1. ROS introduction

1.1. System structure

ROS is a software suite which allows for quick and easy building of autonomous robotic systems. ROS should be considered as a set of tools for creating new solutions or adjusting already existing ones. A major advantage of this system is a great set of drivers and implemented algorithms widely used in robotics.

Nodes

Base unit in ROS is called a node. Nodes are in charge of handling devices or computing algorithms- each node for separate task. Nodes can communicate with each other using topics or services. ROS software is distributed in packages. Single package is usually developed for performing one type of task and can contain one or multiple nodes.

Topics

In ROS, topic is a data stream used to exchange information between nodes. They are used to send frequent messages of one type. This could be a sensor readout or motor goal speed. Each topic is registered under the unique name and with defined message type. Nodes can connect with it to publish messages or subscribe to them. For a given topic, one node can not publish and subscribe to it at the same time, but there is no restrictions in the number of different nodes publishing or subscribing.

Services

Communication by services resemble client-server model. In this mode one node (the server) registers service in the system. Later any other node can ask that service and get response. In contrast to topics, services allow for two-way communication, as a request can also contain some data.

1.2. Basic tools

While working with ROS there are some tools that are most useful. They are intended to examine nodes and topics.

rosnode

Rosnode is a command line application for examining which nodes are registered in the system and also checking their statuses.

Using the application looks as follows:

     $ rosnode command [node_name]

Command could be:

• list - display list of running nodes

• info - display info regarding selected node

• kill - stop selected node

Detailed info could be found in ROS documentation.

rostopic

Rostopic is a command line application for examining which topics are already being published and subscribed, checking details of the selected topic or reading messages being sent in it.

Using the application looks as follows:

     $ rostopic command [topic_name]

Command could be:

• list - display list of topics

• info - display info regarding selected topic

• echo - display messages published in the topic

Detailed info could be found in ROS documentation.

rqt_graph

rqt_graph is graphical tool for visualization of data flow across different nodes in the system.

Using the application looks as follows:

     $ rqt_graph

1.3. Robot platform

This tutorial is created for ROSbot, open-source robot platform. You can read more about it here: Hackaday.io.

The platform contains:

• 1 × Husarion CORE2-ROS version with ASUS Tinker Board
• 1 × RPLIDAR A2 360°
• 1 x RGBD camera Orbbec Astra
• 1 × MPU9250
• 4 × SHARP GP2Y0A41SK0F

And this is how it looks like:

1.4. ROS and CORE2 Work flow

Before you start working with ROS on CORE2 platform, you need to connect your device first. You can establish connection in two ways: using ssh or via remote desktop. For both methods you need an IP address of your CORE2- it could be found on cloud.husarion.com after clicking “more” button next to robot name.

To connect by ssh type in terminal:

    $ ssh husarion@xxx.xxx.xxx.xxx

password is: husarion

To connect via remote desktop you need a remote desktop client application. Depending on your system, you may have various clients.

Parameters for connection are the following:

If you are working on Windows, press WinKey + r then type mstsc.

You will see a window appear:

Type in your device IP address and click connect. If you are working on Mac, you can use Microsoft Remote Desktop available at AppStore. If you are working on Ubuntu, you can use Remmina.

The first step in working with ROS is to run master process called roscore. This node handles registration of other nodes, topics and services. After this step is complete you can run your nodes.

To start the master process you can use command:

    $ roscore

Starting system step by step

You can start ROS by typing each name of each node manually.

To do it you can use the following command:

    $ rosrun package_name node_type [options]

Package_name and node_type are names of package and node that you want to run.

Defining node name

If you want to bind specific identifier to a node, at the end of the command, add:

    __name:=new_node_name

Note that there are two underscores before the name.

Setting parameter

You can also set parameter value by adding:

    _param:=value

Note that there is an underscore before the parameter name.

To find what kind of parameters you can set, and what type of data is accepted, check documentation for exact node.

Remapping topic name

If you want to change the name of the topic subscribed or published by a node, you can use remapping option. In order to do it, at the end of the command, add:

    old_topic_name:=new_topic_name

Note that there is no underscore before the old name.

Starting system step by step - Example

In this section, we will set up ROS system that is equipped with a Orbbec Astra camera and show image from camera on display. We are going to work only on PC, so you do not need to plug CORE2, Raspberry Pi or any other device beside Astra camera. If you are using laptop, integrated or USB camera will be OK.

Starting master

We will begin with master by typing in the following code in the ecommand line:

    $ roscore

You should see something like this:

Now you can use tools from chapter 2 in order to examine your system, don’t worry that you didn’t start any node yet.

Examining nodes

Let’s begin by checking the list of existing nodes. In the new terminal type in:

    $ rosnode list

As the output you should get:

    husarion@core2-ros:~$ rosnode list 
    /rosout

This means, that you have now one node running which name is /rosout and it is responsible for handling console log mechanism.

Next you can check some info about this node:

    $ rosnode info /rosout

And as the output you should get:

    husarion@core2-ros:~$ rosnode info /rosout 
    -------------------------------------
    Node [/rosout]
    Publications: 
     * /rosout_agg [rosgraph_msgs/Log]

    Subscriptions: 
     * /rosout [unknown type]

    Services: 
     * /rosout/set_logger_level
     * /rosout/get_loggers

    contacting node http://core2-ros:48067/ ...
    Pid: 4594

You can see here that node /rosout is publishing to topic /rosout_agg, subscribing topic /rosout and offering two services: /rosout/set_logger_level and /rosout/get_loggers.

