PiezoelectricAccelerometers

Theory and Application

Published by:

Metra Mess- und Frequenztechnik © 2001

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Contents

1. Introduction 3

1.1. Why Do We Need Accelerometers? 31.2. The Advantages of Piezoelectric Sensors 31.3. Instrumentation 4

2. Operation and Designs 4

2.1. Operation 42.2. Accelerometer Designs 72.3. Built-in Electronics 9

3. Characteristics 12

3.1. Sensitivity 123.2. Frequency Response 133.3. Transverse Sensitivity 133.4. Maximum Acceleration 133.5. Non-Vibration Environments 14

3.5.1. Temperature 143.5.2. Base Strain 153.5.3. Magnetic Fields 153.5.4. Acoustic Noise 15

4. Application Information 16

4.1. Instrumentation 164.1.1. Accelerometers With Charge Output 164.1.2. Accelerometers With Built-in Electronics 164.1.3. Intelligent Accelerometers to IEEE1451.4 17

4.2. Preparing the Measurement 184.2.1. Mounting Location 184.2.2. Choosing the Accelerometer 194.2.3. Mounting Methods 194.2.4. Cabling 214.2.5. Calibration 23

Notice: ICP is a registered trade mark of PCB Piezotronics Inc.

#390 08.01

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1. Introduction1.1. Why Do We Need Accelerometers?

Vibration and shock are present in all areas of our daily lives. Theymay 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 evendestructive. Consequently, there is often a need to understand thecauses of vibrations and to develop methods to measure and preventthem.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 yearsutilize the phenomenon of piezoelectricity. They generate an electriccharge signal proportional to vibration acceleration. The activeelement of Metra accelerometers consists of a specially developedceramic material with excellent piezoelectric properties.Piezoelectric accelerometers are widely accepted as the best choice formeasuring 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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1.3. Instrumentation

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 lowimpedance signal first. For processing the sensor signal a variety ofequipment 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 withoutan accurate sensor signal. In many cases the accelerometer is the mostcritical link in the measurement chain. To obtain precise vibrationsignals some basic knowledge about piezoelectric accelerometers isrequired.

2. Operation and Designs2.1. Operation

The active element of the accelerometer is a piezoelectric material.One side of the piezoelectric material is connected to a rigid post atthe sensor base. A so-called seismic mass is attached to the other side.When the accelerometer is subjected to vibration a force is generatedwhich acts on the piezoelectric element. This force is equal to theproduct of the acceleration and the seismic mass. Due to the piezoe-lectric effect a charge output proportional to the applied force isgenerated. Since the seismic mass is constant the charge output signalis proportional to the acceleration of the mass. Over a wide frequencyrange both sensor base and seismic mass have the same accelerationmagnitude hence the sensor measures the acceleration of the testobject.The piezoelectric element is connected to the Sensor output via a pairof electrodes. A summary of basic calculations shows Figure 1.

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F

F

A

d u

q

electrode areathicknessforcechargevoltagepiezo constants

AdFqud33, e33

piezo disk

q = d33 F

u = Fd33 de33 A

F = m A

charge sensitivity:

Bqa = qa

Bua = ua

voltage sensitivity:

Figure 1: Piezoelectric effect, basic calculations

Some accelerometers feature an integrated electronic circuit whichconverts the high impedance charge output into a low impedancevoltage 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 theamplifier input resistance (see chapter 4.2.4.).The upper frequency limit depends on the resonance frequency of theaccelerometer. In order to have a wider operating frequency range theresonance frequency has to be increased. This is usually achieved byreducing the seismic mass. However, the lower the seismic mass, thelower the sensitivity. Therefore, accelerometers with a high resonancefrequency 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.

f 0 fr0.5fr0.3fr

0.2fr ffL 2fL3fL

0.900.951.001.051.10

1.30

0.71

fLf0fr

lower frequency limitcalibration frequencyresonance frequency

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 %. Forthis 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 approximatelyone 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 individualsuitability for various measurement tasks and varying sensitivity toenvironmental influences. The following table shows advantages anddrawbacks of the 3 designs:

Shear Compression BenderAdvantage

�

• low tem-perature tran-sient sensitiv-ity

• low basestrain sensi-tivity

• high sensi-tivity-to-massratio

• robustness• technological

advantages

• best sensi-tivity-to-massratio

Drawback

�

• lower sensi-tivity-to-massratio

• high tem-perature tran-sient sensitiv-ity

• high basestrain sensi-tivity

• fragile• relatively

high tem-perature tran-sient sensitiv-ity

Examples KS70/71,KS80, KS93,KS943

KD37, KD41,KD93

KB12, KB103

Due to its better performance shear design is used in the majority ofnewly developed accelerometers.The main components of the 3 accelerometer designs are shown in thefollowing illustrations:

