MYRAbot's arm model for simulation (urdf+gazebo)

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Creation of URDF model

URDF (Unified Robot Description Format) is the format used to define the model of the arm (xml) in order to simulate it in gazebo. It consists of a tree of geometrical elements (links) plugged through joints(joints). This joints can be fixed or mobile, the mobile joints can be rotating, linear or floating.

First, we will create a new package with the necessary dependences for our programs (urdf brazo_fer std_msgs sensor_msgs tf roscpp). We will execute the next commands in a terminal:

cd ~/ros_workspace
roscreate-pkg brazo_fer_modelo urdf [[Control brazo MYRAbot (bioloid+arduino)|brazo_fer]] std_msgs sensor_msgs tf roscpp

We have divided the model in three files, main file, macros file and placement file. For the main file, we will create a file named "brazo.xacro" within the folder "urdf" of the created package with the content that is shown below:

<?xml version="1.0"?>

<robot>

	<include filename="$(find brazo_fer_modelo)/urdf/brazo-macros.xacro" />

<macro name="brazo" params="parent *origin">

  <joint name="fixed" type="fixed">
    <parent link="${parent}"/>
    <child link="base_brazo_link"/>
    <insert_block name="origin" />
    <axis xyz="0 0 1" />  
  </joint>
    
  <link name="base_brazo_link">
    <visual>
      <geometry>
        <box size="0.032 0.05 0.04"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 -0.0135 0.020"/>
      <material name="black">
        <color rgba="0 0 0 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 -0.0135 0.020"/>		
      <geometry>
        <box size="0.032 0.05 0.04"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0 -0.0135 0.020"/>			
      <mass value="0.055"/>
	  <default_inertia_servos />
    </inertial>       
  </link>

  <joint name="base" type="revolute">
    <parent link="base_brazo_link"/>
    <child link="hombro_link"/>
    <origin xyz="0 0 0.04" rpy="0 0 0" />
    <axis xyz="0 0 1" />
	<default_limit />
	<default_dynamics />          
  </joint>
  
  <link name="hombro_link">
    <visual>
      <geometry>
        <box size="0.04 0.032 0.05"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0.025"/>
      <material name="black" />
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0.025"/>		
	  <geometry>
        <box size="0.04 0.032 0.05"/>
      </geometry>       
    </collision> 
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0.025"/>			
      <mass value="0.055"/>
	  <default_inertia_servos />
    </inertial>        
  </link>  

  <joint name="arti1" type="revolute">
    <parent link="hombro_link"/>
    <child link="brazo_link"/>  
    <origin xyz="0 0 0.0385" rpy="0 0 0" />
	<axis xyz="1 0 0" />
	<default_limit />
	<default_dynamics />        
  </joint>
  
<!--Brazo-->

<servo nombre="brazo" />
  
  <joint name="servo_arti1_B" type="fixed">
    <parent link="brazo_link"/>
    <child link="brazo_link_B"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  

<base nombre="brazo" /> 
  
  <joint name="servo_arti1_D" type="fixed">
    <parent link="brazo_link"/>
    <child link="brazo_link_SI"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  

<soporte nombre="brazo" simetrico="1" lado="I" />  
  
  <joint name="servo_arti1_I" type="fixed">
    <parent link="brazo_link"/>
    <child link="brazo_link_SD"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
<soporte nombre="brazo" simetrico="-1" lado="D" />     
  
  <joint name="arti2" type="revolute">
    <parent link="brazo_link"/>
    <child link="antebrazo_link"/>
    <origin xyz="0 0 0.104" rpy="0 0 0" />
    <axis xyz="1 0 0" />
	<default_limit />
	<default_dynamics />            
  </joint>
  
<!--Antebrazo-->
  
<servo nombre="antebrazo" /> 
  
  <joint name="servo_arti2_B" type="fixed">
    <parent link="antebrazo_link"/>
    <child link="antebrazo_link_B"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
<base nombre="antebrazo" />
  
  <joint name="servo_arti2_D" type="fixed">
    <parent link="antebrazo_link"/>
    <child link="antebrazo_link_SI"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
<soporte nombre="antebrazo" simetrico="1" lado="I" />  
  
