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The communication between the Raspberry Pi and the Pixhawk works via the RTPS/DDS interface. For implementation manual, see [PX4 Manual](https://docs.px4.io/master/en/ros/ros2_comm.html).
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## Current status
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We were able to establish the connection, using the UART interface of the RPi and the Pixhawk.
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### Hardware Setup
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The setup is shown in the figure below. The TX/RX UART ports are connected accordingly. Therfore we opened a pixhawk telemetry cable and connected it via alligator clips and jumper wires to the RPi. However, this is not a practical solution. Therfore we are aming to use a Micro USB Cable.
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### Software Setup
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#### Pixhawk
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At first, the correct Firmware must be installed on the Pixhawk. Usually this should be done already. If not, follow these steps:
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1. [Install Toolchain on Linux](https://docs.px4.io/master/en/dev_setup/dev_env_linux_ubuntu.html). It says that Ubuntu 20 is not supported, but it does work, so no worry. You can also do this in a virtual machine.
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2. [Install RTPS/DDS](https://docs.px4.io/master/en/dev_setup/fast-dds-installation.html). It is needed to build the proper PX4 Target.
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3. Clone Repository:\
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`git clone https://github.com/PX4/PX4-Autopilot.git --recursive -b release/1.11`
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4. Build Target and upload:\
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`make px4_fmu-v5_rtps upload`
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If the correct Firmware is installed, type the following command into the NuttShell to start the transmission:\
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`micrortps_client start -t UART -d /dev/ttyS3`\
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When the phyiscal Interface is changed, the device '/dev/ttyS3' must be adjusted. Therefore, see [this](https://docs.px4.io/master/en/hardware/serial_port_mapping.html) site.
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#### Raspberry Pi 4
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The Ubuntu 20 SD Cards from Maarten are already set up. \
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On every new startup, the access to the serial port must be granted by typing:\
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`chmod o+rw /dev/ttyS0`\
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The privileges can be checked using\
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`ls -axl /dev/ttyS0`\
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Sometimes, the system resets the priviliges... then you need to be fast to run the next command:\
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`micrortps_agent start -t UART -d /dev/ttyS0`\
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This starts the rtps agent program which handles the connection. To see if the setup works correctly, start the example code. Therfore, open a new terminal and source the ROS2 workspace:\
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`source ~/px4_ros_com_ros2/install/setup.sh`\
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Then, start the supplied program:\
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`ros2 launch px4_ros_com sensor_combined_listener.launch.py`\
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If it shows the realistic and timely changing measurements, the communication works. \
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\
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See [here](https://docs.px4.io/master/en/ros/ros2_comm.html#ros-2-example-applications) for more information on how to read and write data via the rtps interface.
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## Next Steps
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- [ ] Evaluate other ways for physical connection |
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\ No newline at end of file |
