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
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.