  <joint name="servo_arti2_I" type="fixed">
    <parent link="antebrazo_link"/>
    <child link="antebrazo_link_SD"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
<soporte nombre="antebrazo" simetrico="-1" lado="D" />     

  <joint name="arti3" type="revolute">
    <parent link="antebrazo_link"/>
    <child link="muneca_link"/>
    <origin xyz="0 0 0.104" rpy="0 0 0" />
    <axis xyz="1 0 0" />
	<default_limit />
	<default_dynamics />             
  </joint>
  
<!--Muñeca-->
  
  <link name="muneca_link">
    <visual>
      <geometry>
        <box size="0.05 0.04 0.032"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0.00625 0 0.06"/>
      <material name="black" />
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0.00625 0 0.06"/>		
	  <geometry>
        <box size="0.05 0.04 0.032"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0.00625 0 0.06"/>			
      <mass value="0.055"/>
	  <default_inertia_servos />
    </inertial>            
  </link>  
  
  <joint name="servo_arti3_B" type="fixed">
    <parent link="muneca_link"/>
    <child link="muneca_link_B"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
  <link name="muneca_link_B">    
	<visual>
      <geometry>
        <box size="0.05325 0.04 0.005"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0.004625 0 0.0415"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0.004625 0 0.0415"/>		
	  <geometry>
        <box size="0.05325 0.04 0.005"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0.004625 0 0.0415"/>		
      <mass value="0.0005"/>
	  <default_inertia />
    </inertial>              
  </link>
  
  <joint name="servo_arti3_D" type="fixed">
    <parent link="muneca_link"/>
    <child link="muneca_link_SI"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
  <link name="muneca_link_SI">    
	<visual>
      <geometry>
        <box size="0.002 0.032 0.0555"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0.021 0 0.01625"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0.021 0 0.01625"/>		
	  <geometry>
        <box size="0.002 0.032 0.0555"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0.021 0 0.01625"/>		
      <mass value="0.0005"/>
	  <default_inertia />
    </inertial>           
  </link>
  
  <joint name="servo_arti3_I" type="fixed">
    <parent link="muneca_link"/>
    <child link="muneca_link_SD"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
  <link name="muneca_link_SD">    
	<visual>
      <geometry>
        <box size="0.002 0.032 0.0555"/>
      </geometry>
      <origin rpy="0 0 0" xyz="-0.021 0 0.01625"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="-0.021 0 0.01625"/>		
	  <geometry>
        <box size="0.002 0.032 0.0555"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="-0.021 0 0.01625"/>			
      <mass value="0.0005"/>
	  <default_inertia />
    </inertial>            
  </link>
  
  <joint name="servo_arti3_P" type="fixed">
    <parent link="muneca_link"/>
    <child link="muneca_link_P"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  

<!--Pinza fija-->
  
  <link name="muneca_link_P">    
	<visual>
      <geometry>
        <box size="0.002 0.04 0.092"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0.03425 0 0.090"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0.03425 0 0.090"/>		
	  <geometry>
        <box size="0.002 0.04 0.092"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0.03425 0 0.090"/>			
      <mass value="0.001"/>
	  <default_inertia />
    </inertial>           
  </link>
  
  <joint name="servo_arti3_PS" type="fixed">
    <parent link="muneca_link"/>
    <child link="muneca_link_PS"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>
  
  <link name="muneca_link_PS">    
	<visual>
      <geometry>
        <box size="0.002 0.04 0.037"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0.03225 0 0.0575"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0.03225 0 0.0575"/>	
	  <geometry>
        <box size="0.002 0.04 0.037"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0.03225 0 0.0575"/>			
      <mass value="0.0005"/>
	  <default_inertia />
    </inertial>            
  </link>             
  
  <joint name="pinza" type="revolute">
    <parent link="muneca_link"/>
    <child link="pinza_link"/>
    <origin xyz="-0.00725 0 0.06" rpy="0 0 0" />
    <axis xyz="0 1 0" />
	<default_limit />
	<default_dynamics />     
  </joint>
  
