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.
closed loop control system block diagram
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.
• Feedback path element:
It is the transducer that produces feedback signal b(t) proportional to the controlled output c(t).
• Controller:
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).
• Plant:
It is the body or process or machine whose condition is controlled.
Some important terms in closed loop control system are –
(1) Reference input r(t):
It is an external signal applied to a closed loop system in order to obtain a desired output.
(2) Feedback signal b(t):
It is the signal proportional to the controlled output c(t).
(3) Error signal e(t):
It is difference or sum of the r(t) and b(t).
(4) Controller:
It produces a controlled output based on the error signal.
(5) Forward path:
It is the transmission path from the actuating signal to the controlled output.
Example of a closed loop control system
1) Human Beings
Human being is the best example of closed loop control system. Consider that the objective (human being) has to reach for an object (For example, book) on table.
In this case, the brain performs the work of controller and eyes serves as the measuring element which gives continuous feedback regarding the position of hand which is the output.
Here, reference input is objective (book).
The distance between hand and object is the error which brought to zero by the controller i.e. brain by giving instructions to hand to move closer to object.
When error i.e. distance between book and hand is zero, he reach to his destination i.e. he get desired output.
2. Automatic Electric Irons
An automatic iron regulates the temperature of iron such that for a given quality of cloth selection, the temperature remains in a specified range.
Suppose temperature is set at 40°C. When temperature falls below 40°C, heating starts and when it exceeds than 40°C, the heating stops. Hence, it is closed loop system with temperature as control action depends on output temperature. It is shown in Fig.
Automatic Electric Irons
3. Driving an Automobile
A simple block diagram of an automobile steering mechanism is shown in Fig.
Driving an Automobile as Closed-Loop System
The driver senses visually and by body movement the error between the actual and desired directions of the automobile. Additional information is available to driver from sensing of the steering wheel through his hands, these informations forms the feedback signal, which are interpreted by driver's brain, who then signals his hands to adjust the steering wheel accordingly. Hence, the close loop.
Advantages of closed loop control system:
1. High accuracy.
2. High bandwidth.
3. Facilitates automation.
4. Reduced effects of non-linearities.
5. It senses changes in outputs.
Disadvantages closed loop control system:
1. Complex design.
2. Costly maintenance.
3. Unstable system.
