Eliminating Following Error in SINAMICS S120 with SINA_POS

1. Problem Overview

A SINAMICS S120 axis controlled via the SIMATIC SINA_POS telegram block from a higher-level SIMOTION, SIMATIC S7-1500, or T-CPU controller exhibits a non-zero following error after the position setup mode is exited and absolute/relative positioning is started. The drive appears to add the residual following error from the setup phase to the new position setpoint, so the axis overshoots or undershoots the target. This document covers root causes, parameter changes, third-party encoder handling, and the trace-based verification procedure.

The reported symptom on the platform is consistent with one or more of the following field conditions:

• Speed pre-control p2534 is set to 0 % (default) on firmware versions where this parameter can fully eliminate following error during constant velocity.
• Position controller proportional gain p2536 (Kv factor) is too low for the mechanical load.
• The encoder is wired to the CU320-2 via a CUA31 control unit adapter and is not a Siemens DRIVE-CLiQ encoder; pulse number, resolution, or counting direction is mis-configured.
• A third-party induction motor is operated without completing the motor data identification routine, leaving the field-weakening and slip models inaccurate and dragging the torque pre-control output.
• Trace r2665 shows a steady-state DC offset (typical value reported: 2,500-4,500 LU), which is characteristic of an open current loop or encoder scaling mismatch rather than a tuning problem.

2. Following Error Fundamentals

The following error is the closed-loop position error that remains after the position controller has acted on it. In a SIMOVERT/SINAMICS context it is defined in length units (LU) so that the mechanical and software views of the axis share a single unit basis.

Definition:

e_follow [LU] = s_setpoint [LU] − s_actual [LU]

where s_setpoint is the smoothed position setpoint from the interpolator (r2665 is derived from this), and s_actual is the position actual value from the encoder interface (r2521).

Under ideal conditions, a P-only position controller cannot drive the following error to zero for a constant-velocity motion because a non-zero speed is required to produce a non-zero setpoint velocity, and that speed can only be commanded when a non-zero position error exists. The Kv gain defines the linear relationship:

n_setpoint [rpm] = e_follow [LU] × Kv [1000/min]

For constant velocity, the steady-state following error is therefore:

e_follow,ss = v_setpoint / Kv

To bring this error to zero without sacrificing dynamic response, SINAMICS provides setpoint pre-control: a copy of the velocity and acceleration reference is added to the position controller output, bypassing the proportional path during the constant-velocity and constant-acceleration phases.

3. SINAMICS S120 Position Control Architecture

The SINAMICS S120 position control chain with the standard servo control structure (active when p0108 bit 12 is set or under SERVO control mode) is:

1. Position setpoint input via PROFIdrive telegram 110 (SINA_POS) or 111 — the PLC writes MDI_TARPOS, MDI_VELOCITY, MDI_ACCEL, MDI_DECEL and triggers via STW2 bits.
2. Position controller (r2561 setpoint / r2562 output) operates with Kv = p2536.
3. Speed pre-control adds p2534 × v_setpoint to the controller output.
4. Torque pre-control adds p2535 × a_setpoint × total_inertia to the controller output.
5. Speed controller (p1460 P / p1472 I) drives the current setpoint.
6. Current controller (p1715 P / p1720 I, field-oriented) commands the Motor Module.

The relevant firmware requirement for full speed pre-control is documented in the SINAMICS S120/S150 List Manual and Function Manual "Speed and Position Control": speed pre-control at 100 % was officially exposed and recommended starting with firmware 4.8. On older firmware, the parameter is present but the default is 0 % and the recommendation in official Siemens documentation is more conservative.

4. SINA_POS Block Integration

The SINA_POS function block (FB284 for SIMATIC S7-1500/TIA Portal, FB388 for SIMOTION SCOUT, equivalent blocks on SIMOTION V4.x) implements the PROFIdrive positioning interface defined by telegram 110.

Signal mapping summary:

The PLC must signal the axis the correct mode bits before lifting the setpoint acceptance edge. If the axis was previously in "position setup mode" and the mode bits are not updated before the first MDI block, the drive may still be receiving a residual setpoint from the setup profile generator and the resulting motion combines the MDI target with the residual.

