# Control - Block Diagram Question

Discussion in 'Homework Help' started by Fraser_Integration, Apr 4, 2011.

1. ### Fraser_Integration Thread Starter Member

Nov 28, 2009
142
6
Hello. Got this block diagram question the other day. Having a bit of trouble with the last part, and not sure of my answer to part b) either. I attach the question.

a) E = R - Y
but Y = GKR / (1 + GKH)

then E = R - GKR/(1+GKH)

For E = 0,

R = GKR/1+GKH

Rearrange for H = 1 - 1/KG

b) I just worked backwards to achieve:

Y = [ (0 - (1-1/KG)) + Td*P ] *G
= 1 - KG + Td*P*G

c) Not really sure about this one. Not come across Y(s) = 0 transfer functions before. Any general information would be appreciated.

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2. ### Georacer Moderator

Nov 25, 2009
5,151
1,266
I think you have done a mistake on b. You can disregard R and think of Td as the input of your feedback system.

I 'll rewrite it for you:
$Y=\frac G {HK} \cdot P \cdot T_d\\
Y=\frac G {\frac {GK-1}{GK} K} \cdot PT_d\\
Y=\frac {G^2} {GK-1} PT_d$

Now the question is if it is possible for this to happen:
$\frac {G^2} {GK-1}=0$
I think you can answer that.

Last edited: Apr 4, 2011

Nov 28, 2009
142
6
Hi.