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Mitsubishi MR-J4 servo amplifiers are widely used on servo presses in Asia. AL.16 and AL.20 are the most common encoder-related alarms.
## AL.16: Encoder Error 1
This alarm indicates a communication error between the servo amplifier and the encoder. The MR-J4 uses a high-speed serial encoder (MELSERVO protocol) that is sensitive to cable quality and noise.
Causes specific to press applications:
- Cable damage at the slide flex point (most common)
- Electrical noise from the main drive inverter coupling into the encoder cable
- Loose connector at the encoder (vibration from press operation loosens connectors over time)
- Encoder cable too long without proper shielding
Diagnosis:
1. Check the encoder cable at the point where it enters the slide housing - flex the cable manually and watch for alarm triggering
2. Check connector MR-J4CN2: look for bent pins, corrosion, or loose locking mechanism
3. Measure cable shield continuity: should be under 1 ohm from amplifier end to encoder end
4. Check cable routing: minimum 100mm separation from power cables, cross at 90 degrees if they must intersect
Fix:
- Replace cable with Mitsubishi MR-EKCBL flexible cable (rated for 10 million flex cycles)
- Add ferrite core on encoder cable at amplifier end (Mitsubishi part: MR-BFV0.5)
- Re-route cable away from power cables
## AL.20: Encoder Error 2
AL.20 is a more serious encoder fault indicating the encoder itself has detected an internal error.
Causes:
- Encoder overheating (motor temperature above 80C)
- Encoder battery failure (for absolute encoders)
- Physical damage to encoder from press vibration or crash
- Encoder supply voltage out of range
Diagnosis:
1. Check motor temperature: MR-J4 reports motor temperature via parameter display. Above 80C triggers AL.20
2. Check encoder battery voltage: should be above 3.0V. Replace if below 3.2V
3. Check encoder supply voltage at CN2 connector: pin 1 (+5V) should be 4.75-5.25V
## After Encoder Alarm: Absolute Position Recovery
After clearing AL.16 or AL.20 on an absolute encoder system:
1. Perform home position return (parameter PA14 = 1)
2. Verify the home position matches the mechanical reference
3. Run 5 slow-speed strokes and verify position repeatability
4. Do NOT run production until home position is confirmed