05 DRAWSTRINGS

// PROJECT: BMAR — BIOMECHANICAL ROBOTIC ARM | DOCUMENT: BMAR-PH1-001 REV-B
PHASE 1: UPPER ARM
COMPLETE ENGINEERING DRAWING SET
DOC NO.BMAR-PH1-001
REVB — 2025
MATERIALSEE BOM SHT-00
VOLTAGE12–24V DC
UNITSALL DIMS IN mm
PA6/PA66 NYLON
NICHROME Ni90
POLY. THREAD
KEVLAR/ARAMID
PET SLEEVE
TPU ELASTIC
OIL FILL
Al/BRASS BONE
FINE FASCICLE
COARSE FASCICLE
SHT-00
ASSEMBLY OVERVIEW
START HERE — HIERARCHY + BOM + OVERALL DIMENSIONS
FIGURE 00.1 — PART HIERARCHY (LEVEL 1 → LEVEL 5)
LVL 1 LVL 2 LVL 3 LVL 4 LVL 5 FIBER NY+Ni+Thread coiled+heat-set ×2 2-PLY PAIR counter-twisted self-stabilizing 2–3× FASCICLE Pairs + Kevlar in 4mm PET sleeve + oil fill FINE or COARSE → see DWG-03 F+C MUSCLE GROUP Fine + Coarse fascs Elastic tendons MOSFET per fascicle FLEXOR / EXTENSOR → see DWG-04 ×2 UPPER ARM Flexor group Extensor group Al/Brass bone tube PTFE pivot joint Kevlar ligaments ESP32 + MOSFETs → DWG-04 ENVELOPE Length: 250mm Bone OD: 16mm Muscle ring: ~40mm Joint OD: ~30mm Mass (est): ~150g DOF: 1 (elbow flex) Voltage: 12–24V
FIGURE 00.2 — MASTER BILL OF MATERIALS (PHASE 1: UPPER ARM)
ITEMDESCRIPTIONMATERIAL / SPECQTY (PH1)UNITSOURCE / SEARCH TERMDWG REF
P-01Nylon monofilament — coarse fiberPA6 or PA66, ∅0.30mm5mAliExpress — "nylon fishing line 0.3mm PA6" — CONFIRM NOT PVDFDWG-01
P-02Nylon monofilament — fine fiberPA6 or PA66, ∅0.10mm3mAliExpress — "nylon monofilament 0.1mm"DWG-01
P-03Nichrome wire — coarse heaterNi90 (NiCr9010) AWG34, ∅0.15mm, ~17.2Ω/m5mAliExpress — "Nichrome 90 AWG34" or "resistance wire 0.15mm"DWG-01
P-04Nichrome wire — fine heaterNi90 AWG40, ∅0.08mm, ~138.8Ω/m3mAliExpress — "Nichrome 90 AWG40 0.08mm" — HANDLE CAREFULLYDWG-01
P-05Sewing thread binderPolyester, thin gauge, T_rated ≥180°C1spoolAny haberdashery — standard polyester sewing threadDWG-01
P-06Kevlar/Aramid threadBraided Kevlar, ∅0.3–0.5mm approx5mAliExpress — "Kevlar braided thread" or "Aramid fiber cord"DWG-03,05
P-07PET braided sleeve4mm ID expandable PET mesh2mAliExpress — "PET expandable braided sleeve 4mm"DWG-03
P-08Oil fill — fasciclesLight mineral oil or silicone oil, 50cSt50mLPharmacy (mineral oil) or AliExpress (silicone oil 50cSt)DWG-03
P-09TPU elastic cord — tendonsTPU, ∅0.5mm — NOT rubber (creep-resistant)2mAliExpress — "TPU elastic cord 0.5mm"DWG-05
P-10Aluminum tube — bone6061 Al, OD 16mm, ID 12mm, wall 2mm300mmAliExpress — "aluminium tube 16mm OD 12mm ID" or local metal supplierDWG-04
P-11Resin for joint bracketsStandard photopolymer resin, 3D printed2pcs3D print — PTFE spray after curing (see DWG-04 for geometry)DWG-04
P-12PTFE spray — joint coatingPTFE aerosol, dry lubricant1canAny hardware store or AliExpressDWG-04
P-13M3 pivot bolt + nut (joint)M3×20 stainless steel hex bolt2setsAny hardware storeDWG-04
P-14MOSFET — per fascicle channelIRLZ44N (logic-level) or IRF540N8pcsAliExpress — "IRLZ44N N channel MOSFET"DWG-06
