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

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

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

Random PLC input flicker
Encoder pulse miscount
Flying shear length inconsistency
PID oscillation
False strip detection
Intermittent safety faults
Servo following errors
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

Running sensor cable in same conduit as motor cable
No shield termination clamp
Multiple ground points
Using 0–10V in noisy environment
Poor crimping or loose terminals
Ignoring cabinet earth bonding
No suppression diode
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:

Signal cables separated from power cables
Differential encoders used
4–20mA analog preferred
Shield grounding properly implemented
Solenoid suppression installed
Cabinet earth bonding verified
Separate control power supply used
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.