These articles were originally published in Experimental Techniques by the
Society for Experimental Mechanics and Blackwell Publishing under the title of Modal Space - Back to Basics
They are now available in Adobe PDF format (see www.adobe.com for free Acrobat PDF reader)
The following is the list of articles currently available:Feb 98 Could you explain modal analysis for me?
Aug 04 I find overload and underload of the digitizer and range setting difficult to understand.
Oct 04 Do mode shapes of a plate have any particular predetermined order?
Dec 04 Once I have set up a good measurement, is there any reason to watch the time and frequency results for every FRF?
Feb 05 I know that certain shaker excitations have different characteristics - but which one is best to use?
Apr 05 When I perform impact testing, the input spectrum looks distorted - do you think my FFT analyzer has a problem?
Jun 05 Why does my stability diagram of a component on a system show modes that the SUM or MIF don't show?
Aug 05 Sometimes my input force is very smooth just as expected but often looks it is oscillating - Why is that?
Oct 05 I ran a shaker test with a simple beam but the modes don't look right - What's wrong?
Dec 05 Should the measurement bandwidth match the frequency range of interest for impact testing?
Feb 06 Can modal parameters be extracted for heavily damped modes when you can't see them in the measured FRF's?
Apr 06 What effect can the test setup and rigid body modes have on the higher flexible modes of interest?
Jun 06 Is there any problem running a modal test with 2 Khz excitation but only analyzing up to 500 Hz?
Aug 06 What is a good MAC value so I know my model is right?
Oct 06 Someone told me that operating modal analysis produces better results.
Dec 06 What is MRIT? I hear people talk about it for impact testing.
Feb 07 What is the difference between all the mode indicator functions?
Apr 07 How do you select the reference location for a modal test?
Jun 07 How do you interpret the stability diagram?
Aug 07 The Stability Diagram has poles that are not indicated by the MMIF or CMIF. Are they really modes?
Oct 07 Bolted joints are common in structures. Can the frequency change significantly due to joints?
Dec 07 Sometimes the mode shapes appear to be rotated from what is expected. Are the modes wrong? What’s up?
Feb 08 A reference DOF is needed for all modal software systems. Why is the FFT analyzer reference different?
Apr 08 Should more residuals be used to improve a curvefit? The results look better when many extra terms are used.
Jun 08 My coherence is better in some measurements than others when impact
testing. Am I doing something wrong?
Aug 08 My accelerometer is not overloaded but my measurement is terrible. What could be wrong?
Oct 08 Double impacts are undesirable. What about multiple impacts?
Dec 08 Can you describe reciprocity? It just doesn’t make sense to me.
Feb 09 Do we really need an accurate updated model?
Apr 09 I made a stiffness modification to a free-free system. The flexible modes shifted down!
Jun 09 I made a stiffness change to the tip of a cantilever beam but I can only shift the frequency so far.
Aug 09 If I run a shaker test with the input oblique to the global coordinate system, how do I decompose the force into the specific components in each direction?
Oct 09 When the Transfer Function is evaluated along the frequency axis, the damping is zero. Does this mean there is no damping in the system?
Dec 09 Curvefitting still seems like black magic to me. Can you explain transfer function, FRF and parameters to me?
Feb 10 Is there any effect in impact testing when hitting harder or softer with the different tips?
Apr 10 Can the shaker stinger have any effect on the FRF measurements?
Jun 10 I am still confused about how the SDOF in modal space is related to the physical response?
Aug 10 I hear mode participation all the time. What does that really mean?
Oct 10 Hey - I ran a test and got some extra modes I didn't expect. Does the overall test set up have any effect?
Dec 10 Will the support mechanism have any effect on FRFs? Does bungie cord vs fishing line make any difference?
Feb 11 Does the structure need to come to rest between impact measurements? Doesn't the damping window take care of that?
Apr 11 Why can't I run a modal test with one big shaker and just "crank up the signal"?
Jun 11 Is there really a difference running a modal test with MIMO as opposed to SISO?
Aug 11 So if I use multiple reference vs single reference FRFs, is there really a difference in the modal parameters?
Oct 11 Someone told me that you can’t accept a FRF when the input spectrum has more than a 20 dB rolloff
Dec 11 Sometimes when I have a double impact, I switch hammer tips to eliminate it. Is that OK?
Feb 12 I took a measurement one day and the next day the measurement looked different. How can that be?
Apr 12 We compare tests to fully built-in models all the time. Can you really simulate built-in down in the lab?
Jun 12 If you don’t excite a structure at its natural frequency, then how can you know what it is?
Aug 12 How free does a test need to be? Does it really matter that much?
Oct 12 So what really is a drive point FRF? Do you have to impact exactly at the same point?
Dec 12 We updated a composite plate model but the properties seem to not be physical.
Feb 13 Can you explain the Complex Mode Indicator Function (CMIF) again? What are these crossover frequencies?
Apr 13 If the frequencies between test and model are close, then is the model correlated?
Jun 13 Do you really need to measure FRFs? Or is a Transmissibility OK?
Aug 13 If we perform a roving impact hammer test and impact many points, is there any possibility of missing a mode?
Oct 13 So now I have another question…if the hammer impacts a node, then there is no response elsewhere?
Dec 13 Someone told me they used a Hanning window for an impact test. That doesn’t seem right…Can that distort data?
Feb 14 An eigensolution gives us frequencies. But how do we get mode shapes?
Apr 14 Do you need to mount triaxial accelerometers at all locations? That can result in more channels needed and more cost.
Jun 14 Does gravity come into play when testing to find frequencies? Does orientation make a difference?
Aug 14 So I am still confused about the MAC? What is a "good" value for MAC?
Oct 14 So what is the best way to make a free-free test set up ... because nothing is really free-free.
Dec 14 So before you end this Modal Space series ... can you provide some last pieces of advice?
A compilation of the all of the articles is located here.
If you have a question on some aspect of Modal Analysis, just send me an e-mail
In January 2001, Sound & Vibration magazine published an article entitled Experimental Modal Analysis - A Simple Non-Mathematical Overview which is a very good summary on the main aspects relating to experimental modal analysis.
This is a copy of the notes that are used for the Society for Experimental Mechanics,
Young Engineer Program that is offered every year at the International Modal Analysis Conference.
For any information, e-mail at Peter_Avitabile@uml.edu
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