Package testing - Measuring the shock experienced by a packaged product
compared to the level of actual shock exposure allows determination of the
effectiveness of a packaging material. Package testing can also be used to
monitor vibration and shock that a product may experience during transport.
Shock - Accelerometers may be used to determine maximum impact acceleration
levels experienced by vehicles and crash dummies. Shock accelerometers also
measure shock exposure experienced by space vehicles and cargo during stage
A wide variety of piezoelectric accelerometer configurations are available.
Each method has its own particular advantages and disadvantages for different
applications. Since selecting a sensor is not trivial, applications assistance
from PCB field representatives or factory application engineers is available to
assist with the selection process.
Function of Piezoelectric Accelerometers
Piezoelectric accelerometers rely on the piezoelectric effect of quartz or
ceramic crystals to generate an electrical output that is proportional to
applied acceleration. The piezoelectric effect produces an opposed accumulation
of charged particles on the crystal. This charge is proportional to applied
force or stress. A force applied to a quartz crystal lattice structure alters
alignment of positive and negative ions, which results in an accumulation of
these charged ions on opposed surfaces. These charged ions accumulate on an
electrode that is ultimately conditioned by transistor microelectronics.
In an accelerometer, the stress on the crystals occurs as a result of the
seismic mass imposing a force on the crystal. Over its specified frequency
range, this structure approximately obeys Newton's law of motion, F=ma.
Therefore, the total amount of accumulated charge is proportional to the
applied force, and the applied force is proportional to acceleration.
Electrodes collect and wires transmit the charge to a signal conditioner that
may be remote or built into the accelerometer. Sensors containing built-in
signal conditioners are classified as Integrated Electronics Piezoelectric
(IEPE) or voltage mode; charge mode sensors require external or remote signal
conditioning. Once the charge is conditioned by the signal conditioning
electronics, the signal is available for display, recording, analysis, or
control. PCB sensors containing integral electronics are known by their
trademarked term, Integrated Circuit - Piezoelectric, or ICP®.
Structure of Piezoelectric Accelerometers
A variety of mechanical configurations are available to perform the transduction
principles of a piezoelectric accelerometer. These configurations are defined
by the nature in which the inertial force of an accelerated mass acts upon the
piezoelectric material. At PCB, there are two primary configurations in use
today: Shear and Flexural Beam. A third configuration, Compression, is used
less now than previously at PCB, but is included herein as an alternative
Shear mode designs bond, or "sandwich," the sensing crystals between a center
post and seismic mass. A compression ring or stud applies a preload force
required to create a rigid linear structure. Under acceleration, the mass
causes a shear stress to be applied to the sensing crystals. By isolating the
sensing crystals from the base and housing, shear accelerometers excel in
rejecting thermal transient and base bending effects. Also, the shear geometry
lends itself to small size, which minimizes mass loading effects on the test
structure. With this combination of ideal characteristics, shear mode
accelerometers offer optimum performance.
Flexural mode designs utilize beam-shaped sensing crystals, which are supported
to create strain on the crystal when accelerated. The crystal may be bonded to
a carrier beam that increases the amount of strain when accelerated. This
design offers a low profile, light weight, excellent thermal stability, and an
economical price. Insensitivity to transverse motion is also an inherent
feature of this design. Generally, flexural beam designs are well suited for
low-frequency, low-g-level applications like those which may be encountered
during structural testing.
Compression mode accelerometers offer simple structure, high rigidity, and
historical availability. There are basically three types of compression
designs: upright, inverted, and isolated.
Upright compression designs sandwich the piezoelectric crystal between a seismic
mass and rigid mounting base. An elastic stud or screw secures the sensing
element to the mounting base. When the sensor is accelerated, the seismic mass
increases or decreases the amount of force acting upon the crystal, and a
proportional electrical output results. The larger the seismic mass is, the
greater the stress and, hence, the output are.
Due to their inherently stiff structure, the upright compression design
offers high resonant frequencies, resulting in a broad, accurate frequency
response range. This design is generally very rugged and can withstand high-g
shock levels. However, due to the intimate contact of the sensing crystals with
the external mounting base, upright compression designs tend to be more
sensitive to base bending (strain) and thermal transient effects. These effects
can contribute to erroneous output signals when used on thin, sheet-metal
structures or at low frequencies in thermally unstable environments, such as
outdoors or near fans and blowers.
