Result of building and launching a robot by a SDF (Simulation description format) file for a simple differential drive robot
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SDF (Simulation Description Format) is a file format for simulators, like Gazebo, that describes the simulator environment, models (including its links, connections, and physics simulator metadata), and appropriate plugins.
[sudo apt install ros-<ros2-distro>-sdformat-urdf] . I am using Ros Jazzy so the ros-distro here is jazzy. The correct statement is [sudo apt install ros-jazzy-sdformat-urdf]
Default model path to a URDF (Unified Robot Description Format) file
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A create new SDF (Simulation Description Format) file at src\description with it's contain look like
<?xml version="1.0" ?>
<sdf version="1.8" xmlns:xacro="http://ros.org/wiki/xacro">
<model name='thomas_bot' canonical_link='base_link'>
<!-- Define robot constants -->
<xacro:property name="base_width" value="0.31"/>
<xacro:property name="base_length" value="0.42"/>
<xacro:property name="base_height" value="0.18"/>
<xacro:property name="wheel_radius" value="0.10"/>
<xacro:property name="wheel_width" value="0.04"/>
<xacro:property name="wheel_ygap" value="0.025"/>
<xacro:property name="wheel_zoff" value="0.05"/>
<xacro:property name="wheel_xoff" value="0.12"/>
<xacro:property name="caster_xoff" value="0.14"/>
<!-- Define some commonly used inertial properties -->
<xacro:macro name="box_inertia" params="m w h d">
<inertial>
<pose>0 0 0 ${pi/2} 0 ${pi/2}</pose>
<mass>${m}</mass>
<inertia>
<ixx>${(m/12) * (h*h + d*d)}</ixx>
<ixy>0.0</ixy>
<ixz>0.0</ixz>
<iyy>${(m/12) * (w*w + d*d)}</iyy>
<iyz>0.0</iyz>
<izz>${(m/12) * (w*w + h*h)}</izz>
</inertia>
</inertial>
</xacro:macro>
<xacro:macro name="cylinder_inertia" params="m r h">
<inertial>
<pose>0 0 0 ${pi/2} 0 0</pose>
<mass>${m}</mass>
<inertia>
<ixx>${(m/12) * (3*r*r + h*h)}</ixx>
<ixy>0.0</ixy>
<ixz>0.0</ixz>
<iyy>${(m/12) * (3*r*r + h*h)}</iyy>
<iyz>0.0</iyz>
<izz>${(m/2) * (r*r)}</izz>
</inertia>
</inertial>
</xacro:macro>
<xacro:macro name="sphere_inertia" params="m r">
<inertial>
<mass>${m}</mass>
<inertia>
<ixx>${(2/5) * m * (r*r)}</ixx>
<ixy>0.0</ixy>
<ixz>0.0</ixz>
<iyy>${(2/5) * m * (r*r)}</iyy>
<iyz>0.0</iyz>
<izz>${(2/5) * m * (r*r)}</izz>
</inertia>
</inertial>
</xacro:macro>
<!-- Robot Base -->
<link name='base_link'>
<must_be_base_link>true</must_be_base_link>
<visual name="base_link_visual">
<geometry>
<box><size>
${base_length} ${base_width} ${base_height}
</size></box>
</geometry>
<material>
<ambient>0 1 1 1</ambient>
<diffuse>0 1 1 1</diffuse>
</material>
</visual>
<collision name="base_link_collision">
<geometry>
<box><size>
${base_length} ${base_width} ${base_height}
</size></box>
</geometry>
</collision>
</link>
<!-- Robot Footprint -->
<link name='base_footprint'>
<pose relative_to="base_joint"/>
<xacro:box_inertia m="15" w="${base_width}" d="${base_length}" h="${base_height}"/>
</link>
<joint name='base_joint' type='fixed'>
<parent>base_link</parent>
<child>base_footprint</child>
<pose relative_to="base_link">0.0 0.0 ${-(wheel_radius+wheel_zoff)} 0 0 0</pose>
</joint>
<!-- Wheels -->
<xacro:macro name="wheel" params="prefix x_reflect y_reflect">
<link name="${prefix}_link">
<pose relative_to="${prefix}_joint"/>
<visual name="${prefix}_link_visual">
<pose relative_to="${prefix}_link">0 0 0 ${pi/2} 0 0</pose>
<geometry>
<cylinder>
<radius>${wheel_radius}</radius>
<length>${wheel_width}</length>
</cylinder>
</geometry>
<material>
<ambient>0.3 0.3 0.3 1.0</ambient>
<diffuse>0.7 0.7 0.7 1.0</diffuse>
</material>
</visual>
<collision name="${prefix}_link_collision">
<pose relative_to="${prefix}_link">0 0 0 ${pi/2} 0 0</pose>
<geometry>
<cylinder>
<radius>${wheel_radius}</radius>
<length>${wheel_width}</length>
</cylinder>
</geometry>
</collision>
<xacro:cylinder_inertia m="0.5" r="${wheel_radius}" h="${wheel_width}"/>
</link>
<joint name="${prefix}_joint" type="revolute">
<parent>base_link</parent>
<child>${prefix}_link</child>
<pose relative_to="base_link">${x_reflect*wheel_xoff} ${y_reflect*(base_width/2+wheel_ygap)} ${-wheel_zoff} 0 0 0</pose>
<axis>
<xyz>0 1 0</xyz>
<limit>
<lower>-inf</lower>
<upper>inf</upper>
</limit>
</axis>
</joint>
</xacro:macro>
<xacro:wheel prefix="drivewhl_l" x_reflect="-1" y_reflect="1" />
<xacro:wheel prefix="drivewhl_r" x_reflect="-1" y_reflect="-1" />
<link name="front_caster">
<pose relative_to="caster_joint"/>
<visual name="front_caster_visual">
<geometry>
<sphere>
<radius>${(wheel_radius+wheel_zoff-(base_height/2))}</radius>
</sphere>
</geometry>
<material>
<ambient>0 1 1 1</ambient>
<diffuse>0 1 1 1</diffuse>
</material>
</visual>
<collision name="front_caster_collision">
<geometry>
<sphere>
<radius>${(wheel_radius+wheel_zoff-(base_height/2))}</radius>
</sphere>
</geometry>
</collision>
<xacro:sphere_inertia m="0.5" r="${(wheel_radius+wheel_zoff-(base_height/2))}"/>
</link>
<joint name="caster_joint" type="fixed">
<parent>base_link</parent>
<child>front_caster</child>
<pose relative_to="base_link">${caster_xoff} 0.0 ${-(base_height/2)} 0 0 0</pose>
</joint>
</model>
</sdf>
Use thomas_bot_description sdf file as a default model path
default_model_path = os.path.join(pkg_share, 'src', 'description', 'thomas_bot_description.sdf')
Clip - Build and launch robot by the SDF (Simulation Description Format) file
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