Rockwell Automation 2755 Atomscan Bar Code Scanner - Manual (Page 27)

Chapter 3
Designing the System
3–16
Step 1 + Step 2. With both of these aspects working together, the Read
Range is shorter with a pitched symbol than with one that is not pitched. This
results in:
a smaller depth of field, the region within which the symbol must be
located if it is to be read.
a smaller scan width, which is required in the formula for calculating
the number of scans in picket fence orientation.
In Figure 3.9, if the farther end of the symbol extends past 6.5” (16.5 cm),
the scanner will not be able to read part of the symbol, causing a No-Read. In
such a case, move the scanner closer to the conveyor. Conversely, if the
nearer end extends past 2.2” (5.6 cm), a No-Read will result; move the
scanner farther from the conveyor.
Figure 3.10
Calculating the Effects of Pitch
Θ
Symbol Length
L = 3”
Tan (
Θ
) x 3”
= 1.7”
Θ Actual
Narrow Element Width
= 10 mils
Cos (
Θ
) x 10 mils
= 8.7 mils
Apparent Narrow Element Width
(Step 1)
Lost Read Range
(Step 2)
Pitch (
Θ) =
30
_
Read Range allowing for
Narrow Element width
Loss due to distance between
near & far ends of the symbol
Read Range
with 0
_
pitch
Read Range
with 30
_
pitch
2.0” to 7.0” (5.1 to 17.8 cm)
Scan Width Scan Width
2.2” to 6.5” (5.6 to 16.5 cm)
1.7” (4.4 cm)
2.5” to 7.5” (6.4 to 19.0 cm)
Useable Read Range 2.2” to 4.8” (5.6 to 12.2 cm)
Depth of field 5.0” (12.7 cm) Depth of field 2.6” (6.6 cm)
2.8” to 5.2” (7.1 to 13.2 cm)
Read Range
In picket fence orientation, the scanner and symbol are set as far apart as
possible within the Read Range, to give the greatest number of scans per
symbol. However with pitched symbols, the range is so narrow that the
symbol is often placed in the center of the depth of field, where it is less apt
to exceed the Read Range limits in either direction.