Creating Alarms

8/10/2019 Creating Alarms

1/15

CHAPTER23 Alarm&EventProgramming

Objectives

Aftercompletingthischapter,youshouldhavetheknowledgeto:

Understand

alarm

programming.

Applythevarioustechniquestocapturealarmandeventinformation.

ManyPLCapplicationsrequiresomeformofalarmingwhenabnormalconditionsoreventsoccur.PLCsare

wellsuitedforthistaskandinfact,manyPLCmanufacturersprovidespecialinstructionsdesignedjustfor

thispurpose. Someofthesespecialinstructionswillbecoveredlaterinthischapter.

Alertingoperatingormaintenancepersonnelofabnormalconditionsorproblemssavestimeandincreases

machineorprocessuptime.Whendetailedalarmingisprovided,manyproblemscanberesolvedby

operatorsthemselvesratherthancallingatechniciantofindtheproblemsavingtimeandincreasing

productivity.

WhenaPLCsystemisconnectedtooneofthemanyHMIinterfacesavailableonthemarkettoday,theHMI

canprovideallofthealarmingfunctionsnecessarywithminimaladditionalPLCprogramming.MostHMI

softwarepackagesprovidesomeformofalarmingandeventrecording. TheseHMIsystemscontinually

monitorthePLCsinternaldatafilesortagsandbasedonhowtheHMIisprogrammedwillalarmaccordingly.

WhenHMIinterfaceswithbuiltinalarmingfeaturesarenotpartoftheoverallsystemorwheneventsmust

becapturedfasterthananHMIsystemcanmonitor,thealarmingandeventrecordingmustbeprogrammed

inthePLC.Thischapterisdedicatedtoshowingthereadersomeofthetechniquesandinstructionsusedto

programalarmandeventinformationinPLCs.

BASIC

ALARM

LOGIC

Alarmsandeventscanbetriggeredfromdigitalinformationsuchasamotorfailingtostartorrun. Theycan

betriggeredfromprocessanalogvaluessuchasahightemperatureorlowpressure. Whethermonitoringa

digitaloranalogvaluetheendresultisthesame,abitortagistypicallysettoa1orONindicatingthata

problemhasbeendetected. Figure231illustratesanexampleofthisbasiclogic.


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12/15

Figure2312AlarmLogicExampleForUpTo80Alarms


13/15

TheresetoutputinstructionthatfollowseachFALinstructionensuresthattheFALisresetbeforethenext

programscansothatnoalarmsarelost.

Theexamplesshownsofarinthischapterareonlysomeofthemanywaysthatalarminginformationcanbe

handledinthePLC.MostHMIsystemstodayhaveveryextensivealarmingandeventrecordingcapabilities

thatmakeprogrammingalarmlogicinthePLCalmostobsolete.Thequestionsyoumustaskyourselfis,does

theHMIupdatefastenoughtocaptureyouralarmswithoutaddingadditionalPLClogic.

ALLENBRADLEYLOGIX5000ALMD&ALMAINSTRUCTIONS

TheAllenBradleyLogixfamilyofPLCprocessorshastwoalarm/eventinstructionsdesignedspecificallyfor

monitoringdigitalandanalogdataforabnormalconditionsasprogrammedbytheuser. TheseLogixbased

alarminstructionsareavailableforuseinrelayladder,structuredtext,andfunctionblockdiagramprograms.

Theadvantagetousingthesetwoinstructionsisthatwhenanalarmisdetected,thecontrollerwillpublish

theeventtoFactoryTalkViewAlarmsandEventsserversthatpropagatealarmstoFactoryTalkViewSE

clients(HMIs)thatsubscribetoreceivenotifications.

TheDigital

Alarm

(ALMD)

instruction

detects

digital

alarms

based

on

Boolean

(true/false)

conditions.

The

instructionisanoutputinstructionthatobtainsitsalarmconditionfromtherungcondition.Someofthe

featuresoftheALMDinstructionsincludealarmacknowledge,latch,minimumdurationtimer,alarm

counter,timestamp,severity,etc.. Figure2313Rung1showsandexampleoftheALMDinstruction.

TheAnalogAlarm(ALMA)instructiondetectsanalogalarmsbasedonthelevelorrateofchangeofananalog

value.Theinstructionisanoutputinstructionandistypicallyplacedonanunconditionalrung.Someofthe

featuresoftheALMAinstructionsincludealarmacknowledge,highandhigh/highlimits,lowandlow/low

limits,deadband,rateofchange,minimumdurationtimer,alarmcounter,timestamp,severity,etc.. Figure

2313Rung2showsanexampleoftheALMAinstruction.

Becauseof

the

many

features

of

both

of

these

instructions

it

is

not

practical

to

include

acomplete

descriptionhere.Pleaserefertothemanufacturersinstructionmanualforacompletedescription.


14/15

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15/15

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Forfallbelow

Documents

Creating Alarms