Technical Note 20154059 Optimization of Vehicle Handling ...

Copyright 2015 Society of Automotive Engineers of Japan, Inc. All rights reserved

Optimization of Vehicle Handling Performance Using a Full Vehicle Model

with Multi-Body System (MBS) Suspensions in Multiple Real Time –

Applying the DoE Method

Steffen Schmidt 1)

Eberhard Schmidt 2)

Waldemar Schick 3)

Josef Henning 4)

1)-4) IPG Automotive GmbHBannwaldallee 60, 76185 Karlsruhe, Germany (E-mail: [email protected])

Received on August 1, 2014

Presented at the JSAE Annual Congress on May 22, 2014

ABSTRACT: The application of a full vehicle model with MBS (>100 DOF) in real time has now become reality. This

makes it possible to efficiently evaluate vehicle handling performance in the whole vehicle. An array of chassis design

parameters (e.g. geometries, kinematics, hard points) can be evaluated in combination with systems (e.g. air suspensions,

steering, powertrain). New applications are possible, such as the MIL, SIL and HIL tool chain, to validate the interactions

with controllers and their variants. Tools and methods (e.g. automation, analysis) can be added. Best combinations can be

achieved by using DoE methods – transferred from powertrain ECU calibration.

KEY WORDS: Vehicle Dynamics, suspension system, chassis/control, multi-body dynamics,multi-body system (MBS) suspensions, real time, Design-of-Experiment [B1]

1. Introduction

Today’s vehicles contain a large number of components,

systems and sub-systems which are inter-connected. The

complexity and integration effort has risen enormously.

Relatively new and emerging technologies such as electrified

and hybrid powertrains, advanced driver assistance systems

(ADAS) and distributed control systems are part of a vehicle and

are complemented by multi-media systems for on-board

communications and entertainment. This results in an elevated

testing and validation curve, while the right decisions,

particularly the inevitable trade-offs of targets versus design

variants, should be made as early as possible in the development

process to avoid time-consuming and costly errors.

Not only the possible different technical variants, but also

the customer’s needs have to be taken into account. A vehicle

can be driven in normal or sportive mode, it is used in the city as

well as on country roads and highways, each resulting in

different demands. At the same time, the driver has to feel the

specific vehicle DNA of the OEM. One of the defining aspects

of a vehicle’s handling characteristics is steering, which includes

comfort as well as stability. For testing the interactions of the

multitude of different systems, the use of powerful virtualization

tools such as the open integration and test platform CarMaker

can enormously reduce time and costs. Furthermore, a

completely new method has been developed to simulate and

analyze multi-body system (MBS) suspensions in real time. Up

to now, the domains of MBS suspensions and real-time

simulation have been completely separate entities. They have

been brought together so that a simulation of a full-vehicle

model with detailed suspensions with more than 100 degrees of

freedom in real time will be possible from now on.

2. Motivation: Early Validation

Driving pleasure is a very subjective and complex matter.

As the car manufacturers have established a special brand DNA,

it is important to meet the expectations concerning sportiness,

ride comfort and handling to offer a satisfying driver experience.

Fig. 1 Virtual integration in the whole vehicle

Therefore, virtual test driving is used in the whole process

and models or real-world components such as steering, ESP or

torque vectoring can be integrated (Figure 1) at a very early

stage in a virtual vehicle and simulated in real time or faster.

Interaction phenomena, such as how ESP reacts on a new axle

configuration, can be examined in virtual test driving.

But not only the technical possibilities, but also the product

diversity addresses an increasingly wide range of consumer

demands, which tend to vary in the local and regional markets of

the global marketplace. In terms of technical diversity, aside

from the ‘traditional’ options, such as different engines,

Technical Note 20154059

Steffen Schmidt et al./International Journal of Automotive Engineering 6 (2015) 53-57

53


automatic or manual transmissions, a consumer, today, may

have the choice between two-wheel drive, four-wheel drive(1)

, a

conventional or a hybrid power train, and perhaps even different

chassis and suspension designs. With respect to sufficiently

testing, verifying and validating their performance, design or

development engineers benefit from a very early validation in

the global vehicle context. Vehicle dynamics evaluation is

much more efficient when virtual test driving is used in every

step of the development process (Figure 2).

