saltylab-firmware/chassis/uwb_assembly.md

SaltyBot UWB System — Assembly Notes + BOM

Rev A — 2026-03-01 — sl-mechanical Issues: #57, #61, #62

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System Overview

Follow-me UWB ranging using 3× MaUWB ESP32-S3 modules:

Role	Qty	Location	File
Tag	1	Operator belt ("Tee")	uwb_tag_enclosure.scad
Anchor	2	Robot stem, 250 mm apart	uwb_anchor_mount.scad

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Antenna Clearance — CRITICAL

The DW3000 UWB chip antenna requires:

• 10 mm clear zone around antenna — no metal, no carbon fibre, no metal-fill filament
• Use plain PLA or PETG only in these parts (no conductive additives)
• Tag lid has an open window directly over the PCB antenna area
• Anchor bracket back wall has a cutout behind the antenna section of the PCB
• When installing modules, orient antenna end away from stem (faces outward)

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Part A — UWB Tag Enclosure (uwb_tag_enclosure.scad)

Printed parts

#	RENDER	Qty	Material	Settings
1	body	1	PETG	4 perims, 35% infill, 0.2 mm layer
2	lid	1	PETG	4 perims, 35% infill
3	tpu_bumper	1	TPU 95A	3 perims, 20% infill, 25 mm/s

Print orientation: body — floor on bed; lid — inside face down; TPU bumper — flat.

Internal layout diagram

X−      ←────────── ~75 mm ──────────→   X+
         ┌──────────────────────────────┐
  USB → │[TP4056]  [   18650 cell    ] │
         │─────────────────────────────│
         │  [  MaUWB ESP32-S3 PCB   ]  │ ← antenna faces UP
         └──────────────────────────────┘
               ↑                   ↑
          micro-USB            LED window
          (charging)          (Y+ face, small hole)

  Power switch cutout: Y− face, centred
  Belt clip: Y− face (back)

IP44 seam

Lid rim overlaps body 4 mm with a 2-turn labyrinth ridge (water must turn 90° twice). No holes on the top face. All ports on side/back faces. Not suitable for rain immersion — use for light splash protection only.

Heat-set insert option

For improved thread strength, replace M2 clearance standoffs with M2 × 3 mm heat-set brass inserts pressed into the standoff posts with a soldering iron.

Fasteners

#	Spec	Qty	Use
4	M2 × 6 BHCS	4	MaUWB PCB to standoffs
5	M2 hex nut	4	Alternative if no heat-set inserts
6	M3 × 8 SHCS	2	Belt clip to enclosure back
7	M3 hex nut	2	Captured in clip back plate

Assembly sequence — tag

1. Install TP4056 board in body, micro-USB port aligned with cutout
2. Solder TP4056 leads to 18650 spring contacts (or use 18650 holder PCB)
3. Snap 18650 into battery cradle (+ end toward spring contact at X+)
4. Seat MaUWB PCB on M2 standoffs; tighten M2 × 6 bolts (0.3 N·m — soft)
5. Route USB data + power cable from MaUWB to TP4056 B+ / B− pads
6. Press power switch into cutout (Y− face)
7. Close lid — press corners until 4 snap clips engage (audible click ×4)
8. Slide TPU bumper sleeve onto body from below
9. Attach belt clip to Y− face with 2× M3 × 8

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Part B — Anchor Mounts (uwb_anchor_mount.scad)

Printed parts (per mount × 2 mounts)

#	RENDER	Qty per mount	Total	Material	Notes
10	collar_front	1	2	PETG	Flat-face-down; has thumbscrew recesses
11	collar_rear	1	2	PETG	Has hex nut pockets + USB cable channel
12	bracket	1	2	PETG	Flat-face-down; back wall on bed

Anti-rotation

The front collar half has a flat tab (3 × 8 mm radial tab at +X) that protrudes from the collar outer surface. This tab:

• Bears against the module bracket arm, preventing rotation during clamping
• Creates a positive mechanical register — mount always faces the same direction
• No modification to the stem required

