saltylab-firmware/chassis/ISSUE_505_CHARGING_DOCK_24V_DESIGN.md

# Issue #505: 24V Charging Dock Hardware Design

**Agent:** sl-mechanical
**Status:** In Progress
**Date Started:** 2026-03-06
**Related Issues:** #159 (5V dock), #489 (docking node)

---

## Design Overview

Upgraded charging dock system for 24V DC power delivery with improved reliability, higher power capacity, and integrated ArUco marker (ID 42) for precision alignment.

### Key Specifications

| Parameter | Specification | Notes |
|-----------|---------------|-------|
| **Voltage** | 24 V DC | Upgrade from 5V (Issue #159) |
| **Power capacity** | 480 W (20 A @ 24V) | Supports battery charging + auxiliary systems |
| **Contact type** | Spring-loaded brass pads (Ø12 mm, 2 pads) | 20 mm CL-to-CL spacing |
| **Alignment method** | V-channel rails + ArUco marker ID 42 | Precision ±15 mm tolerance |
| **Docking nodes** | Compatible with Issue #489 (ROS2 docking node) | MQTT status reporting |
| **Frame material** | PETG (3D-printable) | All parts exportable as STL |
| **Contact height** | 35 mm above dock floor (configurable per robot) | Same as Issue #159 |

---

## Subsystem Design

### A. Power Distribution

#### PSU Selection (24V upgrade)

**Primary:** Mean Well IRM-240-24 or equivalent
- 240W / 10A @ 24V, open frame
- Input: 100-240V AC 50/60Hz
- Output: 24V ±5% regulated
- Recommended alternatives:
  - HLK-240M24 (Hi-Link, 240W, compact)
  - RECOM R-120-24 (half-power option, 120W)
  - TDK-Lambda DRB-240-24 (industrial grade)

**Specifications:**
- PCB-mount or chassis-mount (via aluminum bracket)
- 2× PG7 cable glands for AC input + 24V output
- Thermal shutdown at 70°C (add heatsink if needed)

#### Power Delivery Cables

| Component | Spec | Notes |
|-----------|------|-------|
| PSU to pogo pins | 12 AWG silicone wire (red/black) | 600V rated, max 20A |
| Cable gland exits | PG7, M20 thread, 5-8 mm cable | IP67 rated |
| Strain relief | Silicone sleeve, 5 mm ID | 150 mm sections at terminations |
| Crimp terminals | M3/M4 ring lug, 12 AWG | Solder + crimped (both) |

#### Contact Resistance & Safety

- **Target contact resistance:** <50 mΩ (brass pad to pogo pin)
- **Transient voltage suppression:** Varistor (MOV) across 24V rail (14-28V clamping)
- **Inrush current limiting:** NTC thermistor (10Ω @ 25°C) or soft-start relay
- **Over-current protection:** 25A fuse (slow-blow) on PSU output

---

### B. Mechanical Structure

#### Dock Base Plate

**Material:** PETG (3D-printed)
**Dimensions:** 300 × 280 × 12 mm (L×W×H)
**Ballast:** 8× M20 hex nuts (4 pockets, 2 nuts per pocket) = ~690 g stabilization

**Features:**
- 4× M4 threaded inserts (deck mounting)
- 4× ballast pockets (underside, 32×32×8 mm each)
- Wiring channel routing (10×10 mm), PSU mounting rails
- Cable exit slot with strain relief

#### Back Wall / Pogo Housing

**Material:** PETG
**Dimensions:** 250 × 85 × 10 mm (W×H×T)
**Contact face:** 2× pogo pin bores (Ø5.7 mm, 20 mm deep)

**Features:**
- Pogo pin spring pre-load: 4 mm travel (contact engage at ~3 mm approach)
- LED status bezel mount (4× 5 mm LED holes)
- Smooth contact surface (0.4 mm finish to reduce arcing)

#### V-Guide Rails (Left & Right)

**Material:** PETG
**Function:** Self-aligning funnel for robot receiver plate

**Geometry:**
- V-channel depth: 15 mm (±7.5 mm from centerline)
- Channel angle: 60° (Vee angle) for self-centering
- Guide length: 250 mm (front edge to back wall)
- 2.5 mm wall thickness (resists impact deformation)

