ISO/TR 8726) X XLimit braking (V = 90km/h) Xa Xa Xa Xa X XAcceleration in a straight line X X X X X XTraction in a curve X X X X X XChange of µ in a curve (R=100m) X XPedal force calibration X(1). Open/closed loop manoeuvre - active system OFF(2). Open loop manoeuvre - active system ON (except ISO Lane Change w hich is close d loop - active system ON)(a). tests done on different surfaces and on longitudinal/transverse split surfaces

ConditionsDry Wet Snow/Ice

3.2 TEST SURFACES

The above tests were carried out on dry, wet, snowy and icy roads by the different project partners. Friction measurements were taken on each surface, either with the SCRIM or with the GRIPTESTER in order to assess their microtexture level, and with the RUGO to assess their macrotexture.

4. RESULTS/ANALYSIS This section summarises the key results from the different manoeuvres carried out during WP3 of the VERTEC project. This paper does not contain all of the results but only summarises some of the key findings. More details of the results can be found in VERTEC deliverable R3.1 (dry tests) and R3.2 (wet, snow & ice tests).

4.1 Steady State at Constant Radius (ISO 4138)

Figure 3 shows the results from the tests carried out by CETE on a dry 100 m radius circle with new Pirelli ‘A’ tyres. Tests were carried out with the vehicle at its normal weight and also fully laden. The fully laden tests were only performed in the linear range to highlight the effect of the load distribution on the balance of the vehicle. Figure 3 shows, that a lateral acceleration of 0.7 g was reached well before the saturation of the front axle, and the fully laden tests show a minimal increase in the under steer gradient.


Appendix A

"Steady-State" test - Normal load and Fully Laden - Dry surface

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Figure 4 shows a comparison between tests on dry stone mastic asphalt (SMA) and tests on a wet SMA. The graph shows that the vehicle approaches saturation of the front axle at a lower lateral acceleration on wet SMA than in the dry. This is not unexpected because the coefficient of friction for the wet surface is lower than that for the dry.

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Tests were also carried out on snow and ice, however the results were difficult to analyse, especially with the summer tyres, because the limit of lateral acceleration was very low and was reached suddenly. Tests were carried out with “ESP-on” and “ESP-off” although significant differences could not be seen because the ESP was usually active after the vehicle had started sliding and was already in an un-recoverable spin, this was especially the case with the summer tyres.


Appendix A

4.2 Step Steer (ISO 7401)

For this manoeuvre, tests were carried out in dry and snow/icy conditions. A comparison of time histories between the “ESP-on” and “ESP-off” configurations on a dry track surface is shown in Figure 5. All of the values in the graph have been normalised against different values to enable plotting on the same scale.

Step Steer - VEL = 100km/h - SWA = 110deg

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Figure 5: Time histories of Step Steer Manoeuvre at 100 km/h – “ESP-on” and “ESP-off”

In terms of vehicle dynamics, the ESP showed its ability in reducing the side slip angle and consequently the rear axle lateral slip. The ESP was also able to influence the yaw rate oscillations, helping in stabilising them more easily. As shown in Figure 6, the ESP increased the braking pressure on the external corner of the front axle and so generated the stabilising moment.

Step Steer - VEL=100km/h - SWA=110deg - ESP On

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Figure 6: Description of ESP intervention – Step Steer manoeuvre at 100 km/h

For the tests on ice many combinations of speed and steering wheel angle were used to identify the vehicles behaviour. The steady state acceleration could not be achieved very often because the vehicle


Appendix A

was sliding or understeering or both. During the step steer tests the ESP was frequently activated and it usually helped the vehicle to turn by braking the inside rear wheel (PRSX) when turning left. Occasionally this made the yaw rate too high (causing over-steer), and so the outside front wheel (PFDX) was braked to correct the situation, as shown in Figure 7.

Figure 7: Steering wheel angle and brake pressure during step steer test on ice at 50 km/h

4.3 ISO Lane Change (ISO 3888)

For the tests on a dry surface the initial vehicle velocity was 90 km/h. Figure 8 shows an example of the time history for two manoeuvres, one with the ESP switched on and one with the ESP switched off. The trajectory followed by the vehicle was very similar and in both cases shows how the ESP allowed the driver to steer later and with lower angles, resulting in a smaller side slip angle and yaw rate.

ISO Lane Change - V=90km/h

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Figure 8: Comparison of time history between “ESP-on” and “ESP-off” – ISO Lane Change at 90 km/h

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Appendix A

Figure 9 describes the ESP intervention. Its contribution was very similar to the one described in section 4.2 for the step steer response and the brake pressure peaks generated by the ESP system were approximately 30 bars.

Figure 9: ESP Intervention ISO Lane Change Manoeuvre at 90 km/h

The purpose of the tests on the icy surface was not to find the maximum velocity of the vehicle, but to test at a high speed which would produce smooth data suitable for the model validation. An example of the time history for one of these tests is shown in Figure 10.

Figure 10: Example time history for ISO lane change manoeuvre on icy road at 50 km/h

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Appendix A

4.4 Braking in a Curve (ISO 7975)

The manoeuvres performed by CRF in the dry were at a speed of 80 km/h on a radius of 100 m. The ratio between the maximum value and the initial value of the yaw rate and side slip angle were plotted against the mean value of the longitudinal deceleration during the brake, as shown in Figure 11. The analysis was unable to distinguish any significant difference between the “ESP-on” and “ESP-off” configurations.

Figure 11: Ratio between the maximum value and the initial value of the yaw rate and of side slip angle against the mean of the longitudinal acceleration –

Braking in turn at 80 km/h and at AY0 = 0.50 g – “ESP-on” and “ESP-off”

The effect of the ESP system is illustrated in Figure 12. It shows that, as the wheel velocities on the internal side of the vehicle (VFSX & VRSX) start to decrease more than the others and the vehicle speed, the system detects the situation with a short delay and reduces the braking pressures on the sliding corners, moreover it cuts the pressure to the other rear corner. Therefore, in addition to the improvement of the longitudinal performance of the vehicle, a stabilizing yawing moment is generated by the differentiation of the front braking pressures.

Figure 12: Description of the braking system action – Braking in curve at 80 km/h and at mean(AX) = 0.50 g – “ESP-on”

Similar tests by CRF and TRL in wet conditions, albeit at a lower speed than in the dry, again found that it was not possible to distinguish any significant difference between the “ESP-on” and “ESP-off” configurations. Overall the results showed that the deviation from the original circular path increased as the initial lateral acceleration increased and that the deviation decreased as the longitudinal

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Appendix A

deceleration increased. In some conditions, the new Nokian C tyre (NCN) was more prone to lose lateral grip at the front axle. This was reflected in a sudden decrease in the yaw rate modulus and a positive deviation from the original path. In other words, the car tended to go straight along the tangent to the circle. No considerable change in behaviour was obtained by loading the vehicle.

Brake in a turn tests were also carried out in a split-µ condition. CRF simulated this condition using a water controlled depth pool. The results shown in Figure 13 show how the ESP configuration was easier to control and consequently how the manoeuvres were more repeatable.

Figure 13: Comparison of time histories between “ESP-on” and “ESP-off”–R = 100 m at 100 km/h

4.5 Power off in a curve

This manoeuvre required the driver to maintain a constant velocity around a specified radius of path at the desired speed, before suddenly releasing the throttle and allowing the vehicle to slow, while still maintaining a constant steering wheel angle. Figure 14 shows the time history for a manoeuvre with the ESP system on. It shows that the ESP applied brake pressure to the outside front wheel (PFDX) and both rear wheels (PRSX & PRDX). This resulted in an increase in longitudinal deceleration and also reduced the yaw rate and lateral acceleration of the vehicle.


Appendix A

Figure 14: Time history – Power off in a curve – “ESP-on”

4.6 Random Steer Input (ISO 7401 and ISO/TR 8726)

The purpose of these manoeuvres was to investigate the vehicles response by varying the frequency of actuation of the steering wheel angle. Different levels of steering wheel angle were chosen, corresponding to different level of stationery lateral acceleration (AY0): 0.28 g, 0.40 g, 0.58 g and 0.75 g. The vehicle velocity was kept constant at 100 km/h.

At the first level of lateral acceleration (AY0=0.28 g) the ESP did not show any intervention. At the second level (AY0=0.40 g) the ESP system started to act. At the third (AY0=0.58 g) and at the fourth level (AY0=0.75 g) the timing and distribution of the ESP intervention were similar to that at 0.40 g, but the magnitude was much greater. Moreover for the transfer function of yaw rate/steering wheel angle (Figure 15), a reduction of the resonance peak is evident.

Figure 15: Transfer function - yaw rate/steering wheel angle vs. frequency (Hz) – Random steer at 100 km/h and AY0=0.75 g

4.7 Limit braking in a straight line

Tests were carried out in all conditions (dry, wet, snow and ice) by several of the project partners. The results consistently showed that there was no ESP intervention and that the modulation of the brake pressure was due to the ABS system. Figure shows a comparison of the stopping distance for the tyres tested by CETE on the very thin asphalt concrete (VTAC – surface S3) in the wet, on the wet polished cement concrete (PCC – surface S4) and on the dry asphalt concrete (AC – surface S5).

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Appendix A

Figure 16 shows the higher level of grip available on the dry track surface. It also shows that the stopping distances for the new tyres were between 2% and 22% lower than the corresponding distance with the worn tyres.

