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What Is An Optical Encoder?

Mar 18, 2017

An optical encoder is a type of rotary encoder that uses a sensor to identify position change as light passes through a patterned encoder wheel or disk.

There are four components in an optical shaft encoder:

l  A light source (an LED light)

l  A sensor

l  A moveable disk

l  A fixed mask

he LED shines through one side of the optical shaft encoder. The encoder wheel or disk has a series of tracks on it, similar to the concentric grooves in an LP. The mask has a corresponding track for every track on the disk of the optical encoder, and small perforations, called windows, are cut along the tracks in the mask. As the disk moves, different windows in the mask are covered or open, showing the movement and position of the optical shaft encoder. Each arc in the rotation indicates a different position and has a different pattern of open/closed windows. The sensor behind the mask identifies the optical encoders’ current pattern.

Each sensor represents one single signal for the optical encoder. A track can contain two sensors, which are offset to give two slightly different signals produced at the same time. These offset signals can be used by the optical encoder engine to determine more detailed motion information, like speed. A second track can be used to give an index pulse once per revolution, providing a method to orient the signals.

An even more reliable cousin to basic mask optical encoders is phased-array optical rotary encoders. Phased-array optical encoders use multiple signal outputs to average together to create a single signal that is delivered by the engine. These multiple signals that are used by an optical shaft encoder are called the array. By using averages instead of a single reading, phased array-optical encoders have much more stable signals so they can be used in less stable environments, such as mining or heavy manufacturing, where vibrations or shock could affect a traditional mask optical shaft encoder. They require less precision during installation than traditional mask optical encoders.