Applications:
Often seismic
transducers are not the best sensor for monitoring vibration on specific
machines. Eddy probes offer a solution to most of the problems inhibiting
the use of traditional seismic sensors. The most common use of eddy current probes is for journal type
bearings. Journal bearings behave different than rolling element bearings in
that they do not transmit the vibration of the shaft into the bearing cap
due to the damping properties of this type of bearing. Another application
for eddy probes, is where a high case to rotor weight ratio exists (turbines
and compressors). In these applications most of the shaft's vibration energy
is dissipated as displacement, which can only be measured by shaft relative
motion.
Frequency considerations also
exist with eddy probes. While eddy probes have no lower frequency limit,
very high frequencies become a problem. An eddy probe senses displacement,
and at high frequencies displacement is often quite small requiring the use
of a seismic accelerometer.
How Eddy Probes Work:
An eddy probe
system consists of a probe, matched extension cable, and driver
(oscillator demodulator). The tip of the probe contains an encapsulated wire
coil which radiates the driver's high frequency as a magnetic field. When a
conductive surface comes into close proximity to the probe tip, eddy
currents are generated on the target surface decreasing the magnetic field
strength, leading to a decrease in the driver's DC output. This DC output is
usually 200mV/mil or in a similar range. This signal is then sent to a CMCP500 Series Transmitter or Monitor for conversion into a 4-20 mA output
relative to displacement (CMCP540 and 540A) or position (CMCP545 and 545A).
The CMCP540A and CMCP545A Monitors allow programmable alert and danger
alarms with relays for the eddy probe driver's signal. |
