Troubleshooting SIMOTION D410-2 Axis Drift on Enable Command

Problem Overview

During commissioning of a SIMOTION D410-2 motion controller with the integrated SINAMICS S120 single-axis drive, the drive control panel responds normally and the SERVO closed-loop auto-tuning converges. The moment the technology object is enabled through the SCOUT axis control panel — and no positioning command has been issued — the axis immediately begins rotating and the drive trips on a standstill monitoring error. Raising the standstill window through p2542, p2543, or p2544 only delays the trip; the axis is physically moving, not drifting inside the window.

This symptom is the field signature of a positive feedback loop in the position-controlled SERVO: the position controller commands a corrective torque, the torque is applied to the motor in the wrong direction relative to the encoder counts, the position error grows, the controller responds with a larger torque, and the cycle continues until the standstill monitoring window is exceeded and the drive generates F07900 (or its SIMOTION technology object equivalent 20005 "Standstill monitoring error").

Affected System Configuration

The reported installation uses:

• Controller: SIMOTION D410-2 compact motion controller (catalog numbers 6SL3040-1LA00-0AA0 for PROFIBUS or 6SL3040-1MA00-0AA0 for PROFINET variants). The D410-2 combines a SIMOTION runtime with an integrated SINAMICS S120 single-axis drive (CU320-2 / CU310-2 control unit functionality) on a single module.
• Drive: SINAMICS S120 Control Unit integrated into the D410-2. PROFIdrive message frames (Standard Telegram 3, 5, 105, or 106) are assigned automatically by the engineering system when the axis is inserted into the project topology.
• Motor: SIMOTICS 1FK7 or 1FT7 servo motor with a DRIVE-CLiQ encoder interface. The encoder and motor temperature data are transferred digitally over DRIVE-CLiQ; no SMC/SME module is required.
• Power cable: A non-Motion Connect cable during the initial install, later replaced with a Siemens Motion Connect cable.
• Engineering tool: SIMOTION SCOUT (TIA Portal with the SIMOTION option is supported on current firmware).

The communication between the SIMOTION axis object and the integrated SINAMICS drive — including the PROFIdrive message frame, BICO interconnections, and the topology configuration — is set up automatically by the engineering system once the drive is inserted into the device topology. This is documented in the SIMOTION D410-2 Commissioning Manual: SIMOTION D410-2 Commissioning Manual (Siemens Support, attachment 61211384).

Root Cause: Positive Feedback From Reversed Counting Direction

The fault was traced to the power cable. A non-Motion Connect cable was used during the initial installation and the three line phases (U, V, W) were wired in a sequence opposite to the convention expected by the SERVO drive. The motor used a DRIVE-CLiQ encoder, so the encoder is identified and its direction is read from the electronic nameplate at first commissioning. The encoder direction does not change when the power cable is reversed — only the electromagnetic rotation direction relative to the encoder counts changes.

The signal flow is:

1. The encoder reports positive position increments for what should be negative mechanical motion — the counting direction is inverted at the mechanical level relative to the load.
2. The position controller interprets the growing position error and commands a torque to push the axis back toward zero.
3. The applied torque accelerates the axis further in the positive direction because the motor phase sequence is reversed at the power stage.
4. The position error grows, the torque command grows, and the axis runs away until the standstill monitoring window is exceeded and F07900 / 20005 is raised.

The fact that the closed-loop auto-tuning completes successfully even with the wrong cable is a feature, not an inconsistency: the auto-tuning routine excites the axis with small motion commands and tunes the speed-controller Kp/Tn to the observed plant response. A reversed cable inverts the plant response but does not change its gain, so the routine converges to a controller that is mathematically stable for the reversed plant. The instability only becomes visible when the position controller is closed on top of the speed controller with a small position error to correct — the moment the positive feedback loop closes.

Diagnostic Procedure

Follow this ordered sequence to isolate the sign error. Do not skip steps, because two of the three candidate causes (cable phasing, encoder inversion, axis configuration polarity) can coexist and a partial fix will still leave the axis unstable.

