Processing math: 100%

Gain Margin

Ask
The gain margin of system under loop unity negative feedback G(s)H(s) = {100 \over s(s+10)^2}
(a)0 dB
(b)20 dB
(c)26 dB
(d)46 dB

Answer is ( c ) 26 dB
Concept
Gain Margin under closed loop unity negative feedback is =
G(s)H(s) = {100 \over s(s+10)^2} ... Eq(1)
Phase \phi = -90° - 2arctan(ω/10)
Phase cross - over frequency = -180°
-180° = -90° - 2arctan(ω/10)
90° = 2arctan(ω/10)
tan(45°) = ω/10 \therefore ω = 10
In Eq(1) , put s = jω
\therefore G(jω)H(jω) = 100/jω(jω+2)^{2}
|G(jω)H(jω)| = {100\over ω(ω2 + 100)}...Eq(2)
Substituting for ω = 10,Eq (2) becomes, {100\over 10(100+100)} = 1/20
Gain Margin = 1 \over |G(jω)H(jω)| = 1/(1/20) = 20
In dB, G.M (in dB) = 20 log1020 = 26dB

No comments:

Post a Comment