Discussion Forum

Posted: 1 decade ago Feb 10, 2011, 3:11 a.m. EST

Hi first of all get updated to 4.1 with latest patch, really its much smoother than the early 4.0 version. And in v4.0 you need regularly to delete your solver case and rebuild it, as if you edit it a few times it get corrupte (which emans you must also redefine all your postprocessing, this is "repaired " in the newer version) For your canteliever, why do you block all sides of your large block ? you can just ignore it, your model will be as precise but just simpler to solve. But if you want to see the effet of the base, then block only one side of the base, and you can simulate the influence of the fixation of your device. You can also, for such simple devices perform a few simplications, and certainly you should check the results w.r.t. an analytical solution, several excellent books give you te formulas to evalaute the modes and stiffness of a cantilever. Yo ucan also apply a gravity "body load" = -g_const*solid.rho [N/m^3] and check tat your first eigenfrequency 2*pi*fo[Hz] = sqrt(k[N/m]/m[kg])=sqrt(g_const[m/s^2]/maxop1(w)[m]) where maxop1(w) is the maximum tip sag under gravity load in the gravity direction assumed here along "-Z" its also fun to prove the equation above ;) -- Good luck Ivar

Posted: 1 decade ago Feb 10, 2011, 3:42 a.m. EST

[QUOTE] Hi first of all get updated to 4.1 with latest patch, really its much smoother than the early 4.0 version. And in v4.0 you need regularly to delete your solver case and rebuild it, as if you edit it a few times it get corrupte (which emans you must also redefine all your postprocessing, this is "repaired " in the newer version)[/quote] Upgrading to 4.1 is on my list of things to do on tomorrow. I'll get that done asap. [quote] For your canteliever, why do you block all sides of your large block ? you can just ignore it, your model will be as precise but just simpler to solve. But if you want to see the effet of the base, then block only one side of the base, and you can simulate the influence of the fixation of your device.[/quote] The reason I put the large block in is because what I need to model is how the displacement changes with different types of undercut edges. In our actual research cantilevers the base is far less uniform, so I'm going to make it more complex and asymmetric once I get the basic model working. I blocked all sides of the large block because I didn't want it going anywhere. No other reason. [quote] You can also, for such simple devices perform a few simplications, and certainly you should check the results w.r.t. an analytical solution, several excellent books give you te formulas to evalaute the modes and stiffness of a cantilever. Yo ucan also apply a gravity "body load" = -g_const*solid.rho [N/m^3] and check tat your first eigenfrequency 2*pi*fo[Hz] = sqrt(k[N/m]/m[kg])=sqrt(g_const[m/s^2]/maxop1(w)[m]) where maxop1(w) is the maximum tip sag under gravity load in the gravity direction assumed here along "-Z" its also fun to prove the equation above ;) -- Good luck Ivar [/QUOTE] I've checked it with some basic equations and Comsol wasn't giving me the same deflection. It was giving me the "error in solver" I described above and then stopping wherever, which did not give the same results. Did you try to run the solver? Did it error out?

Posted: 1 decade ago Feb 10, 2011, 5:37 a.m. EST

Hi no I did not runmit ("my" COMSOL licence is used by a colleage ;) but I ahve made many of those examples and thay have always worked very nicely so I'm sure you must have a little flaw in a BC or in a selection, that is quickly done. But also if you fix all sides of your "fixed" part your material does not have much freedom to behave, that could perhaps overconstrain somewhat the solver (just a guess) -- Good luck Ivar

Posted: 1 decade ago Feb 10, 2011, 3:48 p.m. EST

[QUOTE] Hi no I did not runmit ("my" COMSOL licence is used by a colleage ;) but I ahve made many of those examples and thay have always worked very nicely so I'm sure you must have a little flaw in a BC or in a selection, that is quickly done. But also if you fix all sides of your "fixed" part your material does not have much freedom to behave, that could perhaps overconstrain somewhat the solver (just a guess) -- Good luck Ivar [/QUOTE] Oddly enough, even though the simulation error'd out every time saying it was unable to converge, it did give a displacement value which, when I checked it against an analytical solution, gave an accurate result. I don't know what the error was for, but it is working (although it still says it errors out). Thanks for the advice!

Posted: 1 decade ago Feb 10, 2011, 4:00 p.m. EST

Hi I believe its rather one of the weaknesses in the 4.0, often if you changed a few time yxour solver settings, you need to delete it and rebuild it from scratch as it got corrupt. Really consider to upgrade, you will be less frsutrated, chekc it out with your rep or sysmanager -- Good luck Ivar

Posted: 8 years ago Aug 30, 2016, 8:53 a.m. EDT

hi Ivar, how do you know which abreviation should be added to the beginning of definition? I mean like adding ''solid.'' to ''rho''. cheers, Shirin

Posted: 8 years ago Sep 1, 2016, 12:17 p.m. EDT

Apologies as this was mentioned already, but this error in my experience typically results from the system being mechanically unconstrained: applying a force without sufficient zero-displacement boundaries to prevent acceleration of the entire structure.