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From: mnentwig on 3 Dec 2007 02:53
Hi,

I would take a full number of cycles (!) of the signal and FFT it. Then
notch out the spectral line corresponding to the fundamental, IFFT back to
time domain, and you've got the double-frequency anomaly.
That is, it's a single bin as long as the frequency is stable enough.

If the one-per-rotation signal causes harmonics, I think there is no way
to separate it from the two-per-rotation anomaly without additional
information.



From: Rune Allnor on 3 Dec 2007 02:55
On 2 Des, 19:08, Greg Berchin <gberc...(a)comicast.net> wrote:
> On Sun, 02 Dec 2007 12:09:07 -0500, Jerry Avins <j...(a)ieee.org> wrote:
> >Can we assume that the twice-per-revolution anomaly is actually a
> >once-around for a part geared 2:1?
>
> No. The "once-per" is a physical situation that occurs at only one
> rotation angle; the "twice-per" is a physical situation that occurs at
> two different rotation angles.
>
> >If not, does it actually consist of
> >dub dub dub dub 180 degrees apart, or might it be a syncopated lub-dub
> >... lub-dub ...? (I hope that's not too cryptic!)
>
> Nominally 180ยบ, but this signal is indicative of severe wear, so there
> will be plenty of room for slop.

This is the detail I would *try* to exploit: The 1/rev event will
*probably*
have a more coherent spectrum in the sense that the phase relation
between harmonics is "fixed". The 2/rev event is, from what you say,
a more random event where the harmonics take a more random
nature.

You can check this hypothesis by making a long recording and
check the coherence of spectrum lines at increasing time
separations. If the hypothesis is correct, the overharmonics of
the 1/rev event will be a stable foundation under the more
fluctuating
harmonics of the 2/rev event. So the harmonics with contributions
only from the 2/rev event would be expected to be far less coherent
over time than the harmonics with contributions from both events.

If that made sense to anyone but me...

Rune
From: Greg Berchin on 3 Dec 2007 07:20
On Dec 2, 1:23 pm, Vladimir Vassilevsky <antispam_bo...(a)hotmail.com>
wrote:

> What if you integrate the signal in the two dimensions per revolution
> and per half revolution?

Hmmm; that's got me thinking. The fundamental and odd harmonics of
the combined signal MUST come from the 1-per phenomenon. So this is
really a matter of separating the even harmonics of the 1-per from the
entirety of the 2-per.

Even harmonics are generated by asymmetrical waveforms (asymmetry
above and below the zero-amplitude line).

Odd harmonics are generated by symmetrical waveforms.

There's got to be a way to separate the symmetrical portions of the
waveform from the asymmetrical portions of the waveform -- much like
decomposing a waveform into its even- and odd-symmetric parts.

Thanks,
Greg
From: Greg Berchin on 3 Dec 2007 07:22
On Dec 2, 12:52 pm, John O'Flaherty <quias...(a)yeeha.com> wrote:

> Just exploring this, how do you know the 2/per isn't just a harmonic
> of the 1/per?

They are entirely separate physical phenomena.

> If they're separate anomalies, can you eliminate either
> temporarily, to find out what the harmonic structure of the other is?

No, unfortunately. I have no control over them. All that I can do is
observe their effects.

Thanks,
Greg
From: Rune Allnor on 3 Dec 2007 08:01
On 3 Des, 08:55, Rune Allnor <all...(a)tele.ntnu.no> wrote:

> If that made sense to anyone but me...

Ouch! That came out a bit different than was intended!

I just tried to indicate that I had a more or less clear
idea about what I tried to say when I wrote the previous
post, but after reading over it I could only make sense
of the text because I knew what I tried to say. No sinister
intentions in any way.

Sorry.

Rune
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