Resolving SIMOTION D425 External Encoder Gearing Modulo 360° Speed Spikes

Electronic gearing on a SIMOTION D425 controller using a Drive-CLiQ external encoder as master and a 1FK7 servo as slave is a common topology for printing, web handling, and converting machinery. When the external encoder is configured as a 360° modulo rotary axis and a sudden velocity/acceleration jump appears exactly at the rollover boundary (359.999° → 0.000°), the slave axis inherits the spike as a real mechanical excursion on the belt. This article walks through the root-cause analysis, the encoder Technology Object (TO) configuration that eliminates the artifact, and the field verification steps.

1. Problem Description

The reported symptoms on the original commissioning platform are:

• Topology: SIMOTION D425 + SINAMICS S120 (or similar) with one Drive-CLiQ external encoder (absolute, rotary) as the master and a 1FK7042 servo with an absolute encoder as the slave.
• Application: Electronic gearing. The slave follows the master encoder position with a configurable transmission ratio.
• Master encoder type: Rotary, modulo 360°.
• Observed fault: Master speed trace shows a sharp velocity/acceleration excursion at the 0°/360° rollover boundary even though the mechanical rotation is steady.
• Downstream effect: The 1FK7042 slave reproduces the spike. The connected belt shows a visible jerk at every modulo transition.

Because the disturbance is exactly periodic with the encoder zero transition, it is almost always a software/math artifact in the encoder Technology Object — not a mechanical phenomenon.

2. Root-Cause Analysis

There are four realistic root causes for a modulo rollover spike in a SIMOTION electronic-gearing configuration, ordered by probability for this specific symptom set:

3. Confirm the Encoder Technology Object Type

The single most common root cause for this exact symptom is an External Encoder TO that was inserted as LINEAR and later had its modulo values forced on. SIMOTION must be told explicitly that the encoder is rotary so that it performs continuous modulo arithmetic on the position, and continuous (wrapped) differentiation on the velocity.

1. In SCOUT / TIA Portal (with SIMOTION option pack), open the project and navigate to the External Encoder Technology Object in the axis tree.
2. Open Configuration → Mechanics.
3. Set Type of axis = ROTARY. The setting is binary; the alternatives are LINEAR and ROTARY.
4. Confirm Modulo range = 360° (or the value matching the physical encoder, typically 360° for a single-turn rotary encoder).
5. Compile and download to the D425.

This procedure and the rotary/linear selector are documented in the SIMATIC AX online manual under the "Configure the type of an external encoder" topic: Configure the type of an external encoder – SIMATIC AX Manual.

4. Select the Correct Gearing Mode

SIMOTION offers two gearing primitives. The choice matters when the master is rotary and modulo:

Implementation in Structured Text on the D425:

// IPO task, called every IPO cycle
IF bEnableGearing THEN
    // _setMasterPosition accepts a rotary-modulo source
    _setMasterPosition(encoder := TO_EncoderExternal,
                       position := TO_EncoderExternal.actualPosition);
    _posGearing(slave := TO_Axis_Slave,
                master := TO_EncoderExternal,
                gearRatioNumerator := 1,
                gearRatioDenominator := 1,
                enable := TRUE);
ELSE
    _posGearing(enable := FALSE);
END_IF;

If the original program is using _velGearing, replace it with _posGearing on a rotary-modulo master. If the application genuinely requires a velocity-coupling behavior, the recommended pattern is to keep _posGearing and instead add a velocity feedforward or a low-pass filter on the slave velocity setpoint (see §6).

5. Check the Encoder Drive-CLiQ Configuration

The Drive-CLiQ encoder is auto-registered in the topology editor when it is plugged into a free Drive-CLiQ port of the D425 or its connected SINAMICS Line Module. Verify the following:

1. Open Topology in SCOUT. The encoder appears under the D425 (or NX module) with its part number and firmware version. Accept the actual topology if it differs from the offline project.
2. Open the Drive object that was created for the encoder. The default is Sensor Module + Drive pair.
3. Confirm the encoder type: Configuration → Encoder → Type should read Rotary, single-turn or Rotary, multiturn, never Linear.
4. Confirm Fine resolution (increments per revolution) and Gear factor are at the default 1:1 unless a mechanical ratio is required.
5. Save, compile HW Config, and download to the D425.

6. Add a Low-Pass Filter if Mechanical Ripple Remains

After steps 3–5, the modulo-rollover derivative discontinuity is gone. If a smaller, broadband ripple is still present (for example, from an eccentric encoder pulley or from the 1FK7042 load-side coupling), apply a low-pass filter to the encoder TO output:

1. Open the External Encoder TO → Configuration → Filter.
2. Enable Velocity filter with a time constant T_filter in the range 1–10 ms (start at 3 ms).
3. If the slave still follows the residual ripple, add the same filter to the slave axis (TO_Axis_Slave → Position control → Velocity setpoint filter).
4. Re-trace and confirm.

Do not increase T_filter beyond ~10 ms for a belt-driven slave — the added phase lag shows up as a lag error on every slave position command and can be misinterpreted as a gearing problem on its own.

7. Capture a Verification Trace

Use the SIMOTION trace to confirm the fix:

1. Open Trace → New in SCOUT.
2. Add the following signals at the IPO-cycle sampling rate (typically 1–4 ms):
   • TO_EncoderExternal.actualPosition
   • TO_EncoderExternal.actualVelocity
   • TO_Axis_Slave.actualVelocity
   • TO_Axis_Slave.positionCommand (slave following error context)
3. Record for at least 10 s and trigger on a single modulo rollover.
4. Expected result after the fix: actualVelocity is a smooth sinusoid (or constant, depending on the master motion), with no impulse at 0°/360°. The slave velocity follows it with a one-cycle delay equal to the filter time constant, but no spike.

8. Fallback Diagnostic: Disable Modulo and Use Relative Gearing

If the spike persists after the encoder TO has been confirmed rotary, perform an isolation test:

1. Temporarily set the external encoder TO to LINEAR with no modulo.
2. Run the master at a constant speed. The reported position will grow without bound (a 32-bit or 64-bit counter).
3. Re-enable gearing with relative gearing on the slave (synchronize once at start-up, then follow deltas).
4. If the spike disappears, the modulo handling on the encoder TO is the cause — escalate to Siemens Application Support with the project archive.
5. If the spike remains, the cause is mechanical; inspect the encoder coupling and the belt.

9. Quick-Reference Parameter Table

10. Troubleshooting Matrix

11. Summary

A periodic velocity/acceleration jump exactly at the 360° modulo rollover of a Drive-CLiQ external encoder master, on a SIMOTION D425 driving a 1FK7 slave, is in 9 out of 10 cases a Technology Object type mismatch or a velocity-gearing primitive used on a modulo master. The fix is twofold:

1. Set the External Encoder TO to ROTARY with modulo 360°.
2. Use _posGearing (position gearing), not _velGearing.

Re-trace both master and slave at IPO cycle, confirm the rollover spike is gone, and apply a 3 ms velocity low-pass filter on the encoder TO if a smaller residual ripple is observed.