Optical Shaft Encoders

Shaft encoders are used to track a rotating shaft's speed, position, and/or direction.

Optical shaft encoders, as the name suggests, are a subset of encoders which use light to do this.

This type of encoder is used in a wide array of applications. You'll find them in photocopiers, telescopes, factory robots and even in giant mining equipment.

In this lesson we are going to look at the different types of optical shaft encoders and how they work.

How It Works

The components used to make an optical shaft encoder are a light source, a photodetector, and an encoding disk. Some designs also include a mask.

Light Source
Encoding Disk
Opaque Area

The encoding disk is commonly a piece of glass or plastic with a pattern of transparent tracks within opaque areas.

The transparent areas allow light to pass through the disk and the opaque areas either reflect or absorb light. Either way, light is not allowed to pass through the disk.

Occasionally encoder disks are made of metal with slots etched into them.

The light from the light source (usually an LED) is directed toward the photodetector.

The encoding disk is placed in the path of the light.

Depending on where the disk is in its rotation, light is either blocked or allowed to pass through.

A mask is used to block light bleed that may be trying to sneak through adjacent slots in the disk.

This is particularily useful if the slots are so close together that the light is shining at several at the same time.

It can also be helpful in sharpening the dividing line between when light is blocked and is detectable.

An electronic circuit processes the light detection events from the photodetector and produces a clean square wave signal.

A computer or controller can then use this signal to determine the status of the shaft to which the encoder is attached, the speed, position, and/or direction the shaft is spinning.

These properties can be determined using different types of encoder disks.

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