Bad influence of modal shaker
Bad influence of modal shaker
(OP)
Hi
A surprising mode shape obtained with a modal shaker seems to figure me out that the shaker could act as an added eccentric mass.
But advertisements for this type of shaker says that they have no influence due to the use of a thin rod connecting the shaker to the structure..
Could specialists give me their point of view?
Thanks
A surprising mode shape obtained with a modal shaker seems to figure me out that the shaker could act as an added eccentric mass.
But advertisements for this type of shaker says that they have no influence due to the use of a thin rod connecting the shaker to the structure..
Could specialists give me their point of view?
Thanks





RE: Bad influence of modal shaker
TTFN
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RE: Bad influence of modal shaker
Incidentally you should always do a survey of shaker positions before doing a modal analysis, I've tried anything up to 5 locations/directions on a car before deciding on the best one for the full survey.
Cheers
Greg Locock
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RE: Bad influence of modal shaker
Did you get big alterations of your results depending on the location of the shaker?
In our case, we wanted to identify by experiment a torsional mode which was given by simulation. The frequency of this mode was well approximated by the simple formula omega=sqrt(I/K) where I is the moment of inertia and K is the torsional stiffness.
But, from my point of view, the mass of the shaker have to change the moment of inertia of the structure (even though the mass of the shaker is not so high...)
It is well known that the mass of the accelerometers can modify results of modal analysis, si I could guess that it's also the same case with shaker, isn't it ?
RE: Bad influence of modal shaker
TTFN
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RE: Bad influence of modal shaker
This assumes that your stinger system is effective at isolating the test object from forces that are not measured by the load cell.
Cheers
Greg Locock
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RE: Bad influence of modal shaker
TTFN

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RE: Bad influence of modal shaker
I don't absolutely agree with the last greg's anwser. Indeed, When two structures are coupled together, we have to consider both the coupling force and the coupling velocity. This coupling have to be consider from the point of view of matching impedance.
Actually, there is two extreme cases that correspond to the two ways of finding modes.
If the structure has a mobility (V/F) much lower than the excitation source( a modal shaker for example), then it can be assumed that only force is transmitted from the shaker to the structure. In this case, we can do modal analysis by measuring mobility or inertance.
The other case is when the structure has a mobility much higher than the exitation source (a big/heavy shaker for instance), then only velocity is transmitted from the shaker to the structure. In this case, we can do modal analysis by measuring transmissibility.
RE: Bad influence of modal shaker
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Bad influence of modal shaker
RE: Bad influence of modal shaker
I can hear that, in a perfect world where we know the 6 DOF's of injected forces and moments, it can be possible to get the modal parameters. But how can you explain that parasitic modes appear when the shaker is moved from one location/posture to another and measurements carry out again?
How can you also explain that it is possible to get the modal parameters directly from the transmissibility (that is without knowing the injected force) with a huge shaker?
I confess I don't know what is 101 (maybe 100+1 or 99+2 or something like this...)
RE: Bad influence of modal shaker
- Steve
RE: Bad influence of modal shaker
As to moving the shaker around, well hello, that's where engineering practice bumps into the theory. That's called experience.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Bad influence of modal shaker
TTFN
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Need help writing a question or understanding a reply? forum1529: Translation Assistance for Engineers