# Homework Help: Open-loop & Close-loop system

1. Jul 23, 2012

### dominicfhk

1. The problem statement, all variables and given/known data
Hi, guys. I am watching this guy on youtube explaining block diagram of close and open loop system. While I understand how a close-loop transfer function is equals to C(s)/R(s) = G(s)/{1+G(s)H(s)}, I don't quite understand how the transfer function of an open-loop system equals B(s)/E(s) = G(s)H(s).
http://img35.imageshack.us/img35/5268/captureisk.jpg [Broken]
Shot at 2012-07-23

2. Relevant equations
(none)

3. The attempt at a solution
I thought a transfer function is defined as the laplace transformation of a system output over the laplace transformation of a system input, which is C(s)/R(s). So why not the open loop transfer function equals C(s)/R(s) = G(s)? The feedback loop is disconnected, why we still need to take that into account? Or is he redefining the feedback as the output of the system, instead of C(s)? It so, it is a standard practice even outside his class? Thank you so much!

Edit:

Last edited by a moderator: May 6, 2017
2. Jul 24, 2012

### rude man

It's a matter of semantics, or nomenclature.You open the loop at the summing junction and look at the gain from the input R the output of H.

You have a good point, but that's the jargon and you have to go with the flow!

In studying the stability of the closed loop the open-loop transfer function is the prime tool for doing that.

3. Jul 25, 2012

### ENgez

the 'open loop' transfer function is what you would get if you were to disconnect the feedback wire, where it is fed back to the input, and call the endpoint your 'output'. (i hope that's not too convoluted ..).

it is a very useful tool for analyzing the effect of the feedback on the positions of the closed loop poles, which control the stability and dynamic behavior of the system.