COMSOL Forums: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/
Most recent forum messagesTue, 28 Apr 2015 19:56:40 +0000COMSOL Forums: beam vibration by a shakerhttp://www.comsol.com/shared/images/logos/comsol_logo.gif
http://www.comsol.com/community/forums/general/thread/21964/
Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p172581
Hi Ivar,<br />
<br />
Firstly (I know that this post is now a bit old...), thank you for your precise answers. I read the discussion carefully but at a given point I'm no more able to understand your explanations...<br />
<br />
I have a DUT on shaker measurement, in order to know the physical property of this material (Young's, Poissons's, loss factor), I would like to match to simulation with the measurement. On one face of my solid I assign a prescribed acceleration (around 5 m/s^2), on the other side a body load (comes from added known mass in the real measurement) and add the gravity to the solid. The excitation is 10^{range(0,0.1,3)} my question is for the post-processing part.<br />
<br />
I want to perform the transfer function of the acceleration from the topmost part of the solid (accelerometer placed on the top of the added mass in the real measurement) on the acceleration from the base of my solid (an other acceleration from an accelerometer placed on the base of the shaker).<br />
<br />
How can I extract the average acceleration of the topmost surface, the average acceleration of the base surface and make the ratio?<br />
<br />
I know that my question is really basic, I'm far from an expert in COMSOL but this discussion is really useful!<br />
<br />
Thank you in advance, Luis. Tue, 28 Apr 2015 19:56:40 +000021964.1430251000.172581Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p115903
Hi all <br />
I am working in such a thing you talking about please how can I make a vibrating sphere inside water can anyone help me for that <br />
thanks,<br />
MSHTue, 25 Mar 2014 17:45:17 +000021964.1395769517.115903Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p115853
Hi all <br />
I am working in such a thing you talking about please how can I make a vibrating sphere inside water can anyone help me for that <br />
thanks,<br />
MSHTue, 25 Mar 2014 07:15:11 +000021964.1395731711.115853Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p97937
Hi<br />
<br />
in all generality the "thumb" rules:<br />
<br />
2*pi*freq = sqrt(k_stiff / mass) = sqrt(k_rot / Inertia),<br />
<br />
and for a beam under gravity load, for the first mode gies about the max deflection<br />
<br />
2*pi*freq = sqrt(g_const / Dz_deflection)<br />
<br />
I a rather good approximation. So if you add a mas via your attachment to a shaker, it is clesr that you will influence the mode to. That is why one often attach the FULL system to the shaker, or we shake the base and not the directly part of structure itself under test. At least not with any mass of significance w.r.t. the object mass<br />
<br />
--<br />
Good luck<br />
IvarWed, 13 Mar 2013 15:01:40 +000021964.1363186900.97937Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p97915
Hello all,<br />
<br />
This is a very interesting discussion so far and I am trying to make a vibration model along similar lines. <br />
<br />
I work with a similar excitation, but with a metal beam fixed at both ends (with compliant mounts) and a shaker applies vibration at the center of the beam with a static preload to hold it in place. Experimentally, I noticed that the resonances observed are of the entire system (shaker + beam) and not the beam alone. The frequencies I use are in several kilohertz where the shaker dimensions do matter.<br />
<br />
So, in most of the discussions above where the shaker was attached to the beam, isn't it an issue to consider? Is there a way to include this non-linearity in the comsol model? Wed, 13 Mar 2013 09:43:20 +000021964.1363167800.97915Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p92331
Hmm,may I know why? Because from what I understand, Adding tip mass to the structure increases the strain,which eventually increases the deformation. Body load is assigned on the "large" mass attached to the beam and added mass is assigned at the other end of the beam,together with boudary load<br />
<br />
ThanksThu, 13 Dec 2012 21:59:43 +000021964.1355435983.92331Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p92326
Ok, one more thing to check. Conceptually, if you apply a certain body load and you change the structure by adding mass the deformations should in general be lower and also the voltage.Thu, 13 Dec 2012 21:03:32 +000021964.1355432612.92326Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p92272
Hi,<br />
<br />
thanks for the reply. in the frequency analysis, I've specified a range of frequency, the eigenfrequency lies within that range.<br />
<br />
Thanks.Thu, 13 Dec 2012 09:18:11 +000021964.1355390291.92272Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p92270
The added mass reduces the natural frequency so the voltage may be dropping because the frequency you specified in the frequency domain analysis is no longer close to the beamâ€™s natural frequency.Thu, 13 Dec 2012 08:45:46 +000021964.1355388346.92270Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p92171
Hi<br />
<br />
if you add an "added mass then you must also add a boundary force, as the body load applies ONLY forces to the DOMAIN, not to the added mass that is a separate "fictuous " entitiy do not forget that ;)<br />
<br />
--<br />
Good luck<br />
IvarWed, 12 Dec 2012 10:33:38 +000021964.1355308418.92171Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p92164
Hi,<br />
<br />
I've applied body load to represent base excitation, and assigned "added mass" BC at the free end of the cantilever ( to represent tip mass). Howver, it shows that the voltage output at the eigenfreq keeps decreasing as the "added mass" increases. I believe by adding the tip mass, the voltage will be increased.<br />
<br />
ThanksWed, 12 Dec 2012 07:32:33 +000021964.1355297553.92164Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p90687
Hi<br />
Check the doc about frequency domain solving, and look at the equations, you will understand, it solves far quicker in constant regime sinus type excitation<br />
<br />
--<br />
Good luck<br />
IvarMon, 19 Nov 2012 11:19:40 +000021964.1353323980.90687Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p90654
<div class="quote"><br />
Hi<br />
<br />
In any case you should not talk about comparing eigenfrequencies, as these do not depend on external forces, only on the material, geometry shapes<br />
