whereNandDare factored polynomials and signify numerator and denominator terms,
respectively.We observe the following:Typically,we knowthe factors of the numerators
and denominators of G(s) and H(s). Also, the zeros of T(s) consist of the zeros of G(s)
and the poles of H(s). The poles of T(s) are not immediately known and in fact can
change with K. For example, if GðsÞ ¼ ðs þ 1Þ=½sðs þ 2Þ and HðsÞ ¼ ðs þ 3Þ=ðs þ 4Þ,
the poles of KG(s)H(s) are 0;2; and 4. The zeros of KG(s)H(s) are 1 and 3.
Now, TðsÞ ¼ Kðs þ 1Þðs þ 4Þ=½s3 þ ð6 þ KÞs2þ ð8 þ 4KÞs þ 3K. Thus, the zeros of
T(s) consist of the zeros of G(s) and the poles of H(s). The poles of T(s) are not
immediately known without factoring the denominator, and they are a function of K.
Since the system’s transient response and stability are dependent upon the poles ofT(s),
we have no knowledge of the system’s performance unless we factor the denominator
for specific values ofK. The root locus will be used to give us a vivid picture of the poles
of T(s) as K varies.