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absolute encoder and incremental encoder

DC servo motor encoder is a device that compiles and converts signals (such as bit streams) or data into signal forms that can be used for communication, transmission and storage. Convert angular displacement or linear displacement of encoder into electrical signal, the former is called code wheel, and the latter is called code ruler. According to the reading mode, encoders can be divided into contact type and non-contact type. According to the working principle, encoders can be divided into incremental and absolute types. 

Incremental encoder

Incremental encoder outputs pulses when it rotates, and its position is known by the counting device. When the encoder does not move or loses power, it depends on the internal memory of the counting device to remember the position. In this way, when the power is cut off, the encoder can't move at all. When the incoming call works, the encoder can't lose the pulse due to interference in the process of outputting the pulse. Otherwise, the zero point of the memory of the counting device will shift, and the amount of this shift can't be known until the wrong production results appear.


 Incremental encoder converts the displacement into periodic electrical signals, and then converts the electrical signals into counting pulses, and the number of pulses indicates the displacement. Each position of absolute encoder corresponds to a certain digital code, so its indication value is only related to the starting and ending positions of measurement, and has nothing to do with the intermediate process of measurement. Incremental coding includes UVW signal in addition to ABZ signal of ordinary encoder.

Absolute encoder

Absolute rotary photoelectric encoder has been widely used in angle, length measurement and positioning control in various industrial systems because of its absolute uniqueness, anti-interference and no power-off memory. There are many scribed lines on the code wheel of the absolute encoder, and each scribed line is arranged with 2 lines, 4 lines, 8 lines and 16 lines in turn. In this way, at each position of the encoder, by reading the on and off of each scribed line, a unique set of binary codes (gray codes) from the zero power of 2 to the n-1 power of 2 can be obtained, which is called an n-bit absolute encoder. This encoder is determined by the mechanical position of the encoder, and it is not affected by power failure or interference. 

The uniqueness of each position of the absolute encoder is determined by the mechanical position. It doesn't need to remember, find reference points or count all the time. In this way, the anti-interference characteristics and data reliability of the encoder are greatly improved. Because absolute encoder is obviously superior to incremental encoder in positioning, it has been increasingly used in DC servo motors. 

Because of its high precision and large number of output bits, if parallel output is still used, the output signal of each bit must be well connected, and it must be isolated for more complicated working conditions, and the number of connected cable cores is large, which brings a lot of inconvenience and reduces the reliability.

Therefore, absolute encoders in multi-digit output type generally choose serial output or bus output, and SSI (synchronous serial output) is commonly used for serial output of absolute encoders produced in Germany. 

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