Guide: How to set up the RPM filtering

This is short guide on how to set up the bidirectional DSHOT and RPM filtering in Betaflight. 

What is RPM filtering?

Technical definition: RPM filtering is a set of 36 notch filters on gyro and (optionally) on Dterm which uses the RPM telemetry data to remove motor noise with surgical precision. By default it runs 12 notch filters each on pitch, roll, and yaw, covering the first 3 harmonics of each motor’s noise signature.

In simple words: RPM filtering is filtering precisely the vibrations of the each of the motor – the main source of the vibrations on quadcopter. 

Many pilots say that RPM filtering is by far the best filtering method in Betaflight.

What is bidirectional DSHOT?

Bidirectional DSHOT  is a new feature in Betaflight 4.x which lets the flight controller receive accurate RPM telemetry over each motor’s ESC signal line. No additional wiring or additional telemetry back-channel is needed. Telemetry packet is sent back to the flight controller after the each DSHOT command sent to ESC. 

Bidirectional DSHOT requires ESC with 32-bit BLHeli_32 firmware or ESC with BLHeli_S and special firmware, supporting the DSHOT telemetry.

BLHeli_32 ESCs have ability to send the ESC telemetry (temperature, eRPM, current) through the separate telemetry wire. But this native BLHeli_32 telemetry is sent in relatively slow speed and cannot be used for RPM filtering. Contrary, the bidirectional DSHOT telemetry data is sent over the signal wire at a much higher speed/pace then the standard telemetry.

Requirements for RPM filtering

You will need the following:

  • Betaflight version 4.1 or newer
  • BLHeli_32 ESC’s or BLHeli_S with JazzMaveric BLHeli_S patch 
  • Set up bidirectional DSHOT settings in Betaflight
  • Set up the RPM filter settings in Betaflight

Bidirectional DSHOT on BLHeli_32 ESC’s

You need to have BLHeli_32 firmware version 32.7.0 on your ESC’s to turn on the bidirectional DSHOT. To check the version of your ESC firmware you can run the BLHeliSuite32 and read the ESC information.

If your ESC has the firmware less than 32.7, then you’ll need to update BLHeli_32 firmware. 

Bidirectional DSHOT on BLHeli_S ESC’s

You need to have special BLHeli_S firmware with bidirectional DSHOT support on you ESC’s. Read the Guid: How to flash BLHeli_S firmware with bidirectional DSHOT.

Enabling the Bidirectional DSHOT in Betaflight

You will need to turn the  clock scheduler rate optimization  and dshot burst off in the Betaflight CLI

set dshot_burst = OFF
set scheduler_optimize_rate = ON

Select the ESC protocol DSHOT600 (or DSHOT300). Bidirectional DSHOT works with the DSHOT 300, 600 and 1200 protocols. The preferred protocol should be chosen depending on the ESC, MCU and pid loop time used. It is recommended to run at 4k/4k pid looptime with DShot 600 for your initial test flight.

Now you can turn the bidirectional DSHOT in the betaflight.

Alternatively you can enable the bidirectional DSHOT in the Betaflight CLI:

set dshot_bidir = ON

Don’ t forget to set the motor poles!

ESC telemetry reports the eRPM – the RPM of the motor multiplied by the amount of polar pairs. Betaflight needs to know how many poles does your motor have in order to calculate the right RPM. Motor poles means magnetic poles (magnets), so you have to count the number of the magnets in the motor bell.

Alternatively you can set the motor poles in the Betaflight CLI tab:

motor_poles = 14

If you are running the 2205, 2206, 2207 or 2306 motors, then default setting of the 14 motor poles is most likely correct. Smaller motors, like 080x, 110x, 120x most likely will have 12 magnet poles. Anyway to be sure, you need to count the magnets in the motor bell. The number must be even!

 

Sources used in writing this article:
https://github.com/betaflight/betaflight/wiki/Bidirectional-DSHOT-and-RPM-Filter

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