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Figure 16: Comparison of stopping distances for limit braking tests carried out by CETE

Limit braking tests were also carried out on ice and snow. On rough ice, the studded tyre had good friction even with higher slip values, so it was easier for the ABS to control the wheel speed. This is illustrated in Figure 17 which shows that the wheel speeds of the vehicle using the studded winter tyres did not fall as low as the same vehicle using the un-studded summer tyres.

Figure 17: wheel velocities for test vehicle with summer tyres (top) and studded winter tyres (bottom) when tested on rough ice.

Limit braking tests were also carried out with the ESP switched on with a lateral µ-change (left wheels on the lower friction cement concrete track (S4) and right wheels on the higher friction VTAC 0/10 track (S3), both wet). Figure 18 shows a sample time history for one such test. For this test the stopping distance (47.1 m) was closer to the one obtained with the four wheels braked on section S3 (46.5 m) compared with the one obtained on section S4 (71 m). Figure 18 also shows that the effort of the ABS system was very much biased to the left wheels, which were on the lower friction surface. There was almost no ABS activity on the right wheels, which were on the higher friction surface.

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Appendix A

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Figure 18: Example time history for a lateral split-µµµµ limit braking test

Limit braking tests in straight line with ESP on were carried out across a longitudinal change in friction (braking start on the dry cement concrete track (S4) and braking end on the wet cement concrete track (S4)). Figure 19 shows a sample time history for one such test.

Figure 19 shows that the skidding resistance on the dry surface was almost double that of the wet surface. At the transition (dry - wet) it can be seen that the effort of the ABS system was very much biased to the front wheels as they were very close to locking. The front wheels were very much more sensitive to this rough skidding resistance change than the rear wheels.

In this case the ABS system helped to avoid the locking of the front wheels while minimising the increase in the vehicles stopping distance.

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Figure 19: Example time history for a longitudinal split-µµµµ limit braking test

4.8 Acceleration in a straight line

These tests were carried out on low friction surfaces (snow and ice). They were done with full throttle in first gear with the ESP switched on.


Appendix A

The slip controller (ASR) had two options to control wheel slipping during acceleration. Firstly, it could limit drive wheel speed by controlling engine rotating speed, as shown in Figure 20.

Figure 20: Acceleration in a straight line on low friction surface – ASR controlling engine speed

Secondly, it could use the vehicle’s brakes to slow one of the wheels if is rotating faster than another. In this way it works like a lock in differential gear. This braking activity can be seen in Figure 21.

Figure 21: Acceleration in a straight line on low friction surface – ASR applying brakes

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Appendix A

Acceleration tests with ASR on and off were carried on the snow surface with Nokian WR tyres (NCN). With the ASR switched on the system limited the speed of the engine so that speed difference to rear tyres did not get too big. With the ASR switched off, the engine was running at maximum along with the front wheels, as shown by the increase in wheel slip in Figure 22.

Figure 22: Acceleration in a straight line on low friction surface – “ASR-off”

In theory the test vehicle should have a greater acceleration with the ASR switched on. However, the tests with the ASR switched on gave similar results to the tests with the ASR switched off. One explanation to this result could be that snow got packed into the grooves of the tyres which reduced the friction. In ASR-off, the tyres were rotating very fast, which may have cleaned snow out from tyre grooves.

5 CONCLUSIONS These tests were successful in providing a database of results which helped to design, develop and validate the best possible models for the vehicle, tyre, driver and the road surface. They also gave an indication about the efficiency of the electronic devices, which can help the driver.

The electronic stability program (ESP) was more difficult to operate intentionally than the ABS system because of the activation threshold which was sometimes very high. However, the ESP system was very effective both for the engine effort limitation sent to the front wheels and for the braking of individual wheels which was able to bring the vehicle back in the direction desired by the driver.

The steering-wheel angles for the same lateral acceleration were higher on a wet surface than those obtained on a dry surface. They were also higher with worn tyres than with new tyres, and higher with a fully loaded vehicle than with a normally loaded one.

Variations between tyres are more important on wet surface than on dry surface. Unsurprisingly the winter tyre was among the least effective of the tested ones in summer conditions, and even more so

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Appendix A

when worn. The tyre of 195x65R15 size tended to behave better on wet surfaces with a low skidding resistance compared the tyre of 205x60R15 size.

ACKNOWLEDGEMENTS The authors wish to thank the European Commission for the financial support and the EC manager for the help in the project managing

REFERENCES VERTEC Deliverable R3.1: Vehicle characterisation and reference tests on dry road, February 2004

VERTEC Deliverable R3.2: Reference tests on wet, snowy and icy road, September 2004

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