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Modeling remote driven link

asked 2011-09-28 20:04:27 -0500

Erik gravatar image

updated 2011-09-28 20:12:02 -0500

Hi Everyone,

I'm modeling a robot simulation using gazebo, and have a question about the parent child relationship of the links.

My robot consists of a base and 6 legs. Each leg consist of three links/joints. So lets say I have a configuration which consist of : (base – hip – upper leg – lower leg) all connected by a revolute joint. So in first place you would say that:

Base is parent of the Hip. Hip is parent of the upper leg. Upper leg is parent of the lower leg.

But in real robot it is somewhat different. Here the lower leg is remotely driven. So when the upper leg rotates about the (hip-upper leg) joint, the lower leg will only translate along with the upper leg, but does not change its orientation! w.r.t. the hip.

Here also a figure of the upper and lower leg is given, which should make the question more clear. So when theta_1 changes, vartheta_2 will remain the same. So theta_2 will change!

image description

Anyone an idea to implement this in Gazebo? Can I do something with the parent-child relations, add a massless assistant link to accomplish this, or do I have to do something totally different?

Thanks in advance!


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answered 2011-09-30 09:17:33 -0500

DimitriProsser gravatar image

If I'm correctly interpreting your problem, what you're looking at is a controls problem more than a urdf problem (at least in my opinion). My solution would be to write a controller for your joints that would be able to perform the correct calculations and issue the correct motor commands to preserve the movement that you desire. I don't think that it would be possible with pure urdf.

You can look here and follow the tutorial chain to get you started. I hope that this helps.

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answered 2011-10-10 16:19:42 -0500

hsu gravatar image

one way to solve this problem is to create a new type of joint constraint in ode that constrains the orientations of the "lower leg" relative to the "hip" or "base". Sorry there is no implementation at this time, but this should produce the right behavior I think.

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Asked: 2011-09-28 20:04:27 -0500

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Last updated: Oct 10 '11