Magnetic Sensors

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  • Topic: Sensor, Current source, Temperature
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  • Published : February 22, 2013
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Vibration Sensors Introduction / Selection
Characteristic
Flat Frequency Response
20–1,500 Hz
2–5,000 Hz
Phase Fidelity
2–5,000Hz
Reduced Noise at
Higher Frequencies
Linearity
Mounting in Any Orientation
Temperature Limitation
EMI* Resistance
Mechanical Durability

Coil and Magnet
Velocity Sensor

Piezoelectric
Velocity Sensor

Yes
No

Yes
Yes

Acceptable

Excellent

No
Good
Sensor Dependent
> +707°F (+375°C)
Acceptable
Good

Yes
Good
Yes
+248°F (+120°C)
Excellent
Excellent

*EMI–Electro Magnetic Interference

Traditional velocity sensors are of a mechanical design
that uses an electromagnetic (coil and magnet) system to
generate the velocity signal. Recently, hardier
piezoelectric velocity sensors (internally integrated
accelerometers) have gained in popularity due to their
improved capabilities and more rugged and smaller size
design. A comparison between the traditional coil and
magnetic velocity sensor and the modern piezoelectric
velocity sensor is shown in Table 1.
The electromagnetic (Inductive) velocity sensor does have
a critical place in the proper sensor selection. Because of
its high temperature capability it finds wide application in gas turbine monitoring and is the sensor of choice by
many of the major gas turbine manufacturers.
The high temperature problems for systems using
accelerometers can also be solved by splitting sensor and
electronics (charge amplifiers). The sensor can have high
temperature ranges up to +1,112°F (+600°C).
Some methods of investigating bearing defects and gear
problems may require a higher frequency range and
because the signals are generated by impact, the
sensitivity should be lower. By the same means if the
user is using the SKF “Enveloping Technique” then just
the opposite is applicable.
ACCELEROMETERS
Piezoelectric accelerometers having a constant signal over
a wide frequency range, up to 20 kHz's, for a given
mechanical acceleration level, are very useful for all types of vibration measurements.
Acceleration integrated to velocity can be used for low
frequency measurements. Acceleration signals in the high
frequency range added with various signal processing
techniques like ACC ENV, or HFD are very useful for
bearing and gear measurements.

The basic acceleration sensor has a good signal to noise
ratio over a wide dynamic range.
They are useful for measuring low to very high
frequencies and are available in a wide variety of general
purpose and application specific designs. The
piezoelectric sensor is versatile, reliable and the most
popular vibration sensor for machinery monitoring.
When combined with vibration monitors capable of
integrating from acceleration to velocity, accelerometers
can be a useful component in a Multi-Parameter
Monitoring Program. The user is, therefore, able to
determine both velocity and acceleration values for the
same machine point with a single sensor.

SEE™ SENSORS
As a complementary technology to the sensors discussed
above, SKF has developed and patented sensors based on
SEE Technology. SEE (Spectral Emitted Energy) is
used to detect acoustic emission signals in the range of
250–350 kHz, well beyond the range of conventional
vibration sensors. Such acoustic emission signals are
generated by stress-type defects such as metal-to-metal
impacts and wear. This can happen when rolling over a
local defect or a partical or when there is wear in a dry
sliding contact (metal-to-metal contact). For example,
when a rolling element of an anti-friction bearing breaks
through its lubrication film and slides SEE sensors are
able to detect the condition. The user then has the
opportunity to take proactive steps in order to prevent
further damage.

Piezoelectric Sensors
Accelerometers operate on the piezoelectric principal: a
crystal generates a low voltage or charge when stressed as
for example during compression. (The Greek root word
“piezein”...
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