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Basics

Coding Rules

The following rules apply when writing code with ROS.

Type Naming Rule Example
Package under_scored first_ros_package
Topic, Service under_scored raw_image
File under_scored turtlebot3_fake.cpp
Namespace under_scored ros_awesome_package
Variable under_scored string table_name;
Type camelCased typedef int32_t PropertiesNumber;
Class camelCased class UrlTable
Structure camelCased struct UrlTableProperties
Enumeration Type camelCased enum ChoiceNumber
Function camelCased addTableEntry()
Method camelCased void setNumEntries(int32_t_num_entries)
Constant ALL_CAPITALS const uint8_t DAYS_IN_A_WEEK = T;
Marco ALL_CAPITALS #define PI_ROUNDED 3.0
ROS Robot Programming by TurtleBot3 Developers, section 7.1.3
## Standard Unit in ROS
Quantity Unit
Length Meter
Mass Kilogram
Time Second
Current Ampere
Angle Radian
Frequency Hertz
Force Newton
Power Watt
Voltage Volt
Temperature Celsius
ROS Robot Programming by TurtleBot3 Developers, section 7.1.1

Master

ROS master is a Server tracking all network addresses of all nodes. In addition to network addresses it also tracks other information like parameters. All nodes must know the network address of the master on startup ROS_MASTER_URI.

A master can be started with the roscore command or a roslaunch will also start a master if it doesn't exists already.

roscore

ROS Master Publisher Slave
Figure 1: ROS Master Publisher Slave

Publisher and Subscribers

With help of the master, publisher and subscriber establish a peer-to-peer connection. All nodes must know the network address of the master on startup ROS_MASTER_URI.

ROS Publisher Slave
Figure 2: ROS Publisher Slave

  • Any node can publish a message to any topic
  • Any node can subscribe to any topic
  • Multiple nodes can publish to the same topic
  • Multiple nodes can subscribe to the same topic
  • A node can publish to multiple topics
  • A node can subscribe to multiple topics

Catkin Overview

src/ Folder

Location for create and clone new packages

The command catkin_make searches only in the src/ folder for packages and builds them

It is a good practice to clone the ros packages into a different folder e.g. ~/git/<package_name> and create a symlink into you catkin workspace

ls -s ~/git/<package_name>/ ~/catkin_ws/src/

build/ Folder

catkin_make create buixld files and intermediate cache CMake files inside the build/ folder.

devel/ Folder

catkin_make builds each package, if successful, the target executable le is created. Executables are stored inside the devel/ folder. Current workspace packages can be access by the command line if the following command is used:

# for bash
source ~/<workspace_name>/devel/setup.bash

# for zsh
source ~/<workspace_name>/devel/setup.zsh

It is beneficial to add this the the ~/.bashrc or ~/.zshrc file.

In addtion there is the catkin_tools program which simplifies the use.

See dedicated page: catkin_tools

install/ Folder

After building the executables in the devel/ folder, this executables can be install by:

catkin_make install

Messages

  • Serialization format for structured data
  • Defined in a .msg file
  • Compiled to C++/Python classes before using them