Extending the IPG CarMakerby FMI Compliant Units

Stephan Ziegler and Robert HöplerModelon GmbH München

Agnes-Pockels-Bogen 1, 80992 München, Germany{stephan.ziegler,robert.hoepler}@modelon.com

Abstract

This paper presents a generic interface which en-ables exploiting the multi-physics modeling capabil-ities of Modelica within the virtual test drive simula-tor CarMaker [1] using the Functional Mock-up Inter-face (FMI) [2].

Applications ranging from detailed studies of vehi-cle properties to hardware in the loop test-rigs requiremodels of different complexity. CarMaker is providedan interface which allows for vehicle models being ex-tended by almost arbitrary external component modelsimplementing the FMI. The FMI offers two flavors ofunits which permit the balancing of computational per-formance and numerical robustness within CarMaker.Additional model information provided by the FMI isused for automatic integration of the external modelsas well as for configuring the computations. Involvedaspects are shown by an example truck model.

Keywords: FMI, FMU, CarMaker, HIL, Solver,Model export, Interface, Stability, Co-Simulation

1 Introduction

The CarMaker platform [3] is a full-fledged virtualdriving environment which offers a wide range of ap-plications from offline operation to hardware in theloop (HIL) tests. CarMaker was designed to sup-port the development process from an early conceptualstage to hardware prototype testing.

Therefore the CarMaker suite is composed oftwo main components, the CarMaker InterfaceToolbox (CIT) and the Virtual Vehicle Environ-ment (VVE), see Fig. 1. The CIT contains a collec-tion of tools for simulation control, parameterization,analysis, visualisation, and file management.

The VVE represents the computer modeled com-position of the vehicle with all its components suchas powertrain, tires, brakes and chassis as well as

road and driver. Vehicle components may be imple-mented by default generic models, custom code suchas MATLAB/Simulink controller models or even realhardware on a test rig. Depending on the desired task

Figure 1: Typical VVE scenery including road andtraffic rendered by the CarMaker IPG-Movie anima-tion tool.

the VVE can be operated on a regular office computeror on a real-time system. Real-time operation allowsinvestigation of deterministic behavior, office opera-tion might lack real-time capabilities but is thereforeapplicable on almost any host computer and allows thesimulation to run slower or faster than real-time de-pending on system performance and model complex-ity and does not require special hardware.

Simulation techniques are a key enabler in the de-velopment of nowadays propulsion systems of elec-tric and hybrid vehicles. These systems challenge thepower of vehicle dynamics tools in two ways: On theone hand the number and complexity of topologies ofdrive-trains is almost combinatorial due to the largenumber of involved drives, clutches, or gears. Onthe other hand particularly hybrid vehicles representan almost classical multi-domain system (electronics,

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hydraulics, combustion, chemistry, mechanics) with adramatically rich dynamics in the interaction betweenthe various technical components.

Modelica forms a perfect basis for the modelingof multi-domain systems especially automotive sys-tems [4]. The benefits of its general equation-basedformalism has been shown by numerous publicationsin the fields of vehicle dynamics, climate control, driv-etrain modeling, combustion engines, and hybrid pow-ertrains, see e. g. [5–8].

The increasing complexity in automotive system de-velopment especially for hybrid vehicles demands forversatile modeling tools, realistic and reproducible vir-tual testing as well as seamless test rig integration.An ideal software for modeling and simulation ofall vehicle-related scenarios off-line and in real-timewould combine these aspects. Augmenting the com-prehensive CarMaker VVE suite by the multi-physicscapabilities of Modelica is a first step towards this di-rection.

2 Augmenting CarMaker byFunctional Mock-Up Units

Extending CarMaker

The generality of the CarMaker VVE allows for themodifications of the vehicle models in an almost arbi-trary number of ways. Generic vehicle componentssuch as powertrain, steering, tires, brake system aswell as the complete propulsion system might be con-figured, as shown in Fig. 2. For several reasons cus-

Figure 2: CarMaker interface for vehicle model con-figuration.

tom models serve as a replacement of the predefinedmodules in the generic vehicle model or might evenreplace the vehicle model as a whole: (i) All topolo-gies of hybrid vehicles might not be covered evenby an advanced tool specialized in vehicle dynamics.(ii) These multi-physics systems demand for generalmodeling formalisms and simulation techniques suchas Modelica-based approaches. (iii) Many engineeringcompanies and car manufacturers are particularly re-lying on Modelica during development of componentsand control units, e.g. [6].

Replacing single vehicle components by so-calledcustom code is easily done by dynamically linking ex-ecutable code and registering variables, parameters,and their dimensions and types within the VVE. Therequirements when incorporating external code aremanifold:

1. Real-time compliance: The code may not relyon specific processing hardware since CarMakervehicle models should run on arbitrary real-timeplatforms.

2. Small run-time overhead and memory footprint.

3. Since CarMaker runs its own numerical inte-gration schemes the code of external dynamicalmodels must expose input and output variables.These signals can be physically meaningful, e.g.tire forces, wheel speeds, etc..

4. Implementation details might not be exposed forintellecual property reasons.

5. Standardized, automatic and intuitive inclusion ofthe external code.

In general, almost all of these requirements can be metby the usage of black-box models, except for the lastitem. On the other hand these shift problems to differ-ent classes: Lack of numerical robustness and/or com-putational performance.

