Maximum time step

Maximum time step The discontinuity handling relies on the fact that submodels detect the event leading to a discontinuity. However, the solver can take huge integration steps, so that a crossing event may be totally missed.

The following graph shows the blue curve reaches in theory the black dotted line, but as no solver step is taken inside this particular region, the event is never registered.

Figure 4: solver missing a line crossing event

Such a configuration can happen in models containing cyclic characteristics. A rotating machine containing cam profile is a good example: the cam profile contains tabulated data for angles comprised in the range [0, 360] degrees, whereas the rotating machine following a velocity of 6000 RPM will rapidly reach angles of around 360000 degrees. In this case the solver may take one integration step per turn of the cam. This aliasing effect will result in cam submodel providing a constant radius, as if the cam was a perfect circle.

To prevent such a phenomenon, the solver must be forced to be less aggressive, by limiting the size of integration steps. This limit is called the “maximum time step”. It is set by the user in the “Standard options” tab of the “Run Parameters” dialog.

Figure 5: setting the maximum time step

Note that the maximum time step will limit the integration step size during the whole simulation. Setting it to a too small value will have a direct impact on the CPU time and will slow down the simulation. It is therefore recommended to precisely analyze the evolution of the integration step size during the simulation using the “Performance Analyzer” available in the “Analysis” menu, before setting a maximum time step. Note that the Performance analyzer works only with the “Run stats” RSTAT submodel placed on the sketch.

Figure 6: RSTAT icon

RSTAT Setting the maximum time step has no effect on a fixed step solver. Fixed step solvers only use the constant step. Moreover, the discontinuity handling mechanism is disabled in case of fixed step solver: the discontinuity “flag” is always zero.