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The RPM-filter is a feature in the gyro filtering framework that uses the motor RPM information to identify and filter out vibrations that are coming from the rotating parts in a helicopter, i.e. motor, main rotor, tail rotor. Notch filters are automatically configured for each estimated vibration frequency and it's harmonics. This allows less overall low-pass filtering on the gyro signal, as the notch filters are surgically removing the peaks only on the actual vibration frequencies.

The Rotorflight RPM-filter is based on the BetaFlight RPM-filter, but is more versatile and configurable.

The RPM-filter requires motor speed information to be available in the flight controller. There are three ways for obtaining it. One of these must be configured and available.

• Bidirectional DSHOT RPM telemetry
• Frequency Sensor
• KISS ESC telemetry

The motor gear ratios and pole counts need to be configured as per Motor Setup. This enables the flight controller to know the actual rotor RPM from the motor electrical RPM (eRPM) which is obtained by the motor telemetry.

The RPM-filter can be turned ON with the feature flag RPM_FILTER from the Configuration Tab.

From the Gyro Tab enable the Gyro RPM Notch Filters with the switch.

In general terms the heavier or more filters that are used the greater the gyro delay that is introduced (difference between the actual helicopter movement and the signal seen by the flight controller). We want to be sparing with our filters and only use what is required to remove unwanted noise.

• Single - A Single notch filter is used.
• Double - Two notch filters are used together to provide heavier filtering when required. This option is provided for the Main and Tail Rotor RPM and the Second Harmonic as these will be by far the strongest.

The quality factor (Q factor) is a measure of how selective or narrow the filter notch is. The higher the Q means a narrow notch. Lower Q values provide wider filtering however this is at the expense of an increase in delay. Typically set to the range 1.5 to 3.0.

Fundamental Frequency - Enable the filter, choose Notch Type Single or Double and set a Notch filter Q.

2nd Harmonic - Enable the filter, choose Notch Type Single or Double and set a Notch filter Q.

3nd Harmonic - Enable the filter and set a Notch filter Q.

4nd Harmonic - Enable the filter and set a Notch filter Q.

A well build heli would usually have very minor vibrations. It should be enough to filter out the main rotor fundamental and its three harmonics, the tail motor fundamental and the second harmonic, and the brushless motor fundamental frequency(ies).

Using the Trex-450 example above, the config would look like this:

An older heli with worn out bearings (or bad build!) may have more vibrations. The frequency band could also be wider. It would help to have a dual notch on the main rotor fundamental and second harmonic (at least). If this is still not enough, lowering the Q-value is an options, but it will increase the filter delay!

A config could look like this:

The best thing to do is to fix your heli first.

(PDK) Work in progress.