MR-J4-B torque loss after a stop: EM2, STO and maintenance isolation
An axis stopping does not identify the stop path. Distinguish EM2, controller forced stop and STO using E6, E7 and 63 records, and understand why STO does not prove safe maintenance isolation.
Contents
A maintenance note saying only “the motor does not move after an emergency stop” cannot establish amplifier failure. The origin of the stop request, event order and removal of drive energy are the useful distinctions. This article covers MR-J4-_B_(-RJ) using SH(NA)030106ENG-T, June 2024. It does not transfer directly to MR-J4-A, MR-J4W, MR-J5 or other safety-module combinations. It interprets existing records; it is not an emergency-stop design, wiring or functional-test procedure.
Here, STO inputs turning OFF means that the STO1/STO2 inputs are OFF and safe torque off is activated; it does not mean disabling or bypassing the STO safety function.
1. Separate the source of a stop from its method
The stop table in §4.2(5) and §3.6 associate EM2 opening with forced-stop deceleration, when its conditions are met, and AL. E6. A controller forced-stop command corresponds to AL. E7. Forced-stop deceleration is unavailable in torque control. The table associates STO with base-circuit shutdown and dynamic braking; STO itself does not guarantee controlled deceleration.
An axis at rest is an outcome, not evidence identifying the stop path. E6 does not mean that amplifier power components have failed. Classify the warning source before interpreting other alarms and machine states, rather than replacing an amplifier solely because of its display.
2. Use existing evidence, not bypasses
| Recorded observation | Evidence to examine first | What it does not establish |
|---|---|---|
| AL. E6 | EM2 state at that time, control mode, controller records and machine events | Motor or power-module failure, or successful completion of controlled deceleration |
| AL. E7 | The controller forced-stop event and its upstream cause | A fault specifically in the amplifier EM2 circuit merely because the axis does not move |
| No drive torque in STO | Existing safety-system diagnostics, stop reasons and reset-condition records | Power disconnection, absence of stored energy or permission to enter a hazardous area |
| AL. 63 | Complete alarms, speed records and the order of EM2/STO events around the stop | A particular wire or component that must be replaced |
These are evidence-organizing suggestions, not instructions to touch energized terminals or create new stops. Use only safely accessible existing logs, monitor records and machine documents. Qualified personnel must handle measurements or functional validation under the machine's approved procedures.
3. Why AL. 63 needs an event timeline
Section 3.6.4(3) identifies STO inputs turning OFF during forced-stop deceleration as a cause of AL. 63. Section 13.3 also describes dynamic-brake stopping when STO inputs turn OFF during operation. Put deceleration onset, STO changes, speed changes and alarm appearance on one timeline rather than treating the last displayed alarm as the first cause.
For illustration only, suppose saved records show E6 followed by 63. Ask whether STO acted before deceleration finished; do not declare a failed safety relay. If records have different time bases or sampling is too slow to resolve their order, state “sequence unconfirmed.” Do not alter safety delays, disable STO or bridge inputs to remove an alarm.
4. No torque is not maintenance isolation
Section 13.1.4 distinguishes electronic shutdown from mechanical disconnection: STO provides no electric-shock isolation and cannot guarantee that external forces will not move the axis. Safety page A-2 requires power-off, waiting and safe-voltage verification before inspection. No single STO indication, zero-speed display or charge-lamp state proves complete isolation.
A handover should therefore answer three separate questions: Who requested the stop? Which stop path did the drive take? Have maintenance isolation and mechanical risk controls been confirmed under the machine procedure? Answering the first two does not complete the third. Machine risk assessment must determine safeguards for gravity or other external forces; “STO active” is not a substitute.
5. Write a conclusion that supports further diagnosis
Record the full amplifier model, control mode, complete alarms and subcodes, timestamp sources, saved monitor data and machine safety-document revision. Separate observations, interpretations supported by the manual and points still needing verification. A useful initial conclusion might say: controller forced stop is recorded; amplifier hardware failure is not established; maintenance isolation requires separate confirmation under the site procedure. This supports the next technician better than “servo faulty.”
This article provides no safety certification, stopping distance, parameter setting or permission to restart. Final diagnosis and return to service require the actual configuration, applicable manuals and machine-builder validation requirements.
Reference: MR-J4-_B_(-RJ) Servo Amplifier Instruction Manual, SH(NA)030106ENG-T (2024-06). §4.2(5), printed page 4-7; §§3.6 and 3.6.1, 3-29; §3.6.4, 3-32–3-33; §§13.1.2–13.1.4, 13-1–13-2; §13.3, 13-6; Safety Instructions, A-2. Recordkeeping methods and analytical examples are editorial suggestions, not manufacturer test cases.