How does the encoder achieve precise measurement of motor position?

A device that compiles and converts signals or data into signal forms that can be used for communication, transmission, and storage. Converting angular displacement or linear displacement into electrical signals, the former is called a code disk, and the latter is called a code ruler. It is a commonly used equipment for motor positioning in industry, which can accurately test the angular displacement and rotation position of the motor.
According to the working principle, it can be divided into incremental and absolute forms. An incremental encoder converts displacement into a periodic electrical signal, which is then converted into a counting pulse, and the displacement is represented by the number of pulses. Each position corresponds to a certain numerical code, and the displayed value is only related to the starting and ending positions of the measurement, and is independent of the intermediate process of the measurement.
Incremental 
Usually, there are three output ports, namely A-phase, B-phase, and Z-phase outputs. A phase and B-phase delay each other by 1/4 cycle pulse output, and the positive and negative directions can be distinguished based on the delay relationship. Moreover, by taking the rising and falling edges of A-phase and B-phase, 2 or 4 times the frequency can be achieved; The Z-phase is a single cycle pulse, meaning one pulse is emitted per cycle.
Absolute formula
It corresponds to a circle, and each reference angle emits a unique binary value corresponding to that angle. Multiple positions can be recorded and measured through external circle recording devices.
The big difference between the two is that in the case of incremental encoders, the position is determined by calculating the number of pulses starting from the zero position mark, while in the case of absolute encoders, the position is determined by outputting code readings. Within a circle, each position outputs a unique code reading, so when the power is disconnected, the absolute encoder does not separate from the actual position. If the power is turned on again, the position reading is still current and valid, unlike incremental encoders, which must search for zero marks. 
Working principle of NO.3 encoder Encoder
A photoelectric code disk with a central axis, on which there are circular through and dark lines, and photoelectric emission and reception devices to read, obtains four sets of sine wave signals combined into A, B, C, and D. Each sine wave has a phase difference of 90 degrees (360 degrees relative to a cycle), and the C and D signals are reversed and stacked on the A and B phases to enhance stable signals; Another Z-phase pulse is output per revolution to represent the zero reference position.
Due to the 90 degree difference between A and B phases, the encoder's forward and reverse rotation can be distinguished by comparing whether A phase is in front or B phase is in front. By using zero pulse, the encoder's zero reference position can be obtained. The materials of the encoder encoder encoder disk include glass, metal, and plastic. The glass encoder disk is a thin scribed line deposited on the glass, which has good thermal stability and high accuracy. The metal encoder disk directly passes through and does not have scribed lines, and is not fragile. However, due to the certain thickness of the metal, the accuracy is limited, and its thermal stability is one order of magnitude worse than that of glass. The plastic encoder disk is economical and has low cost, but its accuracy, thermal stability, and lifespan are all slightly worse. 
Resolution - The resolution refers to the number of through or dark lines provided by the encoder per 360 degrees of rotation, also known as the resolution resolution, or directly measuring the number of lines, typically ranging from 5 to 10000 lines per revolution.