Closed Loop System

We have seen that an electronic control system with one or more feedback paths is called a Closed-loop System. Closed-loop control systems are also called “feedback control systems” are very common in process control and electronic control systems. Feedback systems have part of their output signal “fed back” to the input for comparison with the desired set point condition. The type of feedback signal can result either in positive feedback or negative feedback.

In a closed-loop system, a controller is used to compare the output of a system with the required condition and convert the error into a control action designed to reduce the error and bring the output of the system back to the desired response. Then closed-loop control systems use feedback to determine the actual input to the system and can have more than one feedback loop.

Closed-loop control systems have many advantages over open-loop systems. One advantage is the fact that the use of feedback makes the system response relatively insensitive to external disturbances and internal variations in system parameters such as temperature. It is thus possible to use relatively inaccurate and inexpensive components to obtain the accurate control of a given process or plant.
However, system stability can be a major problem especially in badly designed closed-loop systems as they may try to over-correct any errors which could cause the system to loss control and oscillate.

Open Loop System

Process Control
It is control system in which a control action is applied on the output of the system. Such system are simply command by the input, an open loop control system is shown in the below figure,
The process is controlled by providing the input to the control of the open loop system to keep the out put at a desired level. The set point of the open loop system is also called “Command”.

It is necessary to achieve the ideal operating point for the process and observe the result at the out put manually, since the only one input is applied to the control in form of set point, so it is clear that an open loop control system control the process blindly and the controller receive no information in the form of feed back to control the output status of the process.
The open loop control system are cheaper and less complex then the closed loop control system, while the controlling efficiency  of this system is nearly zero.

Open and closed loop system summary
The open loop system has input and corresponding output.
For example pump fitted with on/off switch - once you switch on pump will start, water tank will fill. if you do not switch off at this time, it will keep on running/ overflow of water takes place.

But with closed loop system, output is measured, compared with desired output, difference is fedback to the input so that output is maintained at desired level continuously.
Example- same as above but fitted with sensors in water tank, and relay to actuate on/off the pump/motor. Now if tank is filled with water by running pump, top sensor senses and give signal through relay to stop the pump. When after consumption, water level in tank decreases, lower sensor senses now actuate the relay to start the pump. So tank is neither empty nor overflow. it maintaines the water at desire level.