📋 Project Overview

Tendon-driven biomechanical robotic arm inspired by human anatomy. Staged build: upper arm → forearm → shoulder → wrist → hand/fingers. Actuation via TCP nylon muscle fibers + nichrome heating, mirroring biological muscle hierarchy.

Muscle Hierarchy

Level	Description
Fiber	0.3mm or 0.1mm nylon + nichrome co-inserted, sewing thread bound, coiled, heat-set 150°C
Pair	2 fibers twisted opposite direction — 2-ply, self-stabilizing, no return spring needed
Fascicle	2-ply pair + Kevlar strain limiters in 4mm PET braided sleeve + mineral oil fill
Muscle	3 fascicles, controlled per muscle via MOSFET
Arm	TPU elastic tendons + Kevlar ligaments, aluminum/brass bones, resin joints

Actuation Specs

Parameter	Value
Nylon (coarse)	0.3mm PA6/PA66 monofilament
Nylon (fine)	0.1mm PA6/PA66 monofilament
Nichrome (coarse)	0.15mm AWG34, co-inserted parallel to nylon
Nichrome (fine)	0.08mm AWG40, co-inserted parallel to nylon
Target resistance (coarse)	~10–25Ω / 10cm
Target resistance (fine)	~40Ω / 10cm
Heat-set temperature	150°C, 10–15 min, light tension (hang with weight)
Contraction (coarse)	0.5–2s heating \| 3–8s cooling
Contraction (fine)	~1s heating \| ~1s cooling
Control	ESP32 + N-channel MOSFETs (IRLZ44N) + PWM
Power	12V, MOSFET per muscle group

Skeleton

Component	Material	Notes
Bones	Aluminum/brass tubing + 3D-printed resin joints	Hollow for tendon routing
Elastic Tendons	TPU 0.5mm cord + Kevlar X-wrap	Pre-tension storage, NOT rubber (rubber creeps)
Ligaments	Woven Kevlar panels with X-cross, figure-8, loop+cinch	Joint constraint, prevent dislocation
Joints	PTFE bushings	Low-friction rotation

Build Phases

Phase	Goal
1	Upper arm — prove TCP fiber concept with 2-ply pairs
2	Forearm — add flexor/extensor muscle groups
3	Shoulder — 2 DOF (pitch + yaw)
4	Wrist — pitch + roll
5	Hand/fingers — grip feedback sensors
6	Electromagnetic joint locking — fail-safe locked, energize to unlock
7	CNC/machining head — lock joints, lightweight spindle
8	AI/neural control — Raspberry Pi, closed-loop feedback

Upper Arm Muscles (3 major)

Biceps brachii: Flexion (pulling the forearm up) + supination (rotating palm up). Primarily a pulling muscle.
Triceps brachii: Extension of the elbow (straightening the arm). Primarily a pushing muscle.
Brachialis: Flexes the elbow, underneath the biceps, adds pulling strength.

Shoulder Muscles (9 major)

Deltoid (3 parts: anterior, lateral, posterior) — lifts the arm in various directions
Rotator cuff: Supraspinatus, infraspinatus, teres minor, subscapularis — stabilize and rotate the shoulder
Pectoralis major — flexion and adduction
Latissimus dorsi — extension and adduction
Teres major, Coracobrachialis — assist with rotation and flexion

Shoulder movements: flexion, extension, abduction, adduction, internal/external rotation, circumduction

Forearm Muscles (~20 muscles)

Flexors (anterior compartment) — pull the wrist and fingers inward (toward the palm)
Extensors (posterior compartment) — pull wrist and fingers outward
Pronator teres & pronator quadratus — rotate forearm so palm faces down (pronation)
Supinator — rotate forearm so palm faces up (supination)

These muscles originate mostly from the upper arm bones (humerus and ulna) and insert into the radius and hand bones.

Hand and Fingers (30+ muscles)

Intrinsic muscles — control fine motor skills, finger abduction/adduction, and precise movements
Extrinsic muscles — power grip and gross finger movements (in forearm, controlled via tendons)

Reference Anatomy

📚 Notion Subpages

📄 Source: 🦾 Biomechanical Robotic Arm