General Information

These packages contains example rosnodes and custom messages

Nodes

state_publisher_node

This is a very basic node that exists entirely inside a main function. It periodically sends out example_msgs/State messages

clock_face_node

This is a slightly more complex node that exists inside a main function and uses another class to offload some functionality. It's functionality is pretty useless, it publishes example_msgs/ClockFace messages that show the current EST time using the hands on an analog clock

clock_face_converter_node

This node has a very bare main function, and the functionality exists within a separate class. It converts the example_msgs/ClockFace messages to a visualization_msgs/Marker that can be viewed within Rviz

Launch Files

state_publisher.launch

Launches state_publisher_node

clock_face.launch

Launches clock_face_node

clock_face_converter.launch

Launches clock_face_converter_node

combined_clock.launch

Launches clock_face_node, clock_face_converter_node, and rviz

Parameter Files

clock_face_converter_params.yaml

Contains parameters for clock_face_converter_node

Rviz Files

clock.rviz

Set up to view output of clock_face_converter_node

Setup Instructions

1. Move into the directory where you want to have your workspace
   ex: cd ~/src

2. Create a workspace and source directory
   ex: mkdir -p example_ws/src
   _Note: the name of the workspace directory does not matter (*ws is typical), but you must have a folder named src inside. You can also add these packages into an existing workspace, but creating a new one may be cleaner.

3. Clone the necessary repos
   ex: git clone https://github.com/purdue-arc/ros-examples.git

4. Build the packages
   ex: cd .. && catkin build
   Note: catkin_make can also be used, but catkin build provides a more descriptive command line interface and also builds packages in parallel.

5. Source the packages
   ex: source devel/setup.bash
   Note: This must be done in every new terminal window that you want to use these packages in. It may be convenient to add it to your bashrc, which is a script called every time you open a new one. You can add it via the following command: echo 'source ~/src/example_ws/devel/setup.bash --extend' >> ~/.bashrc The --extend flag is used so multiple workspaces can be sourced simultaneously.

Operation Guide

Launching Nodes

You can launch nodes either via rosrun or roslaunch. Rosrun starts a single node from a specified package, while roslaunch can launch multiple nodes simultaneously. This can be incredibly useful for repeatedly launching a whole system of nodes, but you must have a launch file set up in order to use it. Launch files can also load parameters, remap topics, and perform basic if-else logic. Rosrun can do many of the same things via the command line interface. Roslaunch will also start a ROS master if one is not already running. Rosrun requires you to have one already running.

Rosrun

Start the ROS core if not already running: roscore
Run a node: rosrun <pkg name> <node name>
ex: rosrun example state_publisher_node

Roslaunch

Launch a launch file: roslaunch <pdk name> <launch file name>
ex: roslaunch example combined_clock.launch Note: This will start the ros core automatically if needed

Debugging

There are several command line and GUI tools to help in debugging a network of ROS nodes

Roswtf

This is a command line tool that can do some basic automated debugging. Sometimes its useful, other times it isn't.
ex: roswtf

Rostopic

This is a command line tool used to interface with ROS topics and messages.
ex: rostopic list ex: rostopic echo /<topic name>

Rosnode

This is a command line to interface with ROS nodes.
ex: rosnode list
ex: rosnode info <node name>

Rqt_graph

This is a GUI tool that displays the network of ROS nodes and topics. Be sure to check out the various visibility options in the top left.
ex: rosrun rqt_graph rqt_graph

Coding Challenges

To get better at working in ROS, there are a few coding challenges you can do with this package. Create a new branch for each one. Sample solutions are available in branches named challenge-*, where the text is the title of the challenge from the below list. They are roughly arranged in order of increasing difficulty.

1. Combined Launch File
   Create a launch file that combines the functionality of state_publisher.launch and combined_clock.launch in a single file, using include tags.

2. Timer Conversion
   Modify state_publisher_node to operate off a Timer instead of a Rate object. Parameterize the publish rate with a parameter you pick, and set this parameter in the launch file. Ensure it is robust to the parameter not being set. Measure the output rate using rostopic hz /state for when the parameter is set and for when it is defaulted to.

3. Cyclical State Publisher
   Modify the state publisher node to cycle through the various states defined within the State message. The rate at which it cycles should be parameterized via a parameter you pick and set in the launch file.

4. State Publisher Timeout
   Modify the state publisher node to slowly worsen its published state in response to a subscribed Empty message. For example, it receives the Empty message (on a topic you pick) and is GOOD for 1 second, BAD for the next second ... stopping at ABYSMAL. Make the time duration between transitions parameterized, and publish the Empty message via the command line rostopic interface.

5. Clock Face Numerals
   Create a new node that publishes Marker messages on topic(s) you pick. These messages must have the proper text and position in order to look like a clock face when viewed in Rviz. Modify the Rviz file to display these messages and modify the combined_clock.launch file to launch it. For extra difficulty, include a parameter to set if the numbers are roman numerals or not.

6. ClockFace Message Decoding
   The /face topic publishes ClockFace messages with the current time (in a mostly useless format). Create a new node that subscribes to this and publishes the time (calculated through the ClockFace message) in a useful format in a String message on a topic you pick. Do not modify combined_clock.launch, but instead launch this node directly from the command line with rosrun.