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caseseismic mass (ring-shaped)piezo ceramics (shear element)central postwireconnectorinsulationmounting base

Figure 3: Shear Design

casecentral postspringseismic masspiezo ceramics (compression disks)electrodewireconnectorinsulationmounting base

Figure 4: Compression Design

seismic masscasewireinsulationconnectorcast resinpostelectrodepiezo 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-inpreamplifier. It transforms the high impedance charge output of thepiezo-ceramics into a low impedance voltage signal which can betransmitted over long distances. Metra uses the well-established ICP®

standard for electronic accelerometers ensuring compatibility with avariety of equipment. The abbreviation ICP means “Integrated CircuitPiezoelectric”. The built-in circuit is powered by a constant currentsource (Figure 6). The vibration signal is transmitted back to thesupply as a modulated bias voltage. Both supply current and voltageoutput are transmitted via the same line which can be as long asseveral hundred meters. The capacitor CC removes the sensor biasvoltage from the instrument input. This provides a zero-based ACsignal. Since output impedance of the signal is very low, speciallyshielded sensor cables are not required, thereby allowing the use oflow-cost coaxial cables.

Integrated Converter

U Instrument

I const

s

Q UPiezoSystem

C c R i

C cIconstR i

Coupling Capacitor

Constant Supply Current

Input ResistanceU

s Supply Voltage of Constant Current Source

coaxial cable,over 100 m long

ICP Transducer

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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ICP® CompatibleOutput

Charge Output

Advantage

��

• Fixed sensitivityregardless of cablelength and cablequality

• Low-impedanceoutput can be trans-mitted over long ca-bles in harsh envi-ronments

• Inexpensive signalconditioners

• No power supplyrequired

• No noise, highestresolution

• Wide dynamic range• Higher operating tem-

peratures

Drawback

��

• Constant currentexcitation required

• Inherent noise source Upper operating

temperature limited to120 °C typically

• 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 andthe Vibration Monitor model M10. The constant current source mayalso 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 varieswith supply current and temperature. The output signal of the sensoroscillates around this bias voltage. It never changes to negative val-ues. The upper limit is set by the constant current source supplyvoltage. This supply voltage should be between 20 and 30 V. Thelower limit is the saturation voltage of the built-in amplifier (about0.5 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 constantcurrent 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)

sensor saturation voltage(approx. 0.5 V)

sensor bias voltage(8 to 12 V)

negative overload

dyna

mic

ran

ge

0V

positive overload

Figure 7: Dynamic range of ICP® compatible transducers

In Figure 7 can be seen that ICP® compatible transducers provide anintrinsic self-test feature. By means of the bias voltage at the input ofthe 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 signalconditioners. A multicolor LED indicates the operating condition.The lower frequency limit of Metra´s transducers with integratedelectronics is 0.3Hz for shear accelerometers and 3Hz for compressionand bender systems. The upper frequency limit mainly depends on themechanical properties of the sensor. In case of longer cables theircapacitance has to be taken into consideration. Typical coaxial cablessupplied by Metra have a capacitance of approximately 100pF/m.Figure 8 shows the maximum output as a function of frequency. Thenomogram includes 3 curves for different cable capacitance andsupply current.

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ûV3

2

1.5

1

0,6

Cable capacitance ... at Supply current:

@ 0,1mA@ 4mA

@ 20mA

1 2 3 4 5 10 kHz

5nF200nF

1000nF

1,6nF60nF

320nF

0,5nF20nF

100nF

Limiting frequency (-3 dB)

Out

put s

pan

Figure 8 Output span of integrated preamplifiers

3. Characteristics:

3.1. Sensitivity

A piezoelectric accelerometer can be regarded as either a chargesource or a voltage source with high impedance. Consequently, chargesensitivity and voltage sensitivity are used to describe the relationshipbetween acceleration and output. Both values are measured at 60 or80 Hz at room temperature. The total accuracy of this calibration is1.8 %, 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 ofnominal sensitivity which is specified for some accelerometers. ModelKS80, for instance, has ± 5 % sensitivity tolerance. Charge sensitivitydecreases slightly with increasing frequency. It drops about 2 % perdecade.Before leaving the factory each accelerometer undergoes a thoroughartificial aging process. Nevertheless, further natural aging can not beavoided completely. Typical are -3 % within 3 years. If a high degree

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of accuracy is required, recalibration should be performed (see section4.2.5).

