Fix XACRO joint alignment and precision for IKFast compatibility

[oguzhanbzglu]

Description

This PR fixes the R6bot URDF to make it compatible with IKFast solver generation using tools like pyikfast. The original URDF contained joint misalignments and excessive decimal precision that prevented successful IKFast compilation.

Problem

When attempting to generate an IKFast solver for the R6bot using pyikfast, the process fails because it cannot find solution or try to solve (waited couple hours). The main causes were:

1. Excessive decimal precision: Joint origin values contain too many decimal places

   • Example: xyz="0.518777 0 0.067458" (6 decimals)
   • IKFast/OpenRAVE does not handle large decimal numbers well, as they produce very large fractions and slow down or prevent solver generation

2. Joint misalignment: Non-axis-aligned joints cause solver generation to fail

   • IKFast requires joints aligned to standard axes where possible

Changes

• Rounded joint origin XYZ positions to 3 decimal places and replaced the joint positions
  • Before: xyz="0.518777 0 0.067458"
  • After: xyz="0.519 0 0.067"
• Evaluated RPY expression simpler
  • Before: rpy="${pi/6+pi} 0 ${pi/3}"
  • After: rpy="${pi} 0 ${pi/2}" (or rpy="0 0 0" where applicable)
• Realigned joints to standard axes where geometrically valid
• Kept the original kinematic chain and joint hierarchy unchanged
• No changes to robot geometry or motion behavior

After changes mentioned above, IKFast successfully generated solution.

This is how robot joints look:

[mergify]

@oguzhanbzglu, all pull requests must be targeted towards the master development branch.
Once merged into master, it is possible to backport to jazzy, but it must be in master
to have these changes reflected into new distributions.

[mergify]

This pull request is in conflict. Could you fix it @oguzhanbzglu?

[mergify]

@oguzhanbzglu, all pull requests must be targeted towards the master development branch.
Once merged into master, it is possible to backport to jazzy, but it must be in master
to have these changes reflected into new distributions.

[mergify]

@oguzhanbzglu, all pull requests must be targeted towards the master development branch.
Once merged into master, it is possible to backport to jazzy, but it must be in master
to have these changes reflected into new distributions.

[christophfroehlich]

you change the xacro macro in line 104, but here it is OK? I don't think this is a problem. If IKfast is not capable of importing xacro, then you have to process with xacro first and pass the plain URDF.

[oguzhanbzglu]

I use this xacro and take the plain urdf from the robot_description topic to parse IKfast, but the resulting URDF isn't suitable for IKFast; I'm simplifying it.

[christophfroehlich]

but then for sure the ${pi} macros are evaluated, and no need to change this in the xacro

[christophfroehlich]

so the literal 2 in the PR description above is outdated?

[oguzhanbzglu]

Yes, sorry that was wrong description of the rpy expression (removed).
Basically, IKFast could not find solution for rpy calculations (pi/6 or pi/3) thus, I made them simpler and worked.

[christophfroehlich]

Please rebase your changes on top of master and change the HEAD of this PR.

[mergify]

This pull request is in conflict. Could you fix it @oguzhanbzglu?

[oguzhanbzglu]

This is how robot is positioned in zeros after changes.
Followed the example_7 tutorial and tested with send_trajectory.launch.py, everything works as expected.