MATLAB: Creating “referenced model” that contains physical modeling connection ports

Question

I am creating a model for the simulation of an electrical three phase system. The main-model is supposed to include a subsystem that calls another referenced model. As long as only Simulink-Signals cross the reference borders, this works fine. But I also need physical electrical signals to cross these borders, which does not not seem to be possible.

As a first solution, I tried to work with a current/voltage source at each side of the border which is controlled by a voltage/current measurement-block at the corresponding opposite side of the reference border. Different errors that came up subsequently I could get solved. But right now the model is running quite unstable. As the main model and the referenced model are running in two different time steps, the simulation crashes, as soon as the electrical values change too quick respectively the dynamic of the model states is too high.

Does anyone have any experience with this issue or dealt with a similar model and can help me with that?

Thank you very much in advance!

Best regards, Robin

Answer 1

You're right that model reference boundaries don't work with physical model blocks. The reason your workaround makes the simulation crash is that you're introducing what is known as an algebraic loop in Simulink.

Essentially, by adding Simulink signals at the model boundary, you're "tricking" Simulink into thinking one network's inputs are directly dependent on its outputs. This, in turn, forces Simulink to "guess" the solution through an iterative solution. Generally not a good thing to have if you can avoid it.

A better workaround in my opinion would be to change the model references to libraries. With libraries, you can still split up your design into separate files, create single-sources reusable components, etc. However, physical connection ports can be at the interfaces of library blocks.

The downside of using libraries is that, by keeping all blocks in the same physical network, they all must have the same sample time. Were you trying to run different parts of your model at different rates? If so, I'd stick with your original approach and place a Transfer Fcn block in the loop as a low-pass filter to break the algebraic loop. This requires some manual tweaking that is highly dependent on your model dynamics.

- Sebastian