[example of URDF exercise based on ROS] use of four wheeled robot and camera

Preface

In the last blog section, we systematically learned about URDF The basic usage syntax of , And from the label 、 attribute 、 I have deeply experienced the structural relationship and other aspects URDF The framework and concept of the document , This article focuses on Actual simulation development , Examples are analyzed from cuboid and four-wheel cylindrical robots with cameras , Finally, some applicable development tools are given , Help you understand URDF How to use files .

One 、 Rotating Cameras

1. Target robot model

• demand ： The chassis is a cuboid 、 There is one in front that can go along Z Camera with free rotation axis

• URDF File code ：

<!--  demand :  The chassis is a cuboid 、  There is one in front that can go along Z Camera with free rotation axis   Robot model of  -->
<robot name="mycar">
    <!--  chassis  -->
    <link name="base_link">
        <visual>
            <geometry>
                <box size="0.5 0.2 0.1" />
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0" />
            <material name="blue">
                <color rgba="0 0 1.0 0.5" />
            </material>
        </visual>
    </link>

    <!--  camera  -->
    <link name="camera">
        <visual>
            <geometry>
                <box size="0.02 0.05 0.05" />
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0" />
            <material name="red">
                <color rgba="1 0 0 0.5" />
            </material>
        </visual>
    </link>

    <!--  The joints  -->
    <joint name="camera2baselink" type="continuous">
        <parent link="base_link"/>
        <child link="camera" />
        <!--  You need to calculate two  link  The offset between the physical centers of  -->
        <origin xyz="0.2 0 0.075" rpy="0 0 0" />
        <axis xyz="0 0 1" />
    </joint>

</robot>

2. Startup file

• Pay attention to item ： A status publishing node must be created , Secondly, joint motion control nodes are used to test joint motion , And generate UI

<launch>

    <param name="robot_description" textfile="$(find urdf_rviz_demo)/urdf/urdf/urdf03_joint.urdf" />
    <node pkg="rviz" type="rviz" name="rviz" args="-d $(find urdf_rviz_demo)/config/helloworld.rviz" /> 

    <!--  Add joint status publishing node  -->
    <node pkg="joint_state_publisher" type="joint_state_publisher" name="joint_state_publisher" />
    <!--  Add robot status publishing node  -->
    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" />
    <!--  Optional : The node used to control joint motion  -->
    <node pkg="joint_state_publisher_gui" type="joint_state_publisher_gui" name="joint_state_publisher_gui" />

</launch>

3. Problems and optimization methods

1） Initial model location

• problem ： The robot model generated by the above code will Half sunk to the ground

• reason ： The center of gravity of the chassis is located at the origin of the map

• resolvent ： Set the initial link Set to a very small size link( For example, the radius is 0.001m The sphere of , Or the side length is 0.001m The cube ), Then at the beginning link Add rigid bodies such as chassis on

• Optimize the code ：

<!--  Use  base_footprint  Optimize  -->
<robot name="mycar">
    <!--  Set an origin ( Projection of robot Center ) -->
    <link name="base_footprint">
        <visual>
            <geometry>
                <sphere radius="0.001" />
            </geometry>
        </visual>
    </link>

    <!--  Add chassis  -->
    <link name="base_link">
        <visual>
            <geometry>
                <box size="0.5 0.2 0.1" />
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0" />
            <material name="blue">
                <color rgba="0 0 1.0 0.5" />
            </material>
        </visual>
    </link>

    <!--  The joint between the chassis and the origin  -->
    <joint name="base_link2base_footprint" type="fixed">
        <parent link="base_footprint" />
        <child link="base_link" />
        <origin xyz="0 0 0.05" />
    </joint>

    <!--  Add Cameras  -->
    <link name="camera">
        <visual>
            <geometry>
                <box size="0.02 0.05 0.05" />
            </geometry>
            <origin xyz="0 0 0" rpy="0 0 0" />
            <material name="red">
                <color rgba="1 0 0 0.5" />
            </material>
        </visual>
    </link>
    <!--  The joints  -->
    <joint name="camera2baselink" type="continuous">
        <parent link="base_link"/>
        <child link="camera" />
        <origin xyz="0.2 0 0.075" rpy="0 0 0" />
        <axis xyz="0 0 1" />
    </joint>

