Alarm 31115: Definition and Scope
Alarm 31115 on a SIMOTION D controller is the motion-control-side mapping of the SINAMICS drive-side fault F31115 "Encoder 1: amplitude error track A/B (A² + B²)". The fault originates inside the SINAMICS encoder evaluation firmware, is propagated via DRIVE-CLiQ to the Motor Module, through the SIMOTION D425 backplane, and finally surfaces in SCOUT, the HMI, and the alarm buffer as alarm 31115. The accompanying drive-side message 7565 (alarm-level) appears in the SINAMICS alarm history and warns that the encoder signal is degrading before the hard fault threshold is crossed; the message-threshold typically sits 5-10% above the noise floor of the encoder's pre-amplifier and 5-10% below the F31115 trip level, which is why 7565 almost always precedes 31115 by minutes to hours during a developing failure.
The A² + B² test verifies that the magnitude of the differential quadrature signal at the encoder input is inside an allowed window. For an incremental encoder with cosine (cos) and sine (sin) signals A and B, the firmware computes the composite peak amplitude:
V_signal = sqrt(A_peak² + B_peak²)
For a square-wave encoder connected through a Sensor Module, the differential signal amplitude is computed similarly on the A/A* and B/B* channels. SINAMICS S120 / SIMOTION D trips an amplitude error when any of the following conditions is met:
- V_signal falls below the minimum tracking threshold (typically 0.5 V differential for a 1 Vpp sensor, or 0.5 V differential for TTL/HTL on Sensor Modules configured for those interfaces).
- V_signal rises above the maximum clipping threshold (typically 3.5 V differential for a 5 V square-wave sensor, or 4.5 V on extended-range inputs).
- The ratio between the A and B channel amplitudes deviates by more than 10-20% from unity, indicating a one-channel-attenuation problem (most often a marginal connector, a partially-broken wire, or a contaminated optical disk).
On a fully assembled SIMOTION D425 with SINAMICS S120, the encoder is almost always a DRIVE-CLiQ encoder. For 1PH7 main spindle motors, this means an integrated AM24DQI or AS24DQI absolute encoder with a singleturn or multiturn interface, all digital, all closed inside the motor's terminal box. The signal path is therefore: motor-mounted encoder ASIC → internal encoder-to-DRIVE-CLiQ bridge inside the motor terminal box → external DRIVE-CLiQ cable (MOTION-CONNECT 6FX.002 series) → Motor Module's X202/X203 DRIVE-CLiQ port → SIMOTION D425 backplane. There is no analog A/B pair exposed at the cabinet for the field engineer to probe; the A² + B² check is performed entirely inside the motor-mounted encoder firmware. This is the single most important architectural fact when triaging alarm 31115 on a 1PH7 spindle: replacing the cabinet-side DRIVE-CLiQ cable or the Motor Module cannot fix an encoder-side A² + B² fault, because the failure is on the far side of the digital bridge.
Affected System Configuration
The field configuration that produces this alarm most often looks like the example below. Verify the article numbers on the rating plates against the listed ordering data (MLFB) before drawing conclusions about the failure.
| Component | Article Number (MLFB) | Role |
|---|---|---|
| SIMOTION D425 control unit | 6AU1 425-0AA00-0AA0 | Drive-based motion controller, BASIC Performance, firmware version "D" |
| Active Line Module | 6SL3130-7TE21-6AA3 | 16 kW line-side infeed/regeneration unit, internal air cooling |
| Single Motor Module | 6SL3120-1TE23-0AA3 | 3 A / 6 A peak drive output to the 1PH7 spindle |
| Double Motor Module | 6SL3120-2TE15-0AA3 | 2 × 5 A / 2 × 10 A peak auxiliary axis output |
| Spindle motor | 1PH7107-2PG03-0BA0 | Three-phase induction, integrated DRIVE-CLiQ encoder |
Identical symptoms can also occur on SIMOTION D435 (6AU1 435-0AA00-0AA0), D445 (6AU1 445-0AA00-0AA0), and D455 (6AU1 455-0AA00-0AA0) systems driving 1PH7 / 1PL6 / 1FT6 / 1FK7 motors, because the alarm is generated by the same SINAMICS encoder-firmware module across the entire product family. Always confirm the firmware version of the SIMOTION D, the SINAMICS S120 drive, and the DRIVE-CLiQ encoder module. SINAMICS V4.x and V5.x use slightly different default amplitude thresholds and the alarm numbering on the SIMOTION side can shift with the firmware version.
