What is a Shaft Encoder?

What is a Shaft Encoder?

A shaft encoder, also known as a rotary encoder, is a device that reports the angle of rotation of its shaft. It is typically attached to a motor shaft or other rotating 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 turned on. Unlike an incremental encoder, it tracks the absolute position of the shaft rather than the position relative to where it started. An absolute rotary encoder can use mechanical, magnetic or optical sensors with a rotating disc to determine shaft position. Mechanical encoders use sliding contacts and a disc with metal patterns designed to encode the shaft position. Magnetic encoders detect the position of magnetised strips on a disc, while optical disc devices read specially encoded light and dark areas.

Position data from an absolute shaft encoder is output in digital or analogue form, depending on the design of the device. Digital data is often represented in binary, grey or binary coded decimal code. Gray code is a modified form of binary coding in which adjacent pattern codes differ by only one bit, reducing errors in position data. Digital data can generally be sent in parallel or in a serial format, such as the recommended asynchronous (RS) 422 standard. Standards such as synchronous serial interface (SSI) and controller area network (CAN) are also frequently supported.

Incremental rotary encoders, also known as quadrature encoders, measure the relative motion of the shaft. This type of shaft encoder uses only two optical or mechanical sensors to detect the rotation of the shaft from one angle to the next. To track the current position, external circuitry can be used to count the 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 motion by reading two light- and dark-coded tracks with photodiodes.

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

A rotary shaft optical encoder can generally rotate 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.