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