When the encoder signal drops, the machine literally loses its “eyes.” The following checklist is distilled from thousands of real-world service calls and will get you back to micron-level accuracy in minutes—not hours.
1. Instant Symptom Snapshot
What you see:
· Axis drift or runaway
· Drive alarm: “Position Error,” “Feedback Fault,” or encoder-specific code
· Programmed vs. actual position mismatch > 0.02 mm (or > 0.001″)
· Servo-ready LED off or flashing red
· Axis drift or runaway
· Drive alarm: “Position Error,” “Feedback Fault,” or encoder-specific code
· Programmed vs. actual position mismatch > 0.02 mm (or > 0.001″)
· Servo-ready LED off or flashing red
Quick Go/No-Go Test
1. Jog the axis 1 mm in MDI.
2. Watch the position display: if the number doesn’t change, feedback is dead.
3. If the number changes but the table doesn’t move, the encoder is fine—suspect mechanical backlash or drive enable.
2. Hardware Walk-Down (90 % of cases)
A. Encoder Cable & Connector
· Visually inspect for cuts, kinks, oil soak.
· Wiggle test: flex the cable while monitoring position—if counts jump, replace.
· Pin-out check: continuity < 1 Ω, shield tied at drive end only.
B. Encoder Power
· 5 V ±5 % (TTL) or 8–12 V (line-driver) at the encoder pigtail.
· Brown-out? Swap in a known-good 24 V supply for 30 s test.
C. Encoder Disk / Scale
· Glass scale: look for cracks, haze, finger prints.
· Rotary encoder: spin by hand—LED on drive should flicker; no flicker = disk damage.
· Visually inspect for cuts, kinks, oil soak.
· Wiggle test: flex the cable while monitoring position—if counts jump, replace.
· Pin-out check: continuity < 1 Ω, shield tied at drive end only.
B. Encoder Power
· 5 V ±5 % (TTL) or 8–12 V (line-driver) at the encoder pigtail.
· Brown-out? Swap in a known-good 24 V supply for 30 s test.
C. Encoder Disk / Scale
· Glass scale: look for cracks, haze, finger prints.
· Rotary encoder: spin by hand—LED on drive should flicker; no flicker = disk damage.
3. Drive & Parameter Deep-Dive
A. Fault Buffer
· Pull the last 10 alarms; encoder faults usually log as “A.81,” “AL-21,” or “Enc Err.”
B. Count Verification
· Move to a known mechanical stop (home switch).
· Record encoder counts; power-cycle; re-home; counts should repeat within ±1.
C. Noise Immunity Settings
· Increase “Encoder Filter Time” from 0.2 ms → 1 ms if spikes appear on scope.
· Enable differential input termination (120 Ω) if wiring > 10 m.
· Pull the last 10 alarms; encoder faults usually log as “A.81,” “AL-21,” or “Enc Err.”
B. Count Verification
· Move to a known mechanical stop (home switch).
· Record encoder counts; power-cycle; re-home; counts should repeat within ±1.
C. Noise Immunity Settings
· Increase “Encoder Filter Time” from 0.2 ms → 1 ms if spikes appear on scope.
· Enable differential input termination (120 Ω) if wiring > 10 m.
| Drive Brand | Typical Encoder Alarm Code | Reset Method |
|---|---|---|
| Mitsubishi MR-J4 | AL-21 | Cycle power, then “ALM-RST” |
| Panasonic Minas A6 | Err-21.0 | “PR-CLR” button 3 s |
| Siemens S120 | F31118 | Acknowledge via PLC or HMI |
4. Software & Calibration Loop
A. Backlash & Lead-Screw Comp
· Run a 50 mm bidirectional test; deviation > 0.01 mm → re-map ball-screw pitch error.
B. Absolute Encoder Battery
· Voltage < 3.0 V? Replace CR2032, then re-zero.
C. Final Accuracy Check
· Machine a 20 mm cube; measure with CMM; target ±0.005 mm.
· Run a 50 mm bidirectional test; deviation > 0.01 mm → re-map ball-screw pitch error.
B. Absolute Encoder Battery
· Voltage < 3.0 V? Replace CR2032, then re-zero.
C. Final Accuracy Check
· Machine a 20 mm cube; measure with CMM; target ±0.005 mm.
One-Page Field Sheet
Step 1: Alarm code → Step 2: Cable wiggle → Step 3: Voltage check → Step 4: Count repeatability → Step 5: Calibration cut
Print this, laminate it, stick it on the pendant. Average fix time drops from 45 min to 12 min.
Disclaimer
All experimental data presented in this paper are derived from controlled production environments and standardized test procedures. However, due to differences in equipment models, material batches, and on-site operating conditions, readers are advised to verify and adjust technical parameters according to their specific application scenarios before practical implementation.
The research results and technical insights shared herein are based on the author’s professional experience and experimental observations. The author and the affiliated institution shall not be liable for any direct, indirect, or consequential damages (including but not limited to equipment damage, product quality issues, or production losses) arising from the improper use of the information provided in this paper.
All experimental data presented in this paper are derived from controlled production environments and standardized test procedures. However, due to differences in equipment models, material batches, and on-site operating conditions, readers are advised to verify and adjust technical parameters according to their specific application scenarios before practical implementation.
The research results and technical insights shared herein are based on the author’s professional experience and experimental observations. The author and the affiliated institution shall not be liable for any direct, indirect, or consequential damages (including but not limited to equipment damage, product quality issues, or production losses) arising from the improper use of the information provided in this paper.