Functional Mock-Up Interface

The FMI adresses almost all requirements discussed inthe previous section. It defines an open standard inter-face to be implemented by an executable model calledFunctional Mock-up Unit (FMU). The FMI functionsare used (called) by a simulator to create one or moreinstances of the FMU, and to run these models, typi-cally together with other models. An FMU may eitherbe self-integrating (FMU for co-simulation, FMU-CS)

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or require the simulator to perform numerical integra-tion (FMU for model exchange, FMU-ME) [2]. Theartifacts which csan be packaged in an FMU-file areC-source code, executable code compiled for one ormore platforms, and XML descriptions of variables,parameters, and general solver and model properties.

Interfacing Modelica Models to CarMaker

Choosing the FMI as a basis of the interface is benefi-cial for several reasons since the exposed C-interface isstandardized and XML model description [9] includedin any FMU contains excellent structural informationabout the model. The prerequisite is a modeling tool,e.g. Dymola or SimulationX, which generates this de-scription while exporting the Modelica model as C-code. Alternatively this FMU can be hand-coded orgenerated from any other modeling tool that supportsFMI, e.g. a multibody-package.

When interfacing Modelica models to CarMakerone must consider two key aspects: (i) The executablemodel or code must represent the modeled dynamicalbehavior without any restrictions in order to maintaingenerality and to avoid context dependent model se-mantics. (ii) The resulting model must fulfill require-ments essential for real-time applications, most promi-nently a fixed integration stepsize.

The Interface consists mainly of two parts. Thefirst part governs static type-checking of the input andoutput variables and parameters of FMU to assure asmooth setup of the solver, generation of user inter-faces for parameter input, and automatic extension ofthe so-called data dictionary for tracing relevant vari-ables during simulation. Finally the executable modelis instantiated and connected to the CarMaker solver.Depending on the specific application each type ofFMU can be chosen.

Even if the mean computational performance of twocoupled simulators is sufficient it can be difficult toassure definite turn-around times, an aspect criticalin e.g. HIL systems. CarMaker provides a simula-tion engine for robust fixed stepsize time integrationso including an FMU-ME is a safe solution for sim-ulating the combined vehicle model in the followingcases: When either the model complexity is reason-ably low, the eigenvalues of the system are locatedin the stable region of the integration scheme and nostrong non-linearities are in the imported model. ForFMU generated from very complex or black-box mod-els the option FMU-CS is advantageous, because theFMU contains the appropriate numerical integrationscheme. The interface in combination with CarMaker

serves as the master algorithm in the co-simulation ofCarMaker and the imported block. This leads to anadaptive oversampling of CarMaker’s time grid whichmight rule out usage on embedded systems or HIL,but leads to superior numerical stability. Compared tothe features offered by the full FMU-CS this interfacetakes advantage only of some features. The setup isthe simple one depicted in Fig. 3 with only one singleexecution engine where the interface is the simulationmaster which runs sub-system 2 synchronized to thegrid the solver of tool 1, i.e. CarMaker.

Figure 3: Schematic of run-time architecture.

3 Example Application

The integration of an FMU was investigated for a hy-brid truck model based on the Vehicle Dynamics Li-brary [4, 5]. A schematic of the model is depicted inFigure 4. Either the CarMaker standard drive-train orthe complete vehicle were replaced within the VVE byan FMU exported from Dymola. In case of the drive-train the driver and environment input signals from theCarMaker model were accelerator pedal, clutch, braketorques, gear number, starter and ignition amongstothers. The most prominent model outputs were thewheel speeds.

When using FMU-ME as a replacement the com-plete vehicle model showed unstable numerical behav-ior since the fixed solver stepsize did not comply withthe dynamics of the new drive-train model. The usedstepsize of one millisecond is traditionally used in ve-hicle dynamics HIL applications. Not even the over-sampling feature of CarMaker lead to stable behavioras the constant inputs during the oversampling stepsdeteriorated the dynamical behaviour. Applying anFMI-CS using an appropriate variable stepsize solver

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Figure 4: Truck example model in Modelica.

for the drive-train lead to stable numerical simulationwithin CarMaker. This method allowed the simulationto run in real-time at least in office-mode (soft real-time). Standard techniques to reduce model complex-ity might help to gain hard real-time capability for HILapplications.

4 Conclusion

This paper presented an interface to augment theCarMaker by functional mock-up interfaces generatedfrom Modelica models. The interface supports bothrepresentations of FMI, the one for co-simulation aswell as the one for model exchange, exploiting thespecific advantages of each approach. The first leadsto numerically robust total simulation models wherethe CarMaker simulation engine is the master algo-rithm controlling the FMU model containing its ownnumerical integration scheme and time grid just syn-chronized on the grid imposed by CarMaker. The lat-ter leads to computaionally efficient executable mod-els ideally suited for real-time applications. In the casewhere the modelled dynamics does not fit to the fixedstepsize prescribed by CarMaker integration schemethis might lead to poor numerical robustness as hasbeen shown by a simple example application.

The interface is not restricted to executable mod-els stemming from Modelica-based tools. FMUs canbe generated from dynamic systems by any modelingand simulation tool which supports the FMI standardor can even be hand-coded. The presented work sup-ports a fraction of the features of the current FMI spec-ification V. 1.0. Possible directions are manifold, e. g.network communication between multiple FMUs in atrue co-simulation environment e. g. enabled by Sil-

ver [10], parallelization within CarMaker, or the treat-ment of CarMaker itself as an FMU.

Acknowledgements

The authors would like to thank Christian Schyr,IPG GmbH, and Johan Andreasson, Modelon AB, forvaluable comments.

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