LCM-1 compute module

Summary¶

The LCM-1 (libdrone Compute Module, revision 1) is a Raspberry Pi Zero 2W fitted into the Pi bay on the Backplane. It is the intelligence layer of the platform: it reads flight state from the FC via companion UART, reads sensor data from the payload via WiFi, applies threshold logic to decide what matters, and transmits only meaningful events to the ground operator through the existing ELRS radio link. The LCM-1 is not a sensor and not a camera — it is the decision layer that turns raw data streams into actionable operator alerts. It requires no additional radio hardware and adds no dependency on cellular networks or cloud services.

Concept¶

The problem it solves: data vs information¶

A sensor payload logging at 1 Hz produces 3,600 data points per flight hour. Transmitted raw, this saturates the ELRS telemetry link and gives the operator a firehose they cannot process while also flying. Most of those 3,600 points are "nothing significant happened." The ones that matter — a PM2.5 spike at 67 µg/m³ while flying over a specific GPS coordinate — are buried.

The LCM-1 inverts this. It watches everything, decides what changed significantly, and transmits a single alert with context:

PM2.5: 67 µg/m³ (+55 from baseline) at 48.503°N 15.452°E, altitude 45m, heading NE

The ground operator receives actionable information, not a stream of numbers.

What the LCM-1 sees¶

Via companion UART (UART6, 921,600 baud): - GPS position, altitude, attitude, battery, flight mode, RC switch states - On Betaflight (Pro/Core): polled via MSP at 2–5 Hz - On ArduPilot (Bandit): streamed via MAVLink2 automatically

Via WiFi (payload ESP32-S3 connects to Pi hotspot): - Sensor readings from fitted payload at 1–2 Hz - Payload status

Threshold logic — the intelligence layer¶

The Pi applies configurable thresholds to decide when a reading is significant enough to transmit. Default thresholds:

python THRESHOLDS = { 'PM25': 5.0, # µg/m³ — change triggers transmission 'CO2': 25.0, # ppm 'VOC': 10.0, # index 'RAD': 0.05, # µSv/h — tight: radiation matters early } HEARTBEAT_INTERVAL = 30 # seconds — always transmit something ALTITUDE_MIN_M = 20 # metres — ignore below this (ground contamination)

Below the minimum altitude, all readings are suppressed — the drone is in the downwash contamination zone. Above it, a reading is transmitted when it changes by more than the threshold since the last transmission for that channel. Every 30 seconds, a heartbeat transmits the current state regardless of changes — confirming the link is live and logging is active.

The threshold table is configurable via a JSON file on the Pi's SD card, editable without reflashing. Mission-specific thresholds (lower PM2.5 threshold for air quality research, lower radiation threshold for CBRN deployments) are applied by editing the config file before the flight.

Reference¶

Physical interface¶

Companion harness¶

UART6 is permanently reserved as COMPANION on all libdrone builds. Never reassign it. The companion harness is pre-wired during Phase 5 electronics installation.

Power¶

Pi Zero 2W is powered from a dedicated XL4016 buck converter (5.1V, 1A minimum), independent of the FC BEC. The BEC is already loaded with receiver, GPS, fan, and payload — the Pi's 200 mA typical draw (400 mA peak at boot) does not affect the BEC budget.

Gate: Phase 9 (optional)¶

The LCM-1 is Phase 9 in the WBS — optional, and gated on Phases 1–8 complete and the Pro flying reliably with minimum 10 logged flights. Do not build the LCM-1 until the platform is proven stable.

Procedure¶

LCM-1 first integration¶

1. Print Pi bay cover removal and set aside — bay is already present.
2. Fit Pi Zero 2W on M2.5 × 5 mm standoffs in Pi bay.
3. Connect companion harness JST-SH loom to Pi GPIO pins per pin map.
4. Install companion XL4016 buck converter, output 5.1V, input from battery tap.
5. Configure UART6 in Betaflight as MSP (Pro) or enable MAVLink2 streaming (ArduPilot for Bandit).
6. Flash Pi with Raspberry Pi OS Lite. Install LCM-1 firmware package.
7. Edit /boot/lcm1_config.json with mission-specific thresholds.
8. Power on. Verify Pi boots (LED pattern). Verify FC data arriving via serial log. Verify payload ESP32-S3 connecting to Pi WiFi hotspot.
9. Verify ground station (TX16S → ELRS backpack → tablet QGroundControl) shows position and sensor events.

Rationale¶

Why WiFi for sensor data and not a direct GX12 wired link¶

The GX12 connectors are the payload-to-drone interface, not the payload-to-Pi interface. The Pi reads from the payload via WiFi so that the GX12 wiring remains clean and fixed — adding a third connection to the GX12 would require rerouting wires and potentially creating conflicts. WiFi between the payload ESP32-S3 and the Pi hotspot is short-range (50 cm), effectively lossless, and requires no additional wiring. The GX12 carries power and FC communications; WiFi carries sensor data. The separation is intentional.

Why the LCM-1 is optional and gated on 10 flights¶

Adding the LCM-1 to a drone that is not yet reliable adds an untested component to an already complex system. Any instability — Pi crashing, UART conflict, power draw spike — is much harder to diagnose if the base platform is also unproven. The 10-flight gate ensures the platform behaviour is well understood before the intelligence layer is added.

Connections¶

requires: - flight-controller-hardware - payload-software-protocol - gx12-connector-standard related: - payload-electrical-interface - betaflight-setup - gnss-gps - pro-variant leads_to: - payload-integration