Piezoelectric
`Accelerometers
`
`Theory and Application
`
`Published by:
`
`Metra Mess- und Frequenztechnik © 2001
`
`Page 1 of 24
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`HAPTIC EX2012
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`Contents
`
`1. Introduction
`1.1. Why Do We Need Accelerometers?
`1.2. The Advantages of Piezoelectric Sensors
`1.3. Instrumentation
`2. Operation and Designs
`2.1. Operation
`2.2. Accelerometer Designs
`2.3. Built-in Electronics
`3. Characteristics
`3.1. Sensitivity
`3.2. Frequency Response
`3.3. Transverse Sensitivity
`3.4. Maximum Acceleration
`3.5. Non-Vibration Environments
`3.5.1. Temperature
`3.5.2. Base Strain
`3.5.3. Magnetic Fields
`3.5.4. Acoustic Noise
`4. Application Information
`4.1. Instrumentation
`4.1.1. Accelerometers With Charge Output
`4.1.2. Accelerometers With Built-in Electronics
`4.1.3. Intelligent Accelerometers to IEEE1451.4
`4.2. Preparing the Measurement
`4.2.1. Mounting Location
`4.2.2. Choosing the Accelerometer
`4.2.3. Mounting Methods
`4.2.4. Cabling
`4.2.5. Calibration
`
`Notice: ICP is a registered trade mark of PCB Piezotronics Inc.
`
`#390
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`08.01
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`1. Introduction
`1.1. Why Do We Need Accelerometers?
`Vibration and shock are present in all areas of our daily lives. They
`may be generated and transmitted by motors, turbines, machine-tools,
`bridges, towers, and even by the human body.
`While some vibrations are desirable, others may be disturbing or even
`destructive. Consequently, there is often a need to understand the
`causes of vibrations and to develop methods to measure and prevent
`them.
`The sensor we manufacture serves as a link between vibrating struc-
`tures and electronic measurement equipment.
`1.2. The Advantages of Piezoelectric Sensors
`The accelerometers Metra has been manufacturing for over 40 years
`utilize the phenomenon of piezoelectricity. They generate an electric
`charge signal proportional to vibration acceleration. The active
`element of Metra accelerometers consists of a specially developed
`ceramic material with excellent piezoelectric properties.
`Piezoelectric accelerometers are widely accepted as the best choice for
`measuring absolute vibration. Compared to the other types of sensors,
`piezoelectric accelerometers have important advantages:
`• Extremely wide dynamic range, low output noise - suitable for
`shock measurement as well as for almost imperceptible vibration
`• Excellent linearity over their dynamic range
`• Wide frequency range
`• Compact yet highly sensitive
`• No moving parts - no wear
`• Self-generating - no external power required
`• Great variety of models available for nearly any purpose
`• Acceleration signal can be integrated to provide velocity and dis-
`placement
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`Instrumentation
`1.3.
`The piezoelectric principle requires no external energy.
`Only alternating acceleration can be measured. This type of acceler-
`ometer is not capable of a true DC response, e.g. gravitation accel-
`eration.
`The high impedance sensor output needs to be converted into a low
`impedance signal first. For processing the sensor signal a variety of
`equipment can be used, such as:
`
`• Time domain equipment, e.g. RMS and peak value meters
`• Frequency analyzers
`• Recorders
`• PC instrumentation
`
`However, the capability of such equipment would be wasted without
`an accurate sensor signal. In many cases the accelerometer is the most
`critical link in the measurement chain. To obtain precise vibration
`signals some basic knowledge about piezoelectric accelerometers is
`required.
`2. Operation and Designs
`2.1. Operation
`The active element of the accelerometer is a piezoelectric material.
`One side of the piezoelectric material is connected to a rigid post at
`the sensor base. A so-called seismic mass is attached to the other side.
`When the accelerometer is subjected to vibration a force is generated
`which acts on the piezoelectric element. This force is equal to the
`product of the acceleration and the seismic mass. Due to the piezoe-
`lectric effect a charge output proportional to the applied force is
`generated. Since the seismic mass is constant the charge output signal
`is proportional to the acceleration of the mass. Over a wide frequency
`range both sensor base and seismic mass have the same acceleration
`magnitude hence the sensor measures the acceleration of the test
`object.
