ROS中的TF（坐标变换）

tf

---

一个机器人通常有很多坐标系，tf能够跟踪所有定义的坐标系随时间的变化，以及他们之间的相对位置。

常用tf工具

使用view_frames或rqt_tf_tree工具查看发布的tf坐标系信息：

＃　creates a diagram of the frames being broadcast by tf over ROS.
$ rosrun tf view_frames
$ evince frames.pdf

＃　a runtime tool for visualizing the tree of frames being broadcast over ROS
$ rosrun rqt_tf_tree rqt_tf_tree 

查看目标坐标系相对于参考坐标系的变换，它等于从参考到世界坐标系/world的变换乘以从世界坐标系/world到目标的变换的乘积：

$ rosrun tf tf_echo [reference_frame] [target_frame]

写一个 tf broadcaster (C++)：src/turtle_tf_broadcaster.cpp

#include 
#include 
#include 

std::string turtle_name;



void poseCallback(const turtlesim::PoseConstPtr& msg){
  static tf::TransformBroadcaster br;
  tf::Transform transform;

  // 2D -> 3D
  transform.setOrigin( tf::Vector3(msg->x, msg->y, 0.0) );
  tf::Quaternion q;
  q.setRPY(0, 0, msg->theta);
  transform.setRotation(q);

  //                                   tf::Transform, 时间戳,      parent frame,  child frame
  br.sendTransform(tf::StampedTransform(transform, ros::Time::now(), "world", turtle_name));
}

int main(int argc, char** argv){
  ros::init(argc, argv, "my_tf_broadcaster");
  if (argc != 2){ROS_ERROR("need turtle name as argument"); return -1;};
  turtle_name = argv[1];

  ros::NodeHandle node;
  ros::Subscriber sub = node.subscribe(turtle_name+"/pose", 10, &poseCallback);

  ros::spin();
  return 0;
}

生成一个名字为my_tf_broadcaster的节点，订阅了turtle_name+"/pose"消息，在回调函数里发送tf坐标系信息。

在CMakeList.txt中添加：

add_executable(turtle_tf_broadcaster src/turtle_tf_broadcaster.cpp)
target_link_libraries(turtle_tf_broadcaster ${catkin_LIBRARIES})

在tf/src/下有tfMessage.msg：

写一个tf listener (C++)：src/turtle_tf_listener.cpp

#include 
#include 
#include 
#include 

int main(int argc, char** argv){
  ros::init(argc, argv, "turtle_tf_listener");

  ros::NodeHandle node;

  // call service "spawn" 
  ros::service::waitForService("spawn");
  ros::ServiceClient add_turtle = node.serviceClient("spawn");
  turtlesim::Spawn srv;
  add_turtle.call(srv);

  // publish topic "turtle2/cmd_vel"
  ros::Publisher turtle_vel = 
    node.advertise("turtle2/cmd_vel", 10);

  tf::TransformListener listener;

  ros::Rate rate(10.0);
  while (node.ok()){
    tf::StampedTransform transform;
    try{
　　　//                        目标坐标系　参考坐标系　　最新的tf　　　用来存放tf的对象
      listener.lookupTransform("/turtle2", "/turtle1", ros::Time(0), transform);
    }
    catch (tf::TransformException ex){
      ROS_ERROR("%s",ex.what());
      ros::Duration(1.0).sleep();
    }

　　// 转化成turtle2的角度和速度
    geometry_msgs::Twist vel_msg;
    vel_msg.angular.z = 4.0 * atan2(transform.getOrigin().y(),
                                    transform.getOrigin().x());
    vel_msg.linear.x = 0.5 * sqrt(pow(transform.getOrigin().x(), 2) +
                                  pow(transform.getOrigin().y(), 2));
    turtle_vel.publish(vel_msg);

    rate.sleep();
  }
  return 0;
}

编译节点

在CMakeList.txt中添加：

add_executable(turtle_tf_listener src/turtle_tf_listener.cpp)
target_link_libraries(turtle_tf_listener ${catkin_LIBRARIES})

注意这里的节点，turtle2是在listener节点里画的，所以没有显示出来。只显示了launch文件里的节点和/rosout。

参考

[1] http://wiki.ros.org/tf/Tutorials