mirror of https://github.com/ArduPilot/ardupilot
65 lines
2.5 KiB
Markdown
65 lines
2.5 KiB
Markdown
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# Multicopter CoM compensation
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This script allows for adjusting the control allocation matrix.
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When the Center of Mass (CoM) of an airframe does not coincide with the center
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of its thrusters, then there is a lever arm between the thrust vector and the
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CoM. This often is the case in VTOL fixed-wing aircraft (quadplanes) where
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typically the CoM is more forward than the center of thrust. As a result, the
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thrust produces a pitch-down moment. This produces a disturbance in the pitch
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control and requires significant wind-up in the pitch integrator.
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To compensate for this issue, this script employs the scriptable control
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allocation matrix to request asymmeterical front and back thrust.
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WARNING: This script is applicable only to X-type quadrotors and quadplanes. Do
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not use in any other frame configuration!
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# Parameters
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The script adds 1 parameter to control its behaviour.
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## CGA_RATIO
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This is the desired ratio between the front and back thrust. To have the front
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motors produce more lift that the rear, increase higher than 1.
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Reasonable extreme values are 2 (front works twice as hard as the rear) and 0.5
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(the inverse case). Given an out-of-bounds parameter value, the script will
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revert to the default 1.0.
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# Operation
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## How To Use
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First of all, place this script in the "scripts" directory.
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To tune `CGA_RATIO` on the fly:
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1. Set `FRAME_CLASS` or `Q_FRAME_CLASS` (for quadplanes) to 17 to enable the
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dynamic scriptable mixer.
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2. Enable Lua scripting via the `SCR_ENABLE` parameter.
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3. Reboot.
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4. Fly the vehicle.
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5. Adjust the value of the `CGA_RATIO` parameter. A good indicator of a good
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tune is to monitor the telemetry value `PID_TUNE[2].I` (pitch rate controller
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integrator) until it reaches zero during a stable hover.
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Once you are happy with the tuning, you can fall back to the static motor
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matrix, which consumes no resources from the scripting engine:
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1. Set `FRAME_CLASS` or `Q_FRAME_CLASS` (for quadplanes) to 15 to enable the
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static scriptable mixer.
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2. Ensure Lua scripting is enabled via the `SCR_ENABLE` parameter.
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3. Reboot.
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The aircraft is ready to fly.
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Keep in mind that any further changes to `CGA_RATIO` will now require a reboot.
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## How It Works
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1. The dynamic control allocation matrix is able to change the coefficients
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that convert the throttle command to individual PWM commands for every motor.
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These coefficients have a default value of 1.
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2. The parameter `CGA_RATIO` is used to alter these coefficients, so that the
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front and back thrust commands are not equal.
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