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Testing with DDS/micro-Ros

Architecture

Ardupilot contains the DDS Client library, which can run as SITL. Then, the DDS application runs a ROS 2 node, an eProsima Integration Service, and the MicroXRCE Agent. The two systems communicate over serial or UDP.

---
title: UDP Loopback
---
graph LR

  subgraph Linux Computer

    subgraph Ardupilot SITL
      veh[sim_vehicle.py] <--> xrceClient[EProsima Micro XRCE DDS Client]
      xrceClient <--> port1[udp:2019]
    end

    subgraph DDS Application
      ros[ROS 2 Node] <--> agent[Micro ROS Agent]
      agent <-->port1[udp:2019]
    end

    loopback

  end

---
title: Hardware Serial Port Loopback
---
graph LR

  subgraph Linux Computer

    subgraph Ardupilot SITL
      veh[sim_vehicle.py] <--> xrceClient[EProsima Micro XRCE DDS Client]
      xrceClient <--> port1[devUSB1]
    end

    subgraph DDS Application
      ros[ROS 2 Node] <--> agent[Micro ROS Agent]
      agent <--> port2[devUSB2]
    end

    port1 <--> port2

  end

Installing Build Dependencies

While DDS support in Ardupilot is mostly through git submodules, another tool needs to be available on your system: Micro XRCE DDS Gen.

Follow the wiki here to set up your environment.

Serial Only: Set up serial for SITL with DDS

On Linux, creating a virtual serial port will be necessary to use serial in SITL, because of that install socat.

sudo apt-get update
sudo apt-get install socat

Setup ardupilot for SITL with DDS

Set up your SITL. Run the simulator with the following command. If using UDP, the only parameter you need to set it DDS_ENABLE.

Name Description Default
DDS_ENABLE Set to 1 to enable DDS, or 0 to disable 1
SERIAL1_BAUD The serial baud rate for DDS 57
SERIAL1_PROTOCOL Set this to 45 to use DDS on the serial port 0 
# Wipe params till you see "AP: ArduPilot Ready"
# Select your favorite vehicle type
sim_vehicle.py -w -v ArduPlane --console -DG --enable-dds

# Only set this for Serial, which means 115200 baud
param set SERIAL1_BAUD 115
# See libraries/AP_SerialManager/AP_SerialManager.h AP_SerialManager SerialProtocol_DDS_XRCE
param set SERIAL1_PROTOCOL 45

DDS is currently enabled by default, if it's part of the build. To disable it, run the following and reboot the simulator.

param set DDS_ENABLE 0
REBOOT

Setup ROS 2 and micro-ROS

Follow the steps to use the microROS Agent

  • Install ROS Humble (as described here)

  • Install geographic_msgs

    sudo apt install ros-humble-geographic-msgs

  • Install and run the microROS agent (as described here). Make sure to use the humble branch.

    • Follow the instructions for the following:

      • Do "Installing ROS 2 and the micro-ROS build system"
        • Skip the docker run command, build it locally instead
      • Skip "Creating a new firmware workspace"
      • Skip "Building the firmware"
      • Do "Creating the micro-ROS agent"
      • Source your ROS workspace

Using the ROS 2 CLI to Read Ardupilot Data

After your setups are complete, do the following:

  • Source the ROS 2 installation 
    source /opt/ros/humble/setup.bash

Next, follow the associated section for your chosen transport, and finally you can use the ROS 2 CLI.

UDP (recommended for SITL)

  • Run the microROS agent 
    cd ardupilot/libraries/AP_DDS
ros2 run micro_ros_agent micro_ros_agent udp4 -p 2019
  • Run SITL (remember to kill any terminals running ardupilot SITL beforehand) 
    sim_vehicle.py -v ArduPlane -DG --console --enable-dds

Serial

  • Start a virtual serial port with socat. Take note of the two /dev/pts/* ports. If yours are different, substitute as needed. 
    socat -d -d pty,raw,echo=0 pty,raw,echo=0
>>> 2023/02/21 05:26:06 socat[334] N PTY is /dev/pts/1
>>> 2023/02/21 05:26:06 socat[334] N PTY is /dev/pts/2
>>> 2023/02/21 05:26:06 socat[334] N starting data transfer loop with FDs [5,5] and [7,7]
  • Run the microROS agent 
    cd ardupilot/libraries/AP_DDS
# assuming we are using tty/pts/2 for DDS Application
ros2 run micro_ros_agent micro_ros_agent serial -b 115200 -D /dev/pts/2
  • Run SITL (remember to kill any terminals running ardupilot SITL beforehand) 
    # assuming we are using /dev/pts/1 for Ardupilot SITL
sim_vehicle.py -v ArduPlane -DG --console --enable-dds -A "--serial1=uart:/dev/pts/1"

Use ROS 2 CLI

You should be able to see the agent here and view the data output.

