4.Hardware for a Process Control System

Hardware for a Process Control System  The physical elements (hardware) constituting a control control system as it is implemented in practice for the control of real physical processess.

Hardware Elements of a Control System In every control conguration we can distinguish the following hareware elements : 1. The chemical process : It represents the material equipment together with the physical or chemical operations that occur there. 2. The measuring instruments or sensors : uch instruments are used to measure the distur!ances" the controlled output varia!les" or secondary output varia!les" and the main sources of information a!out what is going on in the process. #. Transducers ransduce rs : $any measurements cannot !e used for control until they are converted to physical quantities (such as electric voltage or curr current ent"" or a pneum pneumati atic c sign signal al"" i.e. i.e."" comp compre ress ssed ed air air or com compres press sed liq liqui uid d) whic which h can !e trans ransmi mitt tted ed easi easily ly..  Transducers  Transducers are used for this purpose. %. Transmission lines :  These are used to carry the measuring signals from from the measuring device to the controller. controller.

&. The controller : 1

 This is the hardware element that has 'intelligence. It receives the information from measuring devices and decides what action should !e taen. *. The nal control element :  This is the hardware element that implements in real life the decision taen !y the controller. The control valve is the most frequently encountered nal element !ut not the only one. +ther typical nal control elements for a chemical processes are : ,elay switches" providing on-o control /aria!le-speed pumps /aria!le-speed compressors 0. Recording elements:  These are used to provide a visual demonstration of how a chemical process !ehaves.

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eTemperatur e recorder Desired

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 Ti

Thermoco uple

Ts Tank  CController

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Control ale Transmission line

!igure " : Hardware elements for the feed#ack control of a stirred tank heater

$se of Digital Computers in Process Control 3arge chemical plants such as petroleum reneries" ethylene plants" ammonia plants" and many others are under digital computer control. The eects have !een very su!stantial" leading to !etter control and reduced operating costs.

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ome applications characteristic of the diverse usage of digital computers : 1. Direct digital control %DDC& :  Today the chemical industry is moving more and more toward the 445 of the plants. 6 typical system of 445s for an ethylene plant can include !etween # and % control loops. 6ll the companies that furnish the control system for the chemical industry rely more and more on 445.

'( Superisory computer control : $any times the human operator does not or cannot nd the !est operating policy for a plant which will minimi7e the operating costs. This deciency is due to the enormous comple8ity of a typical chemical plant. In such cases we can use the speed and the programmed intelligence of a digital computer to analy7e the situation and suggest the !est policy. In doing so the computer coordinates the activities of  the !asic 445 loops ( igure #) #. Scheduling computer control : inally" the computer can !e used to schedule the operation of a plant.

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Distur#ances

)anipula ted .aria#les

Process

)easur ed *utput s

$nmeasured outputs **perator+s Console

Ccomputer,to, Process -nterface

DDigital Computer

PProcess,to, Computer -nterface

!igure ' : Typical DDC conguration

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computer for Superiso ry control

DDC !or $nit "

Process $nit "

DDDC !or $nit /

DDC !or $nit '

Process $nit '

Process $nit /

Chemical Plant !igure 0 : Structure of superisory computer control

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