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*.