Fix for issue #49: add 'base' link (transform to World)

[Levi-Armstrong]

This should not affect existing kinematic plugins or MoveIt configurations:

• the link is not part of the main kinematic chain
• the transform is implemented as a fixed joint

[gavanderhoorn]

According to what I can find this is correct for ABB robots, so +1, and thanks for this.

Perhaps we should clarify that even though ABB has a coord system called Base, it is not necessarily the same as this base link in the urdf. If ABBs Base is modified (if that is even possible), users should not start modifying the base link, I think. They should introduce an additional link + joint. Or maybe they should, I'm not sure.

[shaun-edwards]

@Levi-Armstrong, how did you verify the location and orientation of ABB's robot base. Fanuc and Motoman have odd locations for their base frames. They are not where you would logically expect them (for mounting anyway).

[Levi-Armstrong]

The location of the base is at the same location and orientation of the base frame on an ABB robot. The description I found has it positioned at the base of the mounting face where the origin is coincident with Joint 1 axis along with x-axis forward and z-axis is up. Also from what documentation I can find this can not be changed but ABB also has a World CS which by default is the same as base but can modified. This is opposite of a Fanuc robots which have a World CS (I believe this is what we are considering to be the base in the URDF) which can not be change in the Fanuc controller. I verified the URDF by locating the base frame within RobotStudio and compared it to the rviz output after implementation.

[shaun-edwards]

+1....and +1 for ABB putting it in the right spot.