261 lines
16 KiB
Markdown
261 lines
16 KiB
Markdown
# Hardware Re-spec + Watchdog PCB Brief
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**Date:** 2026-06-19 · **Owner:** Leo Rodriguez (Helgen)
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**Strategy (Leo's call):** **phased** — ship a ruggedized interim to the fleet
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now (Phase 1), design the production PCB in parallel (Phase 2).
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**Drivers:** the RCA (`06-Testing-Validation/RCA-2026-06-field-failures.md`) and the ICAVE client
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review (prototype-grade, vibration, connectors, thermal, 4G failures).
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> This is an engineering brief, not a finished schematic. Specific part numbers
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> are **starting points** to price/validate — confirm against the measured power
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> budget (see §2) and your supplier availability.
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---
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## 1. Design requirements (the bar the fix must clear)
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> **Context update (2026-06-19):** the HBOX sits **inside an industrial cabinet**
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> that already provides surge protection, a breaker, isolated ignition sensing, and
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> a regulated **24 Vdc** supply (Mean Well DDR-60G-24) — see
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> `03-Architecture-Hardware/cabinet-electrical-bom.md`. So the HBOX can assume a **clean 24 V feed**
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> and **need not re-implement input surge/reverse-polarity**. The redesign focus
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> narrows to: **24 V→5 V**, the watchdog, the LiPo decision, and — now the top
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> issue — **thermal management** (see §thermal).
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| Requirement | Target | Why |
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| Operating temp | **−10…+75 °C** internal, survive +85 °C | sealed **ASA** box **inside** a sealed ABS IP67 cabinet that **also holds the heat-dissipating Mean Well supply**, in full sun |
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| **Enclosure material** | **ASA (current) — keep**; UV-stable, ~95–100 °C | confirmed ASA (not PLA): margin over the cabinet interior; the case is not the liability |
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| Input voltage | **clean ~24 V** from the cabinet (design the buck 9–36 V for margin) | cabinet's Mean Well already regulates; isolated ignition provided |
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| Surge/transient | **handled at cabinet level** (Phoenix SPD + breaker); HBOX keeps a modest TVS | don't duplicate the cabinet's protection |
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| Vibration | survive continuous machinery vibration | no Dupont, locking connectors, retained boards |
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| Ingress | **IP67 maintained**, no internal condensation | coastal humidity; sealing constrains cooling (§thermal) |
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| 5 V rail | **≥3 A continuous**, handle LTE TX bursts (~2 A) | modem peaks brown out a 1 A hobby boost |
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| Maintainability | **SIM swap without disassembly**; labeled, keyed | client point #5 |
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| Serviceability | one PCB, documented, test points, repeatable | "apt for production" |
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| Observability | log temp + battery + link health | prove failures in the field |
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---
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## 2. First, measure the power budget (blocks everything)
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Before choosing a DC-DC or fuse, **bench-measure** the stack on the actual
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hardware:
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- Idle, 1 Hz, and **LTE-TX-burst** current at 5 V (modem bursts are the spike).
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- Inrush at power-on (caps + modem).
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- Repeat at 12 V and 24 V input.
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Expected ballpark (validate): avg ~0.3–0.6 A @5 V, **bursts 1.5–2 A** during LTE
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transmit. That burst is almost certainly **beyond the PowerBoost 1000C's ~1 A
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continuous rating** — a strong candidate for both the instability *and* the
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"under-rated fuse" the client flagged. Everything below assumes ~2.5 A peak @5 V;
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correct it once measured.
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---
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## 2b. Thermal & enclosure — now the #1 physical fix
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**The problem is a nested thermal trap.** A sealed **ASA** HBOX sits inside a sealed
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**ABS IP67** cabinet that **also contains the Mean Well supply** (60 W class,
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dissipating several watts continuously), in full Veracruz sun. Two failure drivers
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stack:
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- **The ASA shell is not the problem.** ASA is UV-stable and softens at ~95–100 °C,
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with margin over the cabinet interior, so the HBOX shell will **not sag/warp** at
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these temperatures. (Corrects an earlier PLA assumption.)
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- **The LiPo is in that same hot space** (charge limit 45 °C) → degradation,
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swelling, voltage sag → brownout.
