## Robot Structural Analysis

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# Simple Time History problem - why no resonance

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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?

Solved! Go to Solution.

# Re: Simple Time History problem - why no resonance

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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.

# Re: Simple Time History problem - why no resonance

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bjur,

don't you want this?

FRF Harmonic?

file attached

# Re: Simple Time History problem - why no resonance

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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).

# Re: Simple Time History problem - why no resonance

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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.

# Re: Simple Time History problem - why no resonance

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*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.

# Re: Simple Time History problem - why no resonance

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Thanks Tony. Thats perfect.

# Re: Simple Time History problem - why no resonance

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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.

# Re: Simple Time History problem - why no resonance

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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;

# Re: Simple Time History problem - why no resonance

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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.