`The piezoelectric element is connected to the Sensor output via a pair
`of electrodes. A summary of basic calculations shows Figure 1.
`
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`q = d33 F
`
`d33 d
`u = F
`e33 A
`
`F = m A
`
`charge sensitivity:
`Bqa = q
`a
`
`voltage sensitivity:
`Bua = u
`a
`
`q
`
`u
`
`F
`
`A d
`
`piezo disk
`F
`electrode area
`thickness
`force
`charge
`voltage
`piezo constants
`
`AdFqud
`
`33, e33
`
`Figure 1: Piezoelectric effect, basic calculations
`
`Some accelerometers feature an integrated electronic circuit which
`converts the high impedance charge output into a low impedance
`voltage signal.
`A piezoelectric accelerometer can be regarded as a mechanical low-
`pass with resonance peak.
`Its equivalent circuit is a charge source in parallel to an inner capaci-
`tor.
`Within the useful operating frequency range the sensitivity is inde-
`pendent of frequency, apart from certain limitations mentioned later
`(see section 3.1).
`The low frequency response mainly depends on the chosen preampli-
`fier. Often it can be adjusted. With voltage amplifiers the low fre-
`quency limit is a function of the RC time constant formed by acceler-
`ometer, cable, and amplifier input capacitance together with the
`amplifier input resistance (see chapter 4.2.4.).
`The upper frequency limit depends on the resonance frequency of the
`accelerometer. In order to have a wider operating frequency range the
`resonance frequency has to be increased. This is usually achieved by
`reducing the seismic mass. However, the lower the seismic mass, the
`lower the sensitivity. Therefore, accelerometers with a high resonance
`frequency are usually less sensitive (e.g. shock accelerometers).
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`Figure 2 shows a typical frequency response curve of an accelerome-
`ter’s electrical output when it is excited by a constant vibration level.
`
`fLf 0
`fr
`
`lower frequency limit
`calibration frequency
`resonance frequency
`
`1.30
`
`1.10
`1.05
`1.00
`0.95
`0.90
`
`0.71
`
`fL 2fL3fL
`
`f 0
`
`0.5fr
`0.2fr
`0.3fr
`
`fr
`
`f
`
`Figure 2: Frequency response curve
`Several useful frequency ranges can be derived from this curve:
`• At about 1/5 the resonance frequency the response of the sensor is
`1.05. This means that the measured error compared to lower fre-
`quencies is 5 %.
`• At approximately 1/3 the resonance frequency the error is 10 %. For
`this reason the “linear” frequency range should be considered lim-
`ited to 1/3 the resonance frequency.
`• The 3 dB limit with about 30 % error is obtained at approximately
`one half times the resonance frequency.
`
`
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`2.2. Accelerometer Designs
`Metra employs 3 mechanical construction designs:
`• Shear system (“KS” types)
`• Compression system (“KD” types)
`• Bender system (“KB” types)
`The reason for using different piezoelectric systems is their individual
`suitability for various measurement tasks and varying sensitivity to
`environmental influences. The following table shows advantages and
`drawbacks of the 3 designs:
`
`Compression
`• high sensi-
`tivity-to-mass
`ratio
`• robustness
`• technological
`advantages
`
`• high tem-
`perature tran-
`sient sensitiv-
`ity
`• high base
`strain sensi-
`tivity
`KD37, KD41,
`KD93
`
`Bender
`• best sensi-
`tivity-to-mass
`ratio
`
`• fragile
`• relatively
`high tem-
`perature tran-
`sient sensitiv-
`ity
`
`KB12, KB103
`
`KS70/71,
`KS80, KS93,
`KS943
`Due to its better performance shear design is used in the majority of
`newly developed accelerometers.
