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 | | موضوع: آنظمة التحكم Control Systems الإثنين فبراير 22, 2010 11:58 am | |
| آنظمة التحكم Control Systems
تكنولوجيا نظام التحكم الآلي (Automatic Control Systems Technology) فرع
من العلوم التكنولوجية, ويعني بالسيطرة على العمليات الصناعية والأجهزة
والمعدات, وتشغيلها دون الحاجة إلى مشغل بشري. ويعتبر التحكم الآلي ملتقى
المعارف الهندسية, إذ ينبغي مراقبة وضبط المتغيرات التي تتفاعل في جميع
العمليات الصناعية كي تؤدي المنشآت والتجهيزات الوظائف التي شيدت من أجلها.
ولتكنولوجيا نظام التحكم الآلي تطبيقات في جميع النشاطات الصناعية مثل:
محطات توليد الطاقة الكهربائية وتحلية المياه
مصافي تكرير النفط
مصانع تعبئة المواد الغذائية
صناعة السيارات
الملاحة الجوية والبحرية
التطبيقات العسكرية, ....
كما أن لتكنولوجيا نظم التحكم الآلي دور كبير في تخفيف أعباء الحياة
اليومية, وجعلها أكثر رفاهية, فنجد تطبيقات التحكم الآلي في معظم الأجهزة
المنزلية, مثل:
التبريد والتكييف
التدفئة والأفران
الغسالات والنشافات, ...
ولقد أصبحت مفاهيم التحكم الآلي تستخدم في شتى مجالات المعرفة مثل علوم
الأحياء والاقتصاد والاجتماع والتربية فضلاً عن أنظمة النقل
(Transportation Systems) , والتخطيط العمراني (Urban Planning) , والبيئة
(Environment)..
فيما يلي مقدمة لمبادئ التحكم الآلي, وفي الوصلة, في نهاية الموضوع شرح
مفصّل باللغة العربية لمنهج عربي يتناول الموضوع في كتابين:
الكتاب الأول:
"مقدمة أنظمة التحكم"
الكتاب الثاني:
"تحليل نظم التحكم"
An Introduction To Control Systems
_______________
Some Examples
We want you to imagine that you have a job - actually several
jobs. These jobs will involve designing control systems.
• Imagine that you have been hired by a company that produces specialty
metals. They are setting up a new production line for a new kind of
magnetic material that they are producing for shielding rooms from
magnetic fields produced by high currents. The production of this metal
involves heating it to very specific temperatures and holding those
temperatures for specified periods of time. Actually, the rates of
temperature increase and decrease between the set points are also
critical. If the temperatures vary too much from what is required the
metal produced will have to be scrapped. Are you ready for this job?
It's not tough conceptually, but can you guarantee the company that they
can go forward with confidence that the temperature control systems
will work to produce the required temperature vs time profile?
Here's another situation.
• Imagine that are part of an aircraft control system design team. Your
company has just bid on the control of a new supersonic aircraft. Your
team will need to design the autopilot systems for the aircraft. In
other words, you need to design systems that will keep the aircraft at
the same altitude and on the same heading when the pilot is not
actuating the controls. Can you design a system and guarantee that the
system will work? Remember also that you need to design this system so
that it works when the fuel tanks are full and when they are nearing
empty at the end of a flight, and that the system has to work in all
kinds of conditions including heavy cross winds.
What Is A Control System?
In most systems there will be an input and an output. This
block diagram represents that. (Control system designers and engineers
use block diagrams to represent systems ) Signals flow from the input,
through the system and produce an output.
• The input will usually be an ideal form of the output. In other words
the input is really what we want the output to be. It's the desired
output.
• The output of the system has to be measured. In the figure below, we
show the system we are trying to control - the "plant" - and a sensor
that measures what the controlled system is doing.
• The input to the plant is usually called the control effort, and the
output of the sensor is usually called the measured output, as shown
below in the figure.
For example, if we want the output to be 100oC, then that's the input.
