How fine must the MuJoCo contact settings be for a moving grasp?

What it found

A follow-up to robofab-graspfast. On all seven printed parts of a toy gripper, a jaw assembly accelerating upward at 100, 400 and 800 m/s² needs a simulated minimum force of 0.98–1.04 times the bound at contact time constants of 0.2 ms and below (63 cases). At 0.5 ms the bound is matched at 100 m/s² but no force up to 60 N holds any part at 400 or 800. A scan on one part shows the largest adequate time constant falls with acceleration and is not a single threshold on their product. Stylised: upward load only, rigid box pads, one friction value, 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 it the contact time constant or the timestep that breaks a moving grasp?Is it solref or the timestep that breaks a moving MuJoCo grasp? On a 24-cell grid only 6 cells hold; a coarse step always fails, a finer step does not rescue a marginal solref, and solref 0.1 ms holds at steps of 0.1 ms and below (stylised).
• 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).

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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.