The general block diagram of a closed loop control system which is shown in Fig. 

Reference input is desired input reference value given by the user.
An error detector compares a signal obtained through feedback elements, which is a function of the output response, with the reference input.
Any Difference between these two signals constitutes an error or actuating signal, which actuates the control elements.
The control elements in turn alter the conditions in the plant (controlled
member) in such a manner as to reduce the original error.
This controlled signal is given to plant which gives us the controlled output.
output is measured and converted to electrical signal by Feedback path elements then given to the controller.
control elements and plant are forward path elements.
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