Difference between revisions of "Modal Theorical FAQ"

From OROS Wiki
Jump to navigation Jump to search
Line 1: Line 1:
[[File:modal.png|900px]]
[[File:modal.png|900px]]
==General
[https://orossas.sharepoint.com/:b:/g/support/ESHPvhJIsCBHijgwBeffvcABU-VZCt6ZON1OMcguK2sGoQ?e=qsLNUI What is modal analysis?.]<br>
[https://orossas.sharepoint.com/:b:/g/support/ETV5oBwxLjFHnsfVSL1hit0Bigt_2FuSJIbVmoHjAMPNug?e=Kb0vHk Common mistakes.]


[https://orossas.sharepoint.com/:b:/g/support/ESHPvhJIsCBHijgwBeffvcABU-VZCt6ZON1OMcguK2sGoQ?e=qsLNUI Could you explain modal analysis for me.]
==Windowing==
[https://orossas.sharepoint.com/:b:/g/support/ESeXYLSIKetIp7v1LyevBO0BI0hStYqFdzah5aO9kzgGIA?e=MTVQsF When impact testing, can the use of the exponential window cause any problems?]
[https://orossas.sharepoint.com/:b:/g/support/EUw3GiSkKIdBmFAZLE5vw-IBZ6-2-Yczz7zz4i3cTda2iQ?e=btozQG Which window is most appropriate for the various types of modal tests performed?]
[https://orossas.sharepoint.com/:b:/g/support/EZmiM-le4zpDvsyTb1Kfmc0B7AymLe5HB810BiQ2jtsxhQ?e=vJbs0G I heard someone say Pete doesn't do windows. Whets the scoop.]
==Acquisition==
==Acquisition==
[https://orossas.sharepoint.com/:b:/g/support/ERUU4dr0usxHiAn1Ih8FTAcBwQYcRPpNEMnUTsmT7JBCTg?e=Ag9Nit I ran one test with an x-excitation and can see some modes and another test with a  y-excitation and see some different modes could I use an oblique angle instead ?]<br>
===Generality===
[https://orossas.sharepoint.com/:b:/g/support/ESAbTQrm4yFKi0RMb2BVRTYBexWVHZewsr7hvDLPuEtUSQ?e=AZTle9 Is it really necessary to reject a double impact?]
[https://orossas.sharepoint.com/:b:/g/support/EQKjRyGlsShNg14lgX6gUtwBSq9W2VdBEwiDQ4jTvI8trg?e=1bCxCc basic steps to acquire data]
[https://orossas.sharepoint.com/:b:/g/support/Ee9ZZy7KqVxLjWURcjNtKSYBwxXFR_QjQVKhJofS9c4xKA?e=wdtfHO Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test?]
[https://orossas.sharepoint.com/:b:/g/support/EVWdcqfwhc9Mshv8R6M5BKYB_I8x96uro6ItAhVah31wGA?e=qqb8So Is there a difference between a roving hammer and roving accelerometer test?]
[https://orossas.sharepoint.com/:b:/g/support/EVxxlCgPltJHslo8IlH2Y0wB5T_Z1Mn9mpXMiOhaIRWV8Q?e=h9dWZR Can the test setup have an effect on the measured modal data ? Do the setup boundary conditions and accelerometers have an effect?]
[https://orossas.sharepoint.com/:b:/g/support/EVxxlCgPltJHslo8IlH2Y0wB5T_Z1Mn9mpXMiOhaIRWV8Q?e=h9dWZR Can the test setup have an effect on the measured modal data ? Do the setup boundary conditions and accelerometers have an effect?]
[https://orossas.sharepoint.com/:b:/g/support/EfDJXZ2NsXBIpNnOPouoJLIBlqBNBjgTfnKQ4OUMay4R6g?e=igvjzh How many points are enough when running a modal test]
[https://orossas.sharepoint.com/:b:/g/support/EV4GFjhvcStErAqtxUDAFcUBU6db28UlUtckHR2l09Ad7A?e=3DHX1C is it better to collect averaged FRF data for a modal test? Or collect time data and process it afterwards?]
[https://orossas.sharepoint.com/:b:/g/support/EV4GFjhvcStErAqtxUDAFcUBU6db28UlUtckHR2l09Ad7A?e=3DHX1C is it better to collect averaged FRF data for a modal test? Or collect time data and process it afterwards?]
[https://orossas.sharepoint.com/:b:/g/support/EfDJXZ2NsXBIpNnOPouoJLIBlqBNBjgTfnKQ4OUMay4R6g?e=igvjzh How many points are enough when running a modal test]
[https://orossas.sharepoint.com/:b:/g/support/EZfWJLtRuCtDpCkrd09zwEUBz2GXFlFjmWmBjBPJ7O1igw?e=o5c75e We talked about the number of points needed for a modal test before.]
[https://orossas.sharepoint.com/:b:/g/support/EZfWJLtRuCtDpCkrd09zwEUBz2GXFlFjmWmBjBPJ7O1igw?e=o5c75e We talked about the number of points needed for a modal test before.]
[https://orossas.sharepoint.com/:b:/g/support/EV3I4RVyqhtInP82-HGjH2gBff6U72IhA7orXLTNWH5LLg?e=JGOeAo Is there any benefit to using multiple references?I thought only one reference was need.]
===Sensor and reference===
 
