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Re: Strange behavior of System Modeler

• To: mathgroup at smc.vnet.net
• Subject: [mg130008] Re: Strange behavior of System Modeler
• From: awnl <awnl at gmx-topmail.de>
• Date: Sun, 3 Mar 2013 23:00:57 -0500 (EST)
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Hi,

>
> I have just started studying WSM and the Modelica language by implementing the simple pendulum model (class) es. described on p.33 of Peter Fritzson's book  Introduction to "Modeling and Simulation of Technical and Physical Systems with Modelica". The code (pendulum equation written as a DAE) is:
>
> model DAEExample "DAEExample"
>    constant Real PI=3.14159265358979;
>    parameter Real m=1,g=9.81,l=0.5;
>    output Real F;
>    output Real x(start=0.5),y(start=0);
>    output Real vx,vy;
> equation
>    m*der(vx)=-x/l*F;
>    m*der(vy)=-y/l*F - m*g;
>    der(x)=vx;
>    der(y)=vy;
>    x^2 + y^2=l^2;
> end DAEExample;
>
> I run the example in WSM and I get totally meaningless results.
> The solver is the default one (DASSL) which, I think, is the right one for handling DAEs.
>
> Any ideas?

Just a followup to my recent post: you can use manual state selection 
(dynamic state selection is one of the tricks that Dymola uses to get 
this right) to get correct solutions with WSM like this:

model DAEExample
   constant Real PI=3.14159265358979;
   parameter Real m=1,g=9.81,l=0.5;
   Real F;
   Real x(start=0.5,stateSelect=StateSelect.always);
   Real y(start=0);
   Real vx(start=0);
   Real vy(start=0);
equation
   m*der(vx) = -x/l*F;
   m*der(vy) = -y/l*F - m*g;
   der(x) = vx;
   der(y) = vy;
   x^2 + y^2 = l^2;
end DAEExample;

hth,

albert