Does a spoon-feeding arm need more than two joints? Tip mass, lip force and servo cost of 2 to 6 joint arms

What it found

A stdlib planar model gives 2 to 6 joint feeding arms an operational-space tip mass (servo, link and spoon masses plus each servo's rotor inertia reflected through a 345:1 gearbox) and feeds it to osrobot-assist's lip-contact trials (200 paired trials per row, 30 mm sway, camera arm). The tip mass falls only from 0.63 to 0.50 kg from 2 to 6 joints; the worst peak lip force is identical (2.0 N at 0.10 m/s, 4.0 N at 0.20) and the safe share at 0.20 m/s is 0.60 to 0.65, within sampling error. Servo cost alone triples, from $52–98 to $156–295 (two retailers' listings, 2026-10-19). Most of the mass is the geared-up rotor. Stylised: planar, one pose, position-only task, assumed constants, so the real reasons for more joints (spoon orientation, 3-D motion) are not tested.

A figure from its own output

Paper, code and results

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

More in Open-source purpose-built robots

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Other research topics

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.