Payload integration
Summary¶
A compliant payload consists of a mechanical mast, the PSB-1 shield board, a sensor or actuator, and a companion MCU (ESP32-S3) running payload firmware. It connects to the drone via two GX12-7 female connectors and mounts to the boss pads on the Backplane. The payload is electrically enabled by the pilot via radio switch, sends live readings to the OSD, logs GPS-tagged data to SD card, and is commanded by the FC. Field swap from one payload to another takes under 2 minutes and requires no tools beyond a small hex key for the mast screws.
Concept¶
The modular payload concept¶
libdrone's value proposition is: one airframe, multiple instruments. The mast-based payload architecture achieves this by separating the flying platform from the sensing payload completely. All electrical connections are at the GX12 interface. All mechanical connections are at the M3 boss pads (20 mm spacing) on the Backplane. The Pi bay adds 6 mm of height above the Backplane surface — all mast heights account for this.
Swapping payloads in the field: 1. Remove battery (mandatory — never swap payload live) 2. Unscrew two M3 screws at mast base (~15 s) 3. Unplug both GX12 connectors (~10 s) 4. Plug in new payload GX12 connectors (~10 s) 5. Screw in new mast (~15 s) 6. Fit dust caps on now-empty drone connectors if no second payload ready 7. Reconnect battery and verify OSD shows new payload readings before arming
Total: under 2 minutes. No tools beyond a 2.5 mm hex key.
Mast height selection¶
Three mast heights are available: 40 mm (short), 80 mm (medium), 120 mm (tall).
The minimum height to clear the propeller downwash recirculation zone for air quality sensing is 120 mm. → See induced-velocity for the calculation.
Short and medium masts are appropriate for: non-atmospheric sensors (cameras, radiation detectors shielded from aerodynamic effects), sensors where absolute position rather than atmospheric sampling is the goal, or payloads where mass must be minimised.
Mast height directly affects CG. The CG shift for a 40 g payload on a 120 mm mast is approximately 5–6 mm upward. For payloads above 60 g on tall masts, recalculate CG and consider D-term adjustment. → See pendulum-stability.
EASA mass budget¶
The hard maximum payload mass for EASA Open A2 compliance (staying below 900 g AUW) is 93 g. This includes the mast, PSB-1, MCU, sensor, and all cabling. The air quality mast (SEN66 + ESP32-S3 + PSB-1 + mast body) weighs approximately 55–65 g — within budget with margin for a small camera.
Payloads above 93 g move the drone into EASA Open A3 or Specific category. → See [[safety-regulations]] domain for the regulatory consequences.
Reference¶
Payload mast specifications¶
| Mast | Height | Target mass | Typical use |
|---|---|---|---|
| Short | 40 mm | 15 g | Cameras, compact sensors, tests |
| Medium | 80 mm | 18 g | Near-surface atmospheric sampling with validation |
| Tall | 120 mm | 22 g | Air quality, gas sensors — required height for clean air |
All mast heights use identical base footprint: 2× M3 × 8 mm screws into boss pads at 20 mm spacing on the Backplane surface.
Reference payload: SEN66 air quality mast (medium/tall)¶
| Component | Mass |
|---|---|
| Mast body (PETG, 80 mm) | 18 g |
| PSB-1 shield board (perfboard) | 8 g |
| ESP32-S3 mini dev board | 5 g |
| Sensirion SEN66 module | 6 g |
| GX12-7 female cable connectors (×2) | 8 g |
| Cabling and hardware | 5 g |
| Total | ~50 g |
Remaining EASA A2 payload budget after this mast: ~43 g.
Pre-flight payload checklist¶
- OSD shows payload readings within 10 s of power-on
- GPS position visible in payload serial log (open site required)
- SD card inserted and logging confirmed (LED indicator on PSB-1)
- GX12 lock rings finger-tight on both connectors
- Payload master switch ON (physical switch, not only radio)
- Mast screws torqued (finger-tight + 1/4 turn, not overtightened)
- Payload mass confirmed within EASA budget
Procedure¶
Mating sequence (payload installation)¶
- Battery removed from drone.
- Remove dust caps from both drone-side GX12 connectors. Store in field bag.
- Align Connector A (LEFT, payload) with Connector A (LEFT, drone). Match D-D anti-rotation flats. Seat until fully engaged.
- Screw lock ring clockwise, finger-tight.
- Repeat for Connector B (RIGHT).
- Align mast base with boss pads. Insert 2× M3 × 8 mm screws.
- Tighten screws — finger-tight plus 1/4 turn. Do not crush PETG.
- Connect battery. Verify OSD shows payload readings before arming.
Demating sequence (payload removal)¶
- Disarm. Remove battery.
- Remove 2× mast screws.
- Unscrew Connector B lock ring. Pull straight out.
- Unscrew Connector A lock ring. Pull straight out.
- Fit dust caps on both drone-side connectors immediately.
- Inspect all pins before storage.
Rationale¶
Why battery removal is mandatory before payload swap¶
The GX12 connectors are not hot-swap rated. Connecting or disconnecting under live power can cause brief short circuits between adjacent pins during connector engagement/disengagement. On a 5V rail this is generally harmless, but the payload's ESP32-S3 MCU may experience a brownout or latch-up during the transition. Removing the battery takes 5 seconds and eliminates the risk entirely.
Why M3 screws and not a quick-release¶
Quick-release mechanisms add complexity, wear over many cycles, and can fail in ways that a simple screw fastener cannot. An M3 screw into a PETG boss pad provides positive, inspectable, known-torque retention. Two screws prevent any rotation of the mast on the boss pads. The 15-second per-mast swap time is acceptable for a professional operational context.
Connections¶
requires: - gx12-connector-standard - psb1-shield-board related: - induced-velocity - pendulum-stability - lipo-batteries leads_to: - airframe-integration