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.

Speed Control of vehicle
The object is to maintain a constant speed of the vehicle immaterial of the gradient and the load. The driver observes the speedometer, and based on the speed shown by the speedometer he decides whether the fuel supply should be increased or decreased or change of gear is to made.
Here the speed shown on the speedometer is feedback. Feedback signals from the eye compares the desired speed in the memory of the driver. Error signals are given to the brain. Brain manipulates the error and gives signals to hand and legs. The hand changes the gear and in the process applies the clutch from the leg and increases the fuel supply if the speed is less than the desired speed, otherwise decreases the fuel supply. Changing of gear and increase or decrease of fuel supply, depends on whether it is an upward or downward gradient respectively. Fig. 1.10 shows the concept of speed control.
Water level control of overhead tanks
Fig. 1.11 shows the schematic diagram of a water level control. The level of the water is sensed by the float. The float has an electrical contactor which is positioned between fixed contactors. The inflow regulation valve is electrically operated. The electrical circuit of the system is closed when the float contactor touches fixed contactors and open when it is not making contact with the fixed contactor. When the level of water in the tank falls the float moves down and make contact with the fixed contactor and the circuit is closed and the pump is switched on. When the level of water rises the float moves up and breaks the contact with the fixed contactor and the circuit is opened and the pump is switched off, thereby the required level of water is maintained