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Distinguished Contributor
bjur
Posts: 147
Registered: ‎09-14-2011
Accepted Solution

Simple Time History problem - why no resonance

531 Views, 10 Replies
01-12-2013 02:52 PM

I wanted to see if I could model a simple cantelevered column with a rotating mass on the top that matched the resonance frequency of the column.  The modal analysis calculated two modes, both 10.38 hz (one in each direction X and Y).  I assigned a horizontal equipment load at the top of the column for each direction X and Y.  Then added a time history analysis that would model the horizontal force rotating at the top of the column.  I used the formula sin(360*10.38*t).  I did the same thing for the Y axis but used cos(360*10.38*t).  When I looked at the time history diagram for deflection at the top node, the deflection stabilized and didn't get any bigger, which I thought it would since the rotating force at the top matched the natural frequency of the column.  Attached are the sample files I made.  How would I demonstrate a resonance condition in Robot?

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Distinguished Contributor
bjur
Posts: 147
Registered: ‎09-14-2011

Re: Simple Time History problem - why no resonance

01-12-2013 04:14 PM in reply to: bjur

Might have figured it out...just needed to bang my head on my desk for a little longer.  I made the column longer (more flexable).  I ran analysis on 3 different frequencies.  The first one was 3 hz (not supposed to be a resonance frequency) the other two (1.57, 8,89) were modal frequencies and should amplify deflections over time.  Seams to have worked this time.  Let me know if what I did is the correct way to do this.  Again the goal was to try to model a rotating motor on a cantelevered column to see if I could get robot to show a resonance condition, one where the deflections get bigger over time.

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Valued Mentor
tony.ridley
Posts: 652
Registered: ‎09-07-2011

Re: Simple Time History problem - why no resonance

01-13-2013 05:38 AM in reply to: bjur

bjur,

 

don't you want this?

 

FRF Harmonic?

 

frf pic 2.jpg

 

frf pic.jpg

 

file attached

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Distinguished Contributor
bjur
Posts: 147
Registered: ‎09-14-2011

Re: Simple Time History problem - why no resonance

01-13-2013 04:52 PM in reply to: tony.ridley

When would I want to use a harmonic analysis?  What about a Harmonic in frequency domain FRF?  I've never used those before in Robot.  The others I'm comforable with (Static, Seismic, Modal, Time History, Pushover).  I understand the idea behind Footfall analysis, but have never had to use it on a project.  Same goes with Buckling analysis.  It would be handy to have an example problem for each analysis type (maybe there is already).

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Valued Mentor
tony.ridley
Posts: 652
Registered: ‎09-07-2011

Re: Simple Time History problem - why no resonance

01-13-2013 04:57 PM in reply to: bjur

bjur wrote:

When would I want to use a harmonic analysis?  What about a Harmonic in frequency domain FRF?  I've never used those before in Robot.  The others I'm comforable with (Static, Seismic, Modal, Time History, Pushover).  I understand the idea behind Footfall analysis, but have never had to use it on a project.  Same goes with Buckling analysis.  It would be handy to have an example problem for each analysis type (maybe there is already).


I thought you were looking to find the frequency that caused amplification of displacement in your model?  I thought that this is what would would use FRF for (as mentioned in my previous post)?  That way you get a graph of displacement versus frequency.  You can quickly see which frequency will cause an issue on your model. 

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Valued Mentor
tony.ridley
Posts: 652
Registered: ‎09-07-2011

Re: Simple Time History problem - why no resonance

01-13-2013 05:06 PM in reply to: tony.ridley
From Robot help menu.............
  Harmonic (FRF) Analysis Description
 

The harmonic analysis in the frequency domain consists in performing sequentially the harmonic analysis for successive frequency values in a selected range. In the software, an FRF (Frequency Response Functions) analysis case is a composed case including subcases. Each of subcases has a solution to the harmonic analysis with a specified frequency.

 

The harmonic analysis in the frequency domain is needed to study vibration receptance of the structure. The purpose of this analysis is to obtain a Frequency Response Function (FRF) for a selected node of the model. The Frequency Response Function expresses the response of the structure to given harmonic vibrations in the frequency domain. A plot of the function indicates for which frequency the influence of vibrations on a structure is maximal. You can continue such analysis as the time history analysis for a selected, critical frequency.

 

 

 

So, run the FRF, get the critical frequency, then run your time history at that frequency. 

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Distinguished Contributor
bjur
Posts: 147
Registered: ‎09-14-2011

Re: Simple Time History problem - why no resonance

01-14-2013 07:24 AM in reply to: tony.ridley

Thanks Tony.  Thats perfect.  

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Distinguished Contributor
bjur
Posts: 147
Registered: ‎09-14-2011

Re: Simple Time History problem - why no resonance

01-14-2013 07:40 AM in reply to: tony.ridley

So I ran a FRF analysis on my prevous test model and got a flat line for the results.  No peaks.  I tried to use the same settings as you but didn't get any results.  What am I doing wrong?  Attached is the model with my attempt at the FRF analysis.

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Valued Mentor
tony.ridley
Posts: 652
Registered: ‎09-07-2011

Re: Simple Time History problem - why no resonance

01-14-2013 02:51 PM in reply to: bjur

You need to have the loads added in the FRF case, not the satic case. 

If I change X-force into the FRF case, looks like this;

Capture.JPG

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Distinguished Contributor
bjur
Posts: 147
Registered: ‎09-14-2011

Re: Simple Time History problem - why no resonance

01-14-2013 10:57 PM in reply to: tony.ridley

In the case of an FRF analysis, if I use a different load value, will I get different FRF results?  I've been trying to understand this FRF analysis type better. Does the load direction matter?  Can I just use a unit load?  I'll try these in the morning. Thanks Tony for your help. 

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