**Design goal:** Robot can approach ±20 mm off-center; V-rails funnel it to ±5 mm at dock contact.

#### ArUco Marker Frame

**Design:** 15 cm × 15 cm frame (150×150 mm outer), marker ID 42

**Frame mounting:**
- Material: PETG (3D-printed frame + acrylic cover)
- Marker insertion: Side-slot, captures 100×100 mm laminated ArUco label
- Position: Dock entrance, 1.5 m height for camera visibility
- Lighting: Optional white LED ring around frame for contrast

**Marker specs:**
- Dictionary: `cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_4X4_250)`
- Marker ID: 42 (uint8, fits DICT_4X4_250: 0-249)
- Printed size: 100×100 mm
- Media: Glossy photo paper + 80 µm lamination (weather protection)

#### PSU Bracket

**Material:** PETG
**Attachment:** 4× M4 SHCS to base rear, bolts through PSU flanges

**Features:**
- Mounting pads for PSU feet
- Cable routing guides (AC input + 24V output)
- Thermal airflow clearance (30 mm minimum)
- Optional DIN-rail adapter (for rackmount variant)

#### LED Status Bezel

**Material:** PETG
**Function:** 4× LED indicator display (charging state feedback)

**LEDs & Resistors:**

| LED | Color | State | Vf (typ) | Resistor | Notes |
|-----|-------|-------|----------|----------|-------|
| L1 | Red | SEARCHING | 2.0 V | 180 Ω | No robot contact |
| L2 | Yellow | ALIGNED | 2.1 V | 180 Ω | Contact made, BMS pre-charge |
| L3 | Blue | CHARGING | 3.2 V | 100 Ω | Active charging |
| L4 | Green | FULL | 2.1 V | 180 Ω | Trickle/float mode |

**Current calculation (for 24V rail):**
- Red/Yellow/Green: R = (24 − Vf) / 0.020 ≈ 1000 Ω (use 1.0 kΩ 1/4W)
- Blue: R = (24 − 3.2) / 0.020 = 1040 Ω (use 1.0 kΩ)

**Control:**
- Jetson Orin NX GPIO output (via I2C LED driver or direct GPIO)
- Pulldown resistor (10 kΩ) on each GPIO if using direct drive
- Alternative: TP4056 analog output pins (if in feedback path)

---

### C. Robot Receiver (Mating Interface)

**Cross-variant compliance:** Same receiver design works for SaltyLab, SaltyRover, SaltyTank with different mounting interfaces.

#### Contact Pads

- **Material:** Bare brass (10-12 mm OD, 2 mm thick)
- **Pressing:** 0.1 mm interference fit into PETG housing
- **Polarity marking:** "+" slot on right side (+X), "-" unmarked on left
- **Solder lug:** M3 ring lug on rear face (connects to robot BMS)

#### V-Nose Guide

- **Profile:** Chamfered 14° V-nose (30 mm wide)
- **Function:** Mates with dock V-rails for alignment funnel

#### Mounting Variants

| Robot | Mount Type | Fastener | Height Adjustment |
|-------|-----------|----------|------------------|
| SaltyLab | Stem collar (split, 2×) | M4 × 16 SHCS (2×) | Tune via firmware offset |
| SaltyRover | Deck flange (bolt-on) | M4 × 16 SHCS (4×) | 20 mm shim if needed |
| SaltyTank | Skid plate (bolt-on) | M4 × 16 SHCS (4×) | 55 mm ramp shim recommended |

---

## 3D-Printable Parts (STL Exports)

All parts print in PETG, 0.2 mm layer height, 40-60% infill:

