Algebraic Loops with FMU

When connecting FMUs in co-simulation mode in a loop an algebraic loop errors occurs, which causes termination of simulation. Therefore, I assume Simulink identifies the FMU-blocks as direct feedthrough. In the FMI 3.0 specification it is stated, that there cannot be a direct feedthough in Co-Simulation, as the step-size has to be greater that 0. Why does Simulink handle FMUs in co-simulation mode as direct feedthrough (and not like integrator blocks instead)?

Unfortunately I could not find anything regarding how the C-API functions of the FMUs are called by the Simulink discrete fixed-step solver.

Answer 1

[Ashutosh Thakur]

Hi Julian,

In Simulink, FMU blocks are treated as standalone entities, so they do not contain all the information related to the states from the original model from which they were created. The use of feedback signals can result in an Algebraic Loop during simulation.

Introducing Unit Delay blocks in the Simulink model can resolve the Algebraic Loop. However, the introduction of Unit Delay blocks can affect the system's dynamics and potentially distort the results. You can refer to the following documentation link for assistance in resolving issues related to Algebraic Loops: https://www.mathworks.com/help/simulink/ug/algebraic-loops.html.

I hope this information helps you.

[Julian]

Hi Ashutosh,

Thanks for your reply!

I have found that the solver is not applicable to FMU blocks as it seems that the algebraic loop solver is not able to call the fmiXstep or any other function that actually results in a calculation. So the algebraic loop solver is not able to interact with the model contained in the FMUs. Therefore, I do not know why it was implemented this way, instead of ignoring loops with FMU blocks following an explicit orchestration algorithm scheme.

@MatlabStaff in the Simulink User Guide p.53-44 describes the update phase: "The FMU block computes states at the current time step [...]. This method calls fmi3SetXXX for input". States at the current time step cannot be calculated using FMI. How is this to be understood?

[Ashutosh Thakur]

Hi Julian,

Can you share the link for the Simulink User Guide?

Thanks

[Julian]

Sure! Here can You find the User's Guide: https://de.mathworks.com/help/pdf_doc/simulink/simulink_ug.pdf