ROS2 Topics¶
Prerequisites¶
Start turtlesim, teleop_turtle, and rqt.
Introducing ros2 topic¶
The ros2 topic tool allows you to get information about ROS2 topics.
You can use the help option to get the available sub-commands for ros2 topic
ros2 topic -h
......
Commands:
bw Display bandwidth used by topic
delay Display delay of topic from timestamp in header
echo Output messages from a topic
find Output a list of available topics of a given type
hz Print the average publishing rate to screen
info Print information about a topic
list Output a list of available topics
pub Publish a message to a topic
type Print a topic's type
......
Let's use some of these sub-commands to examine turtlesim.
ros2 topic list¶
ros2 topic list returns a list of available topics currently subscribed to and published.
Example:
ros2 topic list
/parameter_events
/rosout
/turtle1/cmd_vel
/turtle1/color_sensor
/turtle1/pose
Let's get into some topic list sub-command arguments. Run:
ros2 topic list -h
usage: ros2 topic list [-h] [--spin-time SPIN_TIME] [-s] [--no-daemon] [-t]
[-c] [--include-hidden-topics] [-v]
Output a list of available topics
options:
-h, --help show this help message and exit
--spin-time SPIN_TIME
Spin time in seconds to wait for discovery (only
applies when not using an already running daemon)
-s, --use-sim-time Enable ROS simulation time
--no-daemon Do not spawn nor use an already running daemon
-t, --show-types Additionally show the topic type
-c, --count-topics Only display the number of topics discovered
--include-hidden-topics
Consider hidden topics as well
-v, --verbose List full details about each topic
With the -t (show-types) option:
ros2 topic list -t
This gives the same list but with the topic type appended in brackets:
/parameter_events [rcl_interfaces/msg/ParameterEvent]
/rosout [rcl_interfaces/msg/Log]
/turtle1/cmd_vel [geometry_msgs/msg/Twist]
/turtle1/color_sensor [turtlesim/msg/Color]
/turtle1/pose [turtlesim/msg/Pose]
With the -v (verbose) option:
ros2 topic list -v
This displays a verbose list of topics with message type and publisher/subscriber information.
Published topics:
* /parameter_events [rcl_interfaces/msg/ParameterEvent] 3 publishers
* /rosout [rcl_interfaces/msg/Log] 3 publishers
* /turtle1/cmd_vel [geometry_msgs/msg/Twist] 1 publisher
* /turtle1/color_sensor [turtlesim/msg/Color] 1 publisher
* /turtle1/pose [turtlesim/msg/Pose] 1 publisher
Subscribed topics:
* /parameter_events [rcl_interfaces/msg/ParameterEvent] 3 subscribers
* /turtle1/cmd_vel [geometry_msgs/msg/Twist] 1 subscriber
Feel free to try some of the other options.
ros2 topic type¶
Communication on topics happens by sending ROS2 messages between nodes. For a publisher to talk to a subscriber, such as the publisher teleop_turtle and the subscriber turtlesim, the publisher and subscriber must send and receive the same type of the message. The type of the message on a topic can be determined by using ros2 topic type which returns the message type of any topic being published.
Usage:
ros2 topic type [topic_name]
Example:
ros2 topic type /turtle1/cmd_vel
geometry_msgs/msg/Twist
We can look at the details of a message using ros2 interface show:
ros2 interface show geometry_msgs/msg/Twist
# This expresses velocity in free space broken into its linear and angular parts.
Vector3 linear
float64 x
float64 y
float64 z
Vector3 angular
float64 x
float64 y
float64 z
You may also choose to look up certain type of message of its details. From this documentation "geometry_msgs/msg/Twist Message", you can learn that this message contains two 3D vectors of translational and rotational velocity.
ros2 topic echo¶
ros2 topic echo shows the data published on a topic.
