Getting Info About Nodes¶

Setup¶

roscore¶

Remember that we need to have roscore running so nodes can communicate to each other, in a new terminal:

roscore

If you left roscore running from the last tutorial, you may get the error message:

 roscore cannot run as another roscore/master is already running.
 Please kill other roscore/master processes before relaunching

This is fine. Only one roscore needs to be running.

turtlesim¶

For this tutorial we will also use turtlesim. Please run in a new terminal: rosrun turtlesim turtlesim_node

turtle keyboard teleoperation¶

We'll also need something to drive the turtle around with. Lets see what other nodes are in the the turtlesim package. Type

 rosrun turtlesim

and then press TAB twice instead of pressing enter. The following should appear

 leo ~ $ rosrun turtlesim
 draw_square        mimic              turtlesim_node     turtle_teleop_key

These are the four "nodes" in the turtlesim package. Each node is an individual executable, or program, but all are organized into a single package. Let's use the turtle_teleop_key package. in a '''new terminal''', type:

 rosrun turtlesim turtle_teleop_key

The following should appear

 [ INFO] 1254264546.878445000: Started node [/teleop_turtle], pid [5528], bound on [aqy], xmlrpc port [43918], tcpros port [55936], logging to [~/ros/ros/log/teleop_turtle_5528.log], using [real] time
 Reading from keyboard
 ---------------------------
 Use arrow keys to move the turtle.

Now you can use the arrow keys of the keyboard to drive the turtle around. If you can not drive the turtle '''select the terminal window of the turtle_teleop_key''' to make sure that the keys that you type are recorded.

turtle_key

Now that you can drive your turtle around, let's look at some tools we have to find out what's going on behind the scenes.

ROS Topics¶

In order to communicate nodes publish and/or subscribe to ROS Topics. The turtlesim_node and the turtle_teleop_key node are communicating with each other over the ROS topic command_velocity. The turtle_teleop_key publishes key strokes to the ROS topic command_velocity. The turtlesim_node subscribes to that same topic to receive the key strokes. Using rqt_graph we can see this relationship visually.

rqt_graph¶

rqt_graph is part of the rqt package. Unless you already have it installed, run:

 sudo apt install ros-kinetic-rqt
 sudo apt install ros-kinetic-rqt-common-plugins

In a new terminal:

 rosrun rqt_graph rqt_graph

or simply:

 rqt_graph

You will see something similar to:

rqt_graph_turtle_key

If you place your mouse over /turtle1/command_velocity it will highlight the ROS nodes (here blue and green) and topics (here red). As stated before, the turtlesim_node and the turtle_teleop_key nodes are communicating on the topic named /turtle1/command_velocity.

rqt_graph_turtle_key2

rosnode¶

rosnode is a useful tool for finding information about nodes.

rosnode list¶

rosnode list returns a list of all currently running nodes. This will become a more and more useful tool as you run more and more nodes.

In a new terminal run:

 rosnode list

Assuming you still have the two turtlesim nodes running from the previous tutorial you should see something similar to:

 /rosout
 /turtlesim
 /teleop_turtle

rosnode info¶

rosnode info returns information about a specified node. Run:

 rosnode info turtlesim

You should see something similar to:

 --------------------------------------------------------------------------------
 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:53422/ ...
 Pid: 26992
 Connections:
  * topic: /rosout
     * to: /rosout
     * direction: outbound
     * transport: TCPROS

Note that rosnode info allows us to see the publications, subscriptions, and services associated with a certain node.

rosnode ping¶

rosnode ping can be used to test if a node is up. Let's use it now:

 rosnode ping turtlesim

If it works it should return something similar to:

 rosnode: node is [/turtlesim]
 pinging /my_turtle with a timeout of 3.0s
 xmlrpc reply from http://Hulk:53422/     time=1.152992ms
 xmlrpc reply from http://Hulk:53422/     time=1.120090ms
 xmlrpc reply from http://Hulk:53422/     time=1.700878ms
 xmlrpc reply from http://Hulk:53422/     time=1.127958ms

Review¶

What was covered:

ROS Topics = named buses over which nodes exchange messages.
rqt_graph = useful tool for visualizing interactions among nodes
rosnode = ros+node : ROS tool to get information about a node.