CONTROL SYSTEMS

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CONTROL SYSTEMS
DEÜ ENGINEERING FACULTY, DEPARTMENT OF TEXTILE
ENGINEERING

• INTRODUCTION (Kuo, 1995)
• What a control system is.
• Why control systems are important.
• What the basic components of a control system
  are.
• Some examples of control system applications.
• Why feedback is incorporated into most control
  systems.
• Types of control systems.

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What a control system is (Kuo, 1995) To answer
the question, we can cite that in our that in our
daily life there are numerous objectives that
we need to be accomplished. For instance, We
need to regulate the temperature and humidity of
homes and buildings for comfortable living. We
need to control the automobile and airplane to go
from one point to another accurately and safely.
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Manufacturing processes contain numerous
objectives for products that will satisfy the
precision and cost-effectiveness requirements. A
human being is capable of performing a wide range
of tasks, including decision making. Some of
these tasks, such as picking up objects and
wolking from one point to another, are commonly
carried out in a routine fashion. Under certtain
conditions, some of these tasks are to be
performed in the best possible way. For instance,
sprinter running a 100 m has the objective of
running that distanece in the shottest possible
time.
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A marathon runner, on the other hand, not only
must run the distance as quickly as possible, but
in doing so, he or she must control the
consumption of energy and devise the best
strategy for the race. The means of achieving
these objectives usually involve the use of
control systems that implement certain control
strategies.
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In recent years, control systems have assumed an
increasingly impotant role in the development and
advancement of modern civilization and
technology. Practically every aspect of our
day-to-day activities is affected by some type of
control systems. Control systems are found in
abundance in all sectors of industry, such as
quality control of manufactured products,
automatic assembly line, machine-tool control,
space technolgy and weapon systems, computer
control, transportation systems, power systems,
robotics, and many others. Even the control of
inventory and social and economic systems may be
approached from the theory of automatic control.
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• Basic Components of a Control System
• The basic ingredients of a control system can be
  described by
• Objective of control
• Control system components
• Results or outputs
• The basic relationship between these components
  is illustrated in Fig. 1.

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In more technical terms, the objectives can be
identified with inputs, or actuating signals, u,
and the results are also called outputs, or the
controlled variables, y. In general, the
objective of the control system is to control the
outputs in some prescribed manner by the inputs
through the elements of the control system.
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Examples of Control System Applications
Industrial Sewing Machine Sewing, as a basic
joining operation in the garment making process,
is in principle a rather complicated and
laborious operation. For low cost and high
productivity, the sewing industry has to rely on
sophisticated sewing machines to increase the
speed and accuracy of the sewing operations.
Sewing machines can produce different type of
stitches with a typical rate of over 100 stitches
per second. One stirch corresponds to ne
revolution of the machine main shaft, which
translates to top speeds as high as 8000 rpm. An
ideal velocity profile of one start-stop cycle of
the machine is shown in Fig. 2.
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IDLE
RUN COMMAND
STOP COMMAND
A
VELOCITY
POSITION LOOP ON
B
C
ta
tb
tps
TIME (s)
Figure 2. Ideal velocity profile of one
start-stop cycle of an industrial sewing machine.
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Typically, there should be no velocity overshoot
at point A and no undershoot at point B.
Acceleration time ta , deceleration time tb ,
and position search time tps , should be as short
as possible. When the machine reaches the
stopping point, C, there should be zero or
negligible oscillations. To achive these
performance objectives, the control system in the
machine should be designed with stringent
requirements.
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Temperature Control System
Hot water
Cold water
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Fluid Level Control Systems
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Industrial oven
Material
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Types of Control Systems
Open-Loop Control Systems (Nonfeedback
Systems) The elements of an open-loop control
system can usually be divided into two parts the
controller and the controlled process, as shown
below.
An input signal or command r is applied to the
controller, whose output acts as the actuating
signal u the actuating signal then controls the
controlled process so that the controlled
variable y will perform according to prescribed
standards. In simple cases, the controller can be
an amplifier, mechanical linkage, filter, or
other control element, depending on the nature of
the system. Because of the simplicity and economy
of open-loop control systems, we find this type
of system in many noncritical applications.
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Closed-Loop Control Systems (Feedback Control
Systems)
What is missing in the open-loop control system
for more accurate and more adaptive control is a
link or feedback from to the output to the input
of the system. To obtain more accurate control,
the controlled signal y should be fed back and
compared with the reference input, and an
actuating signal proportional to the difference
of the input and the output must be sent through
the system to correct the error. A system with
one or more feedback paths such as that just
described is called a closed-loop system.
Reference input r
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Input, Output, Disturbance
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Taking a Shower
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• Control objectives
• Control objectives when taking a shower include
  the following
• to became clean
• to be comfortable (correct temperature and water
  velocity a it contacts the body)
• to look good (clean hair, etc.)
• to became refreshed

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To simplify our analysis, we discuss how we can
satisfy the second objective (to maintain water
temperature and flow rate at comfortable values).
Similar analysis can be performed for the other
objectives.
Input variables The manuplated input variables
are hot-water and cold-water valve positions.
Some showers can also vary the velocity by
adjustment of the shower head. Another input is
body position-we can move into and out of the
shower stream. Disturbance inputs include a drop
in water pressure (say, owing to toilet flushing)
and changes in hot water temperature owing to
using up the hot water from the heater.
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Output variables The measured output variables
are the temperature and flow rate (or velocity)
of the mixed stream as it contacts our body.
Constraints The are minumum and maximum valve
positions (and therefore flow rates) on both
streams. The maximum mixed temperature is equal
to the hot water temperature and the minumum
mixed temperature is equal to the cold water
temperature. The previous constraints were hard
constraints - they can not be physically
violated. An example of a soft constraints is the
mixed-stream water temperature we do not want
it to be above a certain value because we may get
scalled. This is a soft constraint because it can
physically happen, although yo do not want it to
happen.
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Operating characteristics This process is
continuous while we are taking a shower but is
most likely viewed as a batch process, since it
is a small part of our day.
Safety, environmental and economic
considerations Too high of a temperature can
scald us - this is certainly a safety
consideration. Economically, if our showers are
too long, more energy is consumed to heat the
water, costing money. Environmental (and
economically), more water consumption means that
more water and wastewater must be treated. An
economic objective might be to minimize the
shower time. Howeve, if the shower time is too
short, or nor frequent enough, our clothes will
become dirty and must be washed more often
increasing outr clothes- cleaning bill.
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Control structure This is a multivariable
control problem because adjusting either valve
affects both temperature and flow rate. Control
manipulations must be coordinated, that is, if
the hot-water flow rate is increased to increase
the temperature, the cold-water flow rate must be
decreased to maintain the same total flow rate.
The measurement signals are continuous, but the
manipulated variable changes are likely to be
discrete (unless our hands are continuously
varying the valve positions).
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Feedback control As the body feels the
temperature changing, adjustments to one or both
valves is made. As the body senses a flow rate or
velocity change, one or both valves are adjusted.
h
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Feed-forward control If we hear the toilet
flush, we move our body out of the stream to
avoid the higher temperature that we anticipate.
Notice that we are moking a manipulated variable
change (moving our body) before the effect of an
output (temperature or flow rate) change is
actually detected.