# Control System: Find the Transfer Function From Block Diagram

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

Second Question: 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

#### Attachments

scottdave
Homework Helper
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.

donpacino
Gold Member
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...

• jaus tail
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.

scottdave
Sounds like the problem might be "hosed" • 