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### About adding a fan — read this first
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**A fan *inside* a sealed IP67 box does almost nothing.** It only stirs the same
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trapped air; with no path to outside air, **no heat actually leaves the box** (it
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even adds a little). Cooling requires either **moving heat out by conduction** or
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**exchanging air with the outside** — and air exchange **breaks IP67**, which in a
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salty, humid, dusty port invites corrosion. So:
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| Option | Removes heat? | Keeps IP67? | Verdict |
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| Internal fan only (sealed) | ✗ (just stirs air) | ✓ | **Don't bother** |
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| Filtered vents + extractor fan | ✓ | ✗ (→ ~IP54) | Only with filters + maintenance; risky in this environment |
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| **Conduction: mount hot parts to a metal plate / metal enclosure** | ✓✓ | ✓ | **Best** — heatsink to the outside, stay sealed |
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| **Reduce solar gain** (white/reflective, **sunshade**, relocate out of sun) | ✓ | ✓ | **Cheap, do it now** |
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| **Reduce internal generation** (efficient DC-DC, **remove LiPo**) | ✓ | ✓ | **Do it** |
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| Gore pressure-vent | ✗ (condensation only) | ✓ (membrane) | Keep for humidity, not for cooling |
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| Active (Peltier / vortex / heat-exchanger) | ✓ | ✓ | Overkill/expensive — last resort |
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**Recommended thermal strategy (sealed, no IP compromise):**
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1. **Keep the ASA enclosure** (UV-stable, ~95–100 °C — adequate). Optionally move to
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**metal** for extra heatsinking, or **eliminate the inner box** and
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mount the PCB directly to the cabinet plate.
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2. **Conduct heat out:** bolt the DC-DC (and the new HBOX board's hot spots) to the
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cabinet's **metal mounting plate / gland plate**, or move to a **metal (aluminium)
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cabinet** that acts as a heatsink. Use thermal pads/gap-filler.
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3. **Cut the solar load:** white/reflective cabinet, a **sunshade**, and mount out
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of direct sun where the machine allows.
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4. **Cut internal heat:** a higher-efficiency 24 V→5 V buck and **removing the
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LiPo** both lower the steady-state interior temperature.
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5. **Quantify it:** put a thermocouple/logger inside a representative cabinet for a
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full sunny shift — design to the measured number, not a guess.
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---
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## 3. Phase 1 — Interim ruggedization (do now, on existing modules)
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Goal: stop the bleeding on the deployed fleet without waiting for a PCB. All of
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this is achievable with off-the-shelf parts and rework.
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1. **Kill the Dupont jumpers.** Replace every jumper with **locking, keyed
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connectors** — JST-XH (signal) / Molex Micro-Fit 3.0 (power). Crimped, not
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hand-twisted. This alone removes the #1 vibration intermittent.
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2. **Replace the PowerBoost 1000C** with an **industrial DC-DC** stepping the
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cabinet's **24 V → 5 V, ≥3 A** (e.g., a sealed **Mornsun/Recom** module or a
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**TI LMR33630-class** buck for the LTE bursts). Design the input wide (9–36 V)
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for margin, but the normal feed is the cabinet's clean 24 V. Sized from §2.
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3. **Resolve the battery (decision — see §6).** At minimum **relocate the LiPo**
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to the coolest wall, away from the modem/DC-DC, and add a temperature cutoff;
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better, **replace it** with LiFePO4 or a supercap hold-up, or **drop battery
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mode** if the client doesn't need parked tracking.
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4. **Verify the cabinet fuse.** Read the **blade fuse fitted in the Phoenix PT
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6-FSI/C** holder and compare to the measured worst-case + inrush — this is the
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"under-rated fuse" the client flagged. Surge/breaker are already handled at the
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cabinet (Phoenix SPD + breaker); the HBOX keeps only a **modest local TVS**.
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5. **Mechanical lockdown.** Mount each board on **standoffs with vibration
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isolators/grommets**; retain the simpleRTK2B + 4G master in their sockets with
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a bracket or retention clip; **thread-lock + torque** the SMA bulkheads.
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6. **Conformal-coat** the watchdog/discrete wiring (acrylic or urethane) against
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humidity and to stabilize solder joints under vibration.
