[ROS advanced chapter] Lecture 9 programming optimization of URDF and use of xacro

【ROS Advanced 】 About 9 URDF Programming optimization Xacro Use

Preface

In the last blog section, we learned systematically URDF Specific use methods and example exercises , But in the process of learning, we can also find direct editing URDF There are many problems in the document , for example Joint parameters and code reusability , This blog mainly introduces one About XML Programming optimization method of language ,Xacro, In practice URDF Popular optimization methods in programming , From the basic concept 、 Grammar explanation to actual robot examples for in-depth discussion .

One 、Xacro Basic concepts of

• Concept ：XML Macros Abbreviation , Essentially, A kind of XML Macro language , It's programmable XML

• Purpose ： solve URDF Inherent problems in building robot models ：

1. connecting rod 、 Calculation of important parameters of joints ： Manually calculate through a fixed formula , Prone to mistakes , And the efficiency is very low ;
2. Highly repetitive content ： For the same robot part ,URDF The file code is the same , But it needs to be programmed repeatedly , Lack of reusability ;

• principle ： Optimize the code construction process through variables and functions

1. Declare variables ： Calculate important parameters through mathematical operations , Record data in variables , Improve efficiency and safety ;
2. Packaging function ： Use process control sequence , Encapsulate fixed logic , Improve code reusability

• effect ： Improve writing efficiency , By using macro commands, we can build shorter and more readable XML file , Expand to greater XML Expression range

• characteristic ：
  \qquad 1. Provides a programmable interface , Contains variable declaration calls 、 Syntax implementation such as function declaration and call ;
  \qquad 2. In the use of xacro When generating a file , Root tag robot Namespace declaration must be included ：xmlns:xacro="http://wiki.ros.org/xacro"

Two 、Xacro A detailed explanation of the grammar of

1. Attributes and arithmetic operations ：

\qquad effect ： encapsulation URDF In the field , For example, car size 、 Wheel radius, etc

\qquad Attribute definitions ： Declare variables , The definition format is as follows

		<xacro:property name="xxxx" value="yyyy" />

\qquad Calling a ： Variable call format ：${ The attribute name }

\qquad Arithmetic operations ： Variable calculation format ：${ Mathematical expression }

2. macro ：

\qquad effect ： Function implementation , Improve code reuse rate , Optimize code structure , Improve safety

\qquad Macro definition ： Function definition , The definition format is as follows

		<xacro:macro name=" Macro name " params=" parameter list ( Multiple parameters are separated by spaces )">
		    .....
		
		     Parameter call format : ${ Parameter name }
		</xacro:macro>

\qquad Macro call ： Function call format ：

		<xacro: Macro name   Parameters 1=xxx  Parameters 2=xxx/>

3. The file contains integration ：

\qquad effect ： Package the different parts of the robot into separate xacro file , Integrate files into a complete robot , You can use File Inclusion to implement

\qquad The file contains the format ：

		<robot name="xxx" xmlns:xacro="http://wiki.ros.org/xacro">
		      <xacro:include filename="my_base.xacro" />
		      <xacro:include filename="my_camera.xacro" />
		      <xacro:include filename="my_laser.xacro" />
		      ....
		</robot>

3、 ... and 、 Examples of actual robot models

1. Target robot and implementation process ：

• The goal is ： It has two driving wheels 、 A cylindrical car with two supporting wheels , And add cameras and radar sensors , The effect is as follows ：

• Implementation process ：

1. Write chassis xacro file ;
2. Write camera and radar xacro file ;
3. Write a portfolio , Assemble all parts of the trolley ;
4. To write launch File to start the Rviz According to the model ;

2. To write xacro file ：

• The trolley chassis realizes ：

<!--  Use  xacro  Optimize  URDF  Implementation of car chassis based on version ：  Realize the idea : 1. Put some constants 、 Variables are encapsulated as  xacro:property  such as :PI  value 、 Trolley chassis radius 、 Distance from the ground 、 Wheel radius 、 Width  .... 2. Use   macro   Encapsulate the driving wheel and supporting wheel to realize , Call relevant macros to generate driving wheel and supporting wheel  -->
<!--  Root tag , You must declare  xmlns:xacro -->
<robot name="my_base" xmlns:xacro="http://www.ros.org/wiki/xacro">
    <!--  Encapsulate variables 、 Constant  -->
    <xacro:property name="PI" value="3.141"/>
    <!--  macro : Black setting  -->
    <material name="black">
        <color rgba="0.0 0.0 0.0 1.0" />
    </material>
    <!--  Chassis properties  -->
    <xacro:property name="base_footprint_radius" value="0.001" /> <!-- base_footprint  radius  -->
    <xacro:property name="base_link_radius" value="0.1" /> <!-- base_link  radius  -->
    <xacro:property name="base_link_length" value="0.08" /> <!-- base_link  Long  -->
    <xacro:property name="earth_space" value="0.015" /> <!--  Distance from the ground  -->

