Rolf H. Kuratle, André SignerKistler Instrumente AG Winterthur, Switzerland

The Basic of Piezoelectric Measurement Technology

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Piezoelectric sensors haveproven to be highly successfulfor the measurement of fastand cyclic processes. Force,pressure or acceleration sen-sors are used today for qualityassurance in the widest varietyof manufacturing processes,particularly in productionplants for the automobile andelectronics industries. Appro-priate process knowledgecombined with a suitable mea-suring system allow zerodefect production.The following paper discussesa little known piezoelectricmeasuring technique, andshows recent innovations anddevelopments.

Piezoelectric sensors for measuringforce, pressure and vibration are used inparticular applications in industry, wheredynamic processes need to be reliablymeasured over a long period of time.Measurements are frequently used forquality assurance and documentation.The advantage of piezoelectric sensorscompared with other types of sensors are:� Long life without aging� High sensitivity� Low threshold� Large measuring range� Practically displacement-free mea-

surement� High natural frequency� Wide temperature rangeSome examples for the use of piezo-electric sensors in industry are:� Measurement of mold cavity pres-

sure for injection molding of plastics� Cylinder pressure monitoring on

diesel and gas engines� Press force monitoring and control� Monitoring joining forces on auto-

matic assembly machines

� Monitoring of vibrating machinery� Process monitoring during machining

The piezoelectric sensor

Piezoelectric sensors consist of a piezo-electric material packaged in a suitablehousing. The term «piezoelectric» signi-Þes that when loaded with a force, thesensor produces an electric charge Qstrictly proportional to the force F with theunit [pC] (1 Picocoulomb = 10-12 Coulomb).It is therefore an active measuringelement. With quartz (silicon dioxideSiO2) nature has provided an idealmaterial. Nowadays, quartz is syntheti-cally grown under large pressure andhigh temperature. Other piezoelectricmaterials are also used for specialapplications.Different effects are achieved dependingto the alignment of the quartz elementsin the sensor package (Fig 1). These canbe produced by different cutting angles.

The Basic of Piezoelectric Measurement TechnologyRolf H. Kuratle, André SignerKistler Instrumente AG Winterthur, Switzerland

Piezoelectricity

Fig. 1: Piezoelectric effect – schematic diagram of various sensors

a) b) c)

disks

a) Transversal effect

The charge output occurs at right anglesto the force contact surfaces. The chargeis dependent on the geometry of thequartz or more precisely on its thinness.Quartz elements therefore often have abar shape. The charge output in the caseof a transversal element for the dimen-sions shown in Fig. 1 amounts to:

Pressure sensors and highly sensitiveforce sensors are typical sensors usingthe transversal piezoelectric effect.

b) Longitudinal effect

The charge output occurs at the forcecontact surfaces. In contrast to thetransverse quartz element, the chargelevel is not dependent on the quartzgeometry but solely on the force appliedand amounts to

These quartz elements are frequentlydisk-shaped. The only possibility of in-creasing the charge yield is to connectseveral quartz disks in series withrespect to the force. Electrically thesedisks are wired in parallel. With n disks,the charge then amounts to

Typical sensors applying the longitudinaleffect are force load washers.

c) Shear effect

With shear force, the charge outputoccurs at the force contact surfaces. Thesensitivity amounts to

As with the longitudinal effect, the geo-metry of the quartz does not affect itssensitivity. Typical sensors with sheareffect are 3-component force sensors,moment sensors and accelerometers.

Amplifiers for piezoelectricsensors

Functional principle

The very small electric charge Q must beconverted to a voltage U (5 V or 10 V) ora current Ι (4 � 20 mA) for evaluation,e.g. in a PC or a PLC. The chargeampliÞer is appropriate for this purpose.

The designation charge ampliÞer is ingeneral use in measuring technologyeven though descriptively incorrect,because the charge ampliÞer does notamplify a charge but rather converts itinto a proportionate voltage.

