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What is a Shaft Encoder?

By Kurt Inman
Updated May 16, 2024
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A shaft encoder, also known as a rotary encoder, is a device that reports the rotational angle of its shaft. It is generally attached to a motor shaft or other rotational device and is used to track its current position. Shaft encoders are often used in robotic applications, computer mice and industrial controls. They are also frequently used in machine tool changers, camera lenses and telescopes.

An absolute shaft encoder can determine the current position of its encoder shaft from the moment it is powered up. Unlike an incremental encoder, it tracks the absolute shaft position rather than the position relative to where it started. An absolute rotary encoder may use mechanical, magnetic or optical sensors with a rotating disc to determine the shaft position. Mechanical encoders use sliding contacts and a disc with metal patterns designed to encode the shaft position. Magnetic encoders sense the position of magnetized strips on a disc while optical disc devices read specially-coded light and dark areas.

The position data from an absolute shaft encoder is outputted in either digital or analog form, depending upon the design of the device. Digital data is often represented in binary, gray code or binary coded decimal. Gray code is a modified form of binary encoding in which adjacent pattern codes differ by just one bit, reducing errors in position data. The digital data can usually be output in parallel or in a serial format such as asynchronous Recommended Standard (RS) 422. Standards such as Serial Synchronous Interface (SSI) and Controller Area Network (CAN) are also frequently supported.

Incremental rotary encoders, also known as quadrature encoders, measure relative shaft movement. This type of shaft encoder uses only two optical or mechanical sensors to detect shaft rotation from one angle to the next. To keep track of the current position, external circuitry can be used to count shaft movements from a reference point. In mechanical encoders, cams on the shaft make contact with mechanical sensors to indicate position. Optical encoders can determine movement by reading two light- and dark-coded tracks with photodiodes.

While most incremental encoders output position data with square waves which are 90 degrees out of phase, some can produce sine waves instead. Linear incremental encoders measure distance in a straight line rather than in rotation. They are often used in machine tools. Some incremental encoders include memory with a battery backup to record count information. This type of shaft encoder in combination with a reference point can be used to track absolute position, even at power-up.

An optical rotary shaft encoder can usually turn at high speeds. Some units can rotate up to 30,000 revolutions per minute (RPM). In contrast, most mechanical encoders are much more limited in speed.

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