Functional Mock-up Interface

The functional mock-up interface (or FMI) defines a standardized interface to be used in computer simulations to develop complex cyber-physical systems.

The vision of FMI is to support this approach: if the real product is to be assembled from a wide range of parts interacting in complex ways, each controlled by a complex set of physical laws, then it should be possible to create a virtual product that can be assembled from a set of models that each represent a combination of parts, each a model of the physical laws as well as a model of the control systems (using electronics, hydraulics, digital software, ..) assembled digitally. The FMI standard thus provides the means for model based development of systems and is used for example for designing functions that are driven by electronic devices inside vehicles (e.g. ESP controllers, active safety systems, combustion controllers). Activities from systems modelling, simulation, validation and test can be covered with the FMI based approach.

To create the FMI standard, a large number of software companies and research centers have worked in a cooperation project established through a European consortium that has been conducted by Dassault Systèmes under the name of MODELISAR. The MODELISAR project started in 2008 to define the FMI specifications, deliver technology studies, prove the FMI concepts through Use Cases elaborated by the consortium partners and enable tool vendors to build advanced prototypes or in some cases even products.

The development of the FMI specifications was coordinated by Daimler AG.

After the end of the MODELISAR project in 2011, FMI is managed and developed as a Modelica Association Project (MAP).

The four required FMI aspects of creating models capable of being assembled have been covered in Modelisar project:

• FMI for model exchange,
• FMI for co-simulation,
• FMI for applications,
• FMI for PLM (integration of models and related data in product life-cycle management).

In practice, the FMI implementation by a software modelling tool enables the creation of a simulation model that can be interconnected or the creation of a software library called FMU (Functional Mock-up Unit).

^[1]

The FMI approach

The typical FMI approach is described in the following stages:

• a modelling environment describes a product sub-system by differential, algebraic and discrete equations with time, state and step-events. These models can be large for usage in off-line or on-line simulation or can be used in embedded control systems;
• as an alternative, an engineering tool defines the controller code for controlling a vehicle system;
• such tools generate and export the component in an FMU (Functional Mock-up Unit);
• an FMU can then be imported in another environment to be executed;
• several FMUs can – by this way – cooperate at runtime through a co-simulation environment, thanks to the FMI definitions of their interfaces.

License

The FMI specifications are distributed under open source licenses:

• the specifications are licensed under CC-BY-SA (Creative Commons Attribution-Sharealike 3.0 Unported) CC_BY_SA 3.0
• the C-header and XML-schema files that accompany this document are available under the BSD license with the extension that modifications must also be provided under the BSD license.

Architecture

Each FMU (functional mock-up unit) model is distributed in a zip file with the extension ".fmu" which contains:^[1]

• an XML file containing among other things the definition of the variables used by the FMU;
• all the equations used by the model (defined as a set of C functions);
• optional other data, such as parameter tables, user interface, documentation which may be needed by the model.

Example

below is an example of an FMI model description issued from Modelica.

<?xml version="1.0" encoding="UTF8"?>
<fmiModelDescription
  fmiVersion="1.0"
  modelName="ModelicaExample"
  modelIdentifier="ModelicaExample_Friction"
...
  <UnitDefinitions>
     <BaseUnit unit="rad">
        <DisplayUnitDefinition displayUnit="deg" gain="23.26"/>
     </BaseUnit>
  </UnitDefinitions>
  <TypeDefinitions>
     <Type name="Modelica.SIunits.AngularVelocity">
        <RealType quantity="AngularVelocity" unit="rad/s"/>
     </Type>
  </TypeDefinitions>
  <ModelVariables>
     <ScalarVariable
        name="inertia1.J"
        valueReference="16777217"
        description="Moment of inertia"
        variability="parameter">
        <Real declaredType="Modelica.SIunits.Torque" start="1"/>
     </ScalarVariable>
...
  </ModelVariables>
</fmiModelDescription>

Comparison to Simulink

FMI proponents explain that FMI models have several advantages over Simulink S-Functions:^[2]

• S-Functions format is proprietary, whereas the FMI schema is licensed under a BSD license.
• The building blocks of S-Functions are much more complex than FMI, making it very difficult to integrate in simulators other than Simulink itself.
• Furthermore, the S-Functions format is specific to Simulink.
• S-Functions are not suited for embedded systems, due to the memory overhead of S-Functions.

