1 机器人坐标变换

博主在大三时期学习过机器人控制系统，其中对此部分知识有很细致的介绍，不过时间过去很久了，需要抽时间好好整理下。在此做一个标记，后续补充相关内容。

2 TF功能包

TF坐标变换：1、广播TF变换 2、监听TF变换
机器人坐标变换

3 小海龟跟随实验

$ sudo apt-get install ros-melodic-turtle-tf
$ roslaunch turtle_tf turtle_tf_demo.launch
$ rosrun turtlesim turtle_teleop_key
$ rosrun tf view_frames

TF位置关系（可以看出turtle1和turtle2都与world坐标系相联系）

其中，红色小龟自由移动，绿色小龟跟随移动，最终位置重合。

rosrun tf tf_echo turtle1 turtle2

使用此命令可以得到当前时刻，turtle1 和turtle2两个坐标系之间的坐标变换，Translation(平移)，Rotation(旋转)，分别通过四元数、弧度、角度进行描述。
$T_{turtle1-turtle2} = T_{turtle1-world}*T_{world-turtle2}$

在Rviz中表示

4 tf坐标系广播与监听的编程实现

4.1 创建一个功能包

$ cd ~/catkin_ws/src
$ catkin_create_pkg learning_tf roscpp rospy tf turtlesim

4.2 创建tf广播器代码

4.2.1 C++程序

cd ~/catkin_ws/src/learning_tf/src
touch turtle_tf_broadcaster.cpp

/**
 * 该例程产生tf数据，订阅turtle1和turtle2的位置信息，广播给tf树
 */

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

std::string turtle_name;

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

	// 初始化tf数据
	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);

	// 广播world与海龟坐标系之间的tf数据
	br.sendTransform(tf::StampedTransform(transform, ros::Time::now(), "world", turtle_name));
}

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

	// 输入参数作为海龟的名字
	if (argc != 2)
	{
		ROS_ERROR("need turtle name as argument"); 
		return -1;
	}

	turtle_name = argv[1];

	// 订阅海龟的位姿话题
	ros::NodeHandle node;
	ros::Subscriber sub = node.subscribe(turtle_name+"/pose", 10, &poseCallback);

    // 循环等待回调函数
	ros::spin();

	return 0;
};

代码解释：

初始化ROS节点
输入参数判断是哪一个海龟（使用重映射方法分别命名节点，避免冲突）
订阅海龟的Pose话题
在回调函数里创建一个TF广播器（TransformBroadcaster）
得到海龟的平移矩阵和旋转矩阵，即world与turtle_name的坐标变换关系
借助transform将2个节点的坐标变换关系（sendTransform）广播出去，插入到tf树中

4.2.2 Python程序

touch turtle_tf_broadcaster.py

#!/usr/bin/env python
# -*- coding: utf-8 -*-

#该例程产生tf数据，订阅turtle1和turtle2的位置信息，广播给tf树

import roslib
roslib.load_manifest('learning_tf')
import rospy

import tf
import turtlesim.msg

def handle_turtle_pose(msg, turtlename):
    br = tf.TransformBroadcaster()
    br.sendTransform((msg.x, msg.y, 0),
                     tf.transformations.quaternion_from_euler(0, 0, msg.theta),
                     rospy.Time.now(),
                     turtlename,
                     "world")

if __name__ == '__main__':
    rospy.init_node('turtle_tf_broadcaster')
    turtlename = rospy.get_param('~turtle')
    rospy.Subscriber('/%s/pose' % turtlename,
                     turtlesim.msg.Pose,
                     handle_turtle_pose,
                     turtlename)
    rospy.spin()

4.3 创建tf监听器代码

4.3.1 C++程序

cd ~/catkin_ws/src/learning_tf/src
touch turtle_tf_listener.cpp

/**
 * 该例程监听tf数据，并计算、发布turtle2的速度指令
 */

#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ROS节点
	ros::init(argc, argv, "my_tf_listener");

    // 创建节点句柄
	ros::NodeHandle node;

	// 请求产生turtle2
	ros::service::waitForService("/spawn");
	ros::ServiceClient add_turtle = node.serviceClient<turtlesim::Spawn>("/spawn");
	turtlesim::Spawn srv;
	add_turtle.call(srv);

	// 创建发布turtle2速度控制指令的发布者
	ros::Publisher turtle_vel = node.advertise<geometry_msgs::Twist>("/turtle2/cmd_vel", 10);

	// 创建tf的监听器
	tf::TransformListener listener;

	ros::Rate rate(10.0);
	while (node.ok())
	{
		// 获取turtle1与turtle2坐标系之间的tf数据
		tf::StampedTransform transform;
		try
		{
			listener.waitForTransform("/turtle2", "/turtle1", ros::Time(0), ros::Duration(3.0));
			listener.lookupTransform("/turtle2", "/turtle1", ros::Time(0), transform);
		}
		catch (tf::TransformException &ex) 
		{
			ROS_ERROR("%s",ex.what());
			ros::Duration(1.0).sleep();
			continue;
		}

		// 根据turtle1与turtle2坐标系之间的位置关系，发布turtle2的速度控制指令
		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;
};

代码解释：

初始化ROS节点
代码产生一只新的海龟turtle2
创建turtle2速度控制指令的发布者
创建tf监听器
等待变换waitForTransform
查询变换lookupTransform，查询turtle1和turtle2之间的之间的位姿变换关系
定义turtle2的速度（x方向的线速度和z轴方向的角速度）
通过发布者将turtle2速度发布出去

4.3.2 Python程序

touch turtle_tf_broadcaster.cpp

#!/usr/bin/env python
# -*- coding: utf-8 -*-

#该例程监听tf数据，并计算、发布turtle2的速度指令

import roslib
roslib.load_manifest('learning_tf')
import rospy
import math
import tf
import geometry_msgs.msg
import turtlesim.srv

if __name__ == '__main__':
    rospy.init_node('turtle_tf_listener')

    listener = tf.TransformListener()

    rospy.wait_for_service('spawn')
    spawner = rospy.ServiceProxy('spawn', turtlesim.srv.Spawn)
    spawner(4, 2, 0, 'turtle2')

    turtle_vel = rospy.Publisher('turtle2/cmd_vel', geometry_msgs.msg.Twist,queue_size=1)

    rate = rospy.Rate(10.0)
    while not rospy.is_shutdown():
        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()
        cmd.linear.x = linear
        cmd.angular.z = angular
        turtle_vel.publish(cmd)

        rate.sleep()

4.4 编译设置

在CMakeLists.txt 文件中设置链接库和可执行文件

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

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

在工作空间根目录下使用catkin_make指令编译。

4.5 程序运行

4.5.1 C++程序运行

roscore
rosrun turtlesim turtlesim_node
rosrun learning_tf turtle_tf_broadcaster __name:=turtle1_tf_broadcaster /turtle1
rosrun learning_tf turtle_tf_broadcaster __name:=turtle2_tf_broadcaster /turtle2
rosrun learning_tf turtle_tf_listener
rosrun turtlesim turtle_teleop_key

4.5.2 Python程序运行

roscore
rosrun turtlesim turtlesim_node
rosrun learning_tf turtle_tf_broadcaster.py __name:=turtle1_tf_broadcaster _turtle:=turtle1
rosrun learning_tf turtle_tf_broadcaster.py __name:=turtle2_tf_broadcaster _turtle:=turtle2
rosrun learning_tf turtle_tf_listener.py
rosrun turtlesim turtle_teleop_key