Description
ModelBuilder.add_mjcf() does not recognize the common actuator target attribute jointinparent. The actuator is silently omitted, even though SolverMuJoCo.TrnType.JOINT_IN_PARENT and the final MuJoCo enum mapping already exist.
Why this matters
For ball and free joints, jointinparent expresses the actuator gear in the parent frame. It is not equivalent to an ordinary joint transmission once the joint rotates. Dropping or downgrading it changes generalized actuator moments.
Proposed behavior
- Resolve
jointinparent through the same named-joint/DOF mapping as joint.
- Preserve
JOINT_IN_PARENT as a distinct transmission type and direct-control actuator.
- Export it as
mjTRN_JOINTINPARENT.
- Compare the resulting actuator moment against native MuJoCo at a rotated ball-joint pose.
Description
ModelBuilder.add_mjcf()does not recognize the common actuator target attributejointinparent. The actuator is silently omitted, even thoughSolverMuJoCo.TrnType.JOINT_IN_PARENTand the final MuJoCo enum mapping already exist.Why this matters
For ball and free joints,
jointinparentexpresses the actuator gear in the parent frame. It is not equivalent to an ordinaryjointtransmission once the joint rotates. Dropping or downgrading it changes generalized actuator moments.Proposed behavior
jointinparentthrough the same named-joint/DOF mapping asjoint.JOINT_IN_PARENTas a distinct transmission type and direct-control actuator.mjTRN_JOINTINPARENT.