Alarm management on HMI in PLC Programming - Mini Project: Build & Apply

Alarm management on HMI

📖 Scenario: You are working on a Human-Machine Interface (HMI) for a factory. The HMI needs to manage alarms from different machines. Each alarm has a unique code and a message. You want to create a simple program that stores alarms, sets a threshold for active alarms, filters alarms that are active, and then displays them on the HMI screen.

🎯 Goal: Build a PLC program that stores a list of alarms with their status, sets a threshold for how many alarms can be active before alerting the operator, filters the active alarms, and displays the active alarms on the HMI.

📋 What You'll Learn

Create a data structure to hold alarms with their codes and active status

Add a threshold variable for the maximum allowed active alarms

Write logic to filter only active alarms

Display the active alarms on the HMI

💡 Why This Matters

🌍 Real World

Factories use HMIs to show machine alarms so operators can quickly see and fix problems. Managing alarms clearly helps keep machines safe and running.

💼 Career

PLC programmers and automation engineers often write alarm management code to improve factory safety and efficiency.

1

Create the alarm data structure

Create a variable called alarms as an array of 5 elements. Each element should be a structure with two fields: code (an integer) and active (a boolean). Initialize the array with these exact values: {code: 101, active: TRUE}, {code: 102, active: FALSE}, {code: 103, active: TRUE}, {code: 104, active: FALSE}, {code: 105, active: TRUE}.

PLC Programming

// Define the alarms array with 5 elements
// Your code here

Hint

Use a STRUCT type to define the alarm with code and active fields. Then create an array of 5 alarms with the exact values.

2

Set the active alarm threshold

Create a variable called activeAlarmThreshold of type INT and set it to 2. This variable will represent the maximum number of active alarms allowed before alerting the operator.

PLC Programming

TYPE Alarm : STRUCT
    code : INT;
    active : BOOL;
END_STRUCT
END_TYPE

VAR
    alarms : ARRAY[1..5] OF Alarm := [
        (code := 101, active := TRUE),
        (code := 102, active := FALSE),
        (code := 103, active := TRUE),
        (code := 104, active := FALSE),
        (code := 105, active := TRUE)
    ];
    // Define the active alarm threshold
    // Your code here
END_VAR

Hint

Declare an INT variable named activeAlarmThreshold and assign it the value 2.

3

Filter active alarms

Create a variable called activeAlarms as an array of INT with size 5. Write a FOR loop using the iterator i from 1 to 5. Inside the loop, check if alarms[i].active is TRUE. If yes, assign alarms[i].code to activeAlarms[i], otherwise assign 0.

PLC Programming

TYPE Alarm : STRUCT
    code : INT;
    active : BOOL;
END_STRUCT
END_TYPE

VAR
    alarms : ARRAY[1..5] OF Alarm := [
        (code := 101, active := TRUE),
        (code := 102, active := FALSE),
        (code := 103, active := TRUE),
        (code := 104, active := FALSE),
        (code := 105, active := TRUE)
    ];
    activeAlarmThreshold : INT := 2;
    activeAlarms : ARRAY[1..5] OF INT;
END_VAR

FOR i := 1 TO 5 DO
    // Check if alarm is active and assign code or 0
    // Your code here
END_FOR

Hint

Use a FOR loop from 1 to 5. Inside, use an IF statement to check alarms[i].active. Assign the alarm code to activeAlarms[i] if active, else assign 0.

4

Display active alarms on HMI

Write code to print the active alarms codes from activeAlarms array. Use a FOR loop with iterator i from 1 to 5. For each element, if the value is not 0, print "Active Alarm Code: " followed by the alarm code.

PLC Programming

TYPE Alarm : STRUCT
    code : INT;
    active : BOOL;
END_STRUCT
END_TYPE

VAR
    alarms : ARRAY[1..5] OF Alarm := [
        (code := 101, active := TRUE),
        (code := 102, active := FALSE),
        (code := 103, active := TRUE),
        (code := 104, active := FALSE),
        (code := 105, active := TRUE)
    ];
    activeAlarmThreshold : INT := 2;
    activeAlarms : ARRAY[1..5] OF INT;
    i : INT;
END_VAR

FOR i := 1 TO 5 DO
    IF activeAlarms[i]  0 THEN
        // Print active alarm code
        // Your code here
    END_IF;
END_FOR

Hint

Use a FOR loop to check each element in activeAlarms. If the value is not zero, print the alarm code with the message.

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