---
license: apache-2.0
task_categories: [robotics]
tags: [vla, robotwin, smolvla, vla-mpc, grasp-detection, holding, empty-close]
pretty_name: VLA-Framework holding / empty-close detection investigation (move_pillbottle_pad)
---

# vla_framework — empty-close ("抓空") detection investigation

Does the gripper **empty-close** grasp-failure flag misfire on every grasp, or
only on real failures? Investigated on `move_pillbottle_pad` (SmolVLA,
`pose_source=robotwin_gt`, MPC corrector, `chunk_exec=50`).

## TL;DR

**On SmolVLA `move_pillbottle_pad` the empty-close detector has zero misfires.**
Every grasp is bimodal: a real grasp lands ~40 N on both jaws (`holding`
confirmed, `empty=0`), a miss lands **0.0 N** (gripper closes on air, `empty=1`,
correct). There is no "held-but-judged-empty" middle case. The grasp-instant
red `[C] holding` people see is `holding` **not yet confirmed** (needs a firm
two-jaw ≥2 N contact) — a separate signal from the empty flag, and the MPC then
engages via **REPLAN** (phase stall), not via empty.

## Exact setup (for config alignment)

- Repo commit: **`b524368`** (`feat(robotwin): optional empty-close detection, default OFF`)
  - chain: `51946ea` fingers-met gate → `54802e8` MPC-engage logging → `b524368` toggle
- Env: conda `vla_framework`; `MUJOCO_GL=egl`, `HF_HUB_OFFLINE=1`, `CUDA_VISIBLE_DEVICES=1`
- Config: `config_used/vla_framework_eval.template.yaml` (unmodified template)
- Command:
  ```
  python scripts/eval_robotwin.py \
    --config config/vla_framework_eval.template.yaml --task move_pillbottle_pad \
    --seed <SEED> --num_episodes 1 --max_steps 200 \
    --model_type smolvla --checkpoint weights/smolvla/robotwin_move_pillbottle_pad_50ep_multi \
    --chunk_exec 50 --no_poselib --contact_trace [--empty_close_detection]
  ```
- `empty_close_detection` defaults **OFF** (b524368); pass `--empty_close_detection` to re-enable.

## Detection logic (RoboTwin adapter, `_derive_grasp_flags`)

- `commanded_closed` = gripper target ≤ 0.05
- `contact_grasp` = **both** jaw fingers bear ≥ **2.0 N** on the **same** object
- `raw_holding` = `contact_grasp & commanded_closed`
- `raw_empty_kin` = `commanded_closed & (finger_opening ≤ 0.05)`  (fingers met)
- `raw_empty` = `~raw_holding & raw_empty_kin`  ← fingers-met gate (fix `51946ea`)

Both debounced 2 ticks. `empty_close` feeds the `holding` predicate's −1.0
empty penalty. The `b524368` toggle zeroes the *consumed* `empty_close` (raw
signals stay in the trace).

## 15-seed result — perfectly bimodal, no misfire

| outcome | seeds | contact force | held ticks | empty ticks | MPC engage |
|---|---|---|---|---|---|
| **real grasp** (SUCCESS) | 100000, 100004, 100006, 100008, 100010 | **39–46 N** | 67–78 | **0** | 0 |
| **miss** (FAIL) | 100001/2/3/5/7/9/11 | **0.0 N** | 0 | ~50 | 1 (REPLAN) |

- `seed 100000` (default, empty OFF): closes @tick 60 with **42 N**, `holding`
  confirms @tick 61, step advances (NEXT), `empty=0`, MPC never engages — see
  `videos/pad_seed100000_*` (frame 62: `[S] holding +1.000`, green).
- `seed 100001/100002`: gripper closes with **0.0 N**, raised empty — see
  `videos/pad_seed100002_frame85_empty_gripper.png`. `empty=1` is correct.

The original held-object false positive (`place_bread_skillet` seed 100000 →
226 spurious CORRECTs on a held skillet, SUCCESS→FAIL) was the real bug, fixed
by the fingers-met gate (`51946ea`).

## pi0 + MPC — the genuine "held but unconfirmed" reproduction

SmolVLA grasps land ~40 N so `holding` always confirms. **pi0** on
`place_container_plate` (seed 100017, `pi0_fk_mpc`) does NOT:
`videos/pi0_seed100017_*` shows the gripper grasping and **lifting/carrying the
container** (the container bbox tracks the EEF) while `[C] holding` stays at
**−0.001** (the *unconfirmed* placeholder `_HOLDING_UNCONFIRMED_MARGIN`, red) —
the grasp step never advances and the MPC keeps engaging.

Note this is **not** the empty flag: the container holds the fingers open, so
`empty_kin = 0` and the empty toggle is irrelevant. It is `contact_grasp = 0` —
the detector requires **both** jaws ≥ 2 N on the same body, but pi0 grips the
thin container rim lightly / one-sided, so the grasp is never confirmed.

Observed model/corrector dependence (same case, seed 100017):

| setup | grasp-time MPC intervention / detection anomaly |
|---|---|
| pi0 + MPC | **yes** — holding never confirms, MPC engages at grasp |
| pi0 + passthrough | **no** anomaly |
| smolvla + MPC | **no** (firm ~40 N grasp confirms) |

That pi0+passthrough is clean but pi0+MPC is not points at the **MPC corrector
perturbing the grasp/gripper in the FK→ee path** (under investigation), on top
of the contact-confirmation (2 N, both-fingers) being too strict for pi0's
light rim grasp. Likely fix directions: lower `contact_force_threshold`, relax
`require_both_fingers`, and check the corrector isn't smoothing the gripper dim.

## Contents

- `config_used/` — the exact config used (align yours against this)
- `videos/pad_seed100000_success_grasp.mp4` + `frame62_holding_confirmed.png` — clean grasp, holding green
- `videos/pad_seed10000{1,2}_fail_grasp.mp4` + `frame85_empty_gripper.png` — real misses
- `contact_traces/seed*.csv` — per-tick contact force / grasp / holding / empty