    <!--  chassis  -->
    <link name="base_footprint">
      <visual>
        <geometry>
          <sphere radius="${base_footprint_radius}" />
        </geometry>
      </visual>
    </link>

    <link name="base_link">
      <visual>
        <geometry>
          <cylinder radius="${base_link_radius}" length="${base_link_length}" />
        </geometry>
        <origin xyz="0 0 0" rpy="0 0 0" />
        <material name="yellow">
          <color rgba="0.5 0.3 0.0 0.5" />
        </material>
      </visual>
    </link>

    <joint name="base_link2base_footprint" type="fixed">
      <parent link="base_footprint" />
      <child link="base_link" />
      <origin xyz="0 0 ${earth_space + base_link_length / 2 }" />
    </joint>

    <!--  Driving wheel  -->
    <!--  Drive wheel properties  -->
    <xacro:property name="wheel_radius" value="0.0325" /><!--  radius  -->
    <xacro:property name="wheel_length" value="0.015" /><!--  Width  -->
    <!--  Drive wheel macro implementation  -->
    <xacro:macro name="add_wheels" params="name flag">
      <link name="${name}_wheel">
        <visual>
          <geometry>
            <cylinder radius="${wheel_radius}" length="${wheel_length}" />
          </geometry>
          <origin xyz="0.0 0.0 0.0" rpy="${PI / 2} 0.0 0.0" />
          <material name="black" />
        </visual>
      </link>

      <joint name="${name}_wheel2base_link" type="continuous">
        <parent link="base_link" />
        <child link="${name}_wheel" />
        <origin xyz="0 ${flag * base_link_radius} ${-(earth_space + base_link_length / 2 - wheel_radius) }" />
        <axis xyz="0 1 0" />
      </joint>
    </xacro:macro>
    <xacro:add_wheels name="left" flag="1" />
    <xacro:add_wheels name="right" flag="-1" />
    <!--  Supporting wheel  -->
    <!--  Support wheel properties  -->
    <xacro:property name="support_wheel_radius" value="0.0075" /> <!--  Radius of supporting wheel  -->

    <!--  Support wheel macro  -->
    <xacro:macro name="add_support_wheel" params="name flag" >
      <link name="${name}_wheel">
        <visual>
            <geometry>
                <sphere radius="${support_wheel_radius}" />
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0" />
            <material name="black" />
        </visual>
      </link>

      <joint name="${name}_wheel2base_link" type="continuous">
          <parent link="base_link" />
          <child link="${name}_wheel" />
          <origin xyz="${flag * (base_link_radius - support_wheel_radius)} 0 ${-(base_link_length / 2 + earth_space / 2)}" />
          <axis xyz="1 1 1" />
      </joint>
    </xacro:macro>

    <xacro:add_support_wheel name="front" flag="1" />
    <xacro:add_support_wheel name="back" flag="-1" />

</robot>

• camera xacro file ：

<!--  Camera related  xacro  file  -->
<robot name="my_camera" xmlns:xacro="http://wiki.ros.org/xacro">
    <!--  Camera properties  -->
    <xacro:property name="camera_length" value="0.01" /> <!--  Camera length (x) -->
    <xacro:property name="camera_width" value="0.025" /> <!--  Camera width (y) -->
    <xacro:property name="camera_height" value="0.025" /> <!--  Camera height (z) -->
    <xacro:property name="camera_x" value="0.08" /> <!--  Camera installation x coordinate  -->
    <xacro:property name="camera_y" value="0.0" /> <!--  Camera installation y coordinate  -->
    <xacro:property name="camera_z" value="${base_link_length / 2 + camera_height / 2}" /> <!--  Camera installation z coordinate : Ride height  / 2 +  Camera height  / 2 -->

    <!--  Camera joints and link -->
    <link name="camera">
        <visual>
            <geometry>
                <box size="${camera_length} ${camera_width} ${camera_height}" />
            </geometry>
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" />
            <material name="black" />
        </visual>
    </link>

    <joint name="camera2base_link" type="fixed">
        <parent link="base_link" />
        <child link="camera" />
        <origin xyz="${camera_x} ${camera_y} ${camera_z}" />
    </joint>
</robot>

• radar xacro file ：

<!--  Add radar to the car chassis  -->
<robot name="my_laser" xmlns:xacro="http://wiki.ros.org/xacro">

