flight/pid: add D-term pre-differentiation LPF (dterm_lpf2_hz)#11464
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flight/pid: add D-term pre-differentiation LPF (dterm_lpf2_hz)#1146419379353560 wants to merge 1 commit intoiNavFlight:masterfrom
19379353560 wants to merge 1 commit intoiNavFlight:masterfrom
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Differentiation amplifies high-frequency noise by f_loop/f_cutoff (~9x at 1kHz loop). Add an optional PT1 pre-filter applied before differentiation to reduce this amplification to ~3.6x, at the cost of only +0.6ms latency at the default 250Hz cutoff. This is inspired by Betaflight's architecture where gyro data is filtered before the derivative is computed. New parameter: dterm_lpf2_hz (default 250, 0 = disabled) - 5-inch builds: 250Hz recommended - 7-inch builds: 200Hz recommended Benefits: - D gain headroom increases ~15-25% - Improved propwash rejection - Better altitude/position hold disturbance rejection The added latency is negligible for the position/altitude loop (bandwidth <5Hz). Backward compatible: set dterm_lpf2_hz=0 to restore stock behavior. Closes: iNavFlight#11463
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Problem
In the current multirotor PID controller, differentiation is applied directly to the raw gyro signal:
Differentiation amplifies high-frequency noise by
f_loop / f_cutoff. At 1 kHz loop rate this is roughly 9×. The downstream PT2 then has to suppress that amplified noise, which limits how high D gain can be set before oscillation occurs.Solution
Add an optional PT1 pre-filter applied before differentiation:
This reduces noise amplification from ~9× to ~3.6× at the cost of only +0.6 ms latency (PT1 @ 250 Hz, 1 kHz loop).
Changes
pidState_t: adddtermLpf2State(PT1) andpreviousFilteredGyroRatedTermProcess(): pre-filter path activated whendterm_lpf2_hz > 0pidInitFilters(): initialize pre-diff filter from configpidProfile_t: adddterm_lpf2_hzfieldsettings.yaml: newdterm_lpf2_hzparameter (default 250 Hz, 0 = disabled)Impact
The added latency is negligible for the position/altitude loop (bandwidth < 5 Hz). Setting
dterm_lpf2_hz = 0restores stock behavior exactly.Recommended values
dterm_lpf2_hz = 250dterm_lpf2_hz = 200Related discussion: #11463