Readers are probably familiar with both incremental and absolute encoders. However, they may be less aware of a new technological development called Virtual Absolute (VA^TM). This encoder is absolute in essence or effect without being formally recognized as such—that’s what virtual means. In reality, it’s neither an incremental encoder nor an absolute encoder. It’s an entirely new kind of device which has the same number of code tracks as an incremental encoder, but behaves more like a conventional absolute encoder.

Though like the two traditional encoder types in some ways, Virtual Absolute encoders have aspects of design and behavior which are unique. Thus, while incremental and conventional absolute encoders are hardly obsolete, they’re also no longer your only choice.

A Virtual Absolute encoder uses just cyclic and index tracks, like an incremental encoder. However, you can see the index track is something like a bar code, instead of just a single line (Figure 1). Absolute position is encoded serially along this one track, rather than being dispersed over multiple parallel tracks. You don’t know position immediately upon start-up, as you do when using a conventional absolute device, but after a very short travel, in either direction and starting from anywhere, you know exactly where you are. In a rotary encoder, this initialization angle is typically 1–2º, depending on the encoder’s line count (non-binary resolutions are easily accommodated); in a linear encoder, less than 1 mm motion is needed. From that point on, the encoder is truly absolute. If it bothers you to think of an encoder you move slightly on power up as being absolutely coded, you may also be dismayed to learn the pseudorandom code structure it uses to accomplish this isn’t exactly random.

Pseudorandom output codes read from the disc or scale need decoding into a natural binary format. This decoder can be implemented in a PLD that stands in place of the quadrature decoder and up/down counter used with an incremental unit. Thus, although this type of unit costs little more than an incremental system of comparable resolution, it’s effectively absolute. Since it doesn’t count position increments from the cyclic track(s), it can’t gain or lose "counts" under duress like an incremental encoder system can.

In addition to the binary position output, the decoder provides a status bit. This bit is logic high whenever the supply voltage is interrupted, when the initializing motion is not yet complete, or when some other effect such as electrical noise, damage, or fouling of the disc interferes with the proper code sequence from the indexing track. When these self-tests are all satisfied and the encoder is initialized, the status bit is low, indicating the position output data is valid. Full-time position verification with real-time reporting of any problem is the most important feature of this new encoder in the opinion of some machine designers.

The decoder circuit is preferably located in the host system, as an incremental up/down counter would be, to preserve frequency response. However, in low speed applications, the decoder may be located inside the encoder housing and the absolute position data transmitted serially along with the status bit. (A number of similar encoders make use of pseudorandom discs or scales, but these generally implement a complex readout system that avoids all motion on power up. Consequently, this has a significant impact on frequency response and reliability.)

The principal advantages of this Virtual Absolute technology can be summarized thus:

• The initialization distance or angle is a fixed and very small motion, regardless of the starting position or direction of travel. Just bump it to find out where you are.

• The encoder generates the same whole-word information as a conventional absolute, so interfacing it to digital signal processors, computers, PLCs, servo controls, etc., is straightforward.

• Built-in-test functions not found in any conventional device report various encoder malfunctions, and can also help detect system problems such as excessive heat and speed.

• Simpler optics allow a rotary VA to be smaller than a conventional absolute of equal resolution (or have a larger through-hole).

• Simpler electronics, a reduced parts count, and less critical internal alignments translate into a higher intrinsic reliability when compared to a conventional absolute device.

VA encoders don’t belong in mice and trackballs, nor are they appropriate if you really must know position immediately on power up, without the slightest motion. However, Table 1 lists a few types of applications where this technology may prove ideal.

Most incremental encoders can be retrofitted to use VA technology. For high resolution/high speed applications, the decoder is a separate credit-card-sized printed circuit which also enhances the resolution of the encoder by interpolation, and monitors analog signal quality to provide early warning of a range of optical and mechanical problems.

Martin Gordinier is vice president of encoder sales at Gurley Precision Instruments. Contact him at 514 Fulton Street, Troy, NY 12181-0088; tel: 518-272-6300; fax: 518-274-0336; m.gordinier@gurley.com

Captions

Figure 1. Disc comparisons of (a) incremental, (b) absolute, and (c) virtual absolute encoders.