!

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Moving in a Straight Line

In this tutorial series, we will create python scripts to move our turtle, in order to practice the ROS basics.

You can find the complete package at: https://github.com/clebercoutof/turtlesim_cleaner

Preparing for work

First of all, we have to create a new package.

$ cd ~/catkin_ws/src
$ catkin_create_pkg turtlesim_cleaner geometry_msgs rospy

Now, build your workspace

#At your catkin workspace
$ cd ~/catkin_ws
$ catkin_make

And now, create a a src folder for your scripts

$ cd ~/catkin_ws/src/turtlesim_cleaner
$ mkdir src
$ cd ~/catkin_ws
$ catkin_make

Understanding the code

Our code will receive as inputs the desired speed, distance and a variable which defines if the movement is forwards or backwards. Since we can just publish a velocity to the topic /turtle1/cmd_vel, our logic will have to calculate the distance specified.

The code

Create your move.py (or any name you want) file and save it in your ~/catkin_ws/src/turtlesim_cleaner/src, our code will look like this:

   1 #!/usr/bin/env python
   2 import rospy
   3 from geometry_msgs.msg import Twist
   4 
   5 def move():
   6     # Starts a new node
   7     rospy.init_node('robot_cleaner', anonymous=True)
   8     velocity_publisher = rospy.Publisher('/turtle1/cmd_vel', Twist, queue_size=10)
   9     vel_msg = Twist()
  10 
  11     #Receiveing the user's input
  12     print("Let's move your robot")
  13     speed = input("Input your speed:")
  14     distance = input("Type your distance:")
  15     isForward = input("Foward?: ")#True or False
  16 
  17     #Checking if the movement is forward or backwards
  18     if(isForward):
  19         vel_msg.linear.x = abs(speed)
  20     else:
  21         vel_msg.linear.x = -abs(speed)
  22     #Since we are moving just in x-axis
  23     vel_msg.linear.y = 0
  24     vel_msg.linear.z = 0
  25     vel_msg.angular.x = 0
  26     vel_msg.angular.y = 0
  27     vel_msg.angular.z = 0
  28 
  29     while not rospy.is_shutdown():
  30 
  31         #Setting the current time for distance calculus
  32         t0 = rospy.Time.now().to_sec()
  33         current_distance = 0
  34 
  35         #Loop to move the turtle in an specified distance
  36         while(current_distance < distance):
  37             #Publish the velocity
  38             velocity_publisher.publish(vel_msg)
  39             #Takes actual time to velocity calculus
  40             t1=rospy.Time.now().to_sec()
  41             #Calculates distancePoseStamped
  42             current_distance= speed*(t1-t0)
  43         #After the loop, stops the robot
  44         vel_msg.linear.x = 0
  45         #Force the robot to stop
  46         velocity_publisher.publish(vel_msg)
  47 
  48 if __name__ == '__main__':
  49     try:
  50         #Testing our function
  51         move()
  52     except rospy.ROSInterruptException: pass

Don't forget to make your node executable:

$ chmod u+x ~/catkin_ws/src/turtlesim_cleaner/src/move.py

First we need to import the packages used on our script.The rospy library is the ros python library, it contains the basic functions, like creating a node, getting time and creating a publisher.The geometry_msgs contains the variable type Twist that will be used: Error: No code_block found

Now we declare our function, initiate our node, our publisher and create the Twist variable. Error: No code_block found

The Twist is necessary because our topic '/turtle1/cmd_vel' uses the Twist message, you can check with the following command:

$ rostopic info /turtle1/cmd_vel

You should see the following screen:

The Twist message is composed by 3 linear components and 3 angular components,you can see the message description with the following command:

$ rosmsg show geometry_msgs/Twist

You should see the following screen:

Since we are moving the turtle in a straight line, we just need the x component, and, depending on the user's input we decide if the movement is forwards or backwards. Error: No code_block found

The following statement guarentee that if we press crtl + c our code will stops Error: No code_block found

Now , with the rospy.Time.now().to_sec(). we get the starting time t0, and the time t1 to calculate the distance and while the actual distance is less than the user's input, it will keep publishing: Error: No code_block found

After we get to the specified distance , we order our robot to stop: Error: No code_block found

And then, we have our main loop which calls our function: Error: No code_block found

Now , you can test and move your robot!

Testing the code

In a new terminal, run:

$ roscore

In a new terminal, run:

$ rosrun turtlesim turtlesim_node

The turtlesim window will open:

Now, in a new terminal, run our code:

$ rosrun turtlesim_cleaner move.py

Just type your inputs and the turtle will move! Here we have an example:

Let's move your robot
Input your speed:1
Type your distance:3
Foward?: 0

The turtle will move like this:

Now you can go to the next tutorial! Learn how to rotate your turtle.