computer is ed process control

8/14/2019 Computer is Ed Process Control

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COMPUTERISEDPROCESSPROCESS

CONTROLBY

SADASIVAN J


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Fundamentals of automaticprocess control

Process:

As a set of different sub processes and activitiesbetween them

Control variable: the parameter which we want tocontrol is called control variable

manipulated variable : The variable which ismanipulated to correct the control variable is calledmanipulated variable or controlled variable

In water heating system , we have to control the temp.of the water coming out. This can be controlled byvarying the steam flow rate . Asume that the temp.of the steam is constant. Out put temp. can bevaried by steam flow rate.Here the tem.of the waterwhich we want to control s called control variable


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Temp is controlled bymanipulating steam flow rate .sosteam flow rate is called

manipulated variable orcontrolled variable


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Other factors that affect the

output temp. are temp. of theincoming water , atmospherictemp etc. These are called loadvariables


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AUTOMATIC PROCESSAUTOMATIC PROCESSCONTROLLERCONTROLLER

IS DEFINED AS A MECHANISM THAT MEASURESIS DEFINED AS A MECHANISM THAT MEASURESTHE VALEU OF CONTROL VARIABLE ANDTHE VALEU OF CONTROL VARIABLE ANDMANIPULATES THE MANIPULATED VARIABLE ORMANIPULATES THE MANIPULATED VARIABLE OR

CONTROLLED VRIABLE TO LIMIT THE DEVIATIONCONTROLLED VRIABLE TO LIMIT THE DEVIATIONOF CONTROL VARIABLE FROM THE SET POINTOF CONTROL VARIABLE FROM THE SET POINT


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FEED BACK CONTROL

A closed loop system or feed backsystem measures the actualsystem output compares it with set

value and determines the error .The error value is then used tocontrol the system output in order

to get the desired output


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Basic of principles of f/b control

Here the level to be controlled issensed and transmitted by LevelTransmitter and sends a signal tothe controller proportional to thelevel in the tank

The contoller consists of three basicparts

1. Memory for the desired level orset point in the tank 2. Comparator for set point Vs

measured value as reported by the

transmitter , producing thedifference between the them,


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3. an amplifier amplifies the errorand produces the output of thecontroller


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VARIOUS TYPES OFCONTROLS

1 Two position control

2 Multi position control

3 PID control 4 Ratio control

5 Cascade control

6. Feed forward control

T iti t l ( / ff


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Two position control (on/offcontrol)

` The simplest and cheapest form ofcontrol is two position control

In this mode of control the finalcontrol element is moved quicklyfrom one of the two fixed positionsto other depending upon wheather

the control variable is greater thanthe set point


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On/off control withdifferential gap

The valve is closed as the controlvariable crosses the high set valueand shall remain closed till it

crosses the low set value , then thevalve is opened again


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2.MULTI POSITION CONTROL

In this the final control element ismoved to one of three or morefixed positions .In this control the

valve is closed when the controlvariable is above the high set value. When it ismid band , the valve is

half open and when it is lower thanthe low set value the valve is fullyopen


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PID control

As the name suggests, the PID algorithmconsists ofthree basic modes:

the Proportionalmode,

the Integralmode& the Derivative mode


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Definitions In the time domain:

++=

++=

dttdeTdtte

TteK

dt

tdeKdtteKteKtu

d

t

i

p

d

t

ip

)()(1)(

)()()()(

0

0

i

dd

i

p

iK

KT

K

KTwhere == ,

p ro p o rtio n a l g a in in te g ra l g a in

d e riv a tiv e g a in

e riva tive tim e con sta n tn te g ra l tim e co n sta n t


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Controller Effects

A proportional controller (P) reduces errorresponses to disturbances, but still allowsa steady-state error.

When the controller includes a termproportional to the integral of the error (I),then the steady state error to a constant

input is eliminated, although typically atthe cost of deterioration in the dynamicresponse.

A derivative control typically makes the


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Closed-loop Response

Rise time Maximumovershoot

Settlingtime

Steady-stateerror

P Decrease Increase Smallchange

Decrease

IDecrease

Increase IncreaseEliminate

D Smallchange

Decrease Decrease Smallchange


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Conclusions

Increasing the proportional feedback gainreduces steady-state errors, but highgains almost always destabilize thesystem.

Integral control provides robust reductionin steady-state errors, but often makesthe system less stable.

