在机器人的控制中,坐标系统是非常重要的,在ROS使用tf软件库进行坐标转换。 相关链接:http://www./wiki/tf/Tutorials#Learning_tf 一、tf简介 我们通过一个小小的实例来介绍tf的作用。
1、安装turtle包$ rosdep install turtle_tf rviz $ rosmake turtle_tf rviz
该例程中带有turtlesim仿真,可以在终端激活的情况下进行键盘控制。
可以发现,第二只乌龟会跟随你移动的乌龟进行移动。 3、demo分析 接下来我们就来看一看到底ROS做了什么事情。
这个例程使用tf建立了三个参考系:a world frame, a turtle1 frame, and a turtle2 frame。然后使用tf broadcaster发布乌龟的参考系,并且使用tf listener计算乌龟参考系之间的差异,使得第二只乌龟跟随第一只乌龟。 我们可以使用tf工具来具体研究。 $ rosrun tf view_frames
然后会看到一些提示,并且生成了一个frames.pdf文件。 该文件描述了参考系之间的联系。三个节点分别是三个参考系,而/world是其他两个乌龟参考系的父参考系。还包含一些调试需要的发送频率、最近时间等信息。
tf还提供了一个tf_echo工具来查看两个广播参考系之间的关系。我们可以看一下第二只得乌龟坐标是怎么根据第一只乌龟得出来的。 [plain] view plain copy
控制一只乌龟,在终端中会看到第二只乌龟的坐标转换关系。
我们也可以通过rviz的图形界面更加形象的看到这三者之间的关系。 $ rosrun rviz rviz -d `rospack find turtle_tf`/rviz/tule_rviz.rviz
移动乌龟,可以看到在rviz中的坐标会跟随变化。其中左下角的是/world,其他两个是乌龟的参考系。
下面我们就来详细分析这个实例。 二、Writing a tf broadcaster1、创建包
cd catkin_ws/src/ catkin_create_pkg learning_tf tf roscpp rospy turtlesim
建立你的新包roscd之前: $ cd ~/catkin_ws $ catkin_make $ source ./devel/setup.bash
我们首先创建源文件。 我们刚刚创建的包: $ roscd learning_tf
去 src / 文件夹并打开你最喜欢的编辑器,将下面的代码粘贴到一个新文件 src / 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){ static tf::TransformBroadcaster br; 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); 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; }; 现在我们创建代码,允许编译它。 打开 CMakeLists.txt 底部文件,并添加以下行: add_executable(turtle_tf_broadcaster src/turtle_tf_broadcaster.cpp) target_link_libraries(turtle_tf_broadcaster ${catkin_LIBRARIES})
Build your package; at the top folder of your catkin workspace: $ catkin_make
创建launch文件start_demo.launch: <launch> <!-- Turtlesim Node--> <node pkg="turtlesim" type="turtlesim_node" name="sim"/> <node pkg="turtlesim" type="turtle_teleop_key" name="teleop" output="screen"/> <!-- Axes --> <param name="scale_linear" value="2" type="double"/> <param name="scale_angular" value="2" type="double"/> <node pkg="learning_tf" type="turtle_tf_broadcaster" args="/turtle1" name="turtle1_tf_broadcaster" /> <node pkg="learning_tf" type="turtle_tf_broadcaster" args="/turtle2" name="turtle2_tf_broadcaster" /> </launch>
$ roslaunch learning_tf start_demo.launch
可以看到界面中只有移植乌龟了,打开tf_echo的信息窗口: $ rosrun tf tf_echo /world /turtle1
world参考系的原点在最下角,对于turtle1的转换关系,其实就是turtle1在world参考系中所在的坐标位置以及旋转角度。
三、Writing a tf listener这一步,我们将看到如何使用tf进行参考系转换。首先写一个tf listener(turtle_tf_listener.cpp) . The turtlesim/Velocity.h header is not used anymore(再也不), it has been replaced by geometry_msgs/Twist.h. Furthermore(此外), the topic /turtle/command_velocity is now called /turtle/cmd_vel. In light of this, a few changes are necessary to make it work:
