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Vibration Monitor (Hyde, Wanttaja?)



 
 
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  #1  
Old March 6th 05, 05:47 PM
RST Engineering
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Default Vibration Monitor (Hyde, Wanttaja?)

Necessity, as they say, is a mother.

I am in the process of reinventing a square wheel called a vibration
monitor. The electronics is relatively trivial IF the input and output
parameters are known.

What we know is that the engine is going to have a fundamental frequency at
cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us a
fundamental frequency of 2400/60 or 40 Hz.

But wait, he said. There are going to be other (sub) harmonics of that
frequency that will be of some interest. And, those harmonics will change
as a function of the engine being a two or four stroke, four or six
cylinder.

So, oh wise and noble gurus of engine stuffings, what (sub) harmonics are
going to be of most interest to us and what is their mathematical
relationship to the fundamental?

As an extra bonus question, my sensor is going to be an old phonograph
cartridge. Should I use the lightest weight "needle" that I can find? How
about a tiny little ball of lead at the tip of that needle? Would that help
the sensor? Or hinder it?

Lastly, once I get this sucker up and running with you all's good ideas, is
anybody game to bolt it onto their flying machine and report results? I can
do it for the 182, but I'd really like some other real-world reports.

Jim


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  #2  
Old March 6th 05, 10:37 PM
Robert Bonomi
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Default

In article ,
RST Engineering wrote:
Necessity, as they say, is a mother.

I am in the process of reinventing a square wheel called a vibration
monitor. The electronics is relatively trivial IF the input and output
parameters are known.

What we know is that the engine is going to have a fundamental frequency at
cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us a
fundamental frequency of 2400/60 or 40 Hz.

But wait, he said. There are going to be other (sub) harmonics of that
frequency that will be of some interest. And, those harmonics will change
as a function of the engine being a two or four stroke, four or six
cylinder.


*most* of the stuff 'of interest' is going to occur at the frequency of
ignition in the #1 cylinder.

You'll have "similar things" happening at the appropriate phase delay for
each cylinder.

*IF* everything is behaving exactly the same, that _should_ give you a
composite signal at (no. of cylinders) * (cyl #1 ignition frequency).

One also has to consider any drive-shaft powered 'accessories', that
may be operating at a _different_ speed than the main engine. (gear ratio,
and/or belt drive with different pulley sizes)

One form of 'bad news' is something that is going on in one cylinder that
is _different_ than what is happening in the rest of 'em. This may be
merely 'different amplitude', or it may be 'different wave-form.

Another form is something "recurring" at a frequency _other_ than what
can be explained. e.g., a flat spot on a roller in a roller bearing,
will have a characteristic frequency based on how many times the roller
rotates, per shaft rotation. Which is likely to be some "weird" ratio.

So, oh wise and noble gurus of engine stuffings, what (sub) harmonics are
going to be of most interest to us and what is their mathematical
relationship to the fundamental?

As an extra bonus question, my sensor is going to be an old phonograph
cartridge. Should I use the lightest weight "needle" that I can find? How
about a tiny little ball of lead at the tip of that needle? Would that help
the sensor? Or hinder it?


Oh Lordie!

To get an _accurate_ measure of vibration, the 'system under test', and the
'testing system' must be *isolated* (mechanically, "vibrationally") from each
other. Then you detect the vibration in the system under test, by measuring
the instantaneous differences in position, relative to the testing system.

When the 'testing system' is mounted _on_ the 'system under test', there
is a complication of 'signal' being transferred *through* the mounting, which
is then *not* detected by the pick-up, because _both_ components are affected.

You can 'approximate' isolation with some sort of a 'suspension' system --
e.g. springs. This, however, ends up "complicating" things, because what
it does is just introduce a 'delay' in the transfer through the suspension
mechanism, *and* a probable, delayed, induced "negative" component restoring
'equilibrium'. Theoretical analysis can get *really* hairy real quick.

To accurately track vibration, you have to have a sensor that will _move_
as far as the maximum 'excursion' of the system under test. It has to
have enough structural strength that the sensor, itself doesn't "flex",
yet inertia has to be low, to enable it to 'mimic' every motion of the
system under test.

You've got two *entirely* different kinds of 'sensor' possible:
1) something 'firmly attached' to the airframe, with a "Cats whisker"
in contact with the engine, this measures engine movement relative
to the airframe.
2) something 'firmly attached' to the engine, using a cat's whisker against
a 'suspended' (as supported by a suspension) mass to compare against.
This measures the 'movement' of the sprung mass, relative to the engine.
Which is "more or less" equivalent (though opposite in sign, naturally
to the movement of the engine relative to the "surround".

In either case, the "cat's whisker" should be as light and rigid as possible
In the latter case, "bigger is better" for the 'reference mass', subject to
the suspension mechanism, and resonances, etc. in *it*.

  #3  
Old March 7th 05, 02:07 AM
jc
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RST Engineering wrote:

snip
As an extra bonus question, my sensor is going to be an old phonograph
cartridge. Should I use the lightest weight "needle" that I can find?
How
about a tiny little ball of lead at the tip of that needle? Would that
help
the sensor? Or hinder it?