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7. **Thermal quick wins (see §2b — do NOT add an internal fan to a sealed box):**
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keep the **ASA** inner box (adequate) or remount the boards on the metal plate; cut
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solar gain (white/reflective cabinet + **sunshade**, relocate out of sun);
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**conduct** the DC-DC/modem heat to the cabinet's metal plate; keep a
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**Gore-style pressure-vent** for condensation only.
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8. **SIM access:** bring the SIM to a **front-panel holder** so swaps don't need
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teardown.
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9. **Flash firmware v2** (thermal logging + proper 4G recovery) on every unit.
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10. **Instrument** at least one unit with an interior **thermocouple log** for a
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full sunny shift to quantify the thermal problem.
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Phase 1 is the "permanent-enough" stabilization while Phase 2 is fabricated.
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---
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## 4. Phase 2 — Integrated Watchdog Carrier PCB (the real fix)
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A single **4-layer, industrial-temp** carrier board that replaces all the
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hand-wiring and the hobby power boards, and onto which the simpleRTK2B and 4G
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NTRIP Master mount via **locking board-to-board** (or retained XBee socket).
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### Block diagram
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```
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Cabinet 24V (already SPD/breaker-protected) ─► [keyed plug]
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└► modest TVS + reverse-polarity FET ─► bulk cap
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└► DC-DC buck 24V→5V (design 9–36V), ≥3A ─► 5V rail
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├► STM32 watchdog (3.3V LDO) ── UART ──► simpleRTK2B RX1 (UBX)
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│ ├ sense: Vin, 5V, battery dividers (filtered)
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│ ├ status LED + SWD header + debug UART (dashboard)
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│ └ optional: 4G modem health/RESET line, RTC
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├► high-side LOAD SWITCH / eFuse ─► switched 5V ─► simpleRTK2B + 4G stack
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└► hold-up: supercap bank OR LiFePO4 charger (see §6)
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Antennas: bulkhead SMA pigtails (GNSS L1/L2 + LTE) · SIM: front-panel holder/MFF2
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```
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### Key design choices vs the current discrete design
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| Current (discrete) | PCB replacement | Benefit |
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| 2N3904 + IRF9540N + 10 k pull-up | integrated **high-side load switch / eFuse** (e.g., TPS25200 / TPS2596 / load-switch IC) | current-limited, fewer parts, defined turn-on, no gate-pull-up gotchas |
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| PowerBoost 1000C | automotive **buck ≥3 A** + protection front-end | survives LTE bursts, wide Vin, transient-tolerant |
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| LiPo in hot box | supercap hold-up **or** LiFePO4 (or none) | removes the thermal/safety liability |
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| **ASA inner box (UV-stable, ~95–100 °C)** | keep (adequate); optional metal for extra heatsinking, or mount PCB to the cabinet plate | survives the cabinet's internal temperature |
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| Dupont + screw terminals | **locking JST/Molex** + pluggable terminal block | vibration-proof |
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| Bare boards loose in box | retained board-to-board + isolated standoffs | no movement, no solder flex |
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| SIM buried in stack | **front-panel SIM** or **MFF2 eSIM** | client point #5 solved |
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| No local protection | **modest TVS + RP FET** (cabinet has the SPD/breaker) | survives without duplicating the cabinet |
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| No temp/observability | MCU temp + sense dividers + test points | provable diagnostics |
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### Board-level details to specify
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- **Protection front-end (light — the cabinet does the heavy lifting):** the
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cabinet already provides surge (Phoenix SPD) + breaker + isolated ignition. The
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HBOX board needs only a **reverse-polarity P-FET** and a **modest TVS** on its
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24 V input — don't duplicate the cabinet's SPD/eFuse.
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- **DC-DC:** qualify input 9–36 V; ≥3 A @5 V; good thermal pad to a copper pour /
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enclosure; spread-spectrum or proper filtering for GNSS RF cleanliness
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(switcher noise can hurt the L1/L2 front-end — keep it away from the antenna
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feed and filter it).
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- **MCU:** keep STM32F411 to reuse v2 firmware, or step to an
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**automotive/industrial-temp STM32 (G0/L4)**. Bring out **SWD** + a **debug
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UART** for the dashboard. Add the internal-temp logging already in v2; consider
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an external temp sensor near the LiPo/modem.
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- **GNSS RF integrity:** keep the switcher and digital noise away from the GNSS
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antenna trace; solid ground plane; the ZED-F9P front-end is sensitive — this
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protects fix quality (and therefore the watchdog's view of "link health").
