How it works

Absolute optical encoders consist of a light source, some focusing optics, and a multi-element photodetector array. The light path passes through a code disc patterned with alternating transparent and opaque regions arranged in radial tracks. Each angular position of the disc produces a unique output based on whether or not light falling on each track gets through. The accuracy and repeatability of the encoder depends primarily on the accuracy of the code disc itself.
Select figure to enlarge.

Optical encoders consist of a light source, a code disc, and a detector. In the case of an absolute encoder, the code disc is imprinted with a symmetric pattern of alternating transparent and opaque arcs arranged in concentric tracks. Each track corresponds to an individual sensing element within the photo-detector, representing one bit of resolution. For each angular position of the disc, the system produces a unique “whole word” output whose bits indicate the presence or absence of light through each track.

Absolute encoders are generally available in single and multiturn versions. Single- turn encoders are recommended for back-and-forth rotary motion in which the shaft never makes a full revolution.

Multiturn encoders are designed for moves spanning tens or even hundreds of revolutions. They consist essentially of two encoders in one housing. A “fine” encoder, fixed to the input shaft, provides the resolution per turn of the device. A second encoder, called the “turns counter” or “coarse encoder,” connects to the input shaft through a gear set. The gear ratio is usually adjusted to increment the coarse encoder one position for each full rotation of the input shaft. In some cases — as in electronic multiturn encoders — a counter circuit is used in place of the second encoder.

General sizing procedures

Absolute encoders are often specified in terms of resolution, the number of bits or words contained in the complete code. For incremental and tachometer encoders, resolution is defined as counts per turn. For absolute single-turn encoders, it is called positions per turn. Multiturn encoders are specified as positions per turn of the input shaft and the number of turns of the internal gear ratio.

The amount of resolution required is a function of the number of positions that must be measured. If a machine needs to measure the travel of a 25-in. lead screw in increments of 0.001 in., for example, then it will take an absolute encoder with a resolution of 25,000 words.

Select figure to enlarge.

Resolution is usually defined in terms of bits, however. The term “12-bit encoder,” for example, refers to the binary number 2¹² which is 4,096 decimal. Thus, a 12-bit encoder has a resolution of 4,096 words.

Absolute encoders are also specified on the basis of accuracy and repeatability. Accuracy, which is traceable to the encoding disc, is the deviation between the actual position and the theoretical position of each bit edge. In other words, it refers to the precision of the output signal in relation to shaft position. A good 12 or 13-bit encoder, for example, will be accurate to within 1/2 count on the least significant bit (LSB).

Repeatability, the ability of the encoder to read the same word each time the shaft is in the same position, is defined as the deviation of the actual encoder position between subsequent identical code readings. Although it has no relation to accuracy, it is usually four to ten times better.

Other selection criteria include shaft rating, shaft and bearing seals, output format, and driver type. Shafts, for example, are typically available in standard and instrument grades, and some are installed with watertight seals. Bearing seals come in several grades as well.

Common applications

Absolute encoders are the preferred transducer for applications requiring a true indication of position at all times. If a system is prone to losing power or can’t be returned to a reference or home position, an absolute encoder is the way to go. It’s also a good fit for machines that may be inactive for long periods of time or ones that move very slowly, such as telescopes, cranes, and motorized platforms.

On the other end of the spectrum, absolute encoders are also well suited for high-speed precision instruments because they produce parallel digital outputs. Parallel data can be read quickly, a necessity for sensing and controlling fast moves.

Other applications that work well with absolute encoders are those that involve very fine movements. The ability of absolute encoders to precisely measure angular motion stems from a combination of multiple code tracks and optical sensing, and is available in a relatively compact package, without need of additional (external) support circuitry.

Continue on page 2