Red Lion GEMINI 2 - Manual (Page 50)

APPENDIX “E” - SCALING FOR RATE INDICATION
The Gemini offers a simplified method of scaling a rate indication display.
This unit offers the capability to monitor the entire range of any linear rate
process. Numerous programming capabilities that are available with the Gemini,
make this unit the most versatile rate indicator on the market today. The Gemini
is designed to internally count input pulses for a programmable sample time
period. At the end of the sample time period selected, the unit displays the pulses
accumulated after they have been internally multiplied by a programmable scale
factor. The unit maintains this display until it is updated by the next successive
sample time period. This is an automatic process that provides a very convenient
and adaptable method for rate indication applications.
The first step in determining the sample time and scale factor to be
programmed is to calculate the pulses per second (PPS) generated by the
application set-up at a known process rate. The standard formula for determining
PPS at a known process rate or machine speed is:
FORMULA [A]
Revolutions/Minute (RPM) x Pulses/Revolution (PPR) = PPS
60 Seconds
This formula is an excellent starting point, and will work in many applications;
however, applications will occur that do not fit this formula. In these cases, it will
require simple deductions to determine the correct PPS.
The next step is to choose a sample time to be programmed from those
available (1, 2 ,5, 10, 20, and 50 seconds).
The following guidelines apply:
1. The sample time is also the display update time. Unless the application dictates
a longer update time (i.e. greater precision or fewer display changes), the
basic rule is to start the calculation process with a one-second sample time.
2. When using the Gemini as a control mechanism, it is imperative that the update
time be shorter than the maximum allowable control reaction time. This is
another reason to begin with the one-second sample time.
APPLICATION EXAMPLE [A]
PARAMETERS AT MONITORED SPEED
A) Speed of shaft - 1200 RPM
B) Gear teeth per revolution - 50
C) Web sped at 1200 RPM - 820 FPM
D) Desired Readout - FPM
To begin the process of determining the sample time and scale factor, the
parameters from application (A) should be inserted into Formula (A) to determine
the PPS:
RPM x PPR = PPS = 1200 x 50 = 1000 PPS
60 60
The next step is to choose a scale factor, that when multiplied by 1000 will cause
a display of 820. The following formula will produce the correct scale factor:
FORMULA [B]
Desired Reading = Scale Factor
PS x Sample Time
820 = Scale Factor = 0.8200
1000 x 1
Thus, by programming a scale factor of 0.8200, the Gemini is now set to display
the correct FPM readout throughout the entire process, as it varies in speed.
A majority of the applications will be completed with a one-second sample
time. The following guidelines (next page) will assist in varying the sample time
and other factors, if the calculated scale factor exceeds the 5.9999 scale factor
capability of the Gemini.
1. Review the applications to ascertain if more input pulses can be generated. In all
rate indication applications, it is best to generate the highest pulse rate possible.
2. If the scale factor is calculated to be between 6.0000 and 11.9998, Function
Code 44 (number of count edges) may be programmed [44 2], which will allow
the calculated scale factor to be divided by 2.
3. If the scale factor is over 5.9999, the sample time may be increased to a higher
level (2,5,10,20, or 50 seconds). Keep in mind the update display time and the
control requirements of the application.
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