Sim fixes, batch 1: clean starts, honest grasps — all four findings closed

2026-08-11 18:4x–19:xxZ work session. Executes the fix list from the sim review (findings) and the contact-fidelity lit page (named mechanisms), items 2–4 of that page’s pre-reg-order list. This unblocks the sim-policy-eval-100seeds protocol pre-reg; servo sysid (item 1) is queued next as its own work item.

Plain words. Yesterday’s audit found four problems with our robot simulator: the arm physically couldn’t fold into the pose real episodes start from, the arm sometimes smacked the boat while the scene was being set up, the boat wore an invisible ~4 mm “force field” that let the gripper shove it before visibly touching it, and a gripped boat pivoted in the jaws far too easily. All four are now fixed and re-measured with the same probes that found them: episode starts are identical across all 100 candidate seeds (zero setup collisions), the force field is down to under half a millimeter, and the boat now barely rotates in a closed gripper (6.9° → 0.4°). One regression appeared along the way — the finer collision skin made the resting boat creep across the table — and was traced to the physics solver being given too small an iteration budget, not to the new skin; raising the budget fixed it at zero speed cost.

What changed (all measured, before → after)

Fix 1 — the start state: three layers deep

The review blamed the unreachable home pose on the wrist-camera mount. That was layer one of three:

1. camera_box2 ↔ shoulder: the mount’s collision box wedged 0.46 mm into a shoulder geom, pinning elbow/wrist_flex/wrist_roll at the ±2.94 N·m force limit. The real arm demonstrably folds into this pose (every recorded episode starts there), so the scene now excludes that contact pair (follower + leader). This alone fixed wrist_flex and the seed-dependent wrist_roll bimodality; elbow error dropped 19.9° → ~9.6°.
2. wrist ↔ shoulder: with the mount pair gone, the wrist link’s own collision geom wedged 0.87 mm into the same shoulder geom — same over-approximation class, same evidence, same fix. Elbow error → 7.7°.
3. The model couldn’t represent the rig’s start state at all: menagerie’s shoulder_lift range is ±100° but the rig’s median episode start is −102.7° (half the real episodes start beyond the sim’s limit); elbow_flex 97.0° vs range ±96.8°. Both ranges are widened at load (runtime patch, vendored XML untouched, same pattern as the arm recolor) — the real servos demonstrably reach these values. The servo-sysid item pins final ranges.

The remaining 6.6° elbow residual is real physics, not a jam: at full shoulder lift the jaw tip rests on the table, and folding the elbow further would push it through. This is the review’s second named cause (zero-perfect sim joints vs the rig’s calibration offsets: the same numeric pose is a slightly different physical pose, and on the rig the jaw clears where in sim it touches). Per the review’s fix direction, the eval protocol pins the settled reachable projection of the rig median — now bit-reproducible and identical across seeds (max per-joint spread 0.003° over 12 seeds). The table contact is deliberately not excluded: jaw–table contact is task-relevant.

Fix 2 — reset never touches the boat (0/100 seeds)

reset() now settles the arm first (boat parked far down-table, outside the sweep of the arm rising from mj_resetData’s laid-out pose), then places the boat at its seeded pose and gives it a short settle of its own. SO101Sim.reset_strike_contacts counts arm–boat contacts during the whole reset — the probe reads it through the public API instead of hand-replicating the reset sequence (the old probe re-implemented reset step-by-step, which would have silently drifted from this change).

Re-verified over seeds 0–99 (the candidate protocol list): first pass found 4/100 seeds still struck — a new, subtler channel: at the settled home the jaw tips rest at x = 0.155 m inside the old spawn region, and a boat spawned at x ∈ [0.17, 0.183] with its 3 cm half-length could land on the parked jaw. The spawn near-bound moved 0.17 → 0.195 (≥1 cm hull-to-jaw clearance); the design target is preserved (settled initial boat→disk distance over 100 seeds: mean 9.5 cm, range 7.1–12.1 — the tuned-for value was ~9.5 cm). Final read: 0/100 strikes, max spawn→settled displacement 0.7 mm (the boat’s own settle).