`The main components of the 3 accelerometer designs are shown in the
`following illustrations:
`
`Advantage
`
`
`
`Drawback
`
`
`
`Shear
`• low tem-
`perature tran-
`sient sensitiv-
`ity
`• low base
`strain sensi-
`tivity
`• lower sensi-
`tivity-to-mass
`ratio
`
`Examples
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`case
`seismic mass (ring-shaped)
`piezo ceramics (shear element)
`central post
`wire
`connector
`insulation
`mounting base
`
`Figure 3: Shear Design
`
`case
`central post
`spring
`seismic mass
`piezo ceramics (compression disks)
`electrode
`wire
`connector
`insulation
`mounting base
`Figure 4: Compression Design
`seismic mass
`case
`wire
`insulation
`connector
`cast resin
`post
`electrode
`piezo ceramics (bending beam)
`mounting base
`
`Figure 5: Bender Design
`
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`2.3. Built-in Electronics
`Several of the accelerometers that we manufacture contain a built-in
`preamplifier. It transforms the high impedance charge output of the
`piezo-ceramics into a low impedance voltage signal which can be
`transmitted over long distances. Metra uses the well-established ICP®
`standard for electronic accelerometers ensuring compatibility with a
`variety of equipment. The abbreviation ICP means “Integrated Circuit
`Piezoelectric”. The built-in circuit is powered by a constant current
`source (Figure 6). The vibration signal is transmitted back to the
`supply as a modulated bias voltage. Both supply current and voltage
`output are transmitted via the same line which can be as long as
`several hundred meters. The capacitor CC removes the sensor bias
`voltage from the instrument input. This provides a zero-based AC
`signal. Since output impedance of the signal is very low, specially
`shielded sensor cables are not required, thereby allowing the use of
`low-cost coaxial cables.
`
`ICP Transducer
`
`U
`s
`
`Instrument
`
`Integrated Converter
`Q
`U
`
`Piezo
`System
`
`coaxial cable,
`over 100 m long
`
`I const
`
`C c
`
`R i
`
`C c
`I const
`R i
`Us
`
`Coupling Capacitor
`Constant Supply Current
`Input Resistance
`Supply Voltage of
`Constant Current Source
`
`Figure 6: ICP® principle
`
`The following table shows advantages and drawbacks of ICP® com-
`patible transducers compared to transducers with charge output.
`
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`Advantage
`
`
`
`Drawback
`
`
`
`ICP® Compatible
`Output
`• Fixed sensitivity
`regardless of cable
`length and cable
`quality
`• Low-impedance
`output can be trans-
`mitted over long ca-
`bles in harsh envi-
`ronments
`• Inexpensive signal
`conditioners
`• Constant current
`excitation required
`• Inherent noise source
` Upper operating
`temperature limited to
`120 °C typically
`
`Charge Output
`
`• No power supply
`required
`• No noise, highest
`resolution
`• Wide dynamic range
`• Higher operating tem-
`peratures
`
`• Limited cable length
`• Special low noise cable
`required
`• Charge amplifier re-
`quired
`
`A variety of instruments contain a constant current sensor supply.
`Examples from Metra are the Signal Conditioners of M68 series and
`the Vibration Monitor model M10. The constant current source may
`also be an external unit, for example models M27 and M31.
`Constant current may be between 2 and 20 mA. Zero-bias voltage, i.e.
`the output voltage without excitation, is between 8 and 12V. It varies
`with supply current and temperature. The output signal of the sensor
`oscillates around this bias voltage. It never changes to negative val-
`ues. The upper limit is set by the constant current source supply
`voltage. This supply voltage should be between 20 and 30 V. The
`lower limit is the saturation voltage of the built-in amplifier (about
`0.5 V). Metra guarantees an output span of > ± 6 V for the sensor.
`Figure 7 illustrates the dynamic range of an ICP® compatible sensor.
`Important: Under no circumstances a voltage source without constant
`current regulation should be connected to an ICP® compatible trans-
`ducer.
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`maximum sensor output =
`supply voltage of constant current source
`(20 to 30 V)
`
`positive overload
`
`dynamic range
`
`sensor bias voltage
`(8 to 12 V)
`
`sensor saturation voltage
`(approx. 0.5 V)
`0V
`
`Figure 7: Dynamic range of ICP® compatible transducers
`
`negative overload
`
`In Figure 7 can be seen that ICP® compatible transducers provide an
`intrinsic self-test feature. By means of the bias voltage at the input of
`the instrument the following operating conditions can be detected:
`• UBIAS < 0.5 to 1 V: short-circuit or negative overload
`• 1 V < UBIAS < 18 V: O.K., output within the proper range
`• UBIAS > 18 V: positive overload or input open
`(cable broken or not connected)
`This self-test feature is applied for instance in the M108/116 signal
`conditioners. A multicolor LED indicates the operating condition.
