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

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

# NOTE: use the actual vendor and product IDs (check it by lsusb)
SUBSYSTEM=="tty", ATTRS{idVendor}=="0403", ATTRS{idProduct}=="6015", SYMLINK+="ttyPixhawk"
#SUBSYSTEM=="tty", ATTRS{idVendor}=="1a86", ATTRS{idProduct}=="7523", SYMLINK+="ttyPixhawk"

# optional; can be omitted if Arduino is not being used
SUBSYSTEM=="tty", ATTRS{idVendor}=="2341", ATTRS{idProduct}=="0042", SYMLINK+="ttyArduino"

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

sudo usermod -a -G tty <username>
sudo usermod -a -G dialout <username>

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

PX4 docs: Companion Computer Peripherals
PX4 docs: Companion Computer for Pixhawk Series
PX4 docs: MAVLink Peripherals (GCS/OSD/Companion)
PX4 docs: Serial Port Configuration
Holybro docs: Pixhawk 4 Pinout Manual

Examples for Healthy/Successful Connections

roslaunch mavros px4.launch

auto-starting new master
process[master]: started with pid [25993]
ROS_MASTER_URI=http://localhost:11311

setting /run_id to 52bef7b4-ea8d-11eb-b0f6-204ef6c5ef8f
process[rosout-1]: started with pid [26011]
started core service [/rosout]
process[mavros-2]: started with pid [26029]
[ INFO] [1626917830.064227539]: FCU URL: /dev/ttyPixhawk:921600
[ INFO] [1626917830.075512530]: serial0: device: /dev/ttyPixhawk @ 921600 bps
[ INFO] [1626917830.077993842]: GCS URL: udp://@arcs-gcs:14550
[ INFO] [1626917830.079239602]: udp1: Bind address: 0.0.0.0:14555
[ INFO] [1626917830.079788274]: udp1: Remote address: 192.168.0.121:14550
[ INFO] [1626917830.123951216]: Plugin 3dr_radio loaded
[ INFO] [1626917830.133589392]: Plugin 3dr_radio initialized
[ INFO] [1626917830.134123504]: Plugin actuator_control loaded
...
[ INFO] [1626917831.596142837]: Plugin wind_estimation loaded
[ INFO] [1626917831.599738645]: Plugin wind_estimation initialized
[ INFO] [1626917831.599962517]: Autostarting mavlink via USB on PX4
[ INFO] [1626917831.600359957]: Built-in SIMD instructions: ARM NEON
[ INFO] [1626917831.600489557]: Built-in MAVLink package version: 2021.3.3
[ INFO] [1626917831.600601205]: Known MAVLink dialects: common ardupilotmega ASLUAV all autoquad icarous matrixpilot paparazzi standard uAvionix ualberta
[ INFO] [1626917831.601232277]: MAVROS started. MY ID 1.240, TARGET ID 1.1
[ INFO] [1626917831.607340148]: IMU: High resolution IMU detected!
[ INFO] [1626917831.623283188]: IMU: Attitude quaternion IMU detected!
[ INFO] [1626917831.712271824]: CON: Got HEARTBEAT, connected. FCU: PX4 Autopilot
[ INFO] [1626917831.726061328]: IMU: High resolution IMU detected!
[ INFO] [1626917831.727046608]: IMU: Attitude quaternion IMU detected!
[ INFO] [1626917832.734570666]: GF: Using MISSION_ITEM_INT
[ INFO] [1626917832.734848618]: RP: Using MISSION_ITEM_INT
[ INFO] [1626917832.735039114]: WP: Using MISSION_ITEM_INT
[ INFO] [1626917832.735182506]: VER: 1.1: Capabilities         0x000000000000e4ef
[ INFO] [1626917832.735327146]: VER: 1.1: Flight software:     010b03ff (a6274bc5ed000000)
[ INFO] [1626917832.735467946]: VER: 1.1: Middleware software: 010b03ff (a6274bc5ed000000)
[ INFO] [1626917832.735595530]: VER: 1.1: OS software:         080200ff (ec20f2e6c5cc35b2)
[ INFO] [1626917832.735720650]: VER: 1.1: Board hardware:      00000032
[ INFO] [1626917832.735841738]: VER: 1.1: VID/PID:             26ac:0032
[ INFO] [1626917832.736006506]: VER: 1.1: UID:                 4356500d20383058
[ WARN] [1626917832.736531306]: CMD: Unexpected command 520, result 0
[ INFO] [1626917837.984168559]: udp1: Remote address: 192.168.0.121:14550
[ WARN] [1626917837.999627759]: FCU: Preflight: GPS Speed Accuracy too low
[ INFO] [1626917838.736300702]: VER: 1.1: Capabilities         0x000000000000e4ef
[ INFO] [1626917838.736565246]: VER: 1.1: Flight software:     010b03ff (a6274bc5ed000000)
[ INFO] [1626917838.736837118]: VER: 1.1: Middleware software: 010b03ff (a6274bc5ed000000)
[ INFO] [1626917838.737020766]: VER: 1.1: OS software:         080200ff (ec20f2e6c5cc35b2)
[ INFO] [1626917838.737107582]: VER: 1.1: Board hardware:      00000032
[ INFO] [1626917838.737212190]: VER: 1.1: VID/PID:             26ac:0032
[ INFO] [1626917838.737322942]: VER: 1.1: UID:                 4356500d20383058
[ INFO] [1626917838.929431802]: WP: seems GCS requesting mission
[ INFO] [1626917838.929658650]: WP: scheduling pull after GCS is done
[ INFO] [1626917838.957424154]: GF: seems GCS requesting mission
[ INFO] [1626917838.957704026]: GF: scheduling pull after GCS is done
[ INFO] [1626917838.984743641]: RP: seems GCS requesting mission
[ INFO] [1626917838.984973657]: RP: scheduling pull after GCS is done
[ INFO] [1626917841.716082666]: HP: requesting home position
[ INFO] [1626917843.932448880]: WP: mission received
[ INFO] [1626917843.961392944]: GF: mission received
[ INFO] [1626917843.987520816]: RP: mission received
[ INFO] [1626917845.005917544]: PR: parameters list received
[ INFO] [1626917851.715933397]: HP: requesting home position
[ INFO] [1626917861.715935981]: HP: requesting home position
[ INFO] [1626917871.715917420]: HP: requesting home position