Fig. 2 Validation along the development process

It contains the fully integrated Model-in-the-Loop (MIL),

Software-in-the-Loop (SIL) and Hardware-in-the-Loop (HIL)

tool chain to validate the interaction with multiple controllers

and their vast array of variants. All of the previously existing

time-tested tools and methods, such as automation, analysis and

test bench communications, can be combined in this process.

New investigation and the full optimization potential can be

successfully harnessed by applying these methods.

3. Steering Feel: Subjective and objective evaluation criteria

Steering characteristics, particularly the aspect of ‘good

steering feel,’ are essential to a positive overall perception of a

vehicle that gives the consumer a high level of (subjective)

driving pleasure, and obviously play a key role with respect to

(objective) driving safety as well. In vehicle dynamics, on-center

steering behavior, also referred to as the‘pull & drift’ effect, is a

key factor that determines the steering feel and safe steering of a

vehicle(2)

. The example of straight-line driving on the freeway is

a good way to illustrate this point (Figure 3).

Fig. 3 Pull and drift effect

While the driver operates the vehicle with full

concentration (i.e. hands on the wheel, eyes on the road) the

vehicle will easily and precisely follow the intended trajectory,

i.e. ‘stay on course.’ However, when the driver’s attention is

distracted, his control of the car deteriorates. In vehicle

dynamics terms, the steering torque applied by the driver

‘switches’ to zero in this case and the vehicle, to a larger or

lesser extent depending on its model- or type-specific dynamic

characteristics, starts to leave the intended trajectory, with

excursions either to the left or right. The degree to which the

vehicle departs from the desired line when steering torque

equals zero is referred to as ‘drift’ and is an objective safety

criterion. By contrast, the so-called center-point steering effort,

also referred to as ‘pull’ or ‘straight-running behavior,’ is

defined as the amount of force the driver has to physically exert

in order to keep the vehicle ‘on course’ and is a subjective

comfort criterion.

Obviously, the smaller the physical effort which the driver

has to exert to keep the car on course, i.e. the steering effort at

center point, the higher the driver’s subjective feeling of

assurance and comfort, and vice versa. This subjective straight-

running behavior can be objectively measured, e.g. as the

vehicle’s yaw rate response when the driver’s steering torque

input equals zero (Figure 4).

Fig. 4 Objective evaluation criteria

Vehicle dynamics requirement are almost entirely based on

driving maneuvers and the corresponding evaluation criteria,

both in real-world and virtual test driving. In a simulation and

design space evaluation of comfort (pull) and safety (drift)

against target values, a modified ISO weave on-center handling

test was run with very small steering angle inputs of 5°, which

basically correspond to straight-line driving. Figure 4 shows that

the results obtained in this virtual test drive (HIL simulation)

very closely matched those from a corresponding real-world

track test.

4. Multi-body system suspensions in multiple real time

Influences on steering behavior(3)

exist due to a lot of

design variables. In this connection, suspension parameters play

a major role. Changing settings affect the interaction of the

components and systems and result in a change of the vehicle’s

performance. There is a wide range of parameters which affect

the vehicle’s behavior(4)

(for examples, see Figure 5) and must

be identified, examined, evaluated and optimized. The need to

come up with radically different ways to handle the massive

complexity and volume of integration tasks was the driving


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force behind the development of powerful simulation and

virtualization tools.

Fig. 5 Influencing parameters

As mentioned, the domains of multi-body system

suspensions and real-time simulation have been separate entities.

