Common Fault Patterns

No Counts at All

The controller shows no movement even though the shaft turns.

What to Check

• No encoder supply
• broken A channel
• wrong input common
• incompatible voltage/interface
• HSC disabled
• cable unplugged.

Diagnostic Sequence

1. Check supply at encoder.
2. Check A signal at encoder.
3. Check A at receiver.
4. Check raw HSC enable/configuration.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Do not replace the encoder until you know where the signal disappears.

Counts Only One Direction

Forward count works, reverse does not or always counts positive.

What to Check

• Missing B channel
• wrong quadrature mode
• A/B wiring issue
• direction input configuration
• mechanical one-way slip.

Diagnostic Sequence

1. Scope A/B both directions.
2. Check B continuity and amplitude.
3. Verify HSC quadrature mode.
4. Inspect coupling during reversal.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: A single healthy channel can hide a failed quadrature path.

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Position Slowly Drifts

Position becomes increasingly wrong over many cycles.

What to Check

• Rare missed/false pulses
• mechanical slip
• scaling error
• reference correction logic
• cable intermittency.

Diagnostic Sequence

1. Trend raw count over repeated identical cycles.
2. Check stationary count for noise.
3. Mark coupling.
4. Repeat at lower speed/acceleration.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Determine whether the error is cumulative counts or a fixed mechanical offset.

Fault Only at High Speed

System works in jog but fails in production.

What to Check

• Input frequency limit
• slow rise time
• cable attenuation
• marginal voltage
• EMI
• bearing vibration.

Diagnostic Sequence

1. Calculate maximum frequency.
2. Scope at receiver at full speed.
3. Check input filter.
4. Compare temporary clean cable route.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Production-speed testing is essential for high-frequency faults.

Fault Only When VFD Runs

Counts jump or drive reports feedback fault when a nearby VFD starts.

What to Check

• EMI coupling
• poor motor cable shield
• poor encoder shield bond
• common-mode voltage
• bad routing.

Diagnostic Sequence

1. Observe stationary counts while VFD switches.
2. Inspect routing and shield bonds.
3. Use temporary separated encoder cable.
4. Check VFD output cable grounding.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Noise is a system EMC problem, not automatically an encoder problem.

Fault at One Axis Position

Encoder fails only when carriage reaches a certain location.

What to Check

• Broken flex cable conductor
• connector strain
• drag-chain pinch
• mechanical obstruction.

Diagnostic Sequence

1. Trend error vs position.
2. Flex isolated cable carefully.
3. Inspect bend radius/chain.
4. Bypass with temporary cable.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Position-dependent electrical faults point strongly to moving cable.

Fault After Warm-Up

Feedback is good cold and fails after 20-60 minutes.

What to Check

• Thermal expansion
• encoder electronics
• power supply sag
• hot receiver module
• connector resistance.

Diagnostic Sequence

1. Record temperature and time.
2. Scope supply and signal cold/hot.
3. Check cabinet cooling.
4. Compare known-good channel.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Time and temperature are variables worth trending.

Random Drive Encoder Alarm

Drive occasionally trips with feedback error but no obvious pattern.

What to Check

• Short signal dropout
• serial CRC error
• connector vibration
• power dip
• cable noise.

Diagnostic Sequence

1. Save alarm subcode/history.
2. Trend feedback status.
3. Use triggered scope.
4. Inspect connectors and supply dips.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Preserve fault data before resetting the drive.

Speed Reads Half or Double

Measured speed is exactly 0.5×, 2×, or 4× expected.

What to Check

• Wrong PPR/CPR
• x1/x2/x4 mode
• gear ratio
• channel counting mode.

Diagnostic Sequence

1. Rotate one exact revolution and compare raw counts.
2. Check HSC mode.
3. Check configured PPR and mechanical ratio.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Exact factors usually indicate configuration, not intermittent hardware.

Position Jumps by a Huge Value

Position suddenly changes by a large repeatable amount.

What to Check

• Counter rollover
• signed/unsigned conversion
• absolute wrap
• serial bit alignment
• data type overflow.

Diagnostic Sequence

1. Inspect raw integer before/after jump.
2. Check numeric range.
3. Check absolute rollover point.
4. Review type conversions.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Large deterministic jumps are often arithmetic or wraparound.

Home Position Is Inconsistent

Axis homes to slightly different positions each cycle.

What to Check

• Missed Z
• sensor hysteresis
• too-fast approach
• mechanical slip
• wrong index edge.

Diagnostic Sequence

1. Monitor home sensor + Z.
2. Reduce homing speed.
3. Mark coupling.
4. Check capture configuration.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Homing is a sequence of events; verify each one.

Counts While Shaft Is Stationary

Raw count changes with no mechanical motion.

What to Check

• EMI
• floating open-collector input
• wrong common
• unstable supply
• receiver noise.

Diagnostic Sequence

1. Lock shaft safely.
2. Observe count during contactor/VFD operation.
3. Scope input.
4. Check pull-up/reference/shield.

Common Mistakes

• Changing multiple things before reproducing the fault.
• Clearing diagnostic history before recording it.
• Assuming the encoder itself is always the failed component.

Field Rule: Stationary false counts are one of the clearest noise indicators.