`The lower frequency limit of Metra´s transducers with integrated
`electronics is 0.3Hz for shear accelerometers and 3Hz for compression
`and bender systems. The upper frequency limit mainly depends on the
`mechanical properties of the sensor. In case of longer cables their
`capacitance has to be taken into consideration. Typical coaxial cables
`supplied by Metra have a capacitance of approximately 100pF/m.
`Figure 8 shows the maximum output as a function of frequency. The
`nomogram includes 3 curves for different cable capacitance and
`supply current.
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`Cable capacitance ...
`5nF
`1,6nF
`200nF
`60nF
`1000nF
`320nF
`
`0,5nF
`20nF
`100nF
`
`at Supply current:
`@ 0,1mA
`@ 4mA
`@ 20mA
`
`ûV3 2
`
`1.5
`
`1
`
`0,6
`
`Output span
`
`1
`
`10
`4 5
`3
`2
`Limiting frequency (-3 dB)
`
`kHz
`
`Figure 8 Output span of integrated preamplifiers
`
`3. Characteristics:
`3.1. Sensitivity
`A piezoelectric accelerometer can be regarded as either a charge
`source or a voltage source with high impedance. Consequently, charge
`sensitivity and voltage sensitivity are used to describe the relationship
`between acceleration and output. Both values are measured at 60 or
`80 Hz at room temperature. The total accuracy of this calibration is
`1.8 %, valid under the following conditions:
`f = 80 Hz, T = 21 °C, a = 10 m/s², CCABLE = 150 pF, ICONST = 4 mA.
`The stated accuracy should not be confused with the tolerance of
`nominal sensitivity which is specified for some accelerometers. Model
`KS80, for instance, has ± 5 % sensitivity tolerance. Charge sensitivity
`decreases slightly with increasing frequency. It drops about 2 % per
`decade.
`Before leaving the factory each accelerometer undergoes a thorough
`artificial aging process. Nevertheless, further natural aging can not be
`avoided completely. Typical are -3 % within 3 years. If a high degree
`
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`of accuracy is required, recalibration should be performed (see section
`4.2.5).
`3.2. Frequency Response
`Measurement of frequency response requires mechanical excitation of
`the transducer. Metra uses a specially-designed calibration shaker
`which is driven by a sine generator swept over a frequency range from
`20 (80) up to 40 000 Hz. The acceleration is kept constant over the
`frequency range by means of a feedback signal coming from a refer-
`ence accelerometer. Each accelerometer (except model KD93) is
`supplied with an individual frequency response curve similar to
`Figure 2. The mounted resonance frequency can be identified from
`this curve. The frequency response of the shock accelerometer model
`KD93 is measured electrically.
`Metra performs frequency response measurements under optimum
`operating conditions with the best possible contact between acceler-
`ometer and vibration source. In practice, mounting conditions will be
`less than ideal in many cases and a lower resonance frequency will be
`obtained.
`The frequency response of ICP® compatible transducers may be
`lowered due to long cables (see Figure 8).
`3.3. Transverse Sensitivity
`Transverse sensitivity is the ratio of the output due to acceleration
`applied perpendicular to the sensitive axis divided by the basic sensi-
`tivity. The measurement is made at 40 Hz sine excitation rotating the
`sensor around a vertical axis. A figure-eight curve is obtained for
`transversal sensitivity. Its maximum deflection is the stated value.
`Typical are <5 % for shear accelerometers and <10 % for compres-
`sion and bender models.
`3.4. Maximum Acceleration
`Usually the following limits are specified:
`• â+
`maximum acceleration for positive output direction
`• â-
`maximum acceleration for negative output direction
`• âq
`maximum acceleration for transversal direction.
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`For charge output accelerometers these limits are determined solely by
`the sensor’s construction. If one of these limits is exceeded acciden-
`tally, for example, by dropping the sensor on the ground, the sensor
`will usually still function.