<!--Pinza móvil-->
  
  <link name="pinza_link">    
	<visual>
      <geometry>
        <box size="0.002 0.04 0.092"/>
      </geometry>
      <origin rpy="0 0 0" xyz="-0.027 0 0.030"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="-0.027 0 0.030"/>		
	  <geometry>
        <box size="0.002 0.04 0.092"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="-0.027 0 0.030"/>		
      <mass value="0.001"/>
 	  <default_inertia />
    </inertial>           
  </link>  
  
  <joint name="servo_pinza_B" type="fixed">
    <parent link="pinza_link"/>
    <child link="pinza_link_B"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
  <link name="pinza_link_B">    
	<visual>
      <geometry>
        <box size="0.002 0.04 0.032"/>
      </geometry>
      <origin rpy="0 0 0" xyz="-0.025 0 0"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="-0.025 0 0"/>		
	  <geometry>
        <box size="0.002 0.04 0.032"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="-0.025 0 0"/>		
      <mass value="0.0005"/>
	  <default_inertia />
    </inertial>           
  </link>
  
  <joint name="servo_pinza_D" type="fixed">
    <parent link="pinza_link"/>
    <child link="pinza_link_SI"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
  <link name="pinza_link_SI">    
	<visual>
      <geometry>
        <box size="0.0375 0.002 0.032"/>
      </geometry>
      <origin rpy="0 0 0" xyz="-0.00725 0.021 0"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="-0.00725 0.021 0"/>		
	  <geometry>
        <box size="0.0375 0.002 0.032"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="-0.00725 0.021 0"/>			
      <mass value="0.0005"/>
	  <default_inertia />
    </inertial>        
  </link>
  
  <joint name="servo_pinza_I" type="fixed">
    <parent link="pinza_link"/>
    <child link="pinza_link_SD"/>  
    <origin xyz="0 0 0" rpy="0 0 0" />
	<axis xyz="1 0 0" />    
  </joint>  
  
  <link name="pinza_link_SD">    
	<visual>
      <geometry>
        <box size="0.0375 0.002 0.032"/>
      </geometry>
      <origin rpy="0 0 0" xyz="-0.00725 -0.021 0"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="-0.00725 -0.021 0"/>		
	  <geometry>
        <box size="0.0375 0.002 0.032"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="-0.00725 -0.021 0"/>			
      <mass value="0.0005"/>
	  <default_inertia />
    </inertial>            
  </link>

</macro>
     
</robot>

For the macros file, we will create a file named "brazo-macros.xacro" within the folder "src" of our package with the content that is shown below:

<?xml version="1.0"?>

<robot xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller">

<macro name="default_inertia">
      <inertia ixx="0.001" ixy="0.0" ixz="0.0" iyy="0.001" iyz="0.0" izz="0.001"/>
</macro>

<macro name="default_inertia_servos">
      <inertia ixx="0.01" ixy="0.0" ixz="0.0" iyy="0.01" iyz="0.0" izz="0.01"/>
</macro>

<macro name="default_limit">
    <limit effort="100.0" lower="-2.62" upper="2.62" velocity="3"/>
</macro>

<macro name="default_dynamics">
      <dynamics fricction="0" damping="0" />
</macro>

<macro name="servo" params="nombre">  
  <link name="${nombre}_link">
    <visual>
      <geometry>
        <box size="0.04 0.032 0.05"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0.0905"/>
      <material name="black" />
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0.0905"/>		
	  <geometry>
        <box size="0.04 0.032 0.05"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0.0905"/>		
      <mass value="0.055"/>
	  <default_inertia_servos />
    </inertial>             
  </link>
</macro>

<macro name="base" params="nombre">    
  <link name="${nombre}_link_B">    
	<visual>
      <geometry>
        <box size="0.04 0.032 0.020"/>
      </geometry>
      <origin rpy="0 0 0" xyz="0 0 0.0555"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="0 0 0.0555"/>		
	  <geometry>
        <box size="0.04 0.032 0.02"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="0 0 0.0555"/>			
      <mass value="0.001"/>
	  <default_inertia />
    </inertial>           
  </link>
</macro> 