| Part | File | Qty | Infill | Est. Mass | Notes |
|------|------|-----|--------|----------|-------|
| Dock base | `charging_dock_505.scad` (base_stl) | 1 | 60% | ~420 g | Print on large bed (300×280 mm) |
| Back wall + pogo | `charging_dock_505.scad` (back_wall_stl) | 1 | 40% | ~140 g | Smooth face finish required |
| V-rail left | `charging_dock_505.scad` (guide_rail_stl) | 1 | 50% | ~65 g | Mirror for right side in slicer |
| V-rail right | *(mirror of left)* | 1 | 50% | ~65 g | — |
| ArUco frame | `charging_dock_505.scad` (aruco_frame_stl) | 1 | 30% | ~35 g | Slot accepts 100×100 mm marker |
| PSU bracket | `charging_dock_505.scad` (psu_bracket_stl) | 1 | 40% | ~45 g | — |
| LED bezel | `charging_dock_505.scad` (led_bezel_stl) | 1 | 40% | ~15 g | — |
| **Receiver (Lab)** | `charging_dock_receiver_505.scad` (lab_stl) | 1 | 60% | ~32 g | Stem collar variant |
| **Receiver (Rover)** | `charging_dock_receiver_505.scad` (rover_stl) | 1 | 60% | ~36 g | Deck flange variant |
| **Receiver (Tank)** | `charging_dock_receiver_505.scad` (tank_stl) | 1 | 60% | ~42 g | Extended nose variant |

---

## Bill of Materials (BOM)

### Electrical Components

#### Power Supply & Wiring

| # | Description | Spec | Qty | Unit Cost | Total | Source |
|---|---|---|---|---|---|---|
| E1 | PSU — 24V 10A | Mean Well IRM-240-24 or Hi-Link HLK-240M24 | 1 | ~$40–60 | ~$50 | Digi-Key, Amazon |
| E2 | 12 AWG silicone wire | Red + black, 600V rated, 5 m spool | 1 | ~$15 | ~$15 | McMaster-Carr, AliExpress |
| E3 | PG7 cable gland | M20 thread, IP67, 5–8 mm cable | 2 | ~$3 | ~$6 | AliExpress, Heilind |
| E4 | Varistor (MOV) | 18–28V, 1 kA | 1 | ~$1 | ~$1 | Digi-Key |
| E5 | Fuse — 25A | T25 slow-blow, 5×20 mm | 1 | ~$0.50 | ~$0.50 | Digi-Key |
| E6 | Fuse holder | 5×20 mm inline, 20A rated | 1 | ~$2 | ~$2 | Amazon |
| E7 | Crimp ring terminals | M3, 12 AWG, tin-plated | 8 | ~$0.20 | ~$1.60 | Heilind, AliExpress |
| E8 | Strain relief sleeve | 5 mm ID silicone, 1 m | 1 | ~$5 | ~$5 | McMaster-Carr |

#### Pogo Pins & Contacts

| # | Description | Spec | Qty | Unit Cost | Total | Source |
|---|---|---|---|---|---|---|
| C1 | Pogo pin assembly | Spring-loaded, Ø5.5 mm OD, 20 mm, 20A rated, 4 mm travel | 2 | ~$8–12 | ~$20 | Preci-Dip, Jst, AliExpress |
| C2 | Brass contact pad | Ø12 × 2 mm, H68 brass, bare finish | 2 | ~$3 | ~$6 | Metal supplier (Metals USA, OnlineMetals) |
| C3 | Solder lug — M3 | Copper ring, tin-plated | 4 | ~$0.40 | ~$1.60 | Heilind, Amazon |

#### LED Status Circuit

| # | Description | Spec | Qty | Unit Cost | Total | Source |
|---|---|---|---|---|---|---|
| L1 | 5 mm LED — Red | 2.0 V, 20 mA, diffuse | 1 | ~$0.30 | ~$0.30 | Digi-Key |
| L2 | 5 mm LED — Yellow | 2.1 V, 20 mA, diffuse | 1 | ~$0.30 | ~$0.30 | Digi-Key |
| L3 | 5 mm LED — Blue | 3.2 V, 20 mA, diffuse | 1 | ~$0.50 | ~$0.50 | Digi-Key |
| L4 | 5 mm LED — Green | 2.1 V, 20 mA, diffuse | 1 | ~$0.30 | ~$0.30 | Digi-Key |
| R1–R4 | Resistor — 1 kΩ 1/4W | Metal film, 1% tolerance | 4 | ~$0.10 | ~$0.40 | Digi-Key |
| J1 | Pin header 2.54 mm | 1×6 right-angle | 1 | ~$0.50 | ~$0.50 | Digi-Key |

#### Current Sensing (Optional)

| # | Description | Spec | Qty | Unit Cost | Total | Source |
|---|---|---|---|---|---|---|
| S1 | INA219 I2C shunt monitor | 16-bit, I2C addr 0x40, 26V max | 1 | ~$5 | ~$5 | Adafruit, Digi-Key |
| S2 | SMD resistor — 0.1 Ω | 1206, 1W | 1 | ~$1 | ~$1 | Digi-Key |

