open loop closed loop functions

1. Control Systems: What Are They? (Open-Loop &
Closed-Loop Control System Examples)
What is a Control System?
A control system is defined as a system of devices that manages,
commands, directs, or regulates the behavior of other devices or systems to
achieve a desired result. A control system achieves this through control
loops, which are a process designed to maintain a process variable at a
desired set point.
In other words, the definition of a control system can be simplified as a
system, which controls other systems. As human civilization is being
modernized day by day the demand for automation has increased
alongside it. Automation requires control over systems of interacting
devices.
In recent years, control systems have played a central role in the
development and advancement of modern technology and civilization.

2. Practically every aspect of our day-to-day life is affected more or less by
some type of control system.
Examples of control systems in your day-to-day life include an air
conditioner, a refrigerator, an air conditioner, a bathroom toilet tank, an
automatic iron, and many processes within a car – such as cruise control.
In industrial settings, we find control systems in the quality control of
products, weapons system, transportation systems, power systems, space
technology, robotics, and much more.
The principles of control theory are applicable to both engineering and
non-engineering field. You can learn more about control systems by
studying our control system MCQs.
Features of a Control System
The main feature of a control system is that there should be a clear
mathematical relationship between the 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.
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.

3. 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 the 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 a 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 a system is very small.
Oscillation: A small number of oscillations or constant oscillations of
output tends to indicate the system to be stable.
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
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 a manual control
system.
This system can further be improved by using a 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 the desired
temperature level of the room.

4. There is another improved way of controlling the temperature of the room.
Here one sensor measures the difference between the actual temperature
and desired temperature.
If there are any differences 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 the 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 the heating element produces heat as long as the power supply
is kept on and the final room temperature does not have any control over
the input power supply of the system. This system is referred to as an
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 systems. They are as follow
1. Open-loop control systems
2. Closed-loop control systems
Open Loop Control System
A control system in which the control action is totally independent of the
output of the system then it is called an 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.

5. Practical Examples of Open Loop Control Systems
Examples of open-loop control systems in daily life include:
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 – Lamps glow whenever the 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.
Advantages of Open Loop Control Systems
Advantages of open-loop control systems include:
1. Simple in construction and design.
2. Economical.

6. 3. Easy to maintain.
4. Generally stable.
5. Convenient to use as output is difficult to measure.
Disadvantages of Open Loop Control System
Disadvantages of open-loop control systems include:
1. They are inaccurate.
2. They are unreliable.
3. Any change in output cannot be corrected automatically.
Closed Loop Control System
Control systems 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 a closed-loop control system.
An open-loop control system can be converted into a closed loop control
system by providing feedback. This feedback automatically makes suitable
changes in the output due to external disturbance.
In this way, a closed loop control system is called an automatic control
system. The figure below shows the block diagram of the closed loop
control system in which feedback is taken from the output and fed into the
input.
Practical Examples of Closed Loop Control System
Examples of open-loop control systems in daily life include:
1. Automatic Electric Iron – Heating elements are controlled by the
output temperature of the iron.
2. Servo Voltage Stabilizer – Voltage controller operates depending
upon the output voltage of the system.
3. Water Level Controller – Input water is controlled by the water level
of the reservoir.

7. 4. Missile Launched and Auto Tracked by Radar – The direction of the
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.
Advantages of Closed Loop Control System
Advantages of closed-loop control systems include:
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 the presence
of a feedback signal.
3. The bandwidth range is large.
4. Facilitates automation.
5. The sensitivity of the system may be made small to make the system
more stable.
6. This system is less affected by noise.
Disadvantages of Closed Loop Control System
Disadvantages of a closed-loop control systems include:
1. They are costlier.
2. They are complicated to design.
3. Required more maintenance.
4. Feedback leads to an oscillatory response.
5. Overall gain is reduced due to the presence of feedback.
6. Stability is the major problem and more care is needed to design a
stable closed loop system.