    <!--  Radar support  -->
    <xacro:property name="support_length" value="0.15" /> <!--  Support length  -->
    <xacro:property name="support_radius" value="0.01" /> <!--  Support radius  -->
    <xacro:property name="support_x" value="0.0" /> <!--  Bracket mounted x coordinate  -->
    <xacro:property name="support_y" value="0.0" /> <!--  Bracket mounted y coordinate  -->
    <xacro:property name="support_z" value="${base_link_length / 2 + support_length / 2}" /> <!--  Bracket mounted z coordinate : Ride height  / 2 +  Support height  / 2 -->

    <link name="support">
        <visual>
            <geometry>
                <cylinder radius="${support_radius}" length="${support_length}" />
            </geometry>
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" />
            <material name="red">
                <color rgba="0.8 0.2 0.0 0.8" />
            </material>
        </visual>
    </link>

    <joint name="support2base_link" type="fixed">
        <parent link="base_link" />
        <child link="support" />
        <origin xyz="${support_x} ${support_y} ${support_z}" />
    </joint>


    <!--  Radar properties  -->
    <xacro:property name="laser_length" value="0.05" /> <!--  Radar length  -->
    <xacro:property name="laser_radius" value="0.03" /> <!--  Radar radius  -->
    <xacro:property name="laser_x" value="0.0" /> <!--  Radar installation x coordinate  -->
    <xacro:property name="laser_y" value="0.0" /> <!--  Radar installation y coordinate  -->
    <xacro:property name="laser_z" value="${support_length / 2 + laser_length / 2}" /> <!--  Radar installation z coordinate : Support height  / 2 +  Radar altitude  / 2 -->

    <!--  Radar joints and link -->
    <link name="laser">
        <visual>
            <geometry>
                <cylinder radius="${laser_radius}" length="${laser_length}" />
            </geometry>
            <origin xyz="0.0 0.0 0.0" rpy="0.0 0.0 0.0" />
            <material name="black" />
        </visual>
    </link>

    <joint name="laser2support" type="fixed">
        <parent link="support" />
        <child link="laser" />
        <origin xyz="${laser_x} ${laser_y} ${laser_z}" />
    </joint>
</robot>

3. Combination and startup ：

• Combined trolley model file ：

<!--  Combine the car chassis with camera and radar  -->
<robot name="my_car_camera" xmlns:xacro="http://wiki.ros.org/xacro">
    <xacro:include filename="my_base.urdf.xacro" />
    <xacro:include filename="my_camera.urdf.xacro" />
    <xacro:include filename="my_laser.urdf.xacro" />
</robot>

• Integrate launch file ：

\qquad 1) First convert to urdf Post file integration ：

1. First the xacro The file is parsed into urdf file ：

rosrun xacro xacro xxx.xacro > xxx.urdf

2. Direct integration launch file ：

<launch>
   <param name="robot_description" textfile="$(find demo01_urdf_helloworld)/urdf/xacro/my_base_camera_laser.urdf.xacro" />" />
   <node pkg="rviz" type="rviz" name="rviz" args="-d $(find demo01_urdf_helloworld)/config/helloworld.rviz" />
   <node pkg="joint_state_publisher" type="joint_state_publisher" name="joint_state_publisher" output="screen" />
   <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" output="screen" />
   <node pkg="joint_state_publisher_gui" type="joint_state_publisher_gui" name="joint_state_publisher_gui" output="screen" />
</launch>

\qquad 2) stay launch Load directly into the file xacro

<launch>
    <param name="robot_description" command="$(find xacro)/xacro $(find demo01_urdf_helloworld)/urdf/xacro/my_base_camera_laser.urdf.xacro" />
    <node pkg="rviz" type="rviz" name="rviz" args="-d $(find demo01_urdf_helloworld)/config/helloworld.rviz" />
    <node pkg="joint_state_publisher" type="joint_state_publisher" name="joint_state_publisher" output="screen" />
    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" output="screen" />
    <node pkg="joint_state_publisher_gui" type="joint_state_publisher_gui" name="joint_state_publisher_gui" output="screen" />
</launch>

summary

• Statement ： The blog section of this section refers to CSDN User zhaoxuzuo ROS course , This paper mainly introduces URDF An optimized programming method for files , namely Xacro, Improve code security by declaring variables and functions 、 Efficiency and reusability , From the basic concepts 、 The grammar is explained in detail and the final actual robot example is analyzed in detail , In the following tutorial, we will also introduce about URDF File with the RVIZ and Gazebo The tutorial of Component Co simulation and the content about sensors and navigation , Coming soon .