[ROS Q&A] 066 – buoyancy neutral object goes up with hydrodynamics plugin part2

What will you learn in this post

• Understand how to use the buoyancy plugin in gazebo

List of resources used in this post

• The question on Gazebo Answers
• A live version of this post on YouTube: https://youtu.be/evweU2MjNjc
• The question is: buoyancy neutral object goes up with hydrodynamics plugin
• The answer: Calculate the volume using Blender and its bounding boxes.

Launching a complex robot

In the previous post, we learned the basics of buoyancy plugin in the gazebo simulator using a floating sphere.

Now we are going to dive deeper into how to calculate the buoyancy for a more complex robot like a simulated fish. For that, we launch the main_submarines.launch located in the buoyancy_tests_pkg package mentioned in the previous post. If you launched correctly, you should have a simulation like the one below:

fish with buoyancy in ROSDS

The geometric buoyancy version of the fish is the one in the naro_geometric_buoyancy.urdf.sdf file, which has the following content:

<robot name="naro">

    <material name="blue">
        <color rgba="0 0 0.8 1"/>
    </material>
    <material name="red">
        <color rgba="0.8 0 0 1"/>
    </material>
    <material name="green">
        <color rgba="0 0.8 0 1"/>
    </material>
    <material name="purple">
        <color rgba="0.8 0 0.8 1"/>
    </material>

    <gazebo>
        <plugin name="gazebo_ros_control" filename="libgazebo_ros_control.so">
        </plugin>
    </gazebo>

	<!-- * * * Link Definitions * * * -->
    <link name="base_link">
 	    <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="1.37616837872" />
            <inertia ixx="0.00393997006828" ixy="0.0" ixz="0.0" iyy="0.00393997006828" iyz="0.0" izz="0.00186057964803"/>
        </inertial>
        <collision>
            <origin rpy="0.0 0 0" xyz="0 0 0"/>
            <geometry>
                <cylinder radius="0.052" length="0.162"/>
            </geometry>
        </collision>
        <visual>
            <origin rpy="0.0 0 0" xyz="0 0 0"/>
            <geometry>
                <cylinder radius="0.052" length="0.162"/>
            </geometry>
            <material name="red"/>
        </visual>
	</link>

    <gazebo reference="base_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Red</material>
    </gazebo>

    <link name="NaroBody_2_link">
 	    <inertial>
            <origin xyz="0 0 0.0362" rpy="0 0 0"/>
            <mass value="0.605368047943" />
            <inertia ixx="0.000667233852526" ixy="0.0" ixz="0.0" iyy="0.000667233852526" iyz="0.0" izz="0.000805602035221"/>
        </inertial>
        <collision>
            <origin xyz="0 0 0.0362" rpy="0 0 0"/>
            <geometry>
                <cylinder radius="0.05159" length="0.0724"/>
            </geometry>
        </collision>
        <visual>
            <origin xyz="0 0 0.0362" rpy="0 0 0"/>
            <geometry>
                <cylinder radius="0.05159" length="0.0724"/>
            </geometry>
            <material name="blue"/>
        </visual>
	</link>
    <!-- This is for color and physical properties in Gazebo, color won't work with the material tag in gazebo
    only for URDF coloring -->
    <gazebo reference="NaroBody_2_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Blue</material>
    </gazebo>

    <joint name="NaroBody_1_NaroBody_2_joint" type="revolute">
    	<parent link="base_link"/>
    	<child link="NaroBody_2_link"/>
        <origin xyz="0 0 0.096" rpy="0 0 0"/>
        <limit lower="-0.2" upper="0.2" effort="10.0" velocity="1.0"/>
        <axis xyz="1 0 0"/>
	</joint>
	
    <transmission name="tran1">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="NaroBody_1_NaroBody_2_joint">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
        </joint>
        <actuator name="motor1">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>

    <link name="NaroBody_3_link">
 	    <inertial>
            <origin xyz="0 0 0.0267885" rpy="0 0 0"/>
            <mass value="0.137706811701" />
            <inertia ixx="6.1101464537e-05" ixy="0.0" ixz="0.0" iyy="6.1101464537e-05" iyz="0.0" izz="5.63390256449e-05"/>
        </inertial>
        <collision>
            <origin xyz="0 0 0.0267885" rpy="0 0 0"/>
            <geometry>
                <cylinder radius="0.028605" length="0.05357"/>
            </geometry>
        </collision>
        <visual>
            <origin xyz="0 0 0.0267885" rpy="0 0 0"/>
            <geometry>
                <cylinder radius="0.028605" length="0.05357"/>
            </geometry>
            <material name="green"/>
        </visual>
	</link>
    <!-- This is for color and physical properties in Gazebo, color won't work with the material tag in gazebo
    only for URDF coloring -->
    <gazebo reference="NaroBody_3_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Green</material>
    </gazebo>