Derivative control usually increasesdamping and improves stability, but hasalmost no effect on the steady state error


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MULTIVARIABLE CONTROL

In real ,the control depends on manycontrol variable s, though there isonly one controlled variable for

each control loops. Such controlloops are known as multi variablecontrol

They are Cascade control

Ratio control


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4. CASCADE CONTROL


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C a sca d e S y ste m s

C a sca d e C o n tro l S yste m s co n ta inin te g ra te d se ts o f co n tro l lo o p s

:Prim a ry Lo o p M o n ito rs th e co n tro l

v a ria b le a n d u se s d e v ia tio n fro m itssetp o in t to p ro v id e a n o u tp u t to th e

.se co n d a ry lo o p :S e con d a ry Lo o p R e ceive s its

se tp o in t fro m th e p rim a ry lo o p a n dco n tro ls th e re fe re n ce v a ria b le

.a cco rd in g ly


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B e n e fits o f C a sca d e C o n tro l

E ffe ctiv e ly a cco u n ts fo r e x te rn a ld istu rb a n ce s

R e d u ce s d e a d tim e in v a ria b leresponse

C o m p a tib le w ith o th e r C o n tro l, -S y ste m s su ch a s Fe e d B a c k a n d

-Fe e d Fo rw a rd C o n tro l A rch ite ctu re s


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D isa d va n ta g e s o f C a sca d eC o n tro l

M u ltip le co n tro l lo o p s m a ke p h y sica la n d co m p u ta tio n a l a rch ite ctu re m o re

co m p le x

A d d itio n a l co n tro lle rs a n d se n so rs ca nb e co stly


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RATIO CONTROL

In ratio control system , a dependentvariable ( called secondary variable) is measured as a function of an

independent variable


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Objective:To maintain the ratio of twovariables at a specified value R=u/d

U and d are physical variables


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Examples

1.In automobile. : a particular ratio oftetra ethyl lead to gasoline is tobe maintained to produce the

desired octane number 2 In chemical reactor: particular ratio

of different components is to bemaintained

3. Cement kiln: speed of rotor Vsslurry flow rate is to be controlled

4. In boiler control: control of air flow

Vs steam flow


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FEED FORWARD CONTROL


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Feedback FeedforwardAdvantages

Not require identification andmeasurement of disturbance

Acts before a disturbance has been.felt by the system

.Insensitive to modeling errors Good for slow systems or.significant dead time

Insensitive to parameter.changes

Not introduce instability in the- .closed loop response

Disadvantages

Control action is taken after.disturbances have been felt Requires all disturbances and.their direct measurement

Unsatisfactory for slow and.significant dead time process

Cannot cope with unmeasured.disturbances

It may create instability in- .the closed loop response

Sensitive to process parameter.variations

mpare Feedback with Feedforwardontrol


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ELEMENTS OF PROCESS CONTROL

Process

Measurement

Error Detector

Controller

Control Element


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PROCESS CONTROL BLOCK DIAGRAM

SummingPoint

c

u

b

p

Controller

Control Element

Process

Measurement

r

= -e rb

p = Controller output c = Controlled variable b = measured value ofce = Error signal r = Set point u = Controlling variable

Block diagram of basic process control loop.


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:Example

Physical diagram for flow process control system


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Controller

Measurement

Process

Final ControlElement

( )Controller

/ ,I P Converter,Actuator Valve

&Pipe Flow

,Orifice plate DP, /Cell P I Converter

r

(Error)Detector

Block diagram for flow process control system.


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ANALOG DATA REPRESENTATION

Analog Standards for representing range ofvariable in Process Control Systems:

Current Signal : 4 to 20 mA

Pneumatic Signal : 3 to 15 psi (20 to 100kPa)


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Why Current Signal?Current is used instead of voltage because the

system is then less dependent on load.

Transmission of current signal is independent of line resistance


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FINAL CONTROL OPERATIONActuator

FinalControl

Element

Process

SignalConversionControl

Signal

Elements of Final Control Operation


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ANALOG & DIGITAL I/P


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ANALOG & DIGITAL I/PMODULE

These are the modules whichconnect the process to the realtime system/data processing unit

ANALOG I/P SIGNALS SENSORS measures the physical

quantities like pressure, position,velocity , temp. etc

SIGNAL CONDITIONER:takes the o/pof the sensor, amplied, filtered,isolated sothat it is accetable to the

real time system

DIGITAL I/P SIGNAL


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DIGITAL I/P SIGNAL

refers to the ON/OFF positions of

various valves , limit swithesetc,these signals can be inputteddirectly into the real time system.In some cases signal conditioner is

required to filter the noises .


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Analog i/p module


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The module continuously scans theanalog input signals in thepredefined order and frequency

,converts them into digital and thensends these values to processor

more sophisticated input modulecontains local intelligence , ie itinclude processor also


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Analog I/P module with localintelligence

the processor initiates the multiplexerby sending the address of inputchannel

The multiplexer connectesthe input

channel to ADC The processor sends the start of

convertion signal to ADC . The ADCconverts the analog signal to digital,

puts it at the output and issues end ofconversion signal Processor on receipt of EOC signal ,

reads the ADC output and stores it in

memory

DIGITAL I/O MODULE


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DIGITAL I/O MODULE

The digital iputs can be given directly

to the processor, no ADC isrequired


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DIGITAL I/P MODULE WITH


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DIGITAL I/P MODULE WITHINTELLIGENCE

