Closed loop system is one in which the output signal has direct effect upon the control action i.e. the closed loop control system are feedback system.
Definition of closed loop control system
A system in which the control action is dependent on the output is called the closed loop system.
Fig. shows the general block diagram of the closed loop control system.
r(t) = Reference input,
e(t) = Error or actuating signal,
b(t) = Feedback system.
Error detector:
Error detector compares the reference input r(t) and feedback signal b(t), results in error signal e(t).
Note:
When feedback signal is positive, i.e. system is called positive feedback system and e(t) = r(t) + b(t).
When the feedback signal is negative, system is called negative feedback system and e(t) = r(t) – b(t).
• Control elements:
The error signal r(t) is applied to control element which is used to activate the control element. Therefore, error signal is also called actuating signal.
It is the transducer that produces feedback signal b(t) proportional to the controlled output c(t).
The action of the controller will be to drive the controlled element in such a manner so that error is reduced to zero i.e. the feedback signal is equal to reference input r(t).
It is the body or process or machine whose condition is controlled.
Some important terms in closed loop control system are –
It is an external signal applied to a closed loop system in order to obtain a desired output.
It is the signal proportional to the controlled output c(t).
It is difference or sum of the r(t) and b(t).
It produces a controlled output based on the error signal.
It is the transmission path from the actuating signal to the controlled output.