Didviding and multiplying transfer functions

Marko (view profile)

on 6 Apr 2016
Latest activity Edited by yu-hsien chen

yu-hsien chen (view profile)

on 14 Oct 2018
Hi all,
I have a question on transfer function division and multiplication. If I calculate the closed loop function using:
G/(1+G*H)
I obtain different results than if I try calculating it by hand or using command.
feedback(G,H)
For example
4.244e-12 s^3 + 8.004e-08 s^2 + 8e-07 s
------------------------------------------------------------------------
1.801e-19 s^5 + 6.792e-15 s^4 + 6.407e-11 s^3 + 4.308e-08 s^2 + 0.0008 s
This is the closed loop using first command. This is with feedback command:
0.0001 s + 0.001
-------------------------------------------
4.244e-12 s^3 + 8.004e-08 s^2 + 8e-07 s + 1
While calculating it by hand gives:
0.0001 s + 0.001
-------------------------------------------
4.244e-12 s^3 + 1.224e-07 s^2 + 8e-07 s + 1
Do you know how is this possible?
Thanks!

yu-hsien chen (view profile)

on 14 Oct 2018
Edited by yu-hsien chen

yu-hsien chen (view profile)

on 14 Oct 2018

https://au.mathworks.com/help/control/examples/using-feedback-to-close-feedback-loops.html
This is a really really late reply but the above link explained it.

Debarati Banerjee (view profile)

on 12 Apr 2016
Edited by Debarati Banerjee

Debarati Banerjee (view profile)

on 12 Apr 2016

Can you first apply the function ' minreal ' on your 'G' and 'H' functions before applying the closed loop formula? This 'minreal' function will reduce the systems into minimal order.
Also, can you please provide the G and H functions with which you arrived at these results?
Cheers!
Debarati