P-15Flyback diode1N4007, 1A/1000V8pcsComponent kit or AliExpress — "1N4007 diode"DWG-06
P-16Gate resistor10kΩ, 1/4W, ±5%8pcsComponent kitDWG-06
P-17Decoupling capacitor100nF ceramic, 50V8pcsComponent kitDWG-06
P-18ESP32 microcontrollerESP32 DevKit v1 or equivalent1pcsAliExpress — "ESP32 DevKit"DWG-06
P-19Heavy-duty connector32A/250V, 2-pin (or multi-pin)4pcsAlready sourcedDWG-06
P-20Lab PSU30V/10A adjustable, current-limiting1pcsAlready sourcedDWG-06
P-21CA glue (cyanoacrylate)Standard CA glue, thin viscosity1tubeAny hardware store — for securing thread endsDWG-01
P-22Syringe — oil fill10mL plastic syringe, blunt needle1pcsPharmacyDWG-03
P-23304 SS wire (alt heater)∅0.06mm, soft temper — optional alt to P-042mAliExpress — "soft stainless steel wire 0.06mm 304"DWG-01
BUILD SEQUENCE
STEP 1Make + test individual fibers (DWG-01)
STEP 2Pair fibers into 2-ply (DWG-02)
STEP 3Bundle pairs into fascicles (DWG-03)
STEP 4Machine/print bone tube + joint (DWG-04)
STEP 5Make elastic tendons + ligaments (DWG-05)
STEP 6Assemble arm: route fascicles, attach tendons (DWG-04)
STEP 7Wire circuit, test each fascicle (DWG-06)
STEP 8Commission: tune PWM per fascicle, calibrate
OVERALL ARM DIMENSIONS
BONE TUBE OD16mm — Al 6061
BONE TUBE ID12mm (wall = 2mm)
BONE LENGTH250mm (humerus segment)
PIVOT BOLTM3 × 20mm stainless
JOINT BRACKET3D printed resin, see DWG-04
MUSCLE OFFSETFlexor: 15mm from bone CL Extensor: 12mm
FASCICLE COUNT4 coarse + 4 fine (8 total, phase 1)
ELBOW ROM0–150° (set by ligament length)
DWG-01 — ITEMS P-01 TO P-05
COMPOSITE FIBER — LEVEL 1
ALL DIMS IN mm | COARSE + FINE VARIANTS
SECTION A-A — TRANSVERSE (×500 MAG) | CUTTING PLANE: MID-SPAN PRE-COILING
SEC A-A A NY P-01/P-02 Ni P-03/P-04 CONTACT Δ≈0mm (direct) A [A] NYLON PA6/PA66 COARSE: ∅0.30mm P-01 FINE: ∅0.10mm P-02 B [B] NICHROME Ni90 COARSE: ∅0.15mm P-03 FINE: ∅0.08mm P-04 C [C] POLYESTER THREAD P-05 Tight contiguous wrap Secure ends with CA P-21 ∅ 0.30 (coarse) / ∅ 0.10 (fine) ∅ 0.15 (coarse) / ∅ 0.08 (fine) Thread wrap OD ≈ 0.65mm ±0.08
DETAIL B — TCP COIL GEOMETRY (SIDE ELEVATION)
P≈0.5mm D_coil ≈1.2mm L_active = cut to bone segment length ±5mm NYLON COIL BODY NICHROME TRACK (co-inserted) D ← CONTRACTS ΔL≈10% ON HEATING →
ISOMETRIC SKETCH — COIL 3D FORM (SCHEMATIC)
Ni+ Ni- 3D MODEL GUIDE: NYLON: HELICAL SWEEP ∅0.30 COIL ∅1.2 PITCH 0.5mm | NICHROME: CONCENTRIC HELIX ∅0.15 OFFSET 0.225 FROM NY AXIS | THREAD: HELICAL WRAP ∅0.20
TABLE 01.1 — FABRICATION PROCESS (COARSE FIBER)
STEPOPERATIONPARAMETERVALUE / NOTE
01Cut materialsLength200mm for prototyping. Confirm PA6/PA66 on nylon packaging.
02Lay parallelArrangementNichrome alongside nylon — NOT wrapped. Direct contact across full length.
03Thread-wrap P-05Wrap pitchContiguous — no gaps. Thin polyester T_rated ≥180°C. Secure both ends CA glue P-21.
04Pin + weightPre-load weight5–10g hanging weight. Pin top end to board. Weight keeps tension during twist.
