Disadvantage of Open-loop System
a. Inaccurate since there is no correction for ever.
b. Relatively slow in response to changes in demand, (because it has to be manual)
c. Time to time recalibration of the controller is necessary to the maintain the qualityand accuracy of the output.
d. The control depends on the human judgement which may be inaccurate and may not beefficient.
e. Lack optimization in control.

Closed-loop System
A system in which the input is dependent on the output, i.e. variation in the output influences the input by some means of controlling on the input is called a closed loop system.
A closed loop system is characterised by the presence of a feedback system. Feedback system permits the output to be compared with the reference input so that appropriate controlling action can be achieved. The main feature of a closed loop control system is the comparison of the actual value of the variable with the desired value of that variable and any difference between these two values (called error) causes action to be taken in such a manner as to eliminate this difference or make the error zero.
Elements of closed loop control system
The basic elements of a closed loop control systems are
a. Comparison element
b. Control unit
c. Correction unit
d. Process unit.
e. Feed back unit.

Figure 1.8 shows the basic elements of a closed loop system.
Fig. 1.8 : Basic elements of a closed loop control system
A segmented circle is used as a symbol in general, for an element at which signals are summed up.
Functions of the elements of a closed loop system
Comparison element: This unit compares the reference or required value with that of the measured value or feedback signal (signal which is output of feedback element) and produces an error signal.
error signal = Reference signal (Reference value) ± Measured value.
Control unit: Control unit analyses the error signal and decides what action is to be taken. The unit poduces a signal; may be to operate a switch or a valve or perhaps a signal to proportionally open or close a valve which depends on the size of the error. The manipulation of the control unit is such that the error will approach zero.
Correction unit: The modified signal from the control unit will be received by the correction unit which produces a change in the process to correct or change the controlled condition. Change in process may be, allowing more power to the room heater to increase the temperature or vice versa, varying the valve opening for allowing the variation of liquid flow to enter the process.
Process Unit: Process unit is the one which is being controlled. It could be the speed a motor, speed of a vehicle, central heating unit or a central air conditioner.
Examples
One of the best example of a closed loop system is human being. A. person wants to reach for an object. Position of the object is given as reference feed back signals and the eyes compares the actual position of the hands with reference to the position of the object. Error signal is given to the brain. Brain manipulates this error and gives signals to the hands. This process continues till the hand reaches the object. Fig. 1.9.