Spider DSA User’s Manual
262
Applications
There are several different applications for order tracking. A discussion of some is
given below.
The first application, often referred to as Run Up/Run Down, is used to survey a
machine’s dynamic response when the operating RPM is varied across the entire
operating span. In this case, the RPM range can be very large, from a few RPM to
10,000 RPM. Such tests are run on automotive or aircraft engines and when
commissioning new or refurbished stationary processing equipment. The
measurements can be any physical quantities such as sound, displacement,
velocity, acceleration, torque, etc. The analysis measure can be the amplitude or
the power of an order, the energy over a fixed frequency band, a bin of octave
filter, etc. The most important result for this type of measurement is the
magnitude of the response versus RPM.
The second application is monitoring measured machine displacement, velocity,
acceleration, pressure, current or sound while the machine is performing its
normal duty. The instrument measures the amplitudes of specific orders and their
phase relative to a reference tachometer input signal. The phase is calculated
relative to the tachometer input or a separate reference input. This application is
common for machine diagnosis and balancing. In this case, the operating RPM is
relatively stable. Order tracking technology is useful to increase the accuracy of
the estimation of orders.
Order Track signals with phase are useful in the study of rotating machine during
Run Up/Run Down. This is often presented as a “Bode Plot”, useful in
characterizing resonance/excitation intersections. The Bode Plot is a concept
borrowed from control theory; it provides simultaneous Amplitude and Phase
data over a changing speed range (i.e. Run Up or Coast Down). Some of the setup
information depends on the rate of change of the RPM. The Run Up or Coast
Down could take anywhere from a few minutes to a few hours (such as for a cold
startup on a turbine).
Understanding Order Tracking
Resolution and Span
In fixed-bandwidth operation, an analyzer collects N successive samples from an
analog time-history at a sample rate, f
s
. The analog signal is pre-filtered by a low-
pass anti-aliasing filter set to the desired analysis frequency range, F
span
and the
sample rate is set to k F
span
, where k is a constant specific to the analyzer. Each
captured time-history is transformed to yield a spectrum. The following spans and
resolutions result:
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