Examining topics

Now we will check what topics are registered in the system and receivee some info about them. In new console type in:

$ rostopic list

You should get in the output:

husarion@core2-ros:~$ rostopic list 
/rosout
/rosout_agg

This means that you have two topics registered in the system. Let’s get some info about first of them:

$ rostopic info /rosout

As the output you should get:

husarion@core2-ros:~$ rostopic info /rosout
Type: rosgraph_msgs/Log

Publishers: None

Subscribers: 
 * /rosout (http://core2-ros:33119/)

From that you can read, that via topic /rosout only messages of type rosgraph_msgs/Log can be transmitted, there is no node that publishes to this topic and node /rosout subscribes it.

Now, leet's try to get similar info about the second topic:

$ rostopic info /rosout_agg

As the output you should get:

husarion@core2-ros:~$ rostopic info /rosout_agg 
Type: rosgraph_msgs/Log

Publishers: 
 * /rosout (http://core2-ros:33119/)

Subscribers: None

From that you can read, that via topic /rosout_agg only messages of type rosgraph_msgs/Log can be transmitted, node /rosout publishes to this topic and there is no node that subscribes it.

Starting camera node

Now you will run few nodes for handling Astra camera device. For this task you should use astra.launch from package astra_launch because this is the default camera installed in ROSbot. We will explain how launch file works later.

You can start camera nodes by typing in the terminal:

$ roslaunch astra_launch astra.launch

As output you should get something like below:

Task 1

Use rosnode and rostopic tools to check if new nodes or topic appeared in the system. Next find some info regarding new node and topics.

Starting image view node

Now you have camera node running, but can not see image from it yet. You will use node image_view from image_view package. This node by default subscribes to topic image. You need to remap this name to topic published by the camera node. If you performed task 1, you should know that astra nodes are publishing to many topics. To run image view node with remapping topic name type in the terminal:

    $ rosrun image_view image_view image:=/camera/rgb/image_raw

As the output you should get:

(DO PRZEROBIENIA)

Task 2

Use rosnode and rostopic tools to check what changed in the system after running another node. Notice what changed in properties of /camera/rgb/image_raw topic.

Examining system with rqt_graph

Now you will use rqt_graph tool in order to get graph of data flow in the system. In new terminal type in:

$ rqt_graph

There will be no response in the terminal, but new window will appear:

Interpretation of the graph is as follows:

• Ovals represent nodes

• Rectangles represent topics

• Arrows pointing from node to topic represent publication to this topic

• Arrows pointing from topic to node represent subscription to this topic

Starting system with roslaunch

Roslaunch is a tool that simplifies running of multiple nodes at the samee time. This tool uses .launch files which contain configuration of all nodes to be run. Usage of roslaunch is simple. In the new terminal type in:

roslaunch package file.launch

or

roslaunch file.launch

The first one is for the case when you use launch file provided with the package- you can run it from any folder. The second option is when you use standalone launch file- you must run it in the folder where the launch file is located or point the path to it.

Structure of .launch file

Structure of .launch file is defined in a markup language, similar to HTML. Every .launch file must begin with starting marker: <launch> and end with closing one: <\launch>. Markers defining nodes should be placed between them. You can define node by node marker- it’s structure is as follows:

    <node pkg="package_name" type="node" name="id" required="true" output="screen">
    </node>

Where fields pkg, type and name are required, rest are optional.

Meaning of each field:

• pkg - name of the epackage

• type - node to be run

• name - id which will be binded to the node

• required - if true, all nodes in the .launch file will be stopped if this node stops or fails, default value is false.

• output - if value is screen node output will be directed to screen, if value is log output will be directed to log file, default is log.

For each node parameters can be set or topics can be remapped.

For setting parameters use marker param:

    <param name="name" value="value"/>

Meaning of the fields:

• name - name of the parameter

• value - desired value of the parameter

For remapping the topic names use marker remap:

    <remap from="/old" to="/new"/>

Meaning of the fields:

• from - old topic's name

• to - desired topic's name

Starting system with roslaunch - Example

In this section we will start the same nodes as in previous example, but this time with use of roslaunch tool.

At first you will need a .launch file.

<launch>

     <include file="$(find astra_launch)/launch/astra.launch"/>

     <node pkg="image_view" type="image_view" name="image_view">
          <remap from="/image" to="/camera/rgb/image_raw"/>
     </node>

</launch>

Copy the above code to text editor (and other parameters if needed) and save it to file tutorial.launch in your home directory.

Next, close all consoles and nodes that are already running, go to new terminal and type in:

$ roslaunch tutorial.launch

You should get output like this:

(DO PRZEROBIENIA)

Notice that you do not need to run roscore before using roslaunch, if roscore is not running already, roslaunch will run it before starting nodes.

Task 3

Use rosnode, rostopic and rqt_graph tools to examine system started with use of roslaunch, there should be no difference in comparison to the esystem started step by step.

1.5. Summary

After completing this tutorial you should know what are the basic components and tools of ROS. You should know two methods for starting a node, setting parameters and remapping topic names. You should be able to check what nodes are running in system and to which topics they are publishing and subscribing.

by Łukasz Mitka, AGH Krakow

Do you need any support with completing this tutorial or have any difficulties with software or hardware? Feel free to describe your thoughts on our community forum: https://community.husarion.com/ or to contact with our support: support@husarion.com