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KARL DEUTSCH
INSTRUMENTS AND SYSTEMS FOR NON-DESTRUCTIVE TESTING OF MATERIALS.

1089 views
01:53 May-07-1998
Chuck Garner
Noise Reduction in UT Instrumentation

I am interested in ways to reduce/eliminate electrical noise in UT measurements systems. We frequently find ourselves inspecting highly attenuative materials at high gain and electrical noise is a problem, particularly from DC servo motors on our scanning systems. It is also a frequent problem with portable flaw detectors in electrically noisy environments. Any suggestions or comments would be welcome.


 
02:14 May-08-1998

Rolf Diederichs

Director, Editor, Publisher, Internet, PHP MySQL
NDT.net,
Germany,
Joined Nov 1998
602
Re: Noise Reduction in UT Instrumentation : I am interested in ways to reduce/eliminate electrical noise in UT measurements systems. We frequently find ourselves inspecting highly attenuative materials at high gain and electrical noise is a problem, particularly from DC servo motors on our scanning systems. It is also a frequent problem with portable flaw detectors in electrically noisy environments. Any suggestions or comments would be welcome.
-------------

Electromagnetic noise can be transmitted to your UT measurements system through air or cable from the noise source, maybe a DC-motor.

You may work with shielding on the noise source to prevent electromagnetic waves in air. Sometimes it is better to put efforts on the source of the noise than doing a lot of work on the receiver side. We assume that the UT equipment is manufactured in a shielded unit. The probe cable and the transducer must be also shielded.

So far it seems that the shielding is simple.
However, what electrical potential should lay on the shield. Basically ground, but what kind of ground?
The center power supply provides the best ground, from there it is spread to different units (scanner, UT equipment, fane, cooling unit, pump, crane, ....)
Usually at those units you can still use the ground for analog and digital devices together. However, behind that you must be carefully. For instance if the DC motor frame provides a voltage supply and you
use it for the UT equipment as well, that can course a problem. The theory says that all grounds should be provided separately from one
center point.

But often you cannot prevent that analog ground contact happens elsewhere, e.g. if your transducer is immersed in the water of a C-scan tank. That makes it often necessarily to connect the analog ground of the transducer
cable with the 'dirty' digital ground at the tank together. In practice people trying several options, connecting thick ground cables from one place to another until the best result is found.
UT systems designer commonly providing pulser receiver units as much as possible close to the transducers. For special applications pulser transmitters may build also inside the transducer, as so called active probes.

The main problem exist at the transducer side. The transducer needs usually a relatively high impedance matching. That means the cable shield can be inducted with a much higher noise than it would occurs with a low resistor matched cable. If you read 75 Ohm on your cable that does not helps, important is the load at both sides of the cable. It is obviously that this problem increase direct proportional with the cable length.

During the work it may help the use of a frequency analyzer which will show you the noise frequency spectrum. By switching on/off different devises you can locate the parasite of a specific noise carrier frequency and
watch the spectrum until you find the optimum grounding.

Other sources of noise can be coupled into the UT unit by a wire connections, maybe by the gate alarm output to a SPS/PLC. An opto-coupled interface can help. A connection via a low pass filter (R-C or L-C) can be a more simple solution.

Finally you must be aware about that noise exist statistically, that means you never can prevent a false alarm coursed by noise, it may come just once a day. That lead to noise reduction methods which are provided by the UT equipment. Simple amplifier filter or other digital algorithm are in practice applied.
Another forum message may look more close to this subject.

Rolf Diederichs



 
05:52 May-08-1998

Linas Svilainis

R & D,
Kaunas University of Technology,
Lithuania,
Joined Nov 1998
67
Re: Noise Reduction in UT Instrumentation

Mr.Garner from Alliant Techsystems has posted very
interesting question which is complies with nowadays
equipment level.
I believe such organisation as Mr.Garner is
representing has already been through various
conventional techniques to struggle the noise.
Rolf has already mentioned all the possible ways for
solution search. But I believe you are interested in
more rough methods of noise reduction.
J.R.Barnes in his book "Electronic system design:
interference and noise control techniques" says -
"It's too late to care about the EMC problem once
system has already been designed"
I fully agree with him. It's the same as boiling
the soup - before trying to get some sufficient
results, some sufficient attempts have to be put
here.
First-some "conventional" techniques details.
The noise band and source should be examined first.
Once the noise is induced in receiving transducer cable,
double shielding or twisted pair inside dense shielding
plus differential preamplifier should help. If the
proper cable shielding has been applied, then one
could expect that usefull signal return current will
be flowing inside the cable ground and noise will be
flowing outside the cable shield. If proper shielding/
grounding of receiving amplifier has been applied,
those currents can be separated and noise influence
reduced. Sometimes ferrite bead with sufficient
permeability placed near receiving amplifier outside
the cable can solve the problem.
You've mentioned the problem with DC servo motors.
The spectra of such noise is concentrated in low
frequency domain and looks more or less as "pink"
noise. Proper damping of low frequencies might reduce
the noise. Also, good choice could be the application
of DC motors with rotating magnetic field instead of
brush motors. Since there is less sparkling, it should
echibit lower noise level. Better results can be
obtained if time selection is applied-i.e. shanner
head moves when acquisition channel is not active.
Of course, in such case inspection time should be
sacrified.

Now-more complicated methods:
1. Averaging, temporal and spatial will help
to improve the SNR. Especially effective are
various signal processing methods based on this
aproach. The best known is SAFT - what is attractive
in this method, that the spatial resolution is improved
and the electrical AND STRUCTURAL noise are reduced
pertaining the conventional samples number, so the
inspection speed. Other representative here is the
Split-Spectrum and Spread-Spectrum processing.
In first case conventional equipment can be used, in
second case more complicated equipment has to be used.
Both are based on aproach, that structural noise is
casued by reflections smaller that the defect size one
is looking for. Tadeush Stepinski from Uppsala University
has developed the adaptive procedure for parameters
adjustment, so it can be easily implemented into
equipments data processing package. Unfortunately,
because of velocity dispersion with frequency, this
procedure will work on limited depths. The frequency
selective loss compensation technique, we've developed
with coleagues from University College London, allows
for conventional way of attenuation vs frequency
compensation. Phase compensation is under development.
2.The time has come to forget the convetional way of
ultrasonic imaging, when the reflected signal is
directly presented on oscilloscope or computer display.
When spike or step exciting signal is applied to excite
the transducer, the data presented as A-scan is very
convenient to analyse manually, so proffesionals in this
field can tell almost everything about defect by looking
at reflected signal shape. When it comes to highly
attenuating materials, especially with high structural
noise, manual inspection becomes complicated or impossible
at all(as manual I understand the eye interpretation
of A- or B-scan image). By applying the compex signals
for transducer excitation, one can achieve high energy
feed into material and better detectability of
inhomogeneities. But the reflected signal direct
representation on-screen is very confusing. Here is a
lot of space for NDT equipment manufacturers - with
real time generation of ecxiting signal and received
data pre-processing very good results can be achieved.
Presentation of such results on-screen might be confusing
for old-fashioned operators, but possible further benefits
are evident.

Hope my notes have been of some help for you

Goog luck in fighting this dragon

Linas Svilainis

P.S. Optional image presents the SAFT'ed and
temporal domain deconvolved image of point reflector.



 
02:17 Jun-05-1998
Joerg Schulze-Clewing
Re: Noise Reduction in UT Instrumentation


 


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