Resolving SINAMICS S120 Speed Setpoint Tracking Failures

1. Problem Overview

A SINAMICS S120 drive operating in servo (closed-loop speed) control with a CU320-2PN Control Unit (firmware V4.8) and STARTER V5.3 commissioning software fails to reach its commanded speed setpoint, exhibits large torque fluctuations, and oscillates when coupled to a position controller (EPOS / Basic Positioner). The same symptoms appear on a CU310-2 with traversing blocks driving a hoist with 28:1 gearbox, chain-and-sprocket mechanics, and 1400 mm vertical travel. Both installations share a common root cause class: inadequate speed-loop tuning combined with mechanical coupling effects and scaling artifacts in the trace.

This reference covers the diagnostic sequence, the One Button Tuning (OBT) procedure, manual retuning, mechanical verification, and the precontrol / filter parameters required to achieve stable setpoint tracking. All references are documented in the SINAMICS S120 List Manual and Function Manual.

2. Symptoms Reported in the Field

• Speed actual value (r0063) appears to lag or never reach the speed setpoint (r0062) in STARTER trace.
• Torque setpoint (r0079 / r0080) shows large low-frequency oscillations during constant speed operation.
• Position controller in EPOS mode produces audible oscillation; speed-only mode is stable.
• Detection with automatic controller data and manual retuning both yield the same result.
• Mechanical drivetrain uses a belt drive (pinch rollers) or chain-and-sprocket (hoist) with gearbox ratio 28:1.

3. Root Cause Analysis

Three classes of root cause must be eliminated in order.

3.1 Trace and Scaling Artifacts

STARTER's trace tool allows independent Y-axis ranges for each signal. If r0062 (speed setpoint, smoothed) and r0063 (speed actual, smoothed) are displayed on different scales, the actual value can appear permanently below the setpoint even when the controller is tracking correctly. Verify by overlaying both signals on a single Y-axis, or check that r0062 - r0063 (filtered difference) settles to near zero in steady state.

3.2 Mechanical Coupling Effects

Belt drives and chain-and-sprocket systems introduce compliance, backlash, and torsional resonance modes that interact with the speed control loop. Key checks:

• Belt tension - insufficient tension causes slip-stick behaviour and 1×-rotation modulation of the torque.
• Misalignment between pulley/sheave axes causes side-load and vibration.
• Belt / chain stiffness - a too-soft belt produces a low-frequency torsional resonance that the speed controller excites.
• Gearbox backlash - relevant when EPOS engages the position loop; the position controller will oscillate against backlash.

3.3 Inertia Mismatch

The ratio of load inertia reflected to the motor shaft (J_load,motor) to the motor rotor inertia (J_motor) directly affects achievable speed-loop bandwidth. A mismatch above ~5:1 typically requires detuning and a lower Kp. Compute the reflected load inertia:

J_load,motor = J_load × (1 / i²)

where i is the total gear ratio (28 in the hoist case). For the reported hoist: 1 gearbox revolution = 700 mm linear travel, so 2 revolutions reach 1400 mm. The linear inertia must be converted to rotational inertia at the motor shaft before comparing to p0341 (motor moment of inertia) and p0342 (load moment of inertia / total).

The SINAMICS S120 automatic controller calculation uses p0342 to scale Kp_n (p1460) and Tn_n (p1462). If p0342 is left at default or entered incorrectly, the loop will be either too aggressive (oscillation) or too soft (setpoint lag).

3.4 Speed Setpoint Filter / Precontrol Issues

Following the speed setpoint, the SINAMICS S120 chain contains a setpoint filter (p1414 / p1415 / p1416 / p1417) and a torque precontrol path (p1496 enable, Kp adaptation, p0342). If the torque precontrol is disabled or scaled incorrectly, the speed controller must produce 100% of the accelerating torque, which slows response and amplifies disturbance at low speed. The SINAMICS S120 Function Manual (entry ID 99472695) explicitly describes precontrol of the speed control loop from the speed setpoint to improve command behaviour.

4. Hardware and Firmware Context

5. Pre-Tuning Checklist

1. Capture drive identification: p1910 (motor data identification, standstill) and p1960 (speed controller optimisation, rotating).
2. For a hoist: jog axis to mechanical bottom, force-open the holding brake, then run p1910/p1960 at the bottom so the axis does not free-fall during identification. Use Safe Stop or mechanical blocking for safety during identification at standstill.
3. Verify DRIVE-CLiQ topology - encoder and Motor Module on correct ports.
4. Set p1300 = 21 (sensorless vector) only as a check baseline; restore p1300 = 11 (servo) for closed-loop operation.
5. Confirm r2561 (speed setpoint from EPOS) is smooth at constant speed before any tuning of the speed loop.

6. One Button Tuning (OBT) Procedure

OBT (introduced in FW 4.7) performs automatic identification of the mechanical coupling and calculates Kp, Tn, precontrol weights, and reference model / filter settings for both the speed and position controllers in one pass. For drivetrains with a belt or chain, OBT is strongly preferred over the legacy p1960 automatic controller setting because it tunes the position controller as well.

