Question about H1 Upper boddy IK

[Proguranto]

Hi there,

Thanks so much for open-sourcing Bigym!!

I had a question about the H1 upper body IK solver.
When solving for the joint h1/left_wrist and h1/right_wrist, why do you solve for the cosine difference of up direction?
The reason I am confused is suppose I want to tilt the gripper upwards (assume the red axis is the direction the gripper is facing so I would like to rotate around the green axis) like the image:

setting this as the target position will cause the joint h1/left_wrist and h1/right_wrist only rotate the gripper around the red axis instead because it solves for the cosine difference in the up direction. Something like this:

Is this intended behavior or am I understanding this incorrectly?
Also, I tried fixing this by adding rotation actuators for the site as well in generate_ee_actuators and adding left/right wrist into self._arm_joints, does this work?

# lines 119-124
for joint in all_joints:
            if "wrist" not in joint.name.lower() and \
            (joint not in arm_joints or GRIPPER_NAME_PREFIX in joint.name.lower()):
                joint.remove()
            else:
                joint.damping = JOINT_DAMPING
                if joint.name in JOINT_LIMITS:
                    joint.range = JOINT_LIMITS[joint.name]

# lines 205-227
def _generate_ee_actuators(self, site: str, origin: str):
        x = self._model.actuator.add(
            "position",
            kp=KP,
            kv=KV,
            ctrlrange=RANGE_EE_POSITION,
            name=f"{site}_ee_x",
        )

        y = self._model.actuator.add(
            "position",
            kp=KP,
            kv=KV,
            ctrlrange=RANGE_EE_POSITION,
            name=f"{site}_ee_y",
        )

        z = self._model.actuator.add(
            "position",
            kp=KP,
            kv=KV,
            ctrlrange=RANGE_EE_POSITION,
            name=f"{site}_ee_z",
        )

        rx = self._model.actuator.add(
            "position",
            kp=ROT_KP,
            kv=ROT_KV,
            ctrlrange="-1.571 1.571",
            name=f"{site}_ee_rx",
        )

        ry = self._model.actuator.add(
            "position",
            kp=ROT_KP,
            kv=ROT_KV,
            ctrlrange="-1.571 1.571",
            name=f"{site}_ee_ry",
        )

        rz = self._model.actuator.add(
            "position",
            kp=ROT_KP,
            kv=ROT_KV,
            ctrlrange="-1.571 1.571",
            name=f"{site}_ee_rz",
        )

[chernyadev]

Thanks for the question!

Yes, the behavior is intentional. The BiGym version of H1 has only a single wrist joint, so IK is solved in two parts:

• Position is handled by the arm joints;
• Orientation is partially handled by the wrist using the up direction. This limits wrist rotation to match the target pose's orientation along one axis.

If you want full 6D control, adding rotation actuators and including the wrist joints as you did should work - it just goes beyond the BiGym (and original) H1 design.