Failure to run the textbook simulation example of a pendulum - Error solving nonlinear system residualFunc2 (size 1) at time...

[hurak@…]

The introductory model of a pendulum given by Peter Fritzson in his older book on pages 20 and 21 fails to simulate correctly in the recent version of OpenModelica. The code is here:

model Pendulum
  parameter Real m = 1,g = 9.81,L = 0.5;
  Real F;
  Real x(start = 0.5),y(start = 0.0);
  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 Pendulum;

simulate(Pendulum, stopTime=4);

plot(x)

In OMEdit the simulation output contains this error message repeated a number of times:

stdout | warning | Error solving nonlinear system residualFunc2 (size 1) at time 0.418728

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When run in OMNotebook, the "simulate" command just returns "false".

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Tested both on Windows (OpenModelica 1.9.0 Beta 2 nightly build r13781) and Linux (OpenModelica 1.9.0 Beta 2 nightly build r13764, from the deb repository).

Downgrading to OpenModelica 1.8.1 solves the issue.

[sjoelund.se]

Postponed until 1.9.1

[sjoelund.se]

This was fixed sometime since the problem was reported (over a year ago). The system does not contain any nonlinear system (tearing? I don't know).