$ ros2 node list
/ardupilot_dds

$ ros2 topic list -v
Published topics:
 * /ap/battery/battery0 [sensor_msgs/msg/BatteryState] 1 publisher
 * /ap/clock [rosgraph_msgs/msg/Clock] 1 publisher
 * /ap/geopose/filtered [geographic_msgs/msg/GeoPoseStamped] 1 publisher
 * /ap/gps_global_origin/filtered [geographic_msgs/msg/GeoPointStamped] 1 publisher
 * /ap/imu/experimental/data [sensor_msgs/msg/Imu] 1 publisher
 * /ap/navsat/navsat0 [sensor_msgs/msg/NavSatFix] 1 publisher
 * /ap/pose/filtered [geometry_msgs/msg/PoseStamped] 1 publisher
 * /ap/tf_static [tf2_msgs/msg/TFMessage] 1 publisher
 * /ap/time [builtin_interfaces/msg/Time] 1 publisher
 * /ap/twist/filtered [geometry_msgs/msg/TwistStamped] 1 publisher
 * /parameter_events [rcl_interfaces/msg/ParameterEvent] 1 publisher
 * /rosout [rcl_interfaces/msg/Log] 1 publisher

Subscribed topics:
 * /ap/cmd_gps_pose [ardupilot_msgs/msg/GlobalPosition] 1 subscriber
 * /ap/cmd_vel [geometry_msgs/msg/TwistStamped] 1 subscriber
 * /ap/joy [sensor_msgs/msg/Joy] 1 subscriber
 * /ap/tf [tf2_msgs/msg/TFMessage] 1 subscriber

$ ros2 topic hz /ap/time
average rate: 50.115
        min: 0.012s max: 0.024s std dev: 0.00328s window: 52

$ ros2 topic echo /ap/time
sec: 1678668735
nanosec: 729410000

$ ros2 service list
/ap/arm_motors
/ap/mode_switch
---

The static transforms for enabled sensors are also published, and can be received like so:

ros2 topic echo /ap/tf_static --qos-depth 1 --qos-history keep_last --qos-reliability reliable --qos-durability transient_local --once

In order to consume the transforms, it's highly recommended to create and run a transform broadcaster in ROS 2.

Using ROS 2 services

The AP_DDS library exposes services which are automatically mapped to ROS 2 services using appropriate naming conventions for topics and message and service types. An earlier version of AP_DDS required the use of the eProsima Integration Service to map the request / reply topics from DDS to ROS 2, but this is no longer required.

List the available services:

$ ros2 service list -t
/ap/arm_motors [ardupilot_msgs/srv/ArmMotors]
/ap/mode_switch [ardupilot_msgs/srv/ModeSwitch]

Call the arm motors service:

$ ros2 service call /ap/arm_motors ardupilot_msgs/srv/ArmMotors "{arm: True}"
requester: making request: ardupilot_msgs.srv.ArmMotors_Request(arm=True)

response:
ardupilot_msgs.srv.ArmMotors_Response(result=True)

Call the mode switch service:

$ ros2 service call /ap/mode_switch ardupilot_msgs/srv/ModeSwitch "{mode: 4}"
requester: making request: ardupilot_msgs.srv.ModeSwitch_Request(mode=4)

response:
ardupilot_msgs.srv.ModeSwitch_Response(status=True, curr_mode=4)

Contributing to AP_DDS library

Adding DDS messages to Ardupilot

Unlike the use of ROS 2 .msg files, since Ardupilot supports native DDS, the message files follow OMG IDL DDS v4.2. This package is intended to work with any .idl file complying with those extensions.

Over time, these restrictions will ideally go away.