3.2. Frequency Response

Measurement of frequency response requires mechanical excitation ofthe transducer. Metra uses a specially-designed calibration shakerwhich is driven by a sine generator swept over a frequency range from20 (80) up to 40 000 Hz. The acceleration is kept constant over thefrequency range by means of a feedback signal coming from a refer-ence accelerometer. Each accelerometer (except model KD93) issupplied with an individual frequency response curve similar toFigure 2. The mounted resonance frequency can be identified fromthis curve. The frequency response of the shock accelerometer modelKD93 is measured electrically.Metra performs frequency response measurements under optimumoperating conditions with the best possible contact between acceler-ometer and vibration source. In practice, mounting conditions will beless than ideal in many cases and a lower resonance frequency will beobtained.The frequency response of ICP® compatible transducers may belowered due to long cables (see Figure 8).

3.3. Transverse Sensitivity

Transverse sensitivity is the ratio of the output due to accelerationapplied perpendicular to the sensitive axis divided by the basic sensi-tivity. The measurement is made at 40 Hz sine excitation rotating thesensor around a vertical axis. A figure-eight curve is obtained fortransversal 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 bythe sensor’s construction. If one of these limits is exceeded acciden-tally, for example, by dropping the sensor on the ground, the sensorwill usually still function.However, we recommend recalibrating the accelerometer. Continuousvibration should not exceed 25 % of the stated limits to avoid wear.When highest accuracy is required, acceleration should not be higherthan 10 % of the limit. Transducers with extremely high maximumacceleration are called shock accelerometers, for example modelKD93 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 (seesection 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 piezoelectricmaterial. Above a specified temperature, called Curie point, thepiezoelectric element will begin to depolarize causing a permanentloss in sensitivity. The specified maximum operating temperature isthe 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 aregenerally not suitable for temperatures above 120 °C.

3.5.1.2. Temperature Coefficients

Apart from permanent changes, some characteristics vary over theoperating temperature range. Temperature coefficients are specifiedfor charge sensitivity (TK(Bqa)), voltage sensitivity (TK(Bua)), andinner capacitance (TK(Ci)). For sensors with built-in electronics onlyTK(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 isspecified by temperature transient sensitivity baT. Temperature tran-sient outputs are below 10 Hz. Where low frequency measurementsare made this effect must be taken into consideration. To avoid thisproblem, shear type accelerometers should be chosen for low fre-quency measurements. In practice, they are approximately 100 timesless sensitive to temperature transients than compression sensors.Bender systems are midway between the other two systems in termsof sensitivity to temperature transients. When compression sensors areused the amplifier should be adjusted to a 3 or 10 Hz lower frequencylimit.

3.5.2. Base Strain

When an accelerometer is mounted on a structure which is subjectedto strain variations, an unwanted output may be generated as a resultof strain transmitted to the piezoelectric material. This effect can bedescribed as base strain sensitivity bas. The stated values are deter-mined by means of a bending beam oscillating at 8 or 15 Hz. Basestrain output usually occurs at frequencies below 500 Hz. Shear typeaccelerometers have extremely low base strain sensitivity and shouldbe chosen for strain-critical applications.

3.5.3. Magnetic Fields

Strong magnetic fields often occur around electric machines at 50Hzand multiples. Magnetic field sensitivity baB has been measured atB=0.01 T and 50 Hz for some accelerometers. It is very low and canbe ignored under normal conditions. However, adequate isolationmust be provided against ground loops using accelerometers withinsulated bases (for instance models KS74 and KS80) or insulatingmounting studs. Stray signal pickup can be avoided by proper cableshielding. This is of particular importance for sensors with chargeoutput.

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 outputunder 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 pressureinduced motion of the structure on which it is mounted.

4. Application Information4.1. Instrumentation

4.1.1. Accelerometers With Charge Output

The charge output of piezoelectric accelerometers without integratedelectronics needs to be converted and amplified into a low impedancevoltage. Preferably, charge amplifiers should be used, for exampleMetra M68 series Signal Conditioners and ICP100 series RemoteCharge Converters. Some instruments, e.g. analyzers, recorders anddata 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 section4.2.5).

4.1.2. Accelerometers With Built-in Electronics

These transducers are less susceptible to electromagnetic influencesvia the cable. They can be used with standard coaxial cables of 100 mlength and more. The input of the instrument can either supply theconstant current for the built-in amplifier (e.g. M68 series SignalConditioners, M108/116 Signal Conditioners, M10 Vibration Moni-tors) or an external supply unit may be used instead (models M27 orM31). The principle of ICP® supply is shown in Figure 6.

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4.1.3. Intelligent Accelerometers to IEEE1451.4

The standard IEEE 1451, discussed in recent time, complies with theincreasing importance of digital data acquisition systems. IEEE 1451mainly defines the protocol and network structure for sensors withfully digital output. The part IEEE 1451.4, however, deals with"Mixed Mode Sensors", which have a conventional ICP® compatibleoutput, but contain in addition a memory for an “Electronic DataSheet”. This data storage is named "TEDS" (Transducer ElectronicData Sheet). The memory of 256 bits contains all important technicaldata, which are of interest for the user:• Model and version number• Serial number• Manufacturer• Type of transducer; physical quantity• Sensitivity• Last calibration dateIn addition to this data, programmed by the manufacturer, the user foritself 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 isnot necessary to mark and track the cable, which takes up a greatdeal of time.

• The measuring system reads the calibration data automatically. Tillnow it was necessary to have a data base with the technical specifi-cation of the different transducers, like serial number, measuredquantity, 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 veryoften. The so called TEDS sensor contains all necessary technicalspecification. 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. TEDSsensors, therefore, can be used instead of common ICP® transducers.The communication with the 256 bit non-volatile memory of thetransducer, Type DS2430A, is based on the 1-Wire®-protocol of

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Dallas Semiconductor. The software protocol can be part of theinstrument’s firmware. It is also possible, however, to read and writethe 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. modelVC100) 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 followingquestions should be answered:

• At the selected location, is it likely that can you make unadulteratedmeasurements of the vibration and derive the needed information?

• Does the selected location provide a short and rigid path to thevibration source?

• Is it allowable and possible to prepare a flat, smooth, and cleansurface 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 PropertiesMagnitude and frequency range sensitivity, max. acceleration,

resonance frequencyWeight of test object max. weight of accelerometer

1/10 the weight of test objectTemperature transients, strain,magnetic field, extreme acousticnoise, humidity

assess influence, choose sensoraccording to characteristics

Measurement of vibrationvelocity and displacement

for integration below 20Hzpreferably use shear accelerome-ters

Mounting• quick spot measurement below

1000Hz• temporary measurement

• long-term measurement

use probe

use clamping magnet, wax oradhesiveuse mounting stud, screws, prefersensor with fixed cable

Grounding problems use insulating flange or insulatedsensors

Long distance between sensorand instrument

accelerometer with built-inelectronics (ICP® compatible)

4.2.3. Mounting Methods

Choosing the optimum mounting arrangement can significantly affectthe accuracy.

For best performance at high frequencies, the accelerometer baseand the test object should have flat, smooth, unscratched, burr-freeand, if possible, polished surfaces.

The following mounting accessories are supplied by Metra:

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ProbeNo. 001

Attach the accelerometer via the M5 thread,press onto the test object perpendicularly� for estimating and trending measurements

above 5 Hz and below 1000HzAdhesive WaxNo. 002

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 accelerationMounting StudsNos. 003 (M5) /021 (M3) / 042(M6) / 043 (M8) /045 (adapter M5to UNF 10-32)

Mounting thread required in test object, applythin layer of silicon grease between sensor andtest surface for better high frequency perform-ance, recommended torque: 1 Nm� for best performance, good for permanent

mountingMountingMagnetNo. 008

�

Accelerometer with mounting thread M5 re-quired, magnetic object with smooth surfacerequired, if not available, weld or epoxy a steelmounting pad to the test surface, apply thin layerof silicon grease between sensor and test surfaceand between magnet and sensor for better highfrequency performance.Don’t drop the magnet onto the test object toprotect the sensor from shock acceleration.Gently slide the sensor with the magnet to theplace.� for rapid mounting with limited high

frequency performanceInsulating StudsNo. 006No. 029

Screw onto the accelerometer, 029 foradhesive attachment using cyanoacrylate,006 not recommended above 100 °C� avoids grounding problems

Cable ClampsNo. 004No. 020

To be screwed onto the test object together withthe accelerometer� avoids introduction of force via the cable

into the transducer

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Figure 9 compares the high frequency performance of differentmounting methods as a result of added mass and reduced mountingstiffness.

0.5 1 5 10 200.1

0

10

20

30

40

-10kHz

dB20lg

Bua(f)

Bua(f0)

a b

c

ed

a probe

b insulating stud

c mounting magnet

d adhesive mount

e stud mount

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 tobending 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 sensoroutput. It can be troublesome when measuring low vibration withcharge transducers. Therefore Metra supplies all charge transducerswith a special low noise cable. This cable has a special dielectric withnoise reduction treatment. However, it is still recommended to clampthe 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, particularlywhen charge transducers are used. Therefore it is recommended toroute the sensor cable as far away as possible from electromagneticsources.In compression designs (i.e. Metra´s „KD“ models), bending forcescan be transmitted via the cable connection into the sensing elementand thereby induce errors. Therefore the cable should be preventedfrom vibrating. This can be done by the cable clamp belonging to theaccessories set of compression sensors.

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Before starting the measurement, make sure that all connectors arecarefully tightened. Loose connector nuts are a common source ofmeasuring errors. A small amount of thread-locking compound can beapplied 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 Lengthm

∅mm

TMAX

°CMod.

charge transducers Microdot Microdot 1.5 2.2 80 009charge transducers Microdot Microdot 1.5 2.0 200 009/Tcharge transducers TNC Microdot 1.5 2.2 80 012charge transducers Subminiat. Subminiat. 1.5 2.2 80 013charge transducers Microdot Microdot 5 3.8 80 010/5charge transducers Microdot Microdot 10 3.8 80 010/10charge transducers Microdot Microdot 15 3.8 80 010/15charge transducers Microdot Microdot 20 3.8 80 010/20ICP® transducers Microdot Microdot 1.5 2.5 80 050ICP® transducers BNC Microdot 1.5 2.5 80 051ICP® transducers TNC Microdot 1.5 2.5 80 052ICP® transducers TNC BNC 1.5 2.5 80 053ICP® transducers Subminiat. Microdot 1.5 1 120 054

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Additionally Metra offers a selection of plug adapters:

Purpose ModelAdapter Microdot plug to BNC socket 017Adapter Microdot plug to TNC socket 025Coupler for Microdot plugs 016Microdot socket for front panel mounting 032Adapter Binder 711 to 3 Microdot plugs 033Adapter Binder 711 to 3 BNC plugs 034

4.2.5. Calibration

Under normal conditions, piezoelectric sensors are extremely stableand their calibrated performance characteristics do not change overtime. However, often sensors are exposed to harsh environmentalconditions, like mechanical shock, temperature changes, humidity etc.Therefore it is recommended to establish a recalibration cycle. Werecommend that accelerometers should be recalibrated every timeafter use under severe conditions or at least every 2 years.For factory recalibration service, send the transducer to Metra. Ourcalibration service is based on a transfer standard which is regularlysent to the Physikalisch-Technische Bundesanstalt (PTB).Often it is desirable to calibrate the vibration sensor including allmeasuring instruments as a complete chain by means of a constantvibration signal. This can be performed using a Vibration Calibratorof Metra’s VC10 series. The VC10 calibrator supplies a constantvibration of 10 m/s² acceleration, 10 mm/s velocity, and 10 µmdisplacement at 159.2 Hz controlled by an internal quartz generator.The VC100 Vibration Calibrating System has an adjustable vibrationfrequency between 70 and 10,000 Hz at 1 m/s² vibration level. It canbe controlled by a PC software. An LCD display shows the sensitivityof the sensor to be calibrated.Many companies choose to purchase own calibration equipment toperform 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 electricallyeither 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 magnitudeWhen charge output accelerometers are used together with a highimpedance voltage amplifier, the capacitance of sensor, cable, andamplifier input has to be taken into consideration.

Figure 10 shows how to calculate the actual voltage sensitivity B´ua

and the lower frequency limit.

Actual sensitivity:

B´ua = BuaCi + CK

Ci + C´K + Ce

Lower frequency limit:

fL = 1

2 Re (Ci + C´K + Ce)

Ci

CK

Ce

C´K

Ci C´KBqa Ce

Bua = Bqa

Ci + CK

inner capacitanceof accelerometercable capacitance of supplied 150 pF cableinput capacitancecable capacitance of additional cable

CK

Figure 10: actual voltage sensitivity

Notice: The voltage sensitivity given in the individual characteristicshas been measured with 150pF cable capacitance.

Understand the limitations of transducer calibration. Do not expect theuncertainty of calibration to be better than ± 2 %. In practice, par-ticularly at frequencies above 1 kHz, uncertainty may amount up to± 5 %.