Computed parameter in Modelica

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I have a model in which one of the parameters should be found by solving a nonlinear equation. Is it possible to implement one such case in Modelica? For example:

parameter Real Rs

should be found by solving the equation:

(VmaxP*(Isc+I0_diode-2*ImaxP)-ImaxP*I0_diode*Rs)/(VmaxP-Rs*ImaxP)+I0_diode*exp((VmaxP+Rs*ImaxP)/(a*Ns*Vth_diode))*((Rs*(ImaxP-Isc)+VmaxP-a*Ns*Vth_diode)/(a*Ns*Vth_diode))=0;

in the above nonlinear equation, only Rs is unknown.

modelica parameters
1 Answers

Parameters can be computed in the initial equation section when they are declared with fixed=false. Just put your nonlinear equation into this section and Rs will be computed if all other variables are known.

model FixedFalse

  parameter Real Rs(fixed=false);

  // dummy values to let the model simulate
  Real VmaxP=1; Real Isc=1; Real I0_diode=1;
  Real ImaxP=1; Real a = 1; Real Ns = 1; Real Vth_diode = 1;

initial equation 

  (VmaxP*(Isc+I0_diode-2*ImaxP)-ImaxP*I0_diode*Rs)/(VmaxP-Rs*ImaxP)+I0_diode*exp((VmaxP+Rs*ImaxP)/(a*Ns*Vth_diode))*((Rs*(ImaxP-Isc)+VmaxP-a*Ns*Vth_diode)/(a*Ns*Vth_diode))=0;

end FixedFalse;

To prevent that this parameter is shown in the parameter dialog, you can protect it and add a non-final parameter, e.g. for plotting:

  ...

  final parameter Real Rs = _Rs;

protected 
  parameter Real _Rs(fixed=false);

initial equation 
  // Now use _Rs here
  ...
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