Recommended startup sequence after the setup mode

1. Run CancelTraversing to clear the residual setpoint and ensure ZSW1.13 "drive stop active" is cleared.
2. Write the desired mode bits (absolute positioning: ModePos = 16#0001 or relative: 16#0002, depending on FB version).
3. Write target position, velocity, acceleration, deceleration.
4. Trigger Execute with a positive edge on STW1.6.
5. Monitor ZSW1.10 (MDI setpoint acknowledged) and ZSW1.12 (traversing command acknowledged).

5. Pre-Control Configuration (Speed and Torque)

Speed pre-control is the single most effective parameter change for eliminating steady-state following error during the constant-velocity phase of a motion.

Procedure to activate speed and torque pre-control:

1. Set p2534 = 100 (speed pre-control = 100 %). If the parameter has bit-field semantics on your firmware, ensure bit 0 is set or the value is interpreted as a percentage.
2. Compute or measure the total moment of inertia at the motor shaft: p1117 + p1118 = J_total (motor + load). Use p1959.5 or p1900 = 5 for the rotating measurement if the mechanics permit.
3. Set p2535 = 100 (torque pre-control = 100 %).
4. Verify that the encoder reference sign in r2521[0] matches the rotation sign: rotate the motor shaft by hand and confirm the sign of r2521 matches the sign you set in p0410.0 (signal inversion).
5. Save and RAM-to-ROM (p0977 = 1).
6. Re-run the trace described in section 8.

6. CU320-2 with CUA31 and Third-Party Encoders

The CU320-2 control unit has two onboard encoder interfaces (HTL/TTL on Sub-D connector) and supports DRIVE-CLiQ encoders directly. For non-DRIVE-CLiQ encoders (TTL/HTL, SSI, EnDat 2.1 without DRIVE-CLiQ converter), a CUA31 control unit adapter is inserted into the encoder slot. The CUA31 supports incremental encoders with TTL (RS422) or HTL (24 V) outputs and SSI absolute encoders.

The CUA31 occupies encoder interface interface 2 on the drive object, with parameters p04xx indexed under index [1] in many lists (confirm in r0148 and the topology view in STARTER / Startdrive).

For third-party induction motors, the motor data must be entered manually because the electronic nameplate path is not available. Required parameters:

• p0304 Rated motor voltage
• p0305 Rated motor current
• p0307 Rated motor power
• p0308 Rated power factor cos φ
• p0310 Rated motor frequency
• p0311 Rated motor speed
• p0314 Motor pole pair number
• p0322 Maximum motor speed
• p0326 Stall torque / pull-out torque

Then run motor data identification via p1900 = 2 (standstill measurement) followed by p1910 (rotating measurement if safe). The rotating measurement must be enabled (p1900 = 1 or via the optimization run) and the encoder must be configured to a valid topology before this step; otherwise, the drive will abort with F07995 "Motor data identification fault" or F07085.

7. Third-Party Motor and Encoder: Root Cause of the Reported Symptom

The user explicitly stated: "Whenever I'm using non-Siemens encoder for position control means I'm facing this problem." This is a diagnostic signal that the encoder interface — not the position controller — is the source of the residual error.

The most common failure modes encountered with third-party encoders on a CUA31 are:

1. Counting direction inverted. Pulse A / pulse B wiring reversed, or the encoder specifies CW rotation while the drive is configured for CCW. Fix: invert via p0410.0 = 1 for incremental encoders, or set the relevant bit in p0430.13 for absolute encoders.
2. Pulse count (p0408) mismatch. If the parameter does not match the physical line count on the encoder nameplate, every revolution will accumulate an error of (n_physical − n_parametric) × LU_per_rev. For a 2,500-LU pulley and a 1024 vs 2048 mismatch, this yields exactly the type of residual observed.
3. Track A / B swapped. The position controller can hold a stationary position correctly but loses position proportional to mechanical play during direction reversal — the symptom is a steady-state offset of a few hundred LU that disappears after a single rotation.
4. Reference loss during the setup phase. When the position setup mode runs the drive in an open configuration and the encoder reports index pulses inconsistently, the absolute reference is not captured. Verify with r2685 "Encoder absolute value valid" before the first MDI move.
5. Encoder scaling (LU per motor revolution) wrong. If p2520[0] (load revolutions per LU) or p2521[0] (encoder revolutions per load revolution) is miscalculated, the error scales linearly with motion distance and produces a persistent bias that is independent of the velocity profile.

