Scaling analog values in PLC Programming - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does 'scaling analog values' mean in PLC programming?

It means converting a raw sensor signal (like voltage or current) into a meaningful engineering unit (like temperature or pressure) using a formula.

beginner

Why do we need to scale analog values in automation systems?

Because sensors output raw signals that machines can't understand directly. Scaling translates these signals into real-world values humans and machines can use.

intermediate

What is the basic formula for scaling an analog input value?

Scaled Value = ((Raw Value - Raw Min) / (Raw Max - Raw Min)) * (Scaled Max - Scaled Min) + Scaled Min

beginner

In a 4-20mA sensor signal, what does 4mA and 20mA represent?

4mA is the sensor's minimum signal (Raw Min), and 20mA is the maximum signal (Raw Max). These are used to scale the signal to real values.

intermediate

How can you test if your scaling formula works correctly?

By inputting known raw values and checking if the output matches expected scaled values, like testing 4mA gives minimum and 20mA gives maximum.

What is the first step in scaling an analog value?

A Identify the raw input range

B Multiply by 10

C Convert to digital

D Ignore the raw value

You must know the raw input range (like 4-20mA) before scaling.

If a sensor outputs 12mA in a 4-20mA range, what is the approximate scaled percentage?

A 25%

B 50%

C 75%

D 100%

12mA is halfway between 4mA and 20mA, so it represents about 50% scaled value.

Which formula correctly scales a raw value to a new range?

A Scaled = Raw * 2

B Scaled = Raw / 100

C Scaled = ((Raw - RawMin) / (RawMax - RawMin)) * (ScaledMax - ScaledMin) + ScaledMin

D Scaled = Raw + 10

Option C is the correct general scaling formula.

What does 'RawMin' represent in scaling?

A Average sensor value

B Minimum scaled value

C Maximum raw sensor value

D Minimum raw sensor value

'RawMin' is the smallest raw input value from the sensor.

Why is scaling important before using sensor data in PLC logic?

A To convert raw signals into meaningful units for control decisions

B To save memory

C To slow down the process

D To make the signal digital

Scaling converts raw signals into units that the PLC can use for control and monitoring.

Explain how you would scale a 4-20mA sensor signal to a temperature range of 0-100°C.

Think about the formula that converts one range to another.

Describe why scaling analog values is necessary in automation and what could happen if you skip it.

Consider how raw numbers relate to real-world measurements.

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