6.1 OBT Parameters

6.2 OBT Sequence (Hoist, CU310-2, traversing blocks)

1. Disable the axis (cancel enable via STW1.0 or the traversing block). Brake remains closed under drive sequence control.
2. Jog the axis to a mid-position where the maximum travel in both directions is greater than the OBT distance the motor requires at the gearbox output. The hoist must be free to move at least 2 gearbox revolutions in the OBT travel direction.
3. Set p5308 to the maximum safe motor-shaft travel (e.g. 720° for a 2-revolution vertical move of 1400 mm at 700 mm / rev, gear ratio 28:1, this corresponds to 2 × 360° = 720° at the gearbox input - keep the OBT travel in the safe half of the 1400 mm range).
4. Set p5300 = 1. The drive then executes slow / fast motions automatically. For hoists with a holding brake controlled by drive sequence: the drive opens and re-applies the brake automatically through the integrated brake control as required by the OBT motion profile. The motor may turn either direction depending on p5302 and current position; the drive runs OBT in the direction commanded and within the distance p5308 permits.
5. On completion the drive sets p5300 back to 0 and writes results to p1460 (Kp speed), p1462 (Tn speed), p2538 (Kp position), and the precontrol weights. Read back via p5300 = 2..6 for diagnostics.
6. To roll back to factory tuning: set p5300 = -1.

7. Manual Tuning Procedure

Use manual tuning if OBT is not available (FW < 4.7), if OBT returns alarm "insufficient travel", or if the load inertia is non-linear (e.g. telescopic arm, vertical hoist with varying payload).

7.1 Speed Controller Parameters

7.2 Tuning Method (Symmetric Optimum, Speed Loop)

1. Disable torque precontrol: p1496 = 0%.
2. Reduce Kp_n (p1460) to 30% of p1960 result.
3. Apply step in speed setpoint of ~10% of n_max via the control panel; capture r0063, r0079.
4. Raise Kp_n in 20% increments until the speed response shows 10-15% overshoot.
5. Reduce Tn_n (p1462) from infinity in steps until closed-loop settling is critically damped.
6. Re-enable torque precontrol: p1496 = 100%. Set p0342 to the actual inertia ratio. Verify step response - precontrol removes the steady-state lag that the integrator had to compensate for.
7. Activate setpoint filter (p1414..p1417) only if resonance is visible in the torque trace. Identify resonance frequency with a step test and place a band-stop filter at that frequency.

7.3 Position Controller Tuning (EPOS / Basic Positioner)

After the speed loop is stable:

1. Set p2538 (Kp_x) to a low starting value (typical 10 1/s).
2. Issue small positioning setpoints via the control panel; raise p2538 until position response is critically damped.
3. Verify with r2561 (speed setpoint from position controller) - if r2561 is smooth at constant position, the position loop is correctly tuned.
4. If r2561 oscillates, the position loop is exciting the speed loop. Reduce p2538 or improve speed-loop bandwidth by raising p1460 / lowering p1462.

8. Mechanical Verification

• Re-tension belt to manufacturer specification; mark belt deflection under load for repeatability check.
• Measure gearbox backlash by locking the output and rotating the motor shaft in both directions; record the no-motion angle. Values above 0.5° at the motor shaft are significant for position control.
• Check sprocket alignment with a dial indicator: radial and axial runout < 0.05 mm typical for chain drives.
• Confirm coupling alignment between motor and gearbox input (typical < 0.05 mm offset).
• Inspect for worn bearings, loose foundation bolts - these inject periodic disturbance into the speed loop.

9. Precontrol and Reference Model (FW 4.8 Specific)

From firmware V4.8, SINAMICS S120 exposes additional connectors for the speed setpoint "after comma" (fractional) precision. In projects upgraded from FW < 4.7 these connectors may be unconnected. Check the BICO wiring for p2597 and adjacent connectors; the new fractional connectors deliver higher-resolution speed precontrol and improve setpoint tracking at low speeds. Refer to the SINAMICS S120 Function Manual (entry 99472695) for the full description of the speed setpoint filter and precontrol path.

10. Verification

1. Apply step setpoint 0 → 50% n_max via STARTER control panel. Capture r0062, r0063, r0079, r0080 at 1 ms sample time. The speed actual must match setpoint within 2% steady-state and 15% overshoot transient.
2. Run constant-speed trace at 25%, 50%, 75% n_max for 5 s each. Torque setpoint ripple must be < 3% of M_max.
3. Run EPOS positioning cycle 0 → 1400 mm → 0. Position overshoot < 1 mm; settling time within application specification.
4. Disconnect the motor mechanically, repeat step 1 - the controller should still be stable and respond promptly. This confirms the loop is not being stabilised by friction only.
5. Monitor r0035 (motor temperature) and r0048 (thermal load) over 30 min - retuning that produces high current ripple will heat the motor unnecessarily.

11. Troubleshooting Matrix

12. Field-Proven Cautions

• Do not run p1960 (speed controller optimisation) on a hoist with a suspended load - the rotation of the motor identification can exceed the available travel and trigger an over-speed fault. Move the load to bottom, support mechanically, and then identify.
• When p5300 is set to 1, the drive takes control of the axis including brake sequencing. Do not start OBT without ensuring mechanical safety.
• Always set r2561 to be inspected at constant speed - a non-smooth r2561 indicates the position controller is the source, not the speed controller.
• For drivetrains with high load-to-motor inertia ratio (> 10:1), raise the reference model (p1433) and add a feedforward path to stabilise the position loop.
• STARTER V5.3 is end-of-life; migrate to Startdrive in TIA Portal for FW 4.8 projects and access to OBT parameter diagnostics.

13. FAQ