Resolving SINAMICS S120 Servo Resonance Noise on 1FT7084 Motors

1. Problem Summary: High-Frequency Acoustic Emission from a Coupled Load

A Siemens 1FT7084-1AH71-1CG1 synchronous servo motor driven by a SINAMICS S120 drive produces a continuous, high-pitched metallic screaming noise whenever the drive is enabled. The symptom is independent of commanded speed: the noise is present at standstill (zero RPM) and persists at every operating speed. Removing the metallic wheel coupled to the motor shaft eliminates the noise immediately, which isolates the acoustic path to the mechanical interface and the closed-loop control interaction with the load inertia.

The reported symptom is characteristic of a control-induced mechanical resonance. Because the noise survives the manual adjustment of proportional and integral gain in the speed controller, the problem is not solvable through scalar gain tuning alone. It requires the structured commissioning and optimization procedure described in Chapter 13 of the SINAMICS S120/S150 List Manual, including current setpoint filters, optional reference model activation, and mechanical decoupling verification.

2. Affected Hardware Identification

The 1FT7084 belongs to the high-inertia 1FT7 family used for high-load dynamic applications. The trailing order-code fields encode shaft, brake, encoder, and connection options; the encoder variant must be matched in drive parameter P0400[0] and physically wired to the correct Sensor Module (SMC20 or SME20 for incremental/resolver; SME125/SME126 for absolute encoders). A mismatch between encoder feedback resolution and the configured encoder type in the drive is one of the common root causes of high-frequency controller oscillation when commissioning a new coupled load.

3. Root Cause Analysis: Why the Wheel "Screams"

Three failure modes are the most probable causes of a constant-frequency noise that is independent of commanded speed and is suppressed when the load is removed:

3.1 Mechanical structural resonance excited by the current controller

The SINAMICS S120 current controller operates at the drive's pulse frequency (typically 4 kHz, 8 kHz, or 16 kHz depending on P1800). If the metallic wheel has a structural resonance in the audible band (typically 500 Hz to 12 kHz), the high-bandwidth current loop can excite that resonance through torque ripple and through any non-ideal coupling between rotor and load. Because the drive is in closed-loop speed control, even at zero RPM the integral action of the speed controller continues to generate small torque corrections at the resonant frequency of the mechanical system.

3.2 Inertia mismatch and gain-induced instability

The speed controller proportional gain P1460[0] was tuned for the motor alone, not the motor + wheel system. The effective inertia at the motor shaft now includes the wheel inertia. A Kp value that was stable for the bare motor can become unstable or marginally stable when the load inertia is added. The result is sustained oscillation in the speed actual value at the closed-loop pole frequency, which is rendered as a high-pitched acoustic tone.

3.3 Feedback path issues

If the encoder cable shielding, grounding, or routing has been altered when the wheel assembly was mounted, EMI coupling can introduce a periodic disturbance into the speed actual value. A faulty or loose coupling between motor shaft and wheel can introduce a non-rigid mechanical connection that the speed controller interprets as a periodic speed error, again driving oscillation at the mechanical natural frequency.

4. Why Manual P/I Tuning Fails on This Fault

The speed controller has two main tuning parameters:

Reducing Kp until the noise disappears is not a valid fix because it cripples the dynamic performance of the axis. The proper engineering approach is to identify the resonance frequency, install a bandstop (notch) filter in the current setpoint path to suppress excitation at that frequency, and then re-tune the speed controller with the filter in place. This is exactly the procedure that SINAMICS S120 Chapter 13 commissioning workflow covers.

5. SINAMICS S120 Optimization Workflow (Chapter 13 of the List Manual)

Chapter 13 of the SINAMICS S120/S150 List Manual defines the structured commissioning sequence. The relevant sections for this fault are:

1. Basic commissioning (drive configuration, encoder, motor identification)
2. Motor data identification (automatic measurement of stator resistance, leakage inductance, moment of inertia)
3. Speed controller optimization (auto-tuning or one-button tuning with STARTER / Startdrive)
4. Optimization of current setpoint filters (notch filters and lowpass filters)
5. Suppression of mechanical resonances in the controlled system
6. Function generator / measuring function diagnostics to confirm the fix

The official SINAMICS S120 Commissioning Manual and the SINAMICS S120/S150 List Manual are the canonical references. Confirm any firmware-specific parameter changes against the firmware version installed on the Control Unit (CU320-2) and the Motor Module.

6. Current Setpoint Filters (Notch Filters) — The Primary Fix

SINAMICS S120 provides up to four current setpoint filters per drive object. Each filter can be configured as lowpass, highpass, bandstop (notch), or bandpass. For resonance suppression, the bandstop type is the standard tool.

A bandstop filter with denominator damping around 0.05–0.20 produces a deep, narrow notch at the resonance frequency. Numerator damping set higher than the denominator damping widens the suppression band slightly so the resonance is attenuated across its full quality factor, not just at the exact center frequency.