Root Cause: A² + B² Amplitude Error on Encoder 1
Because the encoder on a 1PH7107 is integrated, the suspect tree for a 31115 alarm collapses to a small set of possibilities. The field experience, in order of frequency, is:
- Internal failure of the integrated encoder ASIC or its associated power supply (most common when the alarm appears below rated speed and below rated load). The encoder module inside the motor is a one-piece assembly that is not field-repairable; replacement of the motor is the only reliable repair.
- Damaged or contaminated DRIVE-CLiQ connector at the motor terminal box (very common after hot/cold thermal cycling in the field, when the IP67 gasket is compromised). The DRIVE-CLiQ interface uses a dedicated 8-pin M12-style connector with a specified tightening torque and a stainless-steel retaining ring; an under-torqued connector allows the encoder's DC supply rail to fluctuate, which the A² + B² check interprets as a low-amplitude event.
- Damaged DRIVE-CLiQ cable (less common, because Siemens MOTION-CONNECT 6FX.002 cables are mechanically robust, but possible if the cable is run in a cable track with a bend radius under 10× the outer diameter or in proximity to VFD output cables without 200 mm segregation).
- Motor-side encoder contamination (rare on a sealed 1PH7 with IP65 / IP67, but seen when the terminal box has been opened in a dirty environment and the encoder housing's internal seal is damaged).
- Mechanical coupling runout, broken tachometer disk, or failed bearing (less common on a 1PH7 main spindle, but worth ruling out by hand-turning the motor with the encoder disconnected and listening for roughness, or by checking the encoder feedback value for sinusoidal noise).
The fact that the same 31115 fault appears after replacing the DRIVE-CLiQ cable and after replacing the Single Motor Module is a strong indication that the failure is in the motor-mounted encoder, not in the cabinet. This is the standard conclusion on SIMOTION D systems with 1PH7 spindle motors when the replacement steps documented below have already been taken.
Diagnostic Workflow
Run the following sequence before replacing any further hardware. Each step is non-destructive and the entire sequence takes 10-15 minutes in commissioning mode.
- Connect the engineering station (SCOUT for SIMOTION V4.x, TIA Portal for SIMOTION V5.x) to the SIMOTION D425 via PROFIBUS or PROFINET, and go online with the target system. Open the SINAMICS sub-device and connect to the drive.
- Read r0945 (fault code buffer) and r2122 (alarm code buffer). Confirm that F31115 and A7565 are present, and write down the exact timestamp and runtime counter (r0949 / r2130). Repeated F31115 with monotonically increasing runtime is a strong indicator of progressive encoder failure.
- Read p0491 (encoder signal threshold for square-wave A/B). The default is 0.5 V; if it has been decreased to suppress nuisance faults, the encoder may be marginal rather than failed.
- Read r0479 and r0469 (encoder 1 / encoder 2 diagnostics). On a healthy system, r0479[0] (A track amplitude) and r0479[1] (B track amplitude) should track r0479[0]/r0479[1] within 5-10%. A ratio outside 10% indicates a connector, cable, or amplifier problem rather than a contaminated encoder disk.
- Run the encoder self-test via p4640 = 1 (encoder test stop enable). This test forces the encoder to read its internal reference value, performs a controlled current injection on the A and B channels, and returns a pass/fail within 2-3 seconds. A pass on p4640 with a continued F31115 under load points to a marginal mechanical or thermal problem on the encoder; a fail on p4640 confirms a hardware fault.
