Placing Held Feature

placing-held-feature

The place stage's geometry: where must the hand be so that the thing it is holding arrives where the goal wants it.

Why this is its own skill

Grasping has a whole ladder of strategies and placing had none, which is backwards for any goal phrased as a relation between a held object's feature and a fixture. The composition is small and the mistake is not:

feature_now = object_position + R(object_rotation) · feature_local
move        = target − feature_now
hand_goal   = hand_now + move

Without it the obvious move is to drive the hand at the target, and that is wrong by the length of the tool. It fails as a refusal rather than as a miss, so it reads like an unreachable workspace rather than a mis-specified goal — a diagnosis that costs iterations, because the fix it suggests (try the other arm, give up on the target) is not the fix.

When to use

• A goal written as object.feature at fixture.feature or in a region: a loop over a rod, a shaft into a hole, a plug at a bore.
• Any time the held object is large relative to the hand.

When NOT to use

• To orient. This translates only; turn first, then mate.
• Before the object is held. It reads the object's live pose, which before a grasp is wherever it is lying.

Recommended subgraph state flow

locate → orient → mate → plan → execute → verify

1. locate — the target point. For a welded fixture with no body and no OBB, that is sim.query on the goal's own atom and refs; its at is the feature's world position.
2. orient — turn the held object so the mating axis lines up, as a locked motion.plan_joint at the current position. Transit first, then turn.
3. mate — script: scripts/<sg>/mate_feature.py. Route on route.
4. plan/execute — motion.plan_linear(end=Ref("mate.hand_goal"), orientation="lock") then robot.execute_trajectory. Locked, because the orientation you turned to is the one that must survive the move.
5. verify — re-run mate and read distance_m. Shrinking means the round worked; flat means the move is not being executed as planned.

Use stop_short_axis/stop_short_m when the fixture is mounted flush to a board: close the two free axes fully and stop short on the board's normal, so the approach does not drive the load into the mounting.

Required end states