Anchor positioning on stem

TOP OF STEM
    │
  ──┼──  ← Sensor head (sensor_head.scad, at ~800–900 mm)
    │
  ══╬══  ← Upper anchor (ANCHOR 2)     ┐
    │                                    │ 250 mm
  ══╬══  ← Lower anchor (ANCHOR 1)     ┘
    │
  ──┼──  ← Battery carousel (stem_battery_clamp.scad)
    │
BOTTOM OF STEM (base plate)

Recommended Z positions (above base plate):

• Anchor 1: 450 mm (just above battery carousel top)
• Anchor 2: 700 mm

This places anchors in the mid-stem region, maximising horizontal separation from each other and from nearby metal (battery packs, base plate).

USB cable routing

Each anchor is powered via USB-A from the Jetson Orin. Cable routing:

1. Cable exits Orin USB hub, runs up stem through cable ties
2. Enters anchor bracket through USB notch on left side wall
3. Plugs into MaUWB USB-C port (programming and power)
4. Excess cable loops into rear collar USB channel groove, held with a cable tie

Fasteners (per mount)

#	Spec	Qty	Use
13	M4 × 25 SHCS (thumbscrew head)	2	Collar clamping bolts (tool-free: slot for coin)
14	M4 hex nut	2	Captured in rear collar half
15	M4 × 10 set screw	1	Collar height lock (front half)
16	M4 × 20 SHCS	2	Bracket to collar arm boss
17	M4 hex nut	2	Under collar boss for bracket bolts
18	M2 × 6 BHCS	4	MaUWB PCB to bracket standoffs
19	M2 hex nut	4	PCB standoff nuts

Assembly sequence — anchor (×2)

1. Press M4 hex nuts into collar rear half nut pockets
2. Press M2 hex nuts into bracket standoff counterbores
3. Seat MaUWB PCB in bracket on M2 standoffs; tighten M2 × 6 BHCS (0.3 N·m)
4. Bolt bracket to collar front-half arm boss with 2× M4 × 20
5. Align anti-rotation tab with bracket arm edge
6. Wrap collar halves around stem at desired height
7. Align micro-USB / USB-C port direction, tighten M4 thumbscrews finger-tight
8. Tighten set screw on front half to lock height
9. Route USB cable through rear channel, cable-tie every ~100 mm along stem

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Full Mass Estimate

Part	Material	Est. mass
Tag body + lid (PETG)	PETG	~45 g
Tag TPU bumper	TPU	~12 g
18650 cell	Li-ion	~47 g
TP4056 + MaUWB PCBs	PCB	~20 g
Tag total		~130 g
Anchor collar set × 2 (×2)	PETG	~80 g
Anchor brackets × 2	PETG	~40 g
MaUWB PCBs × 2	PCB	~30 g
Anchor fasteners	SS	~20 g
Total (tag + 2 anchors)		~300 g

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Bill of Materials Summary

#	Description	Qty	Source hint
A	MaUWB ESP32-S3 module	3	AliExpress / MakerFabs
B	18650 Li-ion cell (≥2000 mAh)	1	Panasonic NCR18650B or equiv.
C	TP4056 micro-USB charger board	1	Standard TP4056 module
D	Slide power switch 13×7 mm	1	SS-12D00 or equiv.
E	M2 × 6 BHCS	12	(4 tag + 4 anchor-1 + 4 anchor-2)
F	M2 hex nut	12
G	M3 × 8 SHCS	2	Belt clip
H	M3 hex nut	2
I	M4 × 25 thumbscrew SHCS	4	Slot-head preferred for tool-free use
J	M4 × 20 SHCS	4	Bracket-to-collar
K	M4 × 10 set screw	2	Height lock
L	M4 hex nut	8
M	USB-A to USB-C cable 1 m	2	Anchor power from Orin
N	PETG filament	~200 g	Any brand
O	TPU 95A filament	~30 g	For tag bumper
P	Loctite 243 blue	1	Structural M4 bolts
Q	Cable ties 100 mm	10	Stem cable management