vibrataion isolation

[Tim86]

Hi guys

I would very much appreciate if someone can help me answer this question. In sdof vibration isolation system with very stiff spring does the system still resonate at the resonant frequency region even if the disturbance force is very small

Many Thanks
Tim

 

[GregLocock]

yes

Cheers

Greg Locock


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[Tim86]

Mnany Thanks, your help is very much appreciated. I thought before the disturbance force must be large enough if the stiff is very stiff

Kind Regards
Hatim

 

[Tim86]

sorry I ment in my previous message if the spring is very stiff

 

[IRstuff]

Resonance is resonance, regardless of "stiffness." The question is what is the disturbance and what is the desired response. A MIL-S-901D isolation system is absurdly stiff, normally, but it's intended to isolate extreme events.

TTFN

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[FeX32]

I agree with the above. You should look into damping fundamentals.
A force that can oppose the external forcing function at resonance is proportional to the first time derivative of the displacement.
This is completely regardless of the stiffness of the 'spring' as the others point out.



Fe

 

[Tim86]

Many thanks guys for your help and sorry about my late reply.To start with my system is a sdof system isoalton system with mass supported by srpring in parallel with an MR damper (see the attached).

The magnitude of disturbance fore applied at the top of the system (i.e mass) is about 100N and spring stiffness is 1.4212e+005N.s/m which means my resonance is at 12Hz. Not sure whether this force is enough to make the mass moves buy few mms at 12Hz (i.e. displacement of mass).

Kind Regards
Tim

 

[Tim86]

Sorry I forgot to attached the file in my previous email (see the attached)

 

[IRstuff]

Is this for school?

TTFN

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[Tim86]

No it is for a job where it is required to design an experimental vibration rig

 

[GregLocock]

You need to know the damping, since that controls the amplitude at resonance.

F=x*2*pi*f*d where d is the damping and x is the maximum displacement, for lightly damped systems at resonance, at a guess. The proper equation is in the book you took the picture from, no doubt.

Cheers

Greg Locock


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[MikeyP]

Presumably, if you are using a MR damper, then you can control the damping over quite a wide range by adjusting the magnetic field strength to suit your purpose.

M

--
Dr Michael F Platten

 

[FeX32]

"Presumably, if you are using a MR damper, then you can control the damping over quite a wide range by adjusting the magnetic field strength to suit your purpose."
I agree. This is a great suggestion.
To the OP:
This is known as semi-active vibration isolation.
A nonlinear stiffness is also sometimes designed to take advantage good properties in certain regions.
To date, best results (research and practice) have been from a combination of passive and active.


Fe

 

[FeX32]

Also about your sdof question. That force is it constant? What is the frequency of excitation? (if there is one)
Without this info you don't have much.


Fe

 

[Tim86]

First of all, I would like to thank everyone for the input. The thing I am concerned about is the system might be over-damped even with no current applied to MR damper.Because MR dampers are known to have a pre-loading damping force. Regarding the excitation, I am using an inertial shaker so I can excite the system at any frequencies within the range of 6HZ to 100Hz frequencies and with magnitude of 100N.

Kind Regards
Tim