Assignment and arithmetic in PLC Programming - Mini Project: Build & Apply

Assignment and Arithmetic in PLC Programming

📖 Scenario: You are programming a simple PLC (Programmable Logic Controller) to control a water tank system. The system needs to calculate the total water volume after filling the tank for a certain time.The PLC program will use variables to store values and perform arithmetic operations to find the total volume.

🎯 Goal: Build a PLC program that assigns initial values to variables and calculates the total water volume by multiplying flow rate and fill time.

📋 What You'll Learn

Create variables for flow rate and fill time with exact values

Create a variable to store the total volume

Use arithmetic multiplication to calculate total volume

Display the total volume value

💡 Why This Matters

🌍 Real World

PLC programs often control machines by reading inputs, performing calculations, and setting outputs. This project shows how to assign and calculate values.

💼 Career

Understanding variable assignment and arithmetic is essential for PLC programmers working in manufacturing, automation, and process control.

1

Create initial variables for flow rate and fill time

Create two variables: flow_rate and fill_time. Assign flow_rate the value 10 and fill_time the value 5.

PLC Programming

# Create variables flow_rate and fill_time with values 10 and 5
# Your code here

Hint

Use the assignment operator := to assign values to variables.

2

Create a variable to store total volume

Create a variable called total_volume and initialize it to 0.

PLC Programming

flow_rate := 10;
fill_time := 5;
# Create variable total_volume and set it to 0
# Your code here

Hint

Initialize total_volume with zero before calculation.

3

Calculate total volume using arithmetic

Calculate the total volume by multiplying flow_rate and fill_time. Store the result in total_volume.

PLC Programming

flow_rate := 10;
fill_time := 5;
total_volume := 0;
# Calculate total_volume = flow_rate * fill_time
# Your code here

Hint

Use the multiplication operator * to calculate the total volume.

4

Display the total volume

Write a statement to display the value of total_volume.

PLC Programming

flow_rate := 10;
fill_time := 5;
total_volume := 0;
total_volume := flow_rate * fill_time;
# Display total_volume
# Your code here

Hint

Use the WriteOutput function to display the value.

Practice

1. In PLC programming, which symbol is used to assign a value to a variable?
speed := 100;

easy

A. The ':=' symbol

B. The '=' symbol

C. The '==' symbol

D. The '->' symbol

Solution

1. Step 1: Understand assignment syntax in PLC

   PLC uses ':=' to assign values to variables, not '=' or '=='.

2. Step 2: Identify correct assignment operator

   The ':=' operator updates the variable with the new value.

3. Final Answer:

   The ':=' symbol -> Option A

4. Quick Check:

   Assignment operator = ':=' [OK]

Hint: Remember ':=' means assign value in PLC [OK]

Common Mistakes:

• Using '=' instead of ':='
• Confusing comparison '==' with assignment
• Using arrow '->' mistakenly
• Forgetting semicolon after assignment

2. Which of the following is the correct syntax to add 5 to a variable count in PLC?

easy

A. count := count + 5;

B. count = count + 5;

C. count += 5;

D. count :=+ 5;

Solution

1. Step 1: Check assignment operator usage

   PLC uses ':=' for assignment, so '=' and '+=' are incorrect here.

2. Step 2: Verify arithmetic addition syntax

   'count := count + 5;' correctly adds 5 to count and assigns it back.

3. Final Answer:

   count := count + 5; -> Option A

4. Quick Check:

   Use ':=' for assignment and '+' for addition [OK]

Hint: Use ':=' for assignment, '+' for addition [OK]

Common Mistakes:

• Using '=' instead of ':='
• Trying '+=' which is not valid in PLC
• Writing ':=+' which is invalid
• Missing semicolon

3. What is the value of result after executing the following PLC code?

temp := 10;
result := temp * 3 + 2;

medium

A. 12

B. 36

C. 30

D. 32

Solution

1. Step 1: Calculate multiplication first

   temp * 3 = 10 * 3 = 30

2. Step 2: Add 2 to the multiplication result

   30 + 2 = 32

3. Final Answer:

   32 -> Option D

4. Quick Check:

   10 * 3 + 2 = 32 [OK]

Hint: Multiply before adding, follow order of operations [OK]

Common Mistakes:

• Adding before multiplying
• Using wrong variable value
• Ignoring operator precedence
• Confusing temp and result variables

4. Identify the error in this PLC code snippet:

speed := 50
speed := speed + 10;

medium

A. Cannot add numbers in PLC

B. Using '=' instead of ':='

C. Missing semicolon after first assignment

D. Variable 'speed' not declared

Solution

1. Step 1: Check statement endings

   PLC statements must end with a semicolon ';'. The first line misses it.

2. Step 2: Confirm other syntax is correct

   Second line uses correct ':=' and addition syntax.

3. Final Answer:

   Missing semicolon after first assignment -> Option C

4. Quick Check:

   Each statement ends with ';' [OK]

Hint: Always end PLC statements with semicolon ';' [OK]

Common Mistakes:

• Forgetting semicolon
• Confusing '=' with ':='
• Assuming addition not allowed
• Ignoring variable declaration rules

5. You want to calculate the average speed from three sensor readings s1, s2, and s3 in PLC. Which code correctly assigns the average to avg_speed?

s1 := 40;
s2 := 50;
s3 := 60;
avg_speed := (s1 + s2 + s3) / 3;

hard

A. avg_speed = (s1 + s2 + s3) / 3;

B. avg_speed := (s1 + s2 + s3) / 3;

C. avg_speed := s1 + s2 + s3 / 3;

D. avg_speed := (s1 + s2 + s3) * 3;

Solution

1. Step 1: Check correct assignment operator

   Use ':=' to assign the calculated value to avg_speed.

2. Step 2: Verify arithmetic expression correctness

   Sum all three readings and divide by 3 inside parentheses for average.

3. Final Answer:

   avg_speed := (s1 + s2 + s3) / 3; -> Option B

4. Quick Check:

   Use ':=' and parentheses for correct average [OK]

Hint: Use parentheses to sum before dividing [OK]

Common Mistakes:

• Using '=' instead of ':='
• Missing parentheses causing wrong division
• Multiplying instead of dividing
• Forgetting semicolon