ardupilot/libraries/AP_Scripting/applets/x-quad-cg-allocation.md

Multicopter CoM compensation

This script allows for adjusting the control allocation matrix.

When the Center of Mass (CoM) of an airframe does not coincide with the center of its thrusters, then there is a lever arm between the thrust vector and the CoM. This often is the case in VTOL fixed-wing aircraft (quadplanes) where typically the CoM is more forward than the center of thrust. As a result, the thrust produces a pitch-down moment. This produces a disturbance in the pitch control and requires significant wind-up in the pitch integrator.

To compensate for this issue, this script employs the scriptable control allocation matrix to request asymmeterical front and back thrust.

WARNING: This script is applicable only to X-type quadrotors and quadplanes. Do not use in any other frame configuration!

Parameters

The script adds 1 parameter to control its behaviour.

CGA_RATIO

This is the desired ratio between the front and back thrust. To have the front motors produce more lift that the rear, increase higher than 1.

Reasonable extreme values are 2 (front works twice as hard as the rear) and 0.5 (the inverse case). Given an out-of-bounds parameter value, the script will revert to the default 1.0.

Operation

How To Use

First of all, place this script in the "scripts" directory.

To tune CGA_RATIO on the fly:

1. Set FRAME_CLASS or Q_FRAME_CLASS (for quadplanes) to 17 to enable the dynamic scriptable mixer.
2. Enable Lua scripting via the SCR_ENABLE parameter.
3. Reboot.
4. Fly the vehicle.
5. Adjust the value of the CGA_RATIO parameter. A good indicator of a good tune is to monitor the telemetry value PID_TUNE[2].I (pitch rate controller integrator) until it reaches zero during a stable hover.

Once you are happy with the tuning, you can fall back to the static motor matrix, which consumes no resources from the scripting engine:

1. Set FRAME_CLASS or Q_FRAME_CLASS (for quadplanes) to 15 to enable the static scriptable mixer.
2. Ensure Lua scripting is enabled via the SCR_ENABLE parameter.
3. Reboot.

The aircraft is ready to fly. Keep in mind that any further changes to CGA_RATIO will now require a reboot.

How It Works

1. The dynamic control allocation matrix is able to change the coefficients that convert the throttle command to individual PWM commands for every motor. These coefficients have a default value of 1.
2. The parameter CGA_RATIO is used to alter these coefficients, so that the front and back thrust commands are not equal.