Integrator problem

Hello,

My question is about Simulink. I designed two systems: - 1. The first system computes an output A which is continuously integrated. The result is stored in B.

- 2. The second system takes A as an entry ("from workspace" block). A is continuously integrated. The result is stored in B2.

I used the same solver (ode4), same time step (4Hz), same initial conditions (0) for the integrator in system 1 and 2. Therefore B and B2 should be strictly equal...but there are differences (up to 10%) between them and I don't understand why. Any idea?

MJ

Answer 1

[Guy Rouleau]

This is probably because the input signal is different during minor steps. You can confirm that by trying:

- Try ode1 solver. This solver doe snot have minor time steps and should not be affected by that

- Try my central submissin "Minor Time Step Logging". This will allow you to observe the phenomenon.

http://www.mathworks.com/matlabcentral/fileexchange/22695-minor-time-step-logging

To make the results match, you can try:

- Verify if the From Workspace is configured to interpolate or not. - Verify the sample time of all blocks in your model. Enable sample time colors. For a continuous system, everything should be black. Grey or Red can lead to the behavior you describe. - Note that minor time steps cannot be logged. Consequently when logging a continuous signal and reloading it, you loose information.

Answer 2

[Maxime Journot]

Thanks a lot Mr Rouleau. I suspected the minor time steps of the RK4 but had no mean to monitor them. I used your "Minor Time Step Logging" Simulink block and the cause of the problem appeared obvious.