Answer / 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

Answer / sam

This is a very good, accurate writeup done by someone else
for a similar question earlier on. Hope it helps.


The vibration sensors used in turbine, generator etc are all
same type i.e proximity type of sensors. The sensor are
mounted at the proximity of the Turbine and generator rotor.
From the sensors one coaxial cable runs to the proximater,
which is mounted in a separate JB nearby. From TSI Monitors,
this Vibration probes gets +24 Volts DC. The proximater
gives a signal 0 to -22.4 Volts DC depending upon the
vibration measured by the sensor. Generally the distance
between the probe tip and the rotor is maintained such that
it gives -10 Volts DC when the turbine is not running.

The sensor works on eddy current principle. As the rotor
vibration increases, the eddy current generation remains
constant, but the absorption of power in terms of eddy
current increases. This loss of power is sensed by the
proximater and DC voltage signal is transmitter to TSI
monitor for Measurement, Controlling & Indication purpose.
Good luck

Answer / atta

An eddy-current probe works by passing an alternating
current through a coil of wire and measuring the coil’s
impedance(Resistance to flow of AC current). The coil is
supplied with a alternating current from the remote
eddy probe driver, which sets up an electro-magnetic field
between the tip and the observed surface.
When any electrically conductive material within this
electromagnetic field, ie. the target material, will have
eddy currents induced in it’s surface. These eddy currents
will vary produce there own magnetic field but in opposite
direction of probe field, and hence the impedance to
energising current, in proportion th the probe target
distance.
The sensing electronics in turn convert this impedance
change to a voltage, providing an electrical output
directly proportional to physical gap.

Answer / m. saqib ali nafees

AN EDDY-current probe works on the basic magnetic principles
(Faraday's law and lenz's law) when an electrically
conductive material comes close to the probe we know that
alternating current pass though coil this alternating
current produce EMF in electrically conductive material
(observed surface) by Faraday's law. This produced EMF
oppose its source by lenz's law. ( when the current through
a coil changes and an induced voltages is created as a
result of the magnetic field, the induced voltages in such a
direction that its magnetic effect will oppose the Change
that produce it.) The output voltage vary according to gap
between probe and observed surface by these two laws.

Answer / guest

ENTLY NEVADA vibration measurement system? and Why it
gives the -ve voltage output? ya

Answer / pj

gr8 answer steve

Answer / dhiraj gosain

Thanks Mr.Steve, at last a long search has come to a result
Dhiraj Gosain

Answer / sudhir

Thanks steve for the elaborative answer.

Answer / keyur

thanks a lot Steve Sabin

i clearing my all doubt after reading your answer....

Once again thanks a lot,

Regards,
Keyur

Answer / ramesh

what is purpose of proximity ? why use proximity with
vibration system?

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