A correct way to implement "fixed" joint?

fixed_joint

asked 2013-02-08 22:21:55 -0600

330 ●11 ●14 ●22

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?

edit retag flag offensive close merge delete

add a comment

4

answered 2013-02-08 23:45:00 -0600

nkoenig
7676 ●8 ●44 ●71 http://gazebosim.org/

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.