Bert Moore

No matter what you make or distribute, chances are you have a conveyor somewhere in the system: either as an integral part of your manufacturing process or something as simple as a roller conveyor to help load and unload trucks.

Admittedly, there are some locations that don't rely on conveyors, using lift trucks, Automated Guided Vehicles (AGVs), or even hand trucks instead. But whichever the case may be, overhead fixed-location bar code scanners can provide automatic scanning for a wide variety of applications – even applications where employees hand-carry items under the scanner.

And, just as there are many possible applications, there are many product options available to meet specific application needs.

Laser Scanners

Laser scanners now come in a sometimes bewildering array of configurations, from the very basic to the very complex. It should be noted, however, that the more complex ones typically come with vendor-supplied setup software that makes configuring the many options a relatively straightforward task.

It should also be noted that for all laser scanners, discussions of "lines" or "patterns" actually refer to the path of a single spot of light.

Moving Beam (Line) Scanners

The basic moving beam laser scanner produces a single scan line. The line can be created either by a rotating, multisided mirror or by a "dithering" mirror that oscillates back and forth. Another method to produce the line is the use of a rotating holographic disk that acts as a lens. The holographic disk produces a relatively constant scanning speed across the path.

Mirrors produce an acceleration/deceleration of the speed of the reading spot across the scan path. Rotating mirrors produce greater speed at the beginning and end of each scan (though the variance may be minimized by limiting the aperture to only the optimal part of that path). Dithering mirrors have to start and stop so are slower at the extremes.

Speed variances are compensated for in decoding. Because of these variances, however, it's not typically wise to expect the scanner to read symbols at the very edges of the scan path. Most scanning diagrams show a "sweet spot" well inside the actual limits of the scan path.

Rastering Scanners

Rastering laser scanners typically use a dithering mirror that also oscillates in a vertical direction, creating a "zig zag" pattern that covers a certain area. A rotating holographic disk can also create a series of parallel lines.

All the same considerations apply to rastering scanners as apply to line scanners.

Rastering scanners still use a single spot so potential, effective scanning speed must be divided by the number of raster "lines." That is, if the scanner produces 12 lines and performs 600 scans per second, then it covers the area with the equivalent of 50 scans per second. The achieved scans per second may be critical if rastering scanners are used to "find" a package label or to read multiple symbols on a conveyor moving at high speed.

Omnidirectional Scanners

Typically, omnidirectional scanners use a rotating mirror and a beam splitter to produce multiple scan lines (usually in some variation of an "X" pattern). Again, a holographic disk can be used to create the initial moving beam.

Symbol Reconstruction

Laser scanners of all types may employ a symbol reconstruction algorithm to "piece together" underheight symbols, that is, symbols where the scan path does not pass completely through the symbol. Reassembling the "slices" of the symbol it has seen, symbol reconstruction software then decodes the "whole" symbol. Symbol reconstruction is a form of the image processing performed by CCD scanners.

CCD Scanners

Charge Couple Devices (CCD) scanners are gaining popularity in some areas.

Surprisingly, it is linear CCD scanners that are being used most widely for conveyor applications.

Using the ability of CCD scanners to capture an entire picture slice-by-slice, they construct a picture of the entire symbol.

Among the major users of linear CCD scanners has been UPS. Linear CCD scanners are used to read not only a variety of linear bar code symbols on parcels but also the MaxiCode 2D matrix symbology. And at line speeds up to 500 fpm.