I am checking with Autel support on if you have to hold the drone vertical starting with step 3, or you just let it sit on the arms with hands off. Typically you want to hold vertical, but I want confirmation directly from support on this. It would seem to me that you would want hands off, to let the IMU settle on the calibration setting, and the engineers would program what the angle is supposed to be when set on the arms....just a guess. Will post the reply from support when they answer.
*why is IMU calibration crucial? The IMU tells the drone what is level, straight up, down, sideways etc. Here is the "rub"... went through this a long time ago on one of my first drones doing an IMU calibration. You need an absolutely level table, or as close as you can get. When the drone "sits" on its legs with hand off...is this level, as in level in flight, or is there an angle as it sits? The IMU doesn't know it is sitting on a table. You are telling the IMU that this is perfectly level, so that in flight, it will try to keep this setting. What happens if you are off kilter on any of these settings? The IMU will tell the flight controller..."hey...you aren't level" and try to compensate....the flight controller will check GPS and downward sensors and say...wait a minute....we aren't supposed to be moving...and compensate...thus, fights break out between the two, and the drone is constantly trying to correct all the time, wasting CPU cycles. Back in the day, we used to sit our drones as level as possible, meaning the motors are level to the drone, and use a bubble level on the drone body to make sure that the drone body is perfectly level. Once this is achieved for the IMU, you could basically go into ATTI mode with no downward sensors, and hover with hands off sticks and the drone would not move, provided there was no wind, since the IMU would not fight to maintain level....*
Ok, just got a reply from support....Here is what I would recommend....don't hold it. When holding it, no matter how steady you "think" you are, you aren't. I would start with a flat, level table. What I plan on doing is, using a bubble level on the drone body, finding out where each position is perfectly level... step 1, step 2 etc. For 3, 4 etc, I will use the drone against the wall, so it is sitting at 90 degrees. Is this necessary? Probably not, but it can't hurt and won't take much more time...level doesn't always mean level as it sits with hands off. So bottom line, you want vertical at 90 degrees on each placement and as close to level as possible. Support said to hold it, but I won't. Use a wall or some other object so the IMU gets a solid non-moving reading.
*one more reply from support:
order of calibrations:
IMU
Compass
gimbal
The reasoning is, if you do gimbal before compass, when you do compass, you may throw off the gimbal when rotating for compass calibration. (and may need to do it again)This makes sense.