Configuring SIMOTION D425 Absolute Encoder Cyclic Absolute Mode

Problem Overview

On a Siemens SIMOTION D425-2 PN/DP controller driving a 1FK7063-2AF71-1QB0 synchronous servo motor equipped with a single-turn 20-bit absolute encoder on the Drive-CLiQ interface, the axis reports correct position immediately after a homing routine but loses its absolute position reference after the controller is powered down and back up. Despite the position loss, the technology object's Homed status bit remains TRUE, leading to incorrect machine moves on the first motion command after a cold start.

The failure mode is reproducible: execute an absolute encoder calibration homing, save the project to ROM, jog the axis off the calibrated zero, switch off the 24 V control and 600 V DC-link supplies, restart, and observe that the displayed position no longer corresponds to the mechanical axis position while the Homed flag is still set.

Affected Hardware and Firmware

The same fault pattern applies to any SIMOTION D4xx or P320 platform paired with a single-turn absolute encoder (single-turn singleturn on SINAMICS, encoder code on the motor nameplate AS20DQC or AM20DQC prefix), regardless of drive type, as long as the technology object encoder evaluation is left in the default Absolute setting.

Root Cause Analysis

The root cause is the encoder evaluation attribute on the Encoder configuration page of the Axis technology object in SIMOTION SCOUT. SIMOTION differentiates two encoder evaluation modes on the TO Axis:

• Absolute – the encoder position is read once at ramp-up; the actual position value is only considered absolute within one mechanical revolution.
• Cyclic absolute – the encoder position is read every PROFIdrive cycle, and the controller tracks the number of singleturn overflows. The cumulative revolutions are stored remanently in the SIMOTION flash so that the absolute reference is preserved across power-off cycles.

When a single-turn encoder is configured as Absolute only, SIMOTION assumes the axis has not moved while the controller was switched off. After a power-cycle the TO re-initializes the position from the encoder's current singleturn reading plus the previously saved zero offset. If the axis was driven through any number of complete revolutions during the off state, the controller has no way to detect those revolutions and silently resets the displayed position to a value that differs from the true mechanical position by N × 360°.

The Homed status bit is set when the homing procedure (here: Absolute encoder calibration with specification) completes successfully. Because the calibration reference itself is saved remanently to the controller, the bit is restored on next ramp-up even though the underlying position value is now wrong. The result is a "phantom homed" axis: actualPosition <> realPosition, Homed = TRUE.

Encoder Configuration: Absolute vs. Cyclic Absolute

The behavior is documented in the SIMOTION Motion Control technology objects manual: the Configuration.ActualValueSettings.Type or, in the SCOUT dialog, the Encoder evaluation drop-down must be set to CyclicAbsolute for any singleturn encoder used in a position-controlled axis that is expected to maintain absolute referencing across power cycles.

Step-by-Step Resolution in SIMOTION SCOUT

1. Open the project offline. In SCOUT TIA, expand the SIMOTION device in the project tree and navigate to Axes > [AxisName] > Configuration > Encoder.
2. Open the encoder configuration dialog. Double-click Encoder under the Axis TO to launch the configuration window.
3. Switch to Details view. Click the Details button in the bottom-left of the dialog to expose advanced attributes that are hidden by default.
4. Locate the Encoder evaluation attribute. In the Details list find Configuration.ActualValueSettings.Type (label: Encoder evaluation). It is usually set to Absolute.
5. Change the value to CyclicAbsolute. Select CyclicAbsolute from the drop-down. The change is written to the project offline database.
6. Recompile and download. Right-click the SIMOTION device and choose Download to target system > Download project to target system (or Download CPU / drive unit if the SINAMICS is on a separate bus). Tick Replace all configuration data if the controller prompts.
7. Save the project to ROM (Copy RAM to ROM). On the SIMOTON device toolbar click Copy RAM to ROM so that the new encoder attribute is stored in remanent flash.
8. Perform a fresh homing run. Jog the axis to a known mechanical reference (limit switch, datum flag, or absolute reference marker) and re-execute the configured homing mode. Verify that ActualPosition = 0.0 (or the configured homing offset).
9. Power-cycle test. Move the axis off the home position by a known distance, power off the cabinet, wait at least 5 s for the DC-link to discharge and the controller to fully shut down, then power on. After ramp-up confirm that ActualPosition still matches the mechanical position within one revolution, and that Homed is TRUE.

