Is the Routh Stability Test Result Correct for This System?

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Discussion Overview

The discussion revolves around the application of the Routh Stability Test to determine the stability of a given polynomial system. Participants are analyzing the coefficients derived from the polynomial and their implications for stability, with a focus on the conditions for the variable k that affect the system's stability.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant proposes that for the polynomial s^4+2ks^3+2s^2+(1+k)s+2=0, certain conditions must be met for stability, suggesting k must be greater than 2.15.
  • Another participant challenges the initial calculations, stating that the correct condition for stability is k > 1.
  • A different participant presents a range for k, stating that for the line s^1, k must be between -1/3 and 1, while for s^2, k must be greater than 1/3, leading to the conclusion that 1/3 < k < 1.
  • One participant questions the derivation of the coefficient for s^1 and expresses confusion over the stability results, indicating that their own calculations led to different conclusions.
  • Another participant corrects their earlier calculations, stating that the polynomial is unstable for all values of k due to the negative leading coefficient of the derived quadratic equation.
  • One participant expresses confusion over the varying results they obtain when solving the problem, seeking clarification on the coefficient of s^1.
  • A participant suggests a different approach to the stability condition, leading to a conclusion that the system is unstable for all values of k.
  • One participant shares a personal anecdote about their professor's dismissal of the Routh-Hurwitz criterion, indicating a lack of confidence in the method.

Areas of Agreement / Disagreement

Participants express differing views on the conditions for k that ensure stability, with no consensus reached on the correct range or conditions. Multiple competing interpretations of the stability criteria remain unresolved.

Contextual Notes

Participants reference various coefficients derived from the polynomial, but there are indications of errors in calculations and assumptions that may affect the conclusions drawn. The discussion reflects uncertainty regarding the application of the Routh Stability Test.

Who May Find This Useful

This discussion may be of interest to students and professionals in control systems, stability analysis, and those studying the Routh Stability Test in the context of polynomial systems.

angel23
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to find values of k for which the system is stable.

s^4+2ks^3+2s^2+(1+k)s+2=0

first (1+k)must be >0 and 2K must be >0 then i construct routh array
to get 3k-1/2k as a coefficient of s^2 and
(3k-1)/2k *(1+k) - 4k as a coefficient of s .
then k must be>1/3 and K>2.15 and K>-0.154 then k must be >2.15 ?
is this right or there is something wrong??
 
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the idea is right but your numbers are wrong . it will be k > 1 .
 
For the line s^1 I got
-\frac{1}{3} &lt; k &lt; 1
And for the line s^2
\frac{1}{3} &lt; k
So, you should have
\frac{1}{3} &lt; k &lt; 1
 
how did you get that for s^1 k<1
the coeff is (3k+1)(k-1)(2k) so K must be >1 not <1 !
i just want to know how did you get this as i substituted by a value greater than 1 and found system to be unstable.is there anything wrong with my rules or numbers?
 
I did the calculations wrong. The coeff for s is (3k-1)(1+k) - 8k^2 = -5k^2 + 2k -1, whose roots are complex. Since the higher power of k has a negative coeff, the parabola has the concavity down. For all values of k the polynomial has negative value.
Your system is unstable for every k.
 
i am really too confused everytime i solve this problem i get different solution ! i solved now and got the same result as yours.


but please tell me if the coeff of s^1 is(3k+1)(k-1) then how to get the range?
 
i think you reached this for S^1 :

[ (3k-1)(1+k)-8k^2 ] / 3k-1 > 0

you can't multiply by 3k-1 , so :

(1+k) - (8k^2 / 3k-1) > 0

simplify to get : 5k^2 - 2K +1 < 0

and since your first condition from S^3 is K>0 , then :

system is unstable for all values of k ..
 
ha, my control systems prof basically said routh hurwitz was BS and skipped it. Can't help you here.
 

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