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MIMO for gvt : pseudo random and periodic random inputs

[ Edited ]

Good evening,
I have some questions about the possible use of MIMO functions to develop ground vibration tests on aircrafts. At my university, we've always used the spectral testing module with casual excitement, but yesterday a colleague suggested I use the pseudo-random input and the MIMO module for my next tests. So I did some experimental tests on just a cantilever beam and I read the LMS manual, but I would like to have some suggestions before using this input also because I will not have much time to investigate the setting of the parameters during my next gvt. First of all, I would like to know why I have a very limited time to set up the channels (set ranging )and if I can extend it. Furthermore, I don't know if it would be best to use a random input instead of a pseudo-random. Finally, I wonder if the random test with the spectral testing module is equivalent to the MIMO test done with random input. If you have other suggestions, such as the number of blocks to set (which I suppose depends on the structural damping) or on the input profile, they will be greatly appreciated. Thanks for your help.


Re: MIMO for gvt : pseudo random and periodic random inputs

[ Edited ]

Dear Agnese,

thanks a lot for your question. It is very relevant and i will try to reply as simply as possible.

First of all, let's shed some light on the two applications you are mentioning. Spectral Testing is a more general application, while MIMO FRF is newer, more complete and dedicated to perform structural dynamics testing and measure FRF with shakers.

For standard excitation profiles, such as random and burst random, the two applications are exactly equivalent, but MIMO FRF supports more dedicated and customizable excitation profiles, as well as a wider range of input channels.

Now coming to your more specific questions:


  • Limited time for ranging: that is simply related to your measurement settings (block size, resolution and bandwidth): for the ranging, the software will simply apply 1 block..if you want to have longer ranging, simply increase your resolution and the ranging will be longer...but it won't change the results.
  • Random vs pseudo-random vs burst random: this is related to theory and, as you mentioned, it will depend on damping. Pseudo-random (and periodic as well) have a dedicated, optimized excitation and averaging scheme. It ensure consistent and high quality data with the minimum number of averages. It is theoretically more complex, but as a rule of thumb, use more delay blocks when you have highly damped structure and make sure you have at least 3-4 realizations. This way, you will minimize noise and reduce non-linearities in the data, ensuring high coherence and good results. Also, with pseud-random the signal is defined to be periodic, so you do not need window. Burst-random is a simple way of having a periodic random signal, so again no window. But in your block, you will shut off the shaker to ensure periodicity: the lower the damping, the longer this shut-off time need to and the lower the energy you give to the structure. Finally Continuous random will require you to apply a window, normally hanning, to ensure periodicity. Although the software compensates for it, you will bias the FRF calculation, in particular at resonance (which is what you care about). So my suggestion would be to go for Pseudo-random..we tested it on gvts and it gave the best Frf in the shortest time.

I hope i replied to your doubts. In case you have further question, feel free to contact our Customer Support via GTAC ( )


Best regards and have fun with your GVT Smiley Happy


Simone Manzato

Product Manager Structural Testing Solutions