Bionic Arm — 3D-Printed Myoelectric Prosthesis

“Replacing a missing hand exposes the true complexity of the human body.” — project report, Rev 1.41

This project explores a myoelectric prosthetic arm controlled by surface EMG. The goal is to approach natural hand function while keeping the material cost under $500, producing a platform that’s practical for research and affordable for real users. Motivation & scope (from Rev 1.41):

• Physical design: printable, robust mechanics that mimic human hand kinematics.
• Control scheme: intuitive EMG control for basic tasks (grasp, release, pinch).
• Practicality: useful in day-to-day tasks, not just lab demos.
• Affordability: consumer prostheses often cost $20k–$40k; this build targets sub-$500 materials.

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Note: EMG pipeline uses band-pass (~20–450 Hz), notch (50/60 Hz), rectification, and moving RMS/MA envelope before classification.

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• Why tendons? Compact, printable, tolerant to misalignment, easy to service.
• Links & joints: printed phalanges with captive pins and low-friction bushings.
• Pads & contact: TPU grip pads for friction; swappable liners for wear.

1. Acquire: surface electrodes → EMG preamp (gain, BP/Notch).
2. Condition: rectify → moving RMS / moving average envelope.
3. Map: thresholds / simple classifier → grip state machine.
4. Command: target tendon displacements (position/velocity) with soft limits.

EMG_raw → BP(20–450Hz) → Notch(50/60Hz) → 
Rectify → RMS(τ≈100–200ms) → Thresholds/Features → Grips

• Power grasp, pinch, tripod, open.
• Long-press / co-contraction to cycle grips; envelope magnitude → speed scaling.
• Safety: timeout watchdog, stall detection (current/pos error), soft endstops.

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Commercial hands (e.g., predefined-grip systems) often couple finger joints, limiting fine control. Research prostheses push mechanics/control but can suffer in robustness and cost. This build aims at a middle ground: printable mechanics + simple, reliable EMG control suitable for real-world trials.

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• Prints: 0.2–0.28 mm layers; PETG for high-stress parts, TPU for compliant pads.
• Tendons: low-creep cord (e.g., UHMWPE/Dyneema class), crimped or knotted with printed retainers.
• Assembly: captive pins, heat-set inserts where service is expected.
• Serviceability: tendon swap and pad replacement in minutes.

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• Sequential finger actuation confirmed (see video above).
• EMG grip switching stable with simple envelope thresholds.
• Printable mechanics tolerate repeated open/close cycles; pads improve grip.

This is an R&D prototype intended for research/education. It is not a medical device.

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• 📄 Project slides (PDF) — Download
• 📘 Full report Rev 1.41 (PDF) — Download

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• Add adaptive thresholds per user calibration (on-device).
• Evaluate multi-channel EMG for more reliable grip inference.
• Long-term durability testing (cord wear, joint play, pad abrasion).
• Explore haptic feedback (vibration) for closed-loop grip force cues.

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TL;DR A printable, tendon-driven, myoelectric hand under $500 in materials: EMG → simple grip logic → reliable everyday grasps. Built to be hacked on, serviced, and actually used.

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