Modal analysis with dampers
Modal analysis with dampers
(OP)
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
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





RE: Modal analysis with dampers
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RE: Modal analysis with dampers
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
RE: Modal analysis with dampers
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Modal analysis with dampers
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
RE: Modal analysis with dampers
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: Modal analysis with dampers
RE: Modal analysis with dampers
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Modal analysis with dampers
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.
RE: Modal analysis with dampers
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
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: Modal analysis with dampers
The few times we used detuners to solve a vibration problem we always used steel as spring material.
RE: Modal analysis with dampers
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: Modal analysis with dampers
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