Optical Encoders

An Optical Encoder is a device that consists of an LED light source and a photodiode detector enclosed in a light sealed package. They tend to be rotary encoders, a central shaft spinning a disk with a periodically spaced window pattern, though they can also be linear encoders for reading things like code strips. They can be panel mounted, designed to mechanically fit on a shaft or knob or part of some mechanical arrangement to detect motion.

Optical encoders can have an incremental or absolute encoding function. Absolute encoders have optical patterns pertaining to position which are output from the encoder either as a binary signal level combination on output pins or encoded as a format like binary coded decimal or as a gray code. They are usually defined as having a number of positions that are phases around the 360 degrees of a circle. This denotes the rotational measurement accuracy possible for the encoder.

Most optical encoders are incremental encoders. These have a fixed pattern that rotates between the LED light source and the photodiode detector. The output of the encoder is a pulse waveform that has a frequency relating to the rotational velocity of the shaft. Many optical encoders have two outputs that are detecting the pattern offset physically by 90 degrees relative to the periodic pattern. As the shaft rotates, the pattern moves in one direction or the other and the two outputs will have a pulse train present. The lead/lag relationship of the two pulses is used to determine the rotational direction of movement. These are called quadrature encoders. An important parameter is the number of pulses per revolution. This does not define the measurement accuracy of the optical encoder pertaining to rotational velocity, but it does limit the ability of the encoder to detect changes in-between the phases of the positions. Changes in the output pulse train frequency are a measure of the integral of the accelerations over the phase positions.

Optical encoders can be defined by the minimum number of rotations the shaft is guaranteed to work. They can have water and dust sealing for use in extreme environments. They can also be available in high-temperature ranges, temperature independence being a big advantage of optical encoders over other technologies. Sometimes encoders have integrated switches and they are available in a number of orientations and terminal styles.
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