Opened 4 years ago

Closed 4 years ago

#6164 closed defect (wontfix)

complex transfer function block uses wrong order of coefficients

Reported by: ho Owned by: somebody
Priority: high Milestone: 1.16.0
Component: *unknown* Version: v1.16.0-dev
Keywords: complex transferFunction, denominator, numerator Cc:

Description

The coefficients of the denominator of the complex tranferFunction block (Modelica.ComplexBlocks.ComplexMath.TransferFunction) are used in the wrong order.
The information next to the input field of the 'Parameters' 'a' and 'b' reads 'Denominator coefficients of transfer function (e.g., 5*s+6 is specified as {5,6})' but it is the other way around. One has to insert {6,5} for the above example!
The model 'ShowTransferFunction "Test Complex Transfer Function Block"' does look like it works but it does not. This is because the polynomial coefficients of the transfer function 'a' where chosen in a 'symmetric' way 'parameter Real a[:]={1,2*d,1}'. One has to change one of the two '1's to see the effect!

I suggest a modified version of the model with more realistic chosen parameters like:

model ShowTransferFunction "Test Complex Transfer Function Block"
  extends Modelica.Icons.Example;
  parameter Real d=0.01  "Damping coefficient in kg/s (not the damping ratio)";
  parameter Real m=0.1 "mass in kg";
  parameter Real c=0.1 "stiffness in N/m";
  
  
  parameter Real b[:]={-m} "Numerator polynomial coefficients of the transfer function";

  parameter Real a[:]={c,d,m} "Denominator polynomial coefficients of the transfer function
  a = c + d*s + m*s^2";


  parameter Real wMin=0.01 "Lower bound for frequency sweep";
  parameter Real wMax=100 "Upper bound for frequency sweep";
  Real lg_w=log10(logFrequencySweep.y) "Logarithm of frequency";
  Real dB=20*log10(complexToPolar.len) "Magnitude of the transfer function in decibel";
  Modelica.SIunits.Angle phi(displayUnit="deg")=complexToPolar.phi "Argument of the transfer function";
  Modelica.ComplexBlocks.Sources.LogFrequencySweep logFrequencySweep(
    duration=1,
    wMin=wMin,
    wMax=wMax)
    annotation (Placement(transformation(extent={{-80,-40},{-60,-20}})));
  Modelica.ComplexBlocks.Sources.ComplexConstant const(k(re=1, im=0))
    annotation (Placement(transformation(extent={{-80,-10},{-60,10}})));
  Modelica.ComplexBlocks.ComplexMath.TransferFunction transferFunction(b=b,
      a=a)
    annotation (Placement(transformation(extent={{-40,-10},{-20,10}})));
  Modelica.ComplexBlocks.ComplexMath.ComplexToPolar complexToPolar
    annotation (Placement(transformation(extent={{0,-10},{20,10}})));
equation
  connect(const.y, transferFunction.u)
    annotation (Line(points={{-59,0},{-50.5,0},{-42,0}}, color={85,170,255}));
  connect(logFrequencySweep.y, transferFunction.w)
    annotation (Line(points={{-59,-30},{-30,-30},{-30,-12}}, color={0,0,127}));
  connect(transferFunction.y, complexToPolar.u)
    annotation (Line(points={{-19,0},{-2,0}}, color={85,170,255}));
  annotation (
    experiment(StopTime=1, Interval=0.001), Documentation(info=
               "<html>
<p>This example shows the response of a PT2 (mechanical spring-mass-damper- system with an acceleration acting on the mass) defined by its transfer function</p>
<pre>
             -m
H(jw)=-------------------
      c + d* jw + m*(jw)^2
</pre>
<p>Frequency performs a logarithmic ramp from 0.01 to 100 s^-1.</p>
<p>
Plot the magnitude locus (in dB) dB versus lg_w and the phase locus versus lg_w.
</p>
</html>"));
end ShowTransferFunction;

BR,
Wilfried

Change History (1)

comment:1 Changed 4 years ago by casella

  • Resolution set to wontfix
  • Status changed from new to closed

@ho, this is an issue of the Modelica Standard Library, which is maintained by the ​Modelica Association. The OpenModelica tool just uses it as it is.

I guess what is needed is an update of the documentation of the model. Please open a ticket on the Modelica Standard Library issue tracker ​https://github.com/modelica/ModelicaStandardLibrary

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