ROS进二阶学习笔记（1） TF 学习笔记3 -- TF Listener 编写 （Python）and Adding frame(Python)

ROS进二阶学习笔记（1） TF 学习笔记3 -- TF Listener 编写 （Python）

Ref: http://wiki.ros.org/tf/Tutorials/Writing%20a%20tf%20listener%20%28Python%29

In the previous tutorials we created a tf broadcaster to publish the pose of a turtle to tf. In this tutorial we'll create a tf listener to start using tf. and use tf to get access to frame transformations.
还是要请先读了Ref 再来这里sonictl@csdn看总结：

• 这个listen的程序做了3件事情：
  
• 1. 放个 turtle2乌龟到舞台上去，用的turtlesim/Spawn服务；
• 2. 用tf.TransformListener类下的lookupTransform方法，查询turtle2 相对于 turtle1 的位置。
• 3. 用查到的相对位置来发布cmd速度和角速度信息，引导 turtle2 运动

看看代码

我小改了下代码： (path: nodes/learning_tf_listener.py )

#!/usr/bin/env python  import roslibroslib.load_manifest('learning_tf')import rospyimport mathimport tfimport geometry_msgs.msgimport turtlesim.srvif __name__ == '__main__':    rospy.init_node('tf_turtle')    #next three lines: Create another turtle onto the stage    rospy.wait_for_service('spawn')    spawner = rospy.ServiceProxy('spawn', turtlesim.srv.Spawn)     #turtlesim has a service named: Spawn    spawner(4, 4, 0, 'turtle2')  #(x,y,theta,name)    #Publisher for the turtle2's movement    turtle_vel = rospy.Publisher('turtle2/cmd_vel', geometry_msgs.msg.Twist,queue_size=1)    listener = tf.TransformListener() #tf transforListener    rate = rospy.Rate(10.0)    while not rospy.is_shutdown():        #All this is wrapped in a try-except block to catch possible exceptions:        try:            (trans,rot) = listener.lookupTransform('/turtle2', '/turtle1', 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() #declare an Twist object        cmd.linear.x = linear        cmd.angular.z = angular        turtle_vel.publish(cmd)  # publish the cmd out        rate.sleep()

总结一下：

    tf要解决的问题是: 当 A相对B的tf - A_B , C相对B的tf - C_B都已经被发布到 tf 上时，如何查询 A_C or C_A 的tf的问题。

1.  通过/tf 的发布器类：python: tf.TransformBroadcaster 的方法： sendTransform(translation, rotation, time, child, parent) 发布tf
2.  用tf.TransformListener类下的lookupTransform方法，查询 C 相对于 A 的位置
3.  抛出一个问题： frame name是在哪里指定的？ 比如，我们加一个laser_scanner，它的frame name要怎么配置才能让lookupTransform方法看得懂？
4. rospy.Time(0) 这是什么含义? 跟rospy.Time.now() 有啥区别？
   

== 接下来做点测试 ：

Running the listener

With your text editor, open the launch file called start_demo.launch, and add the following lines:

  <launch>    ...    <node pkg="learning_tf" type="turtle_tf_listener.py"           name="listener" />  </launch>

First, make sure you stopped the launch file from the previous tutorial (use ctrl-c). Now you're ready to start your full turtle demo:

 $ roslaunch learning_tf start_demo.launch

You should see the turtlesim with two turtles.

Checking the results

To see if things work, simply drive around the first turtle using the arrow keys (make sure your terminal window is active, not your simulator window), and you'll see the second turtle following the first one!

When the turtlesim starts up you may see:

• [ERROR] 1253915565.300572000: Frame id /turtle2 does not exist! When trying to transform between /turtle1 and /turtle2.[ERROR] 1253915565.401172000: Frame id /turtle2 does not exist! When trying to transform between /turtle1 and /turtle2.

This happens because our listener is trying to compute the transform before messages about turtle 2 have been received because it takes a little time to spawn in turtlesim and start broadcasting a tf frame.

ROS TF 学习笔记 -- Adding a frame （Python）

Ref: Adding a frame (Python) -- This tutorial teaches you how to add an extra fixed frame to tf.

先看Ref, 再来总结：

• 理解add frame的意义和在哪里加frame
• a frame only has one single parent, but it can have multiple children
• 实质还是一个tf broadcaster。
  

看看代码：

Fire up your favorite editor and paste the following code into a new file callednodes/fixed_tf_broadcaster.py

#!/usr/bin/env python  import roslibroslib.load_manifest('learning_tf')import rospyimport tfif __name__ == '__main__':    rospy.init_node('my_tf_broadcaster')    br = tf.TransformBroadcaster()    rate = rospy.Rate(10.0)    while not rospy.is_shutdown():        br.sendTransform((0.0, 2.0, 0.0), #The carrot1 frame is 2 meters offset from the turtle1 frame                         (0.0, 0.0, 0.0, 1.0), #no rotation                         rospy.Time.now(),                         "carrot1",                         "turtle1")        rate.sleep()

后面有个Broadcasting a moving frame的例程，就不贴了，自己看Ref