`However, we recommend recalibrating the accelerometer. Continuous
`vibration should not exceed 25 % of the stated limits to avoid wear.
`When highest accuracy is required, acceleration should not be higher
`than 10 % of the limit. Transducers with extremely high maximum
`acceleration are called shock accelerometers, for example model
`KD93 with â=100 000 m/s².
`If the accelerometer is equipped with built-in electronics the limits â+
`and â- are usually set by the output voltage span of the amplifier (see
`section 2.3).
`3.5. Non-Vibration Environments
`3.5.1. Temperature
`3.5.1.1. Operating Temperature Range
`The maximum operating temperature is limited by the piezoelectric
`material. Above a specified temperature, called Curie point, the
`piezoelectric element will begin to depolarize causing a permanent
`loss in sensitivity. The specified maximum operating temperature is
`the limit at which the permanent change of sensitivity is 3 %. Some-
`times other components limit the operating temperature, for example,
`resins or built-in electronics. Typical temperature ranges are -30 ..
`150 °C and -10 .. 80 °C. Accelerometers with built-in electronics are
`generally not suitable for temperatures above 120 °C.
`3.5.1.2. Temperature Coefficients
`Apart from permanent changes, some characteristics vary over the
`operating temperature range. Temperature coefficients are specified
`for charge sensitivity (TK(Bqa)), voltage sensitivity (TK(Bua)), and
`inner capacitance (TK(Ci)). For sensors with built-in electronics only
`TK(Bua) is stated.
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`3.5.1.3. Temperature Transients
`In addition to the temperature characteristics mentioned above, accel-
`erometers exhibit a slowly varying output when subjected to tem-
`perature transients, caused by so-called pyroelectric effect. This is
`specified by temperature transient sensitivity baT. Temperature tran-
`sient outputs are below 10 Hz. Where low frequency measurements
`are made this effect must be taken into consideration. To avoid this
`problem, shear type accelerometers should be chosen for low fre-
`quency measurements. In practice, they are approximately 100 times
`less sensitive to temperature transients than compression sensors.
`Bender systems are midway between the other two systems in terms
`of sensitivity to temperature transients. When compression sensors are
`used the amplifier should be adjusted to a 3 or 10 Hz lower frequency
`limit.
`3.5.2. Base Strain
`When an accelerometer is mounted on a structure which is subjected
`to strain variations, an unwanted output may be generated as a result
`of strain transmitted to the piezoelectric material. This effect can be
`described as base strain sensitivity bas. The stated values are deter-
`mined by means of a bending beam oscillating at 8 or 15 Hz. Base
`strain output usually occurs at frequencies below 500 Hz. Shear type
`accelerometers have extremely low base strain sensitivity and should
`be chosen for strain-critical applications.
`3.5.3. Magnetic Fields
`Strong magnetic fields often occur around electric machines at 50Hz
`and multiples. Magnetic field sensitivity baB has been measured at
`B=0.01 T and 50 Hz for some accelerometers. It is very low and can
`be ignored under normal conditions. However, adequate isolation
`must be provided against ground loops using accelerometers with
`insulated bases (for instance models KS74 and KS80) or insulating
`mounting studs. Stray signal pickup can be avoided by proper cable
`shielding. This is of particular importance for sensors with charge
`output.
`3.5.4. Acoustic Noise
`If an accelerometer is exposed to a very high noise level, a deforma-
`tion of the sensor case may occur which can be measured as an output
`under extreme conditions. Acoustic noise sensitivity bap as stated for
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`some models is measured at an SPL of 154 dB. Acoustic noise sensi-
`tivity should not be confused with the sensor response to pressure
`induced motion of the structure on which it is mounted.
`4. Application Information
`4.1.
`Instrumentation
`4.1.1. Accelerometers With Charge Output
`The charge output of piezoelectric accelerometers without integrated
`electronics needs to be converted and amplified into a low impedance
`voltage. Preferably, charge amplifiers should be used, for example
`Metra M68 series Signal Conditioners and ICP100 series Remote
`Charge Converters. Some instruments, e.g. analyzers, recorders and
`data acquisition boards, are also equipped with charge inputs.
