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How is the coordinate system working in URDF

asked 2018-03-09 08:58:45 -0500

Prof. xavier gravatar image

updated 2018-03-09 09:19:40 -0500

David Lu gravatar image

I am a beginner and I can't seem to understand as I was going through some examples of URDF that how the Coordinate system is working here though, I am sure it's working normally but it's not making any sense to me. Below is an example.

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

 <link name="base_link">
  <visual>
    <geometry>
     <cylinder length = "0.01" radius="0.2"/>
      </geometry>
      <origin rpy = "0 0 0" xyz = "0 0 0"/>
      <material name = "yellow">
        <color rgba= "1 1 0 1"/>
      <material/>
     </visual>
    </link>

    <joint name = "pan_joint" type = "revolute">
     <parent link = "base_link"/> 
     <child link = "pan_link"/>
     <origin xyz = "0 0 0.1"/>
     <axis xyz = "0 0 1"/>
    <joint/>

   <link name = "pan_link">
    <visual>
     <geometry>
     <cylinder length = "0.4" radius = "0.04"/>
     <geometry/>
     <origin rpy = " 0 0 0" xyz = " 0 0 0.09"/>
      <material name="red"> 
        <color rgba="0 0 1 1"/> 
      <material/>
    <visual/>
   <link/>


    <joint name="tilt_joint" type="revolute">  
       <parent link="pan_link"/>  
        <child link="tilt_link"/>  
         <origin xyz="0 0 0.2"/>  
         <axis xyz="0 1 0" /> 
    </joint>


 <link name="tilt_link">    
   <visual>      
    <geometry> 
      <cylinder length="0.4" radius="0.04"/>   
    </geometry>     
 <origin rpy="0 1.5 0" xyz="0 0 0"/>
      <material name="green">
        <color rgba="1 0 0 1"/>
      </material>
    </visual>
  </link>
 </robot>

How are the Pan_link's and tilt's xyz is calculated ? Whats the differnce b/w origin rpy and xyz ? please help me out here

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answered 2018-03-09 09:20:20 -0500

I recommend reading through
http://wiki.ros.org/urdf/XML/model ,
http://wiki.ros.org/urdf/XML/joint ,
and http://wiki.ros.org/urdf/XML/link
if you have not already.


joints represent connections between link / reference frames:

  • origin rpy attribute: "Represents the rotation around fixed axis: first roll around x, then pitch around y and finally yaw around z. All angles are specified in radians" (Tait-Bryan xyz extrinsic rotations) relative to the parent link's reference frame (effectively the parent joint).

  • origin xyz attribute: Represents the x, y, z translation relative to the parent link's reference frame (effectively the parent joint).

links represent rigid bodies:

  • origin rpy attribute: "Represents the rotation around fixed axis: first roll around x, then pitch around y and finally yaw around z. All angles are specified in radians" (Tait-Bryan xyz extrinsic rotations) relative to the parent joint.

  • origin xyz attribute: Represents the x, y, z translation relative to the parent joint.


If you want to be confused: https://en.wikipedia.org/wiki/Euler_a...

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Comments

2

Also you can watch this guy blather on: https://www.youtube.com/watch?v=g9WHx...

David Lu gravatar imageDavid Lu ( 2018-03-09 09:21:49 -0500 )edit
1

thanks a lot, I did not the read the above links, I will read it and see it though.

Prof. xavier gravatar imageProf. xavier ( 2018-03-09 09:23:55 -0500 )edit

That video helped me when I started learning about urdf/ros!

josephcoombe gravatar imagejosephcoombe ( 2018-03-09 09:28:47 -0500 )edit

you can watch this guy blather on

@David Lu: +1 :)

gvdhoorn gravatar imagegvdhoorn ( 2018-03-09 10:07:03 -0500 )edit

Who has two thumbs and can blather on about URDF? http://itgroove.net/brainlitter/wp-co...

David Lu gravatar imageDavid Lu ( 2018-03-09 10:39:01 -0500 )edit

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Asked: 2018-03-09 08:58:45 -0500

Seen: 727 times

Last updated: Mar 09 '18