ANALOG O/P MODULE


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ANALOG O/P MODULE


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ANALOG O/P MODULE

The objective of the analog output

module is to provide controlsignals to final control elements.The demultiplexer switches the

digital output received from themaster processor to the out putchannel whose address is specifiedby the processor(ie ,the masterprocessor).The DAC of perticularchannel converts the digital valueto equivalent analog signal which is

connected to final control element


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INTERRUPT CONTROL MODULE

Interrupt signals: are generated

when there is abnormal situationsin the environment or in theprocess . Common abnormalities

are 1. exceeding of certain limits of some

parameters like temp. , pressure ,

flow etc. 2. power failure in which case all

parameters must be stored andwork may be suspended

One interrupt signal ill corresponds


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One interrupt signal will correspondsto one particular abnormality

real time systems attendsabnormalities by executing specialprograms called interrupt servicingroutines


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DISPLAY CONTROL MODULE

Display output signals: drive displaydevices like LED, LCD, alarmsetc.signalsrelated to the status of

process , various control valve, limitset for various parameters in theprocess to be monitored are also

displayed

Display control module consists of


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Display control module consists offollowing independent sub

modules 1. manual entry sub module

2. CRT controller sub module

3. LED/LCD control sub module 4. Alarm annunciator sub module

5. Printer controller sub module


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Manual entry sub module: includeON /OFF command switches, key

boards etc.

CRT controller sub module: interfaces

main processor to visual displayunit , which is used to show thestatus of the process by displaying

the transducer values , present setvalue entered through manualentry sub module, historical trendof various parameters etc.


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LCD/LED control sub module:interfaces the LED/LCD to mainprocessor .this module accepts datafrom main processor and displays iton LED/LCD

Alarm annunciatorcontroller submodule: generates ON/OFF signal for

each type of alarm.Thesignals aredigital to light the incandescent bulbsand audio alarms

Printer controller sub module : is printerinterface to main processor .Generallyit has local intelligence for printercontrol , data buffer to store the data

for printing


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SCADA

S C A D A


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S C A D A is S u p e rviso ry C o n tro lA n d

D a ta A cq u isitio n UPERVISORY CONTROL ,o f e q u ip m e n t. . -e g closing and tripping of swith gears

and tap changing of transformers in,power plants controlling process.parameters in process plants

,ATA ACQUISTION . .i e ability to getvarious information from the field bysome means


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The basic functions of SCADA are


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The basic functions of SCADA are

1. channel scanning

2. Conversion into engineering units 3. Data processing


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Channel scanning

Scanning is the process by whichmicroprocessor address variouschannels and read the data.Thera

are two types scanning 1. polling

2. Interrupt scanning


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POLLING

Polling is the process of scanning thechannel to read data

The channel selection may be

sequential or in any perticular orderdecided by the designer

It is also possible to assign priority to

some channels over the other Channel scanning and reading of

data requires the following actions

to be taken

1 sending channel address to


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1. sending channel address tomultiplexer

2. sending start of convertion pulse

to ADC

3. reading the digital data


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Channel scan array


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The channel can be polledsequentially, in which case the

channel address is increased oneby one

In another case a channel scan array

is maintained in memory ,the scanarray contains the address of thechannel in the order they should be

addressed In the above channel scanning array ,

the first channel to be scanned ischannel 9, followed by 10, 1,2 as

If a channel number is repeated in


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If a channel number is repeated inthe array , then that

perticularchannel will be scannedrepeatedly, in the above casechannel 9 is scanned 3 times ,channel 2 is acanned 2 times while

other channels are scanned onceduring a cycle

it is also possible to schedule the

time gap between two channels

h l i i h i


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Channel scanning with time

i


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Interrupt scanning

The transducer check for anyviolation of limits , if the signal isnot within the high and low limit ,

then the limit checking circuitwould send interrupt request tomicroprocessor which inturn wouldmonitor all parameters till theparameter values come back within pre specified limits

Conversion into engineering


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Conversion into engineeringunits

The transmitter output should be inthe range of 0-5v or 4-20ma .Depending on the input range of

measured value for transmitter acalibration factor is determined

If a transmitter is capable ofmeasuring parameter within the

input range x1 and x2 and provides0-5v signal at output then calirationfactor is

1 volt=(x2-x1)/5 units

If a 8bit unipolar ADC is used the


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If a 8bit unipolar ADC is used theoutput rang 0 -255 ie for 0v output

is 0 for 5v output is 255

1v=255/5 =(x2-x1)/5

If ADC output is Y then thecorresponding value in engineeringunits will be Y(x2-x1)/255

The conversion factor is (x2-x1)/255

D t i


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Data processing

The data read from the ADC forvarious channel is processed by themicroprocessor to carryout limit

checking and performanceanalysis . For limit checking thehighest and lowest limits for eachchannels are stored in the array ifany of the two limits are violatedfor any channel , appropriate actionis initiated

Li it h ki ti


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Limit checking routine

Li it h ki ti


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Limit checking routine

from the ADC output , themicroprocessor analysis theperformance of the system for

managerial level. This report willenable the managers to visualizethe problem in the system and totake decision regarding system

modification or alternateoperational strategy to increase thesystem performance . The analysismay include histogram generation,

standard deviation ,plotting one

Di t ib t d SCADA t


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Distributed SCADA system

D i h i fi ti


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Daisy chain configuratin

R t t i l it(RTU)


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Remote terminal unit(RTU)

computer is ed process control

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