Scaling analog values in PLC Programming - Mini Project: Build & Apply

Scaling analog values

📖 Scenario: You work in a factory where sensors measure temperature as raw analog values from 0 to 4095. You need to convert these raw values into real temperatures in degrees Celsius for monitoring.

🎯 Goal: Build a simple PLC program that scales raw analog input values (0-4095) to temperature values (0-100 °C).

📋 What You'll Learn

Create a variable raw_value with a sample analog input value.

Create a variable max_raw to hold the maximum raw input value (4095).

Write the scaling logic to convert raw_value to temperature in temperature_celsius.

Print the scaled temperature value.

💡 Why This Matters

🌍 Real World

Scaling raw sensor data to meaningful units is common in industrial automation to monitor and control processes accurately.

💼 Career

Understanding how to scale analog inputs is essential for PLC programmers and automation engineers working with sensors and control systems.

1

DATA SETUP: Create the raw analog input variable

Create a variable called raw_value and set it to 2048 to represent a sample analog input.

PLC Programming

# Create a variable called raw_value and set it to 2048
# Your code here

Hint

Think of raw_value as the sensor reading from 0 to 4095.

2

CONFIGURATION: Define the maximum raw input value

Create a variable called max_raw and set it to 4095 to represent the maximum raw analog input value.

PLC Programming

raw_value = 2048
# Create a variable called max_raw and set it to 4095
# Your code here

Hint

This value is the highest possible reading from the sensor.

3

CORE LOGIC: Scale the raw value to temperature

Create a variable called temperature_celsius that scales raw_value from 0 to max_raw into a temperature range from 0 to 100 degrees Celsius using the formula: temperature_celsius = (raw_value / max_raw) * 100.

PLC Programming

raw_value = 2048
max_raw = 4095
# Calculate temperature_celsius by scaling raw_value to 0-100 range
# Your code here

Hint

Divide the raw value by max_raw, then multiply by 100 to get degrees Celsius.

4

OUTPUT: Display the scaled temperature

Write a print statement to display the value of temperature_celsius.

PLC Programming

raw_value = 2048
max_raw = 4095
temperature_celsius = (raw_value / max_raw) * 100
# Print the temperature_celsius value
# Your code here

Hint

The printed value should be approximately 49.96 degrees Celsius.

Practice

1. What is the main purpose of scaling analog values in PLC programming?

easy

A. To reduce the size of the PLC program

B. To increase the speed of the PLC processor

C. To convert raw sensor signals into meaningful engineering units

D. To change digital signals into analog signals

Solution

1. Step 1: Understand analog input signals

   Raw sensor signals are usually voltage or current values that need interpretation.

2. Step 2: Purpose of scaling

   Scaling converts these raw signals into meaningful units like temperature or pressure.

3. Final Answer:

   To convert raw sensor signals into meaningful engineering units -> Option C

4. Quick Check:

   Scaling = Convert raw to useful units [OK]

Hint: Scaling means converting raw sensor data to real-world units [OK]

Common Mistakes:

• Confusing scaling with speeding up the PLC
• Thinking scaling reduces program size
• Mixing analog and digital signal conversion

2. Which of the following is the correct formula to scale a raw analog input raw from range raw_min to raw_max into a scaled output range scale_min to scale_max?

easy

A. scaled = (raw - raw_min) * (scale_max - scale_min) / (raw_max - raw_min) + scale_min

B. scaled = raw * (scale_max - scale_min) / (raw_max + raw_min) + scale_min

C. scaled = (raw + raw_min) * (scale_max + scale_min) / (raw_max - raw_min)

D. scaled = (raw - raw_max) * (scale_min - scale_max) / (raw_min - raw_max) + scale_max

Solution

1. Step 1: Recall the scaling formula

   The formula maps raw input linearly to the scaled range using subtraction and division.

2. Step 2: Check each formula

   scaled = (raw - raw_min) * (scale_max - scale_min) / (raw_max - raw_min) + scale_min correctly subtracts raw_min, scales by ratio of output to input range, then adds scale_min.

