Different results for Simulink's External and Normal mode!

In Real-Time Windows Target (Simulink), a simple DC motor model works fine in Normal mode (unit step input), but becomes unstable and blows up (10^301) in External mode. I've noticed that a time step size of 1e-3 causes blowup while 1e-4 does not. Why? They should behave the same.

Answer 1

[Jan Houska]

The behavior you describe is most likely not because of External or Normal mode. It looks like the cause is a different ODE solver used in each of the modes. Namely, you can use a variable-step solver in Normal Mode, but it is necessary to use fixed-step solver in External Mode. And it looks like the fixed-step solver is not able to solve your system at your selected fixed time step of 1e-3. The fact that reducing the time step to 1e-4 helps only supports this theory.

To verify, please try to simulate your model in Normal Mode with exactly the same solver settings - fixed step, 1e-3. I would expect you to see the same unstable behavior too.

[John]

Interesting idea, could be. I tried running both Normal/Ext modes, and looked at the Config Params, and both are set to Fixed Step.