Control System: Find the Transfer Function From Block Diagram

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Homework Statement


upload_2017-12-7_17-15-7.png


Homework Equations


To find Ca / Rb we take Ra = 0 --- is this right?
I used mason's formula
P1 = -G2 G1 H1
Loop = G1, G2, -H1, -H2
Using Mason's Gain we get:
Delta1 = 1
Delta = 1 - G1G2H1H2
Using Mason's Gain
TF = -G1G2H1 Divided by ( 1 - G1G2H1H2)
But substituting the values I don't get the answer that matches any options.


The Attempt at a Solution


Substituting I get:
upload_2017-12-7_17-26-30.png

This gives answer in cube as second term of numerator will go in cube for s. None of options have s cube.


Second Question:
upload_2017-12-7_17-24-17.png

I think the options are wrong.
For left TF we get C(s)/R(s) = 10/11
For Right we get C(s)/R(s) = 5/3.5
Multiplying them doesn't give the answer
 

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Answers and Replies

  • #2
scottdave
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It's been awhile since I've done these, but I got the same result as you, for the second one: C/R = 100/77. I haven't worked through the first one, yet.
 
  • #3
donpacino
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To find Ca / Rb we take Ra = 0 --- is this right?
yes
I got the same thing as you for the second problem.

In problem number one you can dive both the top and bottom side by s...
 
  • #4
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Not sure if that helps.
I'm getting: Ca/Rb = -G2 H1 G1 divided by (1 + ( -G2 H1 G1 H2 ) )
And that's not reducing to any of the options.
 
  • #5
scottdave
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Not sure if that helps.
I'm getting: Ca/Rb = -G2 H1 G1 divided by (1 + ( -G2 H1 G1 H2 ) )
And that's not reducing to any of the options.
Sounds like the problem might be "hosed":eek:
 
  • #6
613
47
Could be. Even I think the question has some typo.
 

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