    <joint name="NaroBody_2_NaroBody_3_joint" type="revolute">
    	<parent link="NaroBody_2_link"/>
    	<child link="NaroBody_3_link"/>
        <origin xyz="0 0 0.0874" rpy="0 0 0"/>
        <limit lower="-0.2" upper="0.2" effort="5.0" velocity="1.0"/>
        <axis xyz="1 0 0"/>
	</joint>
	
    <transmission name="tran2">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="NaroBody_2_NaroBody_3_joint">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
        </joint>
        <actuator name="motor2">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    
    
    <link name="Naro_caudal_fin_link">
 	    <inertial>
            <origin xyz="0 0.07 0" rpy="0 0 0"/>
            <mass value="0.0923628240155" />
            <inertia ixx="0.000113421547891" ixy="0.0" ixz="0.0" iyy="0.000113421547891" iyz="0.0" izz="0.000226288918838"/>
        </inertial>
        <collision>
            <origin xyz="0 0.07 0" rpy="0 0 0"/>
            <geometry>
                <cylinder radius="0.07" length="0.006"/>
            </geometry>
        </collision>
        <visual>
            <origin xyz="0 0.07 0" rpy="0 0 0"/>
            <geometry>
                <cylinder radius="0.07" length="0.006"/>
            </geometry>
            <material name="purple"/>
        </visual>
	</link>
    <!-- This is for color and physical properties in Gazebo, color won't work with the material tag in gazebo
    only for URDF coloring -->
    <gazebo reference="Naro_caudal_fin_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Purple</material>
    </gazebo>

    <joint name="NaroBody_3_Naro_caudal_fin_joint" type="revolute">
    	<parent link="NaroBody_3_link"/>
    	<child link="Naro_caudal_fin_link"/>
        <origin xyz="0 0 0.05357" rpy="1.5707963267948966 0 0"/>
        <limit lower="-0.2" upper="0.2" effort="5.0" velocity="1.0"/>
        <axis xyz="1 0 0"/>
	</joint>
	
    <transmission name="tran3">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="NaroBody_3_Naro_caudal_fin_joint">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
        </joint>
        <actuator name="motor3">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    
    
    <link name="Naro_pectoral_fin_right_link">
 	    <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="0.005272596" />
            <inertia ixx="1.66514662741e-06" ixy="0.0" ixz="0.0" iyy="1.58601814294e-06" iyz="0.0" izz="1.1076406047e-07"/>
        </inertial>
        <collision>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.006 0.0147 0.05978" />
            </geometry>
        </collision>
        <visual>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.006 0.0147 0.05978" />
            </geometry>
            <material name="purple"/>
        </visual>
	</link>
    <!-- This is for color and physical properties in Gazebo, color won't work with the material tag in gazebo
    only for URDF coloring -->
    <gazebo reference="Naro_pectoral_fin_right_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Purple</material>
    </gazebo>

    <joint name="NaroBody_1_Naro_pectoral_fin_right_joint" type="revolute">
    	<parent link="base_link"/>
    	<child link="Naro_pectoral_fin_right_link"/>
        <origin xyz="-0.04275 -0.05489 0.04478" rpy="1.5707963267948966 0 0"/>
        <limit lower="-0.2" upper="0.2" effort="0.1" velocity="0.5"/>
        <axis xyz="0 0 1"/>
	</joint>
	
    <transmission name="tran4">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="NaroBody_1_Naro_pectoral_fin_right_joint">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
        </joint>
        <actuator name="motor4">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    
    
    <link name="Naro_pectoral_fin_left_link">
 	    <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="0.005272596" />
            <inertia ixx="1.66514662741e-06" ixy="0.0" ixz="0.0" iyy="1.58601814294e-06" iyz="0.0" izz="1.1076406047e-07"/>
        </inertial>
        <collision>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.006 0.0147 0.05978" />
            </geometry>
        </collision>
        <visual>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.006 0.0147 0.05978" />
            </geometry>
            <material name="purple"/>
        </visual>
	</link>
    <!-- This is for color and physical properties in Gazebo, color won't work with the material tag in gazebo
    only for URDF coloring -->
    <gazebo reference="Naro_pectoral_fin_left_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Purple</material>
    </gazebo>

    <joint name="NaroBody_1_Naro_pectoral_fin_left_joint" type="revolute">
    	<parent link="base_link"/>
    	<child link="Naro_pectoral_fin_left_link"/>
        <origin xyz="-0.04275 0.05489 0.04478" rpy="1.5707963267948966 0 0"/>
        <limit lower="-0.2" upper="0.2" effort="0.1" velocity="0.5"/>
        <axis xyz="0 0 1"/>
	</joint>
	
    <transmission name="tran5">
        <type>transmission_interface/SimpleTransmission</type>
        <joint name="NaroBody_1_Naro_pectoral_fin_left_joint">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
        </joint>
        <actuator name="motor5">
            <hardwareInterface>EffortJointInterface</hardwareInterface>
            <mechanicalReduction>1</mechanicalReduction>
        </actuator>
    </transmission>
    
