Determining the range of a controller for a stable system

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Discussion Overview

The discussion revolves around determining the range of the proportional gain Kp for a controller in a closed-loop system to ensure stability. It involves analyzing the characteristic equation derived from the system's transfer function.

Discussion Character

  • Homework-related
  • Mathematical reasoning

Main Points Raised

  • One participant states the characteristic equation is (1.5s^2 + 2.5s - 1 + Kp) and recalls that for stability, all roots must have negative real parts.
  • Another participant suggests solving for the roots of the denominator using a formula, noting it is a second-order polynomial.
  • A participant expresses confusion about incorporating Kp into the quadratic formula, prompting further clarification.
  • Further replies indicate that Kp can be treated as a constant within the quadratic formula, and participants are encouraged to analyze how varying Kp affects the roots.
  • One participant acknowledges understanding the approach after the discussion.

Areas of Agreement / Disagreement

The discussion shows some participants agreeing on the method of using the quadratic formula while others express confusion about the inclusion of Kp. The overall approach remains unresolved as participants are still working through the implications of Kp on stability.

Contextual Notes

Participants have not fully resolved how to systematically determine the range of Kp that ensures stability, and there may be missing assumptions regarding the nature of the roots and their dependence on Kp.

MattH150197
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Homework Statement


The controller K(s) = Kp, determine the range of Kp over which the closed loop system is stable.

Homework Equations


I found the transfer function of system = Y(s)/V(s)=Kp/(1.5s^2 + 2.5s - 1 + Kp)

The Attempt at a Solution


So i know (1.5s^2 + 2.5s - 1 + Kp) is the characteristic equation is been a while since I've done these i think i remember that all roots of the polynomial for a stable system must be negative real parts and i know its probably obvious but i just can't remember how to find the range of Kp. Thanks!
 
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Solve for the roots of the denominator. It's only second order, so you can use a formula.
 
yeah it's just the Kp being part of the denominator that's throwing me a bit, how do I include that in the quadratic formula
 
MattH150197 said:
yeah it's just the Kp being part of the denominator that's throwing me a bit, how do I include that in the quadratic formula
Um, by including it in the quadratic formula? Treat Kp as a constant and write out the formula. Then see how the value of Kp affects the roots. What range of values for Kp will satisfy your requirements? Be discriminating :smile:
 
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MattH150197 said:
... how do I include that in the quadratic formula

As far as I can see IT IS included in the formula (see your own post).
 
Yeah I got it now, cheers guys.
 

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