Red Lion LEGEND - Manual (Page 65)

APPENDIX “A” - SCALING FOR COUNT INDICATION
SCALING FOR COUNT INDICATION
The Legend Plus is factory set to provide 1 count on the display for each
pulse that is input to the unit. In many applications, there is not a one to one
correspondence between input pulses and display units. In these applications
it is necessary for the Legend Plus to scale or multiply the input pulses by a
scaling factor to achieve the proper display units desired (feet, meters,
gallons, etc.). The Count Scale Factor, the Count Scale Multiplier, and the
Number of Count Edges are used in scaling the input pulses to the desired
reading. The number of count edges is determined in the Program Counter
Module. For example, the number of edges for the CNT+DIRX1 mode is one
and the QUAD X4 mode is four. All three are factored together to provide the
Scaling necessary for the Process display. The Count Scale Factor and Scale
Multiplier are programmed in the Program Scaling section.
The first step in scaling requires the Number of Pulses per Display Units to
be obtained. This may require a small amount of deductive reasoning.
Example: A 48-tooth gear is mounted to a 2 ft circumference feed roll in a
paper processing plant. It is desired to display the footage of paper
processed per day. In this example, the display units are in feet. A sensor
sensing the gear teeth provides 48 pulses for each revolution of the feed
roll. Each revolution equates to a linear distance of 2 feet. The number of
Display Units desired is 2. The Number of Pulses per Display Units is 48.
When the number of Display Units and the Number of Pulses have been
obtained, the Total Scaling can be calculated.
The Total Scaling, denoted as “K T ”, is simply the total amount of scaling
required for the Process. It is obtained by dividing the Display Units by the
Number of Pulses as shown in Formula #1 below.
FORMULA #1: K
T
= Display Units ÷ Number of Pulses
WHERE:
K
T
= Total Scaling.
Display Units = The number of desired units (revolutions, feet, 10ths
of feet, meters, etc.) that would be acquired after
the Number of Pulses has occurred.
Number of Pulses = The Number of pulses required to achieve the
number of Display Units.
For the preceding example, the Total Scaling, “K T ”, is calculated by
plugging in 2 and 48 in the formula:
K T = 2 ÷ 48 = 0.041667
As previously stated, the Total Scaling, “K
T
”, is the combination of the
Count Scale Factor, Scale Multiplier, and Number of Count edges. In many
applications the Total Scaling, “K
T
”, can be programmed directly into the
Count Scale Factor, “SF”, in which case the Scale Multiplier and Number of
Count Edges can be left at the factory settings of X1.
In some applications, more display resolution may be required. When the
Total Scaling (K
T
) is greater than 1.0000 and only one edge per count pulse is
used, there may not be enough display resolution.
Example: With a Total Scaling of 2.000, when an input pulse is generated,
the display increments by 2. If the display units are in feet, when 3 feet have
gone by, the display still reads 2. It will not increment again until 4 feet
have been accumulated. With this amount of display resolution it would be
impossible to set the Preset and have the output respond at odd feet
intervals (1, 3, 5, etc.). To increase resolution, the Number of Count edges
must be increased. This can be achieved by selecting the CT+DIRX2 mode
(Count plus direction times 2) in the Program Count Module or by
selecting the QUAD X4 mode, if quadrature counting is being used. If
enough resolution still has not been attained, more input pulses need to be
generated per display unit.
The amount of resolution required varies depending on the particular
application. In cut-to-length applications, a high amount of resolution is
often necessary. However, in totalizing applications, display resolution
may not be important. It should be noted that whenever the number of count
edges is increased to 2 or 4, the maximum count frequency decreases. (See
Specifications for maximum count frequency).
Note: When using 2 or 4 edge counting for length sensor, on/off duty cycle must
be 50% to maintain max. accuracy (mag. pickup will not work).
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