Notch filter for suppressing mechanical resonances
What are notch filters?
Notch filters (band-stop filters) are narrowband filters that specifically remove a certain frequency from the control signal. They are used in servo drives to suppress mechanical resonance frequencies caused by elastic elements in the drive train.
Why is this important?
Real mechanical systems are not rigid bodies. Gearboxes, clutches, belts, and long shafts form spring-mass systems with characteristic natural frequencies. If such resonance is excited by the controller, the following occurs:
- Audible vibrations and mechanical wear
- Oscillations in the speed or position signal that destroy the control quality
- Restriction of the controller bandwidth — the gains must be reduced to avoid instability.
- Damage to mechanics and workpiece due to uncontrolled vibrations
The problem: Higher controller gains improve positioning accuracy and dynamics, but can also cause resonance. Notch filters make it possible to keep the gains high while specifically suppressing the problematic frequency.
How does it work?
A notch filter is a second-order IIR filter defined by three parameters:
- Center frequency — the resonance frequency to be suppressed (30–2000 Hz)
- Bandwidth (Rejection Band) — the width of the suppressed frequency range (3–2000 Hz)
- Depth — the degree of attenuation at the center frequency, in decibels (dB)
A narrow bandwidth (high Q factor) only suppresses the resonance frequency itself and has little effect on the rest of the control behavior. However, a bandwidth that is too narrow can become ineffective if the resonance frequency drifts slightly.
Supplementary low-pass filters: In addition to the notch filter, SOMANET drives offer low-pass filters for speed feedback (0x2021, cut-off frequency 5–2000 Hz) and position feedback (0x2022, cut-off frequency 5–2000 Hz). These are more broadband and generally reduce sensor noise, but lead to phase delay.
Identify resonance frequency: System identification in OBLAC Drives analyzes the frequency response of the drive train and displays resonance points in the Bode diagram. The identified frequency is entered as the center frequency of the notch filter.
Important: Notch filters should only be used for high-frequency resonances. For low-frequency resonances, the phase shift is too large and can negatively affect the control behavior.
How does SOMANET implement this?
The notch filter is configured via the Object Dictionary (0x2023) and is located at the input of the torque controller — it therefore filters the torque setpoint. Frequency prewarping is applied automatically to minimize discretization errors.
The configuration comprises four sub-indices: activation (0x2023:1), center frequency (0x2023:2, default: 1000 Hz), bandwidth (0x2023:3, default: 10 Hz), and depth (0x2023:4).
The function is available on all SOMANET platforms and is typically activated after auto-tuning if the system identification detects mechanical resonance.
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