Getting-fast-into-ROS

In robot programming, implementing planning processes that is customized into the specific application is very essential. In this regard, ROS has a big advantage to control robot with user defined process plans. However, it is not easy to find a precise tutorial that helps to devive into ROS in short time.

Let us consider a turtle sim, 2D robot and dive fast into ROS. Assuming you have a complete installation of ROS packages,and turtle sim;

Content

Explained tutorial using Command terminal with Turtle simulation
Explained tutorial with Python script

Explained tutorial using Command terminal with Turtle simulation

This tutorial helps to get familiar with ROS environment using Turtle sim using command line.

Open terminal command and run;
```
$ rosrun turtlesim turtlesim_node
```

If you want check rostopic lists using

$ rostopic list

Output;

 /turtle1/cmd_vel
 /turtle1/color_sensor
 /turtle1/pose 

To control the turtle movement with your keyboard,
```
$ rosrun turtlesim turtle_teleop_key
```
To subscribe / display the message data in the rostopic, e.g.,cmd_vel ;

$ rostopic echo /turtle1/cmd_vel

To know the types of the topic
```
$ rostopic info /turtle1/cmd_vel
```
Output; ``` Type: geometry_msgs/Twist

Publishers: None

Subscribers:

/rostopic_7099_1643797240683 (http://xxx:36929/)
/turtlesim (http://xxx:38021/) ```

To show types of messages,

rosmsg show geometry_msgs/Twist

Output;

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

If you want to publish message to rostopic node, e.g., Twist, change the desired parameter value and run ;

$ rostopic pub turtle1/cmd_vel geometry_msgs/Twist "linear:
x: 2.0
y: 0.0
z: 0.0
angular:
x: 0.0
y: 0.0
z: 3.0" 

Explained tutorial with Python script

Subscriber function

Save the Subscriber function as “subscriber_tutorial.py” in your src folder in catkin_ws.

import sys
import rospy
from geometry_msgs.msg import Twist
import datetime

def callback_pose(data):
    """Get the ROS message"""   
    print(data)


def listener():
   """Subscribe to ROS topic node"""

    rospy.init_node('listener', anonymous=True)
    rospy.Subscriber("/turtle1/cmd_vel", Twist, callback_pose)
    rospy.spin()

if __name__ == '__main__':
    listener()

Publisher function

Save the publisher function as “publisher_tutorial.py” in your src folder in catkin_ws.

import rospy
from geometry_msgs.msg import Twist
import sys

def moveFunction_turtle(linear_velocity, angular_velocity):
    """Move turtle robot with input velocity"""
    rospy.init_node('moveFunction_turtle', anonymous=True)
    pub = rospy.Publisher("/turtle1/cmd_vel", Twist, queue_size=10)
    rate = rospy.Rate(10) # 10hz
    velocity = Twist()
    while not rospy.is_shutdown():
	velocity.linear.x = linear_velocity
	velocity.angular.z = angular_velocity

        rospy.loginfo("Linear velocity = %f: Angular velocity = %f: ", linear_velocity, angular_velocity)
        pub.publish(velocity)
        rate.sleep()


if __name__ == '__main__':
    try:
        moveFunction_turtle(float(sys.argv[1]), float(sys.argv[2]))
    except rospy.ROSInterruptException:
        pass

How to run the script

Open new terminal and change the directory to the publisher and subscriber function containing folder and run the functions separately as follows;

python publisher_tutorial.py 2.0 4.0

python subscriber_tutorial.py

Congratulations, now you are able to subscribe and publish to rostopic node!!