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.