8. Optimization Procedure (Step-by-Step)

The following procedure is the recommended commissioning sequence for a SINAMICS S120 axis driven by SINA_POS from a higher-level controller, with a third-party motor and third-party encoder on the CUA31.

Prerequisites

• STARTER (≤ V5.x) or Startdrive (V15 and later) commissioned project online.
• Drive READY state, no active faults (r0949 empty).
• Controller connected via PROFIBUS or PROFINET to the CU320-2, with telegram 110 mapped.
• Mechanics confirmed safe for motion (E-stop chain, software limit switches in p2570).

Step-by-Step Procedure

1. Verify topology. In STARTER / Startdrive, open the topology view and confirm that the CUA31 is online. Check r0148[1] for the sensor module firmware version.
2. Configure the encoder. Set p0400[1] = 9999 (third-party encoder), enter pulse count, supply voltage type, and counting direction. Run the encoder commissioning test in p1990 if available. Verify r0061[0] tracks manual rotation in real time.
3. Enter the motor data. Enter the third-party motor data in p0300-p0349 with p0300 = 1 or the appropriate motor type code.
4. Run motor identification. p1900 = 2, then power-cycle, then pulse enable, then trigger p1910 = 1 for the rotating measurement (if safe). Verify r0047.30 = 1 after a successful identification.
5. Configure the mechanical system. Set p2502[0] = encoder interface, p2503[0] = load gear ratio, p2504[0] = feed constant (LU per load revolution), p2505[0] = load revolutions per motor revolution, p2506[0] = feed constant units.
6. Activate the position controller. Set p2501 = 6 or appropriate to enable position control. Set p2530 (LR configuration) to enable setpoint pre-control.
7. Set Kv. Start with p2536 = 10 (1000/min default for 1 m/s = 16.6 LU/ms). Increase stepwise while monitoring the step response.
8. Set pre-control. p2534 = 100, p2535 = 100.
9. Configure the SINA_POS telegram. In the controller, instantiate SINA_POS, connect AxisNo, and wire the cyclic I/O to the axis telegram.
10. Run the trace. Open the trace recorder, add r2665, r2666, r2561, r2521, r2565, r2524, r898, r899 with a sample time of 1 ms and trigger on MDI start.
11. Validate. Command a 100,000 LU move at 50,000 LU/min. Confirm r2665 stays within ±5 LU during the constant-velocity phase and settles to zero within p2544.

9. Diagnostic Trace Parameters

The following parameters, recorded with the SINAMICS internal trace or STARTER trace tool, are the canonical set requested by Siemens support for following-error investigations.

When evaluating the trace:

• r2561 - r2521 must equal r2665 - r2521 minus the position-controller output. A persistent non-zero difference indicates a sign or scaling error.
• r2565 during the constant-velocity phase must equal r2666 (within ±1 LU) if speed pre-control is set correctly. If not, the position controller is still doing the work and the proportional gain / Kv is not correctly sized.
• The d/dt of r2561 should match r2666 when pre-control is at 100 %. If it does not, the smoothing time in p2533 is too high.

10. Fault and Warning Reference

11. Verification Checklist

• Position setup mode can be entered and exited without residual error in r2665.
• Speed pre-control p2534 = 100 % and torque pre-control p2535 = 100 %.
• r2521 and r2524 respond correctly to manual rotation in both directions.
• r2665 does not accumulate an offset between successive MDI moves.
• Position controller p2536 is set for the desired Kv with at least 20 % margin before instability.
• r2565 matches r2666 during the constant-velocity phase.
• Following error r2523 (or equivalent) remains within the warning threshold p2544 at standstill.
• Drive does not report F07451, F07452, F07995, F07085, or F31110 under any of the test motions.

12. Troubleshooting Matrix

13. Related Documentation

• SINAMICS S120 Function Manual — Speed and Position Control (entry ID 60593549)
• SINAMICS S120/S150 List Manual (Firmware 5.x)
• S120 Commissioning Manual with STARTER
• SIMATIC S7-1500 / T-CPU SINA_POS FB284 manual
• PROFIdrive Profile Drive Technology (PNO)