On the one hand, the MBS techniques are state-of-the-art for

suspension analysis. But they are hardly appropriate for global

vehicle evaluation with integrated systems, especially when it

comes to real-time applications such as controller development

and XIL methods. In this case, real-world components have to

be integrated in a virtual vehicle, because the simulation

performance is much slower than real time (~1000 time slower).

On the other hand, the current real-time suspension models are

often not detailed enough to represent dynamic chassis

characteristics, because they are based on static maps. In

addition, the user is not able to modify design parameters such

as hard points and bushings in the simulation environment,

which in some applications are essential for gaining knowledge

about the interaction of the systems.

The two domains (Figure 6) can be seen as two major

fractions, split into two sides. The fraction on the left-hand side

(detailed multi-body system suspension models, without real-

time capability) essentially deals with applications such as stress,

fatigue, noise vibration and harshness (NVH), ride comfort,

kinematic compliance and handling. The fraction on the right-

hand side (multi-body systems with suspension mappings, with

real-time capability) essentially addresses factors such as

handling, integrated controls, virtual (multi-domain) integration

and global vehicle functions.

Fig. 6 MBS tool domains

The new development of detailed MBS suspension models

in CarMaker extends the scope of applications from the right-

hand side more toward the left-hand side. It enhances the MIL,

SIL, HIL and driving simulator applications, where real-time

capability is absolutely essential, with the benefit of having new

effects from the left-hand side.

IPG Automotive has started to establish a new axle

interface for the generic use of external suspension models but

also for use with its own internal suspension model. The latest

development has been the extension of the mapping-based

suspension model to a detailed multi-body suspension model

with the combined functionality of both. Usually, when relying

on the mapping approach, a performance of 40 times faster than

real time is possible. In order to keep this advantage, the new

modeling approach comes with two simulation modes.

4.1. Static and Dynamic MBS

The static mode is based on an elongated preprocessing

phase (milliseconds to seconds), where the look-up tables are

generated automatically in the background. Once the mappings

are available, the full performance can be achieved, even though

the user works on a suspension design level, modifying

parameters such as geometry or bushings in the same simulation

environment. The K&C behavior will be calculated in the

initialization phase of CarMaker simulation. In the static mode a

much faster factor than real time is possible.

The dynamic mode simulates all components of the

suspension (linkage movement, accelerations and forces) during

the simulation and passes back the generalized forces and

torques calculated in wheel center. On the benchmark machine

(standard single-core PC, Core i7 3 GHz), a configuration with a

fully resolved McPherson front suspension and a 4-link rear

suspension still runs 1.8 times faster than real time. The level of

detail is shown through the degrees of freedom: over 40 per axle

(for an example see figure 7) and 110 for the whole vehicle.

Fig. 7 McPherson - MBS suspensions (41 DOF)

A fixed step solver with a step size of >0.15 ms is used.

Input data to the MBS suspension model is the typical data

volume such as hard-points, masses, inertias, bushes,

stiffness/damping used in common tools such as ADAMS or

SIMPACK. Another benefit of having all dynamics suspension

effects is that the model is able to calculate ‘on the fly’ typical

suspension values: kinematic & compliance contribution, roll

center height, caster angle, kingpin offset and secondary spring.

4.2. Development tool

The MBS axle models were developed using MESA

VERDE (Mechanism, Satellite, Vehicle and Robots Dynamic


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Equations), a tool created by Prof. Dr.-Ing. Jens Wittenburg and

Dr.-Ing. Udo Wolz at the University of Karlsruhe. The major

benefit is that this formalism represents full non-linearity of all

effects and is able to generate the differential equations of

motion automatically. A symbolical and alphanumerical

structure for dynamic allocation as well as intelligent

substitution algorithms for minimal differential equations leads

to a significant reduction of matrix operations. Another major

benefit is C Code export. Adding the service-based software

architecture with multi-threading used by CarMaker to the

MESA VERDE modelization provides the key to success in

meeting the major challenge of the real-time target and can

clearly be described as the ‘engine’ behind this development.

