Making the grasp honest: contact fidelity for a MuJoCo SO-101 twin

Sources: MuJoCo modeling / computation docs + changelog (fetch-verified 2026-08-11) · CoACD (2205.02961) · IPC-GraspSim (2111.01391) · BAM extended friction models (2410.08650, code) · SIMPLER’s physics ablations (2405.05941) · real-to-sim capture: PolaRiS (2512.16881), GSWorld (2510.20813), soft-body GS twins (2511.04665), Real-is-Sim (2504.03597). Read: 2026-08-11, sim lit lane. Fed: the fix list ahead of the sim-policy-eval-100seeds pre-reg — every one of the sim review’s four findings turns out to have a documented mechanism and a named fix.

The problem in plain words. Our simulator review found that the virtual boat is wrapped in an invisible 4-millimeter force field (a by-product of how curved shapes get approximated for collision checking), that the gripper’s settings silently cancel the boat’s carefully tuned friction at the exact moment of grasping (so the boat spins in the jaws), and that the arm’s motors fight themselves in the home position. Before fixing any of that, we read what the simulator’s own documentation and the surrounding literature say. The satisfying answer: all three problems are known, documented failure modes with standard fixes — we were not fighting mysterious physics, we were holding the tools wrong. The more surprising answer: the biggest lever for making a sim rank policies correctly is not contact physics at all — it’s whether the simulated motors respond like the real ones, which one paper measured directly and we can fix with a few recorded real trajectories. Exact friction values, the thing we were most worried about, turn out to be nearly irrelevant for ranking.

Finding 3 (phantom margin): we capped the wrong knob

CoACD’s intended fidelity control is the concavity threshold (-t, default 0.05, sensible grasping range 0.01–0.02), not the hull cap. Our conversion asked for -t 0.05 but capped hulls at 16, forcing CoACD to stop before reaching its concavity target — the measured 0.149 achieved concavity and the p99 3.8 mm phantom margin are exactly this failure mode. The fix is to drive by threshold (and preprocessing resolution -pr), let the hull count float, and accept the runtime cost — MuJoCo’s docs themselves recommend CoACD by name for this workflow, and obj2mjcf (the tool behind menagerie’s own assets) uses CoACD as its backend. V-HACD is the wrong tool here: its known weakness is filling concavities, the exact property a graspable hull must keep.

Two escape hatches if threshold-driven CoACD stays too coarse:

• Native SDF collisions (MuJoCo ≥3.3.5 needs no plugin): a voxelized signed-distance field generated at model compile time from the unmodified mesh — no convexification, so the phantom margin vanishes by construction. Costs: the docs’ most expensive collider class, and contact-finding by gradient descent can miss contacts if sdf_initpoints is low. Bonus that matters to us: the benchy is CC BY-ND, so our CoACD hulls are un-committable derived assets rebuilt per machine (the repro hazard the sim review flagged) — an SDF is generated at load from the original STL, so the repo ships nothing derived. That would close the per-machine-asset hole and the phantom margin in one move, if the per-step cost is acceptable at our ~27 ms/tick budget.
• maxhullvert quality jumped in 3.10.0 (June 2026, Qhull Q9) — relevant if we coarsen hulls for speed later.

Finding 4 (spin in the jaws): documented behavior, three fixes

The priority rule is spec, not bug: “if one of the two geoms has higher priority, its friction coefficients are used” — wholesale, not element-wise. Our gripper’s priority=1 therefore replaces the benchy’s condim-6 friction triple (torsional 0.05) with its own (torsional 5e-3) at every gripper↔boat contact. Fixes, in increasing order of control: set the full friction triple on the priority geom; drop priority and rely on element-wise max; or — cleanest — declare an explicit <contact><pair> for the jaw–boat seam, which overrides both geoms and is also the only place the separate solreffriction works.

The docs’ “Preventing slip” section reads like a checklist written for our probe results: condim ≥ 4 for torsional friction (“prevents rotation around the normal” — our 6.9° in-grip spin), elliptic cones + impratio ≈ 10 + Newton solver for slip suppression, keep multiccd on for flat-on-flat jaw contact, armature/implicitfast against vibration. Worth knowing exists: MuJoCo 3.11.0 (July 2026) added a per-geom/pair adhesion attribute — “a physical stabilizer for grasping” — a legitimate, documented stabilizer if honest friction still under-holds, at the cost of physics realism.

