Alarm management on HMI in PLC Programming - Step-by-Step Execution

Concept Flow - Alarm management on HMI

Start Monitoring Inputs

↓

Check Alarm Conditions

↓

Alarm Triggered?

No→Continue Monitoring

Yes↓

Log Alarm Event

↓

Display Alarm on HMI

↓

Operator Acknowledges?

No→Wait

Yes↓

Clear Alarm Display

↓

Return to Monitoring

The system continuously monitors inputs, triggers alarms when conditions occur, logs and displays them on the HMI, waits for operator acknowledgment, then clears alarms and resumes monitoring.

Execution Sample

PLC Programming

IF temperature > 100 THEN
  alarm_active := TRUE;
  DisplayAlarm('High Temp');
END_IF;

IF operator_ack THEN
  alarm_active := FALSE;
  ClearAlarm();
END_IF;

This code checks if temperature is too high, activates an alarm and shows it on HMI, then clears it when operator acknowledges.

Execution Table

Step	Input (temperature)	Condition (temp > 100)	Alarm Active	HMI Display	Operator Ack	Action
1	95	False	False	No Alarm	False	Continue Monitoring
2	105	True	True	Show 'High Temp'	False	Log and Display Alarm
3	105	True	True	Show 'High Temp'	False	Wait for Ack
4	105	True	False	No Alarm	True	Clear Alarm Display
5	95	False	False	No Alarm	True	Return to Monitoring

💡 Operator acknowledged alarm, alarm cleared, system resumes monitoring normal conditions.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	Final
temperature	95	95	105	105	105	95
alarm_active	False	False	True	True	False	False
HMI Display	No Alarm	No Alarm	Show 'High Temp'	Show 'High Temp'	No Alarm	No Alarm
operator_ack	False	False	False	False	True	True

Key Moments - 2 Insights

Why does the alarm stay active even when temperature remains high?

Because the alarm only clears when the operator acknowledges it (see steps 3 and 4 in execution_table). The system waits for acknowledgment before clearing.

What happens if the temperature drops below the threshold but the alarm is still active?

The alarm remains active until the operator acknowledges it, even if temperature drops (step 5 shows alarm cleared after ack, not just temp change).

Visual Quiz - 3 Questions

Look at the execution table, what is the alarm_active value at step 2?

A Undefined

B True

C False

D Depends on operator_ack

💡 Hint

Check the 'Alarm Active' column at step 2 in the execution_table.

At which step does the operator acknowledge the alarm?

A Step 4

B Step 1

C Step 3

D Step 5

💡 Hint

Look at the 'Operator Ack' column and see when it changes to True.

If the operator never acknowledges, what happens to the alarm display?

A It blinks then clears

B It clears automatically

C It stays displayed

D It resets after temperature normalizes

💡 Hint

Refer to steps 2 and 3 where operator_ack is False but alarm_active remains True.

Concept Snapshot

Alarm Management on HMI:
- Monitor inputs continuously
- Trigger alarm if condition met
- Log and display alarm on HMI
- Wait for operator acknowledgment
- Clear alarm after acknowledgment
- Resume monitoring

Full Transcript

This visual execution trace shows how alarm management works on an HMI in a PLC program. The system checks if temperature exceeds 100. If yes, it sets alarm_active to true and displays 'High Temp' on the HMI. The alarm stays active until the operator acknowledges it. Once acknowledged, the alarm clears and the display resets. The system then continues monitoring inputs. Variables like temperature, alarm_active, HMI display, and operator acknowledgment change step-by-step as shown in the execution table and variable tracker. Key moments clarify why alarms do not clear automatically and depend on operator action. The quiz tests understanding of alarm state changes and operator interaction.

Practice

1. What is the main purpose of alarm management on an HMI?

easy

A. To alert operators about abnormal conditions quickly

B. To slow down the machine operation

C. To display decorative graphics on the screen

D. To store historical data only

Solution

1. Step 1: Understand the role of alarms on HMI

   Alarms notify operators when something unusual or wrong happens in the system.

2. Step 2: Identify the main goal of alarm management

   The goal is to alert operators quickly so they can fix problems fast and keep the system safe.