5. Progress through use of real-time MBS

The use of real-time multi-body system suspensions makes

extensive parameter studies on integrated chassis design in

interaction with control systems possible. This is a crucial

advantage, as engineers desire – and actually need – to run an

increasingly large number of parameters to explore the possible

parameter/design space.

There are many examples of possible use cases of which only

five are listed here to illustrate the point:

Comprehensive parameter studies of integrated chassis

design in terms of handling and comfort performance

with MIL

Interaction of suspension design, steering system and

steering controller in terms of steering feel and

comfort with MIL/SIL/HIL

ESP robustness as a function of chassis variants with

SIL/HIL

Interaction of suspension dynamics and controller with

MIL/HIL

Driving simulator use for authentic transient behavior

with HIL

5.1. Classic vs. improved workflow

When taking a comprehensive look at the topic of chassis

design, including suspension design, the steering system and the

steering controller and their interactions, it is obvious that

optimizations in any of these areas are never owed to a single

sub-system. By using real-time multi-body system suspensions,

optimizations can be achieved across system boundaries since

functional interface definitions are made considerably easier.

Fig. 8 Classic workflow

The benefits even become clearer when comparing the different

workflows. In the classic approach (see Figure 8) a new look-up

table had to be generated and transferred into the simulation

environment for every changed suspension design parameter. As

a result, the duration of the process was very long.

In contrast to this function, the possibilities to adapt (even

minor) changes have risen enormously with the improved

workflow (see Figure 9). Parameter changes can be done

directly in the real-time capable simulation tool which is also

possible in an automated way. With respect to ESP robustness,

various chassis set-ups enter into the picture. Particularly the

hard-points and bushings differ between variants and have to be

examined using the MIL, SIL and HIL tool chain that is now

possible.

Fig. 9 Improved workflow

5.2. Use case: Parametric study for chassis design regarding

handling characteristics with DOE-in-the-Loop

Using this model, a complete vehicle (sedan) with sprung,

unsprung mass, suspension force elements, McPherson front

axle, four-link rear axle, EPS steering system with Pfeffer

Steering Model, MagicFormula tires (v5.2 or v6.1), hydraulic

ESC braking system, aerodynamics, sensors and a complete

front-wheel powertrain with engine, clutch, transmission,

driveline with flexible shafts, was set up. Each part and system

could be parameterized and modified during the simulation

process.

The standard driving maneuver with open-loop and closed-

loop maneuvers such as acceleration, braking, sine sweep,

slowly increasing steering, steady state circular driving, weave

on-center handling test, ISO Lane Change, slalom and handling

course were selected from the existing comprehensive maneuver

catalog. For each maneuver-related evaluation, criteria were

defined. A refinement was possible during the optimization task.

One of the key objectives was to improve on-center and off-

center steering feeling, and primarily the torque contribution,

which is a chassis design function.

After setting up the target values for the vehicle’s handling

behavior, the relevant and possible design parameters such as

geometry and kinematics were identified. With these parameters

a test plan was created and executed in a loop with AVL

CAMEO. The use of the integration platform CarMaker and

calibration optimization software AVL CAMEO reduces the

effort. These tools enable methodologies such as Virtual

Systems Prototyping (i.e. a virtual vehicle built on system level),

design of experiment (DOE), and virtual test driving in real time,

where a virtual driver executes the same driving maneuvers

catalog as used in real-world road tests.

The generated results provided a multidimensional sensitive

model with related interactions to the parameter room that was

used to find the optimum vehicle behavior. This configuration

was applied and validated within the selected maneuvers. For a

double check, the selected configuration was approved in

comfort related maneuvers.

For this purpose, a Cameo/CarMaker interface was set up,

where Cameo remotely controls (Figure 10) the test process of

CarMaker with related driving maneuvers. The test parameters


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and the identified evaluation results are handed over between

Cameo and CarMaker until the test plan has been completed.