3. Final Answer:

   scaled = (raw - raw_min) * (scale_max - scale_min) / (raw_max - raw_min) + scale_min -> Option A

4. Quick Check:

   Scaling formula = scaled = (raw - raw_min) * (scale_max - scale_min) / (raw_max - raw_min) + scale_min [OK]

Hint: Use (raw - raw_min) * output_range / input_range + scale_min [OK]

Common Mistakes:

• Adding raw_min instead of subtracting
• Using addition instead of subtraction in denominator
• Mixing max and min values incorrectly

3. Given a raw input value of 2500 from a sensor with raw range 0-4095, scaled to temperature range 0-100°C using the formula scaled = (raw - 0) * (100 - 0) / (4095 - 0) + 0, what is the scaled temperature?

medium

A. 61.07°C

B. 25.00°C

C. 100.00°C

D. 0.61°C

Solution

1. Step 1: Substitute values into formula

   scaled = (2500 - 0) * (100 - 0) / (4095 - 0) + 0 = 2500 * 100 / 4095

2. Step 2: Calculate the scaled value

   scaled = 250000 / 4095 ≈ 61.07°C

3. Final Answer:

   61.07°C -> Option A

4. Quick Check:

   2500/4095 ≈ 0.61 * 100 = 61.07 [OK]

Hint: Divide raw by max raw, multiply by max scale [OK]

Common Mistakes:

• Using raw_max as 2500 instead of 4095
• Forgetting to subtract raw_min
• Mixing up numerator and denominator

4. A PLC program uses this scaling code snippet:

scaled = (raw - 100) * (200 - 0) / (1000 - 0) + 0

What is the error in this scaling formula?

medium

A. The raw_min value should be 0, not 100

B. The denominator should be (1000 - 100) instead of (1000 - 0)

C. The formula should subtract scale_min, not raw_min

D. The scale_max value should be 100, not 200

Solution

1. Step 1: Identify raw input range

   Raw input minimum is 100, maximum is 1000, so input range is 1000 - 100 = 900.

2. Step 2: Check denominator in formula

   The formula uses (1000 - 0) = 1000, which is incorrect; it should be (1000 - 100) = 900.

3. Final Answer:

   The denominator should be (1000 - 100) instead of (1000 - 0) -> Option B

4. Quick Check:

   Input range = max - min = 900, not 1000 [OK]

Hint: Denominator = raw_max - raw_min, not raw_max - 0 [OK]

Common Mistakes:

• Using wrong input range denominator
• Confusing scale and raw min/max
• Subtracting scale_min instead of adding

5. You have a sensor outputting raw values from 500 to 3500 representing pressure from 20 to 100 psi. Which formula correctly scales a raw input raw to pressure?

hard

A. pressure = (raw - 500) * (100 - 20) / (3500 - 500) + 0

B. pressure = (raw - 0) * (100 - 20) / (3500 - 500) + 20

C. pressure = (raw - 500) * (100 - 20) / (3500 - 0) + 20

D. pressure = (raw - 500) * (100 - 20) / (3500 - 500) + 20

Solution

1. Step 1: Identify raw and scale ranges

   Raw range: 500 to 3500; pressure range: 20 to 100 psi.

2. Step 2: Apply scaling formula

   Use (raw - raw_min) * (scale_max - scale_min) / (raw_max - raw_min) + scale_min.

3. Step 3: Check each option

   pressure = (raw - 500) * (100 - 20) / (3500 - 500) + 20 correctly uses raw_min=500, raw_max=3500, scale_min=20, scale_max=100.

4. Final Answer:

   pressure = (raw - 500) * (100 - 20) / (3500 - 500) + 20 -> Option D

5. Quick Check:

   Correct raw and scale min/max used in formula [OK]

Hint: Always subtract raw_min and add scale_min in formula [OK]

Common Mistakes:

• Using 0 instead of raw_min in subtraction
• Adding 0 instead of scale_min at end
• Mixing raw_max with 0 in denominator