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

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)