Homing Configuration Recommendations

SIMOTION offers several homing modes for the Axis TO. With a single-turn absolute encoder combined with Cyclic Absolute evaluation, the recommended modes are:

• Active homing with encoder zero mark + cam – high precision; requires hardware reference cam.
• Passive homing – executed at runtime by the user program; useful when a higher-level PLC defines the reference event.
• Absolute encoder calibration – sets the current mechanical position to a configured value. Requires that the axis was moved to a known mechanical reference by some external means. Used in the original fault case ("absolute encoder calibration with specification").
• Absolute encoder calibration + homing mark – combines an absolute reference with a zero-pulse for higher precision.

Avoid Absolute encoder calibration without specification on singleturn encoders in machines that may be physically rotated while de-energized. Use it only when the axis is mechanically locked.

Verification Procedure

After applying the fix, run the following validation sequence and record the readings:

1. Bring the controller to RUN. Read AxisName.ActualPosition and AxisName.StatusHomed in the symbol table or via Control Panel > Axis in SCOUT.
2. Jog the axis to position 500.0 user units (or any convenient offset). Stop, observe steady state.
3. Trigger a controlled shutdown via SCOUT or by switching off the 24 V supply to the SIMOTION and SINAMICS.
4. Wait 10 s; restore power.
5. After ramp-up, read ActualPosition again. The expected value is the same as before shutdown (within one encoder increment). The expected value of StatusHomed is TRUE.
6. Compare the displayed position with the mechanical position by manually rotating the motor shaft one full revolution and observing that the display advances by exactly 360°.

If the post-shutdown position drifts by exactly N × 360° from the expected value, the axis is still configured in pure Absolute mode; re-check the Details view.

Diagnostic Buffer and Runtime Behavior

When the encoder evaluation mode does not match the encoder type, SIMOTION logs entries in the technology object's diagnostic buffer at ramp-up. Typical messages include:

• Persistente Daten reinitialisiert / Persistent data reinitialized – indicates that the remanent revolution counter was rejected because the configuration was inconsistent.
• Encoder configuration mismatch – encoder evaluation type conflicts with the encoder hardware class.

If no entries appear and yet the position is wrong, the axis is moving while the controller is off and the singleturn encoder cannot capture those revolutions; in that case upgrade to a multi-turn encoder (motor order code suffix ...1QB1 instead of ...1QB0).

Single-Turn vs. Multi-Turn Encoder Behavior

Related Parameters and Configuration

The encoder evaluation attribute lives in the following configuration paths and structured tags:

• TO Axis > Configuration > Encoder > Details > Encoder evaluation
• Configuration.ActualValueSettings.Type := TYPE_CYCLIC_ABSOLUTE
• Configuration.ActualValueSettings.AbsHomingEncoderDimension := 1 – mandatory for singleturn encoders.
• For multi-revolution absolute tracking set Configuration.ActualValueSettings.AbsHomingEncoderDimension := 4096 (or the multiturn resolution of the encoder).

The remaent storage size for the Axis TO grows when Cyclic Absolute is selected because the revolution counter is stored. Ensure that the project's persistent data budget on the SIMOTION CF card is sufficient; otherwise the controller may refuse to write and fall back to non-remanent behavior.

Troubleshooting Matrix

Safety and Commissioning Notes

• Always de-energize and lock-out the cabinet before disconnecting Drive-CLiQ connectors. Drive-CLiQ carries 24 V logic supply; hot-plugging can damage encoders and the SINAMICS Control Unit.
• After any change to encoder evaluation or homing configuration, run a reduced-speed test cycle and verify safe stop (STO) functionality before returning the machine to production.
• If the axis drives a vertical load, mechanical brakes must be applied before the cabinet is opened; an axis falling during service will invalidate the absolute reference regardless of encoder type.

Related Documentation

• SIMOTION D4xx Operating Instructions
• SIMOTION Motion Control – Technology Objects Manual
• SIMOTION 1FK7 Synchronous Servo Motors
• SIMOTION SCOUT Configuration Manual
• SINAMICS S120 Drive Functions Manual (encoder evaluation)

FAQ