Scaling analog values in PLC Programming - Step-by-Step Execution

Concept Flow - Scaling analog values

Read raw analog input

↓

Apply scaling formula

↓

Calculate scaled value

↓

Use scaled value in program

↓

Output scaled value

The flow reads a raw analog input, applies a scaling formula to convert it to a useful range, then outputs the scaled value for use.

Execution Sample

PLC Programming

raw_input = 512
scaled = (raw_input / 1023.0) * 100
output = scaled

This code converts a raw 10-bit analog input (0-1023) to a percentage scale (0-100).

Execution Table

Step	Variable	Value	Calculation	Result
1	raw_input	512	Read raw analog input	512
2	scaled	(512 / 1023.0) * 100	Divide and multiply	50.05
3	output	scaled	Assign scaled to output	50.05
4	-	-	End of scaling process	-

💡 Scaling complete, output holds scaled value 50.05

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3
raw_input	undefined	512	512	512
scaled	undefined	undefined	50.05	50.05
output	undefined	undefined	undefined	50.05

Key Moments - 3 Insights

Why do we divide raw_input by 1023?

Because 1023 is the maximum value of a 10-bit analog input, dividing by it normalizes the input to a 0-1 range as shown in step 2 of the execution_table.

Why multiply by 100 after dividing?

Multiplying by 100 converts the normalized value (0 to 1) into a percentage scale (0 to 100), as seen in the calculation in step 2.

What happens if raw_input is 0?

If raw_input is 0, scaled becomes 0 after step 2, so output will also be 0, meaning no signal or minimum value.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'scaled' at step 2?

A 100

B 512

C 50.05

D 0

💡 Hint

Check the 'Value' and 'Result' columns for 'scaled' at step 2 in the execution_table.

At which step is the raw analog input first read?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at the 'Calculation' column in the execution_table to find when raw_input is assigned.

If raw_input was 1023, what would 'scaled' be at step 2?

A 0

B 100

C 50.05

D 512

💡 Hint

Use the formula in step 2: (raw_input / 1023) * 100 and substitute raw_input = 1023.

Concept Snapshot

Scaling analog values:
- Read raw input (e.g., 0-1023 for 10-bit ADC)
- Normalize by dividing by max raw value (1023)
- Multiply by desired max scale (e.g., 100 for %)
- Use scaled value in program
- Output scaled value for control or display

Full Transcript

This lesson shows how to scale analog values in PLC programming. We start by reading a raw analog input, which is a number between 0 and 1023 for a 10-bit input. Then we divide this raw input by 1023 to normalize it between 0 and 1. Next, we multiply by 100 to convert it to a percentage scale from 0 to 100. Finally, we assign this scaled value to an output variable for use in the program. The execution table traces these steps with example values, showing how the variables change. Key moments clarify why we divide by 1023 and multiply by 100. The quiz tests understanding of these steps and values. This process helps convert raw sensor data into meaningful units for automation control.

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