Understanding Ultrasonic Signal Analysis by Thomas J. Murphy
C.Eng. The Reliability Team
Introduction
This paper reviews the application of digital portable
ultrasound technology as a diagnostic tool in predictive
maintenance.
Ultrasound
The use of ultrasound in various forms of measurement has been
with us for over 40 years. Today, the use of portable ultrasonic
equipment is becoming the standard tool to use for finding
airborne leaks, testing steam traps, finding corona discharge,
bearing condition testing and tightness testing. But how does
this type of ultrasonic measurement work?
Ultrasound is defined as noise beyond the range of human
hearing. For the application to airborne leak detection, the
frequency at which the most noise is produced by a turbulent
leak is 38.4kHz. So, produce an instrument which listens to this
frequency and you can detect leaks – but the human can still not
hear them. To overcome this problem some electronic processing
is required.
The type of electronic processing of the data used varies
slightly by manufacturer. This work was performed using an SDT
International model 170 which, for the vibration-literate, uses
a demodulation method to extract sum and difference frequencies.
The processor in the SDT170 generates a 2kHz filter above and
below the operating centre frequency and takes the sum and
difference frequencies to produce a new signal with a 2kHz
bandwidth which can be heard – and measured. Using this method
it is possible to listen to ultrasounds. By changing the sensor
arrangement connected to that instrument, it becomes possible to
listen to ultrasounds generated by different mechanisms.
The beauty of this technique is that the audio signal is a
reasonable analogue of the original rather than a series of
Geiger-counter clicks or buzzes. From an analytical viewpoint,
the beauty of this method is that there is a signal which
contains real and useful data.
Fault Finding
Using an airborne ultrasound system it is possible to walk
around even the noisiest of environments and listen for
ultrasonic defect signatures. The reason why, is simple. The
normally high levels of noise that we are used to dealing with
in the plant is almost entirely noise generated solely in the
audio region – very little of the normal noise contains
ultrasonic components.
Taking a walk around motors, bearings, belts, gearboxes, pumps,
vacuum lines, steam traps and compressed air lines will take on
a whole new meaning when listening ultrasonically. Motors with
electrical problems will crackle as you listen to the corona,
bearings will have a smooth rushing sound if they are OK and
will have a popping sound if there is a problem. Belts,
gearboxes and pumps will squeal. Pumps will also pop if they are
cavitating. Perhaps some of the most used applications of
ultrasound are airborne leaks and steam trap testing.
Energy Efficiency
The sad fact is that industry wastes Millions each and every
year.
That this loss is almost entirely controllable and is ultimately
controlled by human error has still to sink in.
Compressed air is one of, if not, the most expensive energy
source in use in industry. Many of these uses are necessary and
good, others are inherently wasteful. Between the point of
supply and the point of delivery there will be leaks. To what
extent and how many depends upon many factors – if you do not
have an active programme of leak detection and control, you
could be losing perhaps 35% of the air you produce.
Using a portable ultrasound system it is possible to find not
only compressed air leaks but also vacuum, steam and nitrogen
leaks, as well as some refrigerant gases.
Changing from an airborne sensor to a contacting ultrasound
probe and you have the perfect tool for listening to steam
traps. Once again it does not matter how noisy the environment
is in terms of audible noise, the ultrasound system picks up the
operation or otherwise of the steam trap.
Lubrication improvement
The same contact probe can be used to listen to bearing
condition.
 By
attaching the contact probe to a modified grease pipe it is
possible to listen to the condition of the bearing when the pipe
is attached to the grease nipple.
It
is becoming increasingly apparent that lubrication must move
from time-based and often infrequent greasing to a comprehensive
on-condition approach. This method will prevent not only under
lubrication but also the equally deadly over lubrication which
is so prevalent in many plants.
A simple and common approach to monitoring and setting baseline
friction levels is with an ultrasonic data collector. Friction
from the bearing excites the sensor which in turn produces a
microvolt signal which is accurately measured and referenced to
a decibel log scale. This data is displayed on-screen and stored
to the internal data collector. It can then be transferred to PC
where for further analysis. As a rule any bearing with a
measured ultrasound value 8-10 dBµV over its baseline should be
greased using this method.
If an ultrasound system can do all of this, the next logical
step must be to consider integrating ultrasound and vibration:
to use an ultrasound system as an intelligent sensor for a
vibration system. But first there are some basic problems to
overcome.
