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  1. [11]
    حسن هادي
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    Controller (control theory)

    From Wikipedia, the free encyclopedia


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    In control theory, a controller is a device which monitors and affects the operational conditions of a given dynamic system. The operational conditions are typically referred to as output variables of the system which can be affected by adjusting certain input variables. For example, the heating system of a house can be equipped with a thermostat (controller) for sensing air temperature (output variable) which can turn on or off a furnace or heater when the air temperature becomes too low or too high.
    In this example, the thermostat is the controller and directs the activities of the heater. The heater is the processor that warms the air inside the house to the desired temperature (setpoint). The air temperature reading inside the house is the feedback. And finally, the house is the environment in which the heating system operates.
    The notion of controllers can be extended to more complex systems. In the natural world, individual organisms also appear to be equipped with controllers that assure the homeostasis necessary for survival of each individual. Both human-made and natural systems exhibit collective behaviors amongst individuals in which the controllers seek some form of equilibrium.

    [edit] Types of control

    In control theory there are two basic types of control. These are feedforward and feedback. The input to a feedback controller is the same as what it is trying to control - the controlled variable is "fed back" into the controller. The thermostat of a house is an example of a feedback controller. This controller relies on measuring the controlled variable, in this case the temperature of the house, and then adjusting the output, whether or not the heater is on. However, feedback control usually results in intermediate periods where the controlled variable is not at the desired setpoint. With the thermostat example, if the door of the house were opened on a cold day, the house would cool down. After it fell below the desired temperature (setpoint), the heater would kick on, but there would be a period when the house was colder than desired.
    Feedforward control can avoid the slowness of feedback control. With feedforward control, the disturbances are measured and accounted for before they have time to affect the system. In the house example, a feedforward system may measure the fact that the door is opened and automatically turn on the heater before the house can get too cold. The difficulty with feedforward control is that the effect of the disturbances on the system must be perfectly predicted, and there must not be any surprise disturbances. For instance, if a window were opened that was not being measured, the feedforward-controlled thermostat might still let the house cool down.
    To achieve the benefits of feedback control (controlling unknown disturbances and not having to know exactly how a system will respond to disturbances) and the benefits of feedforward control (responding to disturbances before they can affect the system), there are combinations of feedback and feedforward that can be used.
    Some examples of where feedback and feedforward control can be used together are dead-time compensation, and inverse response compensation. Dead-time compensation is used to control devices that take a long time to show any change to a change in input, for example, change in composition of flow through a long pipe. A dead-time compensation control uses an element (also called a Smith predictor) to predict how changes made now by the controller will affect the controlled variable in the future. The controlled variable is also measured and used in feedback control. Inverse response compensation involves controlling systems where a change at first affects the measured variable one way but later affects it in the opposite way. An example would be eating candy. At first it will give you lots of energy, but later you will be very tired. As can be imagined, it is difficult to control this system with feedback alone, therefore a predictive feedforward element is necessary to predict the reverse effect that a change will have in the future.

    [edit] Types of controllers

    Most control systems in the past were implemented using mechanical systems or solid state electronics. Pneumatics were often utilized to transmit information and control using pressure. However, most modern control systems in industrial settings now rely on computers for the controller. Obviously it is much easier to implement complex control algorithms on a computer than using a mechanical system.
    For feedback controllers there are a few simple types. The most simple is like the thermostat that just turns the heat on if the temperature falls below a certain value and off it exceeds a certain value (on-off control).
    Another simple type of controller is a proportional controller. With this type of controller, the controller output (control action) is proportional to the error in the measured variable.
    The error is defined as the difference between the current value (measured) and the desired value (setpoint). If the error is large, then the control action is large. Mathematically:
    c(t) = Kc * e(t) + cs
    In the above equation, e(t) represents the error, Kc represents the controller's gain, and cs represents the steady state control action necessary to maintain the variable at the steady state when there is no error.
    The gain Kc will be positive if an increase in the input variable requires an increase in the output variable (direct-acting control), and it will be negative if an increase in the input variable requires a decrease in the output variable (reverse-acting control). A typical example of a direct-acting system is controlling flow of cooling water - if the temperature increases, the flow must be increased to maintain the desired temperature. Conversely, a typical example of a reverse-acting system is controlling flow of steam for heating - if the temperature increases, the flow must be decreased to maintain the desired temperature.
    Although proportional control is simple to understand, it has drawbacks. The largest problem is that for most systems it will never entirely remove error. This is because when error is 0 the controller only provides the steady state control action so the system will settle back to the original steady state (which is probably not the new set point that we want the system to be at). To get the system to operate near the new steady state, the controller gain, Kc, must be very large so the controller will produce the required output when only a very small error is present. Having large gains can lead to system instability or can require physical impossibilities like infinitely large valves.
    Alternates to proportional control are proportional-integral (PI) control and proportional-integral-derivative (PID) control. PID control is commonly used to implement closed-loop control.
    Open-loop control can be used in systems sufficiently well-characterized as to predict what outputs will necessarily achieve the desired states. For example, the rotational velocity of an electric motor may be well enough characterized for the supplied voltage to make feedback unnecessary.
    Drawbacks of open-loop control is that it requires perfect knowledge of the system (i.e. one knows exactly what inputs to give in order to get the desired output), and it assumes there are no disturbances to the system.