### Mechanical Hardware

| # | Description | Spec | Qty | Unit Cost | Total | Source |
|---|---|---|---|---|---|---|
| M1 | M20 hex nut | Steel DIN 934, ~86 g | 8 | ~$0.80 | ~$6.40 | Grainger, Home Depot |
| M2 | M4 × 16 SHCS | Stainless A4 DIN 912 | 16 | ~$0.30 | ~$4.80 | Grainger |
| M3 | M4 × 10 BHCS | Stainless A4 DIN 7380 | 8 | ~$0.25 | ~$2.00 | Grainger |
| M4 | M4 heat-set insert | Brass, threaded, M4 | 20 | ~$0.15 | ~$3.00 | McMaster-Carr |
| M5 | M3 × 16 SHCS | Stainless, LED bezel | 4 | ~$0.20 | ~$0.80 | Grainger |
| M6 | M3 hex nut | DIN 934 | 4 | ~$0.10 | ~$0.40 | Grainger |
| M7 | M8 × 40 BHCS | Zinc-plated, floor anchors (optional) | 4 | ~$0.50 | ~$2.00 | Grainger |
| M8 | Rubber foot | Ø20 × 5 mm, self-adhesive | 4 | ~$0.80 | ~$3.20 | Amazon |

### ArUco Marker & Frame

| # | Description | Spec | Qty | Unit Cost | Total | Source |
|---|---|---|---|---|---|---|
| A1 | ArUco marker print | 100×100 mm, ID=42, DICT_4X4_250, glossy photo paper | 2 | ~$1.50 | ~$3.00 | Print locally or AliExpress |
| A2 | Lamination pouch | A4, 80 µm thick | 2 | ~$0.40 | ~$0.80 | Amazon, Staples |
| A3 | Acrylic cover sheet | Clear, 3 mm, 150×150 mm | 1 | ~$3 | ~$3.00 | McMaster-Carr |

### Consumables & Assembly

| # | Description | Spec | Qty | Unit Cost | Total | Source |
|---|---|---|---|---|---|---|
| X1 | Solder wire | 63/37 Sn/Pb or lead-free, 1 m | 1 | ~$3 | ~$3.00 | Digi-Key |
| X2 | Flux paste | No-clean, 25 mL | 1 | ~$4 | ~$4.00 | Digi-Key |
| X3 | Loctite 243 | Thread-locker (medium strength), 10 mL | 1 | ~$4 | ~$4.00 | Grainger |
| X4 | Epoxy adhesive | Two-part, 25 mL | 1 | ~$6 | ~$6.00 | Home Depot |

---

## Assembly Procedure

### Phase 1: Preparation

1. **Print all PETG parts** (see STL export list above)
   - Base: 0.3 mm layer, 60% infill (heavy/stable)
   - Back wall: 0.2 mm, 40% infill
   - Rails & brackets: 0.2 mm, 40-50% infill
   - Support removal: slow, avoid pogo bore damage

2. **Prepare ballast nuts**
   - Sort 8× M20 hex nuts (stack in 4 pockets, 2 per pocket)
   - Optional: fill pockets with epoxy to prevent rattling

3. **Press brass contact pads**
   - Apply 0.1 mm interference press-fit into receiver housing bores
   - Use arbor press @ ~2 tons force
   - Or use slow manual press (avoid chipping brass edges)

### Phase 2: Base Assembly

4. **Install heat-set M4 inserts** into base plate
   - Back wall attach points (3×)
   - Guide rail attach points (4× each side)
   - ArUco mast feet (4×)
   - PSU bracket mount (4×)
   - Use soldering iron (350°C) or insert tool, press vertically

5. **Ballast installation**
   - Insert M20 hex nuts into base pockets (from underside)
   - Verify pockets are flush, no protrusions into wiring channel
   - Optional: epoxy-lock nuts with 5-minute epoxy

6. **Install pogo pins** into back wall
   - Press spring-loaded pins from front face into Ø5.7 mm bores (20 mm deep)
   - Flange seats against counterbore shoulder at 1.5 mm depth
   - Apply small drop of Loctite 243 to bore wall (prevents rotation)

