Rockwell Automation 2755 High Performance VLD Scanner - Manual (Page 26)

3–13 Installation Considerations
Step Ladder Applications
To calculate minimum scans per symbol for step ladder applications,
use this formula:
A x H
Z
S = Scans per Symbol
A =
H = Symbol Height (length of bars of symbol)
Z = Conveyor Speed
S =
(must be at least 5)
Derated Scan Rate (nominal scan rate – 5%)
Conveyor Speed and Symbol Height 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 Compensating for Pitched Symbols)
Example 1:
A 40 mil case code label that is 1 inch tall and 4.75 inches long is to
be read in step ladder orientation at 125 ft/min. Space is limited so
the scanner must be positioned as close as possible to the labels.
Since the 2755-LD8C1 cannot read a 40 mil label unless the scanner
is 10 inches away, let’s assume the 2755-LD4C1 scanner can read
the label. The usable beam must be long enough to cover the entire
symbol, including quiet zones. At 6 inches, the LD4C1 scanner has
a usable beam length of 5.525 inches.
The calculation below converts Conveyor Speed to similar units
(inches).
125 feet/minute x 12 inch/feet x 1 minute/60second = 25 inch/second
To calculate minimum scans per symbol for this step ladder
application, use the following values:
A = 200 scans/seconds – 5% = 190 scans/second (minimum scan rate)
H = 1 inch
Z = 25 inches/second
S = 190 scans/second x 1 inch / (25 inches / second)
S = 7.6 scans
Since S (scans per symbol) is greater than 5 and the application uses
good quality labels, the 2755-LD4C1 scanner is appropriate for this
step ladder application.