Inverted compression designs isolate the sensing crystals from the
mounting base, reducing base bending effects and minimizing the effects of a
thermally unstable test structure. Many reference standard calibration
accelerometers use this design.
Isolated compression designs reduce erroneous outputs due to base strain
and thermal transients. These benefits are achieved by mechanically isolating
the sensing crystals from the mounting base and utilizing a hollowed-out
seismic mass that acts as a thermal insulation barrier. These mechanical
enhancements allow stable performance at low frequencies, where thermal
transient effects can create signal "drift" with other compression designs.
There are two types of piezoelectric material that are used for PCB
accelerometers: quartz and polycrystalline ceramics. Quartz is a natural
crystal, while ceramics are man-made. Each material offers certain benefits,
and material choice depends on the particular performance features desired of
Quartz is widely known for its ability to perform accurate measurement tasks and
contributes heavily in everyday applications for time and frequency
measurements. Examples include everything from wrist watches and radios to
computers and home appliances. Accelerometers benefit from several unique
properties of quartz. Since quartz is naturally piezoelectric, it has no
tendency to relax to an alternative state and is considered the most stable of
all piezoelectric materials. This important feature provides quartz
accelerometers with long-term stability and repeatability. Also, quartz has
virtually no pyroelectric effect (output due to temperature change), which
provides stability in thermally active environments. Because quartz has a low
capacitance value, the voltage sensitivity is relatively high compared to most
ceramic materials, making it ideal for use in voltage-amplified systems.
Conversely, the charge sensitivity of quartz is low, limiting its usefulness in
charge-amplified systems, where low noise is an inherent feature. The useful
temperature range of quartz is limited to approximately 600 °F (315 °C).
A variety of ceramic materials are used for accelerometers, depending on the
requirements of the particular application. All ceramic materials are man-made
and are forced to become piezoelectric by a polarization process. This process,
known as "poling," exposes the material to a high-intensity electric field.
This process aligns the electric dipoles, causing the material to become
piezoelectric. Unfortunately, this process tends to reverse itself over time
until it exponentially reaches a steady state. If ceramic is exposed to
temperatures exceeding its range or electric fields approaching the poling
voltage, the piezoelectric properties may be drastically altered or destroyed.
Accumulation of high levels of static charge also can have this effect on the
piezoelectric output. PCB uses three classifications of ceramics. First, there
are high-voltage-sensitivity ceramics that are used for accelerometers with
built-in, voltage-amplified circuits. There are high-charge-sensitivity
ceramics that are used for charge mode sensors with temperature ranges to 400
°F (205 °C). This same type of crystal is used in accelerometers that use
built-in charge-amplified circuits to achieve high output signals and high
resolution. Finally, there are high-temperature ceramics that are used for
charge mode accelerometers with temperature ranges to 600 °F (316 °C) for
monitoring of engine manifolds and superheated turbines.
Accelerometer Sensing Systems
Piezoelectric accelerometers can be broken down into two categories that define
their mode of operation. Internally amplified ICP® accelerometers
contain built-in microelectronic signal conditioning. Charge mode
accelerometers contain only the sensing element with no electronics.
ICP, as described earlier, is PCB's registered trademark that stands for
"Integrated Circuit - Piezoelectric" and identifies PCB sensors that
incorporate built-in, signal-conditioning electronics. The built-in electronics
convert the high-impedance charge signal that is generated by the piezoelectric
sensing element into a usable low-impedance voltage signal that can be readily
transmitted, over ordinary two-wire or coaxial cables, to any voltage readout
or recording device. The low-impedance signal can be transmitted over long
cable distances and used in dirty field or factory environments with little
degradation. In addition to providing crucial impedance conversion, ICP sensor
circuitry can also include other signal conditioning features, such as gain,
filtering, and self-test features. The simplicity of use, high accuracy, broad
frequency range, and low cost of ICP accelerometers make them the recommended
type for use in most vibration or shock applications. However, an exception to
this assertion must be made for circumstances in which the temperature, at the
installation point, exceeds the capability of the built-in circuitry. The
routine temperature range of ICP accelerometers is 250 °F (121 °C); specialty
units are available that operate to 350 °F (177 °C).