### Phase 3: Electrical Assembly

7. **Solder wires to pogo pin terminals**
   - 12 AWG red wire → POGO+ pin
   - 12 AWG black wire → POGO- pin
   - Solder both in & out of lug for redundancy
   - Add ~50 mm strain relief sleeve over each joint

8. **Route pogo wires through base wiring channel**
   - Guide down channel (10×10 mm trough)
   - Exit through cable gland slot on rear

9. **Assemble PSU bracket**
   - Bolt Mean Well IRM-240-24 (or equivalent) to bracket pads
   - 4× M4 fasteners through bracket to base rear
   - Orient PSU exhaust away from dock (for ventilation)

10. **Connect 24V wiring**
    - Pogo+ wire (red) → PSU V+ terminal
    - Pogo- wire (black) → PSU COM/GND terminal
    - Observe polarity strictly (reverse = short circuit)

11. **Install power protection**
    - Fuse holder in-line on PSU V+ output (25A slow-blow)
    - Varistor (MOV, 18–28V) across V+/COM rails (clamp transients)
    - Optional: NTC thermistor (10Ω @ 25°C) in series for soft-start

12. **Wire AC mains input** (if not pre-assembled)
    - Route AC input through cable gland on PSU bracket
    - Connect to PSU AC terminals (L, N, PE if applicable)
    - Ensure all connections are soldered + crimped

### Phase 4: LED Assembly

13. **Install LED bezel into back wall**
    - 4× 5 mm LEDs press-fit into bezel holes (bodies recessed ~2 mm)
    - Solder resistors (1 kΩ 1/4W) to LED anodes on rear
    - Connect all LED cathodes to common GND line (black wire to PSU COM)
    - Wire LED control lines to Jetson Orin NX GPIO (via I2C expander if needed)

14. **Connect LED header**
    - 2.54 mm pin header (1×6) plugs into LED control harness
    - Pin 1: LED1 (red, SEARCHING)
    - Pin 2: LED2 (yellow, ALIGNED)
    - Pin 3: LED3 (blue, CHARGING)
    - Pin 4: LED4 (green, FULL)
    - Pins 5–6: GND, +24V (power for LED feedback monitoring)

### Phase 5: Mechanical Assembly

15. **Bolt back wall to base**
    - 3× M4×16 SHCS from underside of base
    - Tighten to ~5 Nm (snug, don't overtighten plastic)
    - Back wall should be perpendicular to base (verify with level)

16. **Attach V-guide rails**
    - Left rail: 4× M4 fasteners into base inserts (front & rear attach)
    - Right rail: Mirror (flip STL in slicer) or manually mirror geometry
    - Verify V-channels are parallel & symmetrical (±2 mm tolerance)

17. **Mount ArUco marker frame**
    - Bolt 4× M4×10 fasteners to frame feet (attach to base front)
    - Insert laminated 100×100 mm ArUco marker (ID 42) into frame slot
    - Verify marker is flat & centered (no curl or shadow)

18. **Attach rubber feet** (or floor anchors)
    - 4× self-adhesive rubber feet on base underside corners
    - OR drill M8 holes through base (optional: permanent floor mounting)

### Phase 6: Robot Receiver Assembly

19. **Assemble robot receiver** (per variant)
    - **SaltyLab:** 2-piece stem collar (M4×16 clamps Ø25 mm stem)
    - **SaltyRover:** Single flange piece (4× M4 to deck underbelly)
    - **SaltyTank:** Single piece w/ extended nose (4× M4 to skid plate)

20. **Press brass pads into receiver**
    - Ø12 mm pads press into 0.1 mm interference bores
    - Apply Loctite 603 retaining compound to bore before pressing
    - Manual arbor press @ ~1-2 tons force; pads should be proud 0.2 mm

21. **Solder receiver wires**
    - 12 AWG wires (red/black) solder to M3 solder lugs on pad rear
    - Route wires through wire channel on mount face
    - Terminate to robot BMS/charging PCB input

