What is the basic principle of vibration measurement by
BENTLY NEVADA vibration measurement system? and Why it
gives the -ve voltage output?
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Answer Posted / steve sabin
As a 20-year Bently Nevada employee, and as someone
responsible for answering such questions over the years
within our company as well as externally, allow me to
explain the rather interesting circumstances behind the use
of negative voltages (-24 vdc) for powering eddy current
vibration oscillator/demodulator devices. I will then
proceed to explain the operating principles behind the eddy
current proximity probe and how it converts physical gap
between the probe and its target into a voltage.
First, the question of negative voltage. The short answer
is that When Don Bently worked on making solid-state
versions of the eddy-current measurement system (it was
actually originally designed in the 1930s by GE engineers
using vacuum tubes), he had a choice between using N-P-N
transistors or P-N-P transistors. At the time, transistors
were quite expensive, so he chose the least expensive of
the two: P-N-P (apparently, PNP transistors they were less
expensive to manufacture 50 years ago than their NPN
counterparts).
Because the circuits used PNP transistors, a negative bias
voltage was required rather than a positive bias voltage.
Don chose -18V. This was later changed to -24V to allow
more linear range from the transducer.
At that time, the industrial instrumentation community had
not yet standardized on +24 vdc, and by the time they did,
there were so many Bently Nevada eddy current vibration
sensors installed that changing to +24V rather than -24V
was not greeting with enthusiasm by users. Hence, it has
remained -24V to this day.
This was not a deliberate effort to "be different" or "non-
conformist" on the part of Bently Nevada. It was quite
literally based on which components were the least
expensive when the technology was originally introduced 50-
plus years ago.
Now, the question of how an eddy-current proximity probe
works:
An eddy-current probe works by passing an alternating
current through a coil of wire and measuring the coil’s
impedance. This impedance changes when the probe is brought
near an electrically conductive material and the impedance
change is proportional to the physical gap between the coil
and the conductive target. The sensing electronics in turn
convert this impedance change to a voltage, providing an
electrical output directly proportional to physical gap.
The above is an excerpt from a recent article in our ORBIT
technical journal. You can read the full-length article
here:
http://www.gepower.com/prod_serv/products/oc/en/orbit/downlo
ads/1Q09_Prox_Probes.pdf
Here are some other articles that may also prove helpful in
understanding how proximity probes work in actual machinery
vibration/position measurement applications:
http://www.gepower.com/prod_serv/products/oc/en/orbit/downlo
ads/1q01sabin3.pdf
http://www.gepower.com/prod_serv/products/oc/en/orbit/downlo
ads/1293jordan.pdf
Finally, I am including a link to a datasheet for our 8mm
proximity probe system. Although it does not go into a lot
of detail on principles of operation, you may find some
useful information therein.
Should you have additional questions, please contact the
undersigned.
Regards,
Steve Sabin
Marketing Communications Manager
Bently Nevada Asset Condition Monitoring
GE Energy Services
| Is This Answer Correct ? | 285 Yes | 14 No |
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