In article ,
RST Engineering wrote:
Proximity probes? Or accelerometers?

The proverbial "it depends". _first_ you have do decide what your
'frame of reference' for vibration is. If you're looking for
excursions in the drive shaft, relative to the engine block, then
a pair of proximity sensors in quadrature to measure excursions from
the shaft axis, plus a strain gauge at a fore-to-aft thrust bearing
gives you complete data. Convert from XYZ to polar co-ordinates,
Fourier transform on the magnitude component, check each passband
against loading-corrected reference values, and illuminate corresponding
indicators.

If you're looking for things that might affect all parts of the engine
equally, then, can you use the airframe for reference, or not? If "yes",
then the same approach works, just on a different scale. If, "no", then
you're pretty much stuck with three _inertial_ accelerometers, mounted at
mutual right angles.

I understand about the analog circuitry and actually plan on making a five
or six channel filter at each of the possible resonance points relative to
the fundamental ... and then strobing the filters to light a "normal",
"low", "high" lamp for each channel.


Sounds like you're talking about using a group of narrow bandpass filters
and sampling the short-term average signal level out of each filter,
"Looking for" a marked increase vs a base-line reference (at that frequency),
as an indicator of problems.


I see a couple of difficulties -- not necessarily insurmountable, but
there, nonetheless. (1) Most, if not all, the 'frequencies of interest'
are dependent on engine speed. thus the filter 'center frequencies'
will have to track engine RPM. (2) "Normal" vibration _magnitude_ will
vary with the load on the engine. The trigger threshold would seemingly
have to adjust to compensate. 3,000 RPM in a 2,000 FPM climb is a
different engine environment than 3,000 RPM in level cruise. I don't
know enough engine mechanics to guess whether just the throttle setting
is going to be sufficient to determine the required adjustments. At
a guess, torque relative to the airframe would be a better indicator.

Lastly, I'd suggest one channel that is comparatively broadband. specifically
to catch unanticipated "out of band" goings-on.

Jim




As is tradition, we tend to get off topic ;-)
I think you can accomplish this using the methods discussed.
Mount 2 proximity probes 90 degrees apart.
Calibrate the readings for normal low vibrations (be sure to account for
any nonlinearities in the probe).
Design the circuit to trip the buzzer/lamp when the vibration exceeds the
normal level.
You may need some analog circuitry to help (gain, etc).
But you don't need to get much fancier than that.

Adam