Did you see the pretty warning that says

rx <> 0.0

will not work as expected?

Anyway, you can tell DASSL what tolerance to use as when you get close to the 0, the system changes too fast. You also need to change the cutoff point from 0.0 to some epsilon close to zero.

loadString("

class Kulon

  Real rx(start=0.1);

  Real vx(start=2);

  Real ax;

  parameter Real q = 1;

  parameter Real m = 1;

equation

  m*ax = if noEvent(abs(rx) > 1e-12) then -(q*q)/rx else 0;

  ax = der(vx);

  vx = der(rx);

end Kulon;

");



simulate(Kulon,stopTime=5.0,tolerance=1e-3);getErrorString();

Thank you for help!

sjoelund.se wrote:

Did you see the pretty warning that says

Code:

rx <> 0.0

will not work as expected?

Ah, I missed it, I stuck on that error... Thank you!

sjoelund.se wrote:

Anyway, you can tell DASSL what tolerance to use as when you get close to the 0, the system changes too fast. You also need to change the cutoff point from 0.0 to some epsilon close to zero.

Using this equations I'd like to move one particle (with negative charge) around another particle (with positive charge). For this I use Coulomb law (actually I had an error in my original equation, it should be rx^2 in denominator). So as particle is moving by circle it'll have rx=0 and ry=0 sometimes. How this task could be solved in Modelica?

sjoelund.se wrote:

Code:

loadString("

class Kulon

  Real rx(start=0.1);

  Real vx(start=2);

  Real ax;

  parameter Real q = 1;

  parameter Real m = 1;

equation

  m*ax = if noEvent(abs(rx) > 1e-12) then -(q*q)/rx else 0;

  ax = der(vx);

  vx = der(rx);

end Kulon;

");



simulate(Kulon,stopTime=5.0,tolerance=1e-3);getErrorString();

Thank you for corrections, it's working now! Unless rx == 0...

So you x=y=0 and the charges are opposite so they attract each other and should stick there? The problem is the infinite power and low resolution of double precision floating point (as you could have guessed). I'm not sure what the best approach would be. Something similar to the above approach or possibly a when-clause that uses an external C function (then you can deal with the case that you get infinite acceleration and you can reinit() rx and ry to 0).

sjoelund.se wrote:

So you x=y=0 and the charges are opposite so they attract each other and should stick there? The problem is the infinite power and low resolution of double precision floating point (as you could have guessed). I'm not sure what the best approach would be. Something similar to the above approach or possibly a when-clause that uses an external C function (then you can deal with the case that you get infinite acceleration and you can reinit() rx and ry to 0).

M, I thought it would be easier

That negative particle won't fall on positive particle because it have some speed, enough to compensate attraction force. So there is balance between attraction force (opposite charges attract each other) and repulsion force (centrifugal force). I use orthogonal coordinates and have 2 set of equations - for x and y projections. And when particle is flying around another particle, x and y will be equal to 0 periodically. Seems this is the reason why Modelica throws error.