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Accelerometers
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Learn More about Accelerometers ] |
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Accelerometers are sensors and instruments for measuring,
displaying and analyzing acceleration and vibration. They can
be used on a stand-alone basis, or in conjunction with a data
acquisition system. Accelerometers are available in many
forms. They can be raw sensing elements, packaged transducers,
or as a sensor system or instrument, incorporating features
such as totalizing, local or remote display and data
recording. Accelerometers can have from one axis to three axes
of measurement, the multiple axes typically being orthogonal
to each other. These devices work on many operating
principles. The most common types of accelerometers are
piezoelectric, capacitance, null-balance, strain gage,
resonance, piezoresistive and magnetic induction.
Three
main features must be considered when selecting
accelerometers: amplitude range, frequency range, and ambient
conditions. Acceleration amplitude range is measured in Gs,
whereas frequency is measured in Hz. For the ambient
conditions, such things as temperature should be considered,
as well as the maximum shock and vibration the accelerometers
will be able to handle. This is the rating of how much abuse
the device can stand before it stops performing, much
different from how much vibration or acceleration
accelerometers can measure.
Electrical output options
depend on the system being used with the accelerometers.
Common analog options are voltage, current or frequency.
Digital output choices are the standard parallel and serial
signals. Another option is to use accelerometers with an
output of a change in state of switches or alarms.
When
mounting accelerometers, many choices must be weighed based on
application and ability. Probably the most secure method is
stud mounting. Many accelerometers have the option of a
threaded section that can be fastened to the machinery or
object being monitored. For applications where this is not
possible or desirable, many other options are available: wax,
magnets and adhesive. Some applications require accelerometers
to be mounted on an electrically isolated surface to provide
ground isolation between the mounting surface and signals from
the accelerometers. Triaxial mounting cubes can also be
purchased to mount three accelerometers together in an
orthogonal configuration to each other. This way, only one
mounting surface on the monitored device has to be used for
all three.
To minimize frequency response errors, care
must be taken to relieve cable strain by securing the cables
attached to accelerometers. To do this, affix the cables to
the same device the accelerometers are attached to. This will
prevent flexing of the cables between the anchor point and the
vibrating surface, thus keeping the accuracy of the readings
as high as possible. |
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Acceleration
Instruments -[
Learn More about Acceleration Instruments ] |
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Acceleration instruments are used for measuring,
displaying and analyzing acceleration. Typically these
instruments comprise a transducer, data acquisition and either
a local display or some sort of output to a computer or
another instrument. Acceleration instruments may have
many features, including incorporating features such as
totalizing, local or remote display and data recording. They
may be stationary or else portable field-type
instruments.
Acceleration instruments may
have sensors that measure from one axis to three axes of
measurement, the multiple axes typically being orthogonal to
each other. These devices work on many operating principles.
The most common types of acceleration instruments are
piezoelectric, capacitive, null-balance, strain gage,
resonance, piezoresistive and magnetic induction. On top of
all this, acceleration instruments can sometimes accommodate
multiple sensors so the user can keep track of many different
processes or machines at one time.
Three main features
must be considered when selecting acceleration instruments:
amplitude range, frequency range, and ambient conditions.
Acceleration amplitude range is measured in Gs, whereas
frequency is measured in Hz. For ambient conditions, such
things as temperature should be considered, as well as the
maximum shock and vibration the acceleration instruments will
be able to handle. This is the rating of how much abuse the
devices can stand before it stops performing, much different
from how much vibration or acceleration, acceleration
instruments can measure.
Electrical output options
depend on the system being used with the acceleration
instruments. Common analog options are voltage, current or
frequency. Digital output choices are the standard parallel
and serial signals. Another option is to use acceleration
instruments with an output of a change in state of switches or
alarms. Two further output options are important to consider.
Acceleration instruments can often output velocity or
displacement values as well as standard acceleration
readings.
