Sensor Noise Interference Troubleshooting in Roll Forming Machines (EMI & PLC Stability Guide)

Sensor Noise Interference Troubleshooting

EMI, Grounding & Signal Stability in Roll Forming Machines

Electrical noise interference is one of the most common — and most misunderstood — causes of instability in roll forming and coil processing machines.

Symptoms often blamed on:

• PLC faults

• Servo drive errors

• Encoder failure

• Hydraulic instability

• Random cut variation

Are frequently caused by:

• Poor grounding

• Incorrect shielding

• Improper cable routing

• VFD switching noise

• Ground loops

Noise issues typically appear:

• At high production speed

• During servo acceleration

• When hydraulic solenoids fire

• When large motors start

This guide explains how to systematically diagnose and eliminate sensor noise interference in industrial roll forming environments.

1) What Is Electrical Noise?

Electrical noise is unwanted electrical disturbance that interferes with signal transmission.

In roll forming systems, noise typically comes from:

• VFD output switching (PWM)

• Servo drives

• Hydraulic solenoids

• Contactor coil switching

• High-current busbars

• Poor earth bonding

Noise affects:

• Proximity sensors

• Photoelectric sensors

• Encoders

• Load cells

• Linear transducers

• Analog inputs

2) Common Noise Symptoms

1. Random PLC input flicker

2. Encoder pulse miscount

3. Flying shear length inconsistency

4. PID oscillation

5. False strip detection

6. Intermittent safety faults

7. Servo following errors

8. Analog value jumping

Noise problems often increase with line speed.

3) Primary Noise Source: VFD Motor Cables

VFD output cables generate high-frequency switching pulses.

These create:

• Electromagnetic interference (EMI)

• Capacitive coupling

• Radiated noise

If sensor cables run parallel to VFD output cables:

Signal corruption likely.

Rule:

Never route sensor cables alongside VFD motor cables.

4) Improper Shield Grounding

Correct method:

Shield grounded at cabinet end only.

Incorrect methods:

• Shield grounded at both ends (creates ground loop)

• Shield left floating

• Shield connected to signal common

Ground loops cause signal offset and instability.

5) Ground Loop Identification

Ground loop occurs when:

Multiple ground paths exist between two points.

Symptoms:

• Analog reading drift

• Signal noise increases under load

• Intermittent instability

Solution:

Single-point grounding architecture.

All signal grounds must return to one cabinet reference.

6) Word-Based Shielding Example

Sensor Cable:

• Signal +
• Signal –
• Shield

Shield → Earth Bar (Cabinet Side Only)

Do NOT connect shield to sensor ground.

Use 360° shield clamp at cabinet entry.

7) Separation of Power & Signal Cables

Cabinet layout best practice:

Left trunking → Power cables
Right trunking → Signal cables

If crossing unavoidable:

Cross at 90 degrees.

Maintain physical separation distance.

8) Solenoid & Contactor Coil Suppression

Hydraulic solenoids and contactor coils generate voltage spikes.

If no suppression diode installed:

Spikes travel into control circuit.

Word-Based:

Solenoid Coil → Diode across terminals (for DC coils)

Without suppression:

PLC input corruption possible.

9) Analog Signal Vulnerability

Analog signals (0–10V especially) are highly noise-sensitive.

Symptoms:

• Fluctuating load cell reading

• Oscillating linear transducer feedback

• PID instability

Solution:

• Use 4–20mA instead of 0–10V where possible

• Shielded twisted pair cable

• Proper grounding

4–20mA is more noise-resistant.

10) Encoder Noise Problems

Single-ended encoder signals highly vulnerable.

Symptoms:

• Missed pulses

• Length variation

• Counter overflow

Solution:

Use differential (A+/A–, B+/B–) encoder signals.

Differential signals reject common-mode noise.

11) Power Supply Instability

Poor 24VDC power supply causes:

• Sensor flicker

• PLC input drop

• Random stop

Test:

Measure voltage during motor start.

If voltage drops significantly:

Upgrade power supply or separate control transformer.

12) Cabinet Bonding & Earthing

Cabinet must have:

• Solid earth bar

• Low-resistance connection to machine frame

• Proper bonding of door panel

Loose earth creates unstable reference.

Earth resistance should be tested.

13) Long Cable Run Considerations

Long runs (>20m):

• Increase EMI pickup

• Increase voltage drop

Use:

• Shielded cable

• Twisted pair

• 4–20mA for analog

• Line driver encoder outputs

Never run long unshielded signal cable.

14) Step-by-Step Troubleshooting Procedure

• Step 1: Identify affected signal
• Step 2: Observe when issue occurs (speed? load?)
• Step 3: Inspect cable routing
• Step 4: Check shield grounding
• Step 5: Measure 24VDC stability
• Step 6: Inspect solenoid suppression
• Step 7: Temporarily separate suspect cables
• Step 8: Test under full production speed

Systematic testing required.

15) Using Oscilloscope for Diagnosis

Advanced troubleshooting may require oscilloscope.

Observe:

• Noise spikes

• Signal distortion

• Voltage ripple

Useful for high-speed encoder systems.

16) Common Field Mistakes

1. Running sensor cable in same conduit as motor cable

2. No shield termination clamp

3. Multiple ground points

4. Using 0–10V in noisy environment

5. Poor crimping or loose terminals

6. Ignoring cabinet earth bonding

7. No suppression diode

8. Cheap non-industrial cable

Most noise issues are installation-related.

17) Preventative Design Strategies

Design system to minimize noise from start:

• Use differential encoders

• Use 4–20mA analog

• Install line reactors on VFD

• Separate control transformer

• Structured cabinet layout

• Single-point grounding

Good design eliminates most future faults.

18) Buyer Strategy (30%)

Before purchasing a roll forming machine, verify:

1. Signal cables separated from power cables

2. Differential encoders used

3. 4–20mA analog preferred

4. Shield grounding properly implemented

5. Solenoid suppression installed

6. Cabinet earth bonding verified

7. Separate control power supply used

8. Noise testing performed at full speed

Red flag:

“Length variation only at high speed.”

Often indicates encoder noise.

6 Frequently Asked Questions

1) Why does sensor flicker when motor runs?

Likely EMI from VFD output cable.

2) Should shield be grounded both ends?

No, cabinet end only.

3) Why is 4–20mA better than 0–10V?

More resistant to electrical noise.

4) Why does encoder miscount at high speed?

Single-ended wiring or poor shielding.

5) Can poor earth cause noise?

Yes, unstable reference increases interference.

6) What is most common noise cause?

Running signal cable parallel to motor power cable.

Final Engineering Summary

Sensor noise interference in roll forming machines is typically caused by:

• VFD-generated EMI

• Poor shielding

• Incorrect grounding

• Improper cable routing

• Lack of suppression devices

• Power supply instability

Effective mitigation requires:

• Differential signal wiring

• 4–20mA analog integration

• Single-point grounding

• Shielded twisted-pair cable

• Physical separation of power and signal

• Proper commissioning at full production speed

Noise issues are rarely component defects — they are almost always integration discipline problems.

In high-speed roll forming production, eliminating electrical noise is fundamental to cut accuracy, tension stability, and long-term machine reliability.