Open loop and closed loop control system.

Control system: 
It is an arrangement of components interconnected in such a way as to regulate, direct itself to obtain a required objective.  
Open loop control system:-
An open loop control system is one in which the control action does not depend on the desired output. In the open loop closed loop system, when the input is applied to the control unit which generates the necessary action to control the driver or process which is to be controlled to get the required output. this system is very sensitive to the environment, if there are any changes in the external environment in between the operation, then the desire cannot be achieved.
example: traffic control system, automatic washing machine, electric switch etc.
Advantages

1. highly accurate 
2. convenient to use when the output is difficult to measure
3. comparatively more stable.
4. economical
5. Bandwidth range is large
6. low cost
7. simple in construction

Disadvantages

1. Frequent calibration is required.
2. Required skilled operator
3. Internal disturbance.

Closed loop control system:-

A closed loop control system is one in which the control action depends on the output of the system.in this system, the desired output can be measured and compared with the input of the system using the feedback element. This system is consist of a summing point, take off point, feedback, controller and process. In this system, the output is compared with the input at the summing point and this determines the error based on which control action generated. Difference between the input and output is called as an error. The error signal is used to reduce the error and approaches to the desired output.

Examples: automobile speed control, automatic tank level control system etc.

Advantages 

1. More accurate due to feedback.
2. Less affected by noise or disturbance
3. The error detector is present
4. optimisation is possible
5.  more flexible
6. No skilled operator is required 

Disadvantages

1. More complex and expensive
2. the system is complex.
3. less stable
4. Required high power system.