---

## Wiring Diagram (24V System)

```
┌─────────────────────────────────────────────────────────────┐
│                       MAINS INPUT (AC)                       │
│                        110/220 V AC                          │
└────────────┬────────────────────────────────────────────────┘
             │
             ▼
      ┌──────────────┐
      │  IRM-240-24  │  24V / 10A out (240W)
      │   PSU        │  ±5% regulated, open-frame
      └──┬───────┬───┘
    +24V │       │ GND
         │       │
    ┌────┴┐   ┌─┴────┐
    │ [F] │   │ [F]  │  Fuse holder (25A slow-blow)
    │     │   │      │
    │ +24 │   │ GND  │  12 AWG silicone wire to back wall
    │     │   │      │
    └────┬┘   └─┬────┘
         │     │
    +24V│     │GND
        ▼     ▼
    ┌─────────────────┐
    │  Back wall      │
    │  ┌───────────┐  │
    │  │  POGO+    │  │ Spring-loaded contact pin (+24V)
    │  │  POGO-    │  │ Spring-loaded contact pin (GND)
    │  └────┬──────┘  │
    │       │         │
    │ ┌─────┴─────┐   │
    │ │  LED 1-4  │   │ Red, Yellow, Blue, Green indicators
    │ │  Resistors│   │ 1 kΩ limiting resistors (×4)
    │ │  [GPIO]   │   │ Control from Jetson Orin NX I2C
    │ └───────────┘   │
    └─────┬───────────┘
          │
     ═════╧════════ DOCK / ROBOT AIR GAP (≤50 mm) ═════════════
          │
          ▼
    ┌──────────────────┐
    │  Robot Receiver  │
    │  ┌────────────┐  │
    │  │ Contact +  │  │ Brass pad (Ø12×2 mm) [+24V]
    │  │ Contact -  │  │ Brass pad (Ø12×2 mm) [GND]
    │  └──┬──┬──────┘  │
    │     │  │         │
    │   12 AWG wires   │ Red/black to BMS
    │     │  │         │
    │  ┌──▼──▼──┐      │
    │  │ Robot  │      │
    │  │ BMS    │      │
    │  │Battery │      │ Charging current: 0–15A (typical)
    │  └────────┘      │
    └──────────────────┘

OPTIONAL — CURRENT SENSING (Diagnostic)
         │ +24V
    ┌────┴────┐
    │[INA219] │ I2C current monitor (0.1Ω sense resistor)
    │ I2C 0x40│ Jetson reads dock current → state machine
    └────┬────┘
         │ GND

LED STATE MACHINE CONTROL (from docking_node.py):
    State          GPIO/Signal    LED Output
    ─────────────────────────────────────────
    SEARCHING      GPIO H         Red LED ON (20 mA, 1 kΩ)
    ALIGNED        GPIO H         Yellow LED ON (pre-charge active)
    CHARGING       GPIO H         Blue LED ON (>1 A charging)
    FULL/COMPLETE  GPIO H         Green LED ON (float mode)

    GPIO driven via Jetson Orin NX I2C LED driver (e.g., PCA9685)
    or direct GPIO if firmware implements bitbang logic.
```

---

## Integration with ROS2 Docking Node (#489)

**Docking node location:** `./jetson/ros2_ws/src/saltybot_docking/docking_node.py`

### MQTT Topics

**Status reporting (outbound):**
```
saltybot/docking/status  → { state, robot_id, contact_voltage, charge_current }
saltybot/docking/led     → { red, yellow, blue, green }  [0=OFF, 1=ON, blink_hz]
```

**Command subscriptions (inbound):**
```
saltybot/docking/reset   → trigger dock reset (clear fault)
saltybot/docking/park    → move robot out of dock (e.g., after full charge)
```

### Firmware Integration

**State machine (4 states):**
1. **SEARCHING** — No robot contact; dock waits for approach (ArUco marker detection via Jetson camera)
2. **ALIGNED** — Contact made (BMS pre-charge active); dock supplies trickle current (~100 mA) while robot capacitors charge
3. **CHARGING** — Main charge active; dock measures current via INA219, feedback to BMS
4. **FULL** — Target voltage reached (≥23.5 V, <100 mA draw); dock holds float voltage

**Current sensing feedback:**
- INA219 I2C shunt on 24V rail monitors dock-to-robot current
- Jetson polls at 10 Hz; state transitions trigger LED updates & MQTT publish
- Hysteresis prevents flickering (state valid for ≥2 sec)