05Twist (CW)Turn count80–120 turns per 200mm until strand springs into helix spontaneously.
06Heat-setTemp / Time150°C / 10–15 min OVEN. Keep weight attached. Full cool before removing.
07Inspect + measureResistanceMultimeter across Ni ends. Target: 10–25Ω / 10cm (coarse). Reject if <5Ω or >60Ω.
08Test actuationVoltageApply 5V for 1s. Fiber should visibly shorten. If not, check resistance and Ni wire continuity.
TABLE 01.2 — COARSE vs FINE SPECIFICATION
PARAMETERCOARSE (P-01/P-03)FINE (P-02/P-04)
Nylon OD0.30mm ±0.020.10mm ±0.01
Nichrome OD0.15mm AWG340.08mm AWG40
Ni resistance~17.2Ω/m~138.8Ω/m
Target R/10cm10–25Ω35–50Ω
Power @ 12V~5.8W (25Ω)~3.6W — use 20% PWM
Coil OD (approx)~1.2mm~0.4mm
Heat/cool time0.5–2s / 3–8s~0.3s / ~1s
Thermal mass ratio1× (reference)~1/9× (faster)
PWM mode40–100% burst15–30% continuous
PurposeForce + movementPre-tension + hold
AWG40 handling—⚠ Abrade+flux before solder
⚠ CRITICAL — NYLON MATERIAL: Visually confirm nylon is PA6/PA66 from the packaging. Fluorocarbon (PVDF) fishing line looks identical but does NOT coil-actuate and will not work. PA6/PA66 is printed on the label. | ⚠ AWG40 Nichrome ∅0.08mm is extremely fragile. Abrade tip with 400-grit, apply flux, pre-tin before joining. Order 3× required quantity.
DWG-02
2-PLY TWISTED PAIR — LEVEL 2
TORSION-BALANCED SELF-STABILIZING PAIR | NO RETURN SPRING REQUIRED
FIGURE 02.1 — TORSION BALANCE DIAGRAM (PHYSICS)
WHY IT WORKS SINGLE FIBER — PROBLEM +τ ROTATES ON CONTRACTION 2-PLY PAIR — SOLUTION +τ -τ +τ + (−τ) = 0 LINEAR CONTRACTION ONLY No return spring needed KEY RULE Individual coils: SAME direction (CW) Inter-pair twist: OPPOSITE dir (CCW) 3–5 turns / 100mm Fix BOTH ends
SECTION B-B — 2-PLY CROSS-SECTION
SEC B-B F1 FIBER 1 F2 FIBER 2 PAIR WIDTH ≈ 2.4mm (coarse) ∅≈1.2mm per fiber A B [A]=FIBER-1 [B]=FIBER-2 IDENTICAL SPECS
FIGURE 02.2 — ELECTRICAL WIRING (PAIR IN PARALLEL)
FIBER 1 NICHROME — R≈15Ω COARSE example FIBER 2 NICHROME — R≈15Ω IDENTICAL TO FIBER 1 Ni+ Ni− PARALLEL → R_pair ≈ 7.5Ω | 1 MOSFET DRIVES BOTH Twist all same-end Ni wires together before connecting
TABLE 02.1 — 2-PLY PAIRING SPECIFICATION
PARAMETERVALUECRITICAL NOTE
Individual fiber twist directionClockwise (CW) — consistentBoth fibers in the pair MUST be twisted in the same direction during fabrication (DWG-01)
Inter-pair twist directionCounter-clockwise (CCW) — OPPOSITEIf individual coils are CW, pair twist MUST be CCW. Mixing up directions defeats the torsion balance.
Inter-pair twist density3–5 turns per 100mm lengthNot too tight — just enough to interlock. Excessive twisting reduces working stroke.
End fixationBoth ends of both fibers fixedCrimp with small metal sleeve or tie with Kevlar thread + CA glue. All 4 ends (2 per side) must be captured.
Electrical connectionNi wires in PARALLEL at each endTwist all Ni+ wires together → 1 terminal. Twist all Ni− → 1 terminal. One MOSFET channel per pair.
Return springNOT REQUIREDTorsion balance provides this automatically. An antagonist muscle group or gravity provides extension.
ReferenceHaines et al., Science 2014University of Texas Dallas — canonical TCP paper. Coiled polymer actuators.
