Week 1

Meet Oswin

• Oswin So
  • 2nd Year CS Major
  • Inside RoboJackets: IGVC Software Lead
  • Outside RoboJackets: There is no outside (Just kidding. I'm a pretty big anime fan.)
• Contacts
  • Slack: @oswinso
  • email: oswinso@gatech.edu

Ensure you have virtualization turned on in your BIOS

Guide on how to do this

1. Go to File -> Import Appliance

2. Select the igvc.ova file

3. Increase the amoutn of memory and CPUs

• Allocate depending on your computer's specs

4. Hit Import!

Boot your new VM

• Double click the entry

roscore

• Before we can run any nodes, need to run roscore:

    roscore

• Don't worry about the details yet
• Think of it as something that handles ROS functionality

Launching buzzsim

• Let's launch the simulator:

    rosrun igvc_buzzsim buzzsim

  

rosrun in detail:

rosrun {package name} {executable name}

• ROS package is an organizational unit under which we can put ROS nodes, libraries, scripts and more
• For example, rosrun igvc_buzzsim buzzsim
  • We are running the buzzsim executable which lives in the igvc_buzzsim package

ROS Topic & Messages

• Think of a ROS topic as an an address that messages are sent to
• The ROS message is the actual message sent on that topic

Example: Buzzsim

• Buzzsim node listens to messages on the /oswin/velocity topic to make the turtle move.

Making the turtle move in Buzzsim

• The teleop_twist_keyboard node publishes the velocity commands to the /oswin/velocity topic
• The buzzsim node subscribes to the /oswin/velocity topic and uses the received messages to move the turtle

rostopic list

• Let's try out rostopic list

    rostopic list
    # /oswin/velocity
    # /rosout
    # /rosout_agg
    # /tf

• Notice that the /oswin/velocity topic is shown

rostopic info

• We can get more information about the /oswin/velocity topic with rostopic info:

    rostopic info /oswin/velocity
    # Type: geometry_msgs/Twist
    #
    # Publishers:
    # * /teleop_twist_keyboard (http://robojackets:46001/)
    #
    # Subscribers: 
    #  * /buzzsimsim (http://robojackets:36899/)

• The Type of a topic is the type of message for that topic
• In this case, it is geometry_msgs/Twist

Message types

• Since geometry_msgs/Twist, which comes from the package geometry_msgs package from ROS, we can lookup the definition in the ROS docs (Google "geometry_msgs/Twist" and it should show up)

What about looking at what's getting published on a topic?

rostopic echo

• We can use rostopic echo to look at the messages on a topic:

rostopic echo /oswin/velocity
# linear:
#   x: 0.5
#   y: 0.0
#   z: 0.0
# angular:
#   x: 0.0
#   y: 0.0
#   z: 0.0
# ---

rostopic pub

• What if we wanted to publish to a topic from the command line?
  • We can use rostopic pub
• Close teleop_twist_keyboard
• Publish using rostopic pub

    rostopic pub /oswin/velocity geometry_msgs/Twist "linear:
      x: 1.0
      y: 0.0
      z: 0.0
    angular:
      x: 0.0
      y: 0.0
      z: 0.0"

  • Tab completion is really useful here. Type rostopic pub /oswin/velocity and then keep tabbing
  • The identation matters (for those of you who know YAML, this is YAML)
• The turtle in the simulation should move
• Try playing around with the x component of linear and the z component of angular

.msg files

ROS Messages are defined using .msg files. For example, the twist.msg looks like:

# This expresses velocity in free space broken into its linear and angular parts.
Vector3  linear
Vector3  angular

Each line represents one field in the message, with the type on the left and the name on the right. In this case, the twist.msg message has two fields: "linear" of type Vector3, and "angular" of type Vector3.

The Vector3 type that this message refers to is another type that's defined somewhere else:

# This represents a vector in free space. 
# It is only meant to represent a direction. Therefore, it does not
# make sense to apply a translation to it (e.g., when applying a 
# generic rigid transformation to a Vector3, tf2 will only apply the
# rotation). If you want your data to be translatable too, use the
# geometry_msgs/Point message instead.

float64 x
float64 y
float64 z

In general, messages are described by other messages within them.

We don't have a ROS message type for motor commands yet though, and so we need to make our own custom ROS message.

Some requirements for our message:

1. Our robot has two motors, one for each wheel, so we need to be able to send each motor a separate command
2. We want to know when the message was sent, so we can tell if the message is old or not.
3. The command sent to the motor should be a float

Write your ROS message in the file igvc_training_msgs/msg/motor_command.msg.

Tips:

• Check out the std_msgs. It should contain everything you need
• Remember to catkin_make to compile the message!
• Try doing rostopic pub and rostopic echo to verify that your message works.

Summary

The presentation slides are available on github, under the ros-training repository on the RoboJackets github.

There's also a link to the markdown version of the same content in the README of the repository.