Apart from the technological advantages summarized above, the FMI enables tool coupling without having Simulink as an integration/communication platform. Omitting Simulink as integration platform has a positive impact on simulation performance and reduces the administrative burden of keeping the entire tool chain in sync.

Tools support

As of November 2011, FMI is supported on the following simulation frameworks:^[1] See full, up-to-date list and details in FMI web pages.

• AMESim – Simulation software for the modeling and analysis of multi-domain systems from LMS International
• ASIM – AUTOSAR Builder from Dassault Systèmes
• Adams - High end multibody dynamics simulation software from MSC Software
• Atego Ace – Co-simulation environment with AUTOSAR and HIL support
• CATIA V6R2012 – Environment for Product Design and Innovation, including systems engineering tools based on Modelica, by Dassault Systèmes
• Cybernetica CENIT - Industrial product for nonlinear Model Predictive Control (NMPC) from Cybernetica
• Cybernetica ModelFit - Software for model verification, state and parameter estimation, using logged process data. By Cybernetica
• ControlBuild – Environment for IEC 61131-3 control applications from Dassault Systèmes
• CosiMate– Co-simulation Environment from ChiasTek
• DSHplus – Fluid power simulation software from FLUIDON
• Dymola 7.4 – Modelica environment from Dassault Systèmes
• FMI Add-In for Excel – Batch simulation of FMUs in Microsoft Excel
• FMU compliance checker – Software for verifying FMI standard compliance of FMUs
• FMI Library – C library for importing FMUs in custom applications
• FMU Trust Centre - cryptographic protection and signature of models including their safe PLM storage; secure authentication and authorization for protected (co-)simulation
• FMU SDK – FMU Software Development Kit from QTronic
• GT-SUITE - Multi-Physics Simulation Platform for Powertrain and Vehicle Systems
• Hopsan - Distributed system simulation tool using the TLM method
• ICOS Independent Co-Simulation – independent co-simulation environment from Virtual Vehicle Research Center
• IPG CarMaker – via Modeling and Co-Simulation environment by Modelon
• JModelica.org – Open source Modelica environment from Modelon
• MapleSim - via the MapleSim Connector for FMI from Maplesoft
• MATLAB – via FMI Toolbox from Modelon
• OPTIMICA Studio – Modelica environment from Modelon
• MWorks 2.5 – Modelica environment from Suzhou Tongyuan
• NI VeriStand – Real-Time Testing and Simulation Software from National Instruments
• LabVIEW – Graphical programming environment for measurement, test, and control systems from National Instruments
• OpenModelica – Open source Modelica environment from OSMC
• Python – via PyFMI from Modelon, also available as part of JModelica.org
• Silver 2.0 – Virtual integration platform for Software in the Loop from QTronic
• SIMPACK 9 – High end multi-body simulation software from SIMPACK AG
• SimulationX 3.4 – Modelica environment from ITI
• Simulink – via Dymola 7.4 using Real-Time Workshop
• Simulink – via @Source
• Simulink – via FMI Toolbox from Modelon
• TISC – Co-simulation environment from TLK-Thermo
• TWT Co-Simulation Framework - Communication layer tool to flexibly plug together models for performing a co-simulation; front-end for set-up, monitoring and post-processing included
• TWT Matlab/Simulink FMU Interface - FMI-compatible plug-and-play interface to Matlab/Simulink, available as an integrated block
• Vertex – Modelica environment from deltatheta
• Virtual.Lab Motion - Virtual.Lab Motion is a high end multi body software from LMS International
• Wolfram SystemModeler - Modelica environment from Wolfram Research
• xMOD - Heterogeneous model integration environment & virtual instrumentation and experimentation laboratory from IFPEN distributed by D2T.
• ePHASORsim from OPAL-RT Technologies Inc., via OpenModelica for transient stability simulations of power systems

See also

• Modelica
• Simulink

References

External links

• FMI main site
• FMI overview
• A free software development kit for FMU