Control System | Closed Loop Open
Loop Control System
When
a number of elements are combined together to form a system to produce desired
output then the system is referred to as control system. As
this system controls the output, it is so referred. Each element connected to
the system has its own effect on the output.
A. Definition of Control System
A
control system is a system of devices or set of devices, that manages,
commands, directs or regulates the behavior of other devices or systems to
achieve desired results. In other words, the definition of a control system
can be simplified as a system, which controls other systems. As the human
civilization is being modernized day by day the demand for automation is
increasing accordingly. Automation highly requires control of devices.
In
recent years, control systems have played a central role in the development and
advancement of modern technology and civilization. Practically every aspect of
our day-to-day life is affected less or more by some control system. A bathroom
toilet tank, a refrigerator, an air conditioner, a geezer, an automatic iron,
an automobile all are control system. These systems are also used in industrial
processes for more output. We find control systems in the quality control of
products, weapons system, transportation systems, power system, space
technology, robotics and many more. The principles of control theory
are applicable to both engineering and nonengineering field. You can learn more
about control systems by studying our control system MCQs.
B. Features of a Control System
The main feature
of a control system is that there should be a clear mathematical
relationship between input and output of the system. When the relation between
input and output of the system can be represented by a linear proportionality,
the system is called a linear control system. Again when the relationship
between input and output cannot be represented by single linear
proportionality, rather the input and output are related by some non-linear
relation, the system is referred to as a non-linear control system.
C. Requirements of a Good Control
System
Accuracy: Accuracy is the measurement tolerance of the instrument and
defines the limits of the errors made when the instrument is used in normal
operating conditions. Accuracy can be improved by using feedback elements. To
increase the accuracy of any control system error detector should be present in the control system.
Sensitivity: The parameters of a control system are always changing with
the change in surrounding conditions, internal disturbance or any other
parameters. This change can be expressed in terms of sensitivity. Any control
system should be insensitive to such parameters but sensitive to input signals
only.
Noise: An undesired input signal is known as noise. A good control
system should be able to reduce the noise effect for better performance.
Stability: It is an important characteristic of the control system.
For the bounded input signal, the output must be bounded and if the input is
zero then output must be zero then such a control system is said to be a stable
system.
Bandwidth: An operating
frequency range decides the bandwidth of the control system. Bandwidth should
be as large as possible for the frequency response of good control system.
Speed: It is the time taken by
the control system to achieve its stable output. A good control system
possesses high speed. The transient period for such system is very small.
Oscillation: A small numbers of
oscillation or constant oscillation of output tend to indicate the system to be
stable.
D. Types of Control Systems
There
are various types of control systems, but all of them are
created to control outputs. The system used for controlling the position,
velocity, acceleration, temperature, pressure, voltage and current etc. are
examples of control systems.Let
us take an example of the simple temperature controller of the room, to clear
the concept. Suppose there is a simple heating element, which is heated up as
long as the electric power supply is switched on. As long as the power supply
switch of the heater is on the temperature of the room rises and after
achieving the desired temperature of the room, the power supply is switched
off. Again due to ambient temperature, the room temperature falls and then
manually the heater element is switched on to achieve the desired room
temperature again. In this way, one can manually control the room temperature
at the desired level. This is an example of manual control system.
This
system can further be improved by using timer switching arrangement of the
power supply where the supply to the heating element is switched on and off in
a predetermined interval to achieve desired temperature level of the room.
There is another improved way of controlling the temperature of the room. Here
one sensor measures the difference between actual
temperature and desired temperature. If there is any difference between them,
the heating element functions to reduce the difference and when the difference
becomes lower than a predetermined level, the heating elements stop
functioning.
Both
forms of the system are automatic control system. In former one, the
input of the system is entirely independent of the output of the system. The
temperature of the room (output) increases as long as the power supply switch
is kept on. That means heating element produces heat as long as the power
supply is kept on and final room temperature does not have any control over the
input power supply of the system. This system is referred as open loop
control system.
But in the latter case, the heating elements of the system function, depending upon the difference between, actual temperature and desired temperature. This difference is called the error of the system. This error signal is fed back to the system to control the input. As the input to the output path and the error feedback path create a closed loop, this type of control system is referred to as a closed loop control system.