<macro name="soporte" params="nombre simetrico lado">    
  <link name="${nombre}_link_S${lado}">    
	<visual>
      <geometry>
        <box size="0.002 0.032 0.067"/>
      </geometry>
      <origin rpy="0 0 0" xyz="${simetrico*0.021} 0 0.022"/>
      <material name="white">
		<color rgba="1 1 1 1"/>
      </material>
    </visual>
    <collision>
      <origin rpy="0 0 0" xyz="${simetrico*0.021} 0 0.022"/>		
	  <geometry>
        <box size="0.002 0.032 0.067"/>
      </geometry>
    </collision>
    <inertial>
      <origin rpy="0 0 0" xyz="${simetrico*0.021} 0 0.022"/>			
      <mass value="0.001"/>
	  <default_inertia />
    </inertial>           
  </link>
</macro>

</robot>

For the placement file, we will create a file named "brazo.urdf.xacro" within the folder "src" of our package with the content that is shown below:

<?xml version="1.0"?>

<robot name="MYRAbot-arm"
       xmlns:xi="http://www.w3.org/2001/XInclude"
       xmlns:gazebo="http://playerstage.sourceforge.net/gazebo/xmlschema/#gz"
       xmlns:model="http://playerstage.sourceforge.net/gazebo/xmlschema/#model"
       xmlns:sensor="http://playerstage.sourceforge.net/gazebo/xmlschema/#sensor"
       xmlns:body="http://playerstage.sourceforge.net/gazebo/xmlschema/#body"
       xmlns:geom="http://playerstage.sourceforge.net/gazebo/xmlschema/#geom"
       xmlns:joint="http://playerstage.sourceforge.net/gazebo/xmlschema/#joint"
       xmlns:interface="http://playerstage.sourceforge.net/gazebo/xmlschema/#interface"
       xmlns:rendering="http://playerstage.sourceforge.net/gazebo/xmlschema/#rendering"
       xmlns:renderable="http://playerstage.sourceforge.net/gazebo/xmlschema/#renderable"
       xmlns:controller="http://playerstage.sourceforge.net/gazebo/xmlschema/#controller"
       xmlns:physics="http://playerstage.sourceforge.net/gazebo/xmlschema/#physics">
	
	<include filename="$(find brazo_fer_modelo)/urdf/brazo.xacro" />
	
	<link name="world" />
	
	<brazo parent="world">
		<origin rpy="0 0 0" xyz="0 0 0"/>
	</brazo>

</robot>

Model analysis

As we have used xacro in order to simplify and to make the code more editable, we will execute the next commands in a terminal in order to obtain the URDF file processed:

roscd brazo_fer_modelo
cd urdf
rosrun xacro xacro.py brazo.urdf.xacro > brazo.urdf

We will execute the next command in a terminal to check the model:

rosrun urdf_parser check_urdf brazo.urdf

We must obtain something similar to the following:

robot name is: MYRAbot_arm
---------- Successfully Parsed XML ---------------
root Link: world has 1 child(ren)
    child(1):  base_link
        child(1):  hombro_link
            child(1):  brazo_link
                child(1):  antebrazo_link
                    child(1):  muneca_link
                        child(1):  pinza_link
                            child(1):  pinza_link_B
                            child(2):  pinza_link_SI
                            child(3):  pinza_link_SD
                        child(2):  muneca_link_B
                        child(3):  muneca_link_SI
                        child(4):  muneca_link_SD
                        child(5):  muneca_link_P
                        child(6):  muneca_link_PS
                    child(2):  antebrazo_link_B
                    child(3):  antebrazo_link_SI
                    child(4):  antebrazo_link_SD
                child(2):  brazo_link_B
                child(3):  brazo_link_SI
                child(4):  brazo_link_SD

We will execute the next command in a terminal in order to see the graphic tree generated in a .pdf file:

rosrun urdf_parser urdf_to_graphiz brazo.urdf

We must obtain something similar to the following:

Árbol de relaciones modelo URBF para el brazo del MYRAbot

Control of the joints for simulation

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