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.

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 (6 states):

open → compute_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. compute_align — type: script, file scripts/<sg>/compute_align_pose.py (from this bundle's canonical_scripts). Inputs: grasp_pose = Ref("compute_grasp.candidates.poses.0"), target_obb = Ref("in.target_obb"). Returns align_pose.

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

5. descend — type: tool, tool: "robot.go_to_pose", inputs: { pose: Ref("compute_grasp.candidates.poses.0") }.

6. 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).

   "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.

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 align-pose construction in step 3 assumes 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.

Required end states

End state	Meaning
grasped	Gripper has closed on the object after the descend. Route to next subgraph (typically transporting-objects).
failed	Any grasp-attempt failure: planning failure or trajectory execution error (a raise to on_error). Coordinator routes to abort. Lives only in on_error — never declare a failed node.

See also

• references/design_align_then_descend.md — why pre-rotate-then-descend beats blended rotate+descend.
• scripts/compute_align_pose.py — the canonical align-pose construction.