Stanford Research Systems SR785, Руководство по эксплуатации и программированию

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Order Analysis 2-55
SR785 Dynamic Signal Analyzer
Order Analysis
What is Order Analysis?
In vibration analysis or preventive maintenance of rotating machines, it is often useful to
identify spectral components by their relation to the speed of the rotating machine, rather
than their absolute frequency. For instance, if we identify a potentially troublesome
vibration component at 3 times the rotation speed during a baseline test of a machine at
120 RPM, all we really care about is that the vibration is at "3rd order," not that it's at
6Hz. That way we can compare the results to the 3rd order vibration level in some future
test without having to worry about getting the machine to run at exactly 120 RPM again.
In the Order Analysis measurement group the SR785 calculates spectra whose x-axis is
in orders, i.e. multiples of the machine speed. To do this, it requires information about
the speed and postition of the machine's shaft. This is provided by a tachometer input,
which provides a signal at fixed positions relative to the shaft rotation. With the SR785,
all the traditional measurements of rotating machinery analysis including order maps,
rpm profiles, run-up and run-down plots, can be easily made without any additional
equpment.
Order Spectra and Order Tracking
Two types of measurement are typically made with rotating machinery. The first looks at
the input amplitude as a function of order. This type of measurement is similar to the
standard FFT, except that the x-axis is calibrated in orders instead of in absolute
frequency. Typically, a measurement of the order spectrum is first performed to identify
orders for further study.
Once we've identified the problem orders, we can move to the second type of
measurement, where the amplitude of a specific order, or set of orders is recorded as the
machine speed is changed. We may want to ramp the machine from startup to operating
speed, and track the behavior of the third order vibration.
The SR785 can perform both types of measurements by selecting a value for the
[Freq]<Tracking> softkey. When tracking is turned on the analyzer records the
amplitude for each spectral order as a function of RPM. Using the [Freq]<Track Setup>
submenu, a 2-dimensional plot of the amplitude of any order of either input channel can
be generated. Using RPM trigger arming, the analyzer can be configured so that the
tracked data points are acquired at fixed RPM intervals.
The tracking feature uses the SR785's Tracking/Waterfall memory. Thus when tracking
is turned on, waterfall storage of spectra is not allowed. If waterfall storage is necessary,
turn tracking off.
Order Measurement Setup
Choose the Maximum RPM, Minimum RPM, Maximum Order, and Delta Order to setup
an Order measurement. Maximum RPM affects the available range of Max Order; the
higher the selected Maximum RPM, the lower the maximum order that can be selected.
The realtionship can be summarized as: max(Max RPM) = 60
•
40,000 /(Max Order).
Minimum Order does not affect the measurement at all, it merely sets a default scaling
for measurements whose x-axis is calbrated in RPM, such as Order Track.