In principle, the charge ampliÞer consistsof a high-gain voltage ampliÞer with aMOS or J-FET transistor at the input toachieve the high insulation resistance. Itis connected in negative feedback via ahigh-insulation range capacitor and thusacts as an integrator for input currentsßowing through the charge input. Theseare generated by changes in charge (or mechanical load changes) at thesensor. The integral of the change incharge from the piezoelectric sensor andthus a voltage signal proportional to theentire change in charge (or mechanicalload change) appears at the output.

The relationship (Fig. 2) between inputQ and output Uout is:

F Mechanical load on the sensor(e.g. force)

Q Sensor output charge (e.g. 4 pC/N)

CS Sensor capacitance (e.g. 50 pF)

CK Cable capacitance (approx. 100 pF/m)

CB Capacitance of the range capa-citor

Ri Insulation resistance of the inputcircuit (sensor, cable, amplifierinput)

RG Resistance for time constant(lower cut-off frequency)

RTP Resistance of the low-pass inputÞlter

vi Gain factor of the operational ampliÞer (approx. 100,000)

Uin Voltage at the ampliÞer inputUout Voltage at the ampliÞer outputReset Switch for short-circuiting the

range capacitor (zeroing theampliÞer)

As a result of the large gain of the ope-rational amplifier (ideally ν → ∞), thecapacitances of the sensor and the cableare practically negligible, and the outputvoltage is purely proportional to the

Fig. 2: Schematic diagram of a charge amplifier

Charge amplifier

Q Fba

y y= ⋅ ⋅– 2,3pCN

Q Fx x= ⋅– ,2 3pCN

Q – n Fx x= ⋅ ⋅2 3,pCN

Q F= ⋅– ,4 6pCN

Uout =Q

11 1

+

⋅ + ⋅ +( )ν ν

C C CB S K·CB + (CS + CK)

Reset

RG

CB

UoutRiUinCKCS

RTP

QF

F

�vi

quotient of charge and range capaci-

tance:

Charge amplifiers must be highly insu-

lating on the input side (Ri of the order of

magnitude of 1014 Ohm) – the same goes

for the sensor and the cable including

plug connections – since every finite re-

sistance will cause a current [pC/s = pA]

to flow allowing the output signal to drift.

Drift

As already mentioned, the high insula-

tion at the amplifier input is achieved with

a MOSFET. The best MOSFETs current-

ly available have an input leakage

current of the order of magnitude of

several fA. For good charge amplifiers,

this means a typical drift of ±0.03 pC/s.

The small leakage current of the input

stage is responsible for the fact that no

purely static measurements can be

made over a long period of time with

piezoelectric systems.

The percentage drift of the measuring

signal per minute is calculated from:

Sensitivity of the sensor [E] = pC/N

Force to be measured in [F] = N

The limit for the quasistatic measure-

ment of piezoelectric signals is easily

determined by means of a practical

example with a quartz force sensor

(sensitivity 4 pC/N). With a measuring

range of 1000 N, there is a measuring

error of 0.045 % per minute or a

measuring time of 22 minutes without the

measuring error exceeding 1 %. This drift

is independent of the range capacitor

selected. Nevertheless, the higher the

sensitivity of the sensor and the greater

the force to be measured, the smaller

the error component due to drift.

Reset

The ‘Reset’ switch, which is also highly

insulated, enables the output signal to be

reset to zero by short-circuiting the range

capacitor. In many applications, this type

of taring is desirable so that, for example,

the intrinsic weight of machine parts is

not included in the force measurement.

Normally a reset function will be carried

out before every measuring cycle with

the sensor mechanically unloaded. The

reset time is short at approx. 5 ... 100 ms

and is thus suitable for short cyclic

measuring or monitoring applications.

Depending on the charge amplifier

design, the reset switch is operated

either manually or via an external digital

input signal. Semiconductor switches or

reed relays are used as high insulation

reset switches. They are normally closed

when no current is flowing to prevent

damage to the high insulation input from

static charges.

Measuring ranges

Most charge amplifiers have multiple

measuring ranges. The choice of mea-

suring range is made by switching the

relevant range capacitors CB. Subse-

quent amplifier stages provide a scaled

10 V output voltage signal, thus allowing

the use of a single charge amplifier for

sensors with the widest selection of

sensitivities and measuring ranges.

