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joint orientation (rotation)

asked 2013-09-28 10:22:37 -0600

Arn-O gravatar image

Hello.

It looks like the joint rotation is given with the roll, pitch and yaw in this order:

http://wiki.ros.org/urdf/XML/joint

And too bad, the document of the robot I'm working on is in the ypr order. And moreover, this last oder looks more common, as I found a lot of documentation based on this convention.

How to convert ypr to rpy?

Why has it been changed for ROS? Am I the only one to be disturbed by this?

Thanks in advance for the support.

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answered 2013-09-28 23:27:24 -0600

DamienJadeDuff gravatar image

Is your documentation using a fixed axis or rotating axis (extrinsic or intrinsic rotation?)

The URDF documentation is using a fixed axis rotation. This is described in more detail here: REP 103

Also, here: Rotation Methods (ROS)

If your documentation is using rotating axis (intrinsic rotation) there is no problem as ypr on a rotating axis is same as rpy on a fixed axis.

Otherwise:

The easiest way might to convert rotations between Euler angles is to use an intermediate representation like a quaternion.

In ROS if you are using Python, you can use the transformations.py module mentioned here (with a link to the full source): transformations.py

Example Python session using this (first way is with quaternions, second with matrices):

$ python
Python 2.7.3 (default, Jul  5 2013, 08:52:38) 
[GCC 4.6.3] on linux2
Type "help", "copyright", "credits" or "license" for more information.
>>> import transformations
>>> import math
>>> q=transformations.quaternion_from_euler(math.pi,0,0,'szyx')
>>> q
array([  0.00000000e+00,  -0.00000000e+00,   1.00000000e+00,
        6.12303177e-17])
>>> transformations.euler_from_quaternion(q,'sxyz')
(0.0, -0.0, 3.141592653589793)
>>> m=transformations.euler_matrix(math.pi,0,0,'szyx')
>>> m
array([[ -1.00000000e+00,  -1.22460635e-16,  -0.00000000e+00,
        0.00000000e+00],
    [  1.22460635e-16,  -1.00000000e+00,   0.00000000e+00,
        0.00000000e+00],
    [ -0.00000000e+00,   0.00000000e+00,   1.00000000e+00,
        0.00000000e+00],
    [  0.00000000e+00,   0.00000000e+00,   0.00000000e+00,
        1.00000000e+00]])
>>> transformations.euler_from_matrix(m,'sxyz')
(0.0, 0.0, 3.141592653589793)
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Note that fixed axis rotation can mean in general about any axis, but in the ROS documentation I linked to it is referring to 3 rotations around fixed axes x y and z ('sxyz' in transformations.py). This should be the same as 'rzyx' in transformations.py).

DamienJadeDuff gravatar imageDamienJadeDuff ( 2013-09-29 23:38:01 -0600 )edit
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answered 2013-09-29 05:12:18 -0600

Arn-O gravatar image

Hello.

Great answer!

Thanks to you, I have understood a couple of things that I did not get during the readings:

  • extrinsic rotations are independent (that's what I have seen with a couple of example and that was only an assumption)
  • the yrp notation does not imply extrinsic or intrinsic, I understood that it was always extrinsic, but that was wrong

Thanks also for the python tools. I use python a lot, so this is really helpful.

So I had a fresh look at the documentation and the description is given using intrinsic rotations and that explain why I was not able to use the values directly in ROS. I should be able to manage it now.

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Arno, I am unsure what you mean by independent. You should see that `m=transformations.euler_matrix(0,math.pi,math.pi,'sxyz')` is the same as `m2=transformations.euler_matrix(math.pi,0,0,'sxyz')` for example (try `m2[numpy.abs(m2)<0.001]=0` and `m[numpy.abs(m)<0.001]=0` to clean them up).

DamienJadeDuff gravatar imageDamienJadeDuff ( 2013-09-29 23:32:13 -0600 )edit

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Asked: 2013-09-28 10:22:37 -0600

Seen: 1,690 times

Last updated: Sep 29 '13