Encoder (encoder) is a signal (such as bit-stream) or converts the data compilation, available to communication, transmission and storage of signal equipment. Encoders or linear displacement angular displacement into an electric signal, the former is called code disc, the latter known as yardsticks. According to the readout encoders can be divided into contact and non-contact two; in accordance with the principle of incremental and absolute encoders can be divided into two categories. Incremental encoder is turning displacement into periodic signals, and then converts the electrical signal into a pulse, pulse the number means that the displacement size.