The electronics within ICP accelerometers require excitation power from a
constant-current regulated, DC voltage source. This power source is sometimes
built into vibration meters, FFT analyzers, and vibration data collectors. A
separate signal conditioner is required when none is built into the readout. In
addition to providing the required excitation, power supplies may also
incorporate additional signal conditioning, such as gain, filtering, buffering,
and overload indication. The typical system set-ups for ICP accelerometers are
Charge Mode Accelerometers
Two Typical ICP® System Set-ups
Charge mode sensors output a high-impedance, electrical charge signal that is
generated by the piezoelectric sensing element. This signal is extremely
sensitive to corruption from environmental influences. To conduct accurate
measurements, it is necessary to condition this signal to a low-impedance
voltage before it can be input to a readout or recording device. A charge
amplifier or in-line charge converter is generally used for this purpose. These
devices utilize high-input-impedance, low-output-impedance inverting amplifiers
with capacitive feedback. Adjusting the value of the feedback capacitor alters
the transfer function or gain of the charge amplifier.
Typically, charge mode accelerometers are used when high temperature
survivability is required. If the measurement signal must be transmitted over
long distances, PCB recommends the use of an in-line charge converter, placed
near the accelerometer. This minimizes the chance of noise. In-line charge
converters can be operated from the same constant-current excitation power
source as ICP® accelerometers for a reduced system cost.
Typical In-Line Charge Converter System
Sophisticated laboratory-style charge amplifiers usually include adjustments for
normalizing the input signal and altering the feedback capacitor to provide the
desired system sensitivity and full-scale amplitude range. Filtering also
conditions the high and low frequency response. Some charge amplifiers provide
dual-mode operation, which provides power for ICP® accelerometers and
conditions charge mode sensors.
Laboratory Charge Amplifier
Because of the high-impedance nature of the output signal generated by charge
mode accelerometers, several important precautionary measures must be followed.
Always use special low-noise coaxial cable between the accelerometer and the
charge amplifier. This cable is specially treated to reduce triboelectric
(motion induced) noise effects. Also, always maintain high insulation
resistance of the accelerometer, cabling, and connectors. To insure high
insulation resistance, all components must be kept dry and clean.
Accelerometer Mounting Considerations
One of the most important considerations in dealing with accelerometer mounting
is the effect the mounting technique has on the accuracy of the usable
frequency response. The accelerometer's operating frequency range is
determined, in most cases, by securely stud mounting the test sensor directly
to the reference standard accelerometer. The direct coupling, stud mounted to a
very smooth surface, generally yields the highest mechanical resonant frequency
and, therefore, the broadest usable frequency range. The addition of any mass
to the accelerometer, such as an adhesive or magnetic mounting base, lowers the
resonant frequency of the sensing system and may affect the accuracy and limits
of the accelerometer's usable frequency range. Also, compliant materials, such
as a rubber interface pad, can create a mechanical filtering effect by
isolating and damping high-frequency transmissibility.
For best measurement results, especially at high frequencies, it is important to
prepare a smooth and flat machined surface where the accelerometer is to be
attached. Inspect the area to ensure that no metal burrs or other foreign
particles interfere with the contacting surfaces. The application of a thin
layer of silicone grease between the accelerometer base and the mounting
surface also assists in achieving a high degree of intimate surface contact
required for best high-frequency transmissibility.
