Motion control and Optical encoders

April 3, 2002
Optical encoders are critical components in systems with rotating shafts or parts.

By Jeremy Jones
Lead Engineer,
Baumer Electric

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.

The BPSV/BPMV Series optical encoders from Baumer Electric feature a programmable interchangeable bus-cover system, letting the same encoder support all major buses. They have up to 29-bit multiturn resolutions.

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

The Dignalizer encoder from Baumer Electric offers up to 18-bit resolution in a single-turn system or 36-bit in multiturn versions.

The largest diameter hollow-shaft multiturn encoder measures 2 in. in diameter, with an overall housing diameter of only 4.56 in. Its multiturn resolution ranges up to 25 bits.

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.

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.

When read with the other tracks on the disc, absolute encoders provide signals in binary for certain angular positions within 360° of rotation. The 12-bit encoder mentioned above, for example, could divide 360° into 4,096 steps, giving an angular resolution of approximately 5 min, 16 sec.

Absolute encoders are commonly used in robotics, where accurate positional information is critical to safety. In some robotic arms, absolute encoders are mounted at each joint, with the encoder body mounted to the stationary portion of the joint and the shaft coupled to the arm's axis spindle. In this configuration, encoders track arm angles and positions. Absolute encoders retain positional information despite power interruptions, so the robotic arm "knows" its position when power returns. Increasingly, absolute encoders are being placed in high-end satellite dish and radio-telescope systems to verify extremely precise positioning in the 360° horizontal and 180° vertical planes.

Multiturn absolute encoders
Where absolute encoders provide information over one revolution, multiturn absolute encoders supply absolute position over many revolutions. Like absolute encoders, multiturns have a code for each position within 360° of rotation, and also offer codes for each revolution.

Multiturns use standard absolute technology, but add an internal counting process that monitors and tracks the number of rotations. Some multiturn manufacturers rely upon a gear-driven tracking system, which provides absolute position but can also be complex, expensive, and prone to breakage and wear. Other encoder manufacturers employ a noncontact, longer-life tracking system that counts revolutions and monitors directional information using a two-poled magnetic rotor and array of reed switches. This system, designed with battery backup for multiturn stage electronics, saves all positional information if there is a power loss.

Multiturns are commonly used in elevators, where they monitor, via a pulley shaft, the car's position. Due to the many rotations of the pulley, multiturn encoders are the only ones that can deliver exact positioning information for the entire route of the car. Multiturns are also used in medical equipment, such as CAT scan machines, where they mount on the drive system of the patient carriage and monitor body position for fine scanning. These encoders are also used to monitor the position of screw-driven overhead gantries.

Styles and specs
Mounting encoders requires that certain precautions be taken. For shafted encoders, users must either manufacture or purchase coupling devices that mate with the shaft, as well as equipment to mate encoder housings to the coupling. Hollow-shaft encoders eliminate the need for couplings. Instead, they mount and fix directly to the shaft being monitored, dramatically reducing installation times.

The three encoder types are manufactured in a wide variety of sizes and housing styles to accommodate all industrial mounting needs. They are available in shaft, hollow-shaft, and through-shaft versions. Housing sizes vary from smaller than 18 mm in diameter to well over 150 mm, with shafts ranging from 1.5 to over 40 mm. In hollow and through-shaft versions, encoders commonly accept shafts from less than 2 to 50.8 mm (2 in.) in diameter. Protection ratings run as high as IP 68 (NEMA 6). Cable and connector versions are also available.

Encoder outputs range from 4.5 to 30 Vdc. Options include TTL compatibility, line driver capability, short-circuit and reverse-polarity protection, push-pull outputs, and pnp and npn open-collector transistor outputs. And absolute and multiturn encoders can be configured with different types of binary code outputs, such as natural binary and gray codes, and binary coded decimal.

In today's industrial world, the newest multiturn encoders offer up to 36-bit resolutions (18 for steps per revolution, 18 for total revolutions). This creates a need for other interfaces aside from standard parallel outputs, due to wiring complexities. Therefore, many manufacturers are turning to simpler interfaces, such as the synchronous serial interface (SSI). Additionally, to simplify integration of products into plant systems, many manufacturers are adding bus-system interfaces, such as Profibus-DP, CANbus, CANopen, Interbus-S, DeviceNet, and Suconet-K. RS-485 interface encoders are also available.

Specifying an encoder
Here's what you need to know before ordering or specifying an encoder:

  • Incremental or absolute? Does the application require memorization of position?
  • Is information gathered over one revolution or many?
  • Electrical input/output? What are the electrical requirements?
  • Physical mounting needs (shaft/housing dimensions)
  • Pulse range (ppr or spr)
  • How much environmental protection will it need?
  • Connection requirements (radial or axial, bus system considerations)

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