graph-robots selecting-reachable-grasp
poses is a list of Se3Pose, best first -- a propose skill's poses
output, or the poses field of a GraspCandidates. It is declared here in
prose rather than under required_inputs because the registry has no name for
a bare list of poses, and a GraspCandidates is a dict the script does not take.
One strategy for the select stage: reachability, then clearance, first match wins.
Why this is its own skill
A proposer ranks by the object's geometry, which is the only thing it knows. Whether the arm can hold a pose is a different question, and entry 0 is routinely the one it cannot. Nothing downstream notices — IK returns something, the servo drives to it, and the jaws close on air. A study packet put it exactly: "gripper closed on nothing while grasping ring_spanner_19".
Taking the first that passes rather than the best: the list arrives ranked by a proposer that knows things about the object this skill does not, and re-ranking here would silently overrule it.
When to use
- After every propose rung. There is no case where taking candidate 0 unchecked is better than walking the list.
When NOT to use
- As a substitute for a collision-aware planner on the carry. This gates the grasp configuration, not the path to it. The two are different halves: a corridor test refuses a grasp whose descent goes through a shelf; a planner routes the carry around a beam. Neither substitutes for the other.
Recommended subgraph state flow
select → (selected | none_reachable)
A single script node. Inputs: poses (built from the proposer's stations and
robot.grasp_frame's rotation), arm_id, object_name, check_clearance.
Router on route.
Downstream, execute the selection with
motion.plan_linear(orientation="lock") → robot.execute_trajectory so the
closing heading is actually held — robot.go_to_pose leaves the wrist roll
free and discards it.
Required end states
| End state | Meaning |
|---|---|
selected |
pose and joint_config. Read index to see how far down the list it had to go, and corridor_checked to see whether clearance ran. |
none_reachable |
Nothing in the list solved. Route to another propose rung, not back to this one. |