Control system basics_open and closed loop control system

Automatic Control Engineering
Basics of Control System
Nilesh Bhaskarrao Bahadure Ph.D.
nbahadure@gmail.com
https://www.sites.google.com/site/nileshbbahadure/home
June 29, 2021
Nilesh Bhaskarrao Bahadure Ph.D. () Automatic Control Engineering June 29, 2021 1 / 44

Overview
1 Introduction
2 Feature of Control System
Requirement of Good Control System
3 Types of Control System
Open loop control system
Closed loop control system
Comparison of Closed Loop And Open Loop Control System
4 Signal flow graph
5 Conversion of Block Diagrams into Signal Flow Graphs
6 Questions
7 Thank You

Introduction
What is a System?

Introduction
What is a System?
A collection of components that interact with one another and with their
environment.

Introduction
What is a System?
A collection of components that interact with one another and with their
environment.
Some examples of systems. Human beings, mechanical devices, an
electrical switch, plants, animals, the atmosphere, the stock market, the
political system, etc.

Introduction
What is a Control System?
A control system is a collection of components that is designed to drive a
given system (plant) with a given input to a desired output.
Control System
A control system is a system of devices or set of devices, that manages,
commands, directs or regulates the behavior of other device(s) or
system(s) to achieve desire results. In other words A control system is a
system, which controls other system.
As the human civilization is being modernized day by day the demand of
automation is increasing accordingly. Automation highly requires control
of devices.

Introduction
Control System...
Control System
An interconnection of components forming a system configuration that will
provide a desired response.

Introduction
Control System...
Process
The device, plant, or system under control. The input and output
relationship represents the cause-and-effect relationship of the process.
Figure :

Introduction
Control System...
Controlled Variable
It is the quantity or condition that is measured and Controlled. Normally
controlled variable is the output of the control system.

Introduction
Control System...
Manipulated Variable
It is the quantity of the condition that is varied by the controller so as to
affect the value of controlled variable.

Introduction
Control System...
Control
Control means measuring the value of controlled variable of the system
and applying the manipulated variable to the system to correct or limit the
deviation of the measured value from a desired value.

Feature of Control System
Feature of Control System
The main feature of control system is, there should be a clear
mathematical relation 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 linear control system. Again
when the relation 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 as non-linear control
system.

Feature of Control System Requirement of Good Control System
Requirement of Good Control System I
1 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 accuracy of any control system error
detector should be present in control system.
2 Sensitivity : The parameters of control system are always changing
with 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.
3 Noise : An undesired input signal is known as noise. A good control
system should be able to reduce the noise effect for better
performance.

Feature of Control System Requirement of Good Control System
Requirement of Good Control System II
4 Stability : It is an important characteristic of control system. For the
bounded input signal, the output must be bounded and if input is
zero then output must be zero then such a control system is said to
be stable system.
5 Bandwidth : An operating frequency range decides the bandwidth of
control system. Bandwidth should be large as possible for frequency
response of good control system.
6 Speed : It is the time taken by control system to achieve its stable
output. A good control system possesses high speed. The transient
period for such system is very small.
7 Oscillation : A small numbers of oscillation or constant oscillation of
output tend to system to be stable.

Types of Control System
Types of Control System I
There are various types of control system 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 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 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

Types of Control System II
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.
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 to the input power supply of the system. This system is referred as
open loop control system.

Types of Control System III
But in the later case, the heating elements of the system function,
depending upon the difference between, actual temperature and desired
temperature. This difference is called error of the system. This error signal
is fed back to the system to control the input. As the input to output path
and the error feedback path create a closed loop, this type of control
system is referred as closed loop control system.
Hence, there are two main types of control system. They are as follow

Types of Control System...
Depending on whether a human body (the operator) is physically involved
in the control system, they are divided into Manual Control and Automatic
Control. Due to its efficiency, accuracy and reliability, automatic control is
widely used in most of the systems.

Manual Control System
First start with a simple manual control system, to examine how control is
introduced, how the control system is constructed and how it works.
A diagram of the system is shown in figure 2
Figure :
To begin with the shower is cold. To start the heating process the valve in
the hot water line is opened. The operator can then determine the
effectiveness of the control process by standing in the shower. If the water
is too hot, the valve should be closed a little or even turned off. If the
water is not hot enough then the valve is left open or opened wider.