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  2. [12]
    حسن هادي
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    مبدأ التغذية الرجوعية ((العكسية ))للاشارة /

    For the superhero, see Feedback (Dark Horse Comics).
    For the list of albums named "Feedback", see Feedback (album).
    Feedback is the signal that is looped back to control a system within itself. This loop is called the feedback loop. A control system usually has input and output to the system; when the output of the system is fed back into the system as part of its input, it is called the "feedback."
    In cybernetics and control theory, feedback is a process whereby some proportion of the output signal of a system is passed (fed back) to the input. This is often used to control the dynamic behavior of the system. Examples of feedback can be found in most complex systems, such as engineering, architecture, economics, and biology
    *******************
    Types of feedback

    Figure 1: Ideal feedback model. The feedback is negative if B < 0


    Main articles: Negative feedback, positive feedback, and bipolar feedback
    Feedback may be negative, which tends to reduce output (but in amplifiers, stabilizes and linearizes operation), positive, which tends to increase output, or bipolar, which can either increase or decrease output. Systems which include feedback are prone to hunting, which is oscillation of output resulting from improperly tuned inputs of first positive then negative feedback. Audio feedback typifies this form of oscillation.

    [edit] Electro mechanics


    [edit] In electronic engineering

    The processing and control of feedback is engineered into many electronic devices and may also be embedded in other technologies.
    The most common general-purpose controller is a proportional-integral-derivative (PID) controller. Each term of the PID controller copes with time. The proportional term handles the present state of the system, the integral term handles its past, and the derivative or slope term tries to predict and handle the future.
    If the signal is inverted on its way round the control loop, the system is said to have negative feedback; otherwise, the feedback is said to be positive. Negative feedback is often deliberately introduced to increase the stability and accuracy of a system, as in the feedback amplifier invented by Harold Stephen Black. This scheme can fail if the input changes faster than the system can respond to it. When this happens, the negative feedback signal begins to act as positive feedback, causing the output to oscillate or hunt. Positive feedback is usually an unwanted consequence of system behaviour.
    With mechanical devices, hunting can be severe enough to destroy the device.
    Harry Nyquist was an electrical engineer who contributed the Nyquist plot for determining the stability of feedback systems.

    [edit] In mechanical engineering

    In ancient times, the float valve was used to regulate the flow of water in Greek and Roman water clocks; similar float valves are used to regulate fuel in a carburetor and also used to regulate tank water level in the flush toilet.
    The windmill was enhanced in 1745 by blacksmith Edmund Lee who added a fantail to keep the face of the windmill pointing into the wind. In 1787 Thomas Mead regulated the speed of rotation of a windmill by using a centrifugal pendulum to adjust the distance between the bedstone and the runner stone (i.e. to adjust the load).
    The use of the centrifugal governor by James Watt in 1788 to regulate the speed of his steam engine was one factor leading to the Industrial Revolution. Steam engines also use float valves and pressure release valves as mechanical regulation devices. A mathematical analysis of Watt's governor was done by James Clerk Maxwell in 1868.
    The Great Eastern was one of the largest steamships of its time and employed a steam powered rudder with feedback mechanism designed in 1866 by J.McFarlane Gray. Joseph Farcot coined the word servo in 1873 to describe steam powered steering systems. Hydraulic servos were later used to position guns. Elmer Ambrose Sperry of the Sperry Corporation designed the first autopilot in 1912. Nicolas Minorsky published a theoretical analysis of automatic ship steering in 1922 and described the PID controller.
    Internal combustion engines of the late 20th century employed mechanical feedback mechanisms such as vacuum advance but mechanical feedback was replaced by electronic engine management systems once small, robust and powerful single-chip microcontrollers became affordable.

    [edit] In economics and finance

    A system prone to hunting (oscillating) is the stock market, which has both positive and negative feedback mechanisms. This is due to cognitive and emotional factors belonging to the field of behavioral finance. For example,
    • When stocks are rising (a bull market), the belief that further rises are probable gives investors an incentive to buy (positive feedback, see also stock market bubble); but the increased price of the shares, and the knowledge that there must be a peak after which the market will fall, ends up deterring buyers (negative feedback).
    • Once the market begins to fall regularly (a bear market), some investors may expect further losing days and refrain from buying (positive feedback), but others may buy because stocks become more and more of a bargain (negative feedback).
    George Soros used the word "reflexism" to describe feedback in the financial markets and developed an investment theory based on this principle.
    The conventional economic equilibrium model of supply and demand supports only ideal linear negative feedback and was heavily criticized by Paul Ormerod in his book "The Death of Economics" which in turn was criticized by traditional economists. This book was part of a change of perspective as economists started to recognise that Chaos Theory applied to nonlinear feedback systems including financial markets.

