🔧 Muscle Group Assembly — Full Arm Build

A muscle group is a bundle of multiple fascicles working together to actuate one joint movement (e.g., biceps for elbow flexion). All fascicles in a group share a mechanical termination but have independent or grouped electrical control.

2-Ply Fiber Pairs — Key Concept

Each fascicle contains a 2-ply fiber pair: two identical fibers twisted together in the opposite direction to their coils. A few turns are enough to interlock them. The opposing twist provides passive return — no return spring needed.

The pair contracts in balance when heated and returns passively when cooled. This replaces the older approach of single fibers with separate return springs.

Arm Muscle Groups

Total: 4 muscle groups, 12 fascicles

Fascicle Structure (Per Fascicle)

• 2-ply fiber pair — two fibers twisted together, no return spring
• 4mm PET braided sleeve — NOT 6mm! (corrected from earlier versions)
• Kevlar strain limiters — 2–4 parallel strands inside sleeve, never twisted in
• Mineral oil fill — thermal management

Mechanical Assembly — Bundling

Each muscle group contains N fascicles that:

1. Stay separate along their length — individual 4mm PET sleeves maintain independence
2. Merge at the LOAD END — ferrules combine for shared pull point
3. Share one ferrule — single termination point for the entire bundle
4. Kevlar strain limiters between pairs — run parallel, don't twist

Bundle Cross-Section Diagram

Ferrule Application

1. Trim all 2-ply fiber pairs to the same length at the load end
2. Bundle the pairs together — Kevlar strain limiters run parallel between pairs
3. Slide ferrule (0.8/1.8 round aluminum) over the bundled pairs + limiters
4. Crimp with pliers — firm, even pressure
5. The ferrule becomes the single attachment point for the tendon

Control Variants

Variant 1: MOSFET Per Muscle Group

One MOSFET drives ALL fascicles in a group simultaneously. One GPIO per muscle group.

ESP32 GPIO 0 ──────────────────► MOSFET (BICEPS) ──┬─► NiCr F1
                                                    ├─► NiCr F2
ESP32 GPIO 1 ──────────────────► MOSFET (TRICEPS) ──┼─► NiCr F3
                                                    │
ESP32 GPIO 2 ──────────────────► MOSFET (FLEXORS) ──┼─► NiCr F1
                                                    ├─► NiCr F2
ESP32 GPIO 3 ──────────────────► MOSFET (EXTENSORS) ┴─► NiCr F3

Each group: 3 fascicles in PARALLEL, driven by 1 MOSFET

Pros:

• Simple wiring — only 4 MOSFETs, 4 GPIO pins
• Easier to assemble and debug
• Lower component count

Cons:

• All fascicles in a group fire at 100% or nothing (no gradation within group)
• Cannot modulate individual fascicle for fine control
• One MOSFET failure takes out the entire muscle group

Variant 2: MOSFET Per Fascicle

Each fascicle has its own dedicated MOSFET. Enables individual PWM control per fascicle.

ESP32 GPIO 0 ──► MOSFET F1 (biceps fascicle 1) ──► NiCr F1
ESP32 GPIO 1 ──► MOSFET F2 (biceps fascicle 2) ──► NiCr F2
ESP32 GPIO 2 ──► MOSFET F3 (biceps fascicle 3) ──► NiCr F3
... (same pattern for all 4 muscle groups)

Pros:

• Individual PWM control per fascicle — smooth gradation
• Partial contraction possible (not just on/off)
• Better fault isolation (one short only affects one fascicle)
• More realistic mimicking of biological muscle control

Cons:

• 12 MOSFETs required (vs 4)
• 12 GPIO pins needed — use I2C GPIO expander (e.g. MCP23017) to reduce
• More wiring complexity on the arm
• 12 diodes, more complex PCB or wiring harness

Parts Summary

Component Placement

Inside Upper Arm

Inside Forearm

Controller (ESP32)

Assembly Sequence

1. Build individual fibers — nylon + nichrome co-inserted, sewing thread bound, coiled, heat-set
2. Make 2-ply pairs — twist two identical fibers opposite their coil direction, a few turns to interlock
3. Build fascicles — sleeve pairs in 4mm PET with Kevlar strain limiters (parallel, not twisted), fill with mineral oil
4. Mechanical bundling — merge at load end, crimp shared ferrule per muscle group
5. Route nichrome leads — separate positive leads from each fascicle to MOSFET board
6. Wire gate signals — V1: one GPIO per group to gate. V2: one GPIO per fascicle (or via expander)
7. Install flyback diodes — across each MOSFET drain-source (1N4007, cathode to +V)
8. Add fuses — one per muscle group on the +V rail
9. Decoupling capacitors — 100µF + 100nF across +V and GND near ESP32
10. Connect power supply — 12V → XT60 → distribution → MOSFET drains

📄 Source: 🔧 Muscle Group Assembly — Full Arm Build