A lot depends on the range of frequencies, any sensor will have its own
fundamental frequency- multiple sensors may be a good idea. Geophones seem
to be good up to a few hundred Hz, cheap but difficult to get qty one. For
the application a speaker may be the clue (as a second sensor).
--

regards

jc

LEGAL - I don't believe what I wrote and neither should you. Sobriety and/or
sanity of the author is not guaranteed

EMAIL - and are not valid email
addresses. news2x at perentie is valid for a while.
  #4  
Old March 7th 05, 04:42 AM
Brian Whatcott
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On Mon, 07 Mar 2005 13:07:25 +1100, jc wrote:

///
A lot depends on the range of frequencies, any sensor will have its own
fundamental frequency- multiple sensors may be a good idea. Geophones seem
to be good up to a few hundred Hz, cheap but difficult to get qty one. For
the application a speaker may be the clue (as a second sensor).



Building on this idea - it appears that piezo speakers with a loaded
wafer have done well in this application.
I'm guessing that loading the ceramic to a natural frequency well
below 40Hz would be good. The output is a function of acceleration
of the casing, I imagine.

Brian Whatcott Altus OK
  #5  
Old March 7th 05, 07:11 AM
Frank van der Hulst
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RST Engineering wrote:
Necessity, as they say, is a mother.

[snip]
As an extra bonus question, my sensor is going to be an old phonograph
cartridge.


And you *need* to use that phonograph needle?

Analog Devices sells cheap MEMS-based accelerometers (e.g. ADXL202). You
could even use two or 3 axes. The output a 5V square wave whose duty
cycle is proportional to acceleration.

Frank
  #6  
Old March 7th 05, 07:09 PM
Joerg
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Hello Jim,

What we know is that the engine is going to have a fundamental frequency at
cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us a
fundamental frequency of 2400/60 or 40 Hz.



Just a thought: Usually the time between the onset of a vibration and
some catastrophic failure is rather short. I believe what also needs to
be detected is the higher frequency stuff that can happen well before
this. Such as friction sounds from a bearing or cam not getting enough
lubrication.

Regards, Joerg

http://www.analogconsultants.com
  #7  
Old March 8th 05, 12:59 AM
Ernest Christley
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RST Engineering wrote:

Lastly, once I get this sucker up and running with you all's good ideas, is
anybody game to bolt it onto their flying machine and report results? I can
do it for the 182, but I'd really like some other real-world reports.

Jim




I should be able to volunteer a rotary by the end of the summer.

Since you won't know what frequencies should be there, could you make it
a sort of historical tracking system, with maybe some sort of alarm that
goes off if something starts going out of whack from historical data?
  #8  
Old March 10th 05, 05:00 PM
Stuart Fields
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Default

Well Jim if you are inventing a vibration monitor, then you need a vibrating
platform to test it on. I have a Safari helicopter that qualifies. I also
have an electronic balancer and accelerometer transducers to provide some
cross checking at engine frequencies of 45hz, rotor frequencies of 8 and
16hz. and possibly with a little fooling, harmonics of all of the above.
Stu Fields

"RST Engineering" wrote in message
...
Necessity, as they say, is a mother.

I am in the process of reinventing a square wheel called a vibration
monitor. The electronics is relatively trivial IF the input and output
parameters are known.

What we know is that the engine is going to have a fundamental frequency

at
cruise RPM. Let's take the math-simple engine RPM of 2400. This gives us

a
fundamental frequency of 2400/60 or 40 Hz.

But wait, he said. There are going to be other (sub) harmonics of that
frequency that will be of some interest. And, those harmonics will change
as a function of the engine being a two or four stroke, four or six
cylinder.

So, oh wise and noble gurus of engine stuffings, what (sub) harmonics are
going to be of most interest to us and what is their mathematical
relationship to the fundamental?

As an extra bonus question, my sensor is going to be an old phonograph
cartridge. Should I use the lightest weight "needle" that I can find?

How
about a tiny little ball of lead at the tip of that needle? Would that

help
the sensor? Or hinder it?

Lastly, once I get this sucker up and running with you all's good ideas,

is
anybody game to bolt it onto their flying machine and report results? I

can
do it for the 182, but I'd really like some other real-world reports.

Jim




  #9  
Old March 12th 05, 01:08 AM
mindenpilot
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Default


Yes, IMHO accelerometers are a better way to go initially.

This sort of thing has been done in commercial products, years ago
although I don't know the details.


Stangely enough, this is what I do for a living. No kidding.
I am an engineer at Bently Nevada, Corp, the industry leader in machinery
diagnostics.
Most of our claim to fame is for rotating machinery; however, we do quite a
bit of work with reciprocating engines as well.

An accelerometer will work well.
So will proximity (vibration) probes.
We do all kinds of little tricks like putting a notch in the crankshaft to
use as a phase reference.
Then we can determine at what point in the cycle an anomaly occurs, which
can lead to different diagnoses.
We do a whole lot more, but I can't give away all of the trade secrets ;-)

Adam
N7966L
Beech Super III


  #10  
Old March 12th 05, 01:25 AM
Morgans
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"mindenpilot" wrote

We do all kinds of little tricks like putting a notch in the crankshaft to
use as a phase reference.
Then we can determine at what point in the cycle an anomaly occurs, which
can lead to different diagnoses.


How does the notch help in telling the rotational position?
--
Jim in NC


 




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