4.7 KiB
Sensor Calibration — 2026-08-17
Calibration used one continuous 83,229-record hardware capture. The enclosure was held stationary on all six faces, then returned flat for three explicitly directed rotations around each enclosure axis. Position changes remained in the recording so freshness behavior could also be checked during motion.
The raw capture is captures/calibration_session_20260817.csv and is
intentionally excluded from source control.
Capture integrity
- Sequence gaps and resets: 0
- Timestamp anomalies: 0
- Rejected or ignored records: 0
- Acquisition-loop overruns: 0
- ADXL345 overruns: 0
- ADXL345 DATA_READY clear: 4,282 records
- L3G4200D ZYXDA clear: 0 records
- L3G4200D ZYXOR set: 2,456 records
Accelerometer
Only records with ADXL345 DATA_READY set were used in the stationary face means. The paired positive and negative faces produced these enclosure-frame coefficients:
| Axis | Offset (counts) | Scale (counts/g) | Scale (mg/LSB) |
|---|---|---|---|
| X | -1.417678 | 258.890661 | 3.862635 |
| Y | -4.400892 | 259.825135 | 3.848742 |
| Z | +11.776132 | 245.755573 | 4.069084 |
The retained digits make the firmware calculation reproducible; they are not a claim of sub-count measurement accuracy. Independent face selection changed the derived values by up to about 0.03 counts, and repositioning changed a face mean by about one count.
Firmware converts a mapped raw value to integer milligravity with:
accel_mg = round((raw_counts - offset_counts) * 1000 / counts_per_g)
The ADXL345 hardware offset registers remain zero. Calibration is deliberately performed in software so raw readings remain recoverable and the coefficients remain explicit.
Gyroscope
The central five thousand samples of the flat stationary interval produced:
| Axis | Zero-rate bias (counts) | Stationary SD (counts) |
|---|---|---|
| X | +9.0482 | 9.8933 |
| Y | -153.6198 | 9.5714 |
| Z | -7.0238 | 10.4259 |
The instructed positive motion was top/USB-edge lift for +X, left-edge lift for +Y, and counterclockwise rotation viewed from the cover for +Z. Accelerometer tilt kinematics independently confirmed X and Y polarity. Rotation around Z while flat is rotation around gravity, so the accelerometer cannot independently check that sign. Z polarity follows from the right-handed enclosure frame, the right-handed L3G4200D frame, the identity gyro mapping, and the verified X/Y polarities. Axis assignment and polarity are therefore confirmed.
Firmware subtracts the measured zero-rate bias and converts with the nominal L3G4200D +/-500 dps scale of 17.5 mdps/LSB. Gyro scale itself was not measured because no controlled angular-rate reference was available.
The compile-time bias is only an initial correction. Twelve quiet stretches in this session showed real zero-rate wander of roughly 50 mdps on X/Z and 72 mdps on Y; the separate smoke capture differed by up to 63 mdps. That is enough to accumulate several degrees per minute if integrated. Runtime re-zeroing during a verified stationary interval is required before fusion, heading integration, or turn counting. Raw gyro counts therefore remain authoritative in the binary record.
Freshness under motion
Using gyro magnitude greater than 500 counts from stationary bias as a conservative motion selector gave 8,233 moving records. Of the 425 records with ADXL345 DATA_READY clear, 300 (70.6%) exactly repeated the preceding XYZ tuple. Of 7,808 records with DATA_READY set, 85 (1.1%) repeated the tuple. Identical fresh samples remain possible from quantization, pauses, or rotation about the gravity vector, but the strong enrichment confirms that the status flag is practically useful under motion.
The clear flag remains conservative: 125 moving records changed values despite DATA_READY being clear at the earlier status transaction, consistent with a new sample arriving between the separate status and data reads.
Firmware verification
The calibrated CSV-v3 firmware was built, flashed to the assembled prototype, and checked with a 3,814-record flat smoke capture:
- Sequence gaps, resets, timestamp anomalies, and rejected records: 0
- Acquisition-loop and ADXL345 overruns: 0
- Independently recomputed calibrated fields differing from firmware output: 0
- Mean calibrated acceleration: (+19.5, -5.6, +999.9) mg
- Mean bias-corrected gyro: (+25.4, -62.9, -54.8) mdps
The small residual horizontal acceleration is consistent with the enclosure not being perfectly level. The largest residual gyro mean is 0.063 dps, which is evidence of the fixed-bias limitation described above rather than a long-term heading guarantee.
The two floating-point startup metadata lines were subsequently observed after a live device reset. They rendered the intended float32 calibration values before the binary stream began.