SINAMICS S120 1FK7 Encoder Config: Fix STARTER Detection

1. Problem Overview

A SINAMICS S120 control unit CU310DP connected to a 1FK7034-5AK71-1DG0 synchronous servo motor reports an encoder type string in STARTER that does not match the motor's MLFB (order number) suffix. The motor carries a 22-bit incremental encoder with 11-bit commutation position and DRIVE-CLiQ interface, identified by the xDxx position in the order code. After automatic configuration STARTER displays the encoder characteristic as:

• Type: rotary
• Encoder code: [2001]
• Resolution: 2048 signals/revolution
• Signal level: 1 Vpp
• Tracks: A/B, C/D with reference (R)

The user observes the standard encoder selection list in STARTER exposes xAxx, xExx, xGxx, xHxx, xJxx, xSxx, xTxx variants of the 1FK7 family but no xDxx. The result is the appearance of an encoder mismatch, even though the Control Unit reads the DRIVE-CLiQ electronic rating plate correctly. Concurrently, the unfiltered actual speed value r63 deviates up to 17% from setpoint at 100 rpm, and the filtered value r21 still drifts approximately 3.5% - outside the required ±1% envelope.

This article documents the underlying cause, the manual encoder parameter path, and the controller optimization that resolves both the encoder label inconsistency and the low-speed accuracy problem.

2. Hardware Identification: CU310DP and 1FK7 Encoder Variants

The CU310DP Control Unit is the standalone (booksize) variant of the SINAMICS S120 platform. It provides PROFIBUS as the fieldbus interface, a DRIVE-CLiQ port for the Sensor Module / motor, and is intended for single-axis high-performance servo applications. Refer to the SINAMICS S120 Function Manual for the full hardware matrix.

The 1FK7 motor family (compact, high-dynamic 3-phase permanent-magnet synchronous servomotors) accepts an encoder in the position 14 of the MLFB. The encoder letter specifies the feedback type and signal interface:

The xDxx suffix on the 1FK7 represents a 22-bit incremental encoder with 11-bit commutation signal (C/D tracks) and integrated DRIVE-CLiQ interface - functionally similar to the AM22DQI concept but delivered as an incremental feedback device rather than an EnDat absolute encoder. The DRIVE-CLiQ encoder is sometimes documented as a 22-bit incremental with commutation offset and is the encoder family that STARTER does not expose in the standard encoder drop-down list, even though the drive firmware contains the correct parameter set.

3. How DRIVE-CLiQ Automatic Configuration Works

When a DRIVE-CLiQ encoder is plugged into a Sensor Module (SMC, SME, or directly into the CU310DP), the SINAMICS S120 performs the following on every power-up:

1. Reads the electronic rating plate (EEPROM) inside the encoder via DRIVE-CLiQ.
2. Writes motor-side parameters (p0300 series) and encoder-side parameters (p0400 series) into the Control Unit automatically.
3. Selects the matching encoder interface (p0400) based on the encoder family code stored in the DRIVE-CLiQ identity record.
4. Reports the active encoder model in the STARTER configuration tree.

According to the SINAMICS S120/S150 List Manual, the parameters that DRIVE-CLiQ auto-populates include p0400 (encoder type), p0401 (encoder selection), p0402 (encoder ratio), p0404 (encoder configuration), p0407 (leading encoder), p0408 (rotary encoder pulses per revolution), p0410 (reversing), and p0411 (measuring gear).

4. Why the Encoder Type Label Appears Incorrect

The display string "rotary, [2001] 2048, 1 Vpp, A/B C/D R" that STARTER shows is not a misdetection - it is the parameterization of the encoder that was actually loaded. The encoder identified by the DRIVE-CLiQ record has:

• p0400 = [2001] (DRIVE-CLiQ incremental encoder with C/D commutation tracks)
• p0408 = 2048 (incremental signal periods per revolution)
• p0420 (encoder voltage) = 1 Vpp
• p0421 (encoder type) = rotary

The mismatch is therefore a display-side issue: the parameter p400 value [2001] and the Enc. type: field in the STARTER GUI both describe a generic DRIVE-CLiQ incremental encoder. The user is searching the standard encoder catalog for the exact xDxx order-number row, but STARTER's catalog table only enumerates the variants listed in section 2. The xDxx variant is a configuration-only entry - the drive firmware recognizes it via the DRIVE-CLiQ electronic rating plate, not via the human-readable MLFB row.

In practice, this means the encoder is already configured correctly. STARTER's drop-down list omits the xDxx row because the same parameter set is reused for several motor variants.

