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Dahmani Merzaka
Dahmani Merzaka

Reputation: 163

Chattering error during simulation of a model with using "when" as control condition

I try to control the volume of a tank, but the simulation chatters after 8 s.

This is the control model which is used to control the valves:

model CONTROLLER

  Modelica.Blocks.Interfaces.RealInput V_min;
  Modelica.Blocks.Interfaces.RealInput V_max;
  Modelica.Blocks.Interfaces.RealInput V;
  Boolean open1(start=true), open2(start=false);

equation 

  when V > V_min then
    open1 = true;
  elsewhen V <= V_min then
    open1 = false;
  end when;

  open2 = not open1;

end CONTROLLER;

Upvotes: 1

Views: 477

Answers (2)

kabdelhak
kabdelhak

Reputation: 697

Try something like

when abs(V - V_min) > 1.E-5 then 
  open = not pre(open);
end when;

This is a when condition that triggers both ways, but has a small tolerance. You have to initialize the variable open(start = false) to make sure it works as intended.

If the surrounding system reacts to this change slowly it does not cause chattering, if this directly influences der(V) it slows the simulation down and may still break.

For good handling of such switches i recommend looking at Modelica.Electrical.Analog.Ideal.IdealDiode (extends from Modelica.Electrical.Analog.Interfaces.IdealSemiconductor) which represents an ideal diode. As you might see it is not really the model of a truly ideal diode, but it is as simplified as it can be without causing chattering.

Upvotes: 1

marco
marco

Reputation: 6655

If you use the same threshold to open and close the valves, chattering is the obvious result.

See this answer and Modelica by Example, which both describe very good what chattering is and how it can be avoided.

Upvotes: 3

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