DWG-03 — ITEMS P-06 TO P-08
FASCICLE BUNDLE — LEVEL 3
4mm PET SLEEVE | OIL-FILLED | COARSE + FINE VARIANTS
SECTION C-C — TRANSVERSE CROSS-SECTION | ∅4mm PET SLEEVE
[P] PET BRAIDED SLEEVE — ∅4.0mm ID (P-07) [O] OIL FILL — mineral/silicone 50cSt (P-08) F1a PAIR 1 F1b F2a PAIR 2 F2b F3a PAIR 3 F3b 0,0 ∅ 4.0mm SLEEVE ID (P-07) ∅1.2mm fiber coil F K [K] KEVLAR strain limiter
TABLE 03.1 — FIBER POSITION COORDINATES (6-FIBER HEX PACK)
FIBER ID
X (mm from CL)
Y (mm from CL)
PAIR
F1a
0.00
+1.20
PAIR 1
F1b
+1.04
+0.60
PAIR 1
F2a
+1.04
−0.60
PAIR 2
F2b
0.00
−1.20
PAIR 2
F3a
−1.04
−0.60
PAIR 3
F3b
−1.04
+0.60
PAIR 3
KEVLAR ID
X (mm)
Y (mm)
NOTE
KV-1
0.00
0.00
Center
KV-2
+0.52
−0.30
Gap F2a-F1b
KV-3
−0.52
−0.30
Gap F2b-F3a
KV-4
0.00
+0.60
Gap F1a-F1b-F3b
ℹ All coordinates in mm from sleeve centerline. Fiber OD ≈ 1.2mm (coarse) so max center-to-wall distance = 2.0 - 0.6 = 1.4mm. All fiber centers within 1.4mm of CL ✓ | Hex packing is theoretical — actual hand assembly will have variation ±0.3mm. Acceptable at prototype stage.
FIGURE 03.2 — SIDE ELEVATION (LENGTH)
Ni+ Ni− L = bone segment length −30mm (for tendon attachment clearance) ∅4mm
TABLE 03.2 — FASCICLE ASSEMBLY PROCEDURE
1
Prepare pairs
Complete 2-ply pairs from DWG-02. Leave 40mm Ni wire leads at each end. Label FINE vs COARSE.
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2
Bundle + Kevlar
Arrange 2–3 pairs in hex layout per Table 03.1. Insert 4 Kevlar strands P-06 at positions KV-1 to KV-4. Do NOT twist Kevlar in.
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3
Slide PET sleeve
Expand 4mm PET sleeve P-07 and slide over bundle. Leave 15mm of wire/Kevlar protruding each end.
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4
Oil fill
10mL syringe P-22 with blunt needle. Insert at one end, inject light mineral/silicone oil P-08 until full. Tap to remove bubbles. Seal end with heat-shrink or cable tie.
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5
Terminate Ni wires
Twist all Ni+ wires together at each end → 1 terminal. Abrade+flux+tin before joining to connector P-19. Label Ni+/Ni− and FINE/COARSE.
DWG-04 — ITEMS P-10 TO P-13
UPPER ARM STRUCTURE
BONE TUBE + ELBOW JOINT + MUSCLE ROUTING | ALL DIMS mm
SECTION D-D — ARM CROSS-SECTION AT MID-HUMERUS
─ ─ OUTER ARM ENVELOPE (REFERENCE) ─ ─ BONE Al 6061 TUBE OD16/ID12 FINE ∅4mm FINE ∅4mm COARSE ∅4mm COARSE ∅4mm FINE ∅4mm FINE ∅4mm COARSE ∅4mm COARSE ∅4mm ANTERIOR — FLEXOR GROUP POSTERIOR — EXTENSOR GROUP ~90mm OD 16 ID 12 A [A] FINE FASCICLES Continuous low PWM Pre-tension elastic tendons B [B] COARSE FASCS Burst recruited
ELEVATION E — UPPER ARM (SIDE VIEW, FULL LENGTH)
HUMERUS Al 6061 OD16/ID12 SHLDR ELBOW FINE CRSE FINE CRSE TENDON INSERT LIGAMENT 250mm BONE LENGTH 15mm offset from bone CL ORIGIN INSERT FLEXOR EXTENSOR
DETAIL F — ELBOW JOINT CONSTRUCTION
HUMERUS Al TUBE 3D RESIN BRACKET PTFE spray M3×20 PIVOT RADIUS/ULNA Al TUBE ∅2mm TENDON ATTACH HOLES LIGAMENT PANEL ANCHOR ZONE ROM: 0–150° BRACKET PRINT NOTES: 3D print resin. M3 through-hole in bracket. PTFE spray all mating surfaces. M3 nut+washer at each side. Bracket height = bone OD (16mm). Width = 30mm. Wall = 3mm. Tendon holes ∅2mm, 8mm from pivot CL.
