Vibration Monitor (Hyde, Wanttaja?)

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

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

"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

"Morgans" wrote in message
...

"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

It takes a little nick out your finger as it goes by?

Rich "Ow ow ow ow owow ow ow ow ow owow" S.

Morgans wrote:
"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?

It adds a known vibration to the system? ;-)

Presumably, it gets used as a timing mark of some kind. I guess that
monitoring the spark time to the No 1 cylinder would almost give you the
same. "Almost" because of course the spark can be advanced/retarded.

Frank

"Morgans" wrote in message
...

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


Without giving too much away...
The notch depth is more significant than any vibration. If using a
proximity measurment, it is easy to determine when the notch occurs.
The notch is aligned with a known position (engineer inserts notch when
pistons/crank is in known position). This gives a phase reference. You can
compare where in the cycle a disturbance (knock, rub, etc) occurs with
relation to the notch. On recip applications, often a multi-tooth wheel is
used in addition to a phase reference for more accuracy.

Adam
N7966L
Beech Super III

In article ,
says...


"Morgans" wrote in message
...

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


Without giving too much away...
The notch depth is more significant than any vibration. If using a
proximity measurment, it is easy to determine when the notch occurs.
The notch is aligned with a known position (engineer inserts notch whe
n
pistons/crank is in known position). This gives a phase reference. Y
ou can
compare where in the cycle a disturbance (knock, rub, etc) occurs with

relation to the notch. On recip applications, often a multi-tooth whe
el is
used in addition to a phase reference for more accuracy.

Adam
N7966L
Beech Super III


I love puzzles.

Start out with a picture in your mind of a perfectly round crank flange
with a dial indicator rideing on the outside edge at the top as you
slowly rotate the crank. The indicator will not move because the
flange is perfectly round.

Under viberation, the crank will move up and down (within the limits of
the bearing clearance) and the dial indicator will show how far the
crank moves.

Now replace the dial indicator with a precision proximity detector.
You get a steady flat line electronic signal until you get movement up
or down. Then the signal will change at the same frequency as the
movement and change amptitude with the amount of movement.

Now add a notch across the edge of the flange so that the notch is seen
by the sensor when #1 piston is at top dead center.

That notch causes a much larger timing blip in the signal than any
crank movement will cause. By measureing the time between the timing
blips you can determine the amount of rotation after a timing blip that
you see an up or down crank movement event.

In other words if you see a minor blip exactly midway between timing
blips then you know the crank moved up or down when #1 piston was at
bottom dead center in our example. The amptitude of the minor blip
will tell you how far it moved. It could even be set up to tell you if
the crank moved up or moved down by the polarity shift of the signal.

Of course this simple setup will not show side to side movement. That
would require a second proximity detector 90 degrees from the first
one. The 2 signals would require integration but would then show all
movement in the rotational plane with the proper signal processing and
calibration.

"sleepy6" wrote

That notch causes a much larger timing blip in the signal than any
crank movement will cause. By measureing the time between the timing
blips you can determine the amount of rotation after a timing blip that
you see an up or down crank movement event.

What sort of technology does a proxomity detector use? Magnetics, sonic,
radio waves?
--
Jim in NC

Without giving too much away............ (yuk yuk yuk)

One can always post your questions at:
http://www.vibinst.org/default.htm

and usually somebody will be happy to answer.

Phase indicators can be a variety of schemes or devices. Proximity
probes, and notch. Strobe light. Magnetic interrupter. Photocell.
Even the old proverbial pencil and masking tape!

Kent Felkins
Tulsa

Jim,

I have a Memsic demo board, which displays LEDS on two axis with
their tilt and motion sensor.

I am done with it, would you like to play with it?

George Graham
RX-7 Powered Graham-EZ, N4449E
Homepage http://bfn.org/~ca266