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URDF vs. SDF

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eterpega edited this page Feb 18, 2018 · 9 revisions

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URDF vs. SDF

- 기본 구성요소 
  - Links
     - 'Link'는 모델상의 하나의 body의 물리적인 속성을 포함한다.(바퀴,링크,실린더,구 등) 
        각각의 'link'는 다수의 collision, visual 및 sensor 요소를 하위요소로 가질 수 있다.
     - Collision : 'collision' 요소는 다른 요소와의 충돌을 체크하기 위한 geometry 요소를 지닌다.
                   'collision geometry'는 단순형상 혹은 삼각형의 메쉬가 될 수 있다. (*.dae, *.stl)  
     - Visual : 'visual' 요소는 'link'요소의 시각화를 위해 사용되어 진다. (*.dae, *.stl 지원)
     - Inertial : 'inertial' 요소는 'link'의 동적 속성을 기술한다. (질량, 회전관성모멘트 등)
  - Sensor
     - 'sensor' 요소는 추가된 plugin을 통해 'world'로부터 특정 가상 센서의 데이터를 취득한다.
  - Joints
     - 'joint' 요소는 서로다른 두개의 'link'간의 구속관계를 표현한다. 
     - 'parent' 와 'child' 간의 회전축/이동조인트/고정조인트 등의 구속조건을 제공한다.
     - 'joint limit' 을 통해 조인트의 이동가능 범위를 제한한다.
  - Transmission
     - link와 link간의 연결되는 joint의 감속기를 적용할때 사용한다.
  - Plugins
     - 모델을 제어하기 위해 controller와 'shared library'를 통해 토픽/프레임 등을 공유한다..

  • URDF(Unified Robot Description Format)

    • An XML format for representing a robot model
    • The description of a robot consists of a set of link (part) elements, and a set of joint elements connecting the links together
      • description of kinematic and dynamic description of the robot
      • description of visual representation of the robot
      • description of collision model of the robot
      • http://wiki.ros.org/urdf/Examples

  • SDF(Simulation Description Format)

    • An XML format that describes objects and environments for robot simulators, visualization, and control
    • It contains a complete description for everything from the world level down to the robot level, including:
      • Scene: Ambient lighting, sky properties, shadows.
      • Physics: Gravity, time step, physics engine.
      • Models: Collection of links, collision objects, joints, and sensors.
      • Lights: Point, spot, and directional light sources.
      • Plugins: World, model, sensor, and system plugins.
      • http://gazebosim.org/sdf.html

  • URDF vs. SDF

    • URDF는 하나의 'robot' 모델에 대해 기술하는 포맷이며, 'world'상에서 'robot'의 포즈를 결정 할 수 없는 단점이 있다.

    • SDF는 'world'상에서 여러개의 'robot' 모델의 포즈를 정의할 수 있을 뿐 만 아니라, 모델간의 'friction' 및 'light', 'heightmaps' 등의 'world'속성 정의도 가능하다.

    • 구조적 차이점

      URDF

      <?xml version="1.0"?>
<robot name="my_robot">

   <link name="link">
     <visual>
       <origin rpy="1.57075 0 0" xyz="0 0 0"/> 
     </visual>
    ...
   </link>

   <joint name="joint1" type="fixed">
    ...
   </joint>

   <plugin filename="libMyPlugin.so" name="my_plugin"/>

</robot>

      SDF

      <?xml version="1.0" ?>
<sdf version="1.5">
  <world name="default">
    <physics type="ode">
     ...
    </physics>

    <scene>
     ...
    </scene>  

    <light>
     ...
    </light>
  </world>

  <model name="box">
    <pose>0 0 0.5 0 0 0</pose>
    <static>false</static>

    <link name="link">
      <pose>0 0 0 0 0 0</pose>
      ...
    </link>

    <joint type="revolute" name="my_joint">
      ...
    </joint>

    <plugin filename="libMyPlugin.so" name="my_plugin"/>

  </model>
</sdf>

xacro

  • Xacro (XML Macros)

    • Xacro 는 XML macro 언어로써, 반복되는 작업을 모듈화하여 재사용성이 용이하도록 도와준다.
    • http://wiki.ros.org/xacro

  • example

  • sample

    <xacro:macro name="pr2_arm" params="suffix parent reflect">
  <pr2_upperarm suffix="${suffix}" reflect="${reflect}" parent="${parent}" />
  <pr2_forearm suffix="${suffix}" reflect="${reflect}" parent="elbow_flex_${suffix}" />
</xacro:macro>

<xacro:pr2_arm suffix="left" reflect="1" parent="torso" />
<xacro:pr2_arm suffix="right" reflect="-1" parent="torso" />

    original

    <pr2_upperarm suffix="left" reflect="1" parent="torso" />
<pr2_forearm suffix="left" reflect="1" parent="elbow_flex_left" />
<pr2_upperarm suffix="right" reflect="-1" parent="torso" />
<pr2_forearm suffix="right" reflect="-1" parent="elbow_flex_right" />

URDF & SDF structure

  • URDF structure

    • 기본 구조
    <robot>
  <link>
    <visual>
       <origin/>
       <geometry/>
       <meterial/>
    </visual>
    <inetial>
       <origin/> 
       <mass/>
       <inertia/>
    </inertial>
    <collision>
       <origin/>
       <geometry/>
    </collision>
  </link>

  <joint name=" " type="fixed">
    <origin/>
    <parent/>
    <child/>
    <axis/>
    <dynamics>
       <damping/>
       <friction/>
    </dynamics>
    <limit>
       <upper/>
       <lower/>
       <effort/>
       <velocity/>
  </joint>
  <sensor>
    <camera>
       <image>
          <width/><height/><format/><hfov/><near/><far/>
       </image>
    </camera>
  </sensor>
</robot>

    .