`Alternatively, high impedance voltage amplifiers are suitable. How-
`ever, some restrictions have to be taken into consideration (see section
`4.2.5).
`4.1.2. Accelerometers With Built-in Electronics
`These transducers are less susceptible to electromagnetic influences
`via the cable. They can be used with standard coaxial cables of 100 m
`length and more. The input of the instrument can either supply the
`constant current for the built-in amplifier (e.g. M68 series Signal
`Conditioners, M108/116 Signal Conditioners, M10 Vibration Moni-
`tors) or an external supply unit may be used instead (models M27 or
`M31). The principle of ICP® supply is shown in Figure 6.
`
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`Intelligent Accelerometers to IEEE1451.4
`4.1.3.
`The standard IEEE 1451, discussed in recent time, complies with the
`increasing importance of digital data acquisition systems. IEEE 1451
`mainly defines the protocol and network structure for sensors with
`fully digital output. The part IEEE 1451.4, however, deals with
`"Mixed Mode Sensors", which have a conventional ICP® compatible
`output, but contain in addition a memory for an “Electronic Data
`Sheet”. This data storage is named "TEDS" (Transducer Electronic
`Data Sheet). The memory of 256 bits contains all important technical
`data, which are of interest for the user:
`• Model and version number
`• Serial number
`• Manufacturer
`• Type of transducer; physical quantity
`• Sensitivity
`• Last calibration date
`In addition to this data, programmed by the manufacturer, the user for
`itself can store information for identification of the measuring point.
`The Transducer Electronic Data Sheet opens up a lot of new possi-
`bilities to the user:
`• When measuring at many measuring points it will make it easier to
`identify the different sensors as belonging to a particular input. It is
`not necessary to mark and track the cable, which takes up a great
`deal of time.
`• The measuring system reads the calibration data automatically. Till
`now it was necessary to have a data base with the technical specifi-
`cation of the different transducers, like serial number, measured
`quantity, sensitivity etc.
`• You can change a transducer with a minimum of time and work
`("Plug & Play"), because of the sensor self-identification.
`• The data sheet of a transducer is a document which disappears very
`often. The so called TEDS sensor contains all necessary technical
`specification. Therefore, you are able to execute the measurement,
`even if the data sheet is just not at hand.
`The standard IEEE 1451.4 is based on the ICP® principle. TEDS
`sensors, therefore, can be used instead of common ICP® transducers.
`The communication with the 256 bit non-volatile memory of the
`transducer, Type DS2430A, is based on the 1-Wire®-protocol of
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`Dallas Semiconductor. The software protocol can be part of the
`instrument’s firmware. It is also possible, however, to read and write
`the TEDS data via a simple hardware adapter by a PC.
`Metra will equip both sensors and instruments with TEDS function.
`Useful instrument applications are vibration calibrators (e.g. model
`VC100) and signal conditioners (e.g. M108/116).
`4.2. Preparing the Measurement
`4.2.1. Mounting Location
`In order to achieve optimum measurement conditions the following
`questions should be answered:
`• At the selected location, is it likely that can you make unadulterated
`measurements of the vibration and derive the needed information?
`• Does the selected location provide a short and rigid path to the
`vibration source?
`• Is it allowable and possible to prepare a flat, smooth, and clean
`surface with mounting thread for the accelerometer?
`• Can the accelerometer be mounted so that it doesn’t alter the vibra-
`tion characteristics of the test object?
`• Which environmental influences (heat, humidity, EMI, bending etc.)
`may occur?
`
`
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`4.2.2. Choosing the Accelerometer
`Criteria
`Accelerometer Properties
`Magnitude and frequency range
`sensitivity, max. acceleration,
`resonance frequency
`max. weight of accelerometer
`1/10 the weight of test object
`assess influence, choose sensor
`according to characteristics
`
`Weight of test object
`
`Temperature transients, strain,
`magnetic field, extreme acoustic
`noise, humidity
`Measurement of vibration
`velocity and displacement
`
`Mounting
`• quick spot measurement below
`1000Hz
`• temporary measurement
`
`• long-term measurement
`
`Grounding problems
`
`for integration below 20Hz
`preferably use shear accelerome-
`ters
`
`use probe
`
`use clamping magnet, wax or
`adhesive
`use mounting stud, screws, prefer
`sensor with fixed cable
`use insulating flange or insulated
`sensors
`accelerometer with built-in
`electronics (ICP® compatible)
`
`Long distance between sensor
`and instrument
`4.2.3. Mounting Methods
`Choosing the optimum mounting arrangement can significantly affect
`the accuracy.
