Lecture-1
Introduction to Control Systems
Bethlehem A. & Nebiyu T.
Addis Ababa University
Addis Ababa Institute of Technology
School of Electrical & Computer Engineering
Introduction to Control Engineering
ECEG-3175
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Overview
Overview
1
Overview
2
Objective
3
Introduction
History
4
Definitions
5
Closed-Loop Vs Open-Loop Control
Closed-Loop Control Systems
Open-Loop Control Systems
Comparison
6
Design and Compensation of Control Systems
Performance Specifications
System Compensation & Design Procedures
7
Examples of Control Systems
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Objective
Lecture Objectives
After this lecture, you will be able to answer the following:
What is a control system?
What is an automatic control system?
How to describe a system?
What are the basic requirements for a control system?
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Introduction
Introduction
Automatic control has played a vital role in the advance of
engineering and science.
In addition to its extreme importance in space-vehicle systems,
missile-guidance systems, robotic systems, and the like, automatic
control has become an important and integral part of modern
manufacturing and industrial processes.
Example
automatic control is essential in the numerical control of machine tools
in the manufacturing industries, in the design of autopilot systems in
the aerospace industries, and in the design of cars and trucks in the
automobile industries. It is also essential in such industrial operations
as controlling pressure, humidity, viscosity, and flow in the process
industries.
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Introduction
History
History of Control Theories and Practices
James Watt’s centrifugal governor for the speed control of a steam
engine in the eighteenth century was the first significant work in
automatic control
1922: Minorsky worked on automatic controllers for steering
ships and showed system modelling and stability analysis from
differential equations
1932: Nyquist developed a relatively simple procedure for
determining the stability of closed-loop systems on the basis of
open-loop response to steady-state sinusoidal inputs
1934: Hazen introduced the term servomechanisms and discussed
the design of relay servomechanisms capable of closely following a
changing input
1940s: frequency-response methods made performance
requirement based designs possible (Bode diagram)
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Introduction
History
1940s and 1950s:
PID controllers were widely used for process controls and Ziegler
and Nichols suggested rules for tuning PID controllers
The end of this era resulted in the full development of the
root-locus method due to Evans
Since about 1960:
The availability of digital computers made possible time-domain
analysis of complex systems
Modern control theory, based on time-domain analysis and
synthesis using state variables
1960s to 1980s: optimal control of both deterministic and
stochastic systems, as well as adaptive and learning control of
complex systems, were fully investigated
Since 1990s: Control systems and theories have been a major
research area
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Definitions
Definitions of Basic Terminologies
Controlled Variable: is the quantity or condition that is
measured and controlled. The controlled variable is the output of
the system.
Manipulated Variable: is the quantity or condition that is
varied by the controller so as to affect the value of the controlled
variable.
Control: means measuring the value of the controlled variable of
the system and applying the manipulated variable to the system
to correct or limit deviation of the measured value from a desired
value.
Plant: may be a piece of equipment, perhaps just a set of
machine parts functioning together, the purpose of which is to
perform a particular operation.
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Definitions
Processe: a natural, progressively continuing operation or
development marked by a series of gradual changes that succeed
one another in a relatively fixed way and lead toward a particular
result or end.
System: is a combination of components that act together and
perform a certain objective.
Disturbance: is a signal that tends to adversely affect the value
of the output of a system. If a disturbance is generated within the
system, it is called internal, while an external disturbance is
generated outside the system and is an input.
Feedback Control: refers to an operation that, in the presence of
disturbances, tends to reduce the difference between the output of
a system and some reference input. (How? )
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Closed-Loop Vs Open-Loop Control
Closed-Loop Control Systems
Closed-Loop Vs Open-Loop control
Closed-Loop Control Systems
Feedback Control Systems
A system that maintains a prescribed relationship between the output
and the reference input by comparing them and using the difference as
a means of control is called a feedback control system.
Feedback control systems are often referred to as closed-loop
control systems.
In practice, the terms feedback control and closed-loop control are
used interchangeably.
In a closed-loop control system the actuating error signal, which is
the difference between the input signal and the feedback signal , is
fed to the controller so as to reduce the error and bring the output
of the system to a desired value.
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Closed-Loop Vs Open-Loop Control
Open-Loop Control Systems
Open-Loop Control Systems
Open-Loop Control Systems
Those systems in which the output has no effect on the control action
are called open-loop control systems.
