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ABSTRACT Some of the
factors that may have limited the popularity of
vibration
analysis on rotating machinery up to now are its relatively high
cost, need for skilled personnel, and sometimes questionable
diagnostic results. In recent years, consumer audio hardware and
software devices have made significant progress, making the
handling of dynamic signals far cheaper than previously
available vibration instrumentation. With very few modifications
to existing audio signal processing technology, the handling of
vibration signals can be made readily available at a much lower
cost and, combined with improved auralisation techniques,
rendered far simpler to use, making the technology accessible to
a wider range of potential users. Some examples of the use of
this new technology (VIBRO-AUDIO) will be presented in
applications for troubleshooting bearings and gears in the
mining industry.
Keywords: cepstrum, composite spectrum, envelope analysis, FFT,
gearbox, helicopter, iron mine, conveyor, shovel,
rolling-element bearings, screw compressor, transmission, USB
audio, vibro-audio, zapper.
INTRODUCTION
The last decade has brought us unprecedented developments in
the field of computers and digital consumer audio, with music
being transferred over the internet, MP3 players the size of a
thumb, and a multitude of other clever multimedia devices which
makes the processing of audio and images a snap compared to what
was available 10 years ago. This evolution in multimedia
technology, which was fueled by the huge demands of the consumer
market, is about to revolutionize other applications, in
particular the acquisition and processing of dynamic signals for
the purpose of condition monitoring and machine diagnosis. The
world of vibration analysis, which in the past was reserved to
specialized personnel using complicated and expensive equipment,
is about to be completely transformed by the readily available
technology and should gain tremendous popularity in the future,
when prices for vibro-audio measuring systems drop and simpler
to use and more powerful software tools for the PC become
available. The use of audio, which enables the user to listen to
the vibration signal, constitutes an important feature in the
fabric of this new technology as even today, many mechanics
still detect faults solely by listening. This is especially true
for the troubleshooting of bearings and gears, as these machine
components usually generate complex vibration patterns which can
often be better understood by combining advanced signal
processing techniques with listening.
FUNDAMENTALS OF VIBRO-AUDIO TECHNOLOGY
To transfer a time signal from a vibration transducer to a
PC, a VIBRO-AUDIO signal interface and a sound card (A/D with
anti-aliasing filters) are required. The purpose of the VIBRO-AUDIO
interface is simply to power the IEPE (ICP) transducer(s) and
sometimes provide gain or attenuation in order to optimize the
signal level(s) to the range of the sound card A/D. Most
built-in PC sound cards have a frequency response which extends
from 20 to 20k Hz (standard sampling frequency is 48000 Hz with
8 or 16 bit resolution), thus imposing some limitations at the
low and high end of the frequency response. However, this is
usually not a problem with most bearing and gears applications.
Microsoft Windows also supports USB audio, where the USB port
can also be used to streamline an audio signal (ASIO), and there
are many inexpensive USB audio devices today which provide an
extended frequency response (from 2 Hz to 40k Hz). USB audio
devices provide in general better quality signals than internal
sound cards, as the A/D conversion takes place outside the PC
where the noise level is usually lower. Many 24-bit USB audio
converters (figure 1) are available today and provide more than
adequate dynamic range (> 110 dB). It will be necessary to
calibrate the sound card or USB audio device with a known signal
(for instance, a 1k Hz tone at 1 g) in order to obtain the right
amplitude levels (note that automatic gain control should be
disabled). Once the basic time signal is acquired, all the other
operations on the signal (integration, differentiation, digital
filtering, FFT and others) can be performed digitally inside the
PC by using the proper application software. An advantage of PC
audio is that because two channels are available (stereo), all
two-channel functions such as cross-spectra, frequency response,
coherence, and more can easily be obtained.
Many techniques are available to remove unwanted components, the
most common being:
- Time averaging
- Digital filtering
- Low-pass, high-pass, band-pass, notch, comb filters
- Self-adaptive noise removal filters (Periodic Signal
Enhancer, Discrete Frequency Zapper)
- Coherence filtering (removal of the contribution from a
coherent source) We will show later how some of these signal
recovery techniques can help in specific bearing and gear
applications.
To be continued in next issue: |
