What is a Rotary Encoder?

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  • Written By: Dorothy Distefano
  • Edited By: Michelle Arevalo
  • Last Modified Date: 10 November 2019
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A rotary encoder, also referred to as a shaft encoder, is a sensor that generates an electrical signal in response to a rotational movement. This signal is used to determine or control the speed or position of a mechanical device. A rotary encoder is mounted on a cylindrical shaft and is often used in combination with mechanical conversion devices, such as linear slides and rack and pinions, to measure linear movement. Rotary encoders are used on a wide variety of precision equipment that requires close control of position or speed, including medical devices, robotics, assembly machines, and testing equipment, among others.

There are two main types of rotary encoder. An incremental encoder generates a series of pulses, or counts, as it rotates. Its output is measured in pulses per revolution, and these pulses are used to measure speed or keep track of position. An absolute encoder generates an output signal that is in digital bits, with each bit corresponding to a known position. As a result, an absolute encoder can be used directly to indicate actual position.


The primary components of a rotary encoder are the disk, sensor, and electronics. The disk contains a pattern of fine lines on its surface or external teeth machined on the outside diameter. A sensor is mounted in close proximity to the disc, and reads the lines or teeth. The electronics consist of an output device that receives the signal from the sensor and converts it into a usable signal that provides speed and position feedback. The disk, sensor, and electronics are all mounted inside a housing, with a single cable extending out of the housing that electrically connects the encoder to other devices.

A rotary encoder can use either optical or magnetic-sensing technology. An optical encoder uses a light emitting diode that shines light through a thin glass disk with a pattern of fine lines on its surface. The light passes through the glass disk and a photo-detector receives the signal and produces an electrical output. A magnetic encoder uses a magneto-resistive sensor that can detect changes in the magnetic field as the patterned disk rotates. These changes correspond to the different angular positions as the disk moves.

Rotary encoders are available in many sizes, with different levels of resolution, and with special features such as indicator lights, custom disks, and improved electronics. Selecting the right encoder for a particular application depends primarily on the speed of rotation, measurement accuracy required, and use environment. A rotary encoder with optical sensing is most often used in applications where high operating speeds and resolution are required. An encoder with magnetic sensing is more resistant to environmental factors like dust, moisture, vibration, and mechanical shock, and is typically used for more rugged applications.


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