Fundamental Concepts

• ROS file system -nomenclature
  • ROS workspace -catkin workspace
    • src space, devel space, build space, install space
  • ROS packages
    • organize different functional modules
• ROS workspace is a folder to organize ROS project files
• catkin - a build tool that compiles source files to binaries
• catkin_workspace - ROS workspace where catkin is used as the build tool

• install dependencies of a ROS package(listed in package.xml)

cd <path_to_folder_with_ROS_packages(s)>
rosdep install <package_name>

• install dependencies of all packages in our source space

cd <path_to_ros_workspace/src>
rosdep install --from-paths . --ignore-src -y

• we dont need the source files of all dependencies of our ROS package

• Devel space

  • Binary executable files for tesing our development
  • files for setting up project specific ROS environment
  • source ROS environment for project in catkin workspace

source <path_to_workspace/devel/setup.bash

ROS launch

• Group Multiple Ros nodes in one file
  • launch file in xml format
• start up all nodes in the launch file
  • arguments, node specific paramters, namespaces
• also possible to include launch files from other packages in the same launch file

example

<launch>
  <node name="counter_with_delay" pkg="hrwros" type="counter_with_delay_as.py" output="screen">
    <param name="counter_delay" type="double" vale="$(arg counter_delay_parameter)"/>

ROS Actions

• No waiting until an execution is complete(non-blocking)
  • waiting is an option if required
• A generalized request-response system - the client-sever infrastructure in ROS
• Actions are defined by the three message types: Goal(request), Result(response) and Feedback
• Requirement: counter with a 1s delay between each count
  • Goal Message - Number of count up to (uint32)
  • Result message - state message(string)
  • feedback message - number of counts completed(unit32)

CounterWithDelay.action

unit32 num_counts <- Goal Field
---
string result_message <- result field
---
unit32 counts_elapsed <- feedback field

• Ros action definitions reside in the ROS package with project specific message definitions
  • /action folder
• generate action messages manually
  • rosrun actionlib_msgs genaction.py
• show the contents of an action message
  • rosmsg show _ msgs/

Processing a goal request

• goalCallbcak function processes a goal request
  • a goal can be pre-empted(and cancelled) before completion
• goal statuses: ACTIVE, SUCCEEDED, ABORTED(실패)

client = actionlb.SimpleActionClient(counter_with_delay,CounterWithDelayAction)

client.wait_for_server()

goal = CounterWithDelayGoal(num_counts=10)
client.send_goal(goal)

ROS Service

• Event-Based execution for ROS applications

float64 measurement_metres <- request field
---
float64 measurement_feet <- response field
Bool success