8. Open Loop vs Closed Loop Control Systems
The table below compares open loop and closed loop control systems.
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 a stable one. It may become unstable.
4 Easy to construct. Complicated construction.
5 It is economical. It is costly.
6 Having a small bandwidth. Having a 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

9. Feedback Loop in a Closed Loop Control System
Feedback is a common and powerful tool when designing a control
system. The feedback loop is the tool that takes the system output into
consideration and enables the system to adjust its performance to meet the
desired result of the system.
In any control system, the output is affected due to a change in
environmental conditions 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 the controller and the output is
corrected. Such a system is called a feedback system. The figure below
shows the block diagram of a feedback system.
When the feedback signal is positive then the system called a positive
feedback system. For a positive feedback system, the error signal is the
addition of a reference input signal and a feedback signal.
When the feedback signal is negative then the system is called a negative
feedback system. For the negative feedback system, the error signal is
given by the difference between the reference input signal and the feedback
signal.
Effect of Feedback in a Control System
The following labels apply to the figure below:
R = Input signal
E = Error signal

10. G = Forward path gain
H = Feedback
C = Output signal
B = Feedback signal
Feedback has the following effects on a control system:
1. The error between system input and system output is reduced.
2. System gain is reduced by a factor 1/(1±GH).
3. Improved insensitivity (i.e. less reactive to change).
4. Stability is improved.

11. Servo Motor Working Principle
Before understanding the working principle of servo motor we should
understand first the basics of a servomechanism.
What is Servomechanism?
A servo system primarily consists of three basic components – a controlled
device, a output sensor, a feedback system.
This is an automatic closed loop control system. Here instead of controlling
a device by applying the variable input signal, the device is controlled by a
feedback signal generated by comparing output signal and reference input
signal.
When reference input signal or command signal is applied to the system, it
is compared with output reference signal of the system produced by
output sensor, and a third signal produced by a feedback system. This
third signal acts as an input signal of controlled device.
This input signal to the device presents as long as there is a logical

12. difference between reference input signal and the output signal of the
system.
After the device achieves its desired output, there will be no longer the
logical difference between reference input signal and reference output
signal of the system. Then, the third signal produced by comparing theses
above said signals will not remain enough to operate the device further
and to produce a further output of the system until the next reference input
signal or command signal is applied to the system.
Hence, the primary task of a servomechanism is to maintain the output of a
system at the desired value in the presence of disturbances.
Working Principle of Servo Motor
A servo motor is basically a DC motor (in some special cases it is AC
motor) along with some other special purpose components that make a DC
motor a servo. In a servo unit, you will find a small DC motor, a
potentiometer, gear arrangement and an intelligent circuitry. The
intelligent circuitry along with the potentiometer makes the servo to rotate
according to our wishes. As we know, a small DC motor will rotate with
high speed but the torque generated by its rotation will not be enough to
move even a light load.
This is where the gear system inside a servomechanism comes into the
picture. The gear mechanism will take high input speed of the motor (fast)
and at the output, we will get an output speed which is slower than
original input speed but more practical and widely applicable.
Say at the initial position of servo motor shaft, the position of the
potentiometer knob is such that there is no electrical signal generated at the
output port of the potentiometer. This output port of the potentiometer is
connected with one of the input terminals of the error detector amplifier.
Now an electrical signal is given to another input terminal of the error
detector amplifier. Now difference between these two signals, one comes
from potentiometer and another comes from external source, will be
amplified in the error detector amplifier and feeds the DC motor.

13. This amplified error signal acts as the input power of the DC motor and the
motor starts rotating in desired direction. As the motor shaft progresses the
potentiometer knob also rotates as it is coupled with motor shaft with help
of gear arrangement.
As the position of the potentiometer knob changes there will be an
electrical signal produced at the potentiometer port. As the angular
position of the potentiometer knob progresses the output or feedback
signal increases. After desired angular position of motor shaft the
potentiometer knob is reaches at such position the electrical signal
generated in the potentiometer becomes same as of external electrical
signal given to amplifier.
At this condition, there will be no output signal from the amplifier to the
motor input as there is no difference between external applied signal and
the signal generated at potentiometer. As the input signal to the motor is
nil at that position, the motor stops rotating. This is how a simple
conceptual servo motor works.