rostopic echo -n1 /diagnostics

header: 
  seq: 461
  stamp: 
    secs: 1626918167
    nsecs: 602185190
  frame_id: ''
status: 
  - 
    level: 0
    name: "mavros: FCU connection"
    message: "connected"
    hardware_id: "/dev/ttyPixhawk:921600"
    values: 
      - 
        key: "Received packets:"
        value: "2169"
      - 
        key: "Dropped packets:"
        value: "0"
      - 
        key: "Buffer overruns:"
        value: "0"
      - 
        key: "Parse errors:"
        value: "0"
      - 
        key: "Rx sequence number:"
        value: "98"
      - 
        key: "Tx sequence number:"
        value: "0"
      - 
        key: "Rx total bytes:"
        value: "7954038"
      - 
        key: "Tx total bytes:"
        value: "194865"
      - 
        key: "Rx speed:"
        value: "36628.000000"
      - 
        key: "Tx speed:"
        value: "875.000000"
  - 
    level: 0
    name: "mavros: GPS"
    message: "3D fix"
    hardware_id: "/dev/ttyPixhawk:921600"
    values: 
      - 
        key: "Satellites visible"
        value: "9"
      - 
        key: "Fix type"
        value: "3"
      - 
        key: "EPH (m)"
        value: "0.95"
      - 
        key: "EPV (m)"
        value: "2.03"
  - 
    level: 0
    name: "mavros: Heartbeat"
    message: "Normal"
    hardware_id: "/dev/ttyPixhawk:921600"
    values: 
      - 
        key: "Heartbeats since startup"
        value: "336"
      - 
        key: "Frequency (Hz)"
        value: "1.000000"
      - 
        key: "Vehicle type"
        value: "Quadrotor"
      - 
        key: "Autopilot type"
        value: "PX4 Autopilot"
      - 
        key: "Mode"
        value: "AUTO.LOITER"
      - 
        key: "System status"
        value: "Standby"
  - 
    level: 2
    name: "mavros: System"
    message: "Sensor health"
    hardware_id: "/dev/ttyPixhawk:921600"
    values: 
      - 
        key: "Sensor present"
        value: "0x122E002F"
      - 
        key: "Sensor enabled"
        value: "0x1221002F"
      - 
        key: "Sensor health"
        value: "0x022E002F"
      - 
        key: "3D gyro"
        value: "Ok"
      - 
        key: "3D accelerometer"
        value: "Ok"
      - 
        key: "3D magnetometer"
        value: "Ok"
      - 
        key: "absolute pressure"
        value: "Ok"
      - 
        key: "GPS"
        value: "Ok"
      - 
        key: "rc receiver"
        value: "Fail"
      - 
        key: "AHRS subsystem health"
        value: "Ok"
      - 
        key: "Battery"
        value: "Ok"
      - 
        key: "pre-arm check status. Always healthy when armed"
        value: "Fail"
      - 
        key: "CPU Load (%)"
        value: "41.4"
      - 
        key: "Drop rate (%)"
        value: "0.0"
      - 
        key: "Errors comm"
        value: "0"
      - 
        key: "Errors count #1"
        value: "0"
      - 
        key: "Errors count #2"
        value: "0"
      - 
        key: "Errors count #3"
        value: "0"
      - 
        key: "Errors count #4"
        value: "0"
  - 
    level: 0
    name: "mavros: Battery"
    message: "Normal"
    hardware_id: "/dev/ttyPixhawk:921600"
    values: 
      - 
        key: "Voltage"
        value: "16.22"
      - 
        key: "Current"
        value: "0.5"
      - 
        key: "Remaining"
        value: "83.0"
  - 
    level: 0
    name: "mavros: Time Sync"
    message: "Normal"
    hardware_id: "/dev/ttyPixhawk:921600"
    values: 
      - 
        key: "Timesyncs since startup"
        value: "3361"
      - 
        key: "Frequency (Hz)"
        value: "10.000005"
      - 
        key: "Last RTT (ms)"
        value: "1.375040"
      - 
        key: "Mean RTT (ms)"
        value: "2.221804"
      - 
        key: "Last remote time (s)"
        value: "384.336849000"
      - 
        key: "Estimated time offset (s)"
        value: "1626917783.223483801"
---

rosrun mavros checkid

OK. I got messages from 1:1.

---
Received 6190 messages, from 1 addresses
sys:comp   list of messages
  1:1     0, 1, 2, 4, 140, 141, 147, 24, 30, 31, 32, 33, 36, 74, 331, 76, 83, 340, 85, 230, 105, 111, 241, 242, 245

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