---

## Testing Checklist

- [ ] **Electrical safety**
  - [ ] 24V output isolated from mains AC (< 2.5 kV isolation @ 60 Hz)
  - [ ] Fuse 25A blocks short-circuit (verify blow @ >30 A)
  - [ ] Varistor clamps transient overvoltage (check 28V limit)
  - [ ] All crimps are soldered + crimped (pull test: no slippage @ 10 lbf)

- [ ] **Mechanical**
  - [ ] Base level on 4 rubber feet (no rocking)
  - [ ] V-rails parallel within ±2 mm across 250 mm length
  - [ ] Back wall perpendicular to base (level ±1°)
  - [ ] Pogo pins extend 4 mm from back wall face (spring preload correct)

- [ ] **Contact alignment**
  - [ ] Robot receiver pads contact pogo pins with ≥3 mm contact face overlap
  - [ ] Contact resistance < 50 mΩ (measure with multimeter on lowest ohm scale during light press)
  - [ ] No visible arcing or pitting (inspect pads after 10 charge cycles)

- [ ] **Power delivery**
  - [ ] 24V output at PSU: 23.5–24.5 V (under load)
  - [ ] 24V at pogo pins: ≥23.5 V (< 0.5 V droop @ 10 A)
  - [ ] Robot receives 24V ± 1 V (measure at BMS input)

- [ ] **LED status**
  - [ ] Red (SEARCHING) steady on before robot approach
  - [ ] Yellow (ALIGNED) turns on when pads make contact
  - [ ] Blue (CHARGING) turns on when charge current > 500 mA
  - [ ] Green (FULL) turns on when current drops < 100 mA (float mode)

- [ ] **ArUco marker**
  - [ ] Marker ID 42 is readable by Jetson camera from 1.5 m @ 90° angle
  - [ ] No glare or shadow on marker (add diffuse lighting if needed)
  - [ ] Marker detected by cv2.aruco in < 100 ms

- [ ] **MQTT integration**
  - [ ] Dock publishes status every 5 sec (or on state change)
  - [ ] LED state matches reported dock state
  - [ ] Current sensing (INA219) reads within ±2% of true dock current

---

## Firmware/Software Requirements

### Jetson Orin NX (Docking controller)

**Python dependencies:**
```bash
pip install opencv-contrib-python  # ArUco marker detection
pip install adafruit-circuitpython-ina219  # Current sensing
pip install rclpy  # ROS2
pip install paho-mqtt  # MQTT status reporting
```

**Key Python modules:**
- `cv2.aruco.getPredefinedDictionary(cv2.aruco.DICT_4X4_250)` → ArUco ID 42 detection
- `Adafruit_INA219` → I2C current monitoring @ 0x40
- GPIO library → LED control (via I2C LED driver or direct GPIO)

**ROS2 node:** `saltybot_docking/docking_node.py` (already present, Issue #489)
- Subscribes to `/docking/approach_request`
- Publishes to `/docking/status`, `/docking/led_state`
- MQTT gateway for legacy systems

---

## Files to Commit

**New files for Issue #505:**
```
chassis/
├── charging_dock_505.scad              [Main dock 24V design]
├── charging_dock_receiver_505.scad     [Robot receiver 24V variant]
├── ISSUE_505_CHARGING_DOCK_24V_DESIGN.md  [This file]
├── charging_dock_505_BOM.csv           [Excel-friendly BOM export]
└── charging_dock_505_WIRING_DIAGRAM.md [Detailed wiring guide]

docs/
└── Issue_505_Assembly_Guide.md  [Step-by-step assembly photos + text]
```

---

## Revision History

| Date | Version | Changes |
|------|---------|---------|
| 2026-03-06 | 1.0 | Initial design (24V upgrade from Issue #159) |

---

## Next Steps

1. ✅ Design specification (this document)
2. ⏳ OpenSCAD CAD files (`charging_dock_505.scad`, `charging_dock_receiver_505.scad`)
3. ⏳ BOM export (CSV format for procurement)
4. ⏳ 3D-printed prototype testing
5. ⏳ Electrical integration with Jetson docking node
6. ⏳ ArUco marker calibration & documentation
7. ⏳ PR submission & merge to `main`

---

**Designer:** sl-mechanical
**Date:** 2026-03-06
**Status:** Design Specification Complete — Awaiting CAD Implementation