ROS/MAVROS Sending Custom Messages
This article has been tested against:
Ubuntu: 18.04
ROS: Melodic
PX4 Firmware: 1.9.0
However these steps are fairly general and so it should work with other distros/versions with little to no modifications.
MAVROS Installation
Follow Source Installation instructions from mavlink/mavros to install "ROS Kinetic".
MAVROS
We start by creating a new MAVROS plugin, in this example named keyboard_command.cpp (in workspace/src/mavros/mavros_extras/src/plugins) by using the code below:
The code subscribes a 'char' message from ROS topic
/mavros/keyboard_command/keyboard_sub
and sends it as a MAVLink message.Edit mavros_plugins.xml (in workspace/src/mavros/mavros_extras) and add the following lines:
Edit CMakeLists.txt (in workspace/src/mavros/mavros_extras) and add the following line in
add_library
.Inside common.xml in (workspace/src/mavlink/message_definitions/v1.0), copy the following lines to add your MAVLink message:
PX4 Changes
Inside common.xml (in PX4-Autopilot/mavlink/include/mavlink/v2.0/message_definitions), add your MAVLink message as following (same procedure as for MAVROS section above):
Remove common, standard directories in (PX4-Autopilot/mavlink/include/mavlink/v2.0).
Git clone "mavlink_generator" to any directory you want and execute it.
You will see a "MAVLink Generator" popup:
For XML, "Browse" to /PX4-Autopilot/mavlink/include/mavlink/v2.0/message_definitions/standard.xml.
For Out, "Browse" to /PX4-Autopilot/mavlink/include/mavlink/v2.0/.
Select Language C
Select Protocol 2.0
Check Validate
Then, press Generate. You will see common, and standard directories created in /PX4-Autopilot/mavlink/include/mavlink/v2.0/.
Make your own uORB message file key_command.msg in (PX4-Autopilot/msg). For this example the "key_command.msg" has only the code:
Then, in CMakeLists.txt (in PX4-Autopilot/msg), include
Edit mavlink_receiver.h (in PX4-Autopilot/src/modules/mavlink)
Edit mavlink_receiver.cpp (in PX4-Autopilot/src/modules/mavlink). This is where PX4 receives the MAVLink message sent from ROS, and publishes it as a uORB topic.
Make your own uORB topic subscriber just like any example subscriber module. For this example lets create the model in (/PX4-Autopilot/src/modules/key_receiver). In this directory, create two files CMakeLists.txt, key_receiver.cpp. Each one looks like following.
-CMakeLists.txt
-key_receiver.cpp
For a more detailed explanation see the topic Writing your first application.
Lastly add your module in the default.px4board file correspondent to your board in PX4-Autopilot/boards/. For example for the Pixhawk 4 add the following code in PX4-Autopilot/boards/px4/fmu-v5/default.px4board:
Now you are ready to build all your work!
Building
Build for ROS
In your workspace enter:
catkin build
.Beforehand, you have to set your "px4.launch" in (/workspace/src/mavros/mavros/launch). Edit "px4.launch" as below. If you are using USB to connect your computer with Pixhawk, you have to set "fcu_url" as shown below. But, if you are using CP2102 to connect your computer with Pixhawk, you have to replace "ttyACM0" with "ttyUSB0". Modifying "gcs_url" is to connect your Pixhawk with UDP, because serial communication cannot accept MAVROS, and your nutshell connection simultaneously.
Write your IP address at "xxx.xx.xxx.xxx"
Build for PX4
Build PX4-Autopilot and upload in the normal way.
For example, to build for Pixhawk 4/FMUv5 execute the following command in the root of the PX4-Autopilot directory:
Running the Code
Next test if the MAVROS message is sent to PX4.
Running ROS
In a terminal enter
In a second terminal run:
This means, publish 97 ('a' in ASCII) to ROS topic "/mavros/keyboard_command/keyboard_sub" in message type "std_msgs/Char". "-r 10" means to publish continuously in "10Hz".
Running PX4
Enter the Pixhawk nutshell through UDP. Replace xxx.xx.xxx.xxx with your IP.
After few seconds, press Enter a couple of times. You should see a prompt in the terminal as below:
Type "key_receiver", to run your subscriber module.
Check if it successfully receives a
from your ROS topic.
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