Question

How to implement Software in the Loop for control systems design with Modelica (OpenModelica, jModelica) for a plant and C/C++ routine for a controller. What approaches you can suggest?
I thought external C Functions would help but seems they have some restrictions such that they should obey reference transparancy property ie return the same values for the same inputs (should not have internal states).

Answer 1

In that case, the issue with calling external C functions does not apply. It is true that Modelica restricts the use of functions in the continuous equations such that the function must return the same value for the same arguments. In those cases, you must find a way to pass the state into the function and have it return the new state (if you want to satisfy this “purity” requirement). This is obviously quite tedious to do with C code (you would have to pass in the state, assign all state variables (probably globals) run your code, then extract the values of all the state variables and return them).

Fortunately for you, you don’t need to worry about this. The reason is because your function only needs to be called from within a when clause. If you are triggering your when clause based on time (e.g. using the sample(…) function), I’m pretty sure you are guaranteed that the function will only be invoked once at each time.

Basically, your setup would be something like this:

algorithm
  when sample(0, sample_rate) then
    u := controllerEvaluation(x, y, t);
  end when;

In this way, you can pass time, the necessary states, x, and inputs, y (or u, depending on your perspective), into the controller and you get back the controller command to the plant, u.

In this context, the when clause is a representation of your scheduler and because it doesn’t include any state events (like you would have, for example, with a tooth wheel encoder or other asynchronous interrupt) the simulator can schedule all these function invocations without any risk of having to repeat them.