    • link 예제
    <link name="base_link">
    <visual>
      <origin rpy="1.57075 0 0" xyz="0 0 0"/>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
      <material name="blue">
        <color rgba="0 0 .8 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="1.57075 0 0" xyz="0 0 0"/>
      <geometry>
        <cylinder length="0.6" radius="0.2"/>
      </geometry>
    </collision>
    <inertial>
      <mass value="10"/>
      <origin xyz="-0.001201 0.024513 -0.098231" rpy="0 0 0"/>
      <inertia ixx="1.0" ixy="0.0" ixz="0.0" iyy="1.0" iyz="0.0" izz="1.0"/>
    </inertial>
  </link>
    • joint 예제
    <joint name="right_gripper_joint" type="revolute">
    <axis xyz="0 0 -1"/>
    <limit effort="1000.0" lower="0.0" upper="0.548" velocity="0.5"/>
    <origin rpy="0 0 0" xyz="0.2 -0.01 0"/>
    <parent link="gripper_pole"/>
    <child link="right_gripper"/>
</joint>
    • plugin 예제
    <gazebo>
  <plugin name="differential_drive_controller" filename="libdiffdrive_plugin.so">
    ... plugin parameters ...
  </plugin>
</gazebo>

  • SDF structure

    • (example) Gazebo-9DOF-arm

    • (example) Willowgarage Model SDF File

      <?xml version="1.0" ?>
<sdf version="1.4">
  <model name="willowgarage">
    <static>true</static>
    <pose>-20 -20 0 0 0 0</pose>
    <link name="walls">
      <collision name="collision">
        <geometry>
          <mesh>
            <uri>model://willowgarage/meshes/willowgarage_collision.dae</uri>
          </mesh>
        </geometry>
      </collision>
      <visual name="visual">
        <geometry>
          <mesh>
            <uri>model://willowgarage/meshes/willowgarage_visual.dae</uri>
          </mesh>
        </geometry>
        <cast_shadows>false</cast_shadows>
      </visual>
    </link>
  </model>
</sdf>

    • link 예제 (link)

      <link name="trunk">
  <pose>0.0 0.0 1.051 0.0 0.0 0.0</pose>
  <!-- Height of leg + thigh + 0.5 * trunk-->
  <inertial>
    <mass>37.0762</mass>
    <pose>0.0 0.0 0.0 0.0 0.0 0.0</pose>
    <!-- Link mass is centered -->
    <inertia>
      <ixx>1.674614776</ixx>
      <ixy>0.0</ixy>
      <ixz>0.0</ixz>
      <iyz>0.0</iyz>
      <iyy>0.01679029</iyy>
      <izz>1.674614776</izz>
    </inertia>
  </inertial>
  <collision name="collision">
    <pose>0 0 0.5 0 0 0</pose>
    <geometry>
      <cylinder>
        <length>0.376</length>
        <radius>0.02</radius>
      </cylinder>
    </geometry>
  </collision>
  <visual name="visual">
    <geometry>
      <cylinder>
        <length>0.376</length>
        <radius>0.02</radius>
      </cylinder>
    </geometry>
  </visual>
  <sensor name="trunk_contact" type="contact">
    <contact>
      <collision>collision</collision>
    </contact>
  </sensor>
</link>

    • joint 예제 (link)

      <joint name="right_ankle" type="revolute">
  <parent>right_leg</parent>
  <child>right_foot</child>
  <pose>0.0 0.0 -0.041 0.0 -1.570796327 0.0</pose>
  <axis>
    <xyz>0.0 1 0.0</xyz>
    <limit>
      <lower>-0.680678408</lower>
      <upper>0.726056969</upper>
    </limit>
    <dynamics>
      <damping>0.05</damping>
    </dynamics>
  </axis>
</joint>

    • plugin 예제

      <model name="your_robot_model">
  <plugin name="differential_drive_controller" filename="libdiffdrive_plugin.so">
  ... plugin parameters ...
  </plugin>
</model>

URDF Check

  • 설치
    sudo apt-get install liburdfdom-tools

  • URDF 문법검사

    • 방법1 $ check_urdf myrobot.urdf
    • 방법2 $ rosrun urdf_parser check_urdf myrobot.urdf

  • URDF 그래프표시

    • 방법1 $ urdf_to_graphiz myrobot.urdf $ evince myrobot.pdf
    • 방법2 $ rosrun urdf_parser urdf_to_graphiz brazo.urdf

URDF-SDF converter

  • Gazebbo 7.0 이전 버전 $ gzsdf print urdfname.urdf > newname.sdf
  • Gazebbo 7.0 이전 버전 $ gz sdf --print my_urdf.urdf > my_sdf.sdf
  • alt) $ /usr/local/Cellar/gazebo8/8.1.0/bin/gz sdf --convert simple_box.urdf

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