Image credit: Avago Technologies | Everight Position Technologies Corp. | RS Components, Ltd.
Absolute rotary encoders are electromechanical devices which provide a unique output for every resolvable movement or shaft rotation.
Absolute vs. Incremental Encoders
Rotary encoders convert the angular position of a shaft or axle to an analog or digital code. In the case of absolute encoders, the device indicates the current position of the shaft and produces a unique code for each distinct shaft angle. This contrasts with incremental rotary encoders, which provide information about the motion of the shaft as opposed to the absolute position.
Absolute and incremental encoders each have specific advantages in different situations. Because absolute encoders use a unique code to describe each position, position data is not lost when power is cycled through the device. When power is cycled through an incremental encoder, position data is lost until the "home" position is once again triggered.
This video provides an overview of absolute encoder operation. This particular encoder is an optical rotary type, which is described below.
Video credit: learncnc.org
When selecting absolute rotary encoders, it is important to understand the different types available. The GlobalSpec SpecSearch database contains information about four types of absolute rotary encoders: optical, mechanical, fiber optic, and magnetic.
Mechanical rotary encoders consist of a metal disc with a ring of cut-out openings as well as a row of sliding contacts fixed to a stationary object. The metal disc is attached to a rotating shaft, and each stationary contact is connected to an electrical sensor. As the disc rotates, some of the contacts touch the disc and switch on, while others fall in the gaps where the metal has been cut out. The combination of switched-on and switched-off contacts creates a unique binary code for each disc position.
Because contacts are susceptible to wear with heavy use, mechanical encoders are well-suited to low-speed manual applications such as radio tuning knobs.
The image below shows a cross-section of a mechanical encoder. In the image at left, note the two rows of cut-out "teeth" labeled A and B. The top of the encoder, shown at the right image, fits on top of the discs, and the brush contacts, also labeled A and B, make contact with the A and B disc tracks. The two sets of contacts labeled "common" are used to maintain contact with the inside of the discs.
Image credit: Robot Room
An optical encoder's disc is made of glass or plastic and features some transparent and some opaque areas. Using a light source and photo detector array, the encoder can read the disc's optical pattern, determine its position, and translate it into a code. The code is then read by a controller to determine the angle of the shaft.
Optical encoders are fairly inexpensive, but because they use LEDs and glass components, they are rarely suitable for operation in extreme temperatures (greater than 70° C) and harsh environments.
A disassembled optical encoder. Image credit: Newport
Magnetic encoders consist of a plastic rotary drum whose circumference is lined with magnetic poles. The drum turns against a magnetic sensor, which then relays an electric signal indicating the drum's position.
Magnetic encoders are relatively new devices and are expensive compared to optical encoders. They do offer several advantages over the latter type:
Improved temperature and environmental resistance due to the materials used
Smaller space requirements due to shorter axial direction
Lower cost of custom encoders
The image below compares the operation of a magnetic and an optical encoder.
Image credit: Astec Corporation, Ltd.
Fiber optic encoders often use similar sensing technology as optical types, and transmit position information by light. Fiber optic transmission is advantageous in applications where noise or electrical interference could occur with signals.
Output and Interface
Output data is important to consider when selecting absolute rotary encoders. An encoder's output code is represented as one of several variations on binary language:
Binary output is simply represented as a series of ones and zeros.
Gray code, also known as reflected binary code, is a binary system in which two consecutive incremental values differ by only one bit. This code is especially useful in rotary encoder applications: because each incremental position change is accompanied by only one bit change, code reading is simplified and errors are reduced.
Binary coded decimal (BCD) outputs are decimal numbers where each digit is signified by four bits.
Buyers of absolute rotary encoders should also consider important electrical or interface data when selecting a product.
Analog signals include voltage (such as 0-10 V) or current (such as 4-20 mA) outputs.
Serial signals include serial synchronous interface (SSI), FOUNDATION Fieldbus, CANbus, INTERBUS, and SUCOnet. SSI in particular represents a general standard for absolute encoders.
Ethernet is a local area network (LAN) protocol and is the basis for the IEEE 802.3 standard for physical software layers.
Performance specs include resolution, accuracy, and maximum update rate.
Resolution refers to the number of bits the encoder uses to encode the position. Resolution can be calculated using the formula 2n, where n is the number of bits. For example, a 9-bit encoder yields 512 counts per disc revolution, whereas a 10-bit encoder yields 1,024 counts per revolution; the resolution of these two distinct encoders would be 9 and 10, respectively. The 10-bit encoder, with twice as many counts as the 9-bit one, would be capable of much more precise position measurement.
Encoders can fall into two groups according to resolution:
Single turn encoders are capable of up to 360° of revolution.
Multi-turn encoders can revolve many times, often up to 4096 turns.
Both single turn and multi-turn encoders retain their position information even after being powered off, but multi-turn encoders also retain their revolution count.
This video describes the difference between single and multi-turn encoders, the importance of performance specifications, and general advantages of an absolute encoder.
Video credit: DesignWorld
Encoder accuracy is typically measured in arc seconds; one degree contains 3600 arc seconds. The accuracy specification refers to the maximum error of a position reading.
Maximum update rate refers to the rate at which new position readings can be generated and updated, and is specified as a variation of Hertz or cycles (Hz, kHz, MHz, etc.)
Encoder diameter/width, shaft speed, and rotor inertia are important mechanical specifications.
Diameter refers to the the body diameter of a circular encoder. If the encoder's body is square, width refers to the maximum side dimension.
Mechanical shaft speed specifies the maximum speed at which the encoder can rotate without sustaining damage.
Rotor inertia is specified as a product of mass, distance, and/or time, such as oz-in-sec2 or kg-m2.
Related Products & Services
Incremental Rotary Encoders
Incremental rotary encoders are multi-turn sensors that use optical, mechanical, or magnetic index-counting for angular measurement. They contain no absolute reference, but instead count relative to the turn-on position.