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© Copyright Brian Brown, 1992-2001. All rights reserved.

İŞLME KONTROLÜ-PROCESS CONTROL
Bu kesimin amacı

• işlem kontrol sistemlerine giriş
• karşılaştırma yapılarak sistemin yapacağı işlemin tanımlanması

Bölüm sonunda aşağıdakileri öğenmiş olacaksınız,

• işlem kontrol sistemlerinin çalışmasını
• işlem kontrolüne uygun uygulamaların örnekleri verilecektir
• karşılaştırma yapılacak sistem ile işlem sisteminin farkı anlaşılacaktır

Process Control Systems
By now, you are familiar with business applications software like accounting packages, spreadsheets and word processors. You are also familiar with operating systems software like MSDOS, UNIX and Windows.

The purpose of this module is to introduce you to another class of software concerning with controlling a process. We call this process control software. The software controls (regulates or changes) the process it is monitoring (like temperature, liquid flow, and air pressure).

Examples Of Process Control Systems

• trafik lambaları
• havaalını sistemleri
• yıkama makineleri
• araba motoru yönetimi sistemleri
• endüstriyel üretim işlemlei (plastikler)
• benzin pompaları

İşlem Kontrolü Nedir
Process control is a general term used to describe the many methods of regulating industrial processes. The process being controlled is monitored for changes by means of sensor devices. These sensor devices provide information about the state of the system. The information provided by the sensor devices is used to calculate some form of feedback, used to manipulate control valves or other control devices.

This provides automatic regulation of the process by computer software. The essential operations are measurement, evaluation and adjustment.

Process control systems operate in real-time. They must respond quickly to changes in the process they are monitoring. For instance, in a nuclear reactor we want the software to react quickly if the water coolant in the reactor rises in temperature above a certain point, and perform changes in the flow of water (increase it) in order to bring the temperature back down again to within acceptable limits.

• Ölçüm:
  This is accomplished by converting measured information into a variable, which is proportional to the quantity being measured. For example, consider a process control system response for monitoring temperature.

  Electronic devices (integrated circuits) are now available which convert temperature into voltage, where each millivolt equals a change of one degree Centigrade. This makes temperature directly related to voltage. Voltage is easily measured by a computer using an analogue to digital converter.

  Similar devices are used to convert air pressure or liquid flow into either current or voltage which can be read by a computer. Measurement is done by a transducer, which converts the measured quantity into a proportional mechanical motion, electrical effect, or pneumatic value. A transmitter converts the transducers output into an electrical signal in a format which can be read by a computer.

• Evaluation:
  Electronic or pneumatic instruments called controllers evaluate the measurements taken by the transducers. Examples of electronic controllers are
  • programmed logic arrays
  • ladder logic
  • micro-controllers
  • computer systems

  The controller compares the measured value to a desired value, called a set point. This produces an error signal , which generates a feedback signal that modifies the process in order to reduce the error signal towards zero in future measurements.

  Error evaluation is done by examining any combination of THREE characteristics about the error signal.

  • actual magnitude of the error relative to the range
    proportional mode control
  • relationship of the error magnitude to the time interval over which the error occurred
    integral (or reset) mode control
  • the rate at which the error occurs
    derivative (or rate) mode control
• Adjustment:
  To perform the automated adjustment, the feedback signal generated by the evaluation section is used to affect the process itself.

  The actual device used to accomplish this is called the final control element. For instance, to control the flow rate of a petrol pump, the final control element is a valve, and the feedback signal determines the speed of the adjustment and size of the valves opening, either letting more petrol go through (by opening the valve more) or less petrol go through (by closing the valve).

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© Copyright Brian Brown, 1992-2001. All rights reserved.