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Control system miscellaneous

  1. If x = Re G(jθ), and y = lm G(jω) then for ω → 0+, the Nyquist plot for
    G(s) =
    1
    s(s + 1)(s + 2)

    becomes asymptotic to the line
    1. x = 0
    2. x = -
      3
      4
    3. x = y – 1 / 6
    4. x =
      y
      3
Correct Option: B

Given , G(s) =
1
s(s + 1)(s + 2)

G(jω) =
1
=
-j(1 - jω)(2 - jω)
jω(1 - jω)(2 + jω)ω(1 - ω²)(4 + ω²)

=
j(2 - 3jω - ω2)
=
-3ω - j(2 - ω2)
ω(1 + ω²)(4 + ω²)ω(1 + ω²)(4 + ω²)

=
- 3
- j
(2 - ω2)
(1 + ω²)(4 + ω²)ω(1 + ω²)(4 + ω²)

⇒ x =
3
(1 + ω²)(4 + ω²)

y = -
(2 - ω2)
ω(1 + ω²)(4 + ω²)

At ω → 0 , x →
3
, y → - ∞
4



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