Download Introduction to Piezoelectric Accelerometers

Introduction to Piezoelectric Accelerometers
Determining phase and amplitude relationships of vibration at various points on a structure permits modal modeling. The resulting modal
model provides valuable information regarding system integrity and operating mode shapes.
Seismic vibration - Sensors are used for measuring ground, building, or bridge vibration on very large structures to detect structural
changes caused by earthquakes, construction, mining activity, and large transportation vehicles.
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 separation.
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 configuration.
Shear Mode
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
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
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.
Upright Compression
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.
Inverted Compression
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.
Isolated Compression
Piezoelectric Material
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 the accelerometer.
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 hightemperature 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® Accelerometers
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 shown below.
Two Typical ICP® System Set-ups
Charge Mode Accelerometers
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
Frequency Response
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.
Surface Preparation
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.
Stud Mounting
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 loosening.
Standard Stud Mount
Screw Mounting
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
transmissibility.
Typical Screw Mount
Adhesive Mounting
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
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 nonmagnetic, 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 measurement data.
Magnet Mounted to Steel
Probe tips
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.
Mass Loading
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 groundisolated 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 accelerometer.
Various Methods for Electrically Isolating Accelerometer
Cables and Connections
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.