Initialize an FMU with non-default values

The interest of using FMUs in Python lies in the ease to change its input / parameter values. This notably enables to study the behavior of the FMU with uncertain inputs / parameters.

Note

A FMU parameter remains constant during simulation (a single value).

A FMU input evolves during simulation (time-dependent values).

See the FMI Standard for more details.

First, retrieve and load the FMU deviation.fmu.

import otfmi.example.utility
from os.path import abspath

path_fmu = otfmi.example.utility.get_path_fmu("deviation")
model = otfmi.fmi.load_fmu(path_fmu)

# check the beam default length and bending load values
print(model.get_variable_start("L"))
print(model.get_variable_start("F"))

250.0
30000.0

We want to set the cantilever beam length to 300 m and the bending load to 25000 N.

Using inline argument

The beam length can be required directly in the simulate function:

result = otfmi.fmi.simulate(model, initialization_parameters=[("L", "F"), (300, 25000)])

# check parameters value and show output value
print("L = %g" % result.final("L"))  # check parameter value
print("F = %g" % result.final("F"))  # check parameter value
print("y = %g" % result.final("y"))  # print output value

Simulation interval    : 0.0 - 1.0 seconds.
Elapsed simulation time: 0.010239019999971788 seconds.
L = 300
F = 25000
y = 18.75

This way of doing is practical when only a few number of parameters / inputs must be set. For larger numbers of variables to initialize, the use of scripts is recommended.

Using initialization scripts

Initialization scripts can gather a large number of initial values. The use of initialization scripts (.mos files) is common in Dymola: - to save the value of all the variables of a model after simulation, - to restart simulation from a given point.

Note

The initialization script can be automatically created in Dymola.

For clarity, we write the initialization script using Python.

temporary_file = "initialization.mos"
with open(temporary_file, "w") as f:
    f.write("L = 300;\n")
    f.write("F = 25000;\n")

Otfmi simulate function launches FMU initialization, using the designated script, then simulates the FMU.

result = otfmi.fmi.simulate(model, initialization_script=abspath(temporary_file))

# check parameters value and show output value
print("L = %g" % result.final("L"))  # check parameter value
print("F = %g" % result.final("F"))  # check parameter value
print("y = %g" % result.final("y"))  # print output value

Simulation interval    : 0.0 - 1.0 seconds.
Elapsed simulation time: 0.009870034000016403 seconds.
L = 300
F = 25000
y = 18.75