运动规划学习笔记2——ROS包_tf2
1、坐标系
2、坐标变换
2.1、坐标变换
2.2、广播坐标变换
2.3、监听坐标转换
3、问题
/usr/bin/env: “python”: 没有那个文件或目录
TypeError: cannot use a string pattern on a bytes-like object
在这里分享了运动规划方面的一些知识，属于《路径规划算法学习笔记》系列的扩展系列，其中内容可能存在不完善和错误之处，如有读者发现，欢迎批评指正。

1、坐标系

1. 一个机器人拥有多个坐标系，这些坐标系在tf2中构成坐标树，tf2通过坐标树维护多个坐标系之间的坐标变换。
2. ROS坐标系为右手坐标系，前X、左Y、上Z。
3. 两坐标系之间关系用6自由度相对位姿表示，tf2中由tf2::Vector3和tf2::Quaternion表示。
4. 查看坐标树

   # 方法一：view_frames工具
   rosrun tf view_frames 
   # 方法二：rqt
   rosrun rqt_tf_tree rqt_tf_tree
   

    查看坐标转换关系

   # 变换关系
   rosrun tf tf_echo /world /turtle1
   

    查看坐标转换时间延迟

   # 时间延迟
   rosrun tf tf_monitor /world /turtle1
   

    2、坐标变换
   2.1、坐标变换
   坐标变换库：tf2_ros，http://docs.ros.org/en/jade/api/tf2_ros/html/c++/namespacetf2__ros.html。
   坐标转换消息类型：geometry_msgs/TransformStamped，包括父坐标系、子坐标系、转换关系

   std_msgs/Header header
     uint32 seq
     time stamp
     string frame_id
   string child_frame_id
   geometry_msgs/Transform transform
     geometry_msgs/Vector3 translation
       float64 x
       float64 y
       float64 z
     geometry_msgs/Quaternion rotation
       float64 x
       float64 y
       float64 z
       float64 w
   

    2.2、广播坐标变换

   广播坐标转换的两种情况

   广播静态坐标转换
   	广播器：tf2_ros::StaticTransformBroadcaster
   	广播方法：tf2_ros::StaticTransformBroadcaster::sendTransform(geometry_msgs/TransformStamped)
   广播动态坐标转换
   	广播器：tf2_ros::TransformBroadcaster
   	广播方法：tf2_ros::TransformBroadcaster::sendTransform(geometry_msgs/TransformStamped)
   广播方法：
   	1、发送一个TransformStamped消息
   	void sendTransform (const geometry_msgs::TransformStamped &transform)
   	2、发送一组TransformStamped消息
   	void sendTransform (const std::vector< geometry_msgs::TransformStamped > &transforms)
   

    广播静态坐标转换

   	static tf2_ros::StaticTransformBroadcaster static_br;
   	geometry_msgs::TransformStamped static_tfStamped;
   
   	static_tfStamped.header.stamp = ros::Time::now();
   	static_tfStamped.header.frame_id = "parent_frame";
   	static_tfStamped.child_frame_id = "child_frame";
   	static_tfStamped.transform.translation.x = atof(x);
   	static_tfStamped.transform.translation.y = atof(y);
   	static_tfStamped.transform.translation.z = atof(z);
   	
   	tf2::Quaternion quat;
   	quat.setRPY(atof(wx), atof(wy), atof(wz);
   	static_tfStamped.transform.rotation.x = quat.x();
   	static_tfStamped.transform.rotation.y = quat.y();
   	static_tfStamped.transform.rotation.z = quat.z();
   	static_tfStamped.transform.rotation.w = quat.w();
   	
   	static_br.sendTransform(static_tfStamped);
   

    广播动态坐标转换

   void callback(const xxx& msg)
   {
   	 static tf2_ros::TransformBroadcaster br;
   	 geometry_msgs::TransformStamped tfStamped;
   	 
   	 tfStamped.header.stamp = ros::Time::now();
   	 tfStamped.header.frame_id = "parent_frame";
   	 tfStamped.child_frame_id = "child_frame";
   	 tfStamped.transform.translation.x = msg->x;
   	 tfStamped.transform.translation.y = msg->y;
   	 tfStamped.transform.translation.z = msg->z;
   	 
   	 tf2::Quaternion q;
   	 q.setRPY(msg->wx, msg->wy, msg->wz);
   	 tfStamped.transform.rotation.x = q.x();
   	 tfStamped.transform.rotation.y = q.y();
   	 tfStamped.transform.rotation.z = q.z();
   	 tfStamped.transform.rotation.w = q.w();
   	
   	 br.sendTransform(tfStamped);
   }
   

    2.3、监听坐标转换

   监听坐标转换

   监听坐标转换
   	监听器：tf2_ros::TransformListener
   监听缓存设置
   	缓存器：tf2_ros::Buffer	默认10s缓存
   监听方法
   	geometry_msgs::TransformStamped 	lookupTransform (const std::string &target_frame, const std::string &source_frame, const ros::Time &time, const ros::Duration timeout) const
   

    

   	#include <tf2_ros/transform_listener.h>
   	 
   	tf2_ros::Buffer tfBuffer;
   	tf2_ros::TransformListener tfListener(tfBuffer);
   
   	try
   	{
   		transformStamped = tfBuffer.lookupTransform("turtle2", "turtle1",ros::Time(0));
   	}
   	catch (tf2::TransformException &ex) 
   	{
   		ROS_WARN("%s",ex.what());
   		ros::Duration(1.0).sleep();
   		continue;
   	}
   

    3、问题

   /usr/bin/env: “python”: 没有那个文件或目录

   背景

   ubuntu20.04
   roslaunch turtle_tf turtle_tf_demo.launch
   

   解决

   sudo update-alternatives --install /usr/bin/python python /usr/bin/python3 1000
   

    

   TypeError: cannot use a string pattern on a bytes-like object

   https://blog.csdn.net/xiaowang_tongxue/article/details/108298544

   1. 问题

   2. Listening to /tf for 5.0 seconds
      Done Listening
      b'dot - graphviz version 2.43.0 (0)\n'
      Traceback (most recent call last):
        File "/opt/ros/noetic/lib/tf/view_frames", line 119, in <module>
          generate(dot_graph)
        File "/opt/ros/noetic/lib/tf/view_frames", line 89, in generate
          m = r.search(vstr)
      TypeError: cannot use a string pattern on a bytes-like object
      

   3. 解决

   4. sudo gedit /opt/ros/noetic/lib/tf/view_frames
      

   5.  88       print(vstr)
                vstr=str(vstr)  //加入这个...
       89       m = r.search(vstr)