Add force offset support to ApplyLinkWrench system and to Link API

[Henrique-BO]

🎉 New feature

• Builds on top of New Apply Link Wrench system #1593

Summary

Adds support for specifying the application point of a force in the ApplyLinkWrench system. This is done through an offset expressed in the link frame (thus, relative to the link origin).

This uses the force_offset field that was already present in the Wrench message, but wasn't being used in the system. Also, an overloaded method AddWorldWrench was added to the Link interface.

Test it

Open a test world that has the ApplyLinkWrench system:

gz sim src/gz-sim/examples/worlds/apply_link_wrench.sdf

Send a message to the /world/<world_name>/wrench topic:

gz topic -t "/world/apply_link_wrench/wrench" -m gz.msgs.EntityWrench  -p "entity: {name: 'cylinder::link', type: LINK}, wrench: {force: {z: 100}, force_offset: {x: 0, y: 100, z: 0}}"

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

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #2026 (5bba53c) into gz-sim7 (e19f26d) will increase coverage by 0.00%.
The diff coverage is 96.42%.

❗ Current head 5bba53c differs from pull request most recent head f36ac69. Consider uploading reports for the commit f36ac69 to get more accurate results

@@           Coverage Diff            @@
##           gz-sim7    #2026   +/-   ##
========================================
  Coverage    65.07%   65.08%           
========================================
  Files          354      354           
  Lines        28720    28741   +21     
========================================
+ Hits         18690    18705   +15     
- Misses       10030    10036    +6     

Files Changed	Coverage Δ
include/gz/sim/Link.hh	100.00% <ø> (ø)
src/systems/apply_link_wrench/ApplyLinkWrench.hh	100.00% <ø> (ø)
src/systems/apply_link_wrench/ApplyLinkWrench.cc	78.83% <88.88%> (+1.05%)	⬆️
src/Link.cc	95.34% <100.00%> (+0.57%)	⬆️

... and 2 files with indirect coverage changes

[ahcorde]

can you add a test to this new method?

[Henrique-BO]

Added a test in 34101f7. Since there already was a LinkAddWorldForce test doing very similar checks and setup, I just added a few lines to the existing function. Should we keep it like this (and so maybe rename the test) or is it better to have a separate test?

[azeey]

@osrf-jenkins run tests please

[quarkytale]

Works for me

[azeey]

This implies that the offset in msgs::EntityWrench and by association, in the component::ExternalWorldWrench. is relative to the COM, but the point of application is the link origin. Am I right? If so, I think we should avoid having two different reference points. If the point of application is the link origin, the offset should also be from the link origin.

[Henrique-BO]

Changed in ee693bc so that the Link API and the ApplyLinkWrench system interpret the offset in the link frame. This way, the semantics of the offset are consistent between them, the msgs::EntityWrench message and the component::ExternalWorldWrench component.

[azeey]

The windows node was killed after building for 3 hours (https://build.osrfoundation.org/job/ign_gazebo-pr-win/5927/). Going to merge without it. /cc @j-rivero.

Feedback has been addressed.