tf一些理解（根据资料）

首先看了开源操作机器人系统-ros这本书（张建伟）第五章slam导航 5.1使用tf配置机器人

还有ros navigation 教程

http://wiki.ros.org/navigation/Tutorials/RobotSetup/TF

这里需要区分两种变换

1、坐标系变换

2、坐标变换

坐标系变换就是指一个坐标系怎么变换成另外一个坐标系

坐标变换就是指一个坐标系下点怎么变换成另外一个坐标系下坐标

这两个略微有所不同

tf tree维护的是坐标系变换

base_link到base_laser坐标系变换就是[0.1,0,0.2]

而base_laser到base_link就是[-0.1,0,-0.2]

坐标变换则是转动坐标系在固定坐标系下运动描述

具体可以看这篇博客

http://blog.exbot.net/archives/1686

博客主要知识点：

这里有个巧合，当然也不是巧合，那就是从child到parent的坐标变换等于从parent到child的frame transform，等于child在parent frame的姿态描述。这里牵扯到了线性代数里的基变换、线性变换、过渡矩阵的概念。

接下来是实际操作部分

下面这段代码实现了从user_set topic读取数据，在call_back中进行tf 广播，在主程序里进行look_up

#include <ros/ros.h>

#include <tf/transform_broadcaster.h>

#include <nav_msgs/Odometry.h>

#include <geometry_msgs/Pose2D.h>

#include <tf/transform_listener.h>

#include <stdio.h>

class test

{

private:

    ros::NodeHandle nh_;

    ros::Subscriber sub_;

    tf::TransformBroadcaster br_;

    bool data_ready;

public:

    test(ros::NodeHandle& nh)

    {

        nh_=nh;

        sub_ = nh_.subscribe("/user_set", 10, &test::call_back,this);

        data_ready = false;

    }

    void call_back(const geometry_msgs::Pose2DPtr& msgs)

    {

        ROS_INFO("recive");

        tf::Transform dest_transform;

        dest_transform.setOrigin(tf::Vector3(msgs->x,msgs->y,0));

        tf::Quaternion q;

        q.setRPY(0, 0, msgs->theta);

        dest_transform.setRotation(q);

        br_.sendTransform(tf::StampedTransform(dest_transform, ros::Time::now(),"world","user_set_frame"));

        data_ready = true;

    }

    bool is_data_ready()

    {

        if(data_ready)

            return true;

        else

            return false;

    }

};

int main(int argc, char** argv){

  ros::init(argc, argv, "tf_broadcaster");

  ros::NodeHandle node;

  test Otest(node);

  tf::TransformListener listener;

  tf::StampedTransform transform;

  while(ros::ok())

  {

    if(!Otest.is_data_ready())

    {

        ros::spinOnce();

        continue;

    }

  ROS_INFO("lookup_transfoem;");

  try

  {

　　　 //查找的是world到user_set_frame“坐标变换”具体可以看见看教程

      //http://wiki.ros.org/tf/Tutorials/Writing%20a%20tf%20listener%20%28C%2B%2B%29

      //rosrun tf tf_echo求的是坐标系变换

      listener.waitForTransform("world","user_set_frame",ros::Time::now(), ros::Duration(1.0));

      listener.lookupTransform("world","user_set_frame",ros::Time(0),transform);

  }

  catch (tf::TransformException ex)

  {

    ROS_ERROR("%s",ex.what());

    break;

  }

  tf::Vector3 vectortf = transform.getOrigin();

  ROS_INFO("transform.x:%f,transform.y:%f,transform.z:%f",vectortf.x(),vectortf.y(),vectortf.z());

  ros::spinOnce();

  }

  return 0;

}

　

Here, the real work is done, we query the listener for a specific transformation. Let's take a look at the four arguments:

1. We want the transform from this frame ...
2. ... to this frame.

3. The time at which we want to transform. Providing ros::Time(0) will just get us the latest available transform.

4. The object in which we store the resulting transform.

知道tf变换后怎么进行坐标变换呢？

开源操作机器人系统-ros这本书（张建伟）第五章slam导航 5.1使用tf配置机器人