Load Cells in Coil Processing & Roll Forming Lines (Tension Control & Wiring Guide)

Load Cells in Coil Processing Lines

Tension Measurement, Wiring & Control Integration in Roll Forming Systems

Load cells are critical in modern coil processing and advanced roll forming lines where strip tension must be precisely controlled.

They are commonly used in:

• Uncoiler tension control

• Bridle roll systems

• Strip accumulators

• Slitting lines

• Cut-to-length systems

• Recoilers

Without properly integrated load cells, tension control becomes unstable, resulting in:

• Strip snapping

• Camber formation

• Edge cracking

• Coil telescoping

• Surface marking

• Panel length variation

• Increased scrap

This guide explains how load cells work, how they are wired, how they integrate into PLC/VFD systems, and how to commission them correctly in coil processing environments.

1) What Is a Load Cell?

A load cell is a force-measuring device that converts mechanical strain into electrical signal.

Most industrial load cells use:

Strain gauge bridge technology.

They measure:

• Strip tension

• Roll force

• Carriage load

In coil processing lines, they are typically installed under:

• Pillow blocks

• Bearing housings

• Support rollers

• Dancer arms

2) Types of Load Cells Used in Coil Lines

1) Tension Load Cells (Pillow Block Style)

Installed under roller bearings.
Measure strip tension directly.

2) Shear Beam Load Cells

Used in bridle and recoiler systems.

3) Compression Load Cells

Used in heavy force applications.

Tension control systems typically use pillow block or shear beam load cells.

3) Basic Load Cell Electrical Output

Load cells produce very small millivolt signals:

Typically 2–3 mV/V at full load.

Because signal is extremely low:

Signal must be amplified using a load cell amplifier or transmitter.

Never connect raw load cell directly to PLC analog input.

4) Word-Based Wiring – 4-Wire Load Cell (Bridge Type)

• Excitation + → +V from amplifier
• Excitation – → 0V return
• Signal + → Amplifier input +
• Signal – → Amplifier input –

Shield → Cabinet Earth Bar (grounded at cabinet only)

Load cell connects to signal conditioner module.

5) Word-Based Wiring – Load Cell Amplifier to PLC

Load Cell → Signal Amplifier

Amplifier Output Options:

• 0–10V

• 4–20mA

Preferred:

4–20mA (better noise immunity).

Word-Based:

• Amplifier +V → +24VDC
• Amplifier 0V → 0V
• Amplifier 4–20mA Output → PLC Analog Input (+)
• PLC Analog Common → 0V

This converts tension into usable PLC signal.

6) Why 4–20mA Is Recommended

In roll forming environments:

• VFD switching noise

• Servo drive interference

• Long cable runs

4–20mA:

• Less sensitive to voltage drop

• More resistant to EMI

• Detects broken wire (0mA fault)

Industrial standard for tension control.

7) Load Cell Placement in Coil Processing Lines

Common mounting locations:

• Uncoiler base → Measures strip pull force
• Bridle roll stand → Measures tension between sections
• Accumulator roller → Measures loop tension
• Recoiler → Controls winding tension

Correct mechanical alignment critical for accurate reading.

8) Tension Control Loop Integration

Word-Based Control Flow:

Load Cell → Amplifier → PLC Analog Input →
PLC PID Control → VFD Speed Adjustment

If tension high → Reduce uncoiler speed
If tension low → Increase uncoiler speed

Closed-loop control stabilizes strip tension.

9) PID Control Considerations

Tension control often uses PID logic.

• Setpoint = Desired tension
• Process Variable = Load cell reading
• Output = Speed correction

Poor PID tuning causes:

• Oscillation

• Strip snapping

• Slack strip

• Speed instability

Mechanical inertia must be considered during tuning.

10) Shielding & Cable Routing

Load cell signal extremely sensitive.

Best practices:

• Use shielded twisted pair cable

• Keep cable short where possible

• Avoid running near motor power cables

• Separate from VFD output cables

• Ground shield at cabinet end only

Improper shielding leads to unstable readings.

11) Grounding Architecture

All analog commons must return to:

Single cabinet ground reference.

Avoid:

Multiple ground paths
Ground loops

Ground loop introduces signal offset.

12) Calibration Procedure

Calibration must be done mechanically and electrically.

Steps:

1. Zero calibration (no load)

2. Apply known tension force

3. Adjust amplifier scaling

4. Verify PLC scaling

5. Test under running condition

Improper calibration causes inaccurate tension control.

13) Common Load Cell Problems

1. Drift over time

2. Noise spikes in reading

3. Broken cable in moving section

4. Loose mounting bolts

5. Overloading beyond rated capacity

6. Amplifier misconfiguration

7. Ground loop interference

Mechanical and electrical issues often combined.

14) Overload Protection

Load cells have rated capacity.

Exceeding rating causes:

• Permanent deformation

• Signal offset

• Failure

Mechanical stops should prevent overload.

Electrical system cannot compensate for structural overload.

15) Temperature Effects

Temperature change affects:

• Strain gauge resistance

• Signal drift

Industrial load cells include temperature compensation.

Cabinet analog input scaling must consider environment.

16) Export Considerations

When exporting roll forming lines:

• Provide load cell rating documentation

• Provide amplifier configuration backup

• Provide calibration data

• Label analog input channels clearly

• Include spare amplifier if possible

Incorrect amplifier replacement common overseas issue.

17) Commissioning Checklist

1. Verify wiring polarity

2. Confirm stable 24VDC supply

3. Check shield grounding

4. Calibrate zero load

5. Apply test load

6. Verify PLC scaling

7. Tune PID gradually

8. Test at full production speed

Commission under actual coil tension conditions.

18) Buyer Strategy (30%)

Before purchasing a roll forming or coil processing line with tension control, verify:

1. Load cells properly rated

2. 4–20mA amplifier used

3. Shielded cabling installed

4. Proper mechanical mounting design

5. Calibration procedure documented

6. PID tuning completed under load

7. Spare amplifier available

8. Tension control tested during coil change

Red flag:

“Load cell installed but tension controlled manually.”

That defeats purpose of closed-loop control.

6 Frequently Asked Questions

1) Why does tension reading fluctuate?

Likely electrical noise or poor shielding.

2) Should I use 0–10V or 4–20mA?

4–20mA preferred in industrial coil lines.

3) Why does load cell drift over time?

Possible mechanical deformation or temperature effect.

4) Can I connect load cell directly to PLC?

No, requires amplifier or signal conditioner.

5) Why does tension oscillate?

PID tuning incorrect or mechanical inertia too high.

6) What is most common integration mistake?

Improper shielding and grounding.

Final Engineering Summary

Load cell integration in coil processing and roll forming lines must ensure:

• Correct load cell type selection

• Proper mechanical mounting

• Amplifier-based signal conditioning

• 4–20mA analog integration

• Shielded twisted-pair wiring

• Single-point grounding

• Accurate calibration

• PID tuning under real load

Improper integration leads to:

• Strip tension instability

• Increased scrap

• Camber and edge defects

• Production downtime

In advanced roll forming systems, load cells are the foundation of stable tension control and must be integrated with both electrical precision and mechanical discipline.