Usage:
ros2 topic echo [topic_name]
Let's look at the command velocity data published by the teleop_turtle node, which is published on the /turtle1/cmd_vel topic.
ros2 topic echo /turtle1/cmd_vel
WARNING: topic [/turtle1/cmd_vel] does not appear to be published yet
Could not determine the type for the passed topic
teleop_turtle publish data by pressing our arrow keys. If the turtle isn't moving, you might need to select the teleop_turtle terminal.
You should now see the following when you press up and then left:
linear:
linear:
x: 2.0
y: 0.0
z: 0.0
angular:
x: 0.0
y: 0.0
z: 0.0
---
linear:
x: 0.0
y: 0.0
z: 0.0
angular:
x: 0.0
y: 0.0
z: -2.0
---
Those are the 3D vectors we saw in the message information. If you'll notice, when you turn, the angular.z changes, and when you move forward and backward, the linear.x changes.
So, what if we wanted to control the turtle with something other than the teleop_turtle node? All we would need to do is to publish geometry_msgs/msg/Twist on the /turtle1/cmd_vel topic. The turtle will turn if we publish messages to the angular.z, and it will move when we publish messages to the linear.x.
Now let's look at rqt. Press the refresh button in the upper-right corner and uncheck the "Debug" box.
The /_ros2cli_26821 node is created by the echo command. As you can see, it is now subscribed to the /turtle1/cmd_vel topic.
ros2 topic pub¶
ros2 topic pub publishes data on to a topic currently advertiseda message to a topic.
Usage:
ros2 topic pub [topic_name] [msg_type] '<args>'
The '
Example:
ros2 topic pub --once /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 4.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 2.0}}"
The optional argument "--once" means “publish one message then exit”. This whole command sends a single message to turtlesim telling it to move with a linear velocity of 4.0 and an angular velocity of 2.0.
The output in the terminal:
publisher: beginning loop
publishing #1: geometry_msgs.msg.Twist(linear=geometry_msgs.msg.Vector3(x=4.0, y=0.0, z=0.0), angular=geometry_msgs.msg.Vector3(x=0.0, y=0.0, z=2.0))
Another example is:
ros2 topic pub --rate 1 /turtle1/cmd_vel geometry_msgs/msg/Twist "{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}"
Instead of the --once option, we use the --rate 1 option here to keep publishing the command in a steady stream at 1 HZ. As you can see the turtle is running in a continuous circle.
This is a pretty complicated example, so let's look at each argument in detail. * This command will publish messages to a given topic:
ros2 topic pub
-
This option causes rostopic to publish messages at a steady stream of 1 Hz:
--rate 1 -
This is the name of the topic to publish to:
/turtle1/cmd_vel -
This is the message type to use when publishing the topic:
geometry_msgs/msg/Twist -
As noted before, a "geometry_msgs/msg/Twist" message has two vectors, and each of the two vectors has three floating point elements:
{linear: {x: 2.0, y: 0.0, z: 0.0}, angular: {x: 0.0, y: 0.0, z: 1.8}}
{x: 2.0, y: 0.0, z: 0.0} is the linear value with x=2.0, y=0.0, and z=0.0, and {x: 0.0, y: 0.0, z: 1.8} is the angular value with x=0.0, y=0.0, and z=1.8.
ros2 topic hz¶
ros2 topic hz reports the rate at which data is published.
Usage:
ros2 topic hz [topic_name]
Let's see how fast the turtlesim node is publishing on /turtle1/pose:
ros2 topic hz /turtle1/pose
average rate: 62.247
min: 0.010s max: 0.022s std dev: 0.00257s window: 64
average rate: 62.513
min: 0.010s max: 0.022s std dev: 0.00209s window: 127
average rate: 62.439
min: 0.010s max: 0.022s std dev: 0.00201s window: 190
Review¶
ros2 topic: a ros2 tool to interface with topicsros2 topic list: output a list of available topicsros2 topic type: print a topic's typeros2 topic echo: output messages from a topicros2 topic pub: publish a message to a topicros2 topic hz: print the average publishing rate to screen