Is it the contact time constant or the timestep that breaks a moving grasp?

What it found

A follow-up to robofab-solrefmap. On the cover part of a toy gripper accelerating upward at 400 and 800 m/s², a MuJoCo jaw-force test holds in only 6 of 24 cells of a grid of contact time constant (0.5, 0.3, 0.2 ms) and timestep (0.4 to 0.05 ms). A coarse timestep always fails, probably because MuJoCo raises the effective constant to twice the timestep. A finer timestep does not rescue a marginal constant (at 800 m/s² and 0.2 ms only a 0.1 ms step holds, finer ones fail for an unexplained reason), while a 0.1 ms constant holds at every step of 0.1 ms and below. Stylised: one part, upward acceleration only, rigid box pads, no arm.

A figure from its own output

Paper, code and results

• README and method
• White paper
• Experiments and results
• Code
• Tests

More in Robot manufacturing loops

• Is MuJoCo's refsafe clamp what breaks a moving grasp at a coarse timestep?Does MuJoCo's refsafe clamp break a moving grasp at a coarse timestep? Switching it off changes 7 of 32 grid cells' outcomes (11 hold on, 10 off), 10 runs go unstable, and the non-monotone timestep behaviour survives (stylised).
• Does an off-centre centre of mass need a harder grasp?Does an off-centre centre of mass need a harder grasp? Ball-pad elliptic-cone bound matches MuJoCo within 1.1% (5× force at 20 mm offset); flat-pad MuJoCo lies between an equal-split and a best-split corner-contact bound (stylised).
• Does the friction-grasp bound survive lift acceleration and an off-centre centre of mass?Does the friction-grasp bound survive lift acceleration and an off-centre centre of mass? Scaled bound matches MuJoCo within 6% over 66 cases; the torque bracket for an off-centre bar is inconclusive and depends on the rotation tolerance (stylised).
• Is the off-centre bar result in robofab-liftload a grasp effect or solver creep?Is the off-centre bar result in robofab-liftload a grasp effect or solver creep? The bar creeps at about 3.7 deg/s from 0.3 to 3 N in MuJoCo, with a rate set by contact settings, so the earlier verdict reflects the simulator (stylised).
• Does a rate-based "holds" criterion make a MuJoCo grasp verdict independent of solver settings?Does a rate-based "holds" criterion make a MuJoCo grasp verdict independent of solver settings? No: the minimum holding force still varies 33x to 400x or more across contact settings, like a 1 degree tolerance (negative, stylised).
• Is any rate threshold settings-robust for a MuJoCo grasp "holds" verdict?Is any rate threshold settings-robust for a MuJoCo "holds" verdict? Spread falls with the allowed tip speed and is 1.6 only at a loose 20 mm/s (0.5 s, 10 mm); 242.7 to 400 at 2 mm/s or less (stylised, preliminary).

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Proper scoring & elicitation28Effort & peer prediction9Verification games & slashing19Detection & tolerances14Markets as routers13Prediction-market design9Paying contributors8Decentralised training systems23Digital twins & sim-to-real164LLM chip design loops40Robot manufacturing loops53Ambient AI assistant46Self-replicating manufacturing34Simulation-first design64Engineered clothing45Task-specific AI accelerators40Algorithmic chip-design loops42Open-source purpose-built robots44The software-engineering loop43Adversarial human imitation41Open-source residential construction41Autonomous multi-angle capture34Sensing, imaging and 3D31Energy supply and dense energy storage39Robot societies among people31Agentic co-design of low-cost robots27Laundry-hanging robots25Machine learning on qualitative data23Robots on the building site27Civilizations & group psychology1History & human sciences1

Produced with AI assistance (Claude). Results are from the code in this repository and are simulated or stylised unless the abstract says otherwise; nothing here implies affiliation with or endorsement by any lab, researcher or company cited. MIT licensed.