TABLE 04.1 — STRUCTURAL SPECIFICATIONS
COMPONENTITEMMATERIALDIMENSIONNOTES
Bone tube (humerus)P-106061 Aluminum alloyOD 16mm / ID 12mm / L 250mmCut to 250mm. Deburr both ends. Drill 2× M3 clearance holes for joint bracket at each end, 8mm from tip.
Joint bracketP-11Standard photo-resin, 3D printedH=16mm / W=30mm / wall=3mm / pivot hole ∅3.2mmPrint at 100% infill. PTFE spray all surfaces after curing. M3 clearance hole through center.
Pivot boltP-13M3×20 stainless steel hex boltM3 × 20mm + M3 nut + M3 washerNyloc nut preferred. Tighten finger-tight + ¼ turn. Should rotate freely but without slop.
Flexor fascicle offset——15mm from bone centerline (anterior)Fascicle centers at 15mm from bone CL. Use 3D printed guide clip or adhesive mounting.
Extensor fascicle offset——12mm from bone centerline (posterior)Extensor slightly closer to bone than flexor — follows biological proportion.
Tendon attachment hole——∅2mm, 8mm from pivot CLDrill through bracket. Kevlar/aramid tendon looped and knotted through hole. CA glue the knot.
Elbow ROM——0–150° nominalSet by medial + lateral ligament panel length. Adjust panel size to change ROM.
DWG-05 — ITEMS P-06, P-09
TENDONS + LIGAMENTS
ELASTIC TENDON | RIGID TENDON | KEVLAR LIGAMENT PANEL
SEC G-G — ELASTIC TENDON CROSS-SECTION
[A] KEVLAR X-WRAP TPU TPU TPU TPU TPU B ∅2.5–3mm (5×∅0.5)
ELASTIC TENDON SPEC
[A] OUTER WRAPKevlar thread P-06
WRAP PATTERNX-cross, 45° angle
[B] CORE CORDS5× TPU ∅0.5mm P-09
TPU vs RUBBERTPU only — resists creep
WRAP TIGHT =Stiffer, less spring travel
WRAP LOOSE =More spring travel
FUNCTIONForce + elastic energy
FIGURE 05.1 — TENDON BUILD SEQUENCE
BUILD STEPS ① 5× TPU ∅0.5mm CORDS parallel, same length ↓ ② LIGHT TWIST (3–4 turns) hold both ends ↓ ③ KEVLAR X-WRAP crossing angle 45–55° ✓ ELASTIC TENDON COMPLETE Loop end through ∅2mm hole in bracket
DWG 05.2 — LIGAMENT PANEL (PLAN VIEW)
⊗ BONE ANCHOR ×4 (∅1.5mm) W = 15mm H ≈ 20–25mm 3 LAYERS OF WEAVE (repeat warp+weft 3×) MEDIAL + LATERAL (2 panels) One panel each side of elbow joint Measure joint gap — cut to fit
TABLE 05.1 — CONNECTIVE STRUCTURE COMPARISON
TYPEMATERIALFUNCTIONSTRETCHCONSTRUCTION
Elastic tendon5× TPU ∅0.5mm + Kevlar X-wrapTransmit muscle force + store elastic energy for speedTunable via wrap tightnessBundle → light twist → Kevlar X-wrap → loop end through bracket hole
Rigid tendonPure Kevlar braid (P-06)Direct force transmission, high-load joints~0% (inextensible)Braid or double-strand Kevlar, knot + CA through ∅2mm hole in bracket
Medial ligamentKevlar 3-layer woven panelPrevent valgus (lateral) dislocation at elbow~0% by designWeave 15mm × 20–25mm × 3 layers, anchor ∅1.5mm holes at 4 corners
Lateral ligamentKevlar 3-layer woven panelPrevent varus (medial) dislocation at elbow~0% by designIdentical to medial, installed opposite side of joint
DWG-06 — ITEMS P-14 TO P-20
DRIVE CIRCUIT SCHEMATIC
ESP32 + MOSFET PWM | 8 CHANNELS PH-1 | ALL DIMS mm
FIGURE 06.1 — SINGLE CHANNEL CIRCUIT (REPEAT FOR ALL 8 CHANNELS)
+V 12–24V 0V GND PSU 12–24V DC P-20 Lab/ATX ESP32 P-18 DevKit GPIO25 GPIO26 GND PWM 15–30% CH1 R=10k IRLZ44N P-14 N-ch, 47A/55V FASCICLE FINE — Ni R=40–80Ω 1N4007 P-15 FLYBACK MANDATORY 100nF P-17 A B C D E F COMPONENT KEY [A] PSU +V RAIL (P-20) 12–24V DC, current-limited [B] NICHROME LOAD Fascicle Ni wire bundle Fine: R=40–80Ω Coarse: R=15–40Ω [C] 1N4007 FLYBACK (P-15) MANDATORY — protects MOSFET Cathode→+V, Anode→Drain [D] IRLZ44N N-ch MOSFET (P-14) Logic-level: ON at 3.3V (ESP32) 47A/55V rated [E] GATE RES 10kΩ (P-16) Prevents oscillation [F] ESP32 GPIO → PWM (P-18) 3.3V logic, 5–15kHz
FIGURE 06.2 — 8-CHANNEL ASSIGNMENT MAP (PHASE 1: UPPER ARM)
CHGPIO PINFASCICLE TYPEMUSCLE GROUPPWM MODER (APPROX)NOTES
CH1GPIO25FINE (∅0.1mm)FLEXOR — anterior15–30% continuous40–80ΩAlways active. Pre-tensions flexor elastic tendon.