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- **Connectors:** all locking/keyed; bulkhead SMA with lock washers; a single
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keyed vehicle-power plug.
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- **Mechanical:** mounting holes with vibration isolators; design for conformal
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coat or selective potting of the power section.
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- **DFM/serviceability:** silkscreen labels, test points on every rail + UART,
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a fab/assembly package, and a documented BOM (this is literally the "apt for
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production" the client asked for).
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---
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## 5. Indicative BOM (validate prices/availability/temp grade)
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| Function | Interim (Phase 1) | PCB (Phase 2) |
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| DC-DC **24→5 V** ≥3 A | Recom/Mornsun/TI buck module | TI LMR33630-class on-board buck |
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| Load switch | keep NPN+P-FET (retained) | TPS25200 / TPS2596 / load-switch IC |
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| Input protection | modest TVS + RP diode (cabinet has SPD/breaker) | RP FET + modest TVS on-board |
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| Hold-up / battery | relocate LiPo + temp cutoff | supercap bank **or** LiFePO4 charger |
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| **Inner enclosure** | **ASA — keep** (adequate); or mount to plate | metal box / mount PCB to cabinet plate |
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| MCU | existing Blackpill | STM32F411 (or G0/L4) on-board |
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| Interconnect | JST-XH / Molex Micro-Fit | board-to-board + locking I/O |
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| Antennas | re-torque bulkhead SMA | bulkhead SMA pigtails |
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| SIM | front-panel holder | front-panel holder or MFF2 eSIM |
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| Coating | acrylic conformal coat | conformal coat / pot power section |
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| Thermal | sunshade + **conduct heat to metal plate** (no internal fan) | metal cabinet as heatsink; sealed (keep IP67) |
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---
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## 6. Open decisions for Leo (these change the design)
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1. **Battery mode — keep, change, or drop?**
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- *Keep tracking while the vehicle is off* (anti-theft / parked location)?
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Then use **LiFePO4** (wider temp, safer) or a relocated/temperature-managed
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cell — **not** a LiPo baking next to the modem.
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- *Only need ride-through* across brief power dips? A **supercap** hold-up is
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simpler, safer, and temperature-tolerant — but won't give long parked
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tracking.
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- *Vehicle-only* acceptable? **Drop the battery** entirely — removes the
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single worst thermal/safety liability. **(Recommended if the client doesn't
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require parked tracking — confirm with ICAVE.)**
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2. **SIM strategy:** swappable **front-panel SIM** vs **MFF2 eSIM** (eliminates
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the swap problem but commits the carrier). Depends on how often SIMs change.
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3. **MCU:** reuse F411 (fast, keeps v2 firmware) vs move to an industrial-temp
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STM32 (more margin, small port effort).
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4. **Inner enclosure material:** **confirmed ASA** (UV-stable, ~95–100 °C) — adequate
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for the nested-cabinet temperatures; no material change required. Optionally
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eliminate the inner box and mount the PCB to the cabinet plate for better conduction.
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5. **Cabinet thermal path:** keep ABS IP67 + sunshade + **conduct heat to a metal
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plate**, or move to a **metal (aluminium) cabinet** as a heatsink (best, costs
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more). **An internal fan in a sealed box is not a fix (see §2b)** — only
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conduction or (IP-breaking) ventilation removes heat.
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6. **Make vs buy the modem:** keep the ArduSimple 4G NTRIP Master, or move to a
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qualified industrial LTE module with a known wide-temp rating.
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---
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## 7. Suggested next actions (feeds `PLAN.md`)
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- Bench power-budget + inrush measurement (§2) → picks the 24→5 V buck.
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- **Quantify cabinet temperature** (thermocouple in a representative cabinet, full
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sunny shift) → design target for §2b.
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- Decide §6.1 (battery), §6.4 (inner-box material), §6.5 (cabinet thermal path) —
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the biggest levers. **The ASA enclosure is adequate — no material swap needed; focus on the conduction path + active thermal.**
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- Phase-1 ruggedization kit BOM + rework one unit as the reference build.
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- Phase-2 schematic → layout → fab/assembly; pilot 2–3 boards before fleet.
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- Cabinet BOM is industrial-grade and stays — see `03-Architecture-Hardware/cabinet-electrical-bom.md`.
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