Apply mimic constraint to joints

[adityapande-1995]

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🎉 New feature

Summary

This PR picks up the mimic joint tag from sdf::JointAxis, and aplies it to the relevant gz::physics joint.
Work in progress !

Test it

Test with the example world included in this PR:

gz sim -v 4 mimic_fast_slow_pendulums_world.sdf \
  --physics-engine gz-physics-bullet-featherstone-plugin

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

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[scpeters]

After finally testing this pull request, I've noticed that dartsim's MimicMotorConstraint is not behaving the same way as the Gearbox joint in gazebo-classic. While the gearbox joint creates a bilateral constraint that applies equal and opposite impulses on the connected joints, the Mimic constraint appears to be unilateral, in that it only applies impulses to the follower joint and does not affect the leader joint. This can be seen in the following animation from the mimic_pendulum_world.sdf that I added to this branch in 554f99d

In this example world, there are three pairs of pendulums. Each pair of pendulums have different lengths, and thus different oscillation frequencies. The middle pair of pendulums are uncoupled, so you can see how they oscillate at different frequencies, and their motions diverge over time. On the left side, the large pendulum mimics the small pendulum with a multiplier of 1.0 and follows it exactly. On the right side, the small pendulum follows the large pendulum with a multiplier of 1.0 and also follows it exactly. With a gearbox joint, I would expect the pendulums to both oscillate at an in-between frequency.

mimic_pendulum.mp4

The behavior of dartsim's MimicMotorConstraint is not wrong, but we need a slightly different constraint to replace the gearbox functionality from gazebo-classic.

[scpeters]

I've opened a feature request in dartsim/dart#1756 to add support for a bilateral linear constraint between joints

[scpeters]

I think this is ready for review

[scpeters]

retargeted to harmonic / main branch

[codecov]

Codecov Report

Merging #1838 (3fcad36) into main (5846393) will decrease coverage by 0.04%.
Report is 1 commits behind head on main.
The diff coverage is 43.47%.

❗ Current head 3fcad36 differs from pull request most recent head 122de88. Consider uploading reports for the commit 122de88 to get more accurate results

@@            Coverage Diff             @@
##             main    #1838      +/-   ##
==========================================
- Coverage   65.32%   65.28%   -0.04%     
==========================================
  Files         321      321              
  Lines       30303    30326      +23     
==========================================
+ Hits        19795    19799       +4     
- Misses      10508    10527      +19     

Files Changed	Coverage Δ
src/systems/physics/Physics.cc	71.84% <43.47%> (-0.41%)	⬇️

... and 1 file with indirect coverage changes

[azeey]

Works for me! Just one minor comment

[azeey]

Since this currently only works with bullet-featherstone, can you provide the command to run here?

[scpeters]

I added the example command in e9b22fc

I noticed that lots of our other example worlds are using command-line strings with -- inside XML comment blocks, which is invalid XML, so I put the example command inside a CDATA block. Now xmllint is happy with the file, and I believe it still works.

[scpeters]

I just merged with main to get the gz-msgs fix. Fingers crossed for good CI...

[scpeters]

I just merged with main to get the gz-msgs fix. Fingers crossed for good CI...

I think we still need #2100 to land

[azeey]

I just merged with main to get the gz-msgs fix. Fingers crossed for good CI...

I think we still need #2100 to land

Yup