[https://orossas.sharepoint.com/:b:/g/support/EQKjRyGlsShNg14lgX6gUtwBSq9W2VdBEwiDQ4jTvI8trg?e=1bCxCc Could you please explain the basic steps to acquire data for modal test]


[https://orossas.sharepoint.com/:b:/g/support/ERUU4dr0usxHiAn1Ih8FTAcBwQYcRPpNEMnUTsmT7JBCTg?e=Ag9Nit I ran one test with an x-excitation and can see some modes and another test with a  y-excitation and see some different modes could I use an oblique angle instead ?]<br>
[https://orossas.sharepoint.com/:b:/g/support/Ee9ZZy7KqVxLjWURcjNtKSYBwxXFR_QjQVKhJofS9c4xKA?e=wdtfHO Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test?]
[https://orossas.sharepoint.com/:b:/g/support/EVWdcqfwhc9Mshv8R6M5BKYB_I8x96uro6ItAhVah31wGA?e=qqb8So Is there a difference between a roving hammer and roving accelerometer test?]
[https://orossas.sharepoint.com/:b:/g/support/ET7xJmNNuyNGnpoX_E0s9MIB7BfU3wKfDjv3gtZlsjGGZA?e=M0mIrO Someone told me that you must have multiple references to identify pseudo repeated.]
[https://orossas.sharepoint.com/:b:/g/support/ET7xJmNNuyNGnpoX_E0s9MIB7BfU3wKfDjv3gtZlsjGGZA?e=M0mIrO Someone told me that you must have multiple references to identify pseudo repeated.]
 
[https://orossas.sharepoint.com/:b:/g/support/EV3I4RVyqhtInP82-HGjH2gBff6U72IhA7orXLTNWH5LLg?e=JGOeAo Is there any benefit to using multiple references?I thought only one reference was need.]
[https://orossas.sharepoint.com/:b:/g/support/ESRlQH3hGgpLgqwRbYFMOHQBb-J3fkac76JMhSCwgAF_pg?e=qQRnZL Should I always use a hard tip for impact testing . . .so the input spectrum is flat over all frequencies]
[https://orossas.sharepoint.com/:b:/g/support/ESRlQH3hGgpLgqwRbYFMOHQBb-J3fkac76JMhSCwgAF_pg?e=qQRnZL Should I always use a hard tip for impact testing . . .so the input spectrum is flat over all frequencies]
Other
[https://orossas.sharepoint.com/:b:/g/support/ESAbTQrm4yFKi0RMb2BVRTYBexWVHZewsr7hvDLPuEtUSQ?e=AZTle9 double impact question]
==Shacker==
==Shacker==
[https://orossas.sharepoint.com/:b:/g/support/EZ0xK6BALQlMgiXqGEh1UNkBJfsAo4aPmTwE7UKcLSppEg?e=NpNUPs Is there any difference between a modal test with a shaker excitation or impact excitation?]
[https://orossas.sharepoint.com/:b:/g/support/EZ0xK6BALQlMgiXqGEh1UNkBJfsAo4aPmTwE7UKcLSppEg?e=NpNUPs Is there any difference between a modal test with a shaker excitation or impact excitation?]
Line 32: Line 30:
[https://orossas.sharepoint.com/:b:/g/support/EWVXgK-KbPBLn3VMLviBuuQB7XgfOpaAPUzLPe0dbMhe9A?e=AVixDw I showed some mode shapes to someone. They asked me if the structural design was ok. What should I tell them ?]
[https://orossas.sharepoint.com/:b:/g/support/EWVXgK-KbPBLn3VMLviBuuQB7XgfOpaAPUzLPe0dbMhe9A?e=AVixDw I showed some mode shapes to someone. They asked me if the structural design was ok. What should I tell them ?]
==Modal theorical==
==Modal theorical==
===Windowing===
[https://orossas.sharepoint.com/:b:/g/support/ESeXYLSIKetIp7v1LyevBO0BI0hStYqFdzah5aO9kzgGIA?e=MTVQsF When impact testing, can the use of the exponential window cause any problems?]
[https://orossas.sharepoint.com/:b:/g/support/EUw3GiSkKIdBmFAZLE5vw-IBZ6-2-Yczz7zz4i3cTda2iQ?e=btozQG Which window is most appropriate for the various types of modal tests performed?]
[https://orossas.sharepoint.com/:b:/g/support/EZmiM-le4zpDvsyTb1Kfmc0B7AymLe5HB810BiQ2jtsxhQ?e=vJbs0G I heard someone say Pete doesn't do windows. Whets the scoop.]
===Curvfitting===
===Curvfitting===
[https://orossas.sharepoint.com/:b:/g/support/EeYfWtoUNHhIgpr2i1_wYkcBZPq3GmF0ETi7I8DtsuWKoQ?e=d5nXIx Curvefitting is so confusing to me!]
[https://orossas.sharepoint.com/:b:/g/support/EeYfWtoUNHhIgpr2i1_wYkcBZPq3GmF0ETi7I8DtsuWKoQ?e=d5nXIx Curvefitting is so confusing to me!]
Line 64: Line 66:




