ROS机器人TF基础（坐标相关概念和实践）_人工智能

TF坐标变换基础

机器人建模和控制必须掌握坐标系和坐标变换等基础知识。机器人在空间中运动主要有两种形式：

平移和旋转

也就是线速度和角速度。坐标关系不仅包括机器人和外部环境，也包括机器人自身各部件，如本体，传感器（摄像头），控制器，执行器（电机）等。 TF概念如下图所示：在日常生活中，如果将现实环境建立数学或物理模型必然也离不开坐标系，如下图：常见的如GPS全局坐标，室内定位局部坐标等，在机器人领域示例如下：教程不仅有理论介绍，还有云平台实践环境：分别展示了两款机器人模型中的TF。下面我们先把坐标系基本概念过一遍：示例演示：

roslaunch turtle_tf2 turtle_tf2_demo_cpp.launch

使用键盘控制控制第一个turtle，第二个跟随，如下图所示：

<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 name="turtle1_tf2_broadcaster" pkg="turtle_tf2" type="turtle_tf2_broadcaster" respawn="false" output="screen" >
    <param name="turtle" type="string" value="turtle1" />
  </node>
  <node name="turtle2_tf2_broadcaster" pkg="turtle_tf2" type="turtle_tf2_broadcaster" respawn="false" output="screen" >
    <param name="turtle" type="string" value="turtle2" />
  </node>
  <node name="turtle_pointer" pkg="turtle_tf2" type="turtle_tf2_listener" respawn="false" output="screen" >
  </node>
 
</launch>

注意：由于noetic版本仅支持python3。为了更方便的调试和查看坐标，ROS-TF2提供了大量的工具：

import rospy
import tf2_py as tf2
import yaml
import subprocess
from tf2_msgs.srv import FrameGraph
import tf2_ros
 
def main():
    rospy.init_node('view_frames')
    
    # listen to tf for 5 seconds
    rospy.loginfo('Listening to tf data during 5 seconds...')
    rospy.sleep(0.00001)
    buffer = tf2_ros.Buffer()
    listener = tf2_ros.TransformListener(buffer)
    rospy.sleep(5.0)
 
    rospy.loginfo('Generating graph in frames.pdf file...')
    rospy.wait_for_service('~tf2_frames')
    srv = rospy.ServiceProxy('~tf2_frames', FrameGraph)
    data = yaml.safe_load(srv().frame_yaml)
    with open('frames.gv', 'w') as f:
        f.write(generate_dot(data))
    subprocess.Popen('dot -Tpdf frames.gv -o frames.pdf'.split(' ')).communicate()
 
def generate_dot(data):
    if len(data) == 0:
        return 'digraph G { "No tf data received" }'
 
    dot = 'digraph G {\n'
    for el in data: 
        map = data[el]
        dot += '"'+map['parent']+'" -> "'+str(el)+'"'
        dot += '[label=" '
        dot += 'Broadcaster: '+map['broadcaster']+'\\n'
        dot += 'Average rate: '+str(map['rate'])+'\\n'
        dot += 'Buffer length: '+str(map['buffer_length'])+'\\n' 
        dot += 'Most recent transform: '+str(map['most_recent_transform'])+'\\n'
        dot += 'Oldest transform: '+str(map['oldest_transform'])+'\\n'
        dot += '"];\n'
        if not map['parent'] in data:
            root = map['parent']
    dot += 'edge [style=invis];\n'
    dot += ' subgraph cluster_legend { style=bold; color=black; label ="view_frames Result";\n'
    dot += '"Recorded at time: '+str(rospy.Time.now().to_sec())+'"[ shape=plaintext ] ;\n'
    dot += '}->"'+root+'";\n}'
    return dot
 
 
if __name__ == '__main__':
    main()

坐标为静态关系：坐标为动态关系：坐标与时间关系：坐标与机器人关系：这个坐标变换TF2工具的意义如上所示，但是可能会觉得不是很清楚，下面举个例子：观察上图绿点的位置，通常我们需要机器人本体坐标，但是传感器并非安装在本体中心，而传感器获取的各类信息都是相对传感器的数据，而非机器人本体，TF2工具可以直接维护这些位置姿态坐标关系，将其转换为机器人本体坐标。 tf2常用功能包： examples-tf2-py: 使用tf2库的Python API示例。 geometry2: 用于在ros，tf2中引入默认软件包第二代坐标变换库的元软件包。 robot-state-publisher: 状态发布后，对于使用tf2的系统中的所有组件都可用。该包使用机器人的运动学树模型将机器人的关节角度作为输入，并发布机器人链接的3D姿势。 ros-base: 一个扩展“ ros_core”并包含其他基本功能（如tf2和urdf）的软件包。 tf2: tf2保持时间缓冲的树结构中的坐标系之间的关系，并允许用户在任意所需的时间点在任意两个坐标系之间转换点，向量等。 tf2-ros: 该软件包包含适用于Python和C ++的tf2库的ROS绑定。其他库：tf2-bullet, tf2-eigen, tf2-geometry-msgs, tf2-kdl, tf2-msgs, tf2-py, tf2-sensor-msgs, tf2-tools。

