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From: Greg Berchin on 2 Dec 2007 07:46
On Sat, 01 Dec 2007 17:01:47 -0600, John O'Flaherty <quiasmox(a)yeeha.com>
wrote:

>Odd harmonics of the 1/per won't be duplicated in the 2/per.
>
>Whoops, said it backwards.

Yes; that is one of the properties that I was hoping to exploit. But
given a signal that is an unknown combination of both, how do I separate
their effects? I tried bispectral analysis, but it's darned near
impossible to interpret the results.

Thanks.
Greg
From: Jerry Avins on 2 Dec 2007 12:09
Greg Berchin wrote:
> On Sat, 01 Dec 2007 17:01:47 -0600, John O'Flaherty <quiasmox(a)yeeha.com>
> wrote:
>
>> Odd harmonics of the 1/per won't be duplicated in the 2/per.
>>
>> Whoops, said it backwards.
>
> Yes; that is one of the properties that I was hoping to exploit. But
> given a signal that is an unknown combination of both, how do I separate
> their effects? I tried bispectral analysis, but it's darned near
> impossible to interpret the results.

Can we assume that the twice-per-revolution anomaly is actually a
once-around for a part geared 2:1? 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!)

Will the harmonics of the anomalies have zero phase when the fundamental
is also zero? There's a good chance of that if the acoustic delay is
small enough. Then you might be able to sort the harmonics into groups
based on their phases.

Jerry
--
Engineering is the art of making what you want from things you can get.
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From: John O'Flaherty on 2 Dec 2007 12:52
On Sun, 02 Dec 2007 07:46:19 -0500, Greg Berchin
<gberchin(a)comicast.net> wrote:

>On Sat, 01 Dec 2007 17:01:47 -0600, John O'Flaherty <quiasmox(a)yeeha.com>
>wrote:
>
>>Odd harmonics of the 1/per won't be duplicated in the 2/per.
>>
>>Whoops, said it backwards.
>
>Yes; that is one of the properties that I was hoping to exploit. But
>given a signal that is an unknown combination of both, how do I separate
>their effects? I tried bispectral analysis, but it's darned near
>impossible to interpret the results.

Just exploring this, how do you know the 2/per isn't just a harmonic
of the 1/per? If they're separate anomalies, can you eliminate either
temporarily, to find out what the harmonic structure of the other is?
Or, alternatively, can you temporarily increase the effect of the
1/per without affecting the 2/per? If you could once measure the
amplitude and phase of the 1/per spectrum, and it was stable, then you
might subsequently be able to infer its complete spectrum from the
measured strength of a non-duplicated odd harmonic, and get the 2/per
by subtraction.
--
John
From: Greg Berchin on 2 Dec 2007 13:08
On Sun, 02 Dec 2007 12:09:07 -0500, Jerry Avins <jya(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 180�, but this signal is indicative of severe wear, so there
will be plenty of room for slop.

>Will the harmonics of the anomalies have zero phase when the fundamental
>is also zero?

There is a fixed phase relationship between the shaft angle, the angle
at which the "once-per" occurs, and the angles at which the "twice-per"
occur. They will vary from device to device, but within a given device
they will be fixed.

>Then you might be able to sort the harmonics into groups
>based on their phases.

That's my working theory. I liked the idea from Ron N about selectively
windowing in the time domain, but I just haven't yet figured out exactly
how to implement it to the greatest effect. I know almost nothing about
the waveforms a priori, except that one occurs once per rotation and the
other occurs twice per rotation.

Greg
From: Vladimir Vassilevsky on 2 Dec 2007 13:23


Greg Berchin wrote:

> That's my working theory. I liked the idea from Ron N about selectively
> windowing in the time domain, but I just haven't yet figured out exactly
> how to implement it to the greatest effect. I know almost nothing about
> the waveforms a priori, except that one occurs once per rotation and the
> other occurs twice per rotation.

Greg,

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

Let's say you have N samples per revolution.
For every sample:

I[n] = x[n] + x[n + N] + x[n + 2N] ....
^^^^^^^^^^^^^^
This accumulates single and double events.

J[n] = x[n] + x[n + N/2] + x[x + N] + x[n + N + N/2]....
^^^^^^^^^^^^
This accumulates 2xdouble events, 1xsingle events and noise which is
hopefully zero mean.

So:
J[n] - I[n] = double events + noise which is averaged out.


The two dimensional FFT also comes to the mind.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com

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