    [edit] In nature

    Bipolar feedback is present in many natural and human systems. Feedback is usually bipolar—that is, positive and negative—in natural environments, which, in their diversity, furnish synergic and antagonistic responses to the output of any system.
    In biological systems such as organisms, ecosystems, or the biosphere, most parameters must stay under control within a narrow range around a certain optimal level under certain environmental conditions. The deviation of the optimal value of the controlled parameter can result from the changes in internal and external environments. A change of some of the environmental conditions may also require change of that range to change for the system to function. The value of the parameter to maintain is recorded by a reception system and conveyed to a regulation module via an information channel.
    Biological systems contain many types of regulatory circuits, both positive and negative. As in other contexts, Positive and negative don't imply consequences of the feedback have good or bad final effect. A negative feedback loop is one that tends to slow down a process, while the positive feedback loop tends to accelerate it.
    Feedback and regulation are self related. The negative feedback helps to maintain stability in a system in spite of external changes. It is related to homeostasis. Positive feedback amplifies possibilities of divergences (evolution, change of goals); it is the condition to change, evolution, growth; it gives the system the ability to access new points of equilibrium.
    For example, in an organism, most positive feedback provide for fast autoexcitation of elements of endocrine and nervous systems (in particular, in stress responses conditions) and play a key role in regulation of morphogenesis, growth, and development of organs, all processes which are in essence a rapid escape from the initial state. Homeostasis is especially visible in the nervous and endocrine systems when considered at organism level.
    The mirror neurons are part of a social feedback system, when an observed action is ´mirrored´ by the brain - like a self performed action.
    Feedback is also central to the operations of genes and gene regulatory networks. repressor (see Lac repressor) and activator proteins are used to create genetic operons, which were identified by Francois Jacob and Jacques Monod in 1961 as feedback loops.
    Any self-regulating natural process involves feedback and is prone to hunting. A well known example in ecology is the oscillation of the population of snowshoe hares due to predation from lynxes.
    In zymology, feedback serves as regulation of activity of an enzyme by its direct product(s) or downstream metabolite(s) in the metabolic pathway (see Allosteric regulation).
    There is an ice-albedo positive feedback loop whereby melting snow exposes more dark ground (of lower albedo), which in turn absorbs heat and causes more snow to melt. This is part of the evidence of the danger of global warming.
    Compare with: feed-forward.

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  3. [13]
    حسن هادي
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    تمنياتنا ان نكون قد اوصلنا افكار الموضوع بتسلسل مشاركاته الى المستوى المطلوب /وبامكانكم اخوتي متابعة الروابط المتداخلة مع تحياتي

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  4. [14]
    شكرى محمد نورى
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    مشرف


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    وسام الاشراف

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    الأخ حسن هادي .

    جزيل الشكر والتقدير وماقصرت يا مبدع .

    اعانك الله على المجهود الذي بذلته ويسر خطاك .

    البغدادي .

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  5. [15]
    حسن هادي
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    عضو متميز
    الصورة الرمزية حسن هادي


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    اقتباس المشاركة الأصلية كتبت بواسطة شكرى محمد نورى مشاهدة المشاركة
    الأخ حسن هادي .

    جزيل الشكر والتقدير وماقصرت يا مبدع .

    اعانك الله على المجهود الذي بذلته ويسر خطاك .

    البغدادي .
    حياك الله يا مشرفنا العزيز وتقبل منا كل المودة والتقدير *

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  6. [16]
    عراااااقي
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    عضو فعال


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    رااائع جدااا

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  7. [17]
    حسن هادي
    حسن هادي غير متواجد حالياً
    عضو متميز
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    تحياتنا لكل الاخوة الاعضاء مع التقدير

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  8. [18]
    حبيب جاسم
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    تاريخ التسجيل: Jul 2007
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    Thanks for the nice information,please if somebody help me to get the following book (Schaum Series ,theory & problem feed back & control system.

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  9. [19]
    fomari6
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    Dear Mr. Hasan
    Thank you for these satisfying explaination .I am looking for books in control desigin system by using simulink,for mechanical application .

    if you have links ,PDFs,or any useful media kindly send it me .

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  10. [20]
    محمد القاضى1
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    طب انا ان شاء الله فى اولى ميكانيكا عايز اعرف اذا كنت ها درس الحاجات الجميله دى ول لا

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