Modal Analysis Basics
These articles were originally published by Dr. Peter Avitabile, Professor 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).
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Could you explain modal analysis for me? |
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Could you explain the difference between the time domain, frequency domain and modal space? |
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Is there any difference between a modal test with a shaker excitation or impact excitation? |
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Is there any difference between a roving hammer and roving accelerometer test? |
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Should I always use a hard tip for impact testing so the input spectrum is flat over all frequencies? |
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Which shaker excitation is best? Is there any difference? |
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Curvefitting is so confusing to me! What do all the different techniques mean? |
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I still don't understand curvefitting. How do I get mode shapes from FRFs? |
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What's the difference between operating deflection shapes and mode shapes? |
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Are you sure you can get mode shapes from one row or column of H? |
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I heard someone say "Pete doesn't do windows" What's the scoop? |
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I'm still overwhelmed by all this modal stuff - give me the big picture! |
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I showed some mode shapes to someone; they asked if the structural design was OK |
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Can I use an oblique angle to run a modal test? |
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How many points are enough when running a modal test? |
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Someone told me SDM will never work since I don't have all the modes |
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Why is mass loading and data consistency so important for modal parameter estimation? |
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I hear about SVD all the time; could you explain it simply to me? |
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Does it make any sense to use actual loads when doing a modal test? |
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When impact testing, can the use of the exponential window cause any problems? |
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Is there any benefit to using multiple references? I thought only one reference was needed? |
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What are some of the most important things to consider when impact testing? |
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Should I use all measurements when estimating modal parameters? |
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Can the test setup have an effect on the measured modal data? |
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Which window is most appropriate for the various types of modal tests performed? |
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What is the difference between a complex mode and a real normal mode? |
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Could you please explain the basic steps to acquire data for modal testing? |
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Is there any real advantage to MIMO testing? Why not just collect SISO and then move the shaker? |
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What are some of the major mistakes that people make in modal tests so I don't make the same ones? |
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Is it better to collect averaged FRF data for a modal test? or collect time data and process it afterwards? |
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Why is cali/pation and mode shape scaling important and does it make a difference? |
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Is it really necessary to reject a double impact? Are they really a problem? |
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Someone told me that you must have multiple references to identify pseudo repeated roots |
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I ran a modal test on a portion of a structure of concern and many modes look the same! What did I do wrong? |
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Why do some measurements have anti-resonances and others do not? |
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What's the difference between local and global curvefitting ?? |
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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. |
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Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test? |
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Mode shape scaling requires a drive point measurement. Is there any way to scale shapes without this? |
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I find overload and underload of the digitizer and range setting difficult to understand. |
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Do mode shapes of a plate have any particular predetermined order? |
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Once I have set up a good measurement, is there any reason to watch the time and frequency results for every FRF? |
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I know that certain shaker excitations have different characteristics - but which one is best to use? |
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When I perform impact testing, the input spectrum looks distorted - do you think my FFT analyzer has a problem? |
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Why does my stability diagram of a component on a system show modes that the SUM or MIF don't show? |
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Sometimes my input force is very smooth just as expected but often looks it is oscillating - Why is that? |
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I ran a shaker test with a simple beam but the modes don't look right - What's wrong? |
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Should the measurement bandwidth match the frequency range of interest for impact testing? |
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Can modal parameters be extracted for heavily damped modes when you can't see them in the measured FRF's? |
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What effect can the test setup and rigid body modes have on the higher flexible modes of interest? |
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Is there any problem running a modal test with 2 Khz excitation but only analyzing up to 500 Hz? |
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What is a good MAC value so I know my model is right? |
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Someone told me that operating modal analysis produces better results. |
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What is MRIT? I hear people talk about it for impact testing. |
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What is the difference between all the mode indicator functions? |
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How do you select the reference location for a modal test? |
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How do you interpret the stability diagram? |
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The Stability Diagram has poles that are not indicated by the MMIF or CMIF. Are they really modes? |
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Bolted joints are common in structures. Can the frequency change significantly due to joints? |
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Sometimes the mode shapes appear to be rotated from what is expected. Are the modes wrong? What's up? |
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A reference DOF is needed for all modal software systems. Why is the FFT analyzer reference different? |
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Should more residuals be used to improve a curvefit? The results look better when many extra terms are used. |
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My coherence is better in some measurements than others when impact testing. Am I doing something wrong? |
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My accelerometer is not overloaded but my measurement is terrible. What could be wrong? |
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Double impacts are undesirable. What about multiple impacts? |
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Can you describe reciprocity? It just doesn't make sense to me. |
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Do we really need an accurate updated model? |
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I made a stiffness modification to a free-free system. The flexible modes shifted down! |
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I made a stiffness change to the tip of a cantilever beam but I can only shift the frequency so far. |
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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? |
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When the Transfer Function is evaluated along the frequency axis, the damping is zero. Does this mean there is no damping in the system? |
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Curvefitting still seems like black magic to me. Can you explain transfer function, FRF and parameters to me? |
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Is there any effect in impact testing when hitting harder or softer with the different tips? |
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Can the shaker stinger have any effect on the FRF measurements? |
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I am still confused about how the SDOF in modal space is related to the physical response? |
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I hear mode participation all the time. What does that really mean? |
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Hey - I ran a test and got some extra modes I didn't expect. Does the overall test set up have any effect? |
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Will the support mechanism have any effect on FRFs? Does bungie cord vs fishing line make any difference? |
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Does the structure need to come to rest between impact measurements? Doesn't the damping window take care of that? |
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Why can't I run a modal test with one big shaker and just "crank up the signal"? |
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Is there really a difference running a modal test with MIMO as opposed to SISO? |
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So if I use multiple reference vs single reference FRFs, is there really a difference in the modal parameters? |
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Someone told me that you can't accept a FRF when the input spectrum has more than a 20 dB rolloff |
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Sometimes when I have a double impact, I switch hammer tips to eliminate it. Is that OK? |
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I took a measurement one day and the next day the measurement looked different. How can that be? |
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We compare tests to fully built-in models all the time. Can you really simulate built-in down in the lab? |
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If you don't excite a structure at its natural frequency, then how can you know what it is? |
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How free does a test need to be? Does it really matter that much? |
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So what really is a drive point FRF? Do you have to impact exactly at the same point? |
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We updated a composite plate model but the properties seem to not be physical. |
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Can you explain the Complex Mode Indicator Function (CMIF) again? What are these crossover frequencies? |
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If the frequencies between test and model are close, then is the model correlated? |
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Do you really need to measure FRFs? Or is a Transmissibility OK? |
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If we perform a roving impact hammer test and impact many points, is there any possibility of missing a mode? |
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So now I have another question…if the hammer impacts a node, then there is no response elsewhere? |
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Someone told me they used a Hanning window for an impact test. That doesn't seem right…Can that distort data? |
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An eigensolution gives us frequencies. But how do we get mode shapes? |
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Do you need to mount triaxial accelerometers at all locations? That can result in more channels needed and more cost. |
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Does gravity come into play when testing to find frequencies? Does orientation make a difference? |
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So I am still confused about the MAC? What is a "good" value for MAC? |
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So what is the best way to make a free-free test set up ... because nothing is really free-free. |
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So before you end this Modal Space series ... can you provide some last pieces of advice? |