ch5 computer j5800

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APPLICATION OF COMPUTERIN PROCESS CONTROL

This topic shall cover the digital andanalogue input terminologies, thedigital and analogue output, data

conversion devices (ADC and DAC),telemetry, multiplex and de-multiplex.


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5/5/12introduction

The use of digital computers to controlindustrial processes had its origins in thecontinuous process industries in late 1950s.

At that time, the computer technology was inits infancy, and only computers available forprocess control were large, expensivemainframes.

The computer that were installed sometimescost more than the process they controlled.

Around 1960s the digital computer started toreplacing the analog controllers in continuous

process control application.


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Example of process control in a paint factoryin which two colors, blue and yellow, aremixed to produce green.

To keep the output color constant, the exactproportional of blue and yellow must bemaintained.

The setup accomplishes this with flow valves

1 & 2, which are manually adjusted until thedesired hue (warna) of green is achieved.

The problem is that, as the level of paint inthe vats (bekas) change, the flow will change

and the mixture will not remain constant.


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To maintain an even flow from the vats, wecould add two electrically operated flowvalves (and their control). Each valve wouldmaintain a specified flow of paint into mixer,

regardless of the upstream pressure.

Theoretically, if the blue and yellow areindependently maintained, the green shouldstay constant.

In practice, however, other factors such astemperature, or humidity may affect themixing chemistry and therefore the outputcolor.


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A better approach might be the system shownin figure (c); a single sensor monitors theoutput color. If the green darkens, thecontroller increases the flow of yellow. If the

green gets too light, the flow of yellow isincreased.

This system is desirable because it monitorsthe actual parameter that needs to be

maintained.

In real life, such a straightforward system maynot be possible because sensors that can bemeasure the output directly may not exits

and/or the process may involve many


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In a large plant such as a refinery, many

processes are occurring simultaneously andmust be coordinated because the output ofone process is the input of another.

In early days of process control, separate

independent controllers were used for eachprocess, as shown in figure (a).

The problem with this approach was that, tochange the overall flow of the product, each

controller had to be readjusted manually.


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In the 1960s, a new system was developed inwhich all independent controllers werereplaced by single large computer. Illustratedin figure (b), this system is called DIRECT

DIGITAL CONTROL (DDC).

The advantage of this approach is that alllocal processes can be implement, monitored,and adjusted from the same place.

Also, because the computer can see thewhole system, it is in position to makeadjustments to enhance total systemperformance.

The drawback is that the whole lant is

DDC


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5/5/12DCS

The advent of small microprocessor-basedcontroller has led to a new approach called aDISTRIBUTED CONTROL SYSTEM (DCS).Illustrated in figure (c).

In this system, each process has its ownseparate controller located at the site. Theselocal controllers are interconnected via a localarea network so that all controllers on the

network can be monitored or reprogrammedfrom a single supervisory computer.

Once programmed, each process is essentiallyoperating independently.


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This makes for a more robust and safesystem, because all the local processes willcontinue to function even if the supervisory

computer or network goes down. For example, a local controller whose job it is

to keep some material at the criticaltemperature will continue to function even if

the supervisory computer is temporarilydisabled.

Increasingly, the components of a controlsystem are being interconnected with the

business office network in a factory, whichallows the status of any process in the factoryto be examined by any computer on anyonesdesk.

You might be sit down at a PC anywhere in thebuildin and determined whether a articular


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5/5/12SEQUENTIALLY CONTROLLED SYSTEM

Defined as a series of tasks to be performed that is, a sequence of operations, one afterthe other.

Each operation in the sequence is performedeither for a certain amount of time, in whichcase it is a time-driven operation, or untilthe task finished, in which case it is an event-driven operation.

Example: automatic washing machine (time-driven operation), traffic signal (event-drivenoperation).


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Many automated industrial processes could beclassified as sequentially controlled systems.

An example is a process where parts areloaded into trays, inserted into furnace for 10minutes, then removed and cooled for 10minutes, and loaded into boxes in groups ofsix.

In the past, most sequentially controlledsystems used limit switches, relays andelectromechanical timers to implement thecontrol logic.

These tasks are now performed more andmore by small computer known as


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5/5/12Programmable logic controllers

A PLCs is a self-contained microprocessor-based unit, designed specifically to be acontroller.

The PLC includes an I/O section that caninterface directly to such system componentsas switches, relays, small motors, and lights.

Developed in the late 1960s to replace relaylogic controllers, PLCs were originally used inapplications where the input and outputs wereall on/off type signals, but over the yearsthey have evolved to be able to handlesophisticated motion control applications as

well.


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Figure below shown a selection of PLCs.


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5/5/12MICROPROCESSOR

The digital integrated circuit (IC) called amicroprocessor [figure (a)], has ushered in awhole new era for control system electronics.

This revolution has occurred because themicroprocessor brings the flexibility ofprogram control and the computational powerof computer to bear on any problem.

Automatic control applications are particularlywell suited to take advantage of thistechnology, and microprocessor-basedcontrollers system are rapidly replacing manyolder control systems based on analog circuit

or electromechanical relays.


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Microprocessor by itself is not a computer;additional components such as memory andinput/output circuit are required to make itoperational.

However, microcontroller [figure (b)], which isa close relative of the microprocessor, doescontain all the computer functions on a singleIC.

Microcontrollers lack some of the power andspeed of the newer microprocessor, but theircompactness is ideal for many controlapplications; most so-called microprocessor-

controller devices, such as vending machines,


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Some specific reason for using adigital microprocessor design incontrol system:

Low-level signals from sensors, onceconverted to digital, can be transmitted longdistances virtually error-free.

Long-term memory is available to keep trackof parameters in slow-moving system.

Changing the control strategy is easy byloading in a new program; no hardwarechanges are required.

Microprocessor-based controlled are moreeasily connected to the computer networkwithin an organization. This allows designersto enter the program changes and readcurrent system status from their desk


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5/5/12Microprocessor system hardware


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Central processing unit (CPU)

Does the actual computing and is composed oftwo subparts: the arithmetic logic unit (ALU) andcontrol sections.

ALU: performs the actual numerical and logiccalculations such as addition, subtraction, AND,OR and so on.

The control section manages the data flow, such

as reading and executing the programinstructions.

If data require calculations, the control sectionhands it over to the ALU for processing.

Memory


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INTERFACING TO A MICROPROCESSORCONTROLLER

In important part of any control system is thelink between the controller and the real world.

For a digital controller, data enter and exitthrough:

a parallel interface or

a serial interface.


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5/5/12Parallel interface

Transfers data 8 bits (or more) at the sametime, using eight separate wires.

The parallel interface is ideal for inputting oroutputting data from devices that are eitheron or off.

In other applications, the controller may use aparallel interface to connect to an analogdevice for example, driving a variable-speeddc motor.

In such a case, the binary output of thecontroller must first be converted into ananalog voltage before it can drive the motor.


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DIGITAL-TO-ANALOG CONVERSION (DAC)

DAC is a circuit that converts a digital word intoan analog voltage.


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ANALOG-TO-DIGITAL CONVERSION (ADC)

ADC is a circuit that converts an analog voltageinto a digital word.

A typical ADC consists of a single IC with fewsupport components.

ADC is more complicated process than DAC.


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5/5/12A control system using parallel

port


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5/5/12Serial interface

The data are sent 1 bit after the other on asingle wire.

There are a number of good reason for doingthis.

1st the cabling is simpler because only twowires are needed (at minimum), those beingdata and return.

2nd shielding a small group of wires, whichnecessary in an electrically noisy industrialenvironment, is easier.

3rd , serial data can make use of existing

single-channel data lines such as the


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Because data always exits in a parallel forminside the computer, it must be converted toserial data before coming out the serial port.

This is accomplished with a special parallel-to-serial converter IC called a universalasynchronous receiver transmitter (UART).

On the other end of the line, a receiver mustconvert the serial data back to parallel data,which is done by another UART. Next Figureshown the basic serial data circuit


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5/5/12Rs 232

In order to make the serial interface practical,a set of specifications called RS 232 standardwas established.

Officially, the RS 232 standard specifies theserial data interface between DATA TERMINALEQUIPMENT (DTE) and DATA COMMUNICATIONEQUIPMENT (DCE).

A common application of RS 232 is theinterface between a PC and the modem, inwhich case the computer is the DTE and themodem is the DCE as shown in next figure.


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5/5/12networking

Probably the most common use of serial datais in networking.

More and more, networking are being used tointerconnect all the units and devices in thecontrol system.

Network cabling differs depending on the typeof LOCAL AREA NETWORK (LAN), but many,such as Ethernet, use the generalized bussystem diagrammed in figure in next slide.


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5/5/12Quiz

1. Give TWO reason for using a digitalmicroprocessor design in control system [4marks]

2. State TWO type of parallel interface. [2marks]

3. Give TWO type of microprocessor hardware[ 2 marks]

4. What is the different between DDC and DCS?[2 marks]


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Refe

rr

ences

W.Bolton.Contro

lEngineering,2ndEdition

.Prentice hall.1998

T. K,Christopher.Moder

ch5 computer j5800

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