Bad influence of modal shaker

[amanuensis]

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

 

[IRstuff]

Model numbers? Pictures? Unit under test?

TTFN
faq731-376

 

[GregLocock]

Repeat the test using hammer excitation. If the mode shape is different with out the shaker, then your shaker setup was bad.

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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[amanuensis]

Thanks Greg for this practical response.

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 ?

 

[IRstuff]

That'll depend on the frequency of the artifact. Again, pictures? Model number? Unit under test? There are instances where slip tables can be affected by bad UUT configurations, resulting in resonances that are induced into the slip table.

TTFN
faq731-376

 

[GregLocock]

No, in a linear system if the load cell is between the shaker mass and the test object then the shaker mass is just part of the excitation force not the mode. This is Modal 101.

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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[IRstuff]

For the shakers I've used, the shaker drives the table upon which the UUT and control accel are installed. However, if the UUT is top-heavy, it will rock the slip table, and being that the major excitation is perpendicular to the driver, the control accel cannot compensate for that. The only solution that appears to work in those instances is to use a hydrostatic bearing table, wherein the slip table is constrained verticaly by the bearings and cannot readily rock. We once spent about 2 months trying to figure out why our UUT had a 36-Hz resonance that was not showing up in the modal analyses, but showed up in the slip table testing. Moved to hydrostatic bearing table, and the resonance disappears.

TTFN
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[amanuensis]

Thanks Irstuff for your concrete anwser. It's perfectly understandable. In short, you say that whether the excitation is caused either by the table or by the impact hammer/modal shaker, then the modal behavior of the structure is hugely changed.

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.

 

[GregLocock]

It doesn't matter to me whether you agree with me. As i said it is 101.

Cheers

Greg Locock


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[btrueblood]

Re-read what Greg said: if you measure the input force with a transducer situated between the shaker device and the UUT, and the transducer is capable of measuring the total force/moment input (i.e. is not hiding something) what more do you need - force input is force input, regardless of how it was generated. Or else everything we learned about superposition is wrong.

 

[amanuensis]

Right,

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...)

 

[SomptingGuy]

101 is an American term expected to be understood by the rest of the World (Like 301k). My understanding is that it's the simplest course in a subject.

- Steve

 

[GregLocock]

Yup. Now, in reality if your force transducer isn't measuring all 6 dof forces on the way in, or you have other constraints on the system such as non free-free suspensions, then of course you aren't testing what you thought you were.

As to moving the shaker around, well hello, that's where engineering practice bumps into the theory. That's called experience.

Cheers

Greg Locock


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[IRstuff]

We, in a multimillion dollar company, spent a couple of man-months trying find our mysterious resonance that turned out to be the slip table. And bear in mind, NO ONE along the way, including the guys who owned the slip table, knew ahead of time that this was going to be a major problem.

TTFN
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