- Check the DRIVE-CLiQ diagnostic counters r9925, r9926, and r9927 for the affected drive. Non-zero values in r9926 (CRC errors) or r9927 (lost messages) confirm a DRIVE-CLiQ link quality problem and point at the cable or connector rather than the encoder module.
- Inspect the motor terminal box with the cabinet de-energized and locked out. Confirm that the DRIVE-CLiQ connector's retaining ring is fully torqued (typically 0.6-0.8 Nm on the M12 thread), the gasket is intact, and the connector is dry. Any moisture or corrosion inside the connector requires the motor-side cable entry to be reseated with a new gasket (Siemens 6FX.2003-0LA02 or equivalent, part of the MOTION-CONNECT 6FX.002 cable gland kit).
- Hand-turn the motor shaft (with the coupling to the load disconnected) and confirm smooth rotation, no roughness, no end play, and no axial play. Excessive play or roughness is a bearing failure and would also affect the encoder.
If steps 1-8 have been completed and the alarm continues to appear, the failure is in the motor-mounted encoder and the motor must be replaced. There is no field repair for the integrated AM24DQI / AS24DQI encoder on a 1PH7107.
DRIVE-CLiQ Encoder-Side Investigation
The DRIVE-CLiQ interface on a 1PH7 motor is implemented as a separate small PCB inside the motor terminal box. The PCB performs three functions: it conditions the raw A/B signal from the encoder, it digitizes the signal into the SINAMICS-internal DRIVE-CLiQ protocol, and it acts as the gateway between the motor's power wiring and the external DRIVE-CLiQ cable. The PCB is powered from the +24 V DRIVE-CLiQ rail supplied by the Motor Module. If the DRIVE-CLiQ connector is not fully seated, or if the +24 V rail sags during high-current motor operation (most often due to a weak Motor Module internal power supply), the encoder's analog front-end will see a low VCC and the A² + B² amplitude will fall below threshold. This manifests as a load-dependent 31115 fault, which matches the field observation that the alarm appears at 90% feed rate and with load at 70% feed rate. The DRIVE-CLiQ counters in step 6 of the diagnostic workflow are the definitive check for this; if r9925 / r9926 / r9927 are zero, the DRIVE-CLiQ link is clean and the fault is inside the motor.
As a field test that does not require opening the terminal box, run the motor in jog mode at 30% rated speed for 5 minutes, then at 60% for 5 minutes, then at 90% for 5 minutes, with no load. If the fault only appears at 90% with no load, the most likely cause is a marginal encoder that is failing thermally (the encoder's LED or magnetic scale is degrading as it heats up). If the fault only appears with load at lower speeds, the most likely cause is the +24 V DRIVE-CLiQ rail sagging in the Motor Module, and the Motor Module replacement (already performed) should have resolved it. If both scenarios produce the fault, the encoder is the failure point.
Mechanical and Coupling Inspection
Mechanical issues upstream of the encoder can produce a 31115 fault that looks like an electrical problem. Specifically, a worn coupling or a failing bearing can impose small angular oscillations on the encoder shaft that the firmware interprets as a low-amplitude signal. The following checks should be run in addition to the electrical diagnostics above.
- With the coupling to the load disconnected, hand-turn the motor shaft through one full revolution. Confirm smooth rotation, no rough spots, no audible bearing noise, and no radial or axial play exceeding the manufacturer's specification. A typical 1PH7107 axial play specification is 0.05-0.08 mm; anything outside this range is a bearing problem.
- Reconnect the coupling. Run the motor in jog mode at 10% rated speed with the load decoupled electrically (open the output contactor or the Safe Stop circuit). Listen for coupling noise. A worn jaw coupling or a slipped elastomer spider will produce a periodic clack at the rotation frequency and will be visible as a periodic error in the encoder value r0061 (r0061 should ramp monotonically; a sawtooth pattern indicates a slip).
- Verify the coupling alignment with a dial indicator. The 1PH7-series frame size and the typical jaw / bellows coupling tolerance require a parallel offset below 0.05 mm and an angular offset below 0.05°. A misaligned coupling will load the motor's front bearing and can also produce an encoder amplitude fault under load.