5. Standard Encoder List in STARTER

The STARTER "Select standard encoder from list" dialog (under Configuration > Encoder selection > Select standard encoder from list > Via order no.) is sourced from the STARTER installation directory and from the connected drive firmware's encoder catalog. The catalog is version-dependent; STARTER V4.2.0.1 (used in the original case) is older and pre-dates several 1FK7 motor expansions.

Upgrading STARTER is optional - if the drive has loaded the encoder parameters correctly, the application can proceed. Verify the loaded parameter set in the expert list (see next section).

6. Manual Encoder Parameter Configuration

If the drive has not loaded the encoder automatically, or if the user wants to perform a manual entry, set the following in the STARTER expert list (right-click the drive > Expert list):

7. p-Parameter Reference for Encoder Setup

The most important encoder parameters for diagnosis and verification are summarized below. All are documented in detail in the SINAMICS S120/S150 List Manual.

The r482 parameter is the raw 32-bit position word (Gn_XIST1) delivered by the encoder to the controller. The 1FK7 xDxx encoder increments r482 with 2048 signal periods and 11-bit commutation position - the controller merges these into a unified position word. A consistent, monotonically increasing r482 during manual shaft rotation confirms the encoder is functional even when the display label looks wrong.

8. Low-Speed Accuracy Diagnostics

The user observed r63 (unfiltered speed actual value) error of up to 17% at 100 rpm setpoint, and r21 (filtered speed actual value) error of 3.5%. The target was ±1% across the entire 100-2000 rpm range. The error pattern - large at low speed, small at high speed - is characteristic of:

1. Unoptimized speed controller - low proportional gain, long integral time, or both.
2. Speed filter not configured - the firmware default p1451 = 0 (no filter) exposes the raw 1-ms quantisation of the speed calculation.
3. Speed setpoint filter not configured - p1441 and p1442 default to 0 ms, allowing step changes to be applied directly to the current setpoint.
4. Mechanical resonance - typical for direct-drive 1FK7 applications where coupling stiffness is low.

At very low speeds, the speed actual value is computed by differentiation of the position (r482) over a fixed time window. The differentiation window of 1 ms and the position resolution of 2048 S/R (with x4 interpolation in the Sensor Module, 8192 counts/rev) yield a quantisation floor of:

v_quant = 60 / (4 * p0408 * 1e-3) = 60 / (4 * 2048 * 0.001) = 7.32 rpm per LSB

That is, a single LSB of the position counter corresponds to ~7.3 rpm at a 1 ms update rate. At 100 rpm, a 1-LSB jitter is already 7.3% - which matches the 17% peak error observed. The mitigation is filtering and, more importantly, a properly tuned speed controller that pre-shapes the speed setpoint from the position difference rather than differentiating the noisy position feedback.

9. Speed Filter Configuration

Three filter blocks determine the smoothness of the actual and setpoint speed signals. Configure them in the expert list before running automatic controller setting:

For a 1FK7034 motor with a 100-2000 rpm operating window and ±1% target accuracy, the p1451 low-pass filter time of 2-4 ms is the most effective single adjustment. The filter is a 2nd-order low-pass implemented in the firmware. Increasing the filter time beyond ~10 ms introduces excessive lag; below 1 ms, the quantisation noise remains visible.

10. Automatic Controller Setting Procedure

1. Connect STARTER to the CU310DP online (PROFIBUS or Ethernet service port).
2. Set the drive to Online > Commissioning mode (top-level toolbar).
3. Navigate to Drive > Commissioning > Automatic controller setting.
4. Configure the limits: maximum speed, maximum acceleration, and maximum permissible travel (if the motor is connected to a load that can move).
5. Start the routine. The drive will perform a series of test movements, then commit the optimised parameters.
6. After completion, perform a Copy RAM to ROM (p0977 = 1) to make the parameters non-volatile.

According to the SINAMICS S120 Function Manual, the automatic controller setting requires the drive to be in operating state S60 (drive commissioning). It is not available in Commissioning Object mode without a preceding online connection.

11. Verification and Acceptance Test

After running the automatic controller setting, verify the speed accuracy by commanding a series of setpoints and recording r21 and r63 via the trace function in STARTER.

The trace recording should be captured at 1-2 ms sampling interval for at least 5 seconds at each setpoint. Use the trace math block to compute (r21 - r62) / r62 * 100 and confirm the maximum deviation is within the ±1% envelope. If the deviation is still outside the envelope, increase p1451 by 1 ms and re-test. If that fails, repeat the automatic controller setting with a higher Maximum acceleration value to force the optimizer into a stiffer control configuration.

12. Encoder Fault Codes and Diagnostics

The following fault codes may appear during the configuration sequence. Refer to the SINAMICS S120/S150 List Manual for the canonical definitions.