Red Lion LEGEND - Manual (Page 53)

APPENDIX “B” - SCALING FOR RATE INDICATION
The Legend 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 so that input
pulses to the unit will be 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 will
automatically scale 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 X2 or X4 count mode. The rate can show
the same reading as the count, but take into account 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 to
multiply the number of pulses by a scaling factor to convert the pulse units to
revolution units. The Legend 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 Factor, “K
T
” To calculate the Total Scaling Factor, “K
T
”, for the
application, the following formula is used.
FORMULA #1: K T
= Display units ¸ Number of pulses
WHERE:
K
T
= Total Scaling Factor.
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 Scale Factor would be 0.020833.
K T = 1 rev ¸ 48 pulses per rev = 0.020833
In many applications the Total Scale Factor, “K
T
”, can be programmed
directly into the Rate Scale Factor, “SF”, in which case the Scale Multiplier
“ SCM” can be lef t at the f actory setting o f 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 the “K T ” factor 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 Factor.
SCM = Rate Scale Multiplier.
In this formula, the Total Scaling Factor, previously calculated, is divided
by the Scale Multiplier Value, “SCM”, to obtain the Scale Factor, “SF”.
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