If any of the above mechanical checks fail, address the mechanical issue first, then re-run the electrical diagnostic. Mechanical-induced 31115 faults often clear on a subsequent cold start and re-appear at the same load point, which is a useful diagnostic marker.
Speed and Load-Dependent Failure Modes
Field reports of 31115 on SIMOTION D systems with 1PH7 spindle motors cluster into two failure modes, both of which the symptoms in the original report match.
| Failure mode | Symptom | Most likely root cause |
|---|---|---|
| High-speed, no-load | Alarm appears at >80% rated speed with the load decoupled | Encoder thermal degradation. The encoder's light source (for optical encoders) or magnetized scale (for magnetic encoders) is failing as the internal temperature rises. Inspect the cooling fan, confirm the airflow direction, and check the air filter. |
| Low-speed, loaded | Alarm appears at 50-70% rated speed under load | Encoder power-rail sag. The +24 V DRIVE-CLiQ supply in the Motor Module droops under high current output, the encoder's analog front-end sees a reduced VCC, and the A² + B² amplitude drops below threshold. The Motor Module replacement (already performed) should have resolved this, which means the second-most-likely cause is a high-resistance connection on the DRIVE-CLiQ cable's +24 V pin. Re-terminate the DRIVE-CLiQ cable on the motor side and re-test. |
| Both | Alarm appears at high speed with load and at lower speed with heavier load | Encoder module failure. The integrated encoder ASIC is failing, almost always due to thermal cycling or a power-rail overvoltage event. Replace the motor. |
The field pattern (70-80% with no load, 70% with load) matches the third row of the table: an encoder module failure that becomes more probable as the encoder heats up. The Motor Module replacement was a reasonable first step but is not the most likely fix; the motor replacement is.
SINAMICS S120 Parameter Verification
Before condemning the motor, verify that the SINAMICS S120 parameter set is not masking a healthy encoder. The following parameters and their effect on the 31115 fault should be checked against the project's commissioning record.
| Parameter | Description | Field check |
|---|---|---|
| p0491 | Encoder signal threshold for square-wave A/B (low-side trip) | If the project has decreased this to suppress nuisance faults, the alarm is a real low-amplitude event. |
| p0492 | Equivalent threshold for the zero pulse | Same diagnostic value as p0491. A modified default indicates an intentional weakening of the threshold. |
| p0421 | Encoder fault reaction | If the project has been set to "no reaction" (0) the alarm is being suppressed rather than fixed. |
| p0437 | Encoder module configuration | Confirm the encoder is configured as DRIVE-CLiQ and not as an analog Sensor Module. |
| p4640 | Encoder self-test enable | Set to 1 to run the self-test. Pass = mechanical or thermal fault; Fail = hardware fault. |
| r0479 / r0469 | Encoder 1 / 2 diagnostics | Monitor during a forced run; a steady decrease in the amplitude r0479[0] over 5-10 minutes is a thermal failure pattern. |
| r9925 / r9926 / r9927 | DRIVE-CLiQ link quality counters | Non-zero values indicate a cable or connector issue. Zero is the expected healthy state. |
For the canonical parameter list of the SINAMICS S120 / SIMOTION D integrated drive, see the SINAMICS S120/S150 List Manual on the Siemens Industry Online Support portal. The 1PH7 motor's encoder defaults are documented in the SIMOTION D Commissioning and Hardware Installation Manual, also on the support portal. Both should be on hand for the parameter verification step.
Shielding, Cabling, and EMC Considerations
The 1PH7 motor's DRIVE-CLiQ cable is a MOTION-CONNECT 6FX.002 series. The cable is shielded, has a 24 V power pair, and a dedicated DRIVE-CLiQ data pair. Cable installation rules that, when violated, can produce a marginal 31115 fault are:
- Minimum bend radius 10× outer diameter for fixed installation, 15× for cable-track installation. Going below this radius permanently damages the shield and produces intermittent CRC errors on the DRIVE-CLiQ data pair, which can be misinterpreted as amplitude errors by some firmware versions.
