modal test phase shifts
modal test phase shifts
(OP)
traditional modal testing involves the sinusoidal steady state with the mode shapes more or less easily identified, mode enumeration is another discussion altogether, but when the modal testing is performed by means of impulse testing (tap tests) the response is no longer a matter of sinusoidal steady state, so the equestion is this
how is the phase shift associated with sensor placement interpreted?
how is the phase shift associated with sensor placement interpreted?





RE: modal test phase shifts
Cheers
Greg Locock
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RE: modal test phase shifts
if I do a tap test with null damping, i should get the same result,
the problem is per an earlier posting in this forum, is what happens when the damping is finite but small, and the wave length of the excitation comparable to the dimensions of the structure being tested.
my quandry is that time is now a factor in the measurements.
so how does this affect the accuracy of the measurements and sensor placement
RE: modal test phase shifts
Propagation time as such isn't an issue, but I still don't see what you are driving at. Perhaps you are puzzled by a possible time delay between the driving point acceleration and the point at the other end of the car? That is, how does the other end of the car 'know' that it has to move in phase with the driving point?
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: modal test phase shifts
in dealing with transient excitation and using fourier analysis to reduce the time series data, it seems that the propagation delays between the source and sensor are going to give rise to phase shifts
thats what puzzles me
RE: modal test phase shifts
How about we set up a 3 mass 2 spring model, masses are 1,2,1 kg, and springs are 10^5 N/m.
The mode shapes are obvious.
Now set up a time based sim of that and see how the third mass responds when the first is struck.
I haven't done this but when i get a free cpu I'll try it.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376: Eng-Tips.com Forum Policies http://eng-tips.com/market.cfm?
RE: modal test phase shifts
for old school types, the modes with phase shift are classified as "complex modes", basically the dynamic mode shapes with damping present. They are in the form of traveliing waves.
"Normal modes" are the stationary modes with identifiable nodes with fix phase relationships, either in-phase (0deg) or out-of-phase(180Deg). They are for the damping free case only.