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Simple Time History problem - why no resonance

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Message 1 of 13
bjur
2160 Views, 12 Replies

Simple Time History problem - why no resonance

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?

12 REPLIES 12
Message 2 of 13
bjur
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.

Message 3 of 13
tony.ridley
in reply to: bjur

bjur,

 

don't you want this?

 

FRF Harmonic?

 

frf pic 2.jpg

 

frf pic.jpg

 

file attached

Message 4 of 13
bjur
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).

Message 5 of 13
tony.ridley
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. 

Message 6 of 13
tony.ridley
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. 

Message 7 of 13
bjur
in reply to: tony.ridley

Thanks Tony.  Thats perfect.  

Message 8 of 13
bjur
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.

Message 9 of 13
tony.ridley
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

Message 10 of 13
bjur
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. 

Message 11 of 13
Pawel.Pulak
in reply to: bjur

            In the case of an FRF analysis, if I use a different load value, will I get different FRF results?

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Yes. Because it is linear dynamic analysis increasing the load for instance twice will result in increasing the results twice.

 

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            I've been trying to understand this FRF analysis type better. Does the load direction matter?  Can I just use a unit load?

----------------------------------

What do you mean by the load direction? Changing the values of  components of the load thus changing the value and direction of the resultant or only changing the signs of load?

In this first case of course it can completely change the response.

In this second case it will results in changing signs of results.

Generally the load defined in FRF analysis is as in harmonic analysis the extreme value (amplitude) defined for specific time moment. It is changing according to sine function with these values as amplitudes. The difference comparing to harmonic analysis is that in harmonic analysis single frequency or period of excitation is defined. In case of FRF analysis some range of frequencies is defined, solver is running a series of harmonic analysis with different frequencies and results of all of them are stored in single compound load case.

The interpretation of results from FRF and harmonic analysis is also the same - these are amplitudes of steady-state harmonic oscillations.

 

As concerns your first post in this thread: using FRF or harmonic analysis you will not be able to model a rotating mass on the top of cantilevered column what was possible in time history analysis using X and Y direction excitation forces with sine and cosine functions respectively. In case of FRF or harmonic analysis only sine function is used so the resultant is not rotating but oscillating in diagonal direction between X and Y.

 

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If this post answers your question please click "Accept as Solution". It will help everyone to find answer more quickly!

 

Regards,

 


Pawel Pulak
Technical Account Specialist
Message 12 of 13

Hi!

 

Concerning example from file "Test of FRF.rtd".

I have modified x-force according to this: If I change X-force into the FRF case.

 

I'm trying to compute this example at Robot 2019 (version 32.0.2.6571).

 

Let look at these graphs of displacement versus frequency below.

The resonance phenomena is on the first graph.

But what is on second one?

Without dampingWithout dampingWith dampingWith damping

Message 13 of 13

Unfortunately such anomalies could be observed on some machines for FRF with damping in Robot 2019. It was improved to Robot 2020.

 

Regards

 


Pawel Pulak
Technical Account Specialist

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