[https://orossas.sharepoint.com/:b:/g/support/ETV5oBwxLjFHnsfVSL1hit0Bigt_2FuSJIbVmoHjAMPNug?e=Kb0vHk What are some of the major mistakes people make in modal tests so I don't make the same one]
 


[https://orossas.sharepoint.com/:b:/g/support/EX4IEP3GWoRPr1V5RTGD5jcBiHNsJ0rJPWpdPaJ_bH_aaQ?e=LIqzlD Why do some measurements have anti-resonances and others not ?]
[https://orossas.sharepoint.com/:b:/g/support/EX4IEP3GWoRPr1V5RTGD5jcBiHNsJ0rJPWpdPaJ_bH_aaQ?e=LIqzlD Why do some measurements have anti-resonances and others not ?]

Revision as of 13:47, 17 July 2020

Modal.png ==General What is modal analysis?.
Common mistakes.

Acquisition

Generality

basic steps to acquire data Can the test setup have an effect on the measured modal data ? Do the setup boundary conditions and accelerometers have an effect? How many points are enough when running a modal test is it better to collect averaged FRF data for a modal test? Or collect time data and process it afterwards? We talked about the number of points needed for a modal test before.

Sensor and reference

I ran one test with an x-excitation and can see some modes and another test with a y-excitation and see some different modes could I use an oblique angle instead ?
Do I need to have an accelerometer mounted in the X, Y and Z directions to do a modal test? Is there a difference between a roving hammer and roving accelerometer test? Someone told me that you must have multiple references to identify pseudo repeated. Is there any benefit to using multiple references?I thought only one reference was need. Should I always use a hard tip for impact testing . . .so the input spectrum is flat over all frequencies Other double impact question

Shacker

Is there any difference between a modal test with a shaker excitation or impact excitation? Is there any real advantage to MIMO testing? Why not just use SISO and then move the shaker?

Which shaker excitation is best? Is there any difference?

results interpretation.

I showed some mode shapes to someone. They asked me if the structural design was ok. What should I tell them ?

Modal theorical

Windowing

When impact testing, can the use of the exponential window cause any problems? Which window is most appropriate for the various types of modal tests performed? I heard someone say Pete doesn't do windows. Whets the scoop.

Curvfitting

Curvefitting is so confusing to me! What's the difference between local and global curvefitting ? I still don't understand curvefitting ...

others questions

What's the difference between a complex mode and a real normal mode? Could you explain the difference between time domain, frequency domain and modal space? I ran a modal test on a portion of a structure of concern and many modes look the same! Someone told me SDM will never work Are you sure you can get mode shapes from one row or column of the H matrix?

I hear about SVD all the time Could you explain it simply to me?


I'am still overwhelmed by all this modal stuff Laplace, Fourier, FRFs, and all that! Does it make any sense to use the actual operating loads as the excitation force for a modal test if the force is a random signal ? Why is calibration and mode shape scaling important? And does it make a difference?


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



What's the difference between operating deflection shapes and mode shapes? sometimes they look the same to me!

Why is mass loading and data consistencyimportant for modal parameter estimation?

Should I use all collected measurements when estimating modal parameters ?



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

Retrieved from "https://wiki.oros.com/index.php?title=Modal_Theorical_FAQ&oldid=7044"