A Resolver is a system that uses a transformer with three windings - A reference winding that rotates (rotor) and sine and cosine stator windings that are 90° from each other on the circumference of the housing.
In the schematic above, a sine-wave is transmitted via a rotary transformer to the rotating reference winding, the 'rotor'. If the rotor is perpendicular to one of the 'stators' none of the signal is received by that stator. If the rotor is parallel a stator, that stator will receive the full signal. Thus the amplitude and polarity of the outputs reveals the quadrant and position within the quadrant. The sub quadrant position is based on the ratio of the sine vs cosine voltages. SIN θ / COS θ = TAN θ, where θ = shaft angle.
There is more than a little hand-waving about how these signals are detected. Comparing the amplitude of the reference vs stator signals could be accomplished with peak detection, but such a system would have poor noise immunity and thus would have poor accuracy. Instead a four quadrant multiplier is used. Peak detection throws out a lot of signal information.
Using a multiplyer, the reference is compared to the signals throughout the whole cycle.
It is also explained that the 'phase information' provides position information - this is true, but misleading - it is better said that the polarity of the signals indicate the quadrant the rotor is in at anyone time.
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