`
`For best performance at high frequencies, the accelerometer base
`and the test object should have flat, smooth, unscratched, burr-free
`and, if possible, polished surfaces.
`
`The following mounting accessories are supplied by Metra:
`
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`

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`Probe
`No. 001
`
`Adhesive Wax
`No. 002
`
`Mounting Studs
`Nos. 003 (M5) /
`021 (M3) / 042
`(M6) / 043 (M8) /
`045 (adapter M5
`to UNF 10-32)
`Mounting
`Magnet
`No. 008
`
`
`
`Insulating Studs
`No. 006
`No. 029
`
`Cable Clamps
`No. 004
`No. 020
`
`Attach the accelerometer via the M5 thread,
`press onto the test object perpendicularly
` for estimating and trending measurements
`above 5 Hz and below 1000Hz
`Roll wax with the fingers to soften,
`smear onto the test surface (not too thick),
`press sensor onto the wax
` for quick mounting of light sensors at room
`temperature and low acceleration
`Mounting thread required in test object, apply
`thin layer of silicon grease between sensor and
`test surface for better high frequency perform-
`ance, recommended torque: 1 Nm
` for best performance, good for permanent
`mounting
`Accelerometer with mounting thread M5 re-
`quired, magnetic object with smooth surface
`required, if not available, weld or epoxy a steel
`mounting pad to the test surface, apply thin layer
`of silicon grease between sensor and test surface
`and between magnet and sensor for better high
`frequency performance.
`Don’t drop the magnet onto the test object to
`protect the sensor from shock acceleration.
`Gently slide the sensor with the magnet to the
`place.
` for rapid mounting with limited high
`frequency performance
`Screw onto the accelerometer, 029 for
`adhesive attachment using cyanoacrylate,
`006 not recommended above 100 °C
` avoids grounding problems
`To be screwed onto the test object together with
`the accelerometer
` avoids introduction of force via the cable
`into the transducer
`
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`Figure 9 compares the high frequency performance of different
`mounting methods as a result of added mass and reduced mounting
`stiffness.
`40
`dB
`30
`
`ed
`
`a probe
`b insulating stud
`c mounting magnet
`d adhesive mount
`e stud mount
`
`20lg
`
`Bua(f)
`Bua(f0)
`
`a
`
`b
`
`c
`
`20
`
`10
`
`0
`
`-10
`
`0.1
`0.5
`1
`5
`10
`20
`kHz
`Figure 9: Resonance frequencies of different mounting methods
`
`4.2.4. Cabling
`Choosing the right sensor cable is of particular importance for accel-
`erometers with charge output. When a coaxial cable is subjected to
`bending or tension this may generate local changes in capacitance.
`They will result in charge transport, the so-called triboelectric effect.
`The produced charge signal cannot be distinguished from the sensor
`output. It can be troublesome when measuring low vibration with
`charge transducers. Therefore Metra supplies all charge transducers
`with a special low noise cable. This cable has a special dielectric with
`noise reduction treatment. However, it is still recommended to clamp
`the cable to the test object, e.g. by adhesive tape.
`ICP® compatible transducers do not require special low noise cables.
`They can be connected with any standard coaxial cable.
`Strong electromagnetic fields can induce error signals, particularly
`when charge transducers are used. Therefore it is recommended to
`route the sensor cable as far away as possible from electromagnetic
`sources.
`In compression designs (i.e. Metra´s „KD“ models), bending forces
`can be transmitted via the cable connection into the sensing element
`and thereby induce errors. Therefore the cable should be prevented
`from vibrating. This can be done by the cable clamp belonging to the
`accessories set of compression sensors.