</robot>

2） Command line error

1. The error message is as follows ：

UnicodeEncodeError: 'ascii' codec can't encode characters in position 463-464: ordinal not in range(128)
[joint_state_publisher-3] process has died [pid 4443, exit code 1, cmd /opt/ros/melodic/lib/joint_state_publisher/joint_state_publisher __name:=joint_state_publisher __log:=/home/rosmelodic/.ros/log/b38967c0-0acb-11eb-aee3-0800278ee10c/joint_state_publisher-3.log].
log file: /home/rosmelodic/.ros/log/b38967c0-0acb-11eb-aee3-0800278ee10c/joint_state_publisher-3*.log

resolvent ： Remove Chinese comments , Avoid coding problems

2. The error message is as follows ：

[ERROR] [1584370263.037038]: Could not find the GUI, install the ‘joint_state_publisher_gui’ package

resolvent ：sudo apt install ros-noetic-joint-state-publisher-gui

Two 、 Four wheeled cylindrical robot

1. Target robot model

• demand ： The chassis is a cylinder , The four wheels are two driving wheels , Two supporting wheels
• Specific data and diagram ：

1. chassis ： radius 10cm、 high 8cm, Distance from the ground 1.5cm
2. wheel ： Driving wheel ： radius 3.25cm、 wide 1.5cm; Supporting wheel ： spherical , radius 0.75cm

• URDF File code ：

<robot name="mycar">
    <!--  Set up  base_footprint -->
	    <link name="base_footprint">
	        <visual>
	            <geometry>
	                <sphere radius="0.001" />
	            </geometry>
	        </visual>
	    </link>

    <!--  Add chassis  -->
		<!--  Parameters   shape : Cylinder   radius :10 cm  Height :8 cm  Off the ground :1.5 cm -->
	    <link name="base_link">
	        <visual>
	            <geometry>
	                <cylinder radius="0.1" length="0.08" />
	            </geometry>
	            <origin xyz="0 0 0" rpy="0 0 0" />
	            <material name="yellow">
	                <color rgba="0.8 0.3 0.1 0.5" />
	            </material>
	        </visual>
	    </link>
	
	    <joint name="base_link2base_footprint" type="fixed">
	        <parent link="base_footprint" />
	        <child link="base_link"/>
	        <origin xyz="0 0 0.055" />
	    </joint>


    <!--  Add drive wheels  -->
	    <!--  The driving wheel is a overturned cylinder   Parameters   radius : 3.25 cm  Width : 1.5 cm  Color :  black   Joint settings : x = 0 y =  The radius of the chassis  +  Tire width  / 2 z =  Distance from the ground  +  Chassis length  / 2 -  Tire radius  = 1.5 + 4 - 3.25 = 2.25(cm) axis = 0 1 0 -->
	    <link name="left_wheel">
	        <visual>
	            <geometry>
	                <cylinder radius="0.0325" length="0.015" />
	            </geometry>
	            <origin xyz="0 0 0" rpy="1.5705 0 0" />
	            <material name="black">
	                <color rgba="0.0 0.0 0.0 1.0" />
	            </material>
	        </visual>
	
	    </link>
	
	    <joint name="left_wheel2base_link" type="continuous">
	        <parent link="base_link" />
	        <child link="left_wheel" />
	        <origin xyz="0 0.1 -0.0225" />
	        <axis xyz="0 1 0" />
	    </joint>
	
	
	    <link name="right_wheel">
	        <visual>
	            <geometry>
	                <cylinder radius="0.0325" length="0.015" />
	            </geometry>
	            <origin xyz="0 0 0" rpy="1.5705 0 0" />
	            <material name="black">
	                <color rgba="0.0 0.0 0.0 1.0" />
	            </material>
	        </visual>
	
	    </link>
	
	    <joint name="right_wheel2base_link" type="continuous">
	        <parent link="base_link" />
	        <child link="right_wheel" />
	        <origin xyz="0 -0.1 -0.0225" />
	        <axis xyz="0 1 0" />
	    </joint>


    <!--  Add Cardan wheel ( Supporting wheel ) -->
	    <!--  Parameters   shape :  sphere   radius : 0.75 cm  Color :  black   Joint settings : x =  Customize ( Chassis radius  -  Radius of universal wheel ) = 0.1 - 0.0075 = 0.0925(cm) y = 0 z =  Chassis length  / 2 +  Distance from the ground  / 2 = 0.08 / 2 + 0.015 / 2 = 0.0475 axis= 1 1 1 -->
	    <link name="front_wheel">
	        <visual>
	            <geometry>
	                <sphere radius="0.0075" />
	            </geometry>
	            <origin xyz="0 0 0" rpy="0 0 0" />
	            <material name="black">
	                <color rgba="0.0 0.0 0.0 1.0" />
	            </material>
	        </visual>
	    </link>
	
	    <joint name="front_wheel2base_link" type="continuous">
	        <parent link="base_link" />
	        <child link="front_wheel" />
	        <origin xyz="0.0925 0 -0.0475" />
	        <axis xyz="1 1 1" />
	    </joint>
	
	    <link name="back_wheel">
	        <visual>
	            <geometry>
	                <sphere radius="0.0075" />
	            </geometry>
	            <origin xyz="0 0 0" rpy="0 0 0" />
	            <material name="black">
	                <color rgba="0.0 0.0 0.0 1.0" />
	            </material>
	        </visual>
	    </link>
	
	    <joint name="back_wheel2base_link" type="continuous">
	        <parent link="base_link" />
	        <child link="back_wheel" />
	        <origin xyz="-0.0925 0 -0.0475" />
	        <axis xyz="1 1 1" />
	    </joint>
</robot>

2. Startup file

• launch File code ：

<launch>
    <!--  take  urdf  The file content is set into the parameter server  -->
    <param name="robot_description" textfile="$(find demo01_urdf_helloworld)/urdf/urdf/test.urdf" />

    <!--  start-up  rivz -->
    <node pkg="rviz" type="rviz" name="rviz_test" args="-d $(find demo01_urdf_helloworld)/config/helloworld.rviz" />

    <!--  Start the robot state and joint state publishing node  -->
    <node pkg="robot_state_publisher" type="robot_state_publisher" name="robot_state_publisher" />
    <node pkg="joint_state_publisher" type="joint_state_publisher" name="joint_state_publisher" />

    <!--  Start the graphical control joint motion node  -->
    <node pkg="joint_state_publisher_gui" type="joint_state_publisher_gui" name="joint_state_publisher_gui" />
</launch>

• Implementation process ： This robot can first create a basic URDF file , After integrating the startup file, gradually add components such as chassis and wheels

3、 ... and 、URDF Authoring tool

1. Syntax check check_urdf

• Tool installation ：

sudo apt install liburdfdom-tools

• Specific tools ：check_urdf, Check for complex urdf Whether there are syntax problems in the file

• Usage method ： Enter the corresponding directory , Use command check_urdf Corresponding urdf file

• Effect indication ：
  

2. Look at the structure urdf_to_graphiz

• Tool installation ： ditto

• Specific tools ：urdf_to_graphiz, see urdf Model structure , Show different link The hierarchy of

• Usage method ： Enter the corresponding directory , Use command urdf_to_graphiz Corresponding urdf file , View... In the current directory PDF File can

• Effect indication ：

summary

• Statement ： The blog section of this section refers to CSDN User zhaoxuzuo ROS course , This blog mainly introduces two uses URDF Specific examples of robot modeling , One is the free rotation camera , The other is a four wheeled robot , Finally, it gives URDF Some of the basics of ROS Tools , Contains syntax error checking and link Structure display, etc , The next blog will introduce how to use Xacro Yes URDF Program optimization , Coming soon .