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
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Copyright 2015 Society of Automotive Engineers of Japan, Inc. All rights reserved
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
Steffen Schmidt et al./International Journal of Automotive Engineering 6 (2015) 53-57
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Copyright 2015 Society of Automotive Engineers of Japan, Inc. All rights reserved
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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Copyright 2015 Society of Automotive Engineers of Japan, Inc. All rights reserved
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
Steffen Schmidt et al./International Journal of Automotive Engineering 6 (2015) 53-57
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Copyright 2015 Society of Automotive Engineers of Japan, Inc. All rights reserved
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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