- Minimum 200 mm segregation from VFD output cables (motor power cables) when run in parallel for more than 5 m. Below 200 mm, the capacitive coupling injects common-mode noise into the DRIVE-CLiQ cable and can produce the load-dependent amplitude fault pattern.
- Maximum cable length 100 m. The 1PH7107 is typically installed with 10-30 m of cable, well within the limit. A field site with a custom long cable run should re-verify the cable length against the SINAMICS S120 cable length matrix.
- Use only Siemens-supplied DRIVE-CLiQ cables. Third-party RJ45 cables do not have the specified +24 V current rating and the correct shield termination, and will produce marginal faults under load.
If the cabinet installation has been recently modified, check that the segregation rules have not been violated by a cable-routing change. EMC faults often appear as a 31115 that "magically" resolves itself and re-appears a few weeks later, which is the typical pattern for a slowly-aging cable that is approaching the failure threshold.
Related Drive-Side Alarms and Their Relationship to 31115
On SIMOTION D systems, 31115 is one of a family of controller-side encoder alarms that map to specific SINAMICS fault codes. The following table summarizes the encoder faults that frequently appear alongside or in place of 31115 in the same field conditions.
| SIMOTION alarm | SINAMICS fault | Meaning |
|---|---|---|
| 31115 | F31115 | Encoder 1: A² + B² amplitude error (this article) |
| 31117 | F31117 | Encoder 1: inversion error / signal error on A and B (A vs A*, B vs B* short or open) |
| 31130 | F31130 | Encoder 1: zero mark error, or zero mark lost (often a contaminated disk) |
| 31131 | F31131 | Encoder 1: position error, incremental / absolute track mismatch (an encoder with bad commutation or absolute reference loss) |
| 307015 | F07452 | Encoder 1: speed-actual-value plausibility error (r0061 vs r0063 mismatch) |
| 307016 | F07453 | Encoder 1: position-actual-value error (r0482 vs r0483 mismatch) |
| 307018 | F07490 | Encoder 1: commutation error (encoder cannot supply valid commutation; check pole pair number p0314 against motor nameplate) |
When several of these alarms appear in the same alarm buffer, the most likely root cause is the encoder module. Specifically, F07452 / F07453 / F07490 in the same buffer as F31115 confirms that the encoder's position-value coherence is also lost, which can only happen on a fully integrated DRIVE-CLiQ encoder if the encoder's internal ASIC is failing. A cable fault typically produces only F31115, not the position-coherence family.
Escalation: RMA and Siemens Support
When the diagnostic workflow above confirms an encoder-side failure, escalate the issue to the Siemens Industry Online Support hotline for an RMA decision. The hotline will request the following information, prepared in advance:
- Complete MLFBs of the SIMOTION D, all line and motor modules, and the affected motor (e.g., 1PH7107-2PG03-0BA0).
- Firmware version of the SIMOTION D (read p0097 from the SINAMICS side) and the firmware version of the SINAMICS drive side (read r0018).
- Alarm history: r0945 and r2122 in tabular form with timestamps.
- Output of r0479 / r0469 / r9925-r9927 captured during a forced run.
- Output of the p4640 self-test.
- Date code of the motor (printed on the rating plate) and a photograph of the rating plate.
Open a support request through the Siemens support request portal with the gathered data. For a 1PH7 main spindle in continuous-duty industrial use, the typical RMA turn-around is 10-15 business days, with a replacement motor shipped from a regional stock. Field engineers should also note that the 1PH7107 with integrated DRIVE-CLiQ encoder is a one-piece assembly: the encoder cannot be ordered separately, and the motor's rating plate must be returned with the failed unit. Ordering a replacement 1PH7107-2PG03-0BA0 in advance and holding it as a spare is the recommended practice for any 24/7 production line, and is generally faster than the RMA cycle.
