Rockwell Automation 2755 Atomscan Bar Code Scanner - Manual (Page 22)

Chapter 3
Designing the System
3–11
Picket Fence Applications
To calculate minimum scans per symbol, use this formula:
R (W–L)
C
S = Scans per label
R =
W = Scan Width at the minimum read distance
L = Symbol Length (including quiet zones)
C = Conveyor Speed
S =
(must be at least 5)
Scan Rate
Scan Width, Symbol Length, and Conveyor Speed must be expressed in
similar units. Calculations assume that the scanner and decoder are triggered
for the entire time the symbol is present and the symbol has a 0
°
pitch. (See
the section at the end of this chapter, “Compensating for Pitched Symbols”.)
Example:
A 40 mil symbol that is 1 inch tall and 4.75 inches long (including quiet
zones) is to be read in a picket fence orientation at 125 ft./min. Space is
limited so the scanner must be positioned as close to the symbols as possible.
The scan width must be large enough to cover the entire symbol, including
the two quiet zones. At 6 inches, the 2655-L6SB scanner has a Scan Width
(from Table 3.B) of 6.5 inches.
The following calculation converts Conveyor Speed to inches per second:
125 feet
x
12 inches
x
1 minute
=
25 inches
minute foot 60 seconds second
5
25
To calculate minimum scans per symbol for this application, insert these
values into the formula:
R = 330 scans/second
W = 6.5 inches
L = 4.75 inches
C = 25 inches/second
330 scans (6.50-4.75) inches
=
second
13.2
23 scans
25 inches
second
1.75
x
Since S (scans per symbol) is greater than 5 and the application uses good
quality symbols, the 2755-L6SB scanner is appropriate for this application.