Modal analysis with dampers

[patrickmech]

Hi,

I am trying to do experimental modal analysis on a structure with tuned mass dampers (TMD) mounted discreetly on it.

When compared to testing an undamped (without TMDs) structure, what are the issues / precautions that need to be considered while testing, curve fitting? Suggestions on how to consider non-proportional damping, etc.

I got more literature on FE analysis of non-proportionally damped structures. Curious to know from experimental side of it.

Thank you in advance.

Pat

 

[electricpete]

I doubt I can answer your question, but I wanted to mention that the tuned mass “dampers” I am familiar with don’t have any (intentional) damping. Just a tuned mass spring system attached to the structures. Do you have something different?

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(2B)+(2B)' ?

 

[patrickmech]

Thank you for the response.

Tuned Mass Dampers have a damping layer along with tuned mass. If it has only tuned mass it is usually termed as dynamic absorber. The targeted mode will look like two split modes without damper. If you add damper it gets smoothed out. Passive dampers are usually viscoelastic tapes, etc.

Having tuned mass dampers discretely placed on structure gives rise to non-proportional damping. Lot of literature is available on how to consider this in FE domain. But I am curious to see if any extra care or techniques are needed while doing experimental modal analysis of such structure.

Pat

 

[GregLocock]

No, no advice. I have seen non proportional damping mentioned in textbooks, but in the rough and dirty world of whole vehicle modal analysis TMDs are pretty insignificant, and when you are tuning them on the powertrain or exhaust (the two most popular locations) they seem quite easy to curve fit.

Essentially I suppose you are asking whether the dmaping of the peak on the FRF physically near the TMD will be similar to that far (spatially) from the TMD. Well post a couple of plots and we'll have a look! Gut feel, not much in it.




Cheers

Greg Locock


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[patrickmech]

Thanks Greg.

I am yet to do testing. I will upload once I get them.

I am not considering automotive structure. It is an aerospace structure with very little damping. But yes I am referring to the same question what you hinted - damping in FRF at and away from TMD will be different. So I am thinking what changes one would adopt in testing a low damped structure and highly damped structure.

The information you provided is really interesting - so are the TMDs being used on powertrain or exhaust?


Kind regards
Pat

 

[GregLocock]

Both, and elsewhere. Look under a 1990s Japanese car and you will probably be able to see 10.

Cheers

Greg Locock


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[rob768]

The effect of a tuned mass damper is optimal when the added mass-spring system has no or minimal damping. Any damping in the added system will decrease the damping effect on the system you want it to dampen.

 

[GregLocock]

Ah, sort of. I agree that for maximum effect at one frequency that that is so, but given the production variability in both the damper and the rest of the car it seems unwise to rely on an undamped solution.

A good example is the crankshaft TV damper, I tune those for both stiffness and damping.

More generally a tuned damper with damping can be used to pull energy out of an unwanted mode across a broader range of frequencies. The other trick is under tuning or over tuning, which results in a differential in the height of the two peaks, so again with a crankshaft damper we might under tune it if the lower peak was under the red line and the upper above it.

Cheers

Greg Locock


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[rob768]

Greg,

I don't know about automotive, but in marine diesels the dampers are usually viscous. These generate damping, in a range with an optimum near the crankshaft natural frequency. Some installations have a rubber dampers which will give a detuning effect, and the better ones have a combination of viscous damping and spring-mass.

This is not a car (so production variation plays no part) but a further undefined structure. I would abstain from any added damping and just use the detuning effect.

How do dampers in a car work? I always assumed they were all spring plates with no damping other than a moving mass.

 

[GregLocock]

Typically they are steel or cast iron masses bonded to a carbon rubber layer, which is bonded to a metal plate that is bolted or welded to the structure of interest. Of course we also use helmholtz resonators acoustically, which are TMDs as well.

I've just realised another reason why you can't get too fancy with the tuning - the stiffness of the rubber varies with temperature. As you increase the carbon content in the rubber the stiffness and the damping increase.


Cheers

Greg Locock


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[rob768]

ah, much different than i pictured in my mind then. I seem to recoil i saw an engine equipped with a mass connected with steel springs in circumferential direction.

The few times we used detuners to solve a vibration problem we always used steel as spring material.

 

[GregLocock]

That was probably a dual mass flywheel. Never tuned one myself.

Cheers

Greg Locock


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[patrickmech]

Thanks Rob and Greg... very useful.

The effect of a tuned mass damper is optimal when the added mass-spring system has no or minimal damping - I agree to this. The damping material has to be selected with optimum loss factor considering the increase in amplitude on higher side and presence of split modes (two peaks on either side of resonance peak) on lower side of optimal value.

Pat