TUTORIAL Introduction to Piezoelectric Sensors

Dytran Instruments, Inc. 1

Installed HUMS BaseDytran sensors are installed on the following airframesfor HUMS applicationsDytran Instruments, Inc. 2

MH60 Blackhawk

A119

AS350

Bell 412

AH-64

MH47

Introduction to PiezoelectricSensorsDave Change

General Topics• Why do we use piezoelectric sensors?• Some common uses for piezo sensors• Dynamic measurement vs. static measurement• Piezoelectric transducer‒ Principle of operation‒ Information provided by the transducer• Charge mode and voltage mode (IEPE)• Transducer selection criteria• Mounting considerations• General handlingDytran Instruments, Inc.10

Stochastic Perturbation TheorySince the solution of Langevin equation will depend on the coupling constant of thetheory, look for the solution as a power expansionIf you insert the previous expansion in the Langevin equation, the tlatter gets translated into ahierarchy of equations, , each for each order, each dependent on lower orders.We already know the solutions for φ 4 theory: Diagrammatically ...+ λ + λ 2 ( + ... ) + O(λ 3 )Now, also observables are expanded... and this is a propagator ...+ 3 λ ( + ) + O(λ 2 )Observation: we can get power expansions from Stochastic Quantizationation’s s main assertion, e.g.

Why do we use piezo sensors?• Small size• Light weight• 2-wire operation(IEPE)• Wide dynamic range• Wide temperaturerange• Broad frequencyrange• Ultra low noise• Simple signalconditioning• Cost effective testimplementationDytran Instruments, Inc. 12

Common Uses for PiezoelectricSensors• Modal Analysis• Environmental StressScreening (ESS)• Health and UsageMonitoring Systems(HUMS)• Predictive /preventativemaintenance• Pyrotechnic events• Aircraft groundvibration test• Aircraft flight testDytran Instruments, Inc. 13

What factory supplied documentationcomes with your accelerometer?• Operating guide• Outline installation drawing• Specification sheet• Calibration certificateDytran Instruments, Inc. 14

Outline installation drawingDytran Instruments, Inc. 15

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Other Piezoelectric Devices• Today’s discussion will focus primarily onpiezoelectric type accelerometers but we use thesame principles to measure dynamic force andpressureDytran Instruments, Inc. 18

Dynamic vs. Static MeasurementDynamic Measurement• The physical quantity (i.e.acceleration, force,pressure) is rapidlychanging over time• Examples of dynamic events‒ Vibration of rotating machinery‒ Pressure in a reciprocatingengine‒ Force input to a structure via ashake tableStatic Measurement• The physical quantity beingmeasured does not changeas a function of time (orchanges very slowly)• Examples of staticmeasurements‒ Force measurement in abathroom scale‒ Constant acceleration due togravityDytran Instruments, Inc. 19

Definition of PiezoelectricityPiezoelectricity is the ability of some materials (notablycrystals and certain ceramics) to generated an electricalpotential in response to applied mechanical stress. Thismay take the form of a separation of electric chargeacross the crystal lattice. If the material is not shortcircuited,the applied charge induces a voltage across thematerial. The word is derive from the Greek wordpiezien, which means to squeeze or press.Dytran Instruments, Inc. 20

What is a Piezoelectric Transducer?• A transducer converts one form of energy into another‒ In the case of piezoelectric transducer the transduction is frommechanical energy to electrical energy• The prefix “piezo” is a Greek word meaning “to squeeze”• Materials that produce an electric charge when a force isapplied to them exhibit what is known as thepiezoelectric effect• Many piezoelectric materials are known toexist‒ Quartz, tourmaline, ceramic (PZT), GAPO4 andmany othersDytran Instruments, Inc.21

Typical IEPE Design• 3211 link to•1060 series link to•2005V seires link to

Piezoelectric Transducer• The active element in all piezoelectric devices is apiece of piezoelectric material. There are manydifferent sensor designs based on various crystalcuts and materials.• The common types (modes) of piezoelectric sensorsin use today are:‒ Voltage mode (IEPE, LIVM, ICP, Piezotron, Isotron)‒ Charge mode‒ Each of these designs have their advantages anddisadvantagesDytran Instruments, Inc.23

Piezoelectric Sensing Element DesignsCompressionShearDytran Instruments, Inc.24

Voltage Mode Transducer• Can utilize either quartz or ceramic• Have built in or integral electronics• Low cost signal conditioning• Stable over broad temperature range• Limited upper temperature range due to built inelectronics• Many modern analyzers and data acquisitionsystems have IEPE power built in• Most voltage mode (IEPE) sensors available with TEDSoption• Ease of useDytran Instruments, Inc.25

Input / Output Characteristics ofVoltage Mode (IEPE) Sensor• Input : 18 – 24 Volts DC, 2-20mA constant current‒ Inputs other than the specified value can be harmful to thesensor• Output : voltage proportional to acceleration orvibrationNote: The IEPE or voltage mode sensor is strictly a two wiredevice (sig / power and ground)Dytran Instruments, Inc.26

Voltage Mode SetupDytran Instruments, Inc.27

Alternate Voltage Mode SetupDytran Instruments, Inc.28

Typical IEPE System DesignDytran Instruments, Inc.29

Bias Voltage Levels in Typical IEPESystemsDytran Instruments, Inc.30

IEPE Sensor Frequency Response• Low frequency response is controