The contact layer · grasp stability

Two fingertips hold an object: force closure from touch

Each contact can only resist a tangential load up to μ·(its normal force). Load the object; if either finger's friction cone is exceeded, it slips and the object falls. The reflex holds the grasp with the minimum pinch force.

Pinch force

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Load

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Friction margin

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Grasp

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Pinch — Load — μ —

Tactile slip sensing is why a hand doesn't drop things. DIGIT-class fingertips read incipient slip per contact; the controller closes the grasp before the object moves.