TF坐标

1.简介

TF是一个让用户随时间跟踪多个坐标系的功能包，它使用树形数据结构，根据时间缓冲并维护多个坐标系之间的坐标变换关系。

2.TF工具

• tf_monitor ：查看TF树中所有坐标系的发布状态
• tf_monitor <source_frame> <target_target> ：查看指定坐标系之间的发布状态
• tf_echo <source_frame> <target_target> ：查看指定坐标系之间的变换关系
• view_frames ：可视化工具，生成pdf文件 rosrun tf view_frames

3.实践

turtle_tf_broadcaster.cpp

#include <ros/ros.h>
#include <tf/transform_broadcaster.h>
#include <turtlesim/Pose.h>

std::string turtle_name;

void poseCallback(const turtlesim::PoseConstPtr& msg)
{
    // tf广播器
    static tf::TransformBroadcaster br;

    // 根据乌龟当前的位姿，设置相对于世界坐标系的坐标变换
    tf::Transform transform;
    transform.setOrigin( tf::Vector3(msg->x, msg->y, 0.0) );
    tf::Quaternion q;
    q.setRPY(0, 0, msg->theta);
    transform.setRotation(q);

    // 发布坐标变换
    br.sendTransform(tf::StampedTransform(transform, ros::Time::now(), "world", turtle_name));
}

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

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

    ros::spin();

    return 0;
};

turtle_tf_lestener.cpp

#include <ros/ros.h>
#include <tf/transform_listener.h>
#include <geometry_msgs/Twist.h>
#include <turtlesim/Spawn.h>

int main(int argc, char** argv)
{
    // 初始化节点
    ros::init(argc, argv, "my_tf_listener");

    ros::NodeHandle node;

    // 通过服务调用，产生第二只乌龟turtle2
    ros::service::waitForService("spawn");
    ros::ServiceClient add_turtle =
    node.serviceClient<turtlesim::Spawn>("spawn");
    turtlesim::Spawn srv;
    add_turtle.call(srv);

    // 定义turtle2的速度控制发布器
    ros::Publisher turtle_vel =
    node.advertise<geometry_msgs::Twist>("turtle2/cmd_vel", 10);

    // tf监听器
    tf::TransformListener listener;

    ros::Rate rate(10.0);
    while (node.ok())
    {
        tf::StampedTransform transform;
        try
        {
            // 查找turtle2与turtle1的坐标变换
            listener.waitForTransform("/turtle2", "/turtle1", ros::Time(0), ros::Duration(3.0));
            listener.lookupTransform("/turtle2", "/turtle1", ros::Time(0), transform);
            //这里求的transform是turtle2到turtle1的变换
        }
        catch (tf::TransformException &ex) 
        {
            ROS_ERROR("%s",ex.what());
            ros::Duration(1.0).sleep();
            continue;
        }

        // 根据turtle1和turtle2之间的坐标变换，计算turtle2需要运动的线速度和角速度
        // 并发布速度控制指令，使turtle2向turtle1移动
        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;
};

rosrun rviz rviz -d 'rospack find turtle_tf' /rviz/turtle_rviz.rviz