TweinStein/RTOS_SensorBoard/IMU_behavior.md

IMU Behavior (GY-521 / MPU-6050)

Empirically verified axis mapping, signs, and scales for the IMU mounted on the racing car.

Mounting

• GY-521 breakout mounted flat on chassis
• Silkscreen Y axis points toward the front bumper
• Chip Z axis points up (out of chassis)

Body-Frame Axis Mapping (right-handed)

Chip axis	Car direction	Physical meaning
+X	RIGHT side of car	lateral
+Y	FORWARD (front bumper)	longitudinal
+Z	UP (away from floor)	vertical

Verification (static gravity tests):

Car orientation	Expected axis = +1 g	Result
Wheels on floor (flat)	AccelZ ≈ +1000 mg	pass
Nose up (on rear bumper)	AccelY ≈ +1000 mg	pass
Laying on LEFT side	AccelX ≈ +1000 mg	pass

Gyro Signs (right-hand rule, verified)

• Counter-clockwise rotation viewed from above → GyroZ positive
• Therefore: left turn → GyroZ > 0, right turn → GyroZ < 0

Steering Sign Convention (project-wide)

steeringAngle > 0 means RIGHT turn.

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:

yaw_rate = -IMU_GyroZ    # now matches steering convention

Accel Signs in Driving

With X-right, Y-forward convention:

Event	Expected sign
Forward throttle (speeding up)	AccelY > 0
Braking (slowing down)	AccelY < 0
Right turn (chassis accel toward right)	AccelX > 0
Left turn (chassis accel toward left)	AccelX < 0
Crash impact	AccelX or AccelY spike, magnitude 20000+ mg

Note: AccelX already matches the steering convention (right turn → positive). No sign flip needed for lateral accel.

Scale Factors (default MPU-6050 ranges)

Quantity	Raw scale	To physical units
Accel	16384 LSB/g	mg = raw * 1000 / 16384
Gyro	131 LSB/(deg/s)	ddps = raw * 10 / 131 (0.1 deg/s)
Temp	—	T_C = raw/340 + 36.53

Ranges: accel ±2 g (±32768 raw), gyro ±250 deg/s (±32768 raw).

Useful vs Unused Axes for Flat-Track Racing

Keep (real signal):

• IMU_GyroZ — yaw rate (king signal)
• IMU_AccelY — longitudinal (braking/throttle)
• IMU_AccelX — lateral (cornering g)

Skip (near-zero on flat ground, wastes RL input dims):

• IMU_GyroX, IMU_GyroY — pitch/roll rates
• IMU_AccelZ — vertical (constant gravity)

Driver Config Summary

See IMU.h for tunable knobs:

• IMU_DLPF_CFG — digital low-pass filter setting (currently 5 = 10 Hz BW, heavy smoothing)
• IMU_AVG_SAMPLES — block average count per IMU_Read call (currently 16)

Both stack: DLPF does analog/digital low-pass in the chip, block averaging reduces residual white noise further.

Notes / TODO

• 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.
• 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.