Search code examples
modelicadymolapid-controller

How to keep the output signal of a PID block unchanged after a fixed time point?

I build a fluid model with a PID control system, and I wanna run my model until reaching a steady state and then keep the output signal of the PID system unchanged, so I could do step excitation stability tests. But right now I am not sure how to keep the output signal of the PID control system unchanged after a fixed time point. So I have to use a logical switch to change the signal source at a fixed time point.
My question is:
How could keep the output signal of a Block Component unchanged after a fixed time point?

enter image description here

Solution

  • I dont't think there is a suitable block in the Modelica Standard Library.

    But would that code do it?

    model KeepValueTime
  extends Modelica.Blocks.Interfaces.SISO;

  parameter Modelica.Units.SI.Time t = Modelica.Constants.inf "Time at which the value shall be kept";
  Real u_keep(start=0) "Value to be output when 'keepValue = true'";

equation 
  if time < t then
    y = u;
  else
    y = u_keep;
  end if;

  when time >= t then
    u_keep = u;
  end when;

  annotation (uses(Modelica(version="4.0.0")));
end KeepValueTime;

    A bit more general with a Boolean input:

    model KeepValue
  extends Modelica.Blocks.Interfaces.SISO;

  Real u_keep(start=0) "Value to be output when 'keepValue = true'";

  Modelica.Blocks.Interfaces.BooleanInput keepValue annotation (Placement(transformation(
        extent={{-20,-20},{20,20}},
        rotation=0,
        origin={-120,80}), iconTransformation(extent={{-140,60},{-100,100}}, rotation=0)));

equation 
  if not keepValue then
    y = u;
  else
    y = u_keep;
  end if;

  when keepValue then
    u_keep = u;
  end when;

  annotation (uses(Modelica(version="4.0.0")));
end KeepValue;

    A quick test:

    Test Example

    ...seems to do what you need: Example Result