Managing Gyro Noise with the Dynamic Harmonic Notch Filters¶
As discussed under the Vibration Damping topic, managing vibration in ArduPilot autopilot installations is extremely important in order to yield predictable control of an aircraft. Typically installations utilize mechanical vibration damping for the autopilot, internally or externally, in order to remove the worst of the vibration. However, mechanical damping can only go so far and software filtering must be used to remove further noise.
To the autopilot, vibration noise looks like any other disturbance (e.g. wind, turbulence, control link slop, etc.) that the autopilot must compensate for in order to control the aircraft. This prevents optimum tuning of the attitude control loops and decreased performance.
ArduPilot provides two filtering mechanisms for noise. Lowpass filters on the accelerometer signals, controlled by the INS_ACCEL_FILTER, and the gyro signals, controlled by INS_GYRO_FILTER, and Harmonic Notch Filters on the gyro signals.
As discussed in Measuring Vibration section, there are basically two classes of noise/vibrations: those generated within the bandwidth of the gyros/accelerometer sampling and noise above those frequencies which appear within the bandwidth and can cause the “leans”. The aliased noise must be eliminated at the source with improved mounting or frame rigidity, but the above filters can deal with the other.
For multicopters and QuadPlanes, virtually all vibrations originate from the motor’s rotational frequency. For helicopters and planes, the vibrations are linked to the main rotor/prop speed.
ArduPilot has support for two notch filters whose filter frequency can be linked to the motor rotational frequency for motors, or the rotor speed for helicopters, and provides notches at a primary frequency and its harmonics.
While the lowpass filters can effectively diminish the impact of this noise, having low frequency set points creates a lot of phase lag and therefore reduces how aggressive the tune can be before oscillation occurs, which results in a poorer tune.
For the gyro based rate controllers, this reduces their ability to respond to fast disturbances. If the gyro lowpass filter can be set higher, the phase lag induced is lower and the tune can be more aggressive. But this allows more noise and vibration, effectively canceling that gain out. The Harmonic Notch filters allow targeting the noise generated by the motors, allowing a higher frequency low pass to be set and tighter tune.
The harmonic notch is enabled overall by setting INS_HNTCH_ENABLE = 1 for the first notch, and INS_HNTC2_ENABLE = 1, for the second. After rebooting, all the relevant parameters will appear.
Various methods of dynamically adjusting the notch(s) center frequency to track motor speed under different thrust conditions is provided, ie Dynamic Harmonic Notch filtering.
Determining Notch Filter Center Frequency¶
Before actually setting up a dynamic notch filter, the frequencies that are desired to be rejected must first be determined. This is crucial if Throttle Based Dynamic Notch Setup is used. While the other methods do not require this knowledge apriori, its still worthwhile as a comparison point for the post filter activation analysis of the filter’s effectiveness
See the Measuring Vibration with IMU Batch Sampler page for this step. Once the noise frequency is determined, the notch filter(s) can be setup.
Notch Filter Control Types¶
Key to the dynamic notch filter operation is control of its center frequency. There are five methods that can be used for doing this:
INS_HNTCH_MODE = 0. Dynamic notch frequency control disabled. The center frequency is fixed and is static. Often used in Traditional Helicopters with external governors for rotor speed, either incorporated in the ESC or separate for ICE motors.
INS_HNTCH_MODE = 1. (Default) Throttle position based, where the frequency at hover throttle is determined by analysis of logs, and then variation of throttle position above this is used to track the increase in noise frequency. Note that the throttle reference only applies to VTOL motors in QuadPlanes, not forward motors, and will not be effective in fixed wing only flight modes. See throttle-based for further setup details.
INS_HNTCH_MODE = 2. RPM sensor based, where an external RPM sensor is used to determine the motor frequency and hence primary vibration source’s frequency for the notch. Often used in Traditional Helicopters (See Helicopters) using the ArduPilot Head Speed Governor feature. See RPM Sensor for further setup instructions.
INS_HNTCH_MODE = 3. ESC Telemetry based, where the ESC provides motor RPM information which is used to set the center frequency. This can also be used for the forward motor in fixed wing flight, if the forward motor(s) ESCs report RPM. This requires that your ESCs are configured correctly to support BLHeli telemetry via a serial port. See ESC Telemetry for further setup instructions. If INS_HNTCH_OPTS, or INS_HNTC2_OPTS if the second set of notches is enabled, has bit 1 set, then a set of notches for each motor will be created, tracking its RPM telemetry, otherwise, the average frequency of all motors will set the center frequency.
INS_HNTCH_MODE = 4. If your autopilot supports it (ie. has more than 2MB of flash, see Firmware Limitations on AutoPilot Hardware), In-Flight FFT, where a running FFT is done in flight to determine the primary noise frequency and adjust the notch’s center frequency to match. This probably the best mode if the autopilot is capable of running this feature. This mode also works on fixed wing only Planes. See In-Flight FFT for further setup instructions.
All of the above are repeated, independently, for the second notch and are prefaced with
INS_HNTC2_ instead of
INS_HNTCH_. The following will explain setup for the first set of notches.
Number of Harmonics Filtered¶
Set INS_HNTCH_HMNCS to enable up to three harmonic (multiples of the center frequency) notches. For ESC INS_HNTCH_MODE tracking, each motor will get a set of these notches. If an octocopter sets up all three harmonics, this results in 8 x 3 = 24 notch filters. Enabling triple notches (see below) would result in 72 filters! This would most certainly cause excessive cpu loading and performance issues. Other modes only provide a single set of harmonic notches.
Always enable only the number of harmonic notch filters actually required and be especially aware of what is being enabled if using ESC (INS_HNTCH_MODE = 3) tracking mode.
FFT Dynamic Harmonic Notch Frequency Tracking¶
FFT mode tracking sets the base frequency to the largest noise peak. Normally, when multiple harmonic notch filters are then enabled, the center frequency of each harmonic is locked to the base frequency of the first filter as an integer multiple, as determined by INS_HNTCH_HMNCS. Setting bit 1 of INS_HNTCH_OPTS, or INS_HNTC2_OPTS, will enable each harmonic filter to track the largest noise peaks, individually.
setting bit 1 of the notch options will also change the default value of INS_HNTCH_HMNCS to 1 instead of its normal 3. This is to maintain backwards compatibility with previous fimware versions. You can set INS_HNTCH_HMNCS back to 3, or whatever is desired, after setting bit 1 of INS_HNTCH_HMNCS.
Checking Notch Filter Effectiveness¶
Once the notch filter(s) are setup, the effectiveness of them can be checked by again measuring the frequency spectrum of the output of the filters (which are the new inputs to the IMU sensors). Refer back to the Measuring Vibration with IMU Batch Sampler page for this.
The software notch filters used are very “spikey” being relatively narrow but good at attenuation at their center. On larger copters the noise profile of the motors is quite dirty covering a broader range of frequencies than can be covered by a single notch filter. In order to address this situation it is possible to configure the harmonic notches as double or triple notches that gives a wider spread of significant attenuation. To utilize this feature set INS_HNTCH_OPTS to “1” for double notches, to “16” for triple notches.
double notch option is no longer recommended since the triple notch option has been added. With a double notch, the maximum attenuation is either side of the center frequency, so on smaller aircraft with a very pronounced peak their use is usually counter productive.
Each notch has some CPU cost so if you configure multiple notches you can end up with many notches on your aircraft. For example, triple single (no harmonics) notches, using ESC telemetry will result in 3 notches per motor or 12 total notches. For example, with F4 cpus this should be acceptable, but enabling a second group of triple notches with INS_HNTC2_ENABLE or multiple harmonic notches, could cause problems.