1. Verify the drive is in SERVO closed-loop control mode. Read parameter p1300 on the drive; it should be 21 (closed-loop speed/torque control with encoder) for a SIMOTICS servo motor.
2. Capture a trace of the actual speed and torque. In SCOUT open the trace tool, record r0061 (actual speed, smoothed), r0079 (torque setpoint), and r0062 (speed setpoint). Trigger on enable. The torque setpoint will saturate positive while the speed ramps positive — the visual signature of positive feedback.
3. Check the "Counting direction" setting on the axis. In the SIMOTION axis configuration, navigate to Axis → Mechanics and inspect the polarity checkbox. The default for a freshly inserted motor is "Standard" and must match the mechanical convention used by the machine.
4. Inspect the SINAMICS output direction parameters. Read p1820 (output phase sequence reversal) and confirm it is 0 (sequence U-V-W). p1820 = 1 forces a phase reversal at the power stage and should be enabled only as a documented workaround for a known cable plant anomaly.
5. Inspect the encoder direction. Read p1821 (direction of rotation reversal) and the encoder inversion flags in p0410. For DRIVE-CLiQ motors these are set automatically from the encoder's electronic nameplate; an inverted value here often indicates a mismatched motor type plate or a motor that was commissioned manually rather than identified automatically.
6. Power-down and physically verify the power cable. Disconnect the motor power connector and ring out U, V, W on the cable against the drive terminal labels.
7. Replace with a Motion Connect cable of the correct length and article number. See the Motion Connect Cable Specification section below.
8. Re-run SERVO optimization. After the cable change, repeat the one-button tuning routine so that the speed-controller Kp/Tn is re-calculated against the corrected torque-to-speed relationship.

Mechanical Axis Configuration in SIMOTION SCOUT

The axis configuration dialog in SCOUT exposes the mechanical interface between the encoder and the load. Three fields govern the sign relationship:

When the counting direction is changed after a project has been downloaded, perform a POWER ON of the D410-2 and re-run the axis homing routine. SIMOTION retains the modulo position from the encoder; inverting the counting direction does not move the home position, but it does change which side of the home mark the axis considers positive.

SINAMICS Standstill Monitoring Parameters

The drive's standstill monitoring function is configured by a small set of parameters in the SINAMICS. These are the values the engineer is most likely to consider increasing when the fault first appears. Increasing them without addressing the positive feedback does not help, but they are documented here for completeness.

The relevant SINAMICS fault is F07900 "Drive: Standstill monitoring has triggered" in the drive fault buffer. In SIMOTION the equivalent technology-object fault is 20005 ("Standstill monitoring error") raised by the Axis object. Both are generated by the same underlying monitoring function and both reset to clear after a fault acknowledge once the position has returned inside the standstill window and the controller is no longer commanding motion.

Encoder Direction and DRIVE-CLiQ

On a D410-2 the encoder is normally a DRIVE-CLiQ device — either integrated into the motor (1FK7/1FT7 with DRIVE-CLiQ interface) or mounted externally and connected through a Sensor Module (SMC20, SME20, etc.). The encoder identification, resolution, and direction are read from the encoder's electronic nameplate over DRIVE-CLiQ at first commissioning. This is why a D410-2 commissioning with a DRIVE-CLiQ motor typically does not require manual encoder parameter entry: the drive auto-configures the encoder after POWER ON.

The two parameters that govern the effective direction of the encoder on the SINAMICS side are:

• p1820 — Output phase sequence reversal (motor terminal side). Setting 1 reverses U-V-W to U-W-V. This is a power-stage fix; it changes the direction of the rotating field relative to the drive's terminal labels.
• p1821 — Direction of rotation reversal (encoder side). Setting 1 inverts the sign of the encoder increments in the drive. This is an encoder-side fix; it changes the sign of the position actual value without touching the power stage.

For a DRIVE-CLiQ motor with phases reversed in the cable, the correct fix is at the cable (swap the cable or set p1820). For a mechanical convention (e.g., a gearbox that reverses the load) the correct fix is p1821 or the SIMOTION axis counting direction checkbox. Mixing these fixes — for example toggling both the checkbox and p1820 to "compensate" — is a common source of intermittent commissioning failures and should be avoided.

Power Cable Phasing Verification

The Siemens power cable color code for SIMOTICS servo motors follows the standard convention U = black (BK), V = brown (BN), W = grey (GY), plus PE = green/yellow. When a third-party cable is used, the conductor colors frequently follow the IEC 60446 power color code (L1 = brown, L2 = black, L3 = grey) which does not correspond to U/V/W. Wiring a cable that follows L1/L2/L3 directly to U/V/W produces a 120° phase rotation error and the motor will run in the opposite direction relative to the encoder counts.

