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How to change the pivot point of a link in URDF joint?

asked 2018-08-19 05:20:53 -0500

Kishore Kumar gravatar image

I have built simple robotic arm, where the Link1 is joined to the base_link through a revolute joint. When i try to move the link1 it is revolving on its own center instead pivoting the bottom to base_link. Here, how to change the pivot point from the center of link1 to its bottom and fix it to the base_link.

My urdf:

<?xml version="1.0"?>
<robot name="myfirst" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <link name="base_link">
    <visual>
      <geometry>
        <box size="0.5 0.5 0.5"/>
      </geometry>
    </visual>
  </link>

<joint name="joint1" type="revolute">
 <axis xyz="0 1 0"/>
<limit effort="1000.0" lower="-0.548" upper="0.548" velocity="0.5"/>
    <parent link="base_link"/>
    <child link="arm2"/>
    <origin xyz="0 0 0.5" rpy="0 0 0" />
  </joint>

<link name="arm2">
<origin xyz="0 0 -0.3" rpy="0 0 0" />
    <visual>
      <geometry>
        <cylinder length="0.6" radius="0.04"/>
      </geometry>
    </visual>
  </link>

</robot>

Result initial pose:

image description

Result end pose:

image description

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2 Answers

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answered 2018-08-19 06:41:30 -0500

gvdhoorn gravatar image

updated 2018-08-19 06:41:47 -0500

A cylinder has its origin in the centre of the shape (all primitive geometry actually). That is why you're seeing the rotation as it is.

If you want the cylinder to rotate at one of its ends, you'll have to translate it half its own length up.

Something like:

<origin xyz="0 0 0.3" rpy="0 0 0" />
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Thank you so much that helped!

Kishore Kumar gravatar imageKishore Kumar ( 2018-08-19 07:26:45 -0500 )edit
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answered 2018-08-19 07:30:49 -0500

Kishore Kumar gravatar image

The origin must be inside the Visual tag, but i had it outside the tag and iterated the origin which didn't help. I added the origin tag inside visual tag as below and changed the origin to fix the issue.

    <?xml version="1.0"?>
    <robot name="myfirst" xmlns:xacro="http://www.ros.org/wiki/xacro">
      <link name="base_link">
        <visual>
          <geometry>
            <box size="0.5 0.5 0.5"/>
          </geometry>
        </visual>
      </link>

    <joint name="joint1" type="revolute">
     <axis xyz="0 1 0"/>
    <limit effort="1000.0" lower="-0.548" upper="0.548" velocity="0.5"/>
        <parent link="base_link"/>
        <child link="arm2"/>
        <origin xyz="0 0 0.5" rpy="0 0 0" />
      </joint>

    <link name="arm2">

        <visual>
<origin xyz="0 0 0.3" rpy="0 0 0" />
          <geometry>
            <cylinder length="0.6" radius="0.04"/>
          </geometry>
        </visual>
      </link>    
    </robot>
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It would be nice to have the figures of the result to see the final origin. Somehow I cannot replicate your results.

hbaqueiro gravatar imagehbaqueiro ( 2018-11-26 06:56:25 -0500 )edit

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Asked: 2018-08-19 05:20:53 -0500

Seen: 383 times

Last updated: Aug 19 '18