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Rework filter tuning, add dynamic notch filters #3467

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Rework filter tuning, add dynamic notch filters
julianoes Nov 20, 2024
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Update en/config_mc/filter_tuning.md
hamishwillee Nov 20, 2024
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Subedit for style and prettier
hamishwillee Nov 21, 2024
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90 changes: 64 additions & 26 deletions en/config_mc/filter_tuning.md
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Expand Up @@ -35,24 +35,57 @@ Below we look at the impact of the low pass filters.

## Filters

This is the filtering pipeline for the controllers in PX4:
The filtering pipeline for the controllers in PX4 is described below.

- On-chip DLPF for the gyro sensor.
This is disabled on all chips where it can be disabled (if not, the cutoff frequency is set to the highest level of the chip).
- A notch filter on the gyro sensor data that is used to filter out narrow band noise, for example harmonics at the rotor blade pass frequency.
This filter can be configured using [IMU_GYRO_NF0_BW](../advanced_config/parameter_reference.md#IMU_GYRO_NF0_BW) and [IMU_GYRO_NF0_FRQ](../advanced_config/parameter_reference.md#IMU_GYRO_NF0_FRQ).
- Low-pass filter on the gyro sensor data.
It can be configured with the [IMU_GYRO_CUTOFF](../advanced_config/parameter_reference.md#IMU_GYRO_CUTOFF) parameter.
::: info
Sampling and filtering is always performed at the full raw sensor rate (commonly 8kHz, depending on the IMU).
:::

### On-chip DLPF for the Gyro Sensor

This is disabled on all chips where it can be disabled (if not, cutoff frequency is set to the highest level of the chip).

### Notch Filters

Setups that have a significant lower-frequency noise spike (e.g. due to harmonics at the rotor blade pass frequency) can benefit from using the notch filter to clean the signal before it is passed to the low pass filter (these harmonics have a similar detrimental impact on motors as other sources of noise).

Without the notch filter you'd have to set the low pass filter cutoff much lower (increasing the latency) in order to avoid passing this noise to the motors.
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Without the notch filter you'd have to set the low pass filter cutoff much lower (increasing the latency) in order to avoid passing this noise to the motors.
Without the notch filter you'd have to set the low pass filter cutoff much lower (increasing the phase lag) in order to avoid passing this noise to the motors.


#### Static Notch Filters

One or two static notch filters on the gyro sensor data that are used to filter out narrow band noise, for example harmonics at the rotor blade pass frequency.
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One or two static notch filters on the gyro sensor data that are used to filter out narrow band noise, for example harmonics at the rotor blade pass frequency.
One or two static notch filters on the gyro sensor data that are used to filter out narrow band noise, for example a bending mode of the airframe.


The static notch filters can be configured using:

- First notch filter: [IMU_GYRO_NF0_BW](../advanced_config/parameter_reference.md#IMU_GYRO_NF0_BW) and [IMU_GYRO_NF0_FRQ](../advanced_config/parameter_reference.md#IMU_GYRO_NF0_FRQ).
- Second notch filter: [IMU_GYRO_NF1_BW](../advanced_config/parameter_reference.md#IMU_GYRO_NF1_BW) and [IMU_GYRO_NF1_FRQ](../advanced_config/parameter_reference.md#IMU_GYRO_NF1_FRQ).

::: info
Only two notch filters are provided.
Airframes with more than two frequency noise spikes typically clean the first two spikes with the notch filters, and subsequent spikes using the low pass filter.
:::

#### Dynamic Notch Filters

Dynamic notch filters use ESC RPM feedback and/or the onboard FFT analysis to track the rotor blade pass frequency and its harmonics.
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The static notch filter appears to serve the same purpose - filter blade frequency and its first harmonic (vs dynamic multiple harmonics).

  1. So when would you use one or the other?
  2. The example in Daniels' original PR filtered crap from props with slight damage - I presume that results in more noise and harmonics?

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I suppose the dynamic one is generally better but only possible when you have ESC feedback. Not sure about the FFT one?

@dagar when would you use the FFT instead of the static notch filter?

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Shall we merge and you chase him for this answer offline? My concern is that he will be too busy to respond, and we end up merging this in 2 years, no better or worse off.

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Let me ping @bresch as well, he might know and answer quicker.

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The FFT is to track a moving vibration peak (e.g.; the blade-pass frequency when ESC telemetry is not available). The static notch is usually to cover a structural mode of the airframe.
I would say that you either use the ESC feedback or the FFT to move the dynamic notches, but you can use a static notch in addition to the dynamic ones.

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::: info
Sampling and filtering is always performed at the full raw sensor rate (commonly 8kHz, depending on the IMU).
:::
ESC RPM feedback requires ESCs capable of providing RPM feedback such as [DShot](../peripherals/esc_motors.md#dshot) with telemetry connected, a bidirectional DShot set up ([work in progress](https://github.com/PX4/PX4-Autopilot/pull/23863)), or [UAVCAN/DroneCAN ESCs](../dronecan/escs.md).
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Before enabling, make sure that the ESC RPM is correct.
You might have to adjust the [pole count of the motors](../advanced_config/parameter_reference.md#MOT_POLE_COUNT).

