PSA: If your copter is suddenly dropping, this might be the reason

lordvon · April 16, 2023, 12:00pm

Aircraft with T/W < 2 and attitude output saturation will suddenly drop.

github.com/ArduPilot/ardupilot

AP_MotorsMatrix: prevent altitude loss when thrust-to-weight is less than 2

ArduPilot:master ← 117-MasterChief:prevent-altitude-loss-dynamic

opened 08:34PM - 15 Apr 23 UTC

117-MasterChief

+8 -3

This change prevents the Copter flight controller from sacrificing altitude for …attitude output when the thrust-to-weight ratio of the aircraft is below 2. In other words, this change prevents the Copter from suddenly dropping under certain conditions. Instead of a hard-coded minimum throttle of `0.5f` reserved for keeping altitude, this introduces a dynamic minimum throttle that preserves desired altitude output. This dynamic minimum throttle utilizes the parameter `MOT_THST_HOVER`. In the cases that this parameter is not set or hover throttle is otherwise not known, this implementation tries to revert to the previous implementation (`0.5f` minimum throttle for altitude preservation). Tests SITL tests were performed using a mission that saturated yaw outputs, demonstrating the problem and solution. [This branch](https://github.com/117-MasterChief/ardupilot/tree/prevent-altitude-loss-SITL-test) (which modifies the default T/W from 2.56 to 1.8) can be used to repeat these tests. The following results are from the aforementioned branch (without this PR), and drop in altitude is observed (note that the timescales may be shifted, due to having to press "Pause" on each graph): ![bug-with-yaw](https://user-images.githubusercontent.com/130329104/232251848-738ceb6e-67d8-4033-b8d5-c8ae8e0304ce.png) With this PR, no drop in altitude is observed: ![fix-with-yaw](https://user-images.githubusercontent.com/130329104/232251862-31afaf01-0b58-49ae-9542-1d99d85bc9da.png) The instructions for reproducing the test are included in a text file in [the test branch](https://github.com/117-MasterChief/ardupilot/tree/prevent-altitude-loss-SITL-test), but repeated here: ``` Go to ardupilot/ArduCopter/ ../Tools/autotest/sim_vehicle.py --wipe-eeprom --frame=quad --vehicle=ArduCopter --map --console --osd Load yaw-saturation-test-3.txt mission using Mission Editor GUI. param set ATC_SLEW_YAW 18000 param set ATC_ACCEL_Y_MAX 72000 param set ATC_RAT_YAW_P 2.5 param set MOT_THST_HOVER 0.556 module load graph graph SERVO_OUTPUT_RAW.servo1_raw SERVO_OUTPUT_RAW.servo2_raw SERVO_OUTPUT_RAW.servo3_raw SERVO_OUTPUT_RAW.servo4_raw graph AHRS2.altitude graph ATTITUDE.yaw rc 3 1000 mode stabilize arm throttle mode auto rc 3 1500 ```