Throttle Based Dynamic Notch Setup

If you do not have ESC telemetry or an RPM sensor, and are not using an autopilot capable of using the in-flight FFT mode, then throttle-based (INS_HNTCH_MODE = 1 , or INS_HNTC2_MODE = 1) center frequency control is best.

In order to configure the throttle-based dynamic harmonic notch filter it is important to identify the motor noise at the hover throttle level. To do this we need to use the Raw IMU Logging for FFT Analysis to obtain logs for analysis.

Once this is done, the center frequency of the notch(s) can be set and other parameters configured:

Note

if the autopilot is capable of running the FFT (most F4 based autopilots are NOT), then by enabling the FFT_ENABLE and using the RC Auxiliary function “162” (Auto Throttle Notch Setup) during a sustained hover, the center frequency, INS_HNTCH_FREQ, INS_HNTCH_ENABLE, and INS_HNTCH_MODE will automatically be setup for the first notch, avoiding log analysis to determine center frequency as outline below:

Harmonic Notch Configuration for Throttle Based Mode

• Set INS_HNTCH_MODE and/or INS_HNTC2_MODE = 1.

• Set INS_HNTCH_ENABLE and/or INS_HNTC2_ENABLE = 1 to enable the harmonic notch

• Set INS_HNTCH_REF and/or INS_HNTC2_REF = hover_thrust to set the harmonic notch reference value

• Set INS_HNTCH_FREQ and/or INS_HNTC2_FREQ = hover_freq to set the harmonic notch reference frequency

• Set INS_HNTCH_BW and/or INS_HNTC2_BW = hover_freq / 2 to set the harmonic notch bandwidth

• Set INS_HNTCH_FM_RAT and/or INS_HNTC2_FM_RAT to the percentage of INS_HNTCH_FREQ and/or INS_HNTC2_FREQ that you desire the notch frequency to track below hover throttle. Note that lower frequency notch filters will have more phase lag and can impact stability. If you want throttle based notch filtering to be effective at a throttle up to 30% below the configured notch frequency then set this parameter to 0.7. The default of 1.0 means the notch will not go below the frequency in the INS_HNTCH_FREQ and/or INS_HNTC2_FREQ parameter.

• Setting bit 1 of INS_HNTCH_OPTS, or INS_HNTC2_OPTS, will enable each harmonic filter to track each motor, individually for its throttle input, rather than by the average overall throttle.

Warning

If you set bit 1 of INS_HNTCH_OPTS, or INS_HNTC2_OPTS, the bandwidth should not be half the frequency(default). It should be greatly reduced as more notches (one for each motor) cause more phase lag (i.e. latency), thus the bandwidth needs to be reduced to maintain a reasonable phase lag, else oscillation and a poorer tune will result despite the more accurate filtering compared to throttle-based filtering. The suggested starting point is setting the _FREQ/_BW params to 4/1 ratio instead of the default 2/1. This is because for notch filters, a wider bandwidth causes a greater phase lag per notch. You can also use the Filter Tool to check the phase lag for your chosen filtering settings. If your phase lag is higher than it was with the single peak filtering, then you can try reducing the bandwidth even further, balancing the phase lag and the amount of noise in the system. Check the performance of the filter(s) after setup by doing another post filter configuration test flight as discussed in the Measuring Vibration with IMU Batch Sampler or Raw IMU Logging for FFT Analysis page for this and analyze the logs.

Advanced Notch Frequency Scaling Adjustment in Throttle Based Mode

The harmonic notch is designed to match the motor noise frequency as it changes by interpreting the throttle value. The frequency is scaled up from the hover frequency and will never go below this frequency. However, in dynamic flight it is quite common to hit quite low motor frequencies during propwash. In order to address this it is possible to change the ref value in order to scale from a lower frequency.

• First perform a long dynamic flight using your current settings and post-filter batch logging. Examine the FFT and look at how far the motor noise peak extends below the hover frequency. Use this frequency - min_freq - as the lower bound of your scaling. Then in order to calculate an updated value of the throttle reference use:

INS_HNTCH_REF and/or INS_HNTC2_REF = hover_thrust * SQUARE(min_freq / hover_freq)