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Problem using initial equations

#4744  Permalink Top

Problem using initial equations

Jun-24-14 12:54:06

Hi,
Please see the follwing model

Code:

model Test

    import SI = Modelica.SIunits;

  parameter SI.Temperature T;

initial equation

    T = (300+400)/2;

end Test;

When I simulate this, I get the following warnings

Code:


[:0:0-0:0] Warning: No system for the symbolic initialization was generated. A method using numerical algorithms will be used instead.

[:0:0-0:0] Warning: It was not possible to solve the over-determined initial system (2 equations and 1 variables)

[:0:0-0:0] Warning: System is over-determined in Equations 1: T = 350.0;

2: T = 288.15;



[:0:0-0:0] Warning: Trying to fix over-determined initial system with 1 variables and 2 equations... [not implemented yet!]

[:3:3-3:29] Warning: Parameter T has no value, and is fixed during initialization (fixed=true), using available start value (start=288.15) as default value.

Firstly, I dont understand why it tells me that there are 2 equations and 1 variable. When instantiating it says that there are 0 equations and 0 variables which is correct.

Secondly, By default, all parameters will have fixed = false by default.  Why is OMEdit taking fixed = true?

#4745  Permalink Top

Re: Problem using initial equations

Jun-24-14 13:03:16

I set fixed = false for the above problem. In this case, I dont get any warnings and the plot shows me T = 350degC. But the Variables browser shows T = 288.15 degC

#4746  Permalink Top

Re: Problem using initial equations

Jun-24-14 13:13:40

The variables browser will show the start-value because that is all that is available to it (the information before simulation starts).

In Modelica, variables will have fixed=false by default. But parameters have fixed=true.

#4747  Permalink Top

Re: Problem using initial equations

Jun-24-14 13:18:49

Can i know why the model was giving that the system was an overdetermined one?

#4748  Permalink Top

Re: Problem using initial equations

Jun-24-14 13:20:32

Code:

model Test 

    import SI = Modelica.SIunits; 

  parameter SI.Temperature T; 

initial equation 

    T = (300+400)/2; 

end Test;

is basically transformed into:

Code:

model Test 

    import SI = Modelica.SIunits; 

  parameter SI.Temperature T(fixed=false); 

initial equation

    T = T.start;

    T = (300+400)/2; 

end Test;

#4749  Permalink Top

Re: Problem using initial equations

Jun-24-14 13:22:51

Thanks current/smile

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