How does HFI (high frequency injection) work with BLDC motors
I skimmed through Google materials and there are two steps.


First it measures the impedance of the coils by sending in current at a particular D-Q angle (I think). Then it changes the angle and compares the two impedances. In other words, it's measuring inductance or the strength of the two flux paths through the motor. This lets you get the rotor position, more or less.




https://www.controleng.com/articles/understanding-permanent-magnet-motors/


But you can see from the graph above that it is symmetrical so there is an ambiguity which needs to be resolved. Unless perfectly centered, the
presence of the PM field will bias the ferromagnetic domains in the
iron core so it saturates in one direction earlier than the other. You can ramp up current in both directions and measure when it saturates to differentiate whether you are near a N or S pole. I'm not sure quite sure when this is done though.


Pretty cool. I'll have to read up on it more tommorrow.