#include <ros/ros.h> #include <tf/transform_listener.h> #include <geometry_msgs/Twist.h> #include <turtlesim/Spawn.h> int main(int argc, char** argv){ ros::init(argc, argv, "my_tf_listener"); ros::NodeHandle node; ros::service::waitForService("spawn"); ros::ServiceClient add_turtle = node.serviceClient<turtlesim::Spawn>("spawn"); turtlesim::Spawn srv; add_turtle.call(srv); ros::Publisher turtle_vel = node.advertise<geometry_msgs::Twist>("turtle2/cmd_vel", 10); tf::TransformListener listener; ros::Rate rate(10.0); while (node.ok()){ tf::StampedTransform transform; try{ listener.lookupTransform("/turtle2", "/turtle1", ros::Time(0), transform); } catch (tf::TransformException &ex) { ROS_ERROR("%s",ex.what()); ros::Duration(1.0).sleep(); continue; } 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; };
修改 CMakeLists.txt gedit CMakeLists.txt
add_executable(turtle_tf_listener src/turtle_tf_listener.cpp) target_link_libraries(turtle_tf_listener ${catkin_LIBRARIES})
Build your package; at the top folder of your catkin workspace: $ catkin_make
修改 launch文件start_demo.launch添加:
<launch> ... <node pkg="learning_tf" type="turtle_tf_listener" name="listener" /> </launch>
然后在运行:
$ roslaunch learning_tf start_demo.launch
就可以看到两只turtle了,也就是我们在最开始见到的那种跟随效果。
四、Adding a frame
在很多应用中,添加一个参考系是很有必要的,比如在一个world参考系下,有很一个激光扫描节点,tf可以帮助我们将激光扫描的信息坐标装换成全局坐标。
1、tf消息结构
tf中的信息是一个树状的结构,world参考系是最顶端的父参考系,其他的参考系都需要向下延伸。如果我们在上文的基础上添加一个参考系,就需要让这个新的参考系成为已有三个参考系中的一个的子参考系。
2、建立固定参考系(fixed frame)
我们以turtle1作为父参考系,建立一个新的参考系“carrot1”。
frame_tf_broadcaster.cpp.
#include <ros/ros.h> #include <tf/transform_broadcaster.h> int main(int argc, char** argv){ ros::init(argc, argv, "my_tf_broadcaster"); ros::NodeHandle node; tf::TransformBroadcaster br; tf::Transform transform; ros::Rate rate(10.0); while (node.ok()){ transform.setOrigin( tf::Vector3(0.0, 2.0, 0.0) ); transform.setRotation( tf::Quaternion(0, 0, 0, 1) ); br.sendTransform(tf::StampedTransform(transform, ros::Time::now(), "turtle1", "carrot1")); rate.sleep(); } return 0; };
修改 CMakeLists.txt gedit CMakeLists.txt add_executable(frame_tf_broadcaster src/frame_tf_broadcaster.cpp) target_link_libraries(frame_tf_broadcaster ${catkin_LIBRARIES})
Build your package; at the top folder of your catkin workspace: $ catkin_make
修改 launch文件start_demo.launch添加: <launch> ... <node pkg="learning_tf" type="frame_tf_broadcaster" name="broadcaster_frame" /> </launch> 然后在运行:
$ roslaunch learning_tf start_demo.launch 发现效果跟以前一样 Open the src/turtle_tf_listener.cpp file, and simple replace "/turtle1" with "/carrot1" in lines 26-27:
listener.lookupTransform("/turtle2", "/carrot1", ros::Time(0), transform);
重新编译
Build your package; at the top folder of your catkin workspace: $ catkin_make ERROR: cannot launch node of type [learning_tf/turtle_tf_broadcaster]: can't locate node [turtle_tf_broadcaster] in package [learning_tf]
ERROR: cannot launch node of type [learning_tf/turtle_tf_broadcaster]: can't locate node [turtle_tf_broadcaster] in package [learning_tf] ERROR: cannot launch node of type [learning_tf/turtle_tf_listener]: can't locate node [turtle_tf_listener] in package [learning_tf] ERROR: cannot launch node of type [learning_tf/frame_tf_broadcaster]: can't locate node [frame_tf_broadcaster] in package [learning_tf] 解决办法:
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