ROS Resources: Documentation | Support | Discussion Forum | Service Status | Q&A answers.ros.org

Naming standards for joints in humanoid robots


#1

Hi,

It was very quiet in this SIG in the last month, so I will try to start a new discussion.
I was trying to use the same code on different humanoid robots with the same 20-DOF joint configuration. The problem was that the URDFs were very differently structured and named. So I tried to find out what the standards are in ROS.
All I could find is REP 120 [1], which specifies the naming for some links and a general frame hierarchy. This is nice, but it doesn’t specify the naming for the joints.
I had a look at some of the URDFs of humanoid robots and the names differ a lot. There are different writing styles and different names for the joints, e.g. HeadPan and neck_yaw is the same thing.
Is there any specification that I missed, or would it maybe be a good idea to make a new REP?

[1] http://www.ros.org/reps/rep-0120.html


#2

First of all HeadPan feels awkward to me, it goes completely against all the naming conventions in ROS.

At the same time, both alternatives you presented are names that encode the axis of rotation the joint performs given some arbitrary reference system.
To further elaborate on this I’d highlight the importance of the convention you use to build your TF tree/URDF.
There are many styles, and I’m going to be probably wrong with the names:

  • Denavit-Hartenberg, where the angle of rotation is around Z and position of X and Y determine the positive direction
  • (my naming) The Japanese convention is the same as Denavit-Hartenberg but they rotate around the Y axis
  • (my naming) The American convention is to assign frames to joints such that all joint frames align with a flat right-handed coordinate frame when the robot is in an arbitrary zero state.

The namings you proposed go well with the American convention that I’ve seen in Valkyrie and the PR2.
I’d however advise for using the Denavit-Hartenberg frame convention as it’s an industry standard plus it gives you freedom over naming, you can number them in the order they appear in a kinematic chain, with REEM-C we used arm_left_1_joint, arm_left_2_joint, arm_left_3_joint, etc. My personal preference is the latter, while with the head you are kind of ok (already weird on Valkyrie, where there are 2 tilt joints) arms will give you a hard time: arm_left_yaw, arm_left_pan, arm_left_pitch you’ve already ran out of meaningful names!

References:
http://www.ros.org/reps/rep-0103.html#axis-orientation
http://wiki.ros.org/Robots/REEM-C