Encoder is an electromechanical device that can measure displacement. Encoder principle, in layman's terms, is a digital displacement sensor composed of mechanical components and sensing heads, generally divided into optical and magnetic induction types. Mechanical components can be disks with deposited or engraved patterns (for rotary encoders) or rulers (for linear encoders). The sensing head includes a light source (LED) and a light sensor (photodetector) for reading the generated code (encoder output).

The principle of optical encoders usually consists of rotating and fixed electronic circuits. The rotor is usually a metal, glass, or plastic disc installed on the encoder shaft. The disk has an optical pattern that generates position information through electronic decoding. The rotor disk in the absolute optical encoder uses opaque and transparent segments arranged in Gray code patterns. The stator has corresponding paired LEDs and phototransistors, and LED light is irradiated through the transparent part of the rotor disk and received by the phototransistor on the other side. After the electronic signal is amplified and converted, it can be used to determine its position.

The most widely used classification refers to the type of motion (linear or rotational). In both cases, they can be incremental, semi absolute, or absolute, and incremental information can be obtained by simply calculating pulses. So, it depends on the previous state and the value of the transition. Its biggest drawback is the need to define the starting position of the zero point: in the event of a sudden power outage, this information will be lost. In contrast, in absolute value encoders, each position uses a unique correct reference code, as shown in the following figure, corresponding to unique bit patterns in various tracks. Therefore, the position is always known, and if the system loses power or shuts down, there is no need to define a reference.

This encoder principle solution with only a sensing head cannot recognize the rotation direction of the disc. To solve this problem, another optical sensing head was used, whose output signal was offset by 90 ° from the first one; In other words, the optical sensing head signal is orthogonal. This layout generates two orthogonal square waves, each corresponding to a sensing head (channels A and B), as shown in the following figure.

Therefore, the direction can be detected by the order in which logical values 0 and 1 appear. That is to say, if 11 is received after 01, it means moving to the right, while receiving 00 means moving to the left. However, this is not the only available advantage, as can be seen from the binary pair sequence separated by the green line in Figure 4. There is only one optical sensing head, with only one pulse per cycle, but if there are two optical heads, one cycle will correspond to four pulses. Figure 4 shows the outputs of two related channels (A and B), which helps to understand the appearance of four pulses in each cycle of a channel (A or B), as well as the logic level formed by combining two waves. With each change in state from 00 to 01, the new combination will trigger another step in the counting operation.

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