The measured anchor for “compliance dominates”: IPC-GraspSim (2111.01391) benchmarked grasp-outcome prediction against 2,000 physical grasps — compliant, intersection-free jaw contact hit F1 = 0.85 and beat both analytic models and Isaac Gym. In MuJoCo terms: condim 4 + softened fingertip solref/solimp is the analog; rigid perfect jaws are the least faithful option, not the most.

Findings 1–2 (home pose, servo saturation): sysid is the first-order term

Two independent sources pin our servo model as the thing to fix first:

• BAM (2410.08650) identified extended friction models for hobby-class servos from a pendulum bench — CMA-ES over 2–11 params, converging in ~5 min — improving trajectory MAE 1.5–2.9× over the standard Coulomb-viscous model, and the repo ships an identified Feetech STS3215 model (our exact servo) with MuJoCo integration. The cheap version of this — replay a few real SO-101 episodes through the sim and fit gains — is SIMPLER’s sysid recipe, and it needs no new hardware bench.
• SIMPLER’s ablation (Table II): degrading controller sysid from control-loss 0.131 to 0.432 moved eval-ranking MMRV from 0.031 to 0.100 — a 3× fidelity loss from controller gains alone. Meanwhile their Table X varied contact parameters (object friction 16×, density, gripper friction) and rankings barely moved (MMRV ≈ 0.055 across nearly all settings). Controller gains are first-order for eval fidelity; friction coefficients are second-order. One caveat cuts back our way: their tasks never stressed in-hand torsion — a qualitatively wrong contact regime (zero torsional friction at the seam, phantom margins) is a different failure class than an imprecise coefficient, so Findings 3–4 still need their fixes; we just shouldn’t tune friction values beyond qualitative correctness.

This lands on a live discrepancy the landscape page surfaced: our menagerie model runs kp 998.22 / forcerange ±2.94 where TheRobotStudio’s upstream publishes kp 17.8 / forcerange ±3.35 for the same servo — and ±2.94 is precisely the saturation ceiling the review measured in the jammed home pose. Fitting gains against real episodes (we have 229 h of rig data) resolves the 56× disagreement empirically instead of by trusting either XML.

What we deliberately don’t need

• Gaussian-splat capture pipelines (PolaRiS: 2–5 min phone scan → eval-grade twin; GSWorld; Real-is-Sim’s 60 Hz-synchronized twin): the field’s answer to visual fidelity plus quick asset capture. Our visual gap is real but unmeasured until real rollouts exist, and our scene is three rigid objects on a table — scanning tooling solves a problem we don’t have yet. Notably, none of this line reports contact-geometry error metrics (our p99 phantom-margin number has no published analogue) — contact quality is asserted via end-task correlation only.
• Eval-time physics randomization: no published result shows ensembling over physics parameters improves rank correlation with real; SIMPLER’s Table X suggests rankings are already stable across wide physics ranges (so the ensemble adds variance, not signal), and the 2026 cross-simulator recipe paper attributes proxy reliability to simulator-level fidelity, not parameter sweeps. Single fixed physics + fixed seeds stays the right design.
• Grasp-quality instrumentation exists if wanted: MuJoCo 3.3.5’s contact/insidesite sensors give clean per-contact readouts — nicer than our probe scripts’ manual mining if the fix-verification probes grow.

The fix list this page feeds

In pre-reg order for sim-policy-eval-100seeds:

1. Servo/controller sysid against real episodes (BAM STS3215 params as the informed prior; resolves the kp discrepancy and should un-jam the home pose with Finding 1’s mount fix).
2. Home pose + spawn-after-settle (Findings 1–2, already specified in the review) — re-verify 0 reset strikes over the registered seed list.
3. Jaw–boat <contact><pair> with condim 4+, elliptic cones, impratio ~10, Newton (Finding 4); re-run the pinch probe, compare spin/tilt.
4. Threshold-driven CoACD (-t 0.01–0.02, uncapped hulls) or the SDF experiment (also fixes the CC-BY-ND per-machine asset hazard); re-run the phantom-volume probe.
5. Friction values: leave at qualitative correctness — SIMPLER Table X is the citation for not tuning them.