CHAPTER 23 Alarm Event Programming - Cengage

1 CHAPTER 23 Alarm & Event Programming Objectives After completing this CHAPTER , you should have the knowledge to: Understand Alarm Programming . Apply the various techniques to capture Alarm and Event information. Many PLC applications require some form of alarming when abnormal conditions or events occur. PLCs are well suited for this task and in fact, many PLC manufacturers provide special instructions designed just for this purpose. Some of these special instructions will be covered later in this CHAPTER . Alerting operating or maintenance personnel of abnormal conditions or problems saves time and increases machine or process up time. When detailed alarming is provided, many problems can be resolved by operators themselves rather than calling a technician to find the problem saving time and increasing productivity.

2 When a PLC system is connected to one of the many HMI interfaces available on the market today, the HMI can provide all of the alarming functions necessary with minimal additional PLC Programming . Most HMI software packages provide some form of alarming and Event recording. These HMI systems continually monitor the PLC s internal data files or tags and based on how the HMI is programmed will Alarm accordingly. When HMI interfaces with built in alarming features are not part of the overall system or when events must be captured faster than an HMI system can monitor, the alarming and Event recording must be programmed in the PLC. This CHAPTER is dedicated to showing the reader some of the techniques and instructions used to program Alarm and Event information in PLCs. BASIC Alarm LOGIC Alarms and events can be triggered from digital information such as a motor failing to start or run.

3 They can be triggered from process analog values such as a high temperature or low pressure. Whether monitoring a digital or analog value the end result is the same, a bit or tag is typically set to a 1 or ON indicating that a problem has been detected. Figure 23 1 illustrates an example of this basic logic. RanananwmAfswthfrotA reprBobeung 1 in Figurn Alarm if thenalog tempern internal bit with an actual monitored by after studying witch is turneherefore the mraction of a sether PLC logicsimilar probleached 101 frocess, the opoth problemselow in Figurere 23 1 monite motor is notrature input (or tag that, woutput that ian HMI interfRung 1 in Figed ON, the stamotor Alarm econd may noc that uses thlem can be sefor only a spliperator may ns described abe 23 2. Figutors the motot running wheN7:1) and alawhen ON, indilluminates a face system.

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7 Larm on a dece in Figure 23 o ensure that the Alarm is sATE OF CHions require thange and alainstructions tion period (30imer preset vF8:1) that waed in F8:2 andd requires a cg value when This logic mocreasing rate Fi 3 that the alathe Alarm remtill active. HANGE ALAthat analog vaarm if such a to create a ra0 seconds). Thvalue. Rung 4 s stored at thd can be chancorrespondingthe PLC is firsonitors for a pof change or igure 23 3 Latarm reset (unmains set for ARM LOGI alues such ascondition exite of change his evaluationcompares thhe end of the nged to any dg unlatch instst started so tpositive (increboth if desiretching Alarm nlatch) rung isthe entire PLC temperatureists. The logicalarm. Rung n period can e current analast evaluatioesired value. truction to resthat a false aleasing) rate ofed. Logic s located aboLC program sce, pressure, lec in Figure 23 2 in Figure 23be set to anyalog value stoon period.

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