Classification Of Control Systems & Difference Between Open Loop And Closed Loop Control System - SKengineers

 

CLASSIFICATION OF CONTROL SYSTEM

A.     Open loop control system (NO feedback control system)

B.     Closed loop control system (with feedback control system)

1.      OPEN LOOP CONTROL SYSTEM

·        The system which doesn’t have any feedback loops connected to it is called “open loop control system”.

·        These type of systems doesn’t depend upon its output i.e. in open loop feedback system, output is not used as the control variable for the system.

·        Open loop systems are one way signal flow systems.

·        As these systems doesn’t contain a feedback loops, these are also known as Non feedback systems.

·        As there is no feedback loops, no data is transferred back to the open loop systems.

·        The output of the open loop system can be determined only by its present state input that means the input of open loop system will always return to zero, before the output returns to zero.

·        There is no chance to correct the transition errors in open loop systems so there is more chance to occur errors.

EXAMPLES OF OPEN LOOP CONTROL SYSTEM

a)      Automatic washing machine

b)     Electric bulb

c)      Electric hand drier

d)     Time based bread toaster

e)     Volume of the audio system

f)       Water faucet

g)      TV remote control

h)     Clothes drier

i)       Shades or blinds on a window

j)       Stepper motor or servo motor

k)      Inkjet printers

l)       Door lock system

ADVANTAGES OF OPEN LOOP CONTROL SYSTEM :

a)      The open loop systems are very simple in design.

b)     These are cheaper in cost than other type of control system.

c)      Maintenance of open loop control system is very simple.

d)     These are stable for some extent of time.

e)     These types of systems are convenient to use.

DISADVANTAGES OF OPEN LOOP CONTROL SYSTEM :

a)      The bandwidth of open loop control system is less.

b)     The non-feedback system doesn’t facilitate the automation of process.

c)      Open loop systems are inaccurate in nature.

d)     As their output is affected by some of the external system disturbances, the non-feedback systems are unreliable.

e)     It cannot correct the output deviations automatically.

APPLICATIONS OF OPEN LOOP CONTROL SYSTEM :

1)     Automatic washing machine :

·        This we use in our home to wash and dry clothes.

·        Internally, the machine runs according to the reset time and doesn’t depend upon the washing process, weather it is completed or not.

2)     Electric bulb :

·        We all know that the electric bulb needs some voltage or current to produce light.

·        When the main supply is available, by switching on the switch, we can make the bulb
to work.

·        And this process does not depend up on the temperature of bulb or any other surrounded parameters.

3)     Electric hand drier :

·        The hand drier that works on electric power supply and when we keep our hands in front of it, it will automatically dry our hands by blowing hot air, irrespective of how much our hand is dried.

4)     Bread toaster :

·        The toaster machine runs as per our adjusted time period.

·        And it toasts the bread place irrespective of the toasting completed or not.

5)     Volume of the audio system :

·        We can adjust the volume of an audio system manually, irrespective of the range of output volume.

2.      CLOSED LOOP CONTROL SYSTEM

·        A system in which input is dependent on the output i.e. variation of the output influences the input by some means of controlling on the input is called a closed loop control system.

·        A closed loop control system is characterized by the presence of feedback system.

·        The feedback system permits the output to be compared with the reference input so that appropriate controlling action can be achieved.

·        The main feature of the closed loop control system is the comparison of the actual value of the variable with the desired value of the variable and any difference between these two values causes action to be taken in such a manner that it eliminates this difference or make the error zero.

1)     COMPARISON ELEMENT :

·        This element compares the required or reference value of the variable condition being controlled with the measured value of what is being achieved and produces an error signal.

·        The error signal is the difference between the reference and measured values.

2)     CONTROL ELEMENT :

·        This element analyses the error signal and decides what action to be taken, may be to operate a switch or valve.

·        The manipulation of the control unit is such that the error will approach zero.

3)     CORRECTION ELEMENT :

·        It produces a change in the process to correct to change the controlled condition. Change in the process may be, allowing more power to the room heater to increase the temperature or vice versa.

4)     PROCESS UNIT :

·        It is the one which is being controlled.

·        It could be a room in a house with its temperature being controlled or a tank of water with its level being controlled.

5)     MEASUREMENT UNIT :

·        This produces a signal related to the variable condition of the process that is being controlled.

·        It might be, for example, a switch which switched on when a particular position is reached or a thermocouple which gives the emf related to the temperature.

PRACTICAL EXAMPLES OF CLOSED LOOP CONTROL SYSTEM

1)     Automatic electric iron :

Heating elements are controlled by output temperature of the iron.

2)     Servo voltage stabilizer :

Voltage controller operates depending upon output voltage of the system.

3)     Water level controller :

Input water is controlled by water level of the reservoir.

4)     Missile launched & auto tracked by radar :

The direction of missile is controlled by comparing the target and position of the missile.

5)     An air conditioner :

An air condition functions depending upon the temperature of the room.

6)     Cooling system in car :

It operates depending upon the temperature which it controls.

ADVANTAGES OF CLOSED LOOP CONTROL SYSTEM

a)      Closed loop control system are more accurate even in the presence of non-linearity.

b)     Highly accurate as any error arising is corrected due to presence of feedback signal.

c)      Bandwidth range is large.

d)     Facilitates automation.

e)     The sensitivity of system may be made small to make system more stable.

f)       This system is less affected by noise.

DISADVANTAGES OF CLOSED LOOP CONTROL SYSTEM

a)      They are costlier.

b)     They are complicated to design.

c)      Required more maintenance.

d)     Feedback leads to oscillatory response.

e)     Overall gain is reduced due to presence of feedback.

f)       Stability is the major problem and more care is needed to design a stable closed loop system.

 

DIFFERENCE BETWEEN OPEN LOOP AND CLOSED LOOP CONTROL SYSTEM

Sr. No.	OPEN LOOP CONTROL SYSTEM	CLOSED LOOP CONTROL SYSTEM
1.	The feedback element is absent.	The feedback element is always present.
2.	An error detector is not present.	An error detector is always present.
3.	It is stable one.	It may become unstable.
4.	Easy to construct.	Complicated construction.
5.	It is an economical.	It is costly.
6.	Having small bandwidth .	Having large bandwidth.
7.	It is inaccurate.	It is accurate.
8.	Less maintenance.	More maintenance.
9.	It is unreliable.	It is reliable.
10.	Examples : hand drier, tea maker.	Examples : servo voltage stabilizer, perspiration.