90 lines
3.3 KiB
Markdown
90 lines
3.3 KiB
Markdown
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# IMU Behavior (GY-521 / MPU-6050)
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Empirically verified axis mapping, signs, and scales for the IMU mounted on the racing car.
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## Mounting
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- GY-521 breakout mounted flat on chassis
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- Silkscreen **Y axis points toward the front bumper**
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- Chip Z axis points up (out of chassis)
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## Body-Frame Axis Mapping (right-handed)
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| Chip axis | Car direction | Physical meaning |
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|-----------|---------------|------------------|
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| **+X** | RIGHT side of car | lateral |
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| **+Y** | FORWARD (front bumper) | longitudinal |
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| **+Z** | UP (away from floor) | vertical |
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Verification (static gravity tests):
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| Car orientation | Expected axis = +1 g | Result |
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|-----------------|----------------------|--------|
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| Wheels on floor (flat) | AccelZ ≈ +1000 mg | pass |
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| Nose up (on rear bumper) | AccelY ≈ +1000 mg | pass |
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| Laying on LEFT side | AccelX ≈ +1000 mg | pass |
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## Gyro Signs (right-hand rule, verified)
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- **Counter-clockwise** rotation viewed from above → **GyroZ positive**
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- Therefore: **left turn → GyroZ > 0**, **right turn → GyroZ < 0**
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## Steering Sign Convention (project-wide)
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**`steeringAngle > 0` means RIGHT turn.**
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Consequence: raw GyroZ sign is opposite to the steering convention. When feeding yaw rate into the model or using it as a turn-rate signal, **negate GyroZ** so that "positive = right turn" throughout:
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```
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yaw_rate = -IMU_GyroZ # now matches steering convention
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```
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## Accel Signs in Driving
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With X-right, Y-forward convention:
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| Event | Expected sign |
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|-------|---------------|
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| Forward throttle (speeding up) | AccelY > 0 |
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| Braking (slowing down) | AccelY < 0 |
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| Right turn (chassis accel toward right) | AccelX > 0 |
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| Left turn (chassis accel toward left) | AccelX < 0 |
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| Crash impact | AccelX or AccelY spike, magnitude 20000+ mg |
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Note: AccelX already matches the steering convention (right turn → positive). No sign flip needed for lateral accel.
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## Scale Factors (default MPU-6050 ranges)
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| Quantity | Raw scale | To physical units |
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|----------|-----------|-------------------|
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| Accel | 16384 LSB/g | `mg = raw * 1000 / 16384` |
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| Gyro | 131 LSB/(deg/s) | `ddps = raw * 10 / 131` (0.1 deg/s) |
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| Temp | — | `T_C = raw/340 + 36.53` |
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Ranges: accel ±2 g (±32768 raw), gyro ±250 deg/s (±32768 raw).
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## Useful vs Unused Axes for Flat-Track Racing
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**Keep (real signal):**
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- `IMU_GyroZ` — yaw rate (king signal)
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- `IMU_AccelY` — longitudinal (braking/throttle)
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- `IMU_AccelX` — lateral (cornering g)
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**Skip (near-zero on flat ground, wastes RL input dims):**
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- `IMU_GyroX`, `IMU_GyroY` — pitch/roll rates
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- `IMU_AccelZ` — vertical (constant gravity)
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## Driver Config Summary
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See `IMU.h` for tunable knobs:
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- `IMU_DLPF_CFG` — digital low-pass filter setting (currently 5 = 10 Hz BW, heavy smoothing)
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- `IMU_AVG_SAMPLES` — block average count per `IMU_Read` call (currently 16)
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Both stack: DLPF does analog/digital low-pass in the chip, block averaging reduces residual white noise further.
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## Notes / TODO
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- Gyro bias calibration at boot is planned but not implemented (`IMU_Calibrate` in pseudocode, not yet in driver). Bias will drift with temperature — recal each boot while car is still.
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- WHO_AM_I check assumes AD0 tied low (address 0x68). If AD0 is ever pulled high, WHO_AM_I still returns 0x68 so the check in `IMU_Init` would fail — compare against hardcoded `0x68` instead of `I2C_ADDRESS` in that case.
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