Root Locus Question: Unraveling Intuition for Stability Analysis

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Discussion Overview

The discussion revolves around the concept of root locus in control systems, specifically addressing the intuition behind its application for stability analysis of closed-loop systems. Participants explore the relationship between open-loop transfer functions and closed-loop stability, as well as the implications of gain on pole locations.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about why the open-loop transfer function is used for analyzing the stability of the closed-loop system, seeking intuitive understanding.
  • Another participant explains that the closed-loop transfer function can be derived using Mason's rule, providing a formula for the characteristic equation.
  • A third participant questions whether the original poster is confusing root locus with Nyquist plots.
  • One participant clarifies that the root locus represents the movement of poles as gain varies, emphasizing that stability is determined by the location of these poles in the s-plane.
  • It is noted that for positive gain, two poles will always remain outside the left half of the s-plane, while for negative gain, one pole will not be in the left half.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of using the open-loop transfer function for stability analysis, with some providing explanations and others questioning the approach. The discussion remains unresolved regarding the original poster's confusion and the relationship between root locus and other stability methods.

Contextual Notes

Some assumptions about the system's configuration and the specific definitions of terms like "open-loop" and "closed-loop" may not be fully articulated, leading to potential misunderstandings. The discussion also highlights the dependence on gain values and their impact on pole locations.

alexmath
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Homework Statement


I've been trying for a week to understand root locus and how it works but what i got is a big confusion that's why I'm asking here for help. Why do we only look at open loop transfer function of the system when we are supposed to know the stability of the closed loop system? I don't even understand the use of this method at all, why don't we look at the poles of the closed loop system which are at 1 / K * Gd in the example below. I don't want you to solve me any problems but please give me the intuition behind this concept.
Thank you so much!
 

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alexmath said:
Why do we only look at open loop transfer function of the system when we are supposed to know the stability of the closed loop system?

The short answer is: You don't.

Say you have a feed forward transfer function, A(s), in the loop, and a feed backword transfer function, B(s). Mason's rule then says, that the transfer function for the closed loop will be:

H(s) = A(s) / ( 1 + A(s) * B(s) ).

In your attached file, A(s) = k / ( s3 - 3s2 - 10s ), B(s) = 1.

Using Mason's rule you will find:

H(s) = k / ( s3 - 3s2 - 10s + k ).

The characteristic equation for the closed loop is: s3 - 3s2 - 10s + k = 0.

By solving this equation with different values of k, you can plot the root locus.

( It seems to be a highly unstable loop. )
 
Last edited:
alexmath said:
Why do we only look at open loop transfer function of the system when we are supposed to know the stability of the closed loop system?
Are you sure you're not thinking of the Nyquist Plot?
 
Yes - Hesch is right. The denominator roots of the closed-loop transfer function are identical to the poles of this function.
And the root locus, therefore, gives you the locus of the poles with the gain as parameter.
Because of stability criteria, these poles (the roots of the denominator) must not move to the right half of the s-plane.
Hence, you can see for which gain values the poles remain within the left half - indicating stability.
 
LvW said:
Hence, you can see for which gain values the poles remain within the left half - indicating stability.

Two of the poles will never be in the left half if gain is positive.

One of the poles will never be in the left half if gain is negative.
 

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