8/7/2019 Piezoelectric Materials for Vibration SensorsPZT Vs. Quartz

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PZT has been

carefully selected as the

best piezoelectric

material for

accelerometers

WILCOXON RESEARCH, INC. 21 FIRSTFIELD ROAD, GAITHERSBURG, MD 20878 301-330-8811 FAX 301-330-8873

Low frequency

amplifier noise is

inversely proportionalto the charge

sensitivity

Low frequency

accelerometers require

very high sensitivity

piezoceramics to

overcome the

electronic noise of thesensor amplifier and

monitoring

instrument

Piezoelectric sensors are used extensivelyfor monitoring structural and machineryvibrations. Piezoceramic PZT and quartzare the most widely used sensingmaterials for accelerometers andpiezovelocity transducers.

Piezoceramics and Quartz

Quartz occurs naturally in a crystalline

form, however, the quartz used in sensorfabrication is artificially grown.Piezoceramic material is also produced in

a laboratory environment through ahighly controlled process specificallydesigned for accelerometer applications.

Lead-Zirconate Titanate (PZT) is atailored piezoceramic that is capable ofmeasuring much lower amplitudevibrations than quartz. For this and otherreasons, PZT has been carefully selectedas the best piezoelectric material foraccelerometers by the worlds leadingsensor research companies. Speciallyformulated PZT provides stable

performance and long-term reliability formodern piezoceramic sensors.

Quartz is used by companies whohistorically manufactured force gaugesand pressure sensors. The lowerefficiency of quartz works well in theseapplications because of the high forcelevels measured in typical pressure and

force gauge applications. However,quartz is not recommended for lowfrequency accelerometer applications.

Applications

Slow Speed Machinery

Slow speed machinery such as papermachines and cooling towers, require thehigher charge output and broaderfrequency range of PZT based sensors.Manufacturers who traditionally used

quartz are now using PZT in many oftheir new accelerometer designs to allowfor a variety of low frequency monitoringapplications.

Low frequency Measurements

Very little machinery vibration, in terms

of acceleration, is excited at lowfrequencies. For example: Whenmonitoring a roll at 60 cpm, 10 mils ppof shaft movement (.03 ips) producesonly .0005 g of acceleration. These low

amplitude levels can approach theelectronic noise floor of standardaccelerometers. Furthermore, a standard100 mV/g accelerometer presents only 50V of output to the data collector andmay introduce instrument noise into themeasurement.

Low frequency accelerometers requirevery high sensitivity piezoceramics toovercome the electronic noise of thesensor amplifier and monitoringinstrument. Low frequency amplifier

noise is inversely proportional to thecharge sensitivity. The charge sensitivityof PZT is 350 picoCoulombs perNewton (pC/N) contrasted to 2.2 pC/Nfor quartz. The use of PZT significantlylowers the accelerometers contributionto low frequency "ski slope" noise and

greatly improves the signal-to-noise ratioof the measurement.

Piezoceramics must be used for lowfrequency 500 mV/g accelerometers and

piezovelocity transducers due to its lownoise characteristics. PZT enables thesehigher output voltage sensors toovercome data collector noise and furtherdecreases system noise for low levelvibration measurements.

High Frequency Measurements

TN19Technical Note 19

Piezoelectric Materials for Vibration SensorsThe TechnicalAdvantages of Piezoceramics Versus Quartz

8/7/2019 Piezoelectric Materials for Vibration SensorsPZT Vs. Quartz

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WILCOXON RESEARCH, INC. 21 FIRSTFIELD ROAD, GAITHERSBURG, MD 20878 301-330-8811 FAX 301-330-8873

TN19PZT.R3 8/9

When monitoring higher frequencies thedifference between PZT and quartz is lessimportant. In terms of acceleration, thegeneral velocity alarm level of 0.3 ips, isequivalent to 2.5 g at 30,000 cpm(500Hz). This excitation level is easilymeasured by most amplifiers and datacollectors. However, the resonance

frequency of a PZT accelerometer will bemuch higher than an equivalent chargeoutput quartz sensor. This can extendthe frequency range and give more

accurate high frequency readings.

Temperature Considerations

Temperature considerations are veryimportant in many industrialapplications. Although quartz crystal isknown for its temperature stability, oncedesigned into an accelerometer it shares

many of the same characteristics aspiezoceramic sensors.

As temperature increases, the sensitivityof both types of accelerometers willchange. The sensitivity of PZT and

quartz accelerometers exhibit 5 to 7%sensitivity shifts from room temperatureto 250F as shown in Figure 1. In veryhigh temperature environments, bothmaterials are used successfully inapplications exceeding 500F.

Thermal Transients

Thermal transient effects must be

considered in some applications such aslow frequency monitoring. Transientchanges in temperature cause thermalexpansion of the sensors metal housing.

Sometimes mistaken for the"pyroelectric" effect, thermal expansionproduces false signals related to the strainsensitivity of the sensor. Accelerometers,whether PZT and quartz, should bedesigned for low mechanical strain

sensitivity to minimize the effects ofthermal transients.

Stability

Recalibration is rarely required for eithertype of sensor for normal industrialapplications unless contractually required.Quartz is naturally stable and will notchange unless mechanically overstressed.Modern PZT sensors are heat treated tostabilize the poling process and eliminatechanges due to long-term temperatureand shock exposure. Properly designedand processed sensors of both types have

been field proven for many years.

Conclusion

Both piezoceramics and quartz areexcellent materials for use in sensordesign. Each material has a cleartechnical advantage over the other fordifferent parameters. Quartz is the bettermaterial for measuring pressure and forcedue to the relatively high forces involved.Piezoceramics are clearly the choice foraccelerometer applications due to the

higher sensitivities required to monitorlow level vibration. Piezoceramics

coupled with an internal micro amplifierare the materials of choice for advancedaccelerometers.

For greater detail, see the article titled,"Accelerometer Noise" by Fred Schlossprinted in the March, 1993, issue of

Sound and Vibration Magazine, page 22.

Accelerometers,

whether PZT andquartz, should be

designed for low

mechanical strain

sensitivity to minimize

the effects of thermal

transients

Piezoceramics are

clearly the choice for

accelerometer

applications due to the

higher sensitivities

required to monitor low

level vibration

Deviation,%

20

10

0

10

2062650 62625 0 25 50 75 100 120

62658 62613 32 77 122 167 212 250

Temperature

CF

TYPICAL TEMPERATURE RESPONSE

627.cdr R.1

Quartz

PZT

Figure 1. Typical Temperature Response of PZT and Quartz.