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Companion Computer

This section introduces how to set up the (on-board) companion computer for PX4-based drones.

Hardware Connection

FTDI Devices / Logic Level Shifters

  • Connect onboard computer and Pixhawk flight controller via an FTDI Chip USB-to-serial adapter.

    • use the TELEM2 port on Pixhawk and a USB port on companion computer

    • make sure the FTDI chip or logic level shifter is compatible in our use case

      • see this section for recommended ones (Pixdev discontinued though)

    • make sure the pin-to-pin connection is correct

      • out of 6 pins, 3 pins must be connected (TX-RX, RX-TX, GND-GND)

      • the red power pin must not be connected

      • CTS and RTS pins are optional; for the use of so called "flow control" for Pixhawk

      • see this section for the full wiring table

  • It is recommended that the FTDI chip operates at 3.3V, but not required

    • the red power pin is not connected anyway

    • according to this section in PX4 docs, all Pixhawk serial ports operate at 3.3V and are 5V level compatible.

  • Example: USB FTDI Serial to JST-GH (Basic)

    • warning: this product is not a ready-to-go adapter; need to switch RX and TX cables (cut them off and then solder together)

      • see this video for a quick recap about soldering two wires together

      • soldering quality can also affect the performance; bad soldering will lead to high noise level in the communication

    • the connection link is: computer --- USB port --- USB to micro USB cable --- micro USB port --- FTDI chip --- 6-pin JST port --- JST cable --- 6-pin TELEM2 port --- Pixhawk

(left) FTDI cable after soldering (right) final cable after hot-glue gun and heat-shrink tubing

WiFi Router

Need to prepare a high-speed low-latency WiFi router

  • edit settings to shrink DHCP range and leave a range of IP addresses unused for static IP

  • in my case, I use 192.168.0.21 for onboard computer and 192.168.0.121 for gcs computer

PX4 Firmware

  • Need to enable MAVLink on a configurable serial port (typically TELEM2).

    • Connect the Pixhawk flight controller to QGroundControl via USB or Telemetry (TELEM1), so that you can change parameters in the parameter table and push them to the vehicle.

    • Set param MAV_1_CONFIG = TELEM 2 (MAV_1_CONFIG is often used to map the TELEM 2 port)

    • Reboot the vehicle to make other parameters visible.

    • Set the remaining parameters if not being set by default.

      • MAV_1_MODE = Onboard

      • SER_TEL2_BAUD = 921600 (921600 or higher recommended for applications like log streaming or FastRTPS)

  • Once this parameter is enabled, when Pixhawk is powered on, it will automatically broadcast messages (drone status) via TELEM2 port. The RX indicator should be flickering accordingly.

Onboard Computer

  • Set up a static IP address for WiFi connection

    • in my case, I use 192.168.0.21 for onboard computer and 192.168.0.121 for gcs computer

    • optional: edit hosts file by sudo gedit /etc/hosts and add item 192.168.0.121 arcs-gcs

  • udev rule: create a new file at /etc/udev/rules.d/99-pixhawk.rules with the following content

    • double check by lsusb the actual vendor and product ID of the FTDI chip

  • Add current user to the tty and dialout groups to avoid to have to execute scripts as root

  • Launch file is the same as mavros/launch/px4.launch except for the following modifications

    • set fcu_url to /dev/ttyPixhawk:921600

    • set gcs_url to udp://@arcs-gcs:14550

    • double check if tgt_system matches MAV_SYS_ID parameter in PX4 firmware (default 1)

    • double check if tgt_component matches MAV_COMP_ID parameter in PX4 firmware (default 1)

Ground Control Station (GSC) Computer

  • Set up a static IP address for WiFi connection

    • in my case, I use 192.168.0.21 for onboard computer and 192.168.0.121 for gcs computer

    • optional: edit hosts file by sudo gedit /etc/hostsand add item 192.168.0.21 arcs-robot

    • optional: set up password-free SSH login by ssh-copy-id arcs@arcs-robot

  • Download QGroundControl Ubuntu amd64 build (arm architecture is not supported)

    • run ./QGroundControl and you can see drone status

    • it will automatically listen to port 14550 for UDP connections

Troubleshooting

Hardware connection

  • lsusb check the vendor ID and product ID of FTDI chip

  • ls -l /dev/ttyPixhawk check the read/write permission

  • use a multi-meter to check voltage level

  • find a chip that has TX/RX indicators to help debugging

mavros connection

  • rostopic echo -n1 /diagnostics

  • rosrun mavros checkid

Issue: FCU not connected

  • If TELEM2 port is indeed broadcasting messages, then cat /dev/ttyPixhawk should print out random symbols/streaming and rostopic echo -n1 /diagnostics should have RX packages/bytes received.

  • See this issue in mavros repository for more details.

References

Examples for Healthy/Successful Connections

roslaunch mavros px4.launch

rostopic echo -n1 /diagnostics

rosrun mavros checkid

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