There are two types of mechanical configurations for optical encoders: rotary encoders and linear encoders.6

Optical rotary encoders are most commonly found in motion control systems while linear encoders are used more specifically for linear-positioning applications such as piston or actuator monitoring systems.

A rotary encoder using optical sensing technology relies on the rotation of an internal code disc that has opaque lines and patterns on it.

The disc is rotated (hence the name rotary encoder) in a beam of light such as an LED and the markings on the disc act as shutters blocking and unblocking the light.

An internal photodetector senses the alternating light beam and the encoder’s electronics convert the pattern into an electrical signal that is then passed on to an external control system through the encoder’s output

Rotary Encoders

Rotary encoders are also available in two formats:

  1. Incremental .
  2. Absolute .    



The simplest and most commonly used encoders are incremental rotary encoders, which provide information about the instantaneous position of a rotating shaft. The encoder does this by producing one square wave cycle per increment of shaft movement. This increment, referred to as the resolution of the encoder, is built directly into the internal hardware of the device on the code disc. By rotating this code disc in the path of a light source, it acts like a shutter to alternately shut off or transmit the light to a photodetector. And encoder resolution of 360 means that 360 square wave cycles are produced in one complete rotation of the shaft. By counting the number of cycles, you can tell the position of the shaft, relative to its starting position.

Incremental encoders are ideal for rotary encoder applications where relative movement is desired.


               An absolute optical rotary encoder provides a unique binary word for each position. Single turn encoders are ideal for motion control applications where the position-sensing device moves at a slow rate under one rotation or is inactive for long periods of time.

Absolute encoders excel in situations where the position information is needed only periodically (not continuously) or in instances where a power outage requires a machine to be able to reorient itself without having to “re-zero” any of its settings. In these instances once the encoder has power again, the controller knows the position as soon as it reads the data lines from the encoder.


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