For permanent installations, where a very secure attachment of the accelerometer
to the test structure is preferred, stud mounting is recommended. First, grind
or machine on the test object a smooth, flat area at least the size of the
sensor base, according to the manufacturer's specifications. Then, prepare a
tapped hole in accordance with the supplied installation drawing, ensuring that
the hole is perpendicular to the mounting surface. Install accelerometers with
the mounting stud and make certain that the stud does not bottom in either the
mounting surface or accelerometer base. Most PCB mounting studs have
depth-limiting shoulders that ensure that the stud cannot bottom-out into the
accelerometer's base. Each base incorporates a counterbore so that the
accelerometer does not rest on the shoulder. Acceleration is transmitted from
the structure's surface into the accelerometer's base. Any stud bottoming or
interfering between the accelerometer base and the structure inhibits
acceleration transmission and affects measurement accuracy. When tightening,
apply only the recommended torque to the accelerometer. A thread-locking
compound may be applied to the threads of the mounting stud to guard against
Standard Stud Mount
When installing accelerometers onto thin-walled structures, a cap screw passing
through a hole of sufficient diameter is an acceptable means for securing the
accelerometer to the structure. The screw engagement length should always be
checked to ensure that the screw does not bottom into the accelerometer base. A
thin layer of silicone grease at the mounting interface ensures high-frequency
Typical Screw Mount
Occasionally, mounting by stud or screw is impractical. For such cases, adhesive
mounting offers an alternative mounting method. The use of separate adhesive
mounting bases is recommended to prevent the adhesive from damaging the
accelerometer base or clogging the mounting threads. (Miniature accelerometers
are provided with the integral stud removed to form a flat base.) Most adhesive
mounting bases available from PCB also provide electrical isolation, which
eliminates potential noise pick-up and ground loop problems. The type of
adhesive recommended depends on the particular application. Petro Wax
(available from PCB) offers a very convenient, easily removable approach for
room temperature use. Two-part epoxies offer stiffness, which maintains
high-frequency response and a permanent mount. Other adhesives, such as dental
cement, hot glues, instant glues, and duct putty are also viable options with a
history of success. A variety of other commonly used adhesives are shown below.
Typical Adhesive Mount
The following table contains a list of adhesive types, brand names, and
suggestions for adhesive mounting accelerometers under different conditions.
There is no one "best" adhesive for all applications because of the many
different structural and environmental considerations, such as temporary or
permanent mount, temperature, type of surface finish, and so forth.
A variety of adhesives are available from many manufacturers, who usually
provide specification charts and application bulletins for their adhesives. A
Consumer Report's article, entitled "Which Glue for Which Job" (Jan. 1988),
provides rating information on adhesives. A Popular Science magazine article,
"Secrets of the Superglues" (Feb. 1989), provides informative data on the use
of superglues. Loctite provides an adhesive "Selector Guide" for its products.
For most accelerometer adhesive mounting applications, PCB Series 080 Adhesive
Mounting Bases are suggested. These mounting pads keep the accelerometer base
clean and free of epoxy that may be very difficult to remove. Also, Series 080
Mounting Bases allow the accelerometer to be easily removed from the test
structure without damage to either the sensor or the test object.
Surface flatness, adhesive stiffness, and adhesion strength affect the usable
frequency range of an accelerometer. Almost any mounting method at low
acceleration levels provides the full frequency range of use if the mounting
surface is very flat and the sensor is pressed hard against the surface to
wring out all extra adhesive. Generally, as surface irregularities or the
thickness of the adhesive increase, the usable frequency range decreases.
The less-stiff, temporary adhesives reduce an accelerometer's usable frequency
range much more than the more rigid, harder adhesives. Generally, temporary
adhesives are recommended more for low-frequency (<500 Hz) structural
testing at room temperature. Petro Wax is generally supplied with most of the
accelerometers for a quick, temporary mounting method used during system set-up
and check-out. When quick installation and removal is required over a wide
frequency range up to 10 kHz, use a Series 080A Adhesive Mounting Base with one
of the stiffer, more permanent adhesives. Also, consider a magnetic mount,
using the Series 080A27 Super Magnet with Model 080A20 Steel Adhesive Mounting
Pad for such measurements. For both, the mounting surface must be very flat to
achieve accurate high-frequency information.
Care should be exercised in selecting and testing an adhesive when concern
exists regarding the possible discoloration or damage to the test structure's
surface finish. Test the adhesive first on a hidden location or a sample of the
structure's finish. Temporary adhesives like Petro Wax or beeswax offer a good
solution for quick installation in room-temperature applications. When higher
temperatures are involved, apply a piece of aluminized mylar tape to the test
structure and mount the accelerometer with adhesive base using one of the other
types of adhesives. After the test, the tape can be easily removed with no
damage to the surface finish of the structure.