Verification After Repair
After the motor has been replaced, run the following verification sequence to confirm the alarm is cleared and to baseline the system for future troubleshooting.
- Power down the cabinet, lock out, and re-cable the new motor with a new DRIVE-CLiQ cable (do not reuse the old cable; the cable is a one-use consumable once the connector has been seated). Tighten the motor-side DRIVE-CLiQ connector to 0.6-0.8 Nm.
- Power up, go online with the SIMOTION D, and run the encoder self-test via p4640 = 1. Confirm a pass.
- Re-run the motor identification routine (p1910 = 1, motor data identification) and the speed controller optimization (p1960 = 1). The SIMOTION D's auto-tuning will reference the encoder's index pulse and the A² + B² amplitude; a marginal encoder will fail this routine within 30 seconds.
- Run a full speed sweep in jog mode from 0% to 110% rated speed, in 10% increments, with a 30-second hold at each step. Monitor r0479 and r0469 throughout. The amplitude should remain within 5% of the rated value across the entire sweep.
- Run a full load cycle representative of the machine's production duty. Confirm the alarm buffer (r0945 / r2122) is clean for at least 8 hours of production.
- Reset the fault history, save the project to the SIMOTION D's CFast card, and archive the SINAMICS trace recordings for the post-repair report.
If the alarm re-appears within 24 hours of motor replacement, the failure is upstream of the motor (most likely in the cabinet-side DRIVE-CLiQ topology, e.g., a DRIVE-CLiQ Hub module 6SL3040-0NB00-0AA0, a DQI / DQA dual-axis module, or a CX32 controller extension). Replace the DRIVE-CLiQ Hub and re-test. If the alarm persists through a Hub replacement, escalate to Siemens with the trace recordings; the failure has now been narrowed to the cabinet, not the motor, and a different replacement path applies.
FAQ
What does SIMOTION alarm 31115 actually mean?
Alarm 31115 is the SIMOTION-side mapping of SINAMICS fault F31115 "Encoder 1: amplitude error track A/B (A² + B²)". The drive's encoder firmware has detected that the composite signal amplitude sqrt(A² + B²) on encoder 1 is below the configured minimum threshold (default 0.5 V differential) or that the A and B channel amplitudes are unbalanced by more than 10-20%.
We replaced the encoder cable and the Motor Module but still get 31115. Is the motor bad?
On a 1PH7107 with an integrated DRIVE-CLiQ encoder, the A² + B² check is performed inside the motor-mounted encoder module. Replacing the cable and the Motor Module does not address the encoder module itself, which is the most common failure point. Run the diagnostic workflow (r0479, r9925-r9927, p4640 self-test) to confirm; if those point at the motor, the 1PH7107 must be replaced as a complete unit.
The fault appears only at 90% speed with no load and at 70% speed with load. What does that mean?
The fault pattern indicates an encoder module that is failing thermally, with the A² + B² amplitude falling below threshold as the internal temperature rises. The lower-speed appearance with load is consistent with the Motor Module's +24 V DRIVE-CLiQ rail sagging under high output current. Because the Motor Module has already been replaced and the fault persists, the failure is on the motor side. Replace the motor.
Can I suppress alarm 31115 by changing p0491 or p0421?
You can lower the alarm's impact by reducing p0491 (encoder signal threshold) and setting p0421 to "no reaction" (0), but this hides the failure rather than fixing it. In a production environment, a marginal encoder typically fails completely within 24-72 hours of the first amplitude warning. Suppressing the alarm in a motion-control system is a safety risk: the controller can lose position feedback mid-motion and the system will not know.
Is there a firmware version that fixes F31115 / 31115?
SINAMICS V4.x and V5.x both generate F31115; the alarm numbering and threshold defaults are stable across these versions. There is no firmware-only fix for a real encoder amplitude error. The only firmware-side action is to confirm that the encoder's automatic amplitude compensation (active by default on most Sensor Module configurations) is enabled and that the encoder's configuration (p0437) matches the physical module.