3. Final Answer:

   To alert operators about abnormal conditions quickly -> Option A

4. Quick Check:

   Alarm management = alert operators fast [OK]

Hint: Alarms warn operators about problems fast [OK]

Common Mistakes:

• Thinking alarms slow down machines
• Confusing alarms with decorative features
• Assuming alarms only store data

2. Which of the following is the correct way to define an alarm condition in a PLC program for HMI?

easy

A. alarm = ON IF temperature == 100

B. alarm = temperature + 100

C. SET alarm WHEN temperature < 100

D. IF temperature > 100 THEN alarm ON

Solution

1. Step 1: Identify correct alarm condition syntax

   Alarm conditions usually use IF statements to check if a value crosses a limit.

2. Step 2: Check each option for proper condition

   IF temperature > 100 THEN alarm ON correctly uses IF temperature > 100 THEN alarm ON, which means alarm triggers when temperature is too high.

3. Final Answer:

   IF temperature > 100 THEN alarm ON -> Option D

4. Quick Check:

   Alarm condition uses IF and comparison [OK]

Hint: Alarm conditions use IF with comparison operators [OK]

Common Mistakes:

• Using assignment instead of condition
• Confusing < and > operators
• Writing incomplete or invalid syntax

3. Given this alarm logic snippet:

IF pressure < 50 THEN alarm = TRUE ELSE alarm = FALSE

What will be the value of alarm if pressure is 45?

medium

A. TRUE

B. 50

C. FALSE

D. Error

Solution

1. Step 1: Analyze the condition with given pressure

   The condition checks if pressure is less than 50. Since 45 < 50, the condition is true.

2. Step 2: Determine alarm value based on condition

   Because the condition is true, alarm is set to TRUE.

3. Final Answer:

   TRUE -> Option A

4. Quick Check:

   45 < 50 means alarm TRUE [OK]

Hint: Check condition result to set alarm TRUE or FALSE [OK]

Common Mistakes:

• Confusing TRUE and FALSE values
• Ignoring ELSE branch
• Assuming alarm equals pressure value

4. You wrote this alarm condition:

IF level > 80 THEN alarm = TRUE ELSE alarm = FALSE

But the alarm never triggers even when level is 90. What is the most likely error?

medium

A. The condition should use < instead of >

B. The alarm variable is not linked to the HMI alarm tag

C. The ELSE part should be removed

D. The level variable is misspelled

Solution

1. Step 1: Check the condition logic

   The condition level > 80 is correct for triggering alarm when level is 90.

2. Step 2: Consider why alarm does not trigger

   If alarm variable is not linked to the HMI alarm tag, the HMI won't show the alarm even if alarm = TRUE.

3. Final Answer:

   The alarm variable is not linked to the HMI alarm tag -> Option B

4. Quick Check:

   Alarm variable must connect to HMI tag [OK]

Hint: Check variable linkage to HMI alarm tag [OK]

Common Mistakes:

• Changing correct condition operator
• Removing ELSE causing undefined alarm state
• Assuming variable spelling error without proof

5. You want to create an alarm on the HMI that triggers only if temperature is above 100°C and pressure is below 30 PSI. Which of these alarm conditions is correct?

hard

A. IF temperature > 100 AND pressure > 30 THEN alarm = TRUE ELSE alarm = FALSE

B. IF temperature > 100 OR pressure < 30 THEN alarm = TRUE ELSE alarm = FALSE

C. IF temperature > 100 AND pressure < 30 THEN alarm = TRUE ELSE alarm = FALSE

D. IF temperature < 100 AND pressure > 30 THEN alarm = TRUE ELSE alarm = FALSE

Solution

1. Step 1: Understand the alarm trigger conditions

   The alarm should trigger only when temperature is above 100 AND pressure is below 30 simultaneously.

2. Step 2: Check each option's logic

   IF temperature > 100 AND pressure < 30 THEN alarm = TRUE ELSE alarm = FALSE uses AND with correct comparisons: temperature > 100 AND pressure < 30, matching the requirement exactly.

3. Final Answer:

   IF temperature > 100 AND pressure < 30 THEN alarm = TRUE ELSE alarm = FALSE -> Option C

4. Quick Check:

   Use AND for both conditions true [OK]

Hint: Use AND to combine multiple alarm conditions [OK]

Common Mistakes:

• Using OR instead of AND
• Reversing comparison operators
• Mixing up greater than and less than