Acceleration instruments can come in
different form factors. As mentioned above, they can be
stationary or portable. Another slightly different option is a
handheld device, meaning that the instrument is actually small
enough to operate in one’s hand, as opposed to being a
portable device with wheels or a handle.
The user
interface can be as simple as an analog readout or as complex
as an actual computer. Acceleration instruments can be
operated either manually or via a host computer, can have
software support for computer interfacing, and can even have
hard drives, removable media or nonvolatile memory
options.
As a complicated piece of equipment,
acceleration instruments can come with lots of other options
that enhance their functionality or usability. Some of these
are event triggering, self-calibration, self-test, built-in
filters, and even capability to withstand extreme
environments, such as those with excessive heat, moisture or
dust. |
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Impact
Hammers -[
Learn More about Impact Hammers ] |
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Impact hammers are used in structural and modal analysis
to determine component or system response to impacts of
varying amplitude and duration. It is simply a hammer with a
force transducer in its head is paired with an accelerometer
on the component being tested to compare impact and response.
Impact hammers are used for modal and structural behavior
analysis for all types of components and systems. The most
critical specifications for impact hammers are the force and
pulse duration, both rated by minimums and maximums. Force is
measured in pounds, kg, N and other similar units. The pulse
duration, a measurement of the time the hammer is imparting a
force on the object being tested, is a very short span of
time, often measured in milliseconds. Another important
specification is the upper limit of the frequency range. This
is the highest frequency for which the response will be
tested, and can be altered by using accessory tips, if
available.
Impact hammers can be controlled either
manually or remotely. That is, the device could resemble an
actual hand-held hammer and is used by manually striking an
object. Remote operation impact hammers are either more like
an automated piece of machinery that the operator tells how
much force to apply or else a handheld power tool that is
operated by actuating a switch.
A range of striking
tips of various materials and hardness is often available,
allowing for different forces and response times. In general,
harder tips will deform less than softer tips during impact
and will therefore have a shorter pulse duration. Typically
the hardest tips are used to measure response at the highest
frequencies. Additional masses that attach to the back of the
hammer head are another common accessory. These are used to
increase excitation force beyond the force of the plain
hammer.
Some impact hammers come as a kit that can
include the hammer, an accelerometer to be mounted to the
object being tested, signal conditioners for the transducer
signals, and a multichannel analyzer for the hammer and
accelerometer. A kit like this would be everything an engineer
needs to do analysis using impact hammers. |
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Inertial
Gyros -[
Learn More about Inertial Gyros ] |
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Inertial gyros are sensors used to measure angular rates
and x, y & z linear acceleration. Applications include
vehicle instrumentation, robotics, and attitude reference
systems.
Up to three angular or rotary axes, as well as
3 linear axes, can be measured using inertial gyros. The most
critical rotary performance specs for inertial gyros are
rotary rate, or fast you can turn the device, and rated
acceleration for linear axes. Both rotary and linear axes have
specifications for accuracy in +/- percentage of the full
scale of measurement, the bandwidth, bias stability and
transverse sensitivity. Axis bandwidth is the frequency range
over which the device meets its accuracy specifications. Bias
stability is the accuracy over operating temperature range.
Transverse sensitivity is the inertial gyro’s sensitivity to a
force that is orthogonal to the desired direction of
measurement.
A number of technology options for angular
measurement are available. Inertial gyros with a gyro
reference have an inertial mass used as a reference for
rotational movements. A vibrating tuning fork or plates
measure Coriolis force in inertial gyros made using MEMS
technology. Gravity reference sensors, another technology and
also known as inclinometers, cannot be used in gravity-free
environments. Fiber optics is a unique way to measure angular
displacement without moving parts.