    
    <link name="Naro_dorsal_fin_up_link">
 	    <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="0.005272596" />
            <inertia ixx="1.66514662741e-06" ixy="0.0" ixz="0.0" iyy="1.58601814294e-06" iyz="0.0" izz="1.1076406047e-07"/>
        </inertial>
        <collision>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.006 0.0147 0.05978" />
            </geometry>
        </collision>
        <visual>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.0147 0.006 0.05978" />
            </geometry>
            <material name="purple"/>
        </visual>
	</link>
	
	<gazebo reference="Naro_dorsal_fin_up_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Purple</material>
    </gazebo>
    
    <joint name="NaroBody_1_Naro_dorsal_fin_up_joint" type="fixed">
        <parent link="NaroBody_2_link"/>
        <child link="Naro_dorsal_fin_up_link"/>
        <origin xyz="0.035795 0 0.0362" rpy="0 1.5707963267948966 0"/>
    </joint>
    
    
        <link name="Naro_dorsal_fin_down_link">
 	    <inertial>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <mass value="0.005272596" />
            <inertia ixx="1.66514662741e-06" ixy="0.0" ixz="0.0" iyy="1.58601814294e-06" iyz="0.0" izz="1.1076406047e-07"/>
        </inertial>
        <collision>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.006 0.0147 0.05978" />
            </geometry>
        </collision>
        <visual>
            <origin xyz="0 0 0" rpy="0 0 0"/>
            <geometry>
                <box size="0.0147 0.006 0.05978" />
            </geometry>
            <material name="purple"/>
        </visual>
	</link>
	
	<gazebo reference="Naro_dorsal_fin_down_link">
        <kp>1000000.0</kp>
        <kd>1000000.0</kd>
        <mu1>10.0</mu1>
        <mu2>10.0</mu2>
        <material>Gazebo/Purple</material>
    </gazebo>
    
    <joint name="NaroBody_1_Naro_dorsal_fin_down_joint" type="fixed">
        <parent link="NaroBody_2_link"/>
        <child link="Naro_dorsal_fin_down_link"/>
        <origin xyz="-0.035795 0 0.0362" rpy="0 1.5707963267948966 0"/>
    </joint>
    
    <gazebo>
        <plugin name="buoyancy_sphere" filename="libBuoyancyPlugin.so">
          <fluid_density>1000</fluid_density>
        </plugin>
    </gazebo>

</robot>

To calculate the volume of a robot with a complex shape we used Blender, by importing the .dae file as explained in this video: https://youtu.be/evweU2MjNjc?t=384

We would ask you to see the video on Youtube now because there it is better to explain how we deal with Blender. If you liked the post and the video, please consider subscribing to our channel so that you can be constantly learning, given that we publish new videos every day.

Thanks for watching and Keep Pushing your ROS Learning.

By watching the video, you can see the robot still rotates after calculating the volume using a 3D shape. Please consider checking the other post to understand how to make the robot stablish: https://www.theconstruct.ai/ros-tutorials-068-buoyancy-neutral-object-goes-hydrodynamics-plugin-part-3/

And below we have the video related to this post:

[ROS Q&A] 065 – YAML dump using C++

Here we show where the ROS Params are dumped when we use the “system()” function in C++ , as asked on this question: https://answers.ros.org/question/267467/yaml-dump-using-c/

Q: How do i dump my param values into a yaml file using C++? I tried system(“rosparam dump example.yaml”); but no luck..

A: ROS Params are dumped at the working directory unless you pass the /full/path/to/dump.yaml as argument, like: rosparam dump /full/path/to/dump.yaml

[ROS Q&A] Catkin workspaces – Problem copying files from one to another

Q: I copied a workspace and now catkin_make doesn’t work

A: I’m using ROS Indigo distribution, I have tried to do the same in my computer and I got the following error:

Then, I have removed the folders build and devel from catkin_02 and tried to compile again. It worked!

I don’t think the compilation problem is related to that, but it’s also important to remove any source catkin_01/devel/setup.bash instruction (maybe you have added to your .bashrc file)

[ROS Q&A] Setting the node log level in run time (C++)

In this video we show how to use C++ to dynamically change the logger level of a node. See the original question: https://answers.ros.org/question/276242/set-log-level-for-each-node-in-ros-cpp/

Q: Set log level for each node in ROS CPP

A: You basically have to use the rosconsole class of roscpp (ros.h) to change the logger level. After importing console.h you need to do the following code call: ros::console::set_logger_level(ROSCONSOLE_DEFAULT_NAME, ros::console::levels::Debug)

As you can see, the last argument indicates the level of debug you want to have for that node. After that call, you must notify, that the logger level has changed. For that, do the following:

ros::console::notifyLoggerLevelsChanged()

That is all! That simple.