Gurley Precision Instruments has developed a unique encoder technology that combines the opto-mechanical simplicity of an incremental encoder with the system reliability and interfacing ease of an absolute encoder. This document refers to rotary encoders, but the technique is equally adaptable to linear encoders. The reason for the selection of the name Virtual Absolute (or VA) will become apparent later in the discussion.

Figure 1: In an incremental encoder, the data track consists of uniformly spaced opaque lines and clear spaces. Source: Gurley

Let's begin with a short review of each type. In an incremental encoder, the data track consists of uniformly spaced opaque lines and clear spaces. The typical incremental encoder output consists of two square waves in quadrature, or 90° out of phase with each other. Most commonly, the user's circuitry "looks at" the relative phase to determine the direction of travel, and counts the four states, or edges, in each cycle. This provides a resolution equal to four times the line count on the disc. (Techniques exist to electronically increase the resolution up to 80 times the line count, or even more, but that's another subject.) A portion of the user's circuitry must be dedicated to "watching" the output at all times so that no information is lost.

To generate "absolute" information from an incremental encoder, an additional track is added to the disc to provide an index signal that occurs once per revolution. (It's also called a marker, reference, home or Z signal.) Once this signal occurs, an absolute home position is known, but it could be almost a full revolution away from the starting point, necessitating a homing start-up procedure.

Figure 2: To generate "absolute" information from an incremental encoder, an additional track is added to the disc to provide an index signal that occurs once per revolution. Source: Gurley

In the conventional absolute encoder, the pattern on the disc consists of a series of concentric incremental tracks, with the number of cycles per revolution doubling on each track of increasing radius. Each track has its own photodetectors, and the tracks are arranged so that reading all the detectors generates a whole "word," usually in either Gray code or natural binary. Thus, an encoder with 13 tracks would generate 2¹³ (= 8,192) words per revolution. (As with incremental encoders, there are electronic techniques to increase the resolution well beyond the number of tracks, but many tracks are still required for high resolution encoding.) The opto-mechanics and related electronics of such an encoder are significantly more complex and expensive than an incremental encoder, but it does provide position information immediately upon start-up.

Two other distinguishing features of an absolute encoder are:

1. Because the encoder generates whole-word information, it is much easier and more straight-forward to interface to a computer.
2. Because the encoder always "knows" where it is, it is not necessary to constantly monitor its output; you can interrogate it whenever you want and find out exactly where it is at that instant.

A VA encoder contains just a data track and an index track, as in an incremental encoder, but the magic is in the index track. Instead of just a single line, it comprises a serial code similar to a bar code. You don't know position immediately upon start-up, as you do in a conventional absolute, but after a very short travel, in either direction and starting from anywhere, you know exactly where you are. (In a 13-bit rotary encoder, for example, this initialization angle is less than 2° of shaft rotation; in a 16-bit encoder, it's about 0.3°.) From then on, the encoder is truly absolute, in the sense of both #1 and #2 above. During its initialization period, the encoder generates a Wait signal; once it's initialized, it generates a Data Valid signal.

As stated, the opto-mechanics of such a device are as simple as an incremental encoder, but the electronics are somewhat more complex. However, the electronics are much more stable and predictable. The encoder costs more than an incremental (but not necessarily a lot more, depending on the software resources available), and less than a conventional absolute.

To summarize, the advantages of VA technology are:

1. The initialization distance is a fixed and very small motion, regardless of the starting position or direction of travel.
2. The encoder contains inherent error detection circuitry not found in a conventional absolute. If the output is not monotonic, for whatever reason, the encoder immediately generates its WAIT signal. DATA VALID will re-appear once it's been re-initialized.
3. The encoder generates the same whole-word information as a conventional absolute, and so is equally easy to interface to a computer.
4. Because of the simpler optics, the encoder can be much smaller than a conventional absolute of the same resolution.
5. A VA encoder is less expensive than a conventional absolute.

Contact Gurley Precision Instruments, Inc. to find out how VA technology can benefit motion measurement applications.

Reprinted with permission from Gurley Precision Instruments