If we want to control the output, we first need to measure the
output. Within the whole system is the system we want to control - the
plant - along with a sensor that measures what the output actually is.
• In our block diagram representation, we show the output signal being
fed to the sensor which produces another signal that is dependent upon
the output.
• A sensor might be an LM35, which produces a vol***e proportional to
temperature - if the output signal is a temperature.
We need the sensor in the system to measure what the system is
doing.
• The sensor measures the output of the system we are controlling.
• It often converts the output into a variable we can use. If the
output is a temperature, we might want to have a vol***e we can use to
control a heater, for example. The LM35 temperature sensor, for
example, produce .01 volts for every 1.0oC change.
To control the system we need to use the information provided by the
sensor.
• Usually, the output, as measured by the sensor is subtracted from the
input (which is the desired output) as shown below. That forms an error
signal that the controller can use to control the plant.
• The device which performs the subtraction to compute the error, E, is a
comparator.
Finally,
the last part of this system is the controller.
• The controller acts on the error signal and uses that information to
produce the signal that actually affects the system we are trying to
control.
• The controller has to provide enough power to drive the system. You
don't want to try to control a large motor with a 741 operational
amplifier. You just can't do that, so the controller has to be able to
compute the control signal, and it has to be able to drive the system
you are trying to control.
• Thus, the controller has two things that it has to achieve.
o The controller has to compute what the control errort should be.
o The controller has to apply the computed control effort.
Consider how this controller works.
• If the gain in the forward path, from the error to the output, is
large, then a small error can produce a much larger ouput.
o There is a certain logic to that strategy. You want a small error,
but you need a control effort large enough to control the system. That
seems to imply that the gain of the controller should be large.
• It looks like a good strategy would be for the controller to be a high
gain power amplifier (for many control situations) because then a small
error could produce the output we want, or something very close to what
we want - because the error would be small.
Now, let's start to refine our model.
• Let's assume that the system we are trying to control is a linear
system.
• To account for the linear dynamics, we'll show the transfer function
of the system. That transfer function will be G(s).
Once we realize that we can describe the system we are
controlling, the plant, we realize that we can describe all of the
components in the sytem with a transfer function de******ion.
• The sensor most likely has an output - typically a vol***e - that is
proportional to the physical variable it measures. That means that the
transfer function is just a constant - a gain. We'll denote that by Ks.
The last item in our system is the controller. Controllers come
in many varieties. The simplest - but certainly not the only one used -
is a proportional controller. That's what we will consider here, but
remember there are also integral controllers, and controllers that blend
integral, proportional and derivative control and lots of others.
• In a proportional controller, the control action is proportional to
the error, and we can represent the controller as a gain, Kp.
That completes a verbal and algebraic de******ion of the system,
but there is also a diagrammatic representation for the system. The
block diagram shown below captures all of the information about the
system as we have developed it above. Note, in this system, we are
assuming that all of the signals are Laplace transform versions of the
time signals we have been discussing, and the de******ions of the blocks
in the block diagram are really transfer functions.
With this block diagram, let's review what we hope happens in
this system.
1. There is an input, u(t), to the system, which we assume starts from
rest. In the block diagram, that is represented by U(s).
2. The output of the system, Y(s), is measured with a sensor that has a
transfer function Ks. That transfer function could have a time
constant, etc., but for now we will examine it as though it is a
constant.
3. There is an error, E(s), developed, particularly because the
controlled system, G(s), cannot respond immediately and the feedback
signal that is subtracted from the input to form the error is zero.
4. The error that is developed acts through the proportional controller,
Kp, to start to move the output of the system to where we want it to
be.
5. As the system continues to operate, the output of the system
(described by G(s)) rises, reducing the error so that the control effort
from the proportional controller gets smaller.
6. Even though the error gets smaller, if the gain of the proportional
controller is large it will still provide enough output to drive the
system close to where we want it to be.
This kind of system is referred to as a closed loop system, since
there is a feedback signal that "closes the loop" in the system.
That's a little jargon you need to learn and remember.
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