In an open-loop control system the output is neither measured nor
fed back for comparison with the input
In the presence of disturbances, an open-loop control system will
not perform the desired task.
Open-loop control can be used, in practice, only if the relationship
between the input and output is known and if there are neither
internal nor external disturbances.
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Closed-Loop Vs Open-Loop Control
Comparison
Closed-Loop Vs Open-Loop control: Comparison
Closed-Loop Control
shows a closed-loop action
(closed control loop);
Open-Loop Control
shows an open-loop action
(controlled chain);
can counteract against
disturbances (negative
feedback);
can become unstable, i.e. the
controlled variable may not
fade away, but grows
(theoretically) to an infinite
value.
can only counteract against
disturbances, for which it has
been designed; other
disturbances cannot be
removed;
cannot become unstable - as
long as the controlled object is
stable.
Note: Combined feedforward plus feedback control can significantly improve
performance over simple feedback control whenever there is a major disturbance
that can be measured before it affects the plant output.
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Design and Compensation of Control Systems
Design and Compensation of Control Systems
In studying a control system, one must model its dynamic
characteristics so that the analysis and design of the system can be
proceeded.
Quantitative mathematical models of physical systems are used to
design and analyze control systems.
The dynamic behavior is, generally, described by ordinary
differential equations ODEs.
The physical systems range over a wide field, including
mechanical, hydraulic, and electrical systems.
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Design and Compensation of Control Systems
Performance Specifications
Performance Specifications
Control systems are designed to perform specific tasks and requirements
are imposed on the control system, and are known as performance specifications.
(a) Stability
A control system must be stable. A stable system is a dynamic system
with a bounded response to a bounded input.
Example
The famous Tacoma Narrows Bridge before it collapsed was found
to oscillate whenever the wind blew.
On November 7, 1940, a wind produced an oscillation that grew in
amplitude until the bridge broke apart.
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Design and Compensation of Control Systems
Performance Specifications
(b) Transient Response
The transient response of a practical control system often exhibits
damped oscillations before reaching steady state.
It is desirable that the transient response to be sufficiently fast
and sufficiently damped.
(c) Steady State Error (ess )
Steady state errors in a control system can be attributed to many
factors
Changes in the reference input will cause unavoidable errors
during transient periods and may also cause steady state errors.
It is desirable that the steady state error be sufficiently small
It can be summerized that: A desired control system should be stable with
sufficiently fast transient response and sufficiently small steady-state error.
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Design and Compensation of Control Systems
System Compensation & Design Procedures
System Compensation & Design Procedures
System Compensation
The compensation is necessary if a control system does not satisfy the
given performance specifications.
Design Procedure
The design procedure of a control system follows the order of
i. Modeling,
ii. System analysis,
iii. System Design, and
iv. Implementation.
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Examples of Control Systems
Examples of Control Systems
Example (Speed Control System)
Figure 1:
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Examples of Control Systems
Example (Speed Control System)
The amount of fuel admitted to the engine is adjusted according
to the difference between the desired and the actual engine speeds.
The sequence of actions may be stated as follows:
The speed governor is adjusted such that, at the desired speed, no
pressured oil will flow into either side of the power cylinder.
If the actual speed drops below the desired value due to
disturbance, then the decrease in the centrifugal force of the speed
governor causes the control valve to move downward, supplying
more fuel, and the speed of the engine increases until the desired
value is reached,
On the other hand, if the speed of the engine increases above the
desired value, then the increase in the centrifugal force of the
governor causes the control valve to move upward. This decreases
the supply of fuel, and the speed of the engine decreases until the
desired value is reached.
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Examples of Control Systems
Control System Examples
Example (Temperature Control System)
Figure 2:
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Examples of Control Systems
Example (Temperature Control System)
The temperature in the electric furnace is measured by a
thermometer, which is an analog device.
The analog temperature is converted to a digital temperature by
an AID converter
The digital temperature is fed to a controller through an interface
This digital temperature is compared with the programmed input
temperature, and if there is any error, the controller sends out a
signal to the heater
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Examples of Control Systems
Example (Liquid-Level Control System)
Figure 3: (a) Liquid-level control system; (b) electromagnetic valve.
The electromagnetic valve is used for controlling the inflow rate.
This valve is either open or closed.
With this two-position control, the water inflow rate is either a
positive constant or zero.
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Questions?
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