IMU layout and representation in flight log

Hi guys,

I have two general questions in regards to IMUs of different Flight Controllers - where my main interest would be in the Pixhawk2. I am aware about the hardware layout for the Pixhawk2 and assume the following design:

FMU Main Board: (Vibration Undamped)

  • STM32F427; flash 2MiB, RAM 256KiB. (microcontroller integrated circuit)
  • On-board 16KiB SPI FRAM
  • All sensors connected via SPI:
    • MPU9250 ( Alternative: ICM20948) - System in Package that combines two Chips: MPU6500 (3-axys Gyroscope + 3-axys Accelerometer) and AK8963 (3-axys digital compass/magnetometer)
    • MS5611 (barometric altitude sensor)

IMU Board: (Vibration damped)

  • All sensors conncected via SPI:
    • MPU9250 ( Alternative: ICM20948)
    • LSM303D (Accel + Mag)
    • L3GD20 (Gyro)
    • MS5611 (Baro)
    • All sensors connected via SPI
Board Parm/Nr./IDIMU Device TypeDe InfoFi
FMU 1 MPU9250 Board is undamped, heated
IMU 2 LSM303D damped, heated (but temperature not measured)
IMU 3 MPU9250 damped, heated

Question1: During a couple of flights and interpretation of data, I’ve made the following observation: IMU1 and IMU3 have smaller Peaks in ACC than IMU2 → Theory: IMU2 is undamped BUT as we see in the Hardwaredesign IMU2 should be damped instead of IMU1 → new Theory: IMU2 has different sensor sensitivity (LSM303D). Can some one confirm the hardware design or tell me specifically which Logdata represents the damped and undamped sensors?

Question2: I’ve read a lot about primary, secondary and tertiary IMUs. But there is no clear indication, which of the specific IMUs is the primary, secondary and tertiary. There is also no information in the log, specifically stating it. In comparison: for the magnetometer there is such option within “COMPASS_PRIMARY”. Therefore I assume, that the log value “IMU” is representing the values from primary IMU, “IMU2” from the secondary and so on. Is this assumption correct?

Thanks for the help

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