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/ Using Luos with ROS1 and ROS2

Luos comes with a package for the Robot Operating System.

ROS 2 is the default version but backward compatibility with ROS 1 is ensured via the official ROS 1 bridge. You can get an example of an application using Luos containers in ROS 2 with the bike sharing example.

In this tutorial, we will assume you're using ROS 2. If you want to communicate with a ROS 1 ecosystem, begin with this quickstart since ROS 2 needs to be installed, and then refer to the Retrocompatibility with ROS 1 page.

Basics: a few ROS-applied-to-Luos concepts

Here is a summary of core concepts in ROS:

  • ROS workspace: this is the directory in which you download all the ROS software that is built from sourcecode. You will likely setup your own projects here but it is also common to download dependencies here, such as luos_ros2. Here we assume your workspace is ~/ros2_ws
  • ROS node: this is the name given to a single program or script being executed
  • ROS launchfile: this is a file that launches multiple nodes at once. Several nodes communicate with each other exchanging information in the form of structured pieces of data called messages, being exchanged through named communication channels called topics. Just like the filesystem of your hardrive, topics are hierarchised. E.g. /something/into/something/else/data
  • ROS package: a package is a folder that contains resources such as nodes, launchfiles, messages, ... Here we have 2 packages named luos_msgs and luos_interface.

With Luos, data coming from/to Luos containers are messages sent on topics. Some messages representing popular types of data comply to existing message formats, e.g. sensor_msgs/Imu representing IMU data: this message is provided by the sensor_msgs package. Some other messages are Luos-specific and are provided by the luos_msgs package.

There is a particular node whose role is to connect the Luos ecosystem with the ROS ecosystem: the broker, provided by the luos_interface package. The broker is strongly associated to a Luos gate since it connects to one (and only one) gate, and ensures the communication seamlessly. A broker can be launched with the command ros2 launch luos_interface broker.launch.py, which then attempts to connect to a serial Luos gate.

With Luos, topics names all start with a prefix being the container's alias and end with suffixes:

  • .../read: this topic is read-only and a .../write topic exists
  • .../write: this topic is write-only and a .../read topic exists. It is often a boolean type that (de)activates some source of data publishing
  • no suffix: this topic is read-only and no other related topic exists

As an example, /Imu_mod/variables/pedometer/read allows to read-only the current pedometer variable from the IMU container named Imu_mod.

Install ROS 2 and Luos

First install ROS 2 Foxy for your OS with FastRTPS. Also install colcon as advised in the guidelines. If you are not familiar with ROS, you should go on with a couple of ROS 2 tutorials to get started.

Then clone luos_ros2 to your workspace and compile it with colcon:

cd ros2_ws/src/    # Or replace by your ROS 2 workspace name, maybe also dev_ws/src
git clone https://github.com/aubrune/luos_ros2.git
cd luos_ros2/luos_interface
pip3 install --no-warn-script-location .
cd ~/ros2_ws && colcon build --symlink-install
source ~/ros2_ws/install/setup.bash

Get started with Luos in ROS 2

Plug in your Luos gate and other containers, such as IMU, Color or State, and run the broker. It will return the detected containers: 

~$ ros2 launch luos_interface broker.launch.py
[INFO] [luos_broker]: Connecting to /dev/ttyUSB0...
[INFO] [luos_broker]: Found containers:
-------------------------------------------------
Type                Alias               ID   
-------------------------------------------------
Gate                gate                1    
Imu                 Imu_mod             2    
State               button_mod          3    
Color               rgb_led_mod         5

Note: If you have several Luos gates, you need several brokers. Specify a port and a unique name for each of them:

ros2 launch luos_interface broker.launch.py device:=/dev/ttyUSB1 name:=brokerUSB1

According to the containers you have plugged-in, the broker will automatically publish the relevant topics in the namespace of your containers' aliases. Here we have plugged a State container (alias button_mod), a Imu container (alias Imu_mod) and a Color container (alias rgb_led_mod) to the gate; thus the broker publishes the following topics:

~$ ros2 topic list
/Imu_mod/acceleration
/Imu_mod/compass
/Imu_mod/imu
/Imu_mod/variables/gravity_vector/read
/Imu_mod/variables/gravity_vector/write
/Imu_mod/variables/heading/read
/Imu_mod/variables/heading/write
/Imu_mod/variables/pedometer/read
/Imu_mod/variables/pedometer/write
/Imu_mod/variables/walk_time/read
/Imu_mod/variables/walk_time/write
/button_mod/events/changed
/button_mod/events/pressed
/button_mod/events/released
/button_mod/variables/state/read
/button_mod/variables/state/write
/parameter_events
/rgb_led_mod/variables/color/read
/rgb_led_mod/variables/color/write
/rgb_led_mod/variables/time/read
/rgb_led_mod/variables/time/write
/rosout

Most variables advertise both /read and /write topics to get data, write data, or (de)activate this data source. Other types of data might be aggregates of Luos variables (such as the imu) or Luos events.

In order to echo messages from the terminal, use a regular ROS subscriber. For instance, here is the current IMU data:

~$ ros2 topic echo /Imu_mod/imu
header:
  stamp:
    sec: 1581267954
    nanosec: 5449
  frame_id: Imu_mod
orientation:
  x: 0.97814
  y: 0.052695
  z: -0.042831
  w: 0.196548
orientation_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
angular_velocity:
  x: 0.0
  y: 0.0
  z: 0.0
angular_velocity_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
linear_acceleration:
  x: 0.177171
  y: 0.04968
  z: 0.176722
linear_acceleration_covariance: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
---

In order to publish messages to the Luos containers, use a regular ROS publisher. For instance, here is how to light up the Luos RGB container, in a pink color:

ros2 topic pub /rgb_led_mod/variables/color/write std_msgs/msg/ColorRGBA "{r: 64, g: 0, b: 64}" --once

Get started with my own ROS2 package using Luos in Python

These command lines will create a new package my_luos_ros2_package relying on luos_interface:

cd ~/ros2_ws/src
ros2 pkg create my_luos_ros2_package --build-type ament_python --dependencies luos_interface

You can then add your ROS Python scripts, by taking example on the bike sharing example page.

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