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Automatic Control System I
Based on the above process, we can easily set up an automatic control
system as shown in the next figure.
Figure :
1 Firstly, we can use a temperature measurement device to measure the
water temperature, which replaces the right hand of the operator.
This addition to the system would have improved accuracy.

Automatic Control System II
2 Instead of manual valves, we use a special kind of valve, called a
control valve, which is driven by compressed air or electricity. This
will replace the left hand of the operator.
3 We put a device called a controller, in this case a temperature
controller, to replace the brain of the operator. This has the functions
of comparison and computation and can give orders to the control
valve.
4 The signal and order connections between the measurement device,
control valve and controller are transfered through cables and wires,
which replace the nerve system in the operator.

Functions of a control system I
It can be seen that this control system, completed by the operator,
possesses the following functions:
1 Measurement
This is essentially an estimate or appraisal of the process being
controlled by the system. In this example, this is achieved by the right
hand of the operator.
2 Comparison
This is an examination of the likeness of the measured values and the
desired values. This is carried out in the brain of the operator.

Functions of a control system II
3 Computation
This is a calculated judgment that indicates how much the measured
value and the desired values differ and what action and how much
should be taken. In this example, the operator will calculate the
difference between the desired temperature and the actual one.
Accordingly the direction and amount of the adjustment of the valve
are worked out and the order for this adjustment is sent to the left
hand from the brain of the operator. If the outlet water temperature
is lower, then the brain of the operator will tell the left hand to open
the steam valve wider. If there is any disturbance, or variation of flow
rate in water to the shower inlet, some adjustment must be made to
keep the outlet water temperature at a desired value.

Functions of a control system III
4 Correction
This is ultimately the materialization of the order for the adjustment.
The left hand of the operator takes the necessary actions following
the order from brain.
Therefore, for a control system to operate satisfactorily, it must have
the abilities of measurement, comparison, computation and correction.
Of course, the manual operation has obvious disadvantages e.g. the
accuracy and the continuous involvement of operators. Although
accuracy of the measurement could be improved by using an
indicator, automatic control must be used to replace the operator. In
industry, it is automatic control that is widely used.

1 Open loop control system
2 Closed loop control system

Types of Control System 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. Manual
control system is also an open loop control system. Fig 4 shows the block
diagram of open loop control system in which process output is totally
independent of controller action
Figure : Open Loop Control System

Open Loop Control System...
1 Since in open loop control systems reference input is not compared
with measured output, for each reference input there is fixed
operating condition.
2 Therefore, the accuracy of the system depends on calibration.
3 The performance of open loop system is severely affected by the
presence of disturbances, or variation in operating/environmental
conditions.
Figure : Generic Open Loop Control System

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.

dried.
2 Automatic Washing Machine - This machine runs according to the
pre-set time irrespective of washing is completed or not.

dried.
3 Bread Toaster - This machine runs as per adjusted time irrespective
of toasting is completed or not.

dried.
4 Automatic Tea/Coffee Maker - These machines also function for pre
adjusted time only.

dried.
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.

dried.
adjusted time only.
6 Light Switch - Lamps glow whenever light switch is on irrespective of
light is required or not.

dried.
adjusted time only.
6 Light Switch - 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.

Advantages of Open Loop Control System
1 Simple in construction and design.

2 Economical.

2 Economical.
3 Easy to maintain.

2 Economical.
3 Easy to maintain.
4 Generally stable.

2 Economical.
3 Easy to maintain.
4 Generally stable.
5 Convenient to use as output is difficult to measure.

Disadvantages of Open Loop Control System
1 They are inaccurate.

2 They are unreliable.

2 They are unreliable.
3 Any change in output cannot be corrected automatically.

Types of Control System Closed loop control system
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.
Figure : Open Loop Control System

Closed Loop Control System...
1 A system that maintains a prescribed relationship between the output
and some reference input by comparing them and using the difference
(i.e. error) as a means of control is called a feedback control system.
2 Feedback can be positive or negative.
3 The performance of open loop system is severely affected by the
presence of disturbances, or variation in operating/environmental
conditions.

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.

