One of the "rules" for sketching the root locus is that for k>0 the RL exists for every part where the number of poles and zeros is odd ("to the right of it"). If we take the simple transfer function P(S) = 1/(S^2)(S+1)
There are even number of poles to the right of s=0 thus i thought the graph should exists there, but i see from wolfram that it does enter image description here

what am i missing?


1 Answer 1


Just read through my old notes, you are not missing anything!

As far as I've always learned the rule is:

'The loci are on the real axis to the left of an odd number of poles and zeros'

as a proof, I have looked up my Control theory book

"Feedback Control of Dynamic Systems, 7th edition" by Gene F. Franklin, J. David Powell and Abbas Emami-Naeini.

If you happen to have that book laying around somewhere, see page 263, expressed after "RULE 2."


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