Support of closed kinematic chains

[osrf-migration]

Original report (archived issue) by Diego Ferigo (Bitbucket: dgferigo).

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Doing some experiments with the SDF of Minitaur quadruped (taken from KodlabPenn/kodlab_gazebo), I noticed that the current state of the dart physics plugin does not yet support closed kinematic chains (parallel robot).

The four legs on Minitaur are composed of a five-bar linkage. The SDFs hosted in the GitHub repo work well in Gazebo 10 Classic. However, the simulation of the model in Ignition Gazebo does not apply the constraint to keep the linkage closed, and the links forming the legs fall apart as soon as they touch the ground.

I found this TODO in the code, therefore I open this issue to track the status of the missing support.

[peci1]

I vouch for resolving this issue. It is in SDF spec that the models can contain closed kinematic chains.

Until this is resolved, I'd expect a note on https://ignitionrobotics.org/docs/citadel/comparison#physics or https://ignitionrobotics.org/docs/all/release-features.

[peci1]

According to dartsim/dart#465 and https://dartsim.github.io/#dynamics it should be possible with DART, maybe with some workarounds.

Other possibly useful resources: https://dartsim.github.io/tutorials_collisions.html.

[peci1]

We have a possible use for closed kinematic chains in the SubT virtual challenge (with the Absolem robot).

[chapulina]

I'd expect a note on

@peci1 , we'd appreciate a PR against the docs adding more rows to the comparison table with high-level features that you identify as missing. Thanks!

[peci1]

I'd expect a note on

@peci1 , we'd appreciate a PR against the docs adding more rows to the comparison table with high-level features that you identify as missing. Thanks!

gazebosim/docs#61

[benediktkreis]

Is this feature going to be implemented soon (e.g. in Garden)? This would help to simulate parallel kinematics e.g. for grippers such as the Robotiq 2F-85 or the OnRobot RG2.

[benediktkreis]

Are there any updates on this feature?

In Gazebo Classic there is the possibility to model 2-finger grippers with closed kinematic chains or use the mimic joints plugin (see here, here and here). Unfortunately, both doesn't work in Ignition Gazebo. That is why it would by of great help if there was the possibility to model closed kinematic chains in Ignition Gazebo.