"Robert Bonomi" wrote in message
...
In article ,
Morgans wrote:

"RST Engineering" wrote in message
...
Yes, all of the below.

Sometimes you drill a hole in a small magnet and use a small screw to
hold
it to the propeller backing plate. Sometimes you use a strip of
reflective
tape on the prop itself. Sometimes you use a pulse from the #1 magneto
lead.

Jim

This guy was talking about a notch and proximity detector for the phase
detection. I know about all of the things you mentioned, but the
proximity
detector's workings are new to me.

Think of a proximity detector as 'ultra short-range radar'. grin
It may use reflected RF energy, or 'optical'.

where the 'excursions' you're trying to measure are smaller than the
wavelength of the measuring 'beam', you can use simple phase-shift
between outgoing and returning signal, to determine distance.

where the distance is much larger than the wavelength, you have to
impress a carrier on the beam, and measure phase-shift in the carrier
frequency. this gets an 'approximate' distance, that can be further
refined by phase angle measurements of the beam itself.

Capacitance tracking is also a possible approach.
and/or "Hall effect".

These can get 'messy', due to inherent non-linearity in the technology,
that has to be compensated for, in 'reading' the signals.

Capacitance tracking works best where there are *very*small* vibrations
involved, and a very _smooth_ surface to measure against. The technique
is capable of mapping individual atoms/molecules in a crystal lattice.
Scientific American had a write-up -- at least 15 years ago -- about
a new 'super microscope' (successor generation to the scanning electron
microscope) that worked in that manner. a _very_fine_ 'needle' was
carefully moved, raster-style, across the object being 'scanned', and
the capacitance changes between the needle and the object were mapped.



Actually, the proximity probes I am familiar with work on the "eddy current"
principle.
That is, RF energy is directed at the shaft through a coil. Some of the
energy is lost into the shaft in the eddy currents.
The energy that is coupled back into the coil is the measured signal.
Since the amount of energy dissipated in the shaft is proportional to the
distance between the shaft and probe, the result is a displacement
measurment.

Adam