Repeatability
Coming from the world of vibration, we are familiar to one
degree or another with the concept of repeatability of our
measurement. Sometimes we use accelerometers with vaguely known
sensitivities in instruments which are poorly calibrated and
then claim a change of 10% in a reading to be significant. This
is perhaps an extreme case of bad measurement method, but in the
world of ultrasound there are other problems to overcome in
terms of repeatability. The greatest problem is to answer the
question “What exactly am I measuring?”.
In High School science experiments, clearly stating the
measurement units was an essential part of good lab technique.
Yet, in the world of ultrasound this most fundamental point has
been almost completely ignored.
Since ultrasound is an acoustic measurement, it seems quite
logical to use the global acoustic unit of amplitude, the
decibel or dB. But, the dB has no meaning unless it is quoted
against a reference. Sadly within the airborne ultrasound world
this basic fact has been overlooked by many of the
manufacturers. It is for this reason that this work was carried
out using the SDT170 because the SDT unit is measured in dBµV,
the sensors are calibrated, traceable and have proven
repeatability.
Again, in our vibration world, we take it for granted that if I
have 3 accelerometers and 3 data collectors I can use any of
these 9 combinations to take a reading on a machine and get the
same result within a small statistical spread with some
repeatability. This has been shown not to be so easy to do with
some of the ultrasound systems on the market. Though this is of
course essential if we are to deploy ultrasound into the
trending and diagnostics world inhabited by the vibration
engineer.
What does an Ultrasound signal show?
It is one thing to go around listening to signals, but is it
possible to use this technology for measurement and for
diagnosis?
Here we see an example of an ultrasound signal produced by a
contact probe and captured by a data collector.
Here is the same bearing captured with an airborne probe:
There is one clear aspect to these two signals: they are
impulsive in nature, not sinusoidal nor repetitive, but
consisting of a series of sharp pulses.
This does not bode well for someone planning to use FFT
processing on these signals. Take a look at this comparison of
the corresponding spectra for the two signal above:
Ignoring the vertical axes and the absolute numbers, it is clear
to see that there is little difference between these two. How
wrong could we be?
The reason for this error goes right back to the basic
mathematics of the FFT. Basically the FFT is only supposed to
work on a repetitive signal from -∞to ∞. Outside this basic
condition, the result may not be as good as you think.
The classic time signal for a bearing defect gives a goldfish
envelope like this:
The spectrum however only really shows the resonance, why
because that is where all of the repetitive energy is in the
signal.
So, relying on spectrum analysis of ultrasound signals in the
case of the transient signals associated with bearings is not
necessarily a good idea. Spectrum measurement works much better
when there is a much lower peak-to-mean ratio. Take for example
in the area of corona and discharge. Here is a comparison of two
time signals, one is nuisance corona and the other is a damaging
corona:
Here we see that in the case of the nuisance signal on the
bottom, the peak-to-mean ratio is quite low and that this ratio
does decrease slightly as the defect deteriorates. In a case
like this there is some safety in applying FFT analysis. In
doing so we get the following:
In the nuisance signal we see a series of harmonics of 120Hz but
in the damaging signal we see a series of harmonics of 60Hz. So
in this case we can use all of the standard methods we use in
vibration analysis to assist in a simple detection method for
damaging corona.
Conclusions
1. Storing the signal generated by an ultrasound system can be
beneficial
2. The ability to visualise a transient event and characterise
it is helpful in the analysis and diagnostic process
3. It is important to select the recording process appropriate
for the diagnosis you wish to perform
4. Storing data in a structured manner inside a predictive
maintenance database program with the analysis tools that
provides will open up new application areas
5. Airborne ultrasound alone can pick up defective bearings
6. Great care must be taken when using spectrum analysis on
signals with large peaks and low energy since the spectrum
calculation process can mask or destroy the pertinent
information
7. Any proprietary predictive maintenance system selected should
include functions which allow the importing, manipulation and
post-processing the time signal
8. Any future developments which would provide the raw broadband
ultrasound signal would mean that the user could apply other
standard DSP analysis tools to the recorded signal.
Biography
Tom Murphy is an Acoustics graduate from Salford University and
has 25 years experience in the world of industrial vibration
measurement.
Tom is the Managing Director of Adash 3TP Limited, based in
Manchester England, a Company specialising in the application of
vibration, infrared and ultrasonic technologies to improve
maintenance.
See Tom Murphy at the
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Maintenance Technology Conference |
