Ping hammer vs. shaker measurements & control accel mounting locations

[davidVz]

Hi,

My knowledge in the field of vibrations is very limited and I’m hoping someone can give me some insight on these questions:

When measuring the natural frequency of a DUT (plastic automotive components in my case), what would be the difference in using a ping hammer/spectrum analyzer instead of a sine sweep on a shaker table? What kind of deviations from the ping hammer results might I expect as the sine sweep acceleration is increased?

When mounting the control accelerometer on a DUT for these shaker table measurements what kind of reasoning should I use to choose the mounting location? I have seen the shaker and product interface used for control but I am not clear why this would be considered the best choice.

Thank you,

David

 

[GregLocock]

Are these questions for a test?



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[davidVz]

No. I take ping hammer and shaker table measurements and would like to understand them more.

I'm not sure if answers can be provided without intimate knowledge about the DUTs but I figured I'd throw the question out here and see if I could learn something.

David

 

[GregLocock]

Well, in that case the best way to learn is to test it both ways and see what the differences are.

Still, to shorten the learning curve

"When measuring the natural frequency of a DUT (plastic automotive components in my case), what would be the difference in using a ping hammer/spectrum analyzer instead of a sine sweep on a shaker table?"

sine sweep will have better S/N, better coherence, when it is properly set up. Hammer will be easier to set up, and can give excellent results. If you are doing a full modal survey (lots of points) use a shaker for preference.

"What kind of deviations from the ping hammer results might I expect as the sine sweep acceleration is increased?"

Smearing, you need to try it to see it, and read up on the maths.

"When mounting the control accelerometer on a DUT for these shaker table measurements what kind of reasoning should I use to choose the mounting location? I have seen the shaker and product interface used for control but I am not clear why this would be considered the best choice."

It isn't a bad location, the response at the driving point is crucial for analytical study. however it is often very insensitive to modes of interest. Again you really need to look at the maths.

It's called EXPERIMENTAL modal analysis for a good reason, there is a lot of messing about.



Cheers

Greg Locock

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Rob45]

You might also visit the B&K website; their writeup on modal data acquisition discusses your question at some length, and answers it.