FAQ S03 - Curves produced by my model look constant whereas they should change with time. What can I do?

Category: Simcenter Amesim

Topic: Simulation

Access Level: End User

Publish Date: November 2010

Question

Curves produced by my model look constant whereas they should change with time. What can I do?

Answer

Reduce the maximum time step:

When a model seems to have low dynamics, the solver tries to increase its integration step size.

The integration step can then become so big that only a few points really correspond to the integration process. Other printout points are interpolated points. This can give the impression that a variable is not varying over time.

This phenomenon can happen when some signals do not directly perturb the state variables of the model, so that the solver does not detect any dynamics.

Reducing the maximum time step to a value close to the printout interval can in such a case solve the problem.

Reduce the print interval:

The value of the print interval must be adapted to the observed phenomenon. The Nyquist-Shannon theorem indicates that the sampling time is strongly linked to the maximum frequency of the observed signal.

This phenomenon is also called "aliasing effect" or "undersampling".

This results in an apparent low frequency in a signal that does not exist and results from a lack of print points.

A good compromise between results file size and precision of the output consists in setting the printout interval to T/4, with T the period of the signal of the observed signal.