Grasping Direct Ik

grasping-direct-ik

Direct-IK grasp: rotate the gripper to grasp orientation at a safe height above the target, then descend straight down, then close. No trajectory planner — works on platforms where CuRobo is not deployed, or in uncluttered scenes where planning is overkill.

Nothing here is written for one hand. The grasp rotation is composed by the connector's robot.grasp_frame from the hand's measured approach and closing axes, the fingertip floor and the hover clearance come from robot.describe_gripper / robot.describe_workspace, so the same subgraph grasps the same way on a hand that closes along tool-local x and on one that closes along tool-local y.

When to use

• The curobo tool bundle is not deployed (no collision-aware planner available).
• The scene is uncluttered enough that a straight-line approach is safe.

When NOT to use

• Cluttered scenes where the arm must thread between obstacles. Prefer grasping-with-planner if available.

Recommended subgraph state flow

The subgraph state machine the agent generates should look like (7 states):

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open → compute_grasp → refine_grasp → compute_align → rotate_align → descend → close → grasped

(grasped is the success-marker noop from sg.add_exit("grasped"), with an edge to END.)

State details:

1. open — type: tool, tool: "robot.open_gripper", inputs: { settle_steps: 40 }.

2. compute_grasp — type: tool, tool: "geometry.top_down_grasp_candidates", inputs: { obb: Ref("in.target_obb") }.

3. refine_grasp — type: script, file scripts/<sg>/refine_top_down_grasp.py (from this bundle's canonical_scripts). Inputs: grasp_pose = Ref("compute_grasp.candidates.poses.0"), target_obb = Ref("in.target_obb"). Returns grasp_pose: the candidate with its rotation rebuilt so this hand's closing axis lies across the OBB's short horizontal axis (robot.describe_gripper + robot.grasp_frame(approach=-z, close_heading_deg)), and its Z raised to the fingertip floor (support_z + finger.reach_m + finger.clearance_m) when the hand states its finger envelope. Keep this state: the candidate fan is world-aligned and a thin object's centred grasp jams the jaws on the table without the floor.

4. compute_align — type: script, file scripts/<sg>/compute_align_pose.py. Inputs: grasp_pose = Ref("refine_grasp.grasp_pose"), target_obb = Ref("in.target_obb"), optionally clearance (a literal in metres). Returns align_pose at the grasp XY and rotation, with z = max(obb_top, grasp_z) + clearance. Omit clearance (or pass 0) and the script asks robot.describe_workspace for align_clearance_m — the hand's own envelope above the fingertips. Pin it (e.g. 0.12) when the held object is what needs the room, such as a long tool that will hang below the fingertips on the way up.

5. rotate_align — type: tool, tool: "robot.go_to_pose", inputs: { pose: Ref("compute_align.align_pose") }.

6. descend — type: tool, tool: "robot.go_to_pose", inputs: { pose: Ref("refine_grasp.grasp_pose") } — the same pose compute_align was given, so hover and grasp share one rotation.

7. close — type: tool, tool: "robot.close_gripper", inputs: { settle_steps: 60 }. Edge directly from close to the grasped success marker; the subgraph's on_error: "failed" catches any raise from earlier steps. Whether the gripper actually closed on the object is checked by the target_held postcondition checkpoint (see ## Checkpoints), NOT by a re-check-and-raise node (none such exists).

   jsonCopy code
   "edges": [ ..., ["close", "grasped"], ["grasped", "END"] ],
   "conditional_edges": {},
   "exit": { "router_field": null, "success_values": ["grasped"] },
   "on_error": "failed"
   

   The lift onto a safe carry height is handled by the next transporting-objects subgraph (its waypoint_move script lifts before lateral motion); do NOT add a lift step here.

scripts/<sg>/plan_to_pose.py is the optional planned variant of a single leg: it calls curobo.plan_to_pose from the observation's joint state to a target pose and returns the trajectory (raising PlanningFailed when the planner refuses) for a platform that deploys CuRobo as a fast single-pose IK fallback without the goalset grasp planner.

Hard rules

1. Use geometry.top_down_grasp_candidates (returns candidates: {poses: list[Se3Pose]}), not geometry.top_down_grasp_from_obb (single bare pose). The refine and align-pose constructions assume compute_grasp.candidates.poses.0 exists.
2. The align_pose descends straight down with the gripper pre-rotated. Do NOT skip the compute_align + rotate_align states — a direct robot.go_to_pose to the grasp pose blends rotation and descent and twists the gripper against the object.
3. descend uses the pose compute_align was given (refine_grasp.grasp_pose). Never descend to the raw candidate after hovering at the refined rotation.

Required end states

See also

• references/design_align_then_descend.md — why pre-rotate-then-descend beats blended rotate+descend.
• scripts/refine_top_down_grasp.py — the close-axis rotation and fingertip floor, read off the live hand.
• scripts/compute_align_pose.py — the canonical align-pose construction.
• scripts/plan_to_pose.py — the optional CuRobo single-pose leg.