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Topic Remapping

Prerequisites

For this tutorial we will be using the joy package. Acquire a joystick and ensure the joy package is installed:

sudo apt install ros-kinetic-joy

Also complete the following tutorial about the use of the joy package: Configuring a Linux Joystick

Connecting Joy and Turtlesim

Joy outputs messages of type sensor_msgs/Joy. Turtlesim requires input messages of type geometry_msgs/Twist. (A good exercise would be to confirm this from the command terminal). In order for Joy to send messages to turtlesim we need to be able to convert messages of type sensor_msgs/Joy to geometry_msgs/Twist.

Let's look a look more in depth at the differences between the two message types. Try the command:

rosmsg show sensor_msgs/Joy

std_msgs/Header header
   uint32 seq
   time stamp
   string frame_id
 float32[] axes
 int32[] buttons
 ```

Now try:
``` bash
rosmsg show geometry_msgs/Twist

geometry_msgs/Vector3 linear
   float64 x
   float64 y
   float64 z
 geometry_msgs/Vector3 angular
   float64 x
   float64 y
   float64 z

As you can see the contents (as well as type) of these two messages are fundamentally different, hence we need a node which will convert the one to the other.

The node we will use to convert sensor_msgs/Joy messages to geometry_msgs/Twist messages is called teleop_twist_joy

Navigate to the src directory of your workspace: 

cd ~/[your_workspace]/src
Now clone the repository from git 
git clone https://github.com/ros-teleop/teleop_twist_joy.git

and build the package. 

cd ~/[your_workspace]
 catkin_make
 ```

Note: If you only wish to build teleop_twist_joy use `catkin_make --pkg teleop_twist_joy`

Now that our package is built let's see if we can command our turtle using a joystick.  In different terminals run:
``` bash
roscore


rosrun turtlesim turtlesim_node

rosrun joy joy_node

rosrun teleop_twist_joy teleop_node

If you get the error message: Error: Package 'teleop_twist_joy' not found, check your source path. To do this you're going to open the .rosrc file with vim. In the terminal run:

vrosrc

This is a short version of vim ~/.rosrc and opens vim so you can change the .rosrc file. Now look for the line of code under ROS_WORKSPACE that starts with source. change it to look like:

source ~/[your_workspace]/devel/setup.bash

To close Vim, hit escape a few times then type: :wq the colon let's it know you're typing a command and wq saves and quits the file.

Now run the following in the terminal:

rospack profile

This command goes through the directory and indexes it. (That means it will find the teleop_twist_joy package so you can use it.)

Now check to make sure that your work space is being source. In the terminal run:

printenv |grep ROS

printenv prints environment variables.

| is sometimes called "pipe." It "pipes" the first command into the second.

As you should know from the linux tutorial, the grep command searches a file and prints out what you tell it to search for.

The whole line basically says "search the printed environment variables for ROS."

Look for ROS_PACKAGE_PATH= and it should be the path you set e.g. catkin_ws/src:[some more stuff after]

Once all that is done, try running the rosrun teleop_twist_joy teleop_node command again.

Now try using the joystick to move the turtle around. You'll notice that it doesn't work. Let's investigate using rqt_graph

rqt_graph

You should see something similar to this:

rosgraph.png

As seen above, all three nodes are running, but /teleop_twist_joy_node is not communicating with /turtlesim. Let's look at what topic /teleop_twist_joy_node is publishing on and /turtlesim is subscribing to.

Let's start with teleop_twist_joy_node:

rosnode info teleop_twist_joy_node

You should see:

--------------------------------------------------------------------------------
 Node [/teleop_twist_joy_node]
 Publications:
  * /rosout [rosgraph_msgs/Log]
  * /cmd_vel [geometry_msgs/Twist]

 Subscriptions:
  * /joy [sensor_msgs/Joy]

 Services:
  * /teleop_twist_joy_node/get_loggers
  * /teleop_twist_joy_node/set_logger_level


 contacting node http://Hulk:39127/ ...
 Pid: 30767
 Connections:
  * topic: /rosout
     * to: /rosout
     * direction: outbound
     * transport: TCPROS
  * topic: /joy
     * to: /joy_node (http://Hulk:59300/)
     * direction: inbound
     * transport: TCPROS
     ```

It appears 'teleop_twist_joy_node' is publishing on the 'cmd_vel' topic. Let's see what turtlesim is subscribing to:

``` bash
rosnode info turtlesim

--------------------------------------------------------------------------------
 Node [/turtlesim]
 Publications:
  * /turtle1/color_sensor [turtlesim/Color]
  * /rosout [rosgraph_msgs/Log]
  * /turtle1/pose [turtlesim/Pose]

 Subscriptions:
  * /turtle1/cmd_vel [unknown type]

 Services:
  * /turtle1/teleport_absolute
  * /turtlesim/get_loggers
  * /turtlesim/set_logger_level
  * /reset
  * /spawn
  * /clear
  * /turtle1/set_pen
  * /turtle1/teleport_relative
  * /kill


 contacting node http://Hulk:59750/ ...
 Pid: 24728
 Connections:
  * topic: /rosout
     * to: /rosout
     * direction: outbound
     * transport: TCPROS

turtlesim is also subscribing to the cmd_vel topic, but within the turtle1 namespace. Let's close turtlesim and fix this by remapping the input of turtlesim to be 'cmd_vel' instead of 'turtle1/cmd_vel' 

rosrun turtlesim turtlesim_node turtle1/cmd_vel:=cmd_vel

Now refresh your rqt_graph. It should now look like:

rosgraph2.png

Now if you hold down the activation button (for Xbox 360 controllers it's the A button) and move the joystick you should be able to command your turtle!