CH2GPIO26FINE (∅0.1mm)FLEXOR — anterior15–30% continuous40–80ΩRedundant fine fascicle for precision holding.
CH3GPIO27COARSE (∅0.3mm)FLEXOR — anterior40–100% burst15–40ΩBurst-recruited for elbow flexion movement.
CH4GPIO14COARSE (∅0.3mm)FLEXOR — anterior40–100% burst15–40ΩSecond coarse fascicle — max load handling.
CH5GPIO12FINE (∅0.1mm)EXTENSOR — posterior15–30% continuous40–80ΩPre-tensions extensor elastic tendon.
CH6GPIO13FINE (∅0.1mm)EXTENSOR — posterior15–30% continuous40–80ΩRedundant fine fascicle for extensor group.
CH7GPIO15COARSE (∅0.3mm)EXTENSOR — posterior40–100% burst15–40ΩBurst-recruited for elbow extension movement.
CH8GPIO2COARSE (∅0.3mm)EXTENSOR — posterior40–100% burst15–40ΩSecond coarse extensor fascicle.
TABLE 06.2 — COMMISSIONING + TEST PROCEDURE
1
Verify resistance
Multimeter across each fascicle terminal before connecting. Record value. Fine: 40–80Ω. Coarse: 15–40Ω. Out of range = check Ni continuity.
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2
Connect diodes first
Install 1N4007 across each load before powering. Cathode to +V rail, Anode to MOSFET Drain. Verify orientation with multimeter diode-check mode.
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3
PSU current limit
Set lab PSU to 12V, current limit = 1A initially. This protects MOSFETs during first power-on if there's a wiring error.
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4
Test each channel
GPIO HIGH → MOSFET ON → fascicle heats. PSU current should rise. Feel fascicle — should get warm within 2s. Fascicle should visibly shorten.
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5
PWM calibration
Set fine channels to 20% duty, coarse to 50%. Observe: fine fascicles maintain slight tension, coarse produce measurable contraction force. Tune from here.
⚠ CRITICAL — 1N4007 FLYBACK DIODE: Nichrome wire has inductance. Without the flyback diode, switching the MOSFET OFF generates a voltage spike that DESTROYS the MOSFET on the first switch cycle. Install one 1N4007 across EVERY fascicle load, cathode to +V rail, before any power-on test. Cost $0.02 each. Ignoring this: replace all MOSFETs. | ⚠ MOSFET SELECTION: Use IRLZ44N for direct ESP32 3.3V drive. IRF540N requires ≥5V at gate — needs a level shifter if using with ESP32 3.3V logic.
PWM PARAMETERS
FREQUENCY5–15 kHz recommended
FINE CH DUTY15–30% continuous
COARSE CH DUTY40–100% burst on demand
ESP32 LOGIC3.3V → IRLZ44N fully ON ✓
RESOLUTION8-bit PWM (256 steps)
CONTROLledcWrite() Arduino/ESP-IDF
NICHROME SOLDERING PROCEDURE
STEP 1Abrade wire tip with 400-grit sandpaper
STEP 2Apply liquid flux to abraded zone
STEP 3Pre-tin wire tip (tin the wire first)
STEP 4Then join to connector pad/pin
IRON TEMP350°C, 60W iron, work fast
SOLDER TYPERosin-core 60/40 or silver solder