Cannot get updated outputs in initialization mode

[AayushSabharwal]

I'm trying to use the low-level FMI functions (as opposed to simulateCS) to integrate FMUs with ModelingToolkit.jl. One problem is that we need to be able to set inputs/states of the FMU and get updated output values during initialization, since the CoSimulation FMU is connected to a larger model in Julia.

I'm trying something equivalent to the following call sequence:

for iteration in newton_solver     # iterative algorithm that solves for the initial state of the system of equations
    instance = FMI.fmi2Instantiate(fmu)
    # all parameters are initial = "exact" variability = "fixed"
    FMI.fmi2SetReal(instance, param_value_references, Csize_t(length(param_value_references)), params)
    FMI.fmi2SetupExperiment(instance, FMI.fmi2True, tolerance, 0.0, FMI.fmi2False, 0.0)
    FMI.fmi2EnterInitializationMode(instance)
    FMI.fmi2SetReal(instance, input_value_references, Csize_t(length(input_value_references)), inputs)
    FMI.fmi2SetReal(instance, state_value_references, Csize_t(length(state_value_references)), states)
    outputs = FMI.fmi2GetReal(instance, output_value_references)
    FMI.fmi2Terminate(instance)
    FMI.fmi2FreeInstance!(instance)

    # other stuff...
end

However, no matter the values of inputs, parameters or states, the outputs have the same value.

[ThummeTo]

(I converted this to discussion)

Hi Aayush,
what you could try:

• activate logging, to print messages from the FMU (depending on the exporting tool, there are nice error messages implemented), see method definition
• within fmi2Instantiate further specify the FMU type, to be sure to be in CS mode (some FMUs support ME and CS)
• add FMI.fmi2ExitInitializationMode(instance) before the call to fmi2GetReal

Performance remarks:

• is it required to allocate/free a new instance within the iterative loop? if not put fmi2Instantiate and fmi2FreeInstance! outside
• if output is always the same size, you can allocate a buffer outside the loop and reuse it via fmi2GetReal!
• this will not change much, but you can skip the fmi2Terminate if you keep the fmi2FreeInstance! (because it's not used anymore after getting the outputs)

Regards!