`
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`Before starting the measurement, make sure that all connectors are
`carefully tightened. Loose connector nuts are a common source of
`measuring errors. A small amount of thread-locking compound can be
`applied on the connector.
`Metra standard accelerometer cables use the following connectors:
`• Microdot: coaxial connector with UNF 10-32 thread
`• Subminiature: coaxial connector with M3 thread
`• TNC: coaxial connector with UNF7/16-28 thread and IP44
`• BNC: coaxial connector with bayonet closure
`• Binder 711: circular 4 pin connector with M8x1 thread
`• Binder 715: circular 4 pin connector with M12 thread and IP67
`The following cables are available from Metra:
`Purpose
`Plug 1
`Plug 2
`Length
`m
`1.5
`charge transducers Microdot Microdot
`1.5
`charge transducers Microdot Microdot
`1.5
`charge transducers TNC
`Microdot
`charge transducers Subminiat. Subminiat. 1.5
`charge transducers Microdot Microdot
`5
`charge transducers Microdot Microdot
`10
`charge transducers Microdot Microdot
`15
`charge transducers Microdot Microdot
`20
`ICP® transducers Microdot Microdot
`1.5
`ICP® transducers BNC
`Microdot
`1.5
`ICP® transducers
`TNC
`Microdot
`1.5
`ICP® transducers
`TNC
`BNC
`1.5
`ICP® transducers
`Subminiat. Microdot
`1.5
`
`TMAX
`°C
`m
`2.2 80
`2.0 200
`2.2 80
`2.2 80
`3.8 80
`3.8 80
`3.8 80
`3.8 80
`2.5 80
`2.5 80
`2.5 80
`2.5 80
`1
`120
`
`∅m
`
`Mod.
`
`009
`009/T
`012
`013
`010/5
`010/10
`010/15
`010/20
`050
`051
`052
`053
`054
`
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`Additionally Metra offers a selection of plug adapters:
`Purpose
`Model
`017
`Adapter Microdot plug to BNC socket
`025
`Adapter Microdot plug to TNC socket
`016
`Coupler for Microdot plugs
`032
`Microdot socket for front panel mounting
`033
`Adapter Binder 711 to 3 Microdot plugs
`034
`Adapter Binder 711 to 3 BNC plugs
`4.2.5. Calibration
`Under normal conditions, piezoelectric sensors are extremely stable
`and their calibrated performance characteristics do not change over
`time. However, often sensors are exposed to harsh environmental
`conditions, like mechanical shock, temperature changes, humidity etc.
`Therefore it is recommended to establish a recalibration cycle. We
`recommend that accelerometers should be recalibrated every time
`after use under severe conditions or at least every 2 years.
`For factory recalibration service, send the transducer to Metra. Our
`calibration service is based on a transfer standard which is regularly
`sent to the Physikalisch-Technische Bundesanstalt (PTB).
`Often it is desirable to calibrate the vibration sensor including all
`measuring instruments as a complete chain by means of a constant
`vibration signal. This can be performed using a Vibration Calibrator
`of Metra’s VC10 series. The VC10 calibrator supplies a constant
`vibration of 10 m/s² acceleration, 10 mm/s velocity, and 10 μm
`displacement at 159.2 Hz controlled by an internal quartz generator.
`The VC100 Vibration Calibrating System has an adjustable vibration
`frequency between 70 and 10,000 Hz at 1 m/s² vibration level. It can
`be controlled by a PC software. An LCD display shows the sensitivity
`of the sensor to be calibrated.
`Many companies choose to purchase own calibration equipment to
`perform recalibration themselves. This may save calibration cost,
`particularly if a larger number of transducers is in use.
`If no calibrator is at hand, calibration can be performed electrically
`either by
`
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`

`• Adjusting the amplifier gain to the stated accelerometer sensitivity
`• Typing in the stated sensitivity when using a PC based data acquisi-
`tion system
`• Replacing the accelerometer by a generator signal and measuring
`the equivalent magnitude
`When charge output accelerometers are used together with a high
`impedance voltage amplifier, the capacitance of sensor, cable, and
`amplifier input has to be taken into consideration.
`Figure 10 shows how to calculate the ac