Adding a new Airframe Configuration

PX4 uses canned configurations as starting point for airframes. Adding a configuration is straightforward: Create a new file which is prepended with a free autostart ID in the init.d folder and build and upload the software.

Developers not wanting to create their own configuration can instead customize existing configurations using text files on the microSD card, as detailed on the custom system startup page.

Airframe configurations

An airframe configuration consists of three main blocks:

  • The apps it should start, e.g. multicopter or fixed wing controllers
  • The physical configuration of the system (e.g. a plane, wing or multicopter). This is called mixer.
  • Tuning gains

These three aspects are mostly independent, which means that many configurations share the same physical layout of the airframe and start the same applications and most differ in their tuning gains.

All configurations are stored in the ROMFS/px4fmu_common/init.d folder. All mixers are stored in the ROMFS/px4fmu_common/mixers folder.

Config file

A typical configuration file is below.

#!nsh
#
# @name Wing Wing (aka Z-84) Flying Wing
#
# @url https://pixhawk.org/platforms/planes/z-84_wing_wing
#
# @type Flying Wing
#
# @output MAIN1 left aileron
# @output MAIN2 right aileron
# @output MAIN4 throttle
#
# @output AUX1 feed-through of RC AUX1 channel
# @output AUX2 feed-through of RC AUX2 channel
# @output AUX3 feed-through of RC AUX3 channel
#
# @maintainer Lorenz Meier <lorenz@px4.io>
#

sh /etc/init.d/rc.fw_defaults

if [ $AUTOCNF == yes ]
then
    param set BAT_N_CELLS 2
    param set FW_AIRSPD_MAX 15
    param set FW_AIRSPD_MIN 10
    param set FW_AIRSPD_TRIM 13
    param set FW_ATT_TC 0.3
    param set FW_L1_DAMPING 0.74
    param set FW_L1_PERIOD 16
    param set FW_LND_ANG 15
    param set FW_LND_FLALT 5
    param set FW_LND_HHDIST 15
    param set FW_LND_HVIRT 13
    param set FW_LND_TLALT 5
    param set FW_THR_LND_MAX 0
    param set FW_PR_FF 0.35
    param set FW_RR_FF 0.6
    param set FW_RR_P 0.04
fi

# Configure this as plane
set MAV_TYPE 1
# Set mixer
set MIXER wingwing
# Provide ESC a constant 1000 us pulse
set PWM_OUT 4
set PWM_DISARMED 1000

IMPORTANT REMARK: If you want to reverse a channel, never do this on your RC transmitter or with e.g RC1_REV. The channels are only reversed when flying in manual mode, when you switch in an autopilot flight mode, the channels output will still be wrong (it only inverts your RC signal). Thus for a correct channel assignment change either your PWM signals with PWM_MAIN_REV1 (e.g. for channel one) or change the signs of the output scaling in the corresponding mixer (see below).

Mixer file

A typical configuration file is below. Note that the mixer file contains several blocks of code, each of which refers to one actuator or ESC. So if you have e.g. two servos and one ESC, the mixer file will contain three blocks of code.

The plugs of the servos / motors go in the order of the mixers in this file.

So MAIN1 would be the left aileron, MAIN2 the right aileron, MAIN3 is empty (note the Z: zero mixer) and MAIN4 is throttle (to keep throttle on output 4 for common fixed wing configurations).

A mixer is encoded in normalized units from -10000 to 10000, corresponding to -1..+1.

M: 2
O:      10000  10000      0 -10000  10000
S: 0 0  -6000  -6000      0 -10000  10000
S: 0 1   6500   6500      0 -10000  10000

Where each number from left to right means:

  • M: Indicates two scalers for two inputs
  • O: Indicates the output scaling (1 in negative, 1 in positive), offset (zero here), and output range (-1..+1 here). If you want to invert your PWM signal, the signs of the output scalings has to be changed. (O: -10000 -10000 0 -10000 10000)
  • S: Indicates the first input scaler: It takes input from control group #0 (attitude controls) and the first input (roll). It scales the input * 0.6 and reverts the sign (-0.6 becomes -6000 in scaled units). It applies no offset (0) and outputs to the full range (-1..+1)
  • S: Indicates the second input scaler: It takes input from control group #0 (attitude controls) and the second input (pitch). It scales the input * 0.65 and reverts the sign (-0.65 becomes -6500 in scaled units). It applies no offset (0) and outputs to the full range (-1..+1)

Behind the scenes, both scalers are added, which for a flying wing means the control surface takes maximum 60% deflection from roll and 65% deflection from pitch, i.e., SERVO = (0.60 roll) + (0.65 pitch). As it is over-committed with 125% total deflection for maximum pitch and roll, it means the first channel (roll here) has priority over the second channel / scaler (pitch).

The complete mixer looks like this:

Delta-wing mixer for PX4FMU
===========================

Designed for Wing Wing Z-84

This file defines mixers suitable for controlling a delta wing aircraft using
PX4FMU. The configuration assumes the elevon servos are connected to PX4FMU
servo outputs 0 and 1 and the motor speed control to output 3. Output 2 is
assumed to be unused.

Inputs to the mixer come from channel group 0 (vehicle attitude), channels 0
(roll), 1 (pitch) and 3 (thrust).

See the README for more information on the scaler format.

Elevon mixers
-------------
Three scalers total (output, roll, pitch).

The scaling factor for roll inputs is adjusted to implement differential travel
for the elevons. 

This first block of code is for Servo 0...

M: 2
O:      10000  10000      0 -10000  10000
S: 0 0  -6000  -6000      0 -10000  10000
S: 0 1   6500   6500      0 -10000  10000

And this is for Servo 1...

M: 2
O:      10000  10000      0 -10000  10000
S: 0 0  -6000  -6000      0 -10000  10000
S: 0 1  -6500  -6500      0 -10000  10000

Note that in principle, you could implement left/right wing asymmetric mixing, but in general the two blocks of code will be numerically equal, and just differ by the sign of the third line (S: 0 1), since to roll the plane, the two ailerons must move in OPPOSITE directions. The signs of the second lines (S: 0 0) are indentical, since to pitch the plane, both servos need to move in the SAME direction. 

Output 2
--------
This mixer is empty.

Z:

Motor speed mixer
-----------------
Two scalers total (output, thrust).

This mixer generates a full-range output (-1 to 1) from an input in the (0 - 1)
range.  Inputs below zero are treated as zero.

M: 1
O:      10000  10000      0 -10000  10000
S: 0 3      0  20000 -10000 -10000  10000

Getting the new airframe to show in QGroundControl

The airframe meta data is bundled in the .px4 firmware file (which is a zipped JSON file).

Ensure to flash the resulting .px4 file in QGroundControl (custom file option) to load the meta data into the application. The new airframe will then be available in the user interface once you restart QGroundControl.

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