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Introduction to Accelerometers
What are Accelerometers?
An
accelerometer is a device that measures the
vibration, or
acceleration of motion of a structure. The force caused by vibration or
a change in motion (acceleration) causes the mass to "squeeze" the
piezoelectric material which produces an electrical charge that is
proportional to the force exerted upon it. Since the charge is
proportional to the force, and the mass is a constant, then the charge
is also proportional to the acceleration. 0
There are two types of piezoelectric
accelerometers (vibration sensors).
The first type is a "high impedance" charge output accelerometer. In
this type of accelerometer the piezoelectric crystal produces an
electrical charge which is connected directly to the measurement
instruments. The charge output requires special accommodations and
instrumentation most commonly found in research facilities. This type of
accelerometer is also used in high temperature applications (>120C)
where low impedance models can not be used.
The second type of accelerometer is a low impedance output
accelerometer. A low impedance accelerometer has a charge accelerometer
as its front end but has a tiny built-in micro-circuit and FET
transistor that converts that charge into a low impedance voltage that
can easily interface with standard instrumentation. This type of
accelerometer is commonly used in industry. An accelerometer power
supply like the ACC-PS1, provides the proper power to the microcircuit
18 to 24 V @ 2 mA constant current and removes the DC bias level, they
typically produces a zero based output signal up to +/- 5V depending
upon the mV/g rating of the accelerometer. All OMEGA(R) accelerometers
are this low impedance type.
Accelerometer Specifications
Dynamic Range is the +/- maximum amplitude that the accelerometer
can measure before distorting or clipping the output signal. Typically
specified in g's.
Frequency Response is determined by the mass, the piezoelectric
properties of the crystal, and the resonance frequency of the case. It
is the frequency range where the output of the accelerometer is within a
specified deviation, typically +/- 5%.
g 1g is the acceleration due to the earth's gravity which is 32.2
ft/sec2, 386 in/sec2 or 9.8 m/sec2.
Grounding - There are two types of signal grounding in
accelerometers. Case Grounded accelerometers have the low side of the
signal connected to their case. As the case is part of the signal path
and may be attached to a conductive material, care must be used when
using this type of accelerometer to avoid noise from the ground plain.
Ground Isolated accelerometers have the electrical components isolated
from the case and are much less susceptible to ground induced noise.
High Frequency Limit is the frequency where the output exceeds
the stated output deviation. It is typically governed by the mechanical
resonance of the accelerometer.
Low Frequency Cut-off is the frequency where the output starts to
fall off below the stated accuracy. The output does not "cut-off " but
the sensitivity decreases rapidly with lower frequencies.
Noise - Electronic noise is generated by the amplifying circuit.
Noise can be specified either broad band (specified over the a frequency
spectrum) or spectral - designated at specific frequencies. Noise levels
are specified in g's, i.e. 0.0025 g 2-25,000 Hz. Noise typically
decreases as frequency increases so noise at low frequencies is more of
a problem than at high frequencies.
Resonance Frequency is the frequency at which the sensor
resonates or rings. Frequency measurements want to be well below the
resonance frequency of the accelerometer.
Sensitivity is the output voltage produced by a certain force
measured in g's. Accelerometers typically fall into two categories -
producing either 10 mV/g or 100 mV/g. The frequency of the AC output
voltage will match the frequency of the vibrations. The output level
will be proportional to the amplitude of the vibrations. Low output
accelerometers are used to measure high vibrational levels while high
output accelerometers are used to measure low level vibrations.
Temperature Sensitivity is the voltage output per degree
of measured temperature. The sensors are temperature compensated to keep
the change in output to within the specified limits for a change in
temperature.
Temperature Range is limited by the electronic micro
circuit that converts the charge to a low impedance output. Typically
the range is -50 to 120C.
Accelerometer Selection When selecting an accelerometer
for your application many parameters must be considered.
- What is the
vibration amplitude to be monitored?
- What is the frequency range to be monitored?
- What is the temperature range of the installation?
- What is the size and shape of the sample to be monitored?
- Are there electromagnetic fields?
- Is there a high level of electrical noise in the area?
- Is the surface where the
accelerometer is to be mounted grounded?
- Is the environment corrosive?
- Does the area require Intrinsically safe or explosion proof instruments?
- Is the area a wet or a wash down area?
Other Considerations:
- The mass of the accelerometers should be significantly smaller than the
mass of the system to be monitored.
- The accelerometer dynamic range should be
broader than the expected vibration amplitude range of the sample.
- The frequency range of the accelerometer should fit the expected
frequency range.
- The Sensitivity of the accelerometer should
produce an electrical output compatible with existing
instrumentation.
- Use a low sensitivity accelerometer to
measure high amplitude vibrations and conversely use a high
sensitivity
accelerometer to measure low amplitude vibrations.
Mounting
The sensor must be mounted directly to the machine surface to correctly
measure the vibrations. This can be accomplished by several types of
mounts:
- Flat Magnet Mount
- 2-pole Magnet Mount
- Adhesives (Epoxy/Cyanoacrylate)
- Mounting Stud
- Isolating Stud
Magnet Mounts are generally temporary mountings.
Magnetic mounts are used to mount accelerometers to ferromagnetic
materials commonly found in machine tools, structures and motors. They
allow the sensor to be easily relocated from site to site for multiple
location readings. Two-pole magnetic mounts are used to mount an
accelerometer to a curved ferromagnetic surface.
Adhesives, and threaded studs are considered permanent mountings.
Adhesives such as epoxy or cyanoacrylate have proven to provide
satisfactory bonding for most applications. Keep the film as thin as
possible to avoid any unwanted dampening of the vibrations due to the
flexibility of the film. To remove an adhesive mounted accelerometer,
use a wrench on the case's wrench flats and twist to break the adhesive
bond. DO NOT USE A HAMMER. Striking the accelerometer will damage it.
Mounting studs are the preferred mounting technique.
They require the structure to be drilled and tapped but provide solid
reliable mountings. Be sure to follow the specified torque settings to
avoid damaging the sensor or stripping the threads.
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