7. Measuring the Resonance Frequency

Before commissioning the notch filter, the resonance must be quantified. Three approaches, in increasing rigour:

7.1 Acoustic measurement with STARTER / Startdrive trace

Enable the drive in speed control with the wheel attached. Trigger the drive to a small step reference (e.g., 10% of rated speed). Use the STARTER or Startdrive trace recorder to capture Speed actual value smoothed (r0061), Current actual value, torque-generating (r0078), and Torque setpoint (r0079) at a sampling rate of at least 4 kHz. Identify the dominant oscillation frequency by FFT or by zero-crossing count.

7.2 Function generator injection

Configure the SINAMICS function generator (P4800 series) to inject a sinusoidal current setpoint superimposed on the torque-generating axis. Sweep from 50 Hz to 1500 Hz in 50 Hz steps. At each frequency, monitor the speed actual value amplitude. The frequency at which the speed response amplitude is highest (or where acoustic emission peaks) is the structural resonance. Notch filter center frequency should be set to this value.

7.3 Hammer-test / impact test with external analyzer

For installations where the drive is not yet trusted to operate with the load, a mechanical impact test on the stationary assembly followed by an FFT of the acceleration response (measured with a piezoelectric accelerometer) identifies the structural mode. The notch filter center frequency is then set to the measured mode.

8. Speed Controller Auto-Tuning Workflow

1. Verify that P1300[0] = 21 (speed control with encoder).
2. Confirm motor identification has completed (P1910 = 0 after completion, results stored in r1912–r1926).
3. Activate online tuning via P5200[0] = 1 or use STARTER's "One Button Tuning" / Startdrive's auto-tuning wizard.
4. The drive performs a series of small motion profiles, measures the closed-loop response, and writes P1460[0] (Kp), P1462[0] (Tn), and optionally P1415/P1416/P1417 (reference model parameters).
5. Verify the result by running the original motion profile that triggered the noise.

Auto-tuning with the load attached is mandatory when the wheel becomes part of the operating load. Tuning with the wheel removed gives values that are not valid for the loaded configuration.

9. Reference Model (Optional Second Defense Layer)

The SINAMICS S120 speed controller includes an optional reference model that linearizes the closed-loop response. Activating it adds a controllable second-order lowpass between the speed setpoint and the I-controller input, which can prevent the speed controller from commanding torque changes faster than the mechanical system can follow.

The reference model is recommended for axes with significant load inertia variation or non-rigid coupling. It is a structural fix that complements — not replaces — the notch filter.

10. Mechanical Verification Before Re-Tuning

Before changing any drive parameters, perform the following checks on the wheel-to-shaft interface:

1. Coupling rigidity: confirm the wheel is mounted with a zero-backlash rigid coupling or keyed clamp. Any backlash or slip introduces non-linearity that excites limit cycles.
2. Concentricity: measure run-out with a dial indicator. Greater than 0.05 mm at the wheel rim indicates an eccentricity that will excite bearing and structural modes at rotation frequency.
3. Balance: a balanced wheel should not produce audible tone at running speed after decoupling the electrical noise. If it does, re-balance the wheel.
4. Encoder mounting: confirm the encoder housing is mechanically isolated from the load and that the encoder cable shield is grounded at the drive end only.
5. Grounding: confirm the motor frame is bonded to the cabinet ground with a short, low-impedance strap. High-frequency noise currents from the inverter must have a low-impedance return path that does not flow through the encoder cable shield.

11. Step-by-Step Commissioning Procedure

1. Save the current drive parameter set to a STARTER / Startdrive project file as a backup.
2. Open the drive in online mode and confirm the encoder and motor data are correctly configured for the 1FT7084-1AH71-1CG1.
3. With the wheel attached, run motor identification (P1900 = 2 or use the wizard).
4. Run a STARTER trace to capture speed and current during a small step command and identify the resonance frequency.
5. Configure a bandstop current setpoint filter (P1656 = 1, P1670 = 2, P1671/P1673 = measured frequency, P1672 = 0.1, P1674 = 1.0).
6. Run speed controller auto-tuning (P5200 = 1 or the wizard).
7. Optionally activate the reference model (P1418 = 1, P1413 and P1414 set for desired bandwidth and damping).
8. Copy RAM-to-ROM to save the new parameter set (P0977 = 1).
9. Power-cycle the drive and verify the noise is eliminated across the full operating speed range.

12. Verification and Acceptance Test

After the commissioning sequence, perform the following checks before returning the machine to production:

13. Troubleshooting Matrix

14. Long-Term Recommendations

• Store the optimized parameter set as the project master. After every drive firmware update, re-verify filter frequencies because motor identification data can shift slightly across firmware versions.
• If the wheel or load mass changes during the machine lifecycle (tooling change, fixture modification), re-run the resonance measurement and re-tune.
• Use the SINAMICS trace recorder at the maximum available sampling rate (typically 8 kHz / 16 kHz depending on P1800) to ensure the FFT identifies the resonance accurately and is not aliased.
• For machines that are produced in volume, document the commissioning values in the machine's electrical documentation so that field service can replicate the tuning after a drive replacement.

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