According to the documentation for Pixhawk Cubes (https://docs.cubepilot.org/user-guides/autopilot/the-cube-module-overview), the Cube Black 2.1 (“Cube+”) has the following IMUs:
According to that document, IMU1 is “non isolated” whereas IMU2 & 3 are isolated on a seperate, vibration damped PCB.
In the context of ArduPilot / ArduCopter, I often read that IMU3 is “non-damped”, which is contrary to the CubePilot Documentation. By default, EK2_IMU_MASK is set to 3 and INS_USE = 1, INS_USE2 = 1 and INS_USE2 = 0. This means IMU3 (at least, what ArduCopter enumerates as IMU3) is not used for EKF2 lane calculation by default. My assumption is, that IMU3 in ArduCopter is what in the CubePilot documentation is physically called IMU1 (the non-isolated one). Meanwhile SB2 strongly recommends to set EK2_IMU_MASK to 7 and all three INS_USE parameters to 1.
When I look at the IDs, that are typically set by ArduCopter for Pixhawk Cube Black:
INS_ACC_ID = 1442082
INS_ACC2_ID = 1114914
INS_ACC3_ID = 1442826
INS_GYR_ID = 2359586
INS_GYR2_ID = 2229282
INS_GYR3_ID = 2360330
(see also https://discuss.px4.io/t/urgent-grounding-notice-for-cubes/10536)
This does not really help in order to understand, which IMU numbers - logically used and logged by ArduCopter - match with which physical IMU number according to the CubePilot documentation.
Particulary I would like to know, which (logical) IMU number in ArduPilot is used for the physically non-isolated IMU.