mikkeli
e8ba581985
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01DDwfd6Ki8UfG4hK6W5YpJz
60 lines
3.6 KiB
Markdown
60 lines
3.6 KiB
Markdown
---
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type: report
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date: 2026-10-15
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---
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# Z27AG CAN: third pass, the drive back
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Sessions `000011-20261015T212904` and `000013-20261015T214544` (the drive back, about 13 minutes,
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no marks) together with pass 2's `000008` and `000009`: 20 km of driving with GNSS and the calibrated
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IMU, no OBD-II. Nothing was marked, so this pass uses what any drive has: corners and distance.
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## Confirmed
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| Signal | Where | Evidence | Registered as |
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|---|---|---|---|
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| Wheel speeds | `235` four words, little-endian (pass 2): bytes 0–1 **front right**, 2–3 **front left**, 4–5 **rear right**, 6–7 **rear left** | from corners, below | `wheel_speed_fr_kph`, `_fl_`, `_rr_`, `_rl_` |
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| Odometer | `570` bytes 4–6, little-endian, **1 km** | steps once per kilometre: 12 steps over 11.9 km (`000009`), 3 over 3.0 km (`000011`); 167966 on the 13th, 167988 at the end of the drive back; valid with the engine off too | `odometer_km` (the layout format gained 24-bit fields) |
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| Distance counter | `14D` bytes 6–7, little-endian, **about 0.40 m** per count | tracks distance from the wheel speeds with correlation 1.0000 in both drives (103.2 and 103.6 m per step of byte 7); wraps every ~26 km | not registered: the odometer covers whole kilometres; this is there if a trip meter needs finer steps |
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### Which word is which wheel
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In a corner the outer wheels run faster than the inner ones, by about half the track width times the
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curvature; and the fronts run on a wider arc than the rears, by about half the wheelbase squared times
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the curvature squared. Fitting each word's share of the four (its speed ÷ their mean) against the
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curvature from the steering angle (`steering_angle_deg` + 2.7° for the straight-ahead offset, ÷ 16.3,
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over a 2.5 m wheelbase), 74 476 frames above 15 km/h:
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| Word | Linear term (track) | Square term (wheelbase) | So |
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|---|---|---|---|
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| 0 | +0.72 m | +1.4 m² | right, front |
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| 1 | −0.72 m | +1.4 m² | left, front |
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| 2 | +0.75 m | −0.9 m² | right, rear |
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| 3 | −0.75 m | −1.9 m² | left, rear |
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Curvature is positive turning left, so a positive linear term is a wheel on the right (outside in a left
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turn). The size, ±0.72–0.75 m, is half the Colt's track (1.45–1.46 m). The square terms split the words
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into a pair above the mean and a pair below it, ±1.5 m² against an expected ±1.6 m² (2.5² ÷ 4). Medians
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by curvature agree: in left turns tighter than 20 m (`κ > 0.05`) the words read 1.068, 0.956, 1.044,
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0.932 of their mean.
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Straight ahead the fronts read 0.6 % more counts than the rears, matching pass 2's per-word scales
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(0.0578 against 0.0581 km/h per count). Driven wheels or tyre sizes, either is ordinary; each pair is
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registered with its own scale, so all four read GNSS speed when cruising.
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Jack stands (checklist 23–26) would still confirm this, but are no longer needed to find it.
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## Candidates
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- **`570` bytes 2 and 3** move together (byte 3 ≈ byte 2 + 48) and fall slowly: byte 2 read 87 on the
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13th and 76 at the end of the drive back, 22 km later. Fuel level fits the fall, but byte 2 also rose
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by about 3 while parked between drives (78 → 81), which fuel would not do on level ground. It may
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be outside temperature instead, falling through the evening. Both read `7F` with the engine off. A
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refuel settles it: fuel jumps, temperature does not. Not registered.
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## Notes on the recording
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- `000012` is a 25-second parked session in between; `000013` ends with the PEAK unplugged.
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- The recorder kept IMU samples as JSON lines (34 MB for `000009`). From this pass on it packs them
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(`imu.bin`, svc-core `imu/packed.rs`): the same samples are 8.5 MB, and all of `000009`'s samples fit
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the packed form exactly.
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