<br />
And amplitudes depends on damping, which is a delicate issue, often ignored, but essential for any amplitude evaluation, you must add thedamping as specific material property nodes, the only thing is to know what to use as values !<br />
<br />
Finally use rather the harmonic development and not the time series, as ou have then all the initial transient behaviour to consider<br />
<br />
--<br />
Good luck<br />
Ivar<br />
</div><br />
<br />
Hi Ivar,<br />
<br />
Thank you for the fast response.<br />
<br />
Comparison of eigenfrequencies is regards to existing lab results and, as you say, of course not dependent on external driving.<br />
<br />
Obviously you are (also) right about damping - I only consider real physical systems why damping is larger than zero.<br />
I have already obtained experimental damping coefficients for the system at hand - I will look at the implementation of damping coefficients in Comsol as you suggested soon...<br />
<br />
Furthermore, I do not understand what you mean by "use rather the harmonic development and not the time series". I have already developed analytical approximate expressions for the steady-state response using perturbation analysis. For these, and relating numerical solution given by a Runge-Kutta solver, I have an estimate for the time period after which the transients are negligible. Thus, I can simulate the system with transients - but as for now, I do not know how to impliment/be able to analyse a time series in Comsol?<br />
<br />
Most importantly, I would like you to elaborate on "use rather the harmonic development and not the time series" if possible - and preferably in relation to Comsol.<br />
<br />
Kind regards,<br />
StefanSun, 18 Nov 2012 22:56:40 +000021964.1353279400.90654Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p90645
Hi<br />
<br />
In any case you should not talk about comparing eigenfrequencies, as these do not depend on external forces, only on the material, geometry shapes<br />
<br />
And amplitudes depends on damping, which is a delicate issue, often ignored, but essential for any amplitude evaluation, you must add thedamping as specific material property nodes, the only thing is to know what to use as values !<br />
<br />
Finally use rather the harmonic development and not the time series, as ou have then all the initial transient behaviour to consider<br />
<br />
--<br />
Good luck<br />
IvarSun, 18 Nov 2012 17:24:42 +000021964.1353259482.90645Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p90638
Hi everybody.<br />
<br />
I have with great interest read this thread.<br />
<br />
I would like to simulate the steady-state response of a vertically base-excited cantilever beam. <br />
<br />
The driving could e.g. be A*cos(Omega*tau) where A is the amplitude, Omega is the frequency, and tau is time.<br />
<br />
(no PZT or multilayer or anything "fancy" - just an isotropic beam of steel).<br />
<br />
I can see from this thread, that several challenges occur when modeling a vertically base-excited cantilever beam. I am completely new to Comsol, and have no experience with it what so ever. It has, in this thread, been proposed to add a comparably large rigid body mass. It seems inappropiate in my case - I would definitely prefer only to consider a cantilever which is driven by an external load (i.e. shaker) and that the resulting values (eigenfrequencies, absolute amplitude etc.) are directly comparable with experimental results.<br />
<br />
The modeling itself should, I guess, be a very simple task for an experienced user and require a minimum of time. Could anybody please provide my with a working example? <br />
<br />
It will of course require subsequent work from me, but I need to start from "steady ground" :)<br />
<br />
Help is highly appreciated!<br />
<br />
Kind regards,<br />
StefanSun, 18 Nov 2012 16:07:33 +000021964.1353254853.90638Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p84239
Hi<br />
<br />
Thanks a lot. you've been really helpfulFri, 03 Aug 2012 20:33:43 +000021964.1344026023.84239Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p84181
Hi<br />
<br />
indeed fixing your block and applying a body load as an acceleration is o good way, in a frequency domain sweep you can get the frequency response too of your system. If you leave a floating potential its as if you have an open circuit, you voltage will be given by the charge build-up and the PZT beam capacity. If you add a CIR physics with an electric impedance of some kind, i.e. representing the entry impedance of your system, then you will see the response and voltage build up of your device in loaded conditions. One way to optimise the energy transfer is to play with the bandpass of your load impedance and the response of the beam<br />
<br />
--<br />
Good luck<br />
IvarFri, 03 Aug 2012 12:41:37 +000021964.1343997697.84181Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p84177
Hi<br />
<br />
I think I'd better stick with your initial suggestion that is to apply body load. I use steel as the large mass due to its stiffness and aluminum beam. I apply fixed constraint to the faces of the mass except for the one that is attached to the beam. <br />
<br />
I'm currently using one physics only that is piezoelectric device. I apply floating potential to the top of the PZT and ground at the bottom of the layer and this is how I extract the voltage generated Do I need to add another physics(i.e electrical circuit) for the impedance?<br />
<br />
Thanks<br />
<br />
Fri, 03 Aug 2012 09:37:45 +000021964.1343986665.84177Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p84127
Hi<br />
<br />
from you image, you should not use fixed and acceleration simulatneously, but you must use two roller conditions on two opposite sides of your sqare mass at right angle to the acceleration direction. Be sure you large mass is "large" compared to the beam, and then get the correct estimate of the true load on the beam<br />
<br />
100V is not necessarily that much, you can get a sparck (>1kV) easily with a stressed PZT, but this also depends strongly on the impedance you connect up on your PZT<br />
<br />
<br />
--<br />
Good luck<br />
IvarThu, 02 Aug 2012 22:29:46 +000021964.1343946586.84127Re: beam vibration by a shaker
http://www.comsol.com/community/forums/general/thread/21964/#p84020
However, by applying prescribed acceleration, the value of voltage is hundreds volts, which is I think is not correctWed, 01 Aug 2012 08:43:23 +000021964.1343810603.84020