The accuracy of the charge amplifier is

mainly determined by the tolerance of

the range capacitors CB. The linearity of

±0.05 % FS is excellent. The charge

amplifier error is thus negligible in its

effect on the calibration of the entire

measuring chain or when tuning the

amplifier to the sensor involved.

Within specific limits, charge amplifiers

are overload-proof. The determining

overload parameters are the signal slew

rate and the magnitude of the charge.

J-FET amplifiers are more insensitive to

static discharge in the event of improper

connection of the sensor as a MOS-FET

amplifier, but have a considerably larger

drift and temperature dependence.

Low-pass input filter

Due to mass oscillation additional forces

are generated while measuring vibrating

machines. These forces are either of no

interest or do not represent disturbances

to the control or monitoring of the

machinery.

These disturbances can be excluded

through the use of a suitable low-pass

input filter. For most applications, filters

covering the range 10 ... 100 Hz have

proven most successful and take the

form of an RC network at the amplifier

input. If the cable and sensor capaci-

tance is used for C, then a low pass filter

is produced with an additional integral

resistor RTP in series with the cable.

Time constants (high pass filters)

A time constant acts like an AC coupling

device, similar to that familiar from os-

cilloscopes. The static signal component

is filtered out and only the dynamic signal

oscillates about zero according to the

waveform. Time constants are produced

with a resistor RG in parallel with the

range capacitor. The insulation resis-

tance is artificially reduced. This is of

course only appropriate for rapid mea-

suring processes.

In the AC mode, the charge amplifier

behaves like a high pass filter. The low-

er cut-off frequency is calculated from

the value of the range capacitor in the

circuit and the time constant resistance

as follows:

Lower cut-off frequency [fu] = Hz

Time constant [t] = s

Time constants are included in the

charge amplifier only when the dynamic

signal component is of interest in rapid

processes. A reset before each cycle is

in the many cases unnecessary for

measurements with time constants.

Industrial charge amplifier ver-sus laboratory charge amplifier

The following overview sets out the most

important features of industrial and labo-

ratory charge amplifiers respectively.

UQC

outB

≈

fTiefpassTP S KR C C

=⋅ ⋅ ⋅ +( )

12 π

fR C

uG B

=⋅ ⋅ ⋅

=⋅ ⋅

12

12π π τ

DriftE F

% /, sec %

MinpC[ ]=

⋅ ⋅⋅

0 03 60 100

RTP · (CS + CK)

·RG ·CB

Application specific

In previous years, Kistler developedcharge amplifiers tailored to specificrequirements. The main emphasis wasto develop speciÞc ampliÞers suitable forindustrial applications.

Miniaturization

Hybrid technology has made it possibleto greatly miniaturize the actual chargeamplifier. Charge amplifiers have alsobecome more rugged. They can belocated signiÞcantly closer to the mea-suring location, e.g. directly on movingparts of machinery, thereby minimizingdistance between the sensor and theampliÞer. Hybrid ampliÞers have a Þxedmeasuring range. Nevertheless, variousmeasuring ranges can be conÞgured bymeans of integral voltage ampliÞers. Thecurrently smallest «In-Line» charge amplifier Type 5027 (Fig. 4) with anenclosure of only 45 x 16 mm is availablewith 3 different measuring ranges. Preci-sion calibration is carried out in eachrange using an integral potentiometer.

Immunity to electrical interference

In industrial production plants, the elec-trical ambient conditions are often lessthan ideal. Ground loops, resulting from

different chassis potentials on themachinery together with electromagneticÞelds, can interfere or distort an original-ly excellent measuring signal. The caus-es are certain to be found in the system(machine/measuring chain/cables/envi-ronment). However, charge amplifiersare available which are practicallyimmune to interference.Basically, all Kistler charge amplifiersand the standard sensors and cables are tested with respect to electromag-netic compatibility EN 50081-1/2 (interfe-rence emission) and EN 50082-1/2 (inter-ference immunity). Safety requirementsaccording to EN 61010-1 are also met.Furthermore, the new amplifier Type5034, for example,is being offered withcomplete electricalisolation. This meansthat the chassispotentials of thesupply and measur-ing signal inputsand outputs areseparated by opto-couplers, so that nointerference (e.g.«power line hum»)caused by groundloops can occur.

