Red Lion LEGEND - Manual (Page 67)

APPENDIX “B” - SCALING FOR RATE INDICATION
The Legend Plus offers a simplified method for scaling the rate portion of the
indicator. The method does not require time unit conversions. The desired time
format (Rate Per Second, Rate Per Minute, Rate Per Hour) is simply selected as
part of the programming procedure. Due to the way the rate is calculated (See
“General Description” section), high resolution and accuracy can be realized at
all input rates, slow or fast.
Note: It is not necessary to increase the pulse information to obtain higher
resolution.
The Rate Minimum Update Time can be programmed from 0.1 up to 99.9 seconds
to provide averaging in applications where the input pulse spacing is not stable. The
Update time selected, however, will not affect the scaling in any manner.
Scaling the Rate channel involves programming the Legend Plus so that
input pulses to the unit are scaled to the desired display units (revolutions, feet,
meters, etc.) and in the desired time format (Rate Per Second, Rate per Minute,
Rate Per Hour).
If the rate indicator is to display the rate at which the counter is counting, the
rate indicator can be programmed with the same scaling parameters as the
counter. The only other requirement is that the desired “Rate Conversion Factor”
be selected to provide the rate display in the desired time format, Rate per Second
(X1), Rate per Minute (X60), or Rate per Hour (X3600). This automatically
scales the rate by X1 (1 pulse per second), X60 (60 pulses per minute), or X3600
(3600 pulses per hour).
Note: The rate uses only the negative edge of the pulse at Input A. The counter
uses both edges of the input pulse for a X2 or X4 count mode. The rate can
show the same reading as the count, but take into consideration the counter
mode selected.
If the rate application is to display a specific Display Unit, then to scale the
rate, it is only necessary to know the number of pulses per display unit desired or
units (feet, revolutions, etc.).
Example: A 48-tooth gear, which is coupled to a shaft, is being sensed and it is
desired to indicate the shaft speed in revolutions, the display units will be in
revolutions. It is obvious that 48 pulses will occur in one revolution. To
convert the pulse units to revolutions, it is necessary for the Legend Plus to
multiply the number of pulses by a scaling factor to convert the pulse units to
revolution units. The Legend Plus has a Rate Scale Factor and a Rate Scale
Multiplier to scale pulse units to the desired display units. They are
programmed in the Program Scaling section. Both are used to attain the Total
Scaling, “K
T
”. To calculate the Total Scaling, “K
T
”, for the application, the
following formula is used.
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.
Using the example previously discussed, the desired display unit would be 1
revolution and the number of pulses per display unit would be 48. Therefore, the
Total Scaling would be 0.020833.
K
T
= 1 rev ¸ 48 pulses per rev = 0.020833
In many applications the Total Scaling, “K
T
”, can be programmed directly into
the Rate Scale Factor, “SF”, in which case the Scale Multiplier “SCM” can be left
at the factory setting of X1. However, in some applications, such as the one above,
it may be desired to obtain more significant digits in the Scale Factor, “SF”. These
situations occur when “K
T ” does not calculate to an even number that will fit into
the four decimal places available to the Scale Factor. The following formula can be
used to calculate the Scale Factor when an SCM value other than X1 is needed.
FORMULA #2: SF = K T
¸ SCM
WHERE:
SF = Rate Scale Factor.
K
T
= Total Scaling.
SCM = Rate Scale Multiplier.
In this formula, the Total Scaling, previously calculated, is divided by the
Scale Multiplier Value, “SCM”, to obtain the Scale Factor, “SF”.
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