Frequency Optimization_increase the natural frequency above a particular value.

I am making a luggage rack to place a motor on it for a hybrid bicycle project. i have to increase the natural frequency of this component, and i find it confusing at the optimization point.

This is the process i followed.

First i made a design space for the luggage space using cad

then i imported this design space, mesh it and and put the static load on it as well.

then i found the natural frequency of the struct with that motor mass on top.

during optimizing

i made the design space for the luggage

then i created a response of volume (Volumefrac)

then another response of frequency, using mode 2.

Then i put the constraint as frequency, with lower bound as 50Hz

Finally, the objective as volume (volumefrac).

Whn i run this, im not getting a proper output, please do recommend me an alteration.

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Altair Forum User

Hi Srujan,

What type of optimization are you trying to do on your model?

Did you select the 1st mode for optimization?

Altair Forum User

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This is the design space i am using. i want to increase the natural fequency of this structure to above 50Hz by changing it volume and to get a optimized shape for my structure. i dont know what mistake i am doing.

Altair Forum User

These are the frequencies of the design space, which frequency should i select for the frequency response? please do help me, i am stuck at this.

if you could tell me the response, constraint and the objective which i should use to get my desired shape, it would be much more helpfull. thank you.

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Altair Forum User

Hi Sarujan,

I apologize as I misspelled your name in one of my replies.

Can you try this... Instead of giving volume fraction as objective, use frequency response for objective. Say maximize 1st frequency which will be your objective.

let us know your findings.

Altair Forum User

FYI,

Refer to this tutorial on OptiStruct help

OS-2020: Increasing Natural Frequencies of an Automotive Splash Shield with Ribs

Altair Forum User

i have tried maximizing the 1st fequency by keeping the volfrac upper boundary as 0.3, and i get a shape, but, the maximum frequency of that shape is about 20Hz, but i want the structure's frequency to be above 50Hz.

am i assigning the constraints right, shudi cahnge the volumefrac to volume?

Thank you for the reference, i will look into it.

Altair Forum User

Hi @Sarujan

Is it possible to share the model file?

If yes, please use the file transfer link in my signature below to share the file.

Altair Forum User

I have sent you the file. U have received a file with the email address stating sarujan123@gmail.com

Thank You.

Altair Forum User

I have attached the files here for your convenience, sorry for any trouble caused.

THis is the design space of the structure which i want, the desired outcome would be a shape of the luggage with a natural fequency of 50Hz or above, how do i givethe constraints accordingly to get that outcome?

Altair Forum User

Hi @Sarujan

Here are my suggestions:

Since the first frequency is quite low, it may not help to target first frequency alone. Please have 4-5 frequency response for first 4-5 modes and try to increase their frequencies.

Also, it is good to go through the out file after optimization as at some intermediate iteration you may the targeted frequencies but left out due to convergence criteria

Altair Forum User

i will try this today. Thank You for your response, btw, i had a doubt with the mass i am using currently, may i please know the unit of CONM2 1D element. i set the control card to KG.i doubt that the 1 D element is set to KG.

Altair Forum User

Hi @Sarujan

Actually HyperMesh don't have a unit system It is upto user to maintain a unit system.

So if your density is in Kg/mm^3, use Kg for mass, mm for length etc...

Altair Forum User

i figured that i have been putting the mass wrong, my density value was tonne/m^3. Thank you for your help.