Measurement Computing Medallion Rotate rev.2.3 - Manual (Page 34)

34
Medallion Rotate Manual October 2000
I
NPUT
S
IGNAL
R
EQUIREMENTS
The fixed sampling rate for the machine speed signal must be high enough
to give a good definition of pulses. Note that the number of tachometer pulses/
revolution determine the maximum torsional frequency of the analysis. In
addition, the sampling rate must be at least 2.5 times the maximum frequency of
interest to avoid aliasing effects. For example, for a maximum torsional
frequency of 10 orders, you must have a tachometer capable of providing at
least 2.5 x 10 pulses/revolution (25 pulses/revolution).
A good rule of thumb is to use 5–10 time oversampling of the tachometer
pulse frequency (not the machine speed) to get good, clean tachometer pulses.
For example, for a tachometer that produces 25 pulses per revolution on a 60
RPM machine, sample the tachometer data at a rate of 125–250 samples/second.
Since the torsional analysis algorithm works in the time domain, do not use
aggressive anti-aliasing filters for the data acquisition.
E
XAMPLE
This example is from a 3-cylinder
diesel engine. The data is from a non-
contact sensor on a 72-teeth gear.
1. The first plot is the time waveform,
zoomed in to show more detail. To
display a time waveform, select the
tachometer channel in the
Channels List window and click
the Plot button.
2. Set the preferences to create the torsion file.
• From the Edit menu choose Preferences.
• Select Create torsion file during tachometer processing so Medallion
Rotate will create the torsion file.
• Medallion Rotate also uses the sampling rate from this dialog box when
processing the signal. This is not the same as the original sample rate
used when collecting the data. A
meaningful sample rate is 1/5 the
rate of the tachometer signal. For
example, for a 1 Hz tachometer
signal, use a sample rate of 0.2 Hz
when processing the tachometer
signal.
25 pulses/rev x 60 RPM x 1 minute
60 seconds
x 5 = 125 samples/second