impact hammer of light structures problem- need help.

[ruimoreira11]

Hi, I am using a small impact hammer from kistler 2722A2000
and recently i realize that for the low frequency range I 'am getting a strange autospectrum from the force signal. Instead of getting a nice curve shape (a near horizontal bounce) I get a irregular bouncing curve like this:

http://files.engineering.com/getfil...-553a-4343-8ed9-63666d970377&file=martelo.bmp

This will affect the transfer FRF function.
The attached figure in the above link presents the autospectrum for the load cell of impact hammer and the other curve is from accelerometer, which is nice and clean.

I already checked cables, ICP feeding (through different icp sources as well), tip link, etc. Any ideas about this issue? Any procedure to check anything?
Please help.
Thanks

 

[snoopnoon]

You have too soft of a tip to supply the energy to the dominate mode it appears your looking for. Try using a tip that is harder.

 

[ruimoreira11]

Thanks for your reply. However i get confused with that solution since I learned that harder tips are useful to extend useful frequency range of the impact (extend first autospectral bounce) whereas soft tips (like rubber tips) are intended to focus the impact energy onto a low frequency range. In this case I use the small tip made of nylon, which would be for larger frequency ranges than the one used here in this test.

On the other hand i understand that softer contact on suspended or foam support can produce some undesired effect.

Any idead?

 

[snoopnoon]

Are you using a pretrigger delay? Also look at your time history of the input force, you maybe double impacting without realizing.

 

[ruimoreira11]

Yes I think am using a pretrigger delay (perhaps too tiny). I think I must make further test on this issue.
In time signal i see a quite narrow peak but i cann't ensure that this peak is really only one (i will make zoom on that just to check)

Is there any recomendations concerning the trigger? or the rectangular window?

thanks

 

[MikeyP]

Looks like a classic double hit to me. Like you say, zoom in on the force time trace and have a look.

M

--
Dr Michael F Platten

 

[GregLocock]

Wander around the lab hitting things with the hammer and look at the time history and the autospectrum. After about 2 years you'll get a good feel for the problems.

Another trap for young players is leaving Hanning switched on for impact tests.

Cheers

Greg Locock

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