Theory of Operation

Gurley Virtual Absolute ( VA ) industrial encoders employ an optical encoding technique which is industrial still relatively new to the motion control industry. Like an incremental encoder, the VA disc (or scale, in a linear encoder) uses only incremental and index code tracks. The incremental track is structured and read by quadrature sensors in the usual way, but neither the encoder nor the host system accumulates incremental counts as the disc turns. Instead, the quadrature signals control the spatial timing for industrial sampling the index track, where the absolute position code is stored. The index track is coded differently from a conventional incremental encoder. On the index track, an opaque or transparent region exactly one full optical cycle wide aligns correspondingly with every opaque/transparent line pair in the adjacent incremental track. The sequence of digital ones and zeros, into which the index marks will be translated by the detector dedicated to this track, is non-repeating around the disc. The absolute position data is encoded serially in this one track, rather than in parallel over many tracks, as is the case for a conventional absolute. This eases opto-mechanical alignment requirements, reduces cost, and improves intrinsic reliability.The encoder disc can contain any number of indices around a full revolution. For illustrative purposes, assume there are 4096 (2) such marks. These are decoded for cycle position, to which 8 bits of interpolated "position within the cycle" are appended in the
12 Model VA electronics. Every possible grouping of 12 consecutive indices in the sequence is unique due to the non-repeating design of the code. Every optical cycle of the incremental timing track is thus tagged by a unique 12-bit code whose first bit is immediately adjacent to the cycle, and whose remaining 11 bits trail behind. This arrangement is sometimes called a VA tag decoder design.When the encoder is first powered up, absolute position is unknown. An initialization procedure must be executed to obtain a complete tag for decoding. Because the index track is viewed by a single sensor, the shaft must be "bumped". The bump can be in either direction, or can be a "wiggle" in both, so long as a minimum net distance is covered. In this example, the initialization distance is (12)(360/4096) = 1.05°. (In a 9710 linear chain code . Because the coding sequence is such that each 12-bit code tag shares 11 bits with its neighboring tags to the left and right, in overlapping fashion, one might conclude the code progression is monostrophic (like Gray code) with only one bit differing between consecutive tags. Not so. While adjacent tags share all but one bit, the positions of all the shared bits within the tags are shifted. Tags which truly differ by only one bit are, as often as not, located in apparently random sections of the disc. Improperly decoded tags would therefore result in gross position errors. Partly for this reason, a sophisticated self-testing capability is an integral part of Gurley's patented VA encoder with ¼-µm resolution, the initialization distance is 0.768 mm.) Cycle boundary phasing with respect to the index marks is such that the indices are sampled only at their centers. For this reason, the incremental track is sometimes called the timing track. There are two very important benefits in this arrangement: As with conventional incremental encoders, the readout accuracy of the VA encoder depends only on the regularity of its incremental track lines. These are easy to create on the disc with precision, and easy for the quadrature detectors to read accurately through slit gratings that average out microscopic optical printing imperfections and moderate amounts of contamination. This also results in healthier signal amplitudes. Conventional absolute encoders have difficulty employing a similar optical averaging technique to improve readout accuracy and signal-to-noise ratio. predicts for that direction of motion. In effect, the decoder "knows" the entire chain code, and reports any disagreement between The serial indices, which must be viewed through a single mask aperture (like a conventional absolute) rather than through a grating, are at their maximal light or dark conditions when sampled. This ensures the greatest electrical noise margin, immunity to contamination, and tolerance of misalignment between disc and read station. Unlike a conventional absolute encoder, the exact position where a transition from light to dark (or vice versa) occurs has no effect on decoding reliability or accuracy.Once the first tag has been obtained, bit-wise, during the initialization traverse, ensuing motions modify the tag based on direction of motion. As new tags become available, they are decoded to produce monotonic natural-binary absolute position words. During the tag decoding process, each new detected bit obtained from the index track is compared against the bit the tag decoder expected and detected tag bits as a fault condition. In this way, the encoder's design embodies a sophisticated and automatic error detection technique that is constantly on guard against malfunction.

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VA

Gurley Precision Instruments514 Fulton StreetTroy, NY 12180 U.S.A.(800) 759-1844, (518) 272-6300, fax (518) 274-0336, Online at www.gurley.com, e-mail: info@encoders.com V3.1

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