Scaling Analog Inputs in Roll Forming PLC Systems — Pressure Transducers and Temperature Sensors

Introduction — Why Analog Scaling Matters in Roll Forming Machines

Most PLC signals in a roll forming machine are digital. They are either ON or OFF, such as a proximity sensor detecting a shear position or a guard door being closed.

However, several important machine systems rely on analog signals, which represent continuously varying values rather than simple ON/OFF states.

Examples include:

• hydraulic pressure sensors
• oil temperature sensors
• motor load signals
• hydraulic oil level transmitters
• line tension sensors
• analog speed references to drives.

Before these signals can be used in the PLC program, they must be scaled.

Scaling converts the raw electrical signal coming from the sensor into meaningful engineering units such as:

• bar or PSI (pressure)
• °C or °F (temperature)
• liters (fluid level)
• meters per minute (speed).

If analog signals are not scaled correctly, the PLC may misinterpret machine conditions, leading to incorrect alarms, unstable control behavior, or unsafe operation.

What an Analog Signal Looks Like to a PLC

Analog sensors do not send values like “100 bar” or “50 °C” directly to the PLC.

Instead, they send an electrical signal proportional to the measurement.

The most common industrial analog signals are:

4–20 mA Current Signal

The sensor outputs a current between:

4 mA = minimum measurement
20 mA = maximum measurement

This is the most common standard in industrial machines because it is resistant to electrical noise.

0–10 V Voltage Signal

The sensor outputs a voltage between:

0 V = minimum measurement
10 V = maximum measurement

Voltage signals are simpler but more sensitive to electrical noise.

In roll forming environments with motors, VFDs, and hydraulic valves, 4–20 mA signals are usually preferred.

Raw PLC Analog Values

PLC analog input modules do not report current or voltage directly. They convert the electrical signal into a digital number.

For example, a PLC may convert a 4–20 mA signal into a digital range such as:

0 to 32767
or
0 to 27648

The PLC program must then convert that digital value into the real measurement.

This conversion process is called analog scaling.

Example of Analog Scaling

Consider a hydraulic pressure sensor.

Sensor range:

0 to 250 bar

Signal range:

4 to 20 mA

PLC raw input range:

0 to 27648

If the PLC reads a raw value of 13824, this corresponds to half of the signal range.

Half of 250 bar equals:

125 bar

The PLC scaling logic converts the raw value into the engineering unit value used in the program.

Why 4–20 mA Is Better Than 0–10 V

In roll forming machines, electrical noise can come from:

• VFD motor drives
• hydraulic valve coils
• servo amplifiers
• large motors.

Current signals are less sensitive to noise because the measurement is based on current flow rather than voltage level.

Another advantage is wire break detection.

If a cable breaks:

• current drops below 4 mA
• the PLC can detect a sensor fault.

With 0–10 V signals, a broken wire may appear as zero volts, which could be misinterpreted as a valid reading.

Analog Input Modules

PLC analog modules convert electrical signals into digital numbers.

Typical features include:

• input resolution
• input type selection
• filtering capability
• diagnostic functions.

Resolution determines how precisely the signal can be measured.

Higher resolution modules produce smoother and more accurate values.

Pressure Transducers in Roll Forming Machines

Hydraulic systems are widely used in roll forming machines.

They power systems such as:

• punching units
• hydraulic shears
• clamp systems
• tension control.

Pressure transducers monitor hydraulic pressure to ensure safe and reliable operation.

Typical pressure ranges include:

• 0–100 bar
• 0–250 bar
• 0–400 bar.

The PLC monitors pressure to detect conditions such as:

• low pressure faults
• overpressure conditions
• pump failures.

Temperature Sensors in Roll Forming Machines

Temperature sensors monitor critical machine systems such as:

• hydraulic oil temperature
• gearbox oil temperature
• electrical cabinet temperature.

High temperatures may indicate:

• overloaded pumps
• cooling failures
• lubrication problems.

Temperature monitoring helps prevent damage to machine components.

Basic Scaling Formula

Analog scaling is typically performed using a linear equation.

General formula:

ScaledValue =
(RawValue – RawMin) × (EngineeringMax – EngineeringMin)
÷ (RawMax – RawMin)

• EngineeringMin

Where:

RawValue = PLC input value
RawMin = minimum raw value
RawMax = maximum raw value
EngineeringMin = minimum measurement value
EngineeringMax = maximum measurement value.

This formula converts the raw signal into meaningful units.

