Discussion - Concept Design Gazebo (Making a Mobile Robot Pt 3)

Articulated Robotics Content Discussions
#22

Main question: How do I fix a link to the Gazebo world so it’s immobile?

Background:
I’ve gone through all the non-physical parts of Making a Mobile Robot before, with a lot of success.
I’m now at the point I want to buy parts for my own robot, so I’ve gone back and updated everything to a simple robot configuration that is interesting to me, and I’m having a bit of trouble with Gazebo.

My robot is a little flashlight holding bot, with an XY prismatic gantry and an AB revolute flashlight pointing head. It’s going to be mounted above my workbench and point a light at things I’m working on.

I want the base XY gantry to just be stuck floating in Gazebo about 1 m off the ground, and completely insensitive to inertial effects, like it’s been fastened to my workbench overhang.

Does anyone know how to do that? I haven’t found a good way to mark a URDF element as kinematic instead of dynamic for Gazebo.

My URDF is below, with placeholder inertial info for each link just to get it working in Gazebo:

<?xml version="1.0"?>
<robot xmlns:xacro="http://www.ros.org/wiki/xacro" >
    <xacro:include filename="inertial_macros.xacro"/>



    <!-- Materials -->
    
    <material name="white">
        <color rgba="1 1 1 1"/>
    </material>

    <material name="orange">
        <color rgba="1 0.3 0.1 1"/>
    </material>

    <material name="blue">
        <color rgba="0.2 0.2 1 1"/>
    </material>

    <material name="black">
        <color rgba="0 0 0 1"/>
    </material>



    <!-- Base Link -->

    <link name="base_link">

    </link>

    <!-- Chassis Link -->

    <joint name="chassis_joint" type="fixed">
        <parent link="base_link" />
        <child link="chassis"/>
        <origin xyz="0 0 1" />
    </joint>

    <link name="chassis">
        <visual>
            <!-- Rear X Rail -->
            <origin xyz="0 -0.25 0" />
            <geometry>
                <box size="1 0.05 0.05"/>
            </geometry>
            <material name="white"/>
        </visual>
        <visual>
            <!-- Front X Rail -->
            <origin xyz="0 0.25 0" />
            <geometry>
                <box size="1 0.05 0.05"/>
            </geometry>
            <material name="white"/>
        </visual>
        <visual>
            <!-- Left Y Support -->
            <origin xyz="-0.5 0 0" />
            <geometry>
                <box size="0.05 0.5 0.05"/>
            </geometry>
            <material name="white"/>
        </visual>
        <visual>
            <!-- Right Y Support -->
            <origin xyz="0.5 0 0" />
            <geometry>
                <box size="0.05 0.5 0.05"/>
            </geometry>
            <material name="white"/>
        </visual>
        <collision>
            <!-- Rear X Rail -->
            <origin xyz="0 -0.25 0" />
            <geometry>
                <box size="1 0.05 0.05"/>
            </geometry>
        </collision>
        <collision>
            <!-- Front X Rail -->
            <origin xyz="0 0.25 0" />
            <geometry>
                <box size="1 0.05 0.05"/>
            </geometry>
        </collision>
        <collision>
            <!-- Left Y Support -->
            <origin xyz="-0.5 0 0" />
            <geometry>
                <box size="0.05 0.5 0.05"/>
            </geometry>
        </collision>
        <collision>
            <!-- Right Y Support -->
            <origin xyz="0.5 0 0" />
            <geometry>
                <box size="0.05 0.5 0.05"/>
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        </xacro:inertial_box>
    </link>

    <!-- <gazebo reference="chassis">
        <material>Gazebo/Grey</material>
        <selfCollide>false</selfCollide>
    </gazebo> -->

    <!-- X Axis Link -->

    <joint name="x_axis_joint" type="prismatic">
        <parent link="chassis" />
        <child link="x_axis"/>
        <origin xyz="-0.45 0 0" />
        <axis xyz="1 0 0"/>
        <limit lower="0" upper="0.9" effort="20" velocity="0.05"/>
    </joint>

    <link name="x_axis">
        <visual>
            <origin xyz="0 0 0"/>
            <geometry>
                <box size="0.025 0.5 0.025"/>
            </geometry>
            <material name="black"/>
        </visual>
        <collision>
            <origin xyz="0 0 0"/>
            <geometry>
                <box size="0.025 0.5 0.025"/>
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        </xacro:inertial_box>
    </link>

    <!-- Y Axis Link -->

    <joint name="y_axis_joint" type="prismatic">
        <parent link="x_axis" />
        <child link="y_axis"/>
        <origin xyz="0 -0.175 -0.025" />
        <axis xyz="0 1 0"/>
        <limit lower="0" upper="0.35" effort="20" velocity="0.05"/>
    </joint>

    <link name="y_axis">
        <visual>
            <origin xyz="0 0 0"/>
            <geometry>
                <box size="0.1 0.1 0.025"/>
            </geometry>
            <material name="orange"/>
        </visual>
        <collision>
            <origin xyz="0 0 0"/>
            <geometry>
                <box size="0.1 0.1 0.025"/>
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        </xacro:inertial_box>
    </link>

    <!-- A Axis Link -->

    <joint name="a_axis_joint" type="revolute">
        <parent link="y_axis" />
        <child link="a_axis"/>
        <origin xyz="0 0 0" />
        <axis xyz="0 0 1"/>
        <limit lower="-3.14" upper="3.14" effort="20" velocity="12"/>

    </joint>

    <link name="a_axis">
        <visual>
            <origin xyz="0 0 0"/>
            <geometry>
                <sphere radius="0.05"/>
            </geometry>
            <material name="orange"/>
        </visual>
        <collision>
            <origin xyz="0 0 0"/>
            <geometry>
                <sphere radius="0.05"/>
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        </xacro:inertial_box>
    </link>

    <!-- B Axis Link -->

    <joint name="b_axis_joint" type="revolute">
        <parent link="a_axis" />
        <child link="b_axis"/>
        <origin  rpy="-1.0 0 0 " xyz="0 0 0" />
        <axis xyz="1 0 0"/>
        <limit lower="0" upper="2.0" effort="20" velocity="12"/>
    </joint>
    
    <link name="b_axis">
        <visual>
            <origin xyz="0 0 -.05"/>
            <geometry>
                <box size="0.025 0.025 0.015"/>
            </geometry>
            <material name="orange"/>
        </visual>
        <collision>
            <origin xyz="0 0 -.05"/>
            <geometry>
                <box size="0.025 0.025 0.015"/>
            </geometry>
        </collision>
        <xacro:inertial_box mass="0.5" x="0.3" y="0.3" z="0.15">
            <origin xyz="0.15 0 0.075" rpy="0 0 0"/>
        </xacro:inertial_box>
    </link>
</robot>
#23

I solved my problem, but wanted to leave this here in case anyone else runs into it.

In order to stick a link to some place in the Gazebo world, add a fixed joint to a link named world.

I attached mine to my base_link, with a Z offset of 1 m:

    <!-- World Fix Link -->
    <link name="world">

    </link>
    
    <joint name="weld" type="fixed">
        <parent link="world" />
        <child link="base_link" />
        <origin xyz="0 0 1" />
    </joint>
#24

I have problems, to run the robot on gazebo harmonic (on ros2 Jazzy) . When I’ll do the command: “ros2 run articubot_one spawn_entity.py -topic robot_description -entity bot_name” the error is: “No executable found”.

I have followed the step of this tutorial, but it still doesn’t work.

any help?