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Dustinsfl
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Homework Statement
Find the stability of ##s^3 + 3s^2 + 2(1 + K_p)s + 2K_i = 0##.
Homework Equations
Routh-Hurwitz criterion
The Attempt at a Solution
By the Routh-Hurwitz stability criterion, we have
$$
\begin{array}{ccc}
s^3 & 1 & 2(1 + K_p)\\
s^2 & 3 & 2K_i\\
s^1 & \frac{6(1 + K_p) - 2K_i}{3} & 0\\
s^0 & 2K_i & 0
\end{array}
$$
From the ##s^3## line, we have that ##2(1 + K_p) > 0##; therefore, ##K_p > -1##. From the ##s^2## and ##s^0## line, we have that ##2K_i > 0##; thus, ##K_i > 0##. From the ##s^1## line, we have that ##6K_p - 2K_i + 6 > 0##; therefore,
$$
\frac{K_i}{K_p} < \frac{3}{K_p} + 3.
$$
I am supposed to conclude
$$
0 < \frac{K_i}{K_p} < 13.5.
$$
From line ##s^2##, we get greater than zero, but what do I do to go from ##\frac{3}{K_p} + 3## to ##13.5##?
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