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Vibration Like Model.

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Patrick

Mon Feb 23 2015, 04:30PM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Is it possible to reduce a vibration problem to math similar to an electrical or thermal circuit? if i could hammer out a circuit diagram, id have an easier time manipulating the variables for a desired result.

Instead of two potentials hot vs cold - low V vs High V, i want a high vibration surfaces dampened down to a lower vibration surface. I can see parallel and series rules applying, but cant pull it all together in my head.

I'm thinking of using all kinds of soft flexible materials in different shapes and lengths, but as a believer in engineering I don't want to guess and hope each time.

Ive looked at the obvious sources like wiki:

but i dont know if a gas or fluid shock is equivalent to capacitance.

Im really wanting to study dampen 100Hz and up. so not a single frequency.

Wolfram

Mon Feb 23 2015, 05:53PM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Check out this section

of the Harmonic Oscillator page on Wikipedia, it shows analogs between mechanical and electrical systems. Using these analogs, you should be able to translate your problem into something you can simulate in LTSpice or whichever electrical simulation package you use.

Dr. Slack

Mon Feb 23 2015, 06:52PM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

There is a complete 1:1 correspondance between electrical and mechanical parameters. The key is energy, resistors and viscosity dissipate it, inductive currents and capacitive voltages, masses in motion and stressed springs store it in two different ways. That bit is easy.

You will find the tricky bit will be modelling the surfaces in sufficient detail to be useful. Circuits make great use of ideal wires to connect the components, and because the ratio between the parameters of a wire and a typical component are so huge normally, that means for most things below RF, a wire doesn't need to be modelled. Wires don't really exist in mechnaical terms, you want the physicst's light inextensible cable for instance. While a steel cable will transmit force and store little energy, its parameters won't be orders of magnitude different from the things it's connecting, so you have a difficult modelling problem.

Take your surface for instance. You can't model it at the molecular level. You could use massive voxels interconnected by springs, but how many? You will need to resolve the elasticity into shear and linear components if you want to do FEA. For special cases, you may be able to extract the eigenmodes, say a stretched drum skin for example. But without a 'thin' surface with lots of symmetry, that's not going to work well.

Patrick

Tue Feb 24 2015, 07:00AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Im thinking of isolating a vibration generating device from a device that needs low vibration. Im wondering if soft-silicone can be cast with specific dimensions to accomplish all of the spring(k)-mass(m)-damper(c) model?

Dr. Slack

Tue Feb 24 2015, 08:25AM

Registered Member #72 Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659

You ask several questions, for which there are several answers

a) can a 'thing that generates vibration' be connected at one point to a 'thing that needs low vibration' by a 'lump of silicone'
yes

b) is it likely that the parameters of the silicone are sufficiently far from the parameters of the other two structural things that you can model the things as rigid masses, and the silicone as a single lumped RLC equivalent?
probably

c) does silicone have the right inherent mix of properties to allow you to control damping and springyness by varying the geometry alone?
if you're lucky

though compared to the mass of the things, I suspect the mass of the silicone buffer will be low, so a first order model for it would be closer to a damped spring alone.

You may want to consider composites where more low-loss springing is provided by springs, or more damping is provided by dashpots.

Andy

Tue Feb 24 2015, 08:52AM

Registered Member #4266 Joined: Fri Dec 16 2011, 03:15AM
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Posts: 874

You could try a mechincal structure like steel washs with hard rubber in between with multiable layers.

At work a virbration machine uses that to isolate the work peice from the electric motor.

Proable only work in one or two axis thought.

Patrick

Tue Feb 24 2015, 08:59AM

Registered Member #2431 Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639

Dr Slack, you sum things up well. These were the matters i was contemplating. Especially (b).

I remembered there are existing solutions which could be scaled up. These little buggers are pressed into a hole, then expand and stay in with out any screws or such. There used on circuit boards and cameras.

look at the above pic, i think black is the softest, red would be the most ridged silicone, right? They are categorizing them for sale purposes to those who arnt experts, but i thinks thats the support limit.

To take the thought experiment further, if i have to ridged surfaces, attached by a "system." So...
the "system" if infinitely thin/soft would transmit infinitely low vibration, but not attach the surfaces and break.
The "system" if infinitely ridged would transmit all vibration generated. like a series of bolts or weld beads.
Therefore i must chose a non-ideal, yet workable place in between?

Shrad

Tue Feb 24 2015, 09:41AM

Registered Member #3215 Joined: Sun Sept 19 2010, 08:42PM
Location:
Posts: 780

idea: latex foam glued to both surfaces to achieve higher structural resilience than with a couple dampers which can be single points of failure

Uspring

Tue Feb 24 2015, 10:02AM

Registered Member #3988 Joined: Thu Jul 07 2011, 03:25PM
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Posts: 711

Dr. Slack wrote:

There is a complete 1:1 correspondance between electrical and mechanical parameters. The key is energy, resistors and viscosity dissipate it, inductive currents and capacitive voltages, masses in motion and stressed springs store it in two different ways. That bit is easy.

I can imagine this for one dimensional mechanical situations, e.g. a vibrating string, which has an analogy as a transmission line.
But generally you'd need to calculate 3D displacements, i.e. vectors for a vibrating system. It is not clear to me, how these can be derived from scalars such as voltages and currents.

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