Page 16
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
99,999 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
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, π 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
Scale Factor Of
Comment
Gives Read Outs In
(π x d‖)
circumference of wheel
inches per second
(π x d‖) ÷ 36
36 inches in a yard
yards per second
((π x d‖) ÷ 36) x 60
60 seconds in a minute
yards per minute
5.2360 x d‖
multiplying the known
yards per minute
Say the diameter (d) is 10 inches. We get that pulses per second = 52.36 yards per minute and our scale factor is thus 52.36 for a
10 inch diameter shaft.
2)
Suppose we have a shaft turning on a conveyor and we know that for each turn of the shaft, the conveyor moves X inches
and we get one pulse into the tachometer. This step eliminates having to calculate the circumference. If we wanted to know
speed in meters per second then review the following.
The Input is measured in pulses per second. There are X inches per pulse, so:
Scale Factor of
Comment
Scales Display To
X
inches per pulse
inches per second
X x 0.914402 ÷ 36
1 yard = 0.914402 meters
meters per second
36 inches in a yard
X x 0.0254
multiply out
meters per second
The scale factor is thus 0.0254 x X (where X is in inches).