Fix 3 — threshold-driven CoACD: phantom margin p99 3.78 → 0.45 mm

Exactly as the lit page prescribed: the conversion now drives CoACD by its concavity threshold (0.05 → 0.015, in the grasping-grade 0.01–0.02 band), removes the 16-hull cap that had forced it to stop at 3× its concavity target, and doubles preprocessing resolution (50 → 100). Result: 340 hulls, collision volume 1.75× → 1.13× the visible boat, phantom margin median 0.34 → −0.06 mm, p99 3.78 → 0.45 mm, max 5.39 → 0.69 mm. The gripper now touches the boat where the cameras show the boat.

340 hulls sounds expensive; measured it is free at this scene scale: 26.7 ms/control-tick vs 26.9 before (render-dominated). The SDF path (which would also close the CC-BY-ND per-machine asset hazard) stays a named alternative if hull count ever bites; it was not needed.

Fix 4 — the jaw seam: priority, not pairs

The review measured the gripper’s priority="1" silently replacing the boat’s tuned condim-6 friction (torsional 0.05) with its own near-zero values (5e-3) at every jaw–boat contact. The queue item called for an explicit <contact><pair>; the landed fix is priority="2" on the generated benchy geoms instead, for a concrete reason: pairs need geom names, and menagerie’s actual jaw contact surfaces (collision_gripper_mesh class) are unnamed in the vendored XML — a pair list would miss the geoms that do the gripping. Priority is the same documented override mechanism (the higher-priority geom’s friction is used wholesale), reaches unnamed geoms, and needs one generated attribute. The rest of the recipe the fix list asked for — elliptic cones, impratio 10, Newton — was verified already present in the model options.

Isolated measurement (priority flip alone, old hulls): in-grip spin 6.9° → 2.7°, lift tilt 0.84 → 0.91. Combined with the new decomposition: spin 0.4°, tilt 0.91, penetration 2.5 → 2.2 mm, still a firm hold (boat rises 7.2 cm with the arm). Friction values stay untuned per SIMPLER Table X.

The regression the fixes exposed: solver budget, not physics

First full-suite run after the new decomposition: rest drift 6.2 mm/10 s (the drift fix’s original 0.000 mm read was the review’s one clean bill of health — this was the owner-visible bug class coming back). Diagnosis ruled out the usual suspects empirically: more free-joint damping made it worse (0.2 → 54.8 mm, 0.5 → 76.4 mm — the creep is a biased contact solve decaying slower, not jiggle to damp), harder solref did nothing. The actual cause: the vendored arm model ships iterations="10" ls_iterations="20" solver caps, tuned for a bare arm — the fine decomposition rests ~30–80 simultaneous keel–table contacts and the Newton line search runs out of budget, leaving a slightly wrong solution every step. ls_iterations 20 → 50 alone collapses the drift to 0.001 mm/10 s at unchanged tick cost (caps only bind when the solve is hard; the scene now sets 50/50 with margin). Lesson worth keeping: collision fidelity and solver budget are coupled knobs — upgrading one can silently invalidate the other’s tuning.

What this unblocks, and what it does not

• Unblocked: the sim-policy-eval-100seeds protocol pre-reg. The start state is now deterministic, seed-independent, strike-free over the candidate seed list, and the initial-distance read is clean (mean 9.5 cm, range 7.1–12.1 over seeds 0–99).
• Still open (queued, sim-servo-sysid): the 56× kp discrepancy vs upstream — SIMPLER’s ablation says controller gains are the first-order eval-fidelity lever, and the fix-list’s item 1 is deliberately its own work item. The 100-seed pre-reg may pin current gains as explicit “v0 physics” if the owner prefers speed.
• Unchanged: cross-machine asset reproducibility (assets are still per-machine builds; the protocol pins one eval machine + menagerie SHA, or adopts SDF later); success()’s missing gripper-open check (distance is the primary metric; noted for the pre-reg’s success-rate column).