IMU coordinates

Blue Robotics Products Flight Controllers
1

Hello everyone,

I’m using a Navigator FC with ArduSub and I’m trying to confirm the physical orientation of its ICM-20602 IMU.

To determine the axes’ orientation, I rotated the board while observing the RAW_IMU accelerometer data. With the Navigator’s forward-arrow as the vehicle’s reference, my tests indicate the IMU’s axes are oriented as follows: +X points South, +Y points West, and +Z points Down (an SWD orientation).

This has confused me, as my understanding is that ArduPilot performs all its internal calculations in a NED (North-East-Down) logical frame. My assumption was that the IMU’s physical orientation would match this.

Furthermore, AHRS ORIENTATION parameter is Roll 90 and I did the tests considering this angle.

Was the IMU actually supposed to be at these coordinates or should it be at NED?

2

Hi @MIguel,

The RAW_IMU messages are in the vehicle’s body frame, so should be forward-right-down (only North-East-down if the autopilot happens to be facing North), which matches what you described.

More generally, the IMU’s physical orientation is not expected to match the autopilot’s orientation - rotation offsets are specified in the relevant hardware definition file, then further adjusted by the configured autopilot orientation within the vehicle.