Chapter 6: Robot Operating System (ROS)

Activity 6: Simulators and Sensor

Solution

1. We start by creating the packages and files:

   cd ~/catkin_ws/src
   catkin_create_pkg activity1 rospy sensor_msgs
   cd  activity1
   mkdir scripts
   cd scripts
   touch observer.py
   touch movement.py
   chmod +x observer.py
   chmod +x movement.py

2. This is the implementation of the image obtainer node:

   Note

   Add the aforementioned code to the observer.py file.

   #!/usr/bin/env python
   import rospy
   from sensor_msgs.msg import Image
   import cv2
   from cv_bridge import CvBridge
   
   class Observer:
       bridge = CvBridge()
       counter = 0
   
       def callback(self, data):
           if self.counter == 20:
               cv_image = self.bridge.imgmsg_to_cv2(data, "bgr8")
               cv2.imshow('Image',cv_image)
               cv2.waitKey(1000)
               cv2.destroyAllWindows()
               self.counter = 0
           else:
               self.counter += 1
   
       def observe(self):
           rospy.Subscriber('/camera/rgb/image_raw', Image, self.callback)
           rospy.init_node('observer', anonymous=True)
           rospy.spin()
   
   
   if __name__ == '__main__':
       obs = Observer()
       obs.observe()

   As you can see, this node is very similar to the one in Exercise 21, Publishers and Subscribers. The only differences are:

3. A counter is used for showing only one image of twenty received.

   We enter 1000 (ms) as the Key() parameter so that each image is shown for a second.

   This is the implementation of the movement node:

   #!/usr/bin/env python
   import rospy
   from geometry_msgs.msg import Twist
   
   def move():
       pub = rospy.Publisher('/mobile_base/commands/velocity', Twist, queue_size=1)
       rospy.init_node('movement', anonymous=True)
       move = Twist()
       move.angular.z = 0.5
       rate = rospy.Rate(10)
       while not rospy.is_shutdown():
           pub.publish(move)
           rate.sleep()
   
   if __name__ == '__main__':
       try:
           move()
       except rospy.ROSInterruptException:
           pass

4. To execute the file, we will execute the code mentioned here.

   Note

   Add this code to observer the .py file.

   cd ~/catkin_ws
   source devel/setup.bash
   roscore
   roslaunch turtlebot_gazebo turtlebot_world.launch
   rosrun activity1 observer.py
   rosrun activity1 movement.py  

5. Run both nodes and check the system functioning. You should see the robot turning on itself while images of what it sees are shown. This is a sequence of the execution:

   The output will look like this:

   

   Figure 6.10: The first sequence of the execution of activity nodes

   

   Figure 6.11: The second sequence of the execution of activity nodes

   

   Figure 6.12: The third sequence of the execution of activity nodes

   Note

   The output will look similar but not exactly look as the one mentioned in figures 6.10, 6.11, and 6.12.

Congratulations! You have completed the activity and at the end, you will have an output which is like figures 6.8, 6.9, and 6.10. By completing this activity successfully, you have been able to implement and work with nodes that let you subscribe to a camera which will show images in the virtual environment. You also learned to rotate a robot on itself that lets you view these images.