Linear acceleration
technologies for inertial gyros are about as numerous. For
piezoelectric devices, a piezoelectric material is compressed
by the proof mass and generates a charge that is measured by a
charge amplifier. For strain gage devices, strain gages
(strain-sensitive variable resistors) are bonded to parts of
the structure that support the proof mass. These strain gages
are typically used as elements in a Wheatstone bridge circuit,
which is used to make the measurement. Capacitance-based
accelerometers measure the variable capacitance between the
support structure and the proof mass. The variable gap between
the two is measured in a capacitance measurement
circuit.
Electrical output options depend on the system
being used with the inertial gyros. Common analog output
options are voltage, current or frequency. Digital output
choices are the standard parallel and serial signals. Another
option is to use inertial gyros with an output of a change in
state of switches or alarms. In addition, these sensors can
have a vertical or artificial horizon output. |
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Piezoelectric
Actuators -[
Learn More about Piezoelectric Actuators ] |
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Piezoelectric actuators are devices that produce a small
displacement with a high force capability when voltage is
applied. There are many applications where a
piezoelectric actuator may be used, such as ultra-precise
positioning and in the generation and handling of high forces
or pressures in static or in dynamic
situations.
Actuator configuration can vary greatly
depending on application. Piezoelectric stack actuators are
manufactured by stacking up piezoelectric disks or plates, the
axis of the stack being the axis of linear motion when a
voltage is applied. Tube actuators are monolithic devices
that contract laterally and longitudinally when a voltage is
applied between the inner and outer electrodes. A disk
actuator is a device in the shape of a planar disk. Ring
actuators are disk actuators with a center bore, making the
actuator axis accessible for optical, mechanical, or
electrical purposes. Other less common configurations include
block, disk, bender, and bimorph styles.
These devices
can also be ultrasonic. Ultrasonic actuators are specifically
designed to produce strokes of several micrometers at
ultrasonic (>20kHz) frequencies. They are especially useful
for controlling vibration, positioning applications and quick
switching. In addition, piezoelectric actuators can be either
direct or amplified. The effect of amplification is a larger
displacement, but it can also result in slower response
times.
The critical specifications for piezoelectric
actuators are the displacement, force and operating voltage of
the actuator. Other factors to consider are stiffness,
resonant frequency and capacitance. Stiffness is a term used
to describe the force needed to achieve a certain deformation
of a structure. For piezoelectric actuators, it is the
force needed to elongate the device by certain amount.
It is normally specified in terms of Newton per micrometer.
Resonance is the frequency at which the actuators respond with
maximum output amplitude. The capacitance is a function of the
excitation voltage frequency.
The size of the actuator,
of course, is important, as are the electrical connectors.
Some of the most common connectors are DB-9, BNC, two wires of
either AWG 26 or AWG 30, or else a LEMO(r) connector, which is
a precision push-pull locking connector for demanding
applications. |
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Shaker
Controllers -[
Learn More about Shaker Controllers ] |
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Shaker controllers are units designed to control shaker
tables. The simplest types of shaker controllers are
controlled manually and depend on the operator to read and
evaluate the feedback signal and adjust the amplifier signal
input voltage accordingly. This type of system can be as
simple as a sine wave signal generator and an accelerometer
monitored by a voltmeter. It is left to the operator to
manually make the necessary gain compensation for changes in
frequency or desired level specifications.
Since most
modern accelerometers require a constant current source/buffer
amplifier and most voltmeters read in either average or RMS
voltage for AC signals, it can be difficult to read and adjust
for peak acceleration with this setup. If the accelerometer
has a sensitivity that is not convenient for conversion to
voltage, mistakes are easy to make. Random acceleration can be
monitored in this fashion more directly because of the RMS
nature of most random acceleration specifications. However,
the RMS calibrated meter will inject another error when
monitoring Gaussian signals. If the vibration specification
involves displacement, it becomes virtually impossible to use
this method.
Shaker controllers vary in sophistication,
but usually provide feedback calibrated in acceleration and
displacement units useful to vibration testing. Simple manual
units are available and provide for frequency and gain
adjustment while providing a calibrated acceleration signal in
peak G’s. More complex units will feature automatic servo
controlled levels with programming and frequency sweep
capabilities. Top end controllers utilizing computer
technology are available and can control to almost any
specification with multiple accelerometers, etc.