- A separate low-pass filter on the D-term.
The D-term is most susceptible to noise while slightly increased latency does not negatively affect performance.
For this reason the D-term has a separately-configurable low-pass filter, [IMU_DGYRO_CUTOFF](../advanced_config/parameter_reference.md#IMU_DGYRO_CUTOFF).
- An optional slew-rate filter on the motor outputs.
This rate may be configured as part of the [Multicopter Geometry](../config/actuators.md#motor-geometry-multicopter) when configuring actuators (which in turn modifies the [CA_Rn_SLEW](../advanced_config/parameter_reference.md#CA_R0_SLEW) parameters for each motor `n`).
The following parameters should be set to enable and configure dynamic notch filters:

| Parameter | Description |
| ------------------------------------------------------------------------------------------------------------- | ------------------------------------------------------------------------ |
| <a href="IMU_GYRO_DNF_EN"></a>[IMU_GYRO_DNF_EN](../advanced_config/parameter_reference.md#IMU_GYRO_DNF_EN) | Enable IMU gyro dynamic notch filtering. `0`: ESK RPM, `1`: Onboard FFT. |
| <a href="IMU_GYRO_FFT_EN"></a>[IMU_GYRO_FFT_EN](../advanced_config/parameter_reference.md#IMU_GYRO_FFT_EN) | Enable onboard FFT (required if `IMU_GYRO_DNF_EN` is set to `1`). |
| <a href="IMU_GYRO_DNF_MIN"></a>[IMU_GYRO_DNF_MIN](../advanced_config/parameter_reference.md#IMU_GYRO_DNF_MIN) | Minimum dynamic notch frequency in Hz. |
| <a href="IMU_GYRO_DNF_BW"></a>[IMU_GYRO_DNF_BW](../advanced_config/parameter_reference.md#IMU_GYRO_DNF_BW) | Bandwidth for each notch filter in Hz. |
| <a href="IMU_GYRO_DNF_HMC"></a>[IMU_GYRO_DNF_HMC](../advanced_config/parameter_reference.md#IMU_GYRO_NF0_BW) | Number of harmonics to filter. |
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### Low-pass Filter

A low pass filter on the gyro data can be configured with the [IMU_GYRO_CUTOFF](../advanced_config/parameter_reference.md#IMU_GYRO_CUTOFF) parameter.

To reduce the control latency, we want to increase the cutoff frequency for the low-pass filters.
The effect on latency of increasing `IMU_GYRO_CUTOFF` is approximated below.
Expand All @@ -70,19 +103,23 @@ Noise on the motors has the following consequences:
- Reduced flight time because the motors continuously change their speed.
- Visible random small twitches.

Setups that have a significant lower-frequency noise spike (e.g. due to harmonics at the rotor blade pass frequency) can benefit from using the notch filter to clean the signal before it is passed to the low pass filter (these harmonics have a similar detrimental impact on motors as other sources of noise).
Without the notch filter you'd have to set the low pass filter cuttoff much lower (increasing the latency) in order to avoid passing this noise to the motors.
### Low-pass Filter on D-term

::: info
Only one notch filter is provided.
Airframes with more than one low frequency noise spike typically clean the first spike with the notch filter, and subsequent spikes using the low pass filter.
:::
The D-term is most susceptible to noise while slightly increased latency does not negatively affect performance.
For this reason the D-term has a separately-configurable low-pass filter, [IMU_DGYRO_CUTOFF](../advanced_config/parameter_reference.md#IMU_DGYRO_CUTOFF).

The best filter settings depend on the vehicle.
The defaults are set conservatively — such that they work on lower-quality setups as well.
### Slew-rate Filter on Motor Outputs

An optional slew-rate filter on the motor outputs.
This rate may be configured as part of the [Multicopter Geometry](../config/actuators.md#motor-geometry-multicopter) when configuring actuators (which in turn modifies the [CA_Rn_SLEW](../advanced_config/parameter_reference.md#CA_R0_SLEW) parameters for each motor `n`).

## Filter Tuning

::: info
The best filter settings depend on the vehicle.
The defaults are set conservatively — such that they work on lower-quality setups as well.
:::

First make sure to have the high-rate logging profile activated ([SDLOG_PROFILE](../advanced_config/parameter_reference.md#SDLOG_PROFILE) parameter).
[Flight Review](../getting_started/flight_reporting.md) will then show an FFT plot for the roll, pitch and yaw controls.

Expand All @@ -91,14 +128,15 @@ First make sure to have the high-rate logging profile activated ([SDLOG_PROFILE]
- Do not try to fix a vehicle that suffers from high vibrations with filter tuning!
Instead fix the vehicle hardware setup.
- Confirm that PID gains, in particular D, are not set too high as this can show up as vibrations.
:::

:::

Filter tuning is best done by reviewing flight logs.
You can do multiple flights right after each other with different parameters and then inspect all logs, but make sure to disarm in between so that separate log files are created.

The performed flight maneuver can simply be hovering in [Stabilized mode](../flight_modes_mc/manual_stabilized.md) with some rolling and pitching to all directions and some increased throttle periods.
The performed flight manoeuvre can simply be hovering in [Stabilized mode](../flight_modes_mc/manual_stabilized.md) with some rolling and pitching to all directions and some increased throttle periods.
The total duration does not need to be more than 30 seconds.
In order to better compare, the maneuver should be similar in all tests.
In order to better compare, the manoeuvre should be similar in all tests.

First tune the gyro filter [IMU_GYRO_CUTOFF](../advanced_config/parameter_reference.md#IMU_GYRO_CUTOFF) by increasing it in steps of 10 Hz while using a low D-term filter value ([IMU_DGYRO_CUTOFF](../advanced_config/parameter_reference.md#IMU_DGYRO_CUTOFF) = 30).
Upload the logs to [Flight Review](https://logs.px4.io) and compare the _Actuator Controls FFT_ plot.
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