How to co-simulate fmu with a Python function?

Question

With Python libraries like FMPy I am able to simulate fmus (using fmpy.simulate_fmu) for given start_time and stop_time. In such case, the function simulate_fmu completes the simulation and return the time-series results.

However, I want to create a closed loop between fmu and a Python function (i.e. in a Python script initialize the fmu, get the results from fmu after every 0.1s and based on that update the input value to the fmu for the next timestep). Is there a way to achieve this using existing libraries like fmpy or pyfmi?

Answer 1

The short answer is yes the tools you mention are set up to do what you are asking.

A long answer can be found for FMPy:

• Example from FMPy
• My own personal variation

But the gist is to perform changes during simulation like you are asking the approach you want you need to go a layer deeper than simulate_fmu and use doStep and associated setup. The functions/approach needed for these are defined by the FMI standard while highler level implmentations like simulate_fmu are not and are therefore tool dependent implementations of the standard.

The cliff notes are:

• Prep the Simulation Inputs
  • e.g., unzip the FMU, define simulation setup, and instantiate the FMU

from fmpy import read_model_description, extract
from fmpy.fmi2 import FMU2Slave

# extract the FMU
unzipdir = extract(fmu_filename)

fmu = FMU2Slave(guid=model_description.guid,
                unzipDirectory=unzipdir,
                modelIdentifier=model_description.coSimulation.modelIdentifier,
                instanceName='instance1')

# initialize
fmu.instantiate()
fmu.setupExperiment(startTime=start_time)
fmu.enterInitializationMode()
fmu.exitInitializationMode()

• Simulation Loop
  • Set/Get values (can do this before and/or after timestep): fmu.setReal and fmu.getReal
  • Solve the time step (fmu.doStep)
  • Store values
  • Loop until complete
• Terminate Simulation: fmu.terminate() and fmu.freeInstance()
• Plot results
• Celebrate!