Modal Analysis Basics

These articles were originally published by Dr. Peter Avitabile, Professor Emeritus of Mechanical Engineering at the University of Massachusetts Lowell, and were later published under the title of "Modal Space - Back to Basics" by Society for Experimental Mechanics and Blackwell Publishing (1998 to 2014).

Feb 98 – Could you explain modal analysis for me?

Apr 98 – Could you explain the difference between the time domain, frequency domain and modal space?

Jun 98 – Is there any difference between a modal test with a shaker excitation or impact excitation?

Aug 98 – Is there any difference between a roving hammer and roving accelerometer test?

Oct 98 – Should I always use a hard tip for impact testing so the input spectrum is flat over all frequencies?

Dec 98 – Which shaker excitation is best? Is there any difference?

Feb 99 – Curvefitting is so confusing to me! What do all the different techniques mean?

Apr 99 – I still don't understand curvefitting. How do I get mode shapes from FRFs?

Jun 99 – What's the difference between operating deflection shapes and mode shapes?

Aug 99 – Are you sure you can get mode shapes from one row or column of H?

Oct 99 – I heard someone say "Pete doesn't do windows" What's the scoop?

Dec 99 – I'm still overwhelmed by all this modal stuff - give me the big picture!

Feb 00 – I showed some mode shapes to someone; they asked if the structural design was OK

Apr 00 – Can I use an oblique angle to run a modal test?

Jun 00 – How many points are enough when running a modal test?

Aug 00 – Someone told me SDM will never work since I don't have all the modes

Oct 00 – Why is mass loading and data consistency so important for modal parameter estimation?

Dec 00 – I hear about SVD all the time; could you explain it simply to me?

Feb 01 – Does it make any sense to use actual loads when doing a modal test?

Apr 01 – When impact testing, can the use of the exponential window cause any problems?

Jun 01 – Is there any benefit to using multiple references? I thought only one reference was needed?

Aug 01 – What are some of the most important things to consider when impact testing?

Oct 01 – Should I use all measurements when estimating modal parameters?

Dec 01 – Can the test setup have an effect on the measured modal data?

Feb 02 – Which window is most appropriate for the various types of modal tests performed?

Apr 02 – What is the difference between a complex mode and a real normal mode?

Jun 02 – Could you please explain the basic steps to acquire data for modal testing?

Aug 02 – Is there any real advantage to MIMO testing? Why not just collect SISO and then move the shaker?

Oct 02 – What are some of the major mistakes that people make in modal tests so I don't make the same ones?

Dec 02 – Is it better to collect averaged FRF data for a modal test? or collect time data and process it afterwards?

Feb 03 – Why is cali/pation and mode shape scaling important and does it make a difference?

Apr 03 – Is it really necessary to reject a double impact? Are they really a problem?

Jun 03 – Someone told me that you must have multiple references to identify pseudo repeated roots

Aug 03 – I ran a modal test on a portion of a structure of concern and many modes look the same!  What did I do wrong?

Oct 03 – Why do some measurements have anti-resonances and others do not?

Dec 03 – What's the difference between local and global curvefitting?

Feb 04 – We talked about the number of points needed for a modal test before. Someone told me that the entire shape may not need to be completely defined.

Apr 04 – Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test?

Jun 04 – Mode shape scaling requires a drive point measurement. Is there any way to scale shapes without this?

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?