Magnetic mounting bases offer a very convenient, temporary attachment to
magnetic surfaces. Magnets offering high pull strengths provide best
high-frequency response. Wedged dual-rail magnetic bases are generally used for
installations on curved surfaces, such as motor and compressor housings and
pipes. However, dual-rail magnets usually significantly decrease the
operational frequency range of an accelerometer. For best results, the magnetic
base should be attached to a smooth, flat surface. A thin layer of silicone
grease should be applied between the sensor and magnetic base, as well as
between the magnetic base and the structure. When surfaces are uneven or
non-magnetic, steel pads can be welded or epoxied in place to accept the
magnetic base. Use of such a pad ensures that periodic measurements are taken
from the exact same location. This is an important consideration when trending
Magnet Mounted to Steel
Handheld vibration probes or probe tips on accelerometers are useful when other
mounting techniques are impractical and for evaluating the relative vibration
characteristics of a structure to determine the best location for installing
the accelerometer. Probes are not recommended for general measurement
applications due to a variety of inconsistencies associated with their use.
Orientation and amount of hand pressure applied create variables, which affect
the measurement accuracy. This method is generally used only for frequencies
less than 1000 Hz.
The vibrational characteristics of a structure can be altered by adding mass to
that structure. Since most measurements are conducted to quantify the
structural vibration, any alteration of the vibration leads to an inaccurate
evaluation of the vibration. An accelerometer that is too heavy, with respect
to the test structure, may produce data that does not correctly represent the
vibration of interest. Use care when selecting an accelerometer and mounting
hardware to avoid the effects of mass loading.
Ground Isolation, Ground Noise, and Ground Loops
When installing accelerometers onto electrically conductive surfaces, a
potential exists for ground noise pick-up. Noise from other electrical
equipment and machines that are grounded to the structure, such as motors,
pumps, and generators, can enter the ground path of the measurement signal
through the base of a standard accelerometer. When the sensor is grounded at a
different electrical potential than the signal conditioning and readout
equipment, ground loops can occur. This phenomenon usually results in current
flow at the line power frequency (and harmonics thereof), potential erroneous
data, and signal drift. Under such conditions, it is advisable to electrically
isolate or "float" the accelerometer from the test structure. This can be
accomplished in several ways. Most accelerometers can be provided with an
integral ground isolation base. Some standard models may already include this
feature, while others offer it as an option. Optional ground-isolated models
are identified by the prefix "J"; for example, Model J353B33. The use of
insulating adhesive mounting bases, isolation mounting studs, isolation bases,
and other insulating materials, such as paper beneath a magnetic base, are
effective ground isolation techniques. Be aware that the additional
ground-isolating hardware can reduce the upper frequency limits of the
Cables and Connections
Various Methods for Electrically Isolating Accelerometer
Cables should be securely fastened to the mounting structure with a clamp, tape,
or other adhesive to minimize cable whip and connector strain. Cable whip can
introduce noise, especially in high-impedance signal paths. This phenomenon is
known as the triboelectric effect. Also, cable strain near either electrical
connector can lead to intermittent or broken connections and loss of data.
To protect against potential moisture and dirt contamination, use RTV sealant or
heat-shrinkable tubing on cable connections. O-rings with heat shrink tubing
have proven to be an effective seal for protecting electrical connections for
short-term underwater use. The use of only RTV sealant is generally only used
to protect the electrical connection against chemical splash or mist.
"Waterproof" Sealed Connection
Under high shock conditions or when cables must undergo large amounts of motion,
as with package drop testing applications, the use of a solder connector
adaptor and lightweight ribbon cables are generally recommended. These solder
connector adaptors provide a more durable connection and can be installed onto
the accelerometer with a thread locking compound to prevent loosening. Use of
lightweight cables helps to minimize induced strain at the connector, which can
create an erroneous output signal. Electrical connection fatigue is also
minimized, reducing the possibility of intermittent or open connections and
loss of data. Solder connector adaptors are installed onto the cable with
solder. This easy connection makes this type of connector user- or
field-repairable in times of crisis. Normally, a flexible plastic plug is
placed over the electrical connections for protection, as well as to provide
cable strain relief.
Cable Secured for High Shock
The solder connector adaptor provides an affordable and simplistic method for
making cables in the field. Only solder and a soldering iron are required. No
special tools or equipment are necessary for installation on a cable end.
Because of the reliability and strength of this connection, these connectors
are recommended for use in shock applications.