Length Control Fundamentals in Roll Forming Machines — Encoder-Based Length Counting

Introduction — Why Length Control Is Critical in Roll Forming

One of the most important requirements in roll forming production is producing panels at the correct length. Roofing panels, cladding sheets, structural deck profiles, and many other products must meet strict dimensional tolerances.

If panel lengths vary even slightly, several problems can occur:

• installation issues on building sites
• product rejection from customers
• material waste
• production inefficiency

For this reason, roll forming machines rely on precise length control systems.

Most modern machines measure panel length using encoder-based length counting controlled by the PLC.

This system continuously tracks strip movement through the machine and triggers the cutting system at the correct position.

Basic Principle of Encoder-Based Length Measurement

Encoder-based length measurement works by converting machine movement into electrical pulses.

An encoder is attached to a rotating component such as:

• a measuring wheel contacting the strip
• the main drive shaft
• a gearbox output shaft

As the shaft rotates, the encoder generates pulses.

The PLC counts these pulses to determine how far the strip has moved.

Once the desired panel length is reached, the PLC triggers the shear to cut the material.

Components of a Length Control System

A typical roll forming length control system includes several key components.

These include:

• rotary encoder
• measuring wheel or drive shaft
• PLC high-speed counter input
• shear control system
• operator HMI interface

Each component plays a role in ensuring accurate panel length measurement.

Rotary Encoders

Rotary encoders are sensors that convert rotational motion into electrical signals.

Encoders generate a series of pulses as the shaft rotates.

The PLC counts these pulses to determine the amount of movement.

Encoders used in roll forming machines typically produce signals such as:

• A channel pulse
• B channel pulse
• index (Z) pulse

The A and B channels allow the PLC to determine direction and movement.

Encoder Pulse Resolution

Encoders are rated by pulses per revolution (PPR).

Common encoder resolutions include:

• 1000 PPR
• 2048 PPR
• 4096 PPR

Higher resolution encoders produce more pulses per revolution, which improves measurement accuracy.

For example, if a measuring wheel rotates once for every meter of strip movement, a 1000 PPR encoder would generate 1000 pulses per meter.

The PLC can then calculate strip length by counting these pulses.

Measuring Wheel Systems

Many roll forming machines use measuring wheels to track strip movement.

A measuring wheel is a rubber-coated wheel that contacts the strip surface.

As the strip moves, the wheel rotates.

The encoder attached to the wheel measures this rotation.

Measuring wheels provide accurate length measurement because they track the actual strip movement rather than machine drive movement.

Encoder Mounting Locations

Encoders may be mounted in different locations depending on machine design.

Common mounting locations include:

Measuring wheel shaft

Provides accurate measurement of strip movement.

Main drive shaft

Measures machine rotation but may not account for strip slip.

Gearbox output shaft

Measures rotation of the drive system.

Measuring wheels usually provide the most accurate length measurement.

PLC High-Speed Counters

Encoder signals generate pulses at very high frequencies.

Standard PLC inputs may not be fast enough to count these pulses accurately.

For this reason, PLCs use high-speed counter inputs designed specifically for encoder signals.

High-speed counters allow the PLC to:

• count encoder pulses
• measure movement accurately
• trigger cutting operations at precise positions.

Length Calculation in the PLC

The PLC converts encoder pulses into length values.

The calculation typically follows this formula:

Length = (Pulse Count) ÷ (Pulses Per Unit Distance)

For example:

If an encoder produces 2000 pulses per meter, and the PLC counts 4000 pulses, the strip has moved:

2 meters

The PLC continuously performs this calculation during machine operation.

Triggering the Cut-to-Length Shear

Once the measured strip length reaches the target value, the PLC triggers the shear system.

The timing must consider:

• shear response time
• machine speed
• strip movement during cutting

To compensate for these factors, the PLC may trigger the shear slightly before the exact target length.

This compensation ensures accurate final panel length.

Length Accuracy Factors

Several factors influence length measurement accuracy.

These include:

• encoder resolution
• measuring wheel diameter
• strip slippage
• machine vibration

Higher encoder resolution and stable mechanical installation improve measurement accuracy.

Calibration of Length Measurement

Length measurement systems must be calibrated during machine commissioning.

Calibration involves comparing the measured panel length with the actual panel length.

Typical calibration procedure:

1. produce a test panel
2. measure the panel length manually
3. compare with the PLC measurement
4. adjust scaling parameters if necessary.

Proper calibration ensures accurate panel lengths during production.

Integration with Recipe Systems

Length settings are often stored in the machine recipe system.

Each product recipe may include:

• target panel length
• shear timing parameters
• speed settings

When a recipe is selected, the PLC loads the correct length settings automatically.

This simplifies production changeovers.

Common Length Control Problems

Several issues can affect encoder-based length measurement.

Strip Slippage

If the strip slips against the measuring wheel, the encoder may measure incorrect movement.

Encoder Signal Loss

Loose wiring or electrical interference may cause encoder signals to be lost.

Incorrect Scaling

If the encoder scaling value is incorrect, the measured length will be inaccurate.

Troubleshooting Length Measurement Systems

When length errors occur, technicians should check:

• encoder wiring and signals
• measuring wheel contact with the strip
• PLC scaling parameters
• shear timing settings

Observing encoder pulse counts during operation can help diagnose problems.

Commissioning Length Control Systems

Commissioning length control involves several important steps.

Typical commissioning tasks include:

1 verifying encoder signal integrity
2 configuring PLC high-speed counter settings
3 calibrating length scaling values
4 testing shear trigger timing.

Testing should be performed at multiple production speeds.

Preventative Maintenance for Length Control Systems

Routine maintenance helps maintain measurement accuracy.

Recommended inspections include:

Monthly checks:

• inspect measuring wheel condition
• verify encoder mounting

Quarterly inspections:

• check encoder wiring
• review PLC length calibration.

Regular maintenance prevents measurement errors.

Benefits of Accurate Length Control

Accurate length measurement provides several benefits.

These include:

• consistent product dimensions
• reduced material waste
• improved customer satisfaction
• efficient production operation.

Reliable length control is therefore essential for roll forming machine performance.

FAQ — Encoder-Based Length Control

Why are encoders used in roll forming machines?

Encoders measure strip movement so the PLC can determine panel length and trigger cutting operations accurately.

What is a measuring wheel used for?

A measuring wheel tracks the actual movement of the steel strip and provides accurate length measurement.

Why do roll forming machines use high-speed counters?

Encoder pulses occur at very high frequencies, so high-speed counters ensure the PLC counts them accurately.

What causes incorrect panel length?

Common causes include strip slippage, encoder faults, or incorrect PLC scaling parameters.

How is length measurement calibrated?

Calibration involves comparing the machine’s measured length with the actual panel length and adjusting the PLC scaling value.

Can panel length settings be stored in recipes?

Yes. Many machines store panel lengths in recipe systems to simplify product changeovers.