Fig. 10 DoE-in-the-Loop with AVL Cameo

Pre-processing in tools such as ADAMS or IPGKinematics(5)

can be reduced from now on. The advantage is that changes in

the parameters do not require a completely new procedure in the

pre-processing-tools but can be done directly in CarMaker.

Another advantage for the customer is the visualization in real

time: Animation of suspension components is possible. Within

the new approach not only the variation of parameters in

CarMaker is possible, but also externally (DoE). Furthermore,

automated investigations on suspension design parameters (e.g.

hard-points, bushings’ stiffness and damping) can be conducted

via DoE or the TestManager.

The ‘DOE-in-the-Loop’ process allows the design engineer

to perform conclusive evaluations of effects and interactive

effects, plot sensitivities and progressively make appropriate

selections through to the ultimate resolution of target conflicts.

In addition, the representation of sensitivities and evaluation

criteria in a sensitivity model enables both targets and

constraints to be identified for the best trade-off decisions

enabled by calculations resulting in a Pareto optimization front

as depicted in Figure 11.

Fig. 11 Sensitivity model, trade-off evaluation and Pareto

optimization

6. Summary

Until now it has taken a great effort in time and material to

test steering characteristics. This can be reduced by using virtual

test driving, nevertheless there are still some areas in testing

which need to imitate real test driving even more. The big

challenge concerning steering characteristics has been the

division into testing either detailed axles or in real time, but

using both of them together has not been possible. The

combination of both of them is now possible with the presented

solution.

The new multi-body system suspension models in

CarMaker afford a much easier and time-reduced way of testing

and validating many variants of suspension design parameters. It

is possible to go through an entire catalog of maneuvers with the

corresponding evaluation criteria across the entire MIL, SIL and

HIL tool chain(6)

in an office environment using normal desktop

PCs. The integration in the whole developing process offers a

high degree of efficiency, effectiveness and ease of use.

It was shown how steering characteristics can be

investigated under certain objective criteria. With the help of

MBS suspensions it is possible to test the interaction of chassis

design parameters and control systems with regard to the OEM’s

specific vehicle DNA. Interaction phenomena can be studied

and tuned even across system boundaries based on optimum

trade-off decisions, ultimately leading to the resolution of target

conflicts. With the method of IPG Automotive presented here,

the gap between detailed and mapping-based MBS has now

been closed. It has been made possible to join multi-body

system suspensions with powerful controlling and calibration

functions as well as sophisticated DoEmethods.

Nevertheless there are still challenges that need to be

solved: One main goal is to find a way to integrate detailed

customer models from different authoring tools (e. g. ADAMS,

SIMPACK) into CarMaker. One possible solution, which could

be realized in the future, would be the integration via the

standardized Functional Mock-up interface (FMI), other ways

are under evaluation. The new development provides a roadmap

for a wide range of future use cases in the virtual domain far

beyond the domain of vehicle dynamics to empower the shift

from traditional approaches to true systems engineering in the

automotive industry.

References

(1) W. Matschinsky: Radführungen der Straßenfahrzeuge,

Springer Verlag, Germany (1998).

(2) H. Flegl: Fahrwerk und Kraftübertragung im Personenwagen.

Part 2, Lecture, Universität Karlsruhe, Germany (1988).

(3) DIN 70000, Straßenfahrzeuge, Begriffe der Fahrdynamik,

Germany (1994).

(4) J. Reimpell: Fahrwerktechnik: Radaufhängungen; First

edition,Vogel Verlag, Germany (1986).

(5) IPGKinematics Reference Manual Version 3.5.2, IPG

Automotive GmbH (2012).

(6) H. Palm, J. Holzmann, St.-A. Schneider, H.-M. Koegeler:

The future of car design: Systems Engineering Based

Optimization, ATZ worldwide, Volume 115, Sixth Issue, pp. 42-

47 (2013).


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