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A correct way to implement "fixed" joint?

asked 2013-02-08 22:21:55 -0500

Boris gravatar image

Am I right that there is no fixed joint type in SDF similar to one in URDF? So how should I implement one?

In one of the tutorials Hokuyo laser is attached to a mobile platform with revolute joint, but with zero limits:

<joint name="hokuyo_joint" type="revolute">
  <child>hokuyo::link</child>
  <parent>chassis</parent>
  <axis>
    <xyz>0 0 1</xyz>
    <limit>
      <upper>0</upper>
      <lower>0</lower>
    </limit>
  </axis>
</joint>

Same approach is used in Simple Arm model, to attach it to the ground.

Although it seems ok, but there is an extra joint control on Joints pane which of course does not control anything. Also there is an extra coordinate frame marker in scene.

So is it the only way to implement fixed joints?

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2 Answers

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4

answered 2013-02-08 23:45:00 -0500

nkoenig gravatar image

A fixed joint does not exist in simulation. The URDF parser auto-magically eliminates fixed joint by restructuring the URDF contents.

There are two ways to attach two models together:

1) Use a revolute joint with zero limits.

2) Include a model inside another (this is the preferred method because it doesn't introduce an additional constraint on the system). Take a look at worlds/model_nesting.world as an example.

There is only one way to attach a model to the world (which means it's fixed at its location of creation), and this is through a joint.

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Thanks for clarification!

Boris gravatar imageBoris ( 2013-02-09 00:10:01 -0500 )edit

model_nesting is in the deprecated folder now, is there another solution to this?

Andrew Hundt gravatar imageAndrew Hundt ( 2014-03-21 13:15:00 -0500 )edit

At the moment only a revolute joint with zero limits between two models is supported.

nkoenig gravatar imagenkoenig ( 2014-03-25 13:34:55 -0500 )edit
6

answered 2015-10-15 02:21:15 -0500

silvio.traversaro gravatar image

For the people finding this question on Google: fixed joints are directly supported in Gazebo since Gazebo6 , using the "fixed" joint type.

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Comments

+1 for raising this to attention (although `fixed` doesn't seem to be available on Model Editor 6.4 yet).

yah130s gravatar imageyah130s ( 2015-10-21 18:44:32 -0500 )edit

Thank you very much for pointing that out!

Dennis Leroy Wigand gravatar imageDennis Leroy Wigand ( 2016-01-28 05:11:19 -0500 )edit

The model editor (8.0 version) does support inserting fixed joint. I tested it with the gazebo8 nightly.

Jose Luis Rivero gravatar imageJose Luis Rivero ( 2016-05-02 17:44:48 -0500 )edit

The model editor (8.0 version) does support inserting fixed joint. I tested it with the gazebo8 nightly package.

Jose Luis Rivero gravatar imageJose Luis Rivero ( 2016-05-02 17:45:08 -0500 )edit
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Asked: 2013-02-08 22:21:55 -0500

Seen: 6,983 times

Last updated: Oct 15 '15