Another favorite part about the International Modal Analysis Conference (IMAC), sponsored by SEM, is spending a little time with Professor David Brown. Dr. Brown, who is now "retired" from University of Cincinnati, is active as ever in teaching, mentoring and innovating in the field of modal analysis and structural dynamics. Catching up, I asked Dave what his observations were on...
Q (Mike): ...the evolution of large channel count sensor arrays… seems the progression over the last decade went from low cost, to cable management, to modular conditioners and to automated channel management. What areas do you see being worked on now?
A (Dave): I'm seeing improvements in sensors in terms of sensitivity… higher sensitivity and lower noise sensors. Obviously, on large structures it's important for good measurements at low frequencies. There are also improvements in bookkeeping and data acquisition software to manage these large channel counts. Part of that includes the ongoing work on TEDS… which is the Transducer Electronic Data Sheet (TEDS) IEEE 1451.4 standard. The standard defines the storing of some types of information into the sensor itself so that when we make the measurements, we can then recall critical information, like calibration. This helps in the bookkeeping of processing large sets of data. We can also improve TEDS with expanded memory that is now common to the digital part… perhaps storing both factory and field calibration curves. Also expanding with template types for all the common sensor types… I see the need for continued cooperation between sensor and data acquisition vendors to ensure smooth handling of scaling of data for large arrays, as well as new uses for expanded memory capability.
The other thing I would like to see done is something we fantasized about back in the 1960’s… That is, anytime we take data I would like to make sure all the time data is stored so that we can go back, post process after the test, in case we want to refine the data analysis. This is becoming much more feasible with Terabyte disks.
Q (Mike): What new technologies have you seen introduced in the last few years?
A (Dave): One that I looked at pretty hard is the optical techniques that have a good advantage of reducing the sensors and cabling. Optical techniques include continuous scanning lasers and optical photogrammetry correlation techniques… meaning that we can basically measure thousands of points on a structure with multi DOF (three to six degrees-of-freedom). They are limited primarily by the line of sight constraint of optical techniques. I imagine they will also be integrated in a hybrid sense with traditional discrete sensing techniques to give us a more complete set of information on a large structure. On aircraft, this could be thousands or tens of thousands of points which could give us a better picture of what’s going on with things like the control surfaces.
Another advancement in technology is simply the progression of integration in hardware. Just like the original ICP® sensors eliminated the hassle of the charge amplifier by integrating a small FET (field effect transistor) into the accelerometer, shakers are now being manufactured with the power amplifier built right into the unit. The simplification makes it easier to distribute more shakers about a structure, meaning more even distribution of the input energy. This also allows for more reference force measurements, providing more information and effectively filling up more columns of the frequency response matrix. Integrating more and easier excitation is also good for sinusoidal excitation, which is the typical input method used with the optical techniques. This is consistent with the spatial sine testing method we worked out in the 90’s. We have a lot of technology for quickly and easily processing large quantities of measured information. At the last IMAC there were some nice demonstrations on the optical technology measuring mode shapes on rotating fans.
Q (Mike): What is your feeling on wireless solutions in the structural dynamics market space? Is wireless ready for primetime yet?
A (Dave): I think there are certain applications where it would be very good. For example, there are lots of situations where we have to telemetry out and that, by nature, is wireless. For very large structures… for things of that nature… bridges… it’s difficult to wire… wireless would be good for those. That application may be first. I think the technology for synchronizing the sensors and the data acquisition together is doable. I think that is the fantasy of almost everybody doing this type of work. The commercial solutions aren’t there yet, but I think they are getting closer.
Feel free to leave a comment or question. We'd be happy to ask Professor Brown for help with answers. Thank you.