@Richard_Parsons did some measurements of the D6374 motor with an LCR meter. He measured the Line-Line inductance of the motor as a function of fairly arbitrary rotor angle steps. See picture below:
The amplitude of the inductance variation is about 20%, which is quite significant. The interesting thing about this is that it opens up the possibility for tracking the rotor angle even at standstill. I’m talking sensorless operation all the way down to zero speed, maybe even with continuous position tracking and/or position control.
There are published methods for a technique called “High Frequency Injection” which basically involves continuously checking the response of the motor inductance to figure out the rotor angle. This can be used together with a regular sensorless estimator that is more accurate at high speed (such as the one we have already).