Modalanalyse - Grundlagen
Diese Artikel wurden ursprünglich von Dr. Peter Avitabile, Professor für Maschinenbau an der Universität von Massachusetts Lowell/USA, veröffentlicht und später unter dem Titel "Modal Space - Back to Basics" von der Society for Experimental Mechanics und Blackwell Publishing (1998 bis 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? |