Hence, there are two main types of control system. They are as follow
1.
Open loop control system
2.
Closed loop control system
1. Open Loop Control System
A control system in which the
control action is totally independent of output of the system then it is called
open loop control system. A manual control system is also an open loop
control system. The figure below shows a control system block diagram of an open loop
control system in which process output is totally independent of the controller
action.
a. Practical Examples of Open Loop
Control System
1.
Electric Hand Drier – Hot
air (output) comes out as long as you keep your hand under the machine,
irrespective of how much your hand is dried.
2.
Automatic Washing Machine –
This machine runs according to the pre-set time irrespective of washing is
completed or not.
3.
Bread Toaster – This
machine runs as per adjusted time irrespective of toasting is completed or not.
4.
Automatic Tea/Coffee Maker –
These machines also function for pre adjusted time only.
5.
Timer Based Clothes Drier –
This machine dries wet clothes for pre-adjusted time, it does not matter how
much the clothes are dried.
6.
Light Switch and Traffic
Light– Lamps glow
whenever light switch is on irrespective of light is required or not.
7.
Volume on Stereo System –
Volume is adjusted manually irrespective of output volume level.
b. Advantages of Open Loop Control
System
1.
Simple in construction and
design.
2.
Economical.
3.
Easy to maintain.
4.
Generally stable.
5.
Convenient to use as output is
difficult to measure.
c. Disadvantages of Open Loop
Control System
1.
They are inaccurate.
2.
They are unreliable.
3.
Any change in output cannot be
corrected automatically.
2. Closed Loop Control System
Control system in which the
output has an effect on the input quantity in such a manner that the input
quantity will adjust itself based on the output generated is called closed
loop control system. Open loop control system can be converted in to
closed loop control system by providing a feedback. This feedback automatically
makes the suitable changes in the output due to external disturbance. In this
way closed loop control system is called automatic control system. Figure below
shows the block diagram of closed loop control system in which feedback is taken
from output and fed in to input.
b. 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 and 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 conditioner functions depending upon the temperature of the room.
6.
Cooling System in Car – It
operates depending upon the temperature which it controls.
7. Roasted
Bread with a Timmer – we
can determine which level of roasting we want based on the time wich we set.
c. Advantages of Closed Loop
Control System
1.
Closed loop control systems are
more accurate even in the presence of non-linearity.
2.
Highly accurate as any error
arising is corrected due to presence of feedback signal.
3.
Bandwidth range is large.
4.
Facilitates automation.
5.
The sensitivity of system may
be made small to make system more stable.
6.
This system is less affected by
noise.
d. Disadvantages of Closed Loop
Control System
1.
They are costlier.
2.
They are complicated to design.
3.
Required more maintenance.
4.
Feedback leads to oscillatory
response.
5.
Overall gain is reduced due to
presence of feedback.
6.
Stability is the major problem
and more care is needed to design a stable closed loop system.
E. Comparison of Closed Loop And
Open 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
|
Feedback
Loop of Control System
A feedback is a common and
powerful tool when designing a control system. Feedback loop is the tool
which take the system output into consideration and enables the system to
adjust its performance to meet a desired result of system.
In any control system, the output is affected due to change in environmental
condition or any kind of disturbance. So one signal is taken from the output
and is fed back to the input. This signal is compared with a reference input
and the error signal is generated. This error signal is applied to controller
and output is corrected. Such a system is called feedback system. The figure
below shows the block diagram of a feedback system.
When the feedback signal is positive then system called positive feedback
system. For positive feedback system, the error signal is the addition of
reference input signal and a feedback signal. When the feedback signal is
negative then the system is called negative feedback system. For negative
feedback system, the error signal is given by the difference of reference input
signal and the feedback signal.
F. Effect of Feedback
Refer figure beside, which represents feedback system where
R = Input signal
E = Error signal
G = Forward path gain
H = Feedback
C = Output signal
B = Feedback signal
1.
Error between system input and
system output is reduced.
2.
System gain is reduced by a
factor 1/(1±GH).
3.
Improvement in sensitivity.
4.
Stability may be affected.
5.
Improve the speed of response.
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