Adding Physical Characteristics to URDF

Question

Hello all,

I am trying to write up a URDF file for my humanoid robot. So far I have followed the tutorials on ROS and written up a visual description of the robot and its manipulator arms (picture). I asked the manufacturer to send me some physical specifications for the robot arms because I intend to simulate its movements in Gazebo later on.

The specifications appear to be Solid Works files but I am unsure of how to implement these into my xml URDF file. Here are the specifications I was sent along with the first links dynamic characteristics.

Here is the rough URDF file for an estimate of the dynamics. (Sorry for the ugly coding, it's my first urdf/xml file)

So I was wondering, what is the best way to implement the physical characteristics into my urdf file. I know there are SW2URDF plugins but I don't know if I can convince the manufacturers to install this and send it to me. Is there another way to add this information so I can have a better model to simulate with in Gazebo.

Kind Regards, Martin

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Originally posted by MartinW on ROS Answers with karma: 464 on 2013-01-08

Post score: 0

Answer 1

For your first pass, I wouldn't worry terribly about getting the moment of inertia numbers correct. You can add in masses (remember to convert to kg), and have some approximate value and it should be ok.

For URDF, the MOI matrix needs to be taken at the center of mass of the link aligned with the link's coordinate system. For most links, the link origin is the same as the joint origin that connects this link to its parent. For the base link, it's the standard ros/gazebo coordinate system.

To take the MOI numbers from the screen grab, you will need to know the coordinate system that they were taken in reference too. For example, if the output coordinate system was just the full assembly origin coordinate system, then you would transform the MOI matrix from that origin to the link's origin.

Is that clear as mud?

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Originally posted by brawner with karma: 581 on 2013-01-08

This answer was ACCEPTED on the original site

Post score: 1

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Original comments

Comment by MartinW on 2013-01-08:
Oh okay, I was hoping I could use approximate values and just make really slow movements to keep the torques low. I know from specifications that the links origins are w.r.t "Reference Frame" for the c.o.m.s but I don't know if the output frame is the finger tips or what. I will ask though. Thank u!