What happens when a feeding arm meets a soft lip instead of a spring, and does a force cap help?
research/osrobot-mjsoft
Replacing the 75,000 N/m spring with a bonded tissue layer (4 to 45 kPa, 5 or 15 mm) cuts simulated peaks to 1 to 3 N in most cells but still breaks the 30 % depth limit in about half the trials; a 0.2 to 0.5 N cap fixes steady pushes, slowing to 0.02 m/s fixes a 10 kPa, 15 mm lip, and nothing fixes a thin 4 kPa one by speed alone (MuJoCo dynamics, closed-form tissue force)
What it found
osrobot-mjlayer found the force that indents a lip 30 % of its thickness is 0.05 to 9 N, far below the 12 to 53 N a kinematic spring produced. Here the osrobot-mjfeed arm (MuJoCo dynamics, same controller and 30 mm sway) meets a lip whose force is that layer law, with assumed viscous damping and a bone stop, across 4 to 45 kPa, 5 and 15 mm and 0.02 to 0.20 m/s, 100 seeds per cell. Peaks are mostly 1 to 3 N, but the indentation stays within 30 % of the layer in only 36 to 100 % of trials: 100 % at 0.02 m/s for a 10 kPa, 15 mm lip (feed time 8.7 s), 52 % at 0.10 m/s, 36 to 53 % for a thin soft lip at any speed. A blocked arm pushes 1 N by its own controller; a 0.2 to 0.5 N cap brings the depth under the limit in the 10 kPa case and a 0.2 N cap in the softest, and the simulated depth matches the closed-form law. Caps barely trim fast impacts. More lip damping raises the share within the limit (52 to 85 %) and the peak force about 8 times. Limits: homogeneous linear bonded layer, MuJoCo used as an integrator with hand-coded contact, unsourced damping, bone stop and controller gains, no pressure limit, no human model. Not a safety case.
A figure from its own output
Paper, code and results
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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.