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Gravity Disabled on Robot Pinned to World

asked 2012-11-08 10:25:27 -0500

liangfok gravatar image

updated 2012-11-29 15:15:10 -0500

I have a simple robot with four links and four revolute joints. The first joint is pinned to the world. When I run the simulation, the robot is not affected by gravity. I know gravity is enabled because when I drag another object like a box and drop it into the world, it falls down. I suspect that gravity is not affecting my four-link robot because it is pinned to the world. Could this be the case? How can I enable gravity for my robot?

Update: The problem was fixed by specifying collision elements for each link in the robot. This is strange since collisions and dynamics should be decoupled. Should this be considered a bug?

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can you post your robot model? (with and without collision bodies?)

hsu gravatar imagehsu ( 2012-11-12 12:24:06 -0500 )edit

can you change your question's title to reflect the issue? i.e. dynamics disabled if missing collision element? thanks.

hsu gravatar imagehsu ( 2012-11-30 20:43:28 -0500 )edit

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answered 2012-11-30 20:42:28 -0500

hsu gravatar image

It appears if a model does not have any collision elements, it behaves statically, ticketed here.

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scpeters gravatar imagescpeters ( 2013-01-11 18:14:25 -0500 )edit

[This issue has been resolved](

scpeters gravatar imagescpeters ( 2013-01-11 18:14:32 -0500 )edit

answered 2012-11-09 13:44:57 -0500

nkoenig gravatar image

Creating a joint between a model and the world (aka pinning the model to the world), will enforce a joint constraint between the robot and world. This is usually only used if you want to the robot to remain in on fixed location.

I think you may have pinned the robot, and it looks like gravity is not working because the joint is just holding the robot in place.

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Asked: 2012-11-08 10:25:27 -0500

Seen: 652 times

Last updated: Nov 30 '12