Motion System Design

Overview and update: Rotary encoders

Whether incremental or absolute, magnetic or optical, rotary encoders track motor shaft rotation to generate digital position and motion information. Their use proliferates in industrial and commercial designs.

Rotary encoders are electromechanical devices used for sensing in myriad applications—on motors paired with drives and automated machinery for everything from consumer electronics, elevators, and conveyor speed monitoring to position control on automated industrial machines and robotics. They track the turning of motor shafts to generate digital position and motion information.

For industrial applications, incremental encoders (used when only relative position is needed, or cost an issue) are typically used with ac induction motors. In contrast, absolute encoders (which give a different binary output at each position, so shaft position is absolutely determined) are often paired with permanent-magnet brushless motors in servo applications. Often, encoder feedback is used to ensure synchronization of the motor stator and rotor positions to drive-supplied current, so current is applied to the windings when the rotor magnets are within a proper position range (to maximize torque.)

Rotary optical encoders

Rotary optical encoders, the most widespread encoder design, consist of an LED light source, light detector, code disc, and signal processor. The disc has opaque and transparent segments and passes between the LED and detector to intermittently interrupt a light beam. The detector tracks the series of light exposures it receives and sends that information to the processor that extracts motion information. Two rotary optical encoder subtypes exist.

Incremental encoders are named for their output, consisting of the two square waves, each corresponding to an increment of rotation. Typically, the LED directs rays through a convex lens that focuses the light into a parallel beam; the beam passes through a grid diaphragm, which splits it to produce a second beam of light 90° out of phase. Light passes from A and B channels through a disc onto the photovoltaic or photodiode array. The disc rotation creates a light-dark pattern through the clear and opaque disc segments.

An absolute encoder has multiple detectors and a disc with multiple, unique tracks. The disc produces Gray code output—a binary numeral system for which (unlike straight binary) successive values differ by one bit. Taking absolute tracking further, multiturn optical encoders clock movement over multiple revolutions. Gearless multiturn encoder designs are also manufactured in both optical and magnetic designs.

Rotary magnetic encoders

Magnetic encoders are inherently rugged and operate reliably under shock and vibration and high temperature. Magnetic debris ingress can degrade rotary magnetic encoder performance, but other contaminants do not. Therefore, rotary magnetic encoders are often used instead of optical encoders. Passive variable reluctance or magnetized strips on a rotating code rotor, wheel, or band are sensed by either a Hall-effect or magnetoresistive sensor. Motor speed and position accuracy dictate which of the two is better suited for an application.

Specifying output

Encoder output data is typically sent to the controller via parallel binary, analog voltage or current, Profinet, EtherNet/IP or Powerlink, Modbus, DeviceNet, Profibus, or CANopen. Standard Serial Output (SSO) is typically leveraged in magnetic-encoder designs, for continuous synchronization during data transmission.

Another option is the Synchronous Serial Interface (SSI), a digital point-to-point interface. Common in Europe, it provides unidirectional communication to 1.5-MHz speeds and uses a six-wire cable—two carrying clock data to the encoder, two carrying data from it, and two for power. Hardware compatible with SSI, connectable via a bus or point-to-point, Bidirectional Synchronous Serial interface (BiSS) is an open protocol that transmits encoder position data whenever the controller asks, for quick recovery after interruptions. It also communicates encoder ID information, and acceleration, temperature, and other data without interfering with realtime operation.

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