change FMI variable/function during simulation

Question

I have a simple FMU file which contains a sine block that takes u as input and outputs y. In this case, u is set to equal to time. In my C++ code I have loaded the FMI library from FMILibrary and had done all the necessary steps up to a point where I want to give my input u a new value of pi(as 3.14). So I went:

fmistatus = fmi2_import_set_real(fmu, &uRef, 1, &pi);
while (timeCurrent < timeEnd){
    fmistatus = fmi2_import_do_step(fmu, timeCurrent , stepSize, fmi2_true);
    timeCurrent += stepSize;
}

u was still set to time even though I tried to give it a new value. Did I miss something?

PS. Is there anywhere I can find a more detailed description on the FMI library functions? Currently I can only find input output descriptions or did I miss something again.

UPDATE: After a few trials, I think this issue might be because I was trying to redefine my equation u = time. In other words when I change my u variable into RealInput block in openmodelica everything goes fine. So what if I really wants to redefine a certain equation? what do I have to do?

Is it not allowed in FMI to "replace" the value of a variable? — will i am, Dec 06 '17 at 00:46
possibly related: https://stackoverflow.com/questions/37571451/parameters-sometimes-not-editable-in-dymola-simulation-tab-and-fmu — matth, Dec 06 '17 at 08:32
It is similar but in my case its not about parameters but rather changing equation. ie change u = time into u = 3.14. — will i am, Dec 06 '17 at 09:33

score 1 · Accepted Answer · answered Dec 07 '17 at 08:53

You shall not be able to set any variable in FMI - and especially not a variable with a binding equation - and I assume your Modelica model has "u=time;". Instead of having "u=time" you need to add a top-level input without any equation (so that the exported FMI has it as an input) - and then connect that to the sine-block.

Details: For a co-simulation FMI the restriction on what you can set are in the state-diagram in section 4.2.4 of FMI2 specification.

Between fmi2DoStep you can only set Real variables that have causality="input" or causality="parameter" and variability="tunable" - and an input with an equation doesn't qualify.

Before starting the integration you could set it for other variables as well, but that is only guess-values for the initialization - and should not over-write the "u=time" equation.

change FMI variable/function during simulation

1 Answers1