Calculating The Number Of Scans; Step Ladder Calculation - Allen-Bradley 2755-LS7-SB User Manual

Calculating the Number of
Scans
To ensure a successful read of a bar code symbol, the bar code
symbol should be scanned 5 times. With fewer scans there will be
more No Read counts, even when the symbol is good. If the symbol
quality is poor, more scans per symbol may also be required.
If the number of scans you derive from one of the following
calculations is less than the minimum for your application, put in the
minimum number of scans and solve for another parameter that
might be changed, such as label speed or scans per second.
Step ladder orientation gives a different number of scans than picket
fence orientation, depending on symbol height, symbol width, and
scan width.

Step Ladder Calculation

To calculate scans per symbol for step ladder applications, use the
following formula.
R x H
S =
C
S = Scans per Symbol (should be greater than 5)
R = Scan Rate
H = Symbol Height (length of bars of the symbol)
C = Conveyor Speed
The –3 accounts for the incomplete scans at the beginning and end and one for the Automatic
Gain Adjustment.
Conveyor speed and symbol height must be expressed in similar
units. The calculation assumes that the reader is triggered for the
entire time the symbol is present and the symbol has a 0° pitch.
Step Ladder Example
A bar code symbol is 1 inch tall. The conveyor speed is 10 inches
per second. The scan rate is 500 scans per second. Substitute in the
following values to determine the number of scans the bar code
symbol will receive.
R = 500 scans/second
H = 1 inch
C = 10 inches/second
500 x 1
– 3
10
Designing Your Scanner System
– 3
= 47 scans
3–9
Publication 2755-6.9