Derivative in simulink causes instability?!

Hi all,

I have attached diagram of a simple system in simulink and I used Linearizing tool t extract the transfer function and its poles as shown in the figure. Any suggestion why this system is unstable? (I used a transfer function tf([N 1/N],[1 N])) instead of the derivative and its stable now.

Best regards Alireza

[Birdman]

Can you share your model?

[Alireza Sadri]

Hi,

Sure, I have attached the block diagram in simulink, but these are all basic simulink elements and I am trying to show that what is happening to derivative block is far more than what is documented and I have no explanation for that, but I do have a solution as mentioned before.

Best regards Alireza

Answer 1

[Ravi Tripathi]

Please use a low pass filter after the derivative according to the signal frequency.

[Alireza Sadri]

Well as I mentioned, if I use tf([N a],[1 N]) and set a to a small positive number, the problem is solved. But I was looking for the reason behind this. Mabe the designer of the derivative should allow people to set a.