Cannot get turtle2 (iRobot) to follow coke can in TFROS Ex2.3

Course Support TF ROS 101
1

I am trying to get the turtle2 robot to follow the coke can in Exercise 2.3

My model1_to_model2_listener.py file looks like this. Can anyone see what I’m doing wrong?

#!/usr/bin/env python
import sys
import rospy
import math
import tf
import geometry_msgs.msg

if __name__ == '__main__':
    rospy.init_node('tf_listener_turtle')

    listener = tf.TransformListener()

    if len(sys.argv) < 3:
        print("usage: turtle_tf_listener.py turtle2 coke_can")
    else:
        turtle2 = sys.argv[1]
        coke_can = sys.argv[2]

        turtle_vel = rospy.Publisher(turtle2+'/cmd_vel', geometry_msgs.msg.Twist,queue_size=1)

        rate = rospy.Rate(10.0)
        ctrl_c = False

        turtle2 = "/"+turtle2
        coke_can = "/"+coke_can

        def shutdownhook():
            # works better than the rospy.is_shut_down()
            global ctrl_c
            print "shutdown time! Stop the robot"
            cmd = geometry_msgs.msg.Twist()
            cmd.linear.x = 0.0
            cmd.angular.z = 0.0
            #turtle_vel.publish(cmd)
            turtle2/cmd_vel.publish(cmd)
            ctrl_c = True

        rospy.on_shutdown(shutdownhook)

        while not ctrl_c:
            try:
                (trans,rot) = listener.lookupTransform(turtle2, coke_can, rospy.Time(0))
            except (tf.LookupException, tf.ConnectivityException, tf.ExtrapolationException):
                continue

            angular = 4 * math.atan2(trans[1], trans[0])
            linear = 0.5 * math.sqrt(trans[0] ** 2 + trans[1] ** 2)
            cmd = geometry_msgs.msg.Twist()
            cmd.linear.x = linear
            cmd.angular.z = angular
            #turtle_vel.publish(cmd)
            turtle2/cmd_vel.publish(cmd)

            rate.sleep()
2

I reccomend that you first check the cmd_vel topics if they are getting publshed.