Visual Tracking on Pixhawk

System Overview

The tracker consists of:

• A quarotor equipped with a Pixhawk, the Drone
• A linux computer connected to a
• HD webcam, and
• 3DR telemetry radio.
• Software running on the linux computer for tracking the drone, and sending the position over the telemetry link
• Modified Pixhawk firmware with sensors added to the ekf2 module

Installation Instructions

Default Pixhawk Firmware

For instructions on setting up the required tool chain and building the standard Pixhawk firmware, follow the tutorial at http://dev.px4.io/starting-installing.html

The Pixhawk team uses Git to synchronize their work. If you are not familiar with git, now would be a good time to familiarize yourself with it.

The Pixhawk firmware is a rather large project. The main part of the software is located at https://github.com/PX4/Firmware.git, but some part reside in so called git submodules.

To get a general idea about, how to make custom modules (programs) for the Pixhawk I suggest reading through the examples at http://dev.px4.io/

Updated Pixhawk Firmware

The important changes in the Pixhawk firmware are located at: <blockqute> src/lib/ecl - this is the submodule for the EKF source code (and some other modules). The reason why it's in a submodule, is because it's ment as a portable estimator and can be used in other projects as well

<blockqute> src/modules/ekf2 - this is a wrapper for the EKF mentioned above. It passes data from uORB topics to the filter and back.

To fetch the code I delvelpped and assign the correct submodule for the ekf run the following commands:

mkdir Pixhawk
cd Pixhawk
git clone https://github.com/skrogh/Firmware.git
cd Firmware
git remote add upstream https://github.com/PX4/Firmware.git
git checkout mocap-ekf2
Tools/check_submodules.sh
cd src/lib/ecl
git remote rename origin upstream
git remote add origin https://github.com/skrogh/ecl.git
git fetch origin
git checkout mocap-ekf2
cd ../../..
make px4fmu-v2_default

This will do:

• Make a new directory for the project
• Enter it
• Clone my fork of the repository. This includes updated wrappers for the filter and some changes to enable Mikrokopter drivers
• Enter the repository
• Add a remote to the original repository. This can later be used to update the software to include changes made by the PX4 team
• All work on motion capture input has ben done on a separate branch mocap-ekf2 so we switch to that
• Tools/check_submodules.sh is a script that ensures, that submodules are up to date. We run it to fetch all submodules
• Go into the submodule for the estimator
• Move the original "origin" remote to "upstream" (these are just names, but this is to keep consistency)
• Add a new remote to the version of this repo I made the changes in
• Fetch changes
• Again work was done on the mocap-ekf2 branch, so switch to this one.
• Go back to the toplevel
• build the default configuration

Pixhawk Setup

General Advice

Not all settings for the Pixhawk are stored in the firmware. A lot of parameters that affects the Pixhawk can be set either from a ground control station, or via the Nuttx shell.

It is recommended to connect to the Nuttx shell through the serial port and not the USB port. This will also give you debut info during boot.

When the Pixhawk boots a script ROMFS/px4fmu-v2_common/init.d/rcS is launched. Based on some parameters and files located on the SD card, the Pixhawk starts the appropriate modules (flight controllers, estimators, sensor drivers etc.)

The boot script does something like this:

1. If the file etc/rc.txt exists on the SD card, this is run instead of the autostart routine (step 2-6).
2. if SYS_AUTOSTART parameter is set, a (automatically generated) script is run that sets parameters associated with the chosen robot. (ex. for SYS_AUTOSTART=4001 https://github.com/PX4/Firmware/blob/master/ROMFS/px4fmu_common/init.d/4001_quad_x is run).
3. If the file etc/config.txt exists on the SD card this script will run, allowing you to overwrite parameters for the robot, without creating your own custom one.
4. Sensor drivers are startet along with output dirvers (PWM and/or Mikrokopter if chosen).
5. Most SYS_AUTOSTART configurations (if not all?) will set the VEHICLE_TYPE variable. This will chose which modules are startet for flight controller and estimator(s). (Note the difference between parameters and variables. Parameters are saved in flash and prevail during poweroff - they are also available in C/C++ code).
6. if the file /etc/extras.txt exists this will run as the last part of the autostart.
7. A Nuttx shell is opened over USB if no SD card is present. Otherwise MAVLINK wil be started on the USB port (on my fork a Nuttx will always be present on the USB, to start mavlink for ground control connection type mavlink start -r 800000 -d /dev/ttyACM0 -m config -x in a Nuttx shell)

Vision Tracking Specific Settings

Instead of making custom startup scrips, we reuse the ones present and set the correct parameters.

On the SD card create the file etc/config.txt:

# EKF2 Setup
echo "Disabeling INAV"
param set INAV_ENABLED 0
echo "Disabeling LPE"
param set LPE_ENABLED 0
echo "This makes the quad use ekf2"
# Mikrokopter setup
echo "Setting MikroKopter BL-ctl settings"
set OUTPUT_MODE mkblctrl
set MKBLCTRL_MODE x

This will tell the Pixhawk to disable the split attitude position estimators and use the combined ekf2 estimator instead. Furthermore the Mikrokopter motor driver will be enabled. Set SYS_AUTOSTART=4001 for a standard x-quadrotor configuration.

In the ekf2 module some parameters have been added:

EKF2_USE_MOCAP (default: 0) - use motion capture data? (if 0 will use GPS)
EKF2_USE_PREDICT (default: 1) - use IMU measurements to propagate the delayed state estimate to hide delay (set to 0 for debug) 
EKF2_MO_x_NOISE - variance of x position measurement
EKF2_MO_y_NOISE - variance of y position measurement
EKF2_MO_z_NOISE - variance of z position measurement
EKF2_MO_r_NOISE - variance of roll measurement (set high, otherwise you have to really carefully align pixhawk with marker)
EKF2_MO_p_NOISE - variance of pitch measurement (set high, otherwise you have to really carefully align pixhawk with marker)
EKF2_MO_h_NOISE - variance of headding measurement

Installing Tracker software

http://docs.opencv.org/3.1.0/d7/d9f/tutorial_linux_install.html

Important notes

Changes in files not concerning the EKF

The following files have been changed, to enable Mikrokopter BL-CTRL motor drivers and ultrasonic rangefinder: cmake/configs/nuttx_px4fmu-v2_default.cmake in the process the ekf2 module was added to the build list along with the useful tool listener. To free up some space, some unused modules were commented out of the build list.