机器人模型与TF2工具应用案例：

ROS1（noetic，melodic，kinetic，indigo）：注意：xacro,sdf等格式适合Gazebo，urdf格式适合rviz。

rosrun xacro xacro -o model_out.urdf model_in.urdf.xacro

使用如下命令打开一个机器人模型（此处不要求掌握，后续详细介绍）：

roslaunch robot1_description display.launch model:=robot1_base1.urdf

rqt-tf-tree:

rosrun tf2_tools echo.py base_link wheel_1

rosrun tf2_tools view_frames.py

ROS2（foxy，dashing）：机器人urdf模型通用！！！

ros2 launch urdf_tutorial display.launch.py model:=src/urdf/robot1_base1.urdf

下图和ros1几乎一样：同样也能使用各类工具：

ros2 run tf2_tools view_frames.py

ROS1和ROS2几乎一致！下一节，将通过URDF构建机器人模型，以ROS2（foxy）讲解为主！主要是ROS1相关URDF功能如何顺利移植到ROS2中！

附录（ros2 foxy）：

display.launch.py

import launch
from launch.substitutions import Command, LaunchConfiguration
import launch_ros
import os
 
 
def generate_launch_description():
    pkg_share = launch_ros.substitutions.FindPackageShare(package='urdf_tutorial').find('urdf_tutorial')
    default_model_path = os.path.join(pkg_share, 'urdf/01-myfirst.urdf')
    default_rviz_config_path = os.path.join(pkg_share, 'rviz/urdf.rviz')
 
    robot_state_publisher_node = launch_ros.actions.Node(
        package='robot_state_publisher',
        executable='robot_state_publisher',
        parameters=[{'robot_description': Command(['xacro ', LaunchConfiguration('model')])}]
    )
    joint_state_publisher_node = launch_ros.actions.Node(
        package='joint_state_publisher',
        executable='joint_state_publisher',
        name='joint_state_publisher',
        condition=launch.conditions.UnlessCondition(LaunchConfiguration('gui'))
    )
    joint_state_publisher_gui_node = launch_ros.actions.Node(
        package='joint_state_publisher_gui',
        executable='joint_state_publisher_gui',
        name='joint_state_publisher_gui',
        condition=launch.conditions.IfCondition(LaunchConfiguration('gui'))
    )
    rviz_node = launch_ros.actions.Node(
        package='rviz2',
        executable='rviz2',
        name='rviz2',
        output='screen',
        arguments=['-d', LaunchConfiguration('rvizconfig')],
    )
 
    return launch.LaunchDescription([
        launch.actions.DeclareLaunchArgument(name='gui', default_value='True',
                                             description='Flag to enable joint_state_publisher_gui'),
        launch.actions.DeclareLaunchArgument(name='model', default_value=default_model_path,
                                             description='Absolute path to robot urdf file'),
        launch.actions.DeclareLaunchArgument(name='rvizconfig', default_value=default_rviz_config_path,
                                             description='Absolute path to rviz config file'),
        joint_state_publisher_node,
        joint_state_publisher_gui_node,
        robot_state_publisher_node,
        rviz_node
    ])

robot1_base1.urdf

<?xml version="1.0" ?>
 
<robot name="robot1_xacro" xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller" xmlns:interface="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface" xmlns:sensor="http://playerstage.sourceforge.net/gazebo/xmlschema/#sensor">
  <link name="base_footprint">
    <visual>
      <geometry>
        <box size="0.001 0.001 0.001"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
    </visual>
    <inertial>
      <mass value="0.0001"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
  <gazebo reference="base_footprint">
    <material>Gazebo/Green</material>
    <turnGravityOff>false</turnGravityOff>
  </gazebo>
  <joint name="base_footprint_joint" type="fixed">
    <origin xyz="0 0 0"/>
    <parent link="base_footprint"/>
    <child link="base_link"/>
  </joint>
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.2 .3 .1"/>
      </geometry>
      <origin rpy="0 0 1.54" xyz="0 0 0.05"/>
      <material name="white">
        <color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <box size="0.2 .3 0.1"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
  <link name="wheel_1">
    <visual>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
  <link name="wheel_2">
    <visual>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="black"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
  <link name="wheel_3">
    <visual>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="black"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
  <link name="wheel_4">
    <visual>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0"/>
      <material name="black"/>
    </visual>
    <collision>
      <geometry>
        <cylinder length="0.05" radius="0.05"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="1"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
  <joint name="base_to_wheel1" type="continuous">
    <parent link="base_link"/>
    <child link="wheel_1"/>
    <origin rpy="1.5707 0 0" xyz="0.1 0.15 0"/>
    <axis xyz="0 0 1"/>
  </joint>
  <joint name="base_to_wheel2" type="continuous">
    <axis xyz="0 0 1"/>
    <anchor xyz="0 0 0"/>
    <limit effort="100" velocity="100"/>
    <parent link="base_link"/>
    <child link="wheel_2"/>
    <origin rpy="1.5707 0 0" xyz="-0.1 0.15 0"/>
  </joint>
  <joint name="base_to_wheel3" type="continuous">
    <parent link="base_link"/>
    <axis xyz="0 0 1"/>
    <child link="wheel_3"/>
    <origin rpy="1.5707 0 0" xyz="0.1 -0.15 0"/>
  </joint>
  <joint name="base_to_wheel4" type="continuous">
    <parent link="base_link"/>
    <axis xyz="0 0 1"/>
    <child link="wheel_4"/>
    <origin rpy="1.5707 0 0" xyz="-0.1 -0.15 0"/>
  </joint>
</robot>

～Fin～

TF坐标变换基础

机器人模型与TF2工具应用案例：

附录（ros2 foxy）：

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