Working with Twist Messages in Python

From the Part 1 publisher exercise, we know how to publish a String type message to a topic in Python, but how do we apply the same principles to a Twist message (on the /cmd_vel topic)? Let's have a look at this...

First, you need to import the rospy library, as well as the Twist message type from the geometry_msgs library:

import rospy
from geometry_msgs.msg import Twist

Then, create an instance of a rospy.Publisher() and assign it to an object called pub. When we create the object we tell the Publisher() method which topic we want to publish this message to (via the first input argument), and also that we will be publishing a message of the Twist type (the second input argument):

pub = rospy.Publisher({topic name}, Twist, queue_size=10) # a queue size of 10 usually works!

Then we need to create a Twist() message instance and assign it to an object (which we'll call vel_cmd):

vel_cmd = Twist()

We know from earlier that the geometry_msgs/Twist message has the format:

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

We also know, that only velocity commands issued to the following two parameters will actually have any effect on the velocity of our robot:

geometry_msgs/Vector3 linear
  float64 x

...and:

geometry_msgs/Vector3 angular
  float64 z

As such, we set appropriate velocity values to these attributes of the Twist() message (assigned to vel_cmd):

vel_cmd.linear.x = 0.0 # m/s
vel_cmd.angular.z = 0.0 # rad/s

We can then publish this to the relevant topic on the ROS network by supplying it to the rospy.Publisher().publish() method (which we instantiated as pub earlier):

pub.publish(vel_cmd)

Use these pointers when working on your move_circle.py node!

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