How to do co-simulation with two FMU models

Question

I have two FMU developed with different simulation tools. I would like to connect them through some variables and run a co-simulation with an open source tool as OpenModelica. This co-simulation has to be rather tight as both FMU have discontinuities.

Is that possible?. Could you provide a simple sample?

Answer 1

Model Exchange vs. Co-Simulation

This depends on how you export your FMUs: You can either use FMI for model-exchange or FMI for co-simulation.

In the model-exchange scenario, the FMU contains only the model and no solver. Therefore the solver of the importing simulator is used.

In the co-simulation scenario, the FMU contains both the model and a solver. Here the importing simulator acts as the master of the co-simulation.

In both cases you will need a simulator that supports the import of the respective FMU type. OpenModelica has support for import of model-exchange FMUs since version 1.8.0. Hence, if you have two FMUs that you have exported using FMI for model-exchange, you can import them both into OpenModelica and simulate them using the OpenModelica solver. Note that strictly speaking, this isn't a real co-simulation since only one solver is involved.

If you want to use a separate solver for each FMU (i.e. perform a real co-simulation) then you should only export one of the models as an FMU using FMI for co-simulation. Import that FMU into the simulator of the other model. Note that OpenModelica 1.8.0 doesn't support FMI for co-simulation.

Model Exchange Example

Here's a working example (tested in Dymola, I don't have OpenModelica installed here right now):

File TestFMU1.mo:

model TestFMU1
  parameter Real p = 1.0;

  connector TestOutputConnector
    output Real value;
  end TestOutputConnector;
  TestOutputConnector c;

equation 
  c.value = p;

end TestFMU1;

File TestFMU2.mo:

model TestFMU2
  Real result;

  connector TestInputConnector
    input Real value;
  end TestInputConnector;
  TestInputConnector c;

equation 
  result = c.value;

end TestFMU2;

Export both of these as FMUs. Then import them and combine them as follows:

model TestConnection
  TestFMU1_fmu OutputFMU;
  TestFMU2_fmu InputFMU;
equation 
  connect(OutputFMU.c_value, InputFMU.c_value);
end TestConnection;

OpenModelica

It seems that OpenModelica currently does not support simulating two FMUs simultaneously. This is a known bug.