OpenModelica doesn't follow the discrete logic as expected

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OpenModelica doesn't follow the discrete logic as expected

#9401 Permalink

OpenModelica doesn't follow the discrete logic as expected

Oct-04-19 08:34:33

Following this and this questions, I'm trying to simulation the below model in OpenModelica:

Code:


package friction1D



  function coulombFriction

    input Real relVel;

    input Real shearForce;

    input Real normalForce;

    input Real statfricco;

    input Real kinfricco;

    input Real inertia;

    input Real relAcc;

    output Real fricForce;

  algorithm

    if (relVel == 0) and (abs(shearForce - inertia * relAcc) < statfricco * normalForce) then

      fricForce := shearForce - inertia * relAcc;

    else

      fricForce := kinfricco * normalForce * sign(relVel);

    end if;

  end coulombFriction;



  model fricexample_1

    //parameters

    parameter Real kco = 0.3;

    parameter Real sco = 0.4;

    parameter Real nfo = 1.0;

    parameter Real mass = 1.0;



    Real sfo;

    Real ffo;

    Real x;

    Real v;



  initial equation

    x = 0;

    v = 0;



  algorithm

    sfo := 0.7 * sin(time);

    ffo := coulombFriction(relVel = v, shearForce = sfo, normalForce = nfo, statfricco = sco, kinfricco = kco, inertia = mass, relAcc = der(v));



  equation

    v = der(x);

    mass * der(v) = sfo - ffo;



  annotation(

      experiment(StartTime = 0, StopTime = 10, Tolerance = 1e-8, Interval = 0.02),

      __OpenModelica_simulationFlags(lv = "LOG_STATS", outputFormat = "mat", s = "dassl"));

      

  end fricexample_1;



end friction1D;

apart from the warning:

Translation Warning
Iteration variables with default zero start attribute in torn nonlinear equation system:
$DER.v:VARIABLE() type: Real

The results don't make sense.

I expect the ffo to rise till 0.4 and then fall to 0.3 at point 1, but it shows weird behavior, which I can't explain at points 2 and 3. I would appreciate it if you could help me understand what is happening here and why I'm not getting the behavior I expect?

P.S. I post this question here on Modelica language Discord group and well as here on Reddit.

Edited by: foadsf - Oct-04-19 08:35:20

foadsf: 20 Posts; Thank you [ 0 ]

#9402 Permalink

Re: OpenModelica doesn't follow the discrete logic as expected

Oct-04-19 08:58:38

I changed the code to :

Code:


package friction1D



  final constant Real eps=1.e-15 "Biggest number such that 1.0 + eps = 1.0";



  function coulombFriction

    input Real relVel;

    input Real shearForce;

    input Real normalForce;

    input Real statfricco;

    input Real kinfricco;

    input Real inertia;

    input Real relAcc;

    output Real fricForce;

  algorithm

    //if (relVel < eps) and (abs(shearForce - inertia * relAcc) < statfricco * normalForce) then

      //fricForce := shearForce - inertia * relAcc;

      if (relVel < eps) and (abs(shearForce) < statfricco * normalForce) then

        fricForce := shearForce;

    else

      fricForce := kinfricco * normalForce * sign(relVel);

    end if;

  end coulombFriction;



  model fricexample_1

    //parameters

    parameter Real kco = 0.3;

    parameter Real sco = 0.4;

    parameter Real nfo = 1.0;

    parameter Real mass = 1.0;



    Real sfo;

    Real ffo;

    Real x;

    Real v;



  initial equation

    x = 0;

    v = 0;



  algorithm

    sfo := 0.7 * sin(time);

    ffo := coulombFriction(relVel = v, shearForce = sfo, normalForce = nfo, statfricco = sco, kinfricco = kco, inertia = mass, relAcc = der(v));



  equation

    v = der(x);

    mass * der(v) = sfo - ffo;



  annotation(

      experiment(StartTime = 0, StopTime = 10, Tolerance = 1e-8, Interval = 0.02),

      __OpenModelica_simulationFlags(lv = "LOG_STATS", outputFormat = "mat", s = "dassl"));

      

  end fricexample_1;



end friction1D;

and now the first issue is solved:

and it makes sense at point 1, but then from point 2 to 3 where ffo simply should be -0.3 it is following the sfo!