Monarch Instrument ACT-3 - Manual (Page 9)

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hysteresis would be 5% of 100 or 5. This value is then subtracted from the set point (it
would be added for a low limit) so that the absolute value is 95. Thus, the alarms will trip for
any input value greater or equal to 100, but will only reset when the input drops below 95.
Without the hysteresis feature, the alarm relays would chatter on and off it the input varied
from 99 to 101, which is undesirable. The user can set the hysteresis to any value from
0.0001 to 99.999% of the set point. Refer to Section 6.0.
NOTE: The instrument recalculates the hysteresis at 5% each time you alter the set point
or change the limit type (e.g. from high to low).
1.3.3 Low Limit Lockout
The low limit lockout is a feature that prevents a low alarm from tripping when the input
starts from zero. The low alarm essentially is locked out and will not operate until the input
exceeds the low limit, at which time the low alarm is enabled and will trip when the input
goes below the set point. This feature enables a motor that has a low speed cut out (low
alarm) to start from rest without having to short out the normally closed relay contacts
externally. This feature may be enabled or disabled by the user. Refer to Section 6.0.
1.4 Current (IO) and Analog (AO) Output
The ACT-3 is unique because it allows simultaneous current sink (4 to 20 mA) and voltage (0 to
5 Vdc) outputs. The current and voltage outputs track one another so that when the current is 4
mA, the voltage is 0 Vdc, and when the current is 20 mA, the voltage is 5 Vdc. The output is linear
to within 0.5%.
The analog outputs are derived from a 12-bit digital to analog converter. This means that the
output voltage (or current) changes in steps. The standard analog output has 4096 steps from
zero to full scale. This implies that each step size is 1/4096 of the full-scale value or about
0.0244% of full scale. The user can set the actual full scale value anywhere from 10 to 999,990.
This full-scale value is the value at which the analog outputs are at a maximum, 20 mA or 5 Vdc.
The zero and full-scale range is usually set to give a reasonable working range for the analog
output. For example, if you are measuring the RPM of a motor that typically runs at 1700 RPM,
you may want to set the zero scale (offset) for 1000 and the full-scale for the analog output at
2000. Note that the zero and full scale ranges are always set in the units you choose to display;
RPM in this case. The output voltage will then be 5 Vdc (20 mA) for an input of 2000. It will be
linear between 1000 (zero scale) and 2000 (full-scale). Thus, at 1700 RPM the output will be:
(1700 - 1000)
—————— X 5 Vdc = 3.5 Vdc
(2000 - 1000)
(2000 - 1000)
Resolution = —————— = 0.2441 RPM
4096
NOTE: For any input below the zero scale setting, the outputs will be at 0 Vdc or 4 mA. For
any input above the full-scale setting, the outputs will be at their maximum value, 5 Vdc
or 20 mA.
1.5 Maximum and Minimum
The unit tracks and saves the maximum and minimum values. These values are continuously
updated and can be viewed at any time by pressing the RECALL button on the front panel. The
first time this button is pressed the MAXimum is shown, indicated by the MAX light to the right
of the display. Pressing the RECALL button a second time shows the MINimum. The user can
also reset these values by pressing and holding the RECALL button and then pressing the RESET
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APPENDIX A - SCALING THE ACT FOR ENGINEERING DISPLAYS
The SCALE Mode must be used to display RPM in applications where there is more than one pulse
per revolution. Below describes how to use this mode and other applications that need to be scaled.
When using the scaling function of the ACT Tachometer it is possible to multiply the input signal by
any value from 0.0001 to 9999.9 making it possible to display the actual output in virtually any
format.
The most important thing to note is that the instrument takes all tachometer measurements in pulses
per second. The RPM Mode requires a 1 pulse per revolution input, so it simply uses a built in scale
factor of 60.
Input Conversions (Scale Factor) Scales Display To
Pulses 1 Rev 60 Seconds Revs
——— x ——— x ————— = ———
Second 1 Pulse Minute Minute
In an application with multiple pulses per rev:
Input Conversions (Scale Factor) Scales Display To
Pulses 1 Rev 60 Seconds Revs
——— x ———— x ————— = ———
Second
N pulses Minute Minute
Therefore, to read out in RPM, the scale factor is 60 ÷ N, where N is the number of pulses.
Thus, if the system gave out 4 pulses per revolution, the scale factor becomes 60 ÷ 4 = 15. The trivial
case is the 60 toothed gearwheel used in older systems which gave out 60 pulses per revolution,
reducing the scale factor to 1, or measuring frequency (cycles per second) directly.
All that is required to scale the unit is a bit of common sense, a basic knowledge of mathematics (you
can of course use a calculator) and some relationships pertaining to your application (e.g. 1 yard = 36
inches, or 1 yard = 0.914402 meters). Refer to Appendix B for some useful conversions.
A very useful formula for this application is knowing the circumference of the shaft you are monitoring.
This could also be a speed wheel, tire etc. The circumference =
π x diameter (π = 3.14159).
In order to scale we need to know what we want as opposed to what we have, and some relationship
between the two. For example:
1) Suppose we have a wheel turning on a roll of paper measuring its linear speed. The wheel has
a diameter of d inches. Each time the wheel turns one complete revolution, x d inches (the
circumference) of paper moves under the wheel and we get one pulse into the tachometer. We
want to know at what speed we are producing paper in yards per minute.
The input is measured in pulses per second. There is one pulse per revolution, so:
Input Conversions (Scale Factor) Scales Display To
Pulses 1 Rev π x d Inches Yard 60 Seconds Yards
——— x ——— x ————— x ———— x ————— = ————
Second
Pulse Rev 36 Inches Minute Minute