The
most critical specifications for shaker controllers are number
of inputs and outputs, output dynamic range and frequency
range, and what types of testing the user needs performed. The
types of tests include simple outputs such as a random
Gaussian output signal, classical shock and arbitrary
transients. More complex signals are available as well.
Sine on random is a test signal in which narrowband sine waves
are combined with broadband random waves for a complex
vibration signal. Random on random produces a test signal that
has narrowband random waves combined with broadband random
waves. A shock response system calculates the responses of a
large number of theoretical, single-degree-of-freedom
spring-mass systems to a given shock pulse. |
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Shakers,
Shock and Vibration Testing -[
Learn More about Shakers, Shock and Vibration Testing ]
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Shakers and vibration and shock testing equipment are
force generators or transducers that provide a vibration,
shock or modal excitation source for testing and analysis.
Shakers are used to determine product or component performance
under vibration or shock loads, detect flaws through modal
analysis, verify product designs, measure structural fatigue
of a system or material or simulate the shock or vibration
conditions found in aerospace, transportation or other
areas.
Shakers can operate under a number of different
principles. Mechanical shakers use a motor with an eccentric
on the shaft to generate vibration. Electrodynamic models use
an electromagnet to create force and vibration. Hydraulic
systems are useful when large force amplitudes are required,
such as in testing large aerospace or marine structures or
when the magnetic fields of electrodynamic generators cannot
be tolerated. Pneumatic systems, known as "air hammer
tables," use pressure air to drive a table. Piezoelectric
shakers work by applying an electrical charge and voltage to
a sensitive piezoelectric crystal or ceramic element to
generate deformation and motion.
Common features of
shakers are an integral slip table and active suspension. An
integral slip allows horizontal or both horizontal and
vertical testing of samples. The slip table is a large
flat plate that rests on an oil film placed on a granite slab
or other stable base. An active suspension system compensates
for environmental or floating platform variations.
The
most important specifications for shakers are peak sinusoidal
force, frequency range, displacement, peak acceleration and
peak velocity. Some of these specifications may be ratings
without a load, as the manufacturers cannot always predict how
the shakers will be used.
The three main test modes
shakers can have are random vibration, sine wave vibration and
shock or pulse mode. In a random vibration test mode, the
force and velocity of the table and test sample will vary
randomly over time. A sine wave test mode varies the force and
velocity of the table and test sample sinusoidally over time.
In a shock test mode, the test sample is exposed to high
amplitude pulses of force. |
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Vibration
Analysis Systems - Systems for vibration testing
and modal analysis such as the instrumentation to acquire data
and/or analyze results. |
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Vibration
Instruments -[
Learn More about Vibration Instruments ] |
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Vibration instruments are used for measuring, displaying
and analyzing vibration. Typically these instruments comprise
a transducer, data acquisition and either a local display or
some sort of output to a computer or another instrument.
Vibration instruments can have many features, including
incorporating features such as totalizing, local or remote
display and data recording. They may be stationary or else
portable field-type instruments.
Vibration instruments
can accept a number of different types of transducers,
including acceleration, linear velocity, proximity and
displacement, rotary velocity and temperature. In
addition, many vibration instruments can take generic signal
inputs, including voltage, current, frequency and serial
inputs. Some of these instruments can even accept
wireless data transmissions.
Four main features must be
considered when selecting vibration instruments: number of
channels, accuracy, sampling frequency and ambient conditions.
The accuracy is usually measured as a percentage of the full
scale of measurement, so an accuracy specification may be
something like 5% or 10% instead of a hard number or range of
values. Sampling frequency is how often the vibration
instruments take readings from the sensors and should not be
confused with measuring ranges of the sensors
themselves. For ambient conditions, such things as
temperature should be considered, as well as the maximum shock
and vibration the vibration instruments will be able to
handle. This is the rating of how much abuse the devices can
stand before it stops performing, much different from how much
shock or vibration the vibration instruments can
measure.
