MATLAB: Simulink time samples

Question

I am running my Simulink model multiple times from workspace. I change one parameter during the simulation (say at t=2 sec, I change the value of x parameter in Simulink model from 10 to 20). I have one s_function written in .m file to do that. The sampling time used in s-function is 0.001 sec.

I have set my input as sine wave with sampling time 0.001 sec.

Clearly, I have 50001 samples. It works for 2 runs (2 different parameters changed). But on the third go, it is giving me 50003 samples instead of 50001.

I checked the time samples and it turns out that it is having some problem around time when parameter is being changed.

For a normal (t=0:0.001:50) command, it is giving me following time samples (around 2 sec):

1.9990    2.0000    2.0010    2.0015    2.0020    2.0028    2.0030    2.0040    2.0050    2.0060    2.0070

But my Simulink model is returning following:

1.9990    2.0000    2.0010    2.0020    2.0030    2.0040    2.0050    2.0060    2.0070    2.0080    2.0090

I have set all the other blocks which have option for sampling time to -1 in the Simulink model.

Answer 1

It is likely that the change of parameter is causing a zero-crossing to occur around t=2, thus forcing the solver to take smaller steps. You can either turn off zero-crossing detection, or, switch to a fixed-step solver if you prefer having a fixed number of time-steps (although that may compromise the accuracy of your data if your system has significant chatter).