Rockwell Automation 2755 Atomscan Bar Code Scanner - Manual (Page 16)

Chapter 3
Designing the System
3–5
Figure 3.3
Step Ladder Orientation
Scanner Mounted
On Side of Conveyor Scanner Mounted
Above Conveyor
Direction of
Travel
Direction of
Travel
In general, ladder orientation (Figure 3.3) is preferred, because each scan
covers a slightly different part of the symbol. This means that:
•
imperfections in the symbol are less liable to prevent a successful read.
•
symbol placement is not as critical.
In picket fence orientation (Figure 3.2) the symbol can be read the whole
time it is in the Read Range, rather than being limited by the height of the bar
code. However picket fence allows scanning of only a small part of the
whole symbol. Slight imperfections such as extraneous ink or voids can
cause misreads or non-reads. The quality of data in picket fence orientation
can be improved by any of the following:
•
Make sure the printing on the symbol is of good quality.
•
Tilt the scan line slightly to allow a larger part of the symbol to be
scanned as it passes through the scan line. This simulates rastering.
•
Use an AtomScan scanner with the raster option, since a raster scanner
places ten scan lines through the symbol while the single-line scanner
places only one.
When to Use a Raster Scanner
Single line scanners are always used in step ladder orientation.
Raster scanners are beneficial in picket fence applications with symbols
printed by a dot matrix printer, or where the bars have ragged edges or voids,
or where the spaces have specks in them.