Modelica.Fluid.Examples.TraceSubstances.RoomCO2WithControls is problematic because of chattering

[casella]

As reported in #4305, Modelica.Fluid.Examples.TraceSubstances.RoomCO2WithControls is problematic: Dymola 2024x fails with it and so does OpenModelica. Apparently there are some reasons to generate chattering.

This should be investigated before the 4.1.0 release.

[maltelenz]

We also see chattering around the same time in System Modeler.

[casella]

I'll have a closer look ASAP

[MatthiasBSchaefer]

This is the commit, causing the chattering according to my tests in Dymola:
930456a

[HansOlsson]

I tried to debug it and preliminarily it seems that the linear part makes sense, but needed to be applied in additional cases.

Specifically the code has:

      if  xi1 < x0 or xi2>x1 then
        // The limits don't make sense, just use linear interpolation
        y := (y1-y0)*(x-x0)/(x1-x0) + y0;
      else

And what happens here is that xi1 and xi2 are practically identical, but x0 and x1 are different.
Adding or abs(xi2-xi1)<100*Modelica.Constants.eps fixes this specific case, but it should be investigated a bit more.

[HansOlsson]

I tried to apply the above and run all the Fluid-tests (without looking at the results) (on maint/4.1.x since I needed MSL-binaries).

In Dymola 2025x (updated) with Advanced.Translation.PlaceDymolaSourceFirst = 2; all fluid examples ran.
I'll make a proper PR of this.

[casella]

It seems that @HansOlsson managed to fix the remaining issues with regfun3 in PR #4522. When that is merged on master we can check if there are any issues popping up with other models and if we have no evidence of that we can back-port it to 4.1.0.