Example Pressure Scaling

Pressure sensor:

0–250 bar

Raw PLC range:

0–27648

Scaling example:

If the PLC reads:

13824

Then:

Pressure = 125 bar

This value can then be used by the PLC program to monitor hydraulic performance.

Example Temperature Scaling

Temperature sensor:

0–150 °C

Signal:

4–20 mA

PLC raw range:

0–27648

If the PLC reads a value corresponding to mid-range, the scaled temperature will be approximately:

75 °C

The PLC can use this value to trigger alarms if the temperature exceeds safe limits.

Why Scaling Must Be Accurate

Incorrect scaling causes many machine problems.

Examples include:

• false low pressure alarms
• missed overpressure conditions
• incorrect temperature warnings
• unstable control loops.

Accurate scaling ensures that machine protection systems function correctly.

Filtering Analog Signals

Analog signals may fluctuate slightly due to noise or mechanical variation.

PLC programs often apply filtering techniques to stabilize readings.

Common methods include:

• moving average filters
• low-pass filters
• sample averaging.

Filtering helps prevent nuisance alarms from small fluctuations.

Alarm Thresholds for Analog Signals

Once analog values are scaled, the PLC can use them to trigger alarms.

Typical alarm conditions include:

Low Pressure Alarm

Hydraulic pressure below the minimum operating level.

High Pressure Alarm

Pressure exceeding safe system limits.

High Temperature Alarm

Oil or motor temperature exceeding safe limits.

Sensor Failure Alarm

Signal outside the expected measurement range.

Detecting Sensor Failures

Analog scaling can also help detect sensor faults.

If a 4–20 mA signal drops below 4 mA, the PLC may interpret this as a broken wire or failed sensor.

The PLC can trigger a sensor fault alarm.

This prevents the machine from relying on invalid measurements.

Analog Scaling in PLC Function Blocks

Many PLC systems provide built-in scaling function blocks.

These blocks allow programmers to enter:

• raw minimum value
• raw maximum value
• engineering minimum value
• engineering maximum value.

The block automatically calculates the scaled output value.

Using standardized scaling blocks improves program consistency.

Troubleshooting Analog Scaling Problems

If analog readings appear incorrect, technicians should check several areas.

Verify Sensor Range

Ensure the sensor’s measurement range matches the PLC scaling parameters.

Check Wiring

Loose or incorrect wiring may cause incorrect readings.

Confirm Input Module Configuration

Analog input modules must be configured for the correct signal type.

Compare with Physical Measurement

Use a pressure gauge or thermometer to confirm the sensor reading.

This helps determine whether the problem lies in the sensor or the PLC scaling logic.

Commissioning Analog Inputs

During machine commissioning, analog inputs should be verified carefully.

Typical commissioning steps include:

1 verify sensor wiring
2 confirm PLC module configuration
3 compare sensor reading with physical measurement
4 adjust scaling parameters if necessary.

Accurate commissioning prevents measurement errors during production.

Preventative Maintenance for Analog Sensors

Analog sensors should be inspected periodically.

Recommended maintenance includes:

Monthly checks:

• inspect sensor wiring
• verify sensor mounting.

Quarterly checks:

• compare sensor readings with manual instruments
• check for contamination or oil leaks.

Maintaining sensors ensures reliable machine monitoring.

Benefits of Proper Analog Scaling

Correct analog scaling provides several benefits.

These include:

• accurate hydraulic pressure monitoring
• reliable temperature protection
• improved machine safety
• better diagnostics and troubleshooting.

For roll forming machines with hydraulic systems and high-speed operation, accurate analog measurements are essential.

FAQ — Scaling Analog Inputs

What does analog scaling mean in PLC programming?

Analog scaling converts raw PLC input values into meaningful engineering units such as pressure or temperature.

Why are 4–20 mA signals commonly used?

They are resistant to electrical noise and allow the PLC to detect wire breaks or sensor failures.

What happens if analog scaling is incorrect?

The PLC may display incorrect measurements and trigger false alarms or miss dangerous conditions.

How can technicians verify analog scaling?

By comparing PLC readings with physical measurements such as pressure gauges or thermometers.

Do PLCs provide built-in scaling functions?

Many PLC platforms include scaling blocks that simplify the conversion of raw signals to engineering units.

Why is filtering used with analog signals?

Filtering reduces noise and stabilizes the measurement so the PLC receives a smoother signal.

The next strong page in the PLC Programming—Roll Forming Patterns category should be:

“PLC Watchdogs and Heartbeat Signals — Detecting Communication Failures.”

This topic is valuable because communication failures between PLCs, drives, and HMIs are common in industrial machines.