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.

the reservoir.
missile.
5 An Air Conditioner - An air conditioner functions depending upon the
temperature of the room.

the reservoir.
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
1 Closed loop control systems are more accurate even in the presence of
non-linearity.

non-linearity.
2 Highly accurate as any error arising is corrected due to presence of
feedback signal.

non-linearity.
feedback signal.
3 Bandwidth range is large.

non-linearity.
feedback signal.
4 Facilitates automation.

non-linearity.
feedback signal.
5 The sensitivity of system may be made small to make system more
stable.

non-linearity.
feedback signal.
5 The sensitivity of system may be made small to make system more
stable.
6 This system is less affected by noise.

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.

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.

Types of Control System Comparison of Closed Loop And Open Loop Control System
Comparison of Closed Loop And Open Loop Control
System
Open loop control system Closed loop control system

System
The feedback element is absent.
The feedback element is always present.

System
An error detector is not present.
An error detector is always present.

System
It is stable one.
It may become unstable.

System
It is stable one.
Easy to construct.
Complicated construction.

System
It is stable one.
Easy to construct.
It is an economical.
It is costly.

System
It is stable one.
Easy to construct.
It is costly.
Having small bandwidth.
Having large bandwidth.

System
It is stable one.
Easy to construct.
It is costly.
It is inaccurate.
It is accurate.

System
It is stable one.
Easy to construct.
It is costly.
It is inaccurate.
It is accurate.
Less maintenance.
More maintenance.

System
It is stable one.
Easy to construct.
It is costly.
It is inaccurate.
It is accurate.
Less maintenance.
More maintenance.
It is unreliable.
It is reliable.

System
It is stable one.
Easy to construct.
It is costly.
It is inaccurate.
It is accurate.
Less maintenance.
More maintenance.
It is unreliable.
It is reliable.
Examples: Hand drier, tea maker
Examples: Servo voltage stabilizer, per-
spiration
Table : Comparision of Open and Closed loop system

Classification of Control Systems

Signal flow graph
Example - 1

Signal flow graph
Example - 1
construct a signal flow graph by considering the following algebraic
equations
y2 = a12y1 + a42y4
y3 = a23y2 + a53y5
y4 = a34y3
y5 = a45y4 + a35y3
y6 = a56y5

Signal flow graph
Solution: Example - 1

Conversion of Block Diagrams into Signal Flow Graphs

Follow these steps for converting a block diagram into its equivalent signal
flow graph
1 Represent all the signals, variables, summing points and take-off
points of block diagram as nodes in signal flow graph.

flow graph
2 Represent the blocks of block diagram as branches in signal flow
graph.

flow graph
graph.
3 Represent the transfer functions inside the blocks of block diagram as
gains of the branches in signal flow graph.

flow graph
graph.
3 Represent the transfer functions inside the blocks of block diagram as
gains of the branches in signal flow graph.
4 Connect the nodes as per the block diagram. If there is connection
between two nodes (but there is no block in between), then represent
the gain of the branch as one. For example, between summing points,
between summing point and takeoff point, between input and
summing point, between take-off point and output.

Example - 2

Example - 2
convert the following block diagram into its equivalent signal flow graph

Example - 2
convert the following block diagram into its equivalent signal flow graph
Represent the input signal R(s) and output signal C(s) of block diagram
as input node R(s) and output node C(s) of signal flow graph.

Solution: Example - 2

Questions
Questions I
1 What is system, explain control system in details
2 Explain Automatic and Manual control systems
3 Explain the types of control systems
4 Explain open and closed loop control systems with examples
5 Differentiate between open and closed loop control system
6 Explain the requirements of good control system
7 Explain the functions of control system
8 Explain the advantages and disadvantages of open and closed loop
control system

Questions
Questions II
9 Construct a signal flow graph by considering the following algebraic
equations
y2 = a12y1 + a42y4
y3 = a23y2 + a53y5
y4 = a34y3
y5 = a45y4 + a35y3
y6 = a56y5
10 What is signals, explain unit step signal and its importance

Thank You
Thank you
Please send your feedback at nbahadure@gmail.com

Thank You
Thank you
Please send your feedback at nbahadure@gmail.com
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Control system basics_open and closed loop control system

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