ROS_Python - RicoJia/notes GitHub Wiki

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======================================================================== 0. Compilation 1. when you first create a python script, you must do this: then catkin_make python catkin_install_python(PROGRAMS scripts/motion_controller.py scripts/test_pub.py DESTINATION ${CATKIN_PACKAGE_BIN_DESTINATION} )

3. get params

   val = rospy.get_param("~val")

   • rospy param: Don't know why, but rospy.get_param("~name", default) works, but not rospy.get_param("name", default)
   • works with rosrun pkg node _name:=val

4. time

   1. 比较时间 rospy.Time.now() - start_time > rospy.Duration(secs = n)
   2. rospy.Timer(rospy.Duration(1), callback)

5. Try except

   try:
       main()
   except rospy.ROSInterruptException:     # this can be thrown by rospy.sleep when ctrl_c. 
       pass

6. Include

   1. numpy: just import numpy in code, no need to include this in CMakelist, package.xml. Cuz this is a script, no need to be compiled
      • But Numpy is not included in rospy!!

7. Subscribers:

   class CmdVelExecutioner(object):
   def __init__(self):
       self.cmd_vel_sub_ = rospy.Subscriber("/cmd_vel", Twist, self.cmdVelCB)
   def cmdVelCB(self, twist_msg):

   1. Each subscriber will start a new thread, but rospy.spin() will simply puts the main thread to sleep until shutdown... So generally, you don't need this .
      • Therefore, rospy has no spinOnce, cuz each subscriber will handle its own cb.
   2. Working with custom message typefrom beginner_tutorials.msg import Person

8. shbang:

   • #!/usr/bin/env python3 for ROS Noetic. Shbang tells which interpreter to use.
   • shbang withenv is better than #!/usr/bin/python because it env will figure out which python to use.

9. ROS-python, naming convention: https://www.ros.org/reps/rep-0008.html#id19 ========================================================================

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1. when using transformlistener, you need to do: link, another link

   listener.waitForTransform('/base_link','/end_effector',rospy.Time(), rospy.Duration(1.0))
   (trans,rot) = listener.lookupTransform( '/base_link','/end_effector', rospy.Time(0))
   rospy.sleep(0.1)

   • Also, the tutorial is misleading: you need to transform from end effector to base link to see the trajectory of end effector in the base frame!!

2. publish transforms: Always make sure there is a child frame when you publish a transform! The child frame cannot be itself. link

   from nav_msgs.msg import Odometry
   from std_msgs.msg import Header
   
   odom_pub = rospy.Publisher('/my_odom', Odometry, queue_size=10);
   odom = Odometry()
   header = Header(); #empty header, stamps, 
   header.frame_id = '/odom';
   
   #try rossrv show Odometry to see what info needs to be filled in. 
   odom.pose.pose = 
   odom.twist.twist = 
   header.stamp = rospy.Time.now()
   odom_pub.publish(odom)

   注意： 你在做tf listerner 的时候， broadcaster 会有delay！

3. what goes into sensor_msgs/joint_state's velocity - the velocity right after we get to the specified position or right before? How does tf use the velocity in its broadcasted transform?
   A: It is just an information. Should be updated is the body velocity in body frame

4. if you have roll pitch yaw like roscpp's tf2::Quaternion::setEulerZYX(yaw, pitch, roll), use Bullet: getEulerFromQuaternion(quat)

5. quat

   tf2::Quaternion quat; 
   quat.setEulerZYX(yaw,pitch,roll)

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1. Rospy.spin() is different than roscpp ros::spin(), which has its own thread. its job is to keep the code from exiting the node until terminated. the thread does not interfere with other threads, such as subscriber callbacks

   • Side Question: Why do timer threads not execute? Because you need to wait a second for the timer callback to fire!

2. there is one main thread. But one subscriber callback has one thread!

3. rospy::spin() is just a sleep function

   try:
       while not rospy.core.is_shutdown() and not self.done:
           time.sleep(0.5)
   except KeyboardInterrupt:
       logdebug("keyboard interrupt, shutting down")

   • in rospy, since spin() is simply waiting for shutdown request:

     # method 1
     while not rospy.is_shutdown():
       #do stuff
     
     #method 2
     rospy.spin()

   • shutdown handler: messages are not guaranteed to be published. But you can do service calls / parameter server calls safely.

     def my_handler():
       #do stuff
     rospy.on_shutdown(my_handler)

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1. message generation: (source code to be checked)

   package_name/msg/Foo.msg → package_name.msg.Foo
   # 2 ways to generate the message
   #1
   import std_msgs.msg.String
   #2
   from std_msgs.msg import String
   msg = String()
   
   # 3 ways to populate
   #1
   msg.data = "lol"
   #2, In-Order Arguments, must provide all args
   msg = String("lol")
   msg = std_msgs.msg.ColorRGBA(255,255,255,128)
   #for this schema:
   float32 r
   float32 g
   float32 b
   float32 a
   
   # 3 keyword args, or kwargs, get default msgs afterwards
   msg = std_msgs.msg.ColorRGBA(b=255)
   

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1. rospy allows only one node, with a unique name. Code?
   • this is a design choice, multiple nodes don't make sense to be in the same process. However, if we call this funtion with the same args, then it's considered the same node.
   • there's a check for args

     global _init_node_args
     if _init_node_args != (name, argv, anonymous, log_level, disable_rostime, disable_signals):
         raise rospy.exceptions.ROSException("rospy.init_node() has already been called with different arguments: "+str(_init_node_args))

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1. you main loop is placed in the init function, and should be AFTER everything else!!
   • e.g, 这样这个sub永远不会被执行！！

     self.strategy2_main()
     self.map_sub = rospy.Subscriber("map",    OccupancyGrid,self.map_callback)

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1. Anaconda changes default path of Python, ROS may not run. Solution:

   echo export PATH=\"/home/$USER/anaconda3/bin:$PATH\" > ~/.bashrc