To verify the phasing with the drive de-energized and locked out per local safety procedure:

1. Disconnect the motor power connector at both ends.
2. Use a continuity tester and ring out each conductor end-to-end, marking U, V, W and PE on both sides.
3. Compare the motor-side pinout against the connector pin assignment in the SIMOTICS motor operating instructions (1FK7: 6SN1192-0AD06-0BPx; 1FT7: 6SN1192-0AD60-0BPx; 1PH8: 6SN1192-0AD84-0BPx).
4. Confirm that the connector backshell is keyed to prevent 180° misorientation. The size 1 (M23) and size 1.5 (M40) Speedtec connectors used on 1FK7/1FT7 motors have a single keyway; a misorientation would prevent mating.
5. After verification, mate the connector and torque to the connector specification (typically 8 Nm for M23, 12 Nm for M40).

Motion Connect Cable Specification

Siemens Motion Connect power cables (6FX5002 series for fixed routing, 6FX8002 series for continuous flex) are factory-built with the correct conductor-to-pin mapping and the cable shield terminated with 360° shield bonding on both ends. Using a non-Motion Connect cable removes the manufacturing guarantee that the U/V/W phase sequence is correct. The cables are ordered by length and motor frame size:

The replacement that resolved the reported incident was a standard Motion Connect power cable of the correct length and frame size for the 1FK7 motor in use. After replacement the axis accepted the enable command without drifting, the standstill window was no longer violated, and the closed-loop auto-tuning converged on the first attempt.

Related DRIVE-CLiQ Issues to Check

Beyond the power cable, a small number of DRIVE-CLiQ-related conditions can produce similar axis-drift symptoms and should be excluded during the same diagnostic session:

• Incorrect motor type plate downloaded. If the SINAMICS was previously commissioned with a different motor and the motor was swapped without a factory reset, p115[0] or the motor parameters p0300-p0360 may still reflect the original motor. A drive POR or motor identification routine clears this.
• Encoder cable integrity. DRIVE-CLiQ cables with intermittent contacts cause F3xxx faults and encoder plausibility errors, not steady drift, but a degraded cable can show as an intermittently inverted encoder reading on some firmware versions. Inspect the DRIVE-CLiQ connectors for bent pins and clean with the approved DRIVE-CLiQ cleaning kit if oxidation is suspected.
• Common-mode noise from adjacent VFDs. If the cabinet shares a supply with a VFD that has poor EMC filtering, common-mode noise can couple into the encoder cable. Use only fully shielded Siemens DRIVE-CLiQ cables (6FX2002-1DCxx) and route them at least 200 mm from power cables.
• Firmware mismatch between Control Unit and Motor Module. On a D410-2 the Control Unit and Motor Module firmware are coupled. Mixing firmware versions can produce unexpected position-controller polarity. Check with the SIMOTION SCOUT online diagnostics → Drive Unit Diagnostics view that the firmware versions are in the compatible release list.

Resolution Summary and Field Commissioning Notes

The reported fault was resolved by replacing the off-brand power cable with a Siemens Motion Connect cable of the correct length and motor frame size. The phases in the original cable were reversed relative to the U/V/W convention expected by the SERVO drive, producing a positive feedback loop the moment the position controller was enabled. After the cable replacement, the axis accepted the enable without moving, the standstill window was no longer violated, and the closed-loop auto-tuning converged.

Operationally, the lesson for future D410-2 commissioning is that the power cable is part of the safety-relevant signal chain. Use only Motion Connect cables (6FX5002 for static, 6FX8002 for flex) with the correct connector size for the motor frame, and verify the U/V/W phase mapping at the connector end with a continuity tester before energizing. The encoder, the counting direction, and the SINAMICS polarity parameters should be left at their defaults unless there is a specific mechanical reason to change them.

Verification Procedure After the Fix

1. With the controller in stop, energize the line module and wait for the ready state (LEDs on the D410-2: green RDY, green RUN).
2. In SCOUT open the axis control panel, but do not enable yet. Confirm r0061 (actual speed) and r0080 (torque actual value) are zero.
3. Assert the controller enable (STW1 bit 0 via the axis control panel or a test program). The axis must remain at rest. Any movement in r0061 indicates the polarity is still wrong.
4. Set a small jog command in the positive direction, then in the negative direction. The position actual value r2521 must increase in the first case and decrease in the second.
5. Run a 5-minute trace capturing r0061, r0079, and the position error. The position error should settle within the following-error window (p2546) without sustained oscillation.
6. Acknowledge any remaining faults and run the axis through one full homing cycle to re-establish the machine reference.

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