To get a new IDL file from ROS 2, follow this process:

cd ardupilot
source /opt/ros/humble/setup.bash

# Find the IDL file
find /opt/ros/$ROS_DISTRO -type f -wholename \*builtin_interfaces/msg/Time.idl

# Create the directory in the source tree if it doesn't exist similar to the one found in the ros directory
mkdir -p libraries/AP_DDS/Idl/builtin_interfaces/msg/

# Copy the IDL
cp /opt/ros/humble/share/builtin_interfaces/msg/Time.idl libraries/AP_DDS/Idl/builtin_interfaces/msg/

# Build the code again with the `--enable-dds` flag as described above

If the message is custom for ardupilot, first create the ROS message in Tools/ros2/ardupilot_msgs/msg/GlobalPosition.msg. Then, build ardupilot_msgs with colcon. Finally, copy the IDL folder from the install directory into the source tree.

Rules for adding topics and services

Topics and services available from AP_DDS are automatically mapped into ROS 2 provided a few rules are followed when defining the entries in the topic and service tables.

ROS 2 message and service interface types

ROS 2 message and interface definitions are mangled by the rosidl_adapter when mapping from ROS 2 to DDS to avoid naming conflicts in the C/C++ libraries. The ROS 2 object namespace::Struct is mangled to namespace::dds_::Struct_ for DDS. The table below provides some example mappings:

ROS 2 DDS
rosgraph_msgs::msg::Clock rosgraph_msgs::msg::dds_::Clock_
sensor_msgs::msg::NavSatFix sensor_msgs::msg::dds_::NavSatFix_
ardupilot_msgs::srv::ArmMotors_Request ardupilot_msgs::srv::dds_::ArmMotors_Request_
ardupilot_msgs::srv::ArmMotors_Response ardupilot_msgs::srv::dds_::ArmMotors_Response_

Note that a service interface always requires a Request / Response pair.

ROS 2 topic and service names

The ROS 2 design article: Topic and Service name mapping to DDS describes the mapping of ROS 2 topic and service names to DDS. Each ROS 2 subsystem is provided a prefix when mapped to DDS. The request / response pair for services require an additional suffix.

ROS 2 subsystem DDS Prefix DDS Suffix
topics rt/
service request rq/ Request
service response rr/ Reply
service rs/
parameter rp/
action ra/

The table below provides example mappings for topics and services

ROS 2 DDS
ap/clock rt/ap/clock
ap/navsat/navsat0 rt/ap/navsat/navsat0
ap/arm_motors rq/ap/arm_motorsRequest, rr/ap/arm_motorsReply

Refer to existing mappings in AP_DDS_Topic_Table and AP_DDS_Service_Table for additional details.

Development Requirements

Astyle is used to format the C++ code in AP_DDS. This is required for CI to pass the build. See Tools/CodeStyle/ardupilot-astyle.sh.

./Tools/CodeStyle/ardupilot-astyle.sh libraries/AP_DDS/*.h libraries/AP_DDS/*.cpp

Pre-commit is used for other things like formatting python and XML code. This will run the tools automatically when you commit. If there are changes, just add them back your staging index and commit again.

  1. Install pre-commit python package.
  2. Install ArduPilot's hooks in the root of the repo, then commit like normal
cd ardupilot
pre-commit install
git commit

Testing DDS on Hardware

With Serial

The easiest way to test DDS is to make use of some boards providing two serial interfaces over USB such as the Pixhawk 6X. The Pixhawk6X/hwdef.dat file has this info.

SERIAL_ORDER OTG1 UART7 UART5 USART1 UART8 USART2 UART4 USART3 OTG2

For example, build, flash, and set up OTG2 for DDS

./waf configure --board Pixhawk6X --enable-dds
./waf plane --upload
mavproxy.py --console
param set DDS_ENABLE 1
# Check the hwdef file for which port is OTG2
param set SERIAL8_PROTOCOL 45
param set SERIAL8_BAUD 115
reboot

Then run the Micro ROS agent

cd /path/to/ros2_ws
source install/setup.bash
cd src/ardupilot/libraries/AP_DDS
ros2 run micro_ros_agent micro_ros_agent serial -b 115200 -D /dev/serial/by-id/usb-ArduPilot_Pixhawk6X_210028000151323131373139-if02

If connection fails, instead of running the Micro ROS agent, debug the stream

python3 -m serial.tools.miniterm /dev/serial/by-id/usb-ArduPilot_Pixhawk6X_210028000151323131373139-if02  115200 --echo --encoding hexlify

The same steps can be done for physical serial ports once the above works to isolate software and hardware issues.