RPM filter

Summary¶

The RPM filter is Betaflight's primary gyroscope noise rejection system. It uses the actual motor RPM — measured every PID loop cycle via bidirectional DShot telemetry — to place narrow notch filters precisely at the motor fundamental frequency and its harmonics, removing motor vibration noise from the gyro signal while leaving attitude signal untouched. On libdrone's H7A3-SLIM running at 8 kHz, the filter simultaneously tracks 36 notch filter positions (4 motors × 3 harmonics × 3 axes), updated every 125 µs. The result is a cleaner gyro signal at lower latency cost than any fixed-frequency filtering approach.

Concept¶

The problem with fixed-frequency filters¶

Before the RPM filter, gyroscope noise was addressed using fixed low-pass filters: remove all signal above a chosen cutoff frequency. This eliminates motor noise (typically 300–800 Hz) but also removes real attitude signal above the cutoff. Every 10 Hz of cutoff reduction adds measurable phase lag to the control loop — the flight controller sees attitude changes later than they happen. Aggressive filtering for noisy builds produced sluggish, unresponsive drones.

The fundamental problem: motor noise frequency changes with throttle. At 20,000 RPM the fundamental is 333 Hz. At 35,000 RPM it is 583 Hz. A fixed notch filter at 400 Hz catches the noise at one throttle level and misses it at another. A broad low-pass filter catches it everywhere but at the cost of latency across the whole spectrum.

How the RPM filter solves this¶

Bidirectional DShot makes each ESC report the motor's exact eRPM to the flight controller every PID cycle. The RPM filter uses this real-time RPM data to calculate the current fundamental frequency of each motor:

fundamental_Hz = (eRPM / pole_pairs) / 60 = mechanical_RPM / 60

It then places notch filters at the fundamental and its harmonics (2× and 3× the fundamental), tracking them continuously as throttle changes. When a motor changes speed, the notch filters follow within one PID cycle (125 µs).

The notch filters are narrow — they remove only a few Hz of bandwidth around each motor frequency. Everything else, including real attitude dynamics below 100 Hz, passes through untouched. The phase lag introduced by narrow notch filters is negligible compared to a broad low-pass filter achieving the same noise rejection.

The computational requirement¶

36 notch filter positions, updated every 125 µs: - 4 motors - 3 harmonics per motor (fundamental, 2nd, 3rd) - 3 gyroscope axes (roll, pitch, yaw) - Updated at 8,000 Hz

This is why the H7A3 (Cortex-M7, 280 MHz) was selected for libdrone. Earlier F4-class flight controllers could run the PID loop at 8 kHz but ran out of processing headroom before the RPM filter computation could be added. The H7 runs both simultaneously with substantial CPU headroom remaining.

The dynamic notch filter¶

On top of the RPM filter, Betaflight runs a dynamic notch filter that analyses the gyro frequency spectrum in real time, identifies high-amplitude noise peaks not accounted for by motor frequencies, and places additional notch filters there. This catches frame structural resonances, aerodynamic turbulence peaks, and any other persistent noise sources.

The RPM filter and dynamic notch filter work together: RPM filter handles predictable, calculated motor noise; dynamic notch handles anything unpredictable that appears in the actual spectrum.

Reference¶

RPM filter dependencies¶

libdrone V2.4.6 RPM filter configuration¶

Verifying RPM filter is working¶

1. In Betaflight Motors tab: spin up all four motors to ~30% throttle with props removed. All four should report eRPM values that are non-zero and proportional to commanded speed.
2. In Betaflight Blackbox Explorer: after a flight, view the gyro spectrum. The characteristic motor frequency peaks (fundamental + harmonics) should be largely absent. Residual peaks indicate the filter is not fully effective — check BiDi DShot is enabled in both FC and ESC configuration.

Procedure¶

Enabling RPM filter on libdrone H7A3-SLIM¶

1. In ESC configurator (AM32 Configurator or BLHeli Suite): enable bidirectional DShot on all four ESC outputs.
2. In Betaflight Configurator → Configuration tab: set Motor Protocol to DShot600, enable Bidirectional DShot.
3. In Betaflight CLI: set motor_poles = 14 (for the BrotherHobby 2507).
4. Save and reboot.
5. Verify in Motors tab: all four motors show eRPM when spun up (props removed).
6. In Betaflight Filtering tab: RPM filter should show as enabled with 3 harmonics.

Rationale¶

Why this article lives in control-systems and not software-stack¶

The RPM filter is a control system concept — it is a real-time adaptive notch filter that makes PID tuning possible on noisy airframes. Understanding what it does (track motor noise and remove it surgically) is essential for understanding why the D term can be set higher than on pre-RPM-filter drones, why BiDi DShot is required, and why the H7 processor matters. The Betaflight configuration steps belong in the software-stack domain. The concept belongs here.

Connections¶

requires: - closed-loop-control - pid-derivative-term - resonance-filtering related: - vibration-isolation-theory - floating-motor-mounts leads_to: - imu-filter-tuning