This is how a resolver[1] type encoder works. It's a rotary transformer with two stator phases at 90 degrees forming a sine and cosine output. The rotor has an AC current passed through, usually a few kHz, and the phase angle of the outputs vs the input is directly related to the shaft angle. There also exists a linear version called an Inductosyn which is similar to the calipers capacitive scale.
They are absolute for a single rotation meaning if power is lost and reapplied, you still know the angle of the shaft (good for robot arms). This absolute positioning is not possible with a quadrature encoder.
They are absolute for a single rotation meaning if power is lost and reapplied, you still know the angle of the shaft (good for robot arms). This absolute positioning is not possible with a quadrature encoder.
[1] https://en.wikipedia.org/wiki/Resolver_(electrical)