Electrical output options depend on the system
being used with the vibration instruments. Common analog
options are voltage, current or frequency. Digital output
choices are the standard parallel and serial signals. Another
option is to use vibration instruments with an output of a
change in state of switches or alarms. Two further output
options are important to consider. Vibration instruments can
often output velocity or displacement values as well as
standard vibration readings.
Vibration instruments come
in different form factors. As mentioned above, they can be
stationary or portable. Another slightly different option is a
handheld device, meaning that the instrument is actually small
enough to operate in one’s hand, as opposed to being a
portable device with wheels or a handle.
The user
interface can be as simple as an analog readout or as complex
as an actual computer. Vibration instruments can be operated
either manually or via a host computer, can have software
support for computer interfacing, and can even have hard
drives, removable media or nonvolatile memory
options.
As a complicated piece of equipment, vibration
instruments can come with lots of other options that enhance
their functionality or usability. Some of these are event
triggering, self-calibration, self-test, built-in filters, and
even capability to withstand extreme environments, such as
those with excessive heat, moisture or dust. |
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Vibration
Sensors -[
Learn More about Vibration Sensors ] |
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Vibration sensors are sensors for measuring, displaying
and analyzing linear velocity, displacement and proximity, or
else acceleration. They can be used on a stand-alone basis, or
in conjunction with a data acquisition system. Vibration
sensors are available in many forms. They can be raw sensing
elements, packaged transducers, or as a sensor system or
instrument, incorporating features such as totalizing, local
or remote display and data recording.
Vibration sensors
can have from one axis to three axes of measurement, the
multiple axes typically being orthogonal to each other. These
devices work on many operating principles. The most common
types of vibration sensors are piezoelectric, capacitance,
null-balance, strain gage, resonance beam, piezoresistive and
magnetic induction. An alternative to traditional vibration
sensors is one manufactured using MEMS technology, a
micro-machining technology that allows for a much smaller
device and thus package design.
Five main features must
be considered when selecting vibration sensors: measuring
range, frequency range, accuracy, transverse sensitivity and
ambient conditions. Measuring range can be in G’s for
acceleration, in/sec for linear velocity (or other distance
over time), and inches or other distance for displacement and
proximity. Frequency is measured in Hz and accuracy is
typically represented as a percentage of allowable error over
the full measurement range of the device. Transverse
sensitivity refers to the effect a force orthogonal to the one
being measured can have on the reading. Again, this is
represented as percentage of full scale of allowable
interference. For the ambient conditions, such things as
temperature should be considered, as well as the maximum shock
and vibration the vibration sensors will be able to handle.
This is the rating of how much abuse the device can stand
before it stops performing, much different from how much
vibration or acceleration vibration sensors can
measure.
Electrical output options depend on the system
being used with the vibration sensors. Common analog options
are voltage, current or frequency. Digital output choices are
the standard parallel and serial signals. Another option is to
use vibration sensors with an output of a change in state of
switches or alarms. In addition, these sensors can have
acceleration, velocity, or displacement as output by either
integrating or differentiating their primary
output.
When mounting vibration sensors, many choices
must be weighed based on application and ability. Probably the
most secure method is stud mounting. Many vibration sensors
have the option of a threaded section that can be fastened to
the machinery or object being monitored. For applications
where this is not possible or desirable, many other options
are available: wax, magnets and adhesive. Some applications
require vibration sensors to be mounted on an electrically
isolated surface to provide ground isolation between the
mounting surface and signals from the vibration sensors.
Triaxial mounting cubes can also be purchased to mount three
vibration sensors together in an orthogonal configuration to
each other. This way, only one mounting surface on the
monitored device has to be used for all three. |
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