Order analysis is a set of functions for converting a signal from time-domain (i.e. signal sampled
at discrete time intervals) into the order-domain (i.e. signal sampled at constant angular
increments of a reference rotating device).
The purpose of this transformation is to provide a better distinction between signal artifacts
introduced through a shaft rotation from other artifacts and to better relate each spectral
component to the shaft rotation speed.
In order to perform this transformation, a separate signal containing RPM (revolutions-per-minute)
values is needed. This signal does not have to be sampled at the same sampling rate as the time
signal but the duration (in seconds) of both time signal and RPM signal should be approximately
the same (+/-10%). If your revolution speed signal is in some other units, for example revolutions-per-seconds, you can easily convert it to RPM by using Scale/Normalize function or the Signal
calculator.
SIGVIEW also includes functions to perform the inverse order transformation, i.e. to convert
order-signal back into the time-signal.
All order analysis functions (forward and inverse transformation) can be found in the Signal
calculator:
Example
For example, let us take a look at the spectrogram of the acceleration signal from a machine
running at a changing shaft rotation speed. The spectrogram shows various artifacts which are
obviously caused by the main shaft rotation speed changes as shown in the corresponding RPM
signal bellow.
Time domain signal (acceleration):
RPM signal:
Spectrogram:
By using Signal calculator, we can convert the original time-signal into the order signal. To do it,
select the input signal, then choose "Convert TO Order signal" function, and finally add the RPM
signal to the expression:
The result will be a signal, similar to the original time-domain signal, but with the X-axis in
"revolutions" units instead of seconds. You can work with this signal just as with any other time-domain signal. For example, you can calculate its FFT, Spectrogram, Time-FFT... Just remember
that the base for this signal are not "samples/sec" but "samples/shaft revolution"
Order signal:
For example, if you calculate the spectrogram of the generated order-signal, you will get this
result:
Order spectrogram:
Instead of displaying Y-axis in Hz (oscillations per second), this graphics shows it in "Order" units
("oscillations per shaft revolution"). The order of 1 means "the same frequency as the shaft", the
order of 2 means "2x shaft frequency" etc.
For most visible artifacts in the spectrogram, you can observe a straight track for each order,
indicating that the vibration occurs at a fixed multiple of the motor rotational speed.
Inverse order transformation
After generating the order-signal, you can work with it just as with any other time-domain signal.
For example, you can perform some of filtering functions. Please
note that all frequency related
functions refer to "orders" instead of "Hz".
For example, you could filter the main shaft frequency and all its harmonics out of the order signal
and then convert the order-signal back to the time-signal again.
You can perform this inverse transformation by using Signal
calculator and its function "Convert
FROM Order signal". You will have to choose the RPM signal used to calculate order-signal as a
second parameter again. The result of this operation will be a time-domain signal again.
Links
For more information about this technique, see some of these external links: