How to figure out which IMU is which?

Hardware Pixhawk family
1

Hi everyone.
To better understand the log data, I’d like to know which IMU in Ardupilot is which in the Cube Orange Plus we are using.
Here you can find a table for the sensors in the CubeOrangePlus: ICM42688, ICM20648, ICM20948. The first is not isolated, the other two are.
Connecting a Cube to MP and checking Setup->MandatoryHardware->HW ID I can find INS_ICM42688, INS_ICM20948, INS_ICM20649. So the first IMU is the same in both, the other IMUs have swapped places and one IMU seems to be different: (ICM20648 vs INS_ICM20649).
Checking the hwdef agrees to the HW ID info, but ICM42688 is titled as both IMU0 and IMU1 there and IMU2 contains ICM45686.

So my guess would be:

  • The first IMU is the non-isolated ICM42688
  • The second IMU is the ICM20948
  • The third IMU is the ICM20649 and there is a typo on the cubepilot website.

Are my conclusions correct?
Why would the non-isolated IMU be the first one when (if I understood correctly) the first core of the EKF uses the first IMU?

2

Check the device ID in the parameter file and use Mission Planners decoder on the ctrl>F screen. Or, review the HWID screen in Mission Planner.

3

Thanks, I did that and it obviously agrees with the HWID screen in Mission Planner, which I already looked at.

Is it usual to use the non-isolated IMU of three as the primary one?

4

I was just wondering that myself. I have a CUAV Nora - marketing talks about built in shock absorption but it isn’t clear which IMUs are isolated. IMU 1 shows much high vibes than 2&3, but by default EKF3 uses IMU 1. Is it better to assign one of the other IMUs to the first core of the EKF?

5

Good question. I see the same thing on a Zealot H743 - the ‘secondary’ IMUs have considerably lower vibes than the one being used. I would like to try one of the quiet IMU.