You want to use an absolute encoder that determines the position even immediately after turning on the power.

The magnetic field vector created by the magnet depends on the rotational angle. Therefore, the absolute angle can be detected by sensing each magnetic field vector and calculating.

For example, AKM rotary angle sensor can detect the absolute angle even when the motor is stopped immediately after the power is turned on, eliminating the need for zero return operation.

The absolute encoder, whose structure is complicated by the optical encoder, can be realized with a simple structure of only "magnet and IC" by using AKM's rotary angle sensor.