External range switching

One of the distinguishing features ofpiezoelectric sensors is their very widemeasuring range. For this to be utilized,charge ampliÞers with switchable mea-suring ranges are required. These willallow, for example, the force characteris-tic in a machine to be measured both inthe low range of a few N and in the rangeof high kN with high resolution. Theamplifier types 5034 and 5039 allowexternally controlled switching of themeasuring ranges. The measuring rangeis switched during measurement bymeans of switching signals.

Multi-channel charge amplifiers

For measurements with a number ofsensors, modular multi-channel ampli-Þers with up to 3 channels are available. Piezoelectric sensors can also beconnected electrically in parallel withoutproblem. A typical application is repre-sented by so-called force plates, e.g. to determine forces in mechanical proces-sing. The force plate is supported on foursensor feet. The charge signals of the individual sensors can be merged, i.e.added together and fed to a single ampliÞer channel.

Industrial LaboratoryConstruction � Rugged metal or impact- � Bench-mounted case or

resistant plastic case 19�rack-System according� Suitable for mounting on to DIN 41494

a machine� Small dimensions� Shock and vibration proof

structure of the electronic system� IP protective class (min. IP64)� Various input connections

Measuring channels � Single or multi-channel � Single or multi-channelMeasuring ranges � Internal adjustable � In-/External adjustable

� Sensor-speciÞc calibration � Numeric display of set valuesavailable

Functions � Semiconductor reset � Adjustable low-pass Þlter� Electrical isolation � Time constant (High pass Þlter)� Differential input adjustable

� Overload monitoring� Zero monitoring

Output signal � ±5 V / ± 10 V � ±10 V� 0 � 20 mA / 4 � 20 mA

Supply � 10 � 36 V DC � 115/230 V ACConnections � Various input connections � BNCInterfaces � none � serial RS-232C

� parallel IEEE-488

Fig. 4: The smallest Kistler charge amplifier Type 5027

Fig. 3: Characteristics of industrial and laboratory charge amplifiers

Fig. 5: Charge amplifier

in a rugged and sealed

enclosure

Type 5034/5038

Fig. 6: Modular design

with plug-in cards

Type 5058

Fig. 7: Latest Control

Monitor «CoMo II-S»

Type 5859

Rack mounting

The charge amplifier Type 5058 isdesigned in the Euro card format. It canbe mounted in racks or switchgear cabi-nets. This allows customized measuringsystems to be designed (Fig. 6).

External actuation

The control inputs for Reset/Operate orrange switching can be directly connect-ed to any machine control system. Allindustrial ampliÞers can be operated withTTL signals or via optocouplers. Inseveral cases, non-electrically isolatedactuation is also possible using a simpleswitch. The power supply for industrialcharge ampliÞers is normally 24 V DC.Current consumption is low at less than100 mA.

Control monitors

Production processes nowadays mustbe constantly monitored for qualityassurance purposes. For this reason,the measuring signals can either beevaluated by a PLC or condenseddirectly on site to provide informativeparameters. Programmable Kistler con-trol monitors are able to distinguishconforming or nonconforming partsdirectly by means of the measuringsignal (Fig. 7). This relieves the PLC ofcomputer-intensive work. Charge ampli-Þers are integrated in the control moni-tors. Moreover, signals can also beconnected from sensors with othermeasuring principles (e.g. displacementsensors) and displayed as a function ofthe Þrst measurand.

Literature

[1] Dipl.-Ing. R. Kail und Dipl.-Ing. W. Mahr; Piezoelectric Measu-ring Instruments and their Appli-cations; Kistler Reprint 20.116e

[2] R. Kuratle; Motorenmesstechnik;Vogel Buchverlag; Würzburg

[3] J. Tichi, G. Gautschi; Piezoelek-trische Messtechnik; Springer-Verlag