By Jeremy Jones
Lead Engineer, Encoders
Baumer Electric
Southington, Conn.

Edited by Stephen Mraz

Most optical encoders operate on the same basic principle. A light source projects a beam of light through a grid or mask, which splits the light into a second channel 90° out of phase, an output style called quadrature. The two light beams pass through a pulse disc attached via a shaft to the monitored process and onto a photodiode array. As the disc turns, its clear and dark segments create a pattern of light and dark, which is read and processed by the array and encoding circuitry. Separate diodes receive each of the two light beams and convert them into two squarewave signals. they get fed to a controller such as a PLC or counter that monitors the number of pulses generated to provide end users with speed, position, distance, or directional data.

Optical rotary encoders are the most widely used way to transform mechanical rotary motion into electrical signals. There are three basic configurations for optical encoders: incremental, absolute, and multiturn absolute encoders. Each delivers different performance characteristics, capabilities, and benefits.

Optical encoders are used everyday in manufacturing environments and high-precision equipment to monitor the presence, position, distance, direction, and speed of rotating equipment. They provide reliable feedback within the process loop, letting designers closely monitor and control motion.

Incremental Encoders

Incremental encoders are the simplest, providing only on/off information and monitoring only speed, direction, or distance of travel. The incremental pulse disc consists of evenly spaced clear and opaque segments. Incremental pulse ranges of more than 10,000 pulses/rev (ppr) are available, but the maximum ppr can be increased by interpolating the quadrature output, making possible values as high as 131,072 ppr or higher.

A controller measures the process' speed simply by timing how fast the encoder supplies pulses. The controller determines whether movement is clockwise or counterclockwise by monitoring which channel, A or B, rises first. It determines travel distance by simply counting the number of pulses supplied since movement began. If power fails, all positional information is lost and a reset or homing cycle is needed to synchronize the encoder with the control device.

Incremental encoders are commonly used in applications where absolute position is not critical.

The BSM/BHM incremental encoders feature an 18-mm-diameter housing and resolution of 2,000 ppr.

An encoder's pulse range is dictated by the number of tracks of clear and opaque lines on its pulse disk. Pure and uninterpolated, a pulse disk with a single track of 200 lines has a pulse range of 200 ppr. Different types of pulse discs let encoders monitor and provide different types and amounts of information.

Incremental encoders can be used to monitor paper feed in web presses and to detect wire-length oversight in winding and unwinding processes. In these applications, the encoder is mounted directly to a spool shaft and the pulse count is converted via the controller into length-of-travel information.

Incrementals are also found in speed control applications such as conveyors, where the encoder is mounted to the drive motor shaft or roller shaft. Certain positioning applications are also suitable for these devices, including X-Y tables and SMD pick-and-place machinery where the encoder would be mounted to the servomotors driving the larger process.

Absolute encoders

Absolute encoders are somewhat more complicated but can monitor distance, speed, direction, and absolute position. They use a special pulse disc with a series of tracks that make up a binary code. Each track, executed as a binary bit, has a series of clear and opaque lines that create an on/off signal. For example, a 12-bit encoder would have 2¹² individual tracks, each acting like a distinct incremental encoder. The absolute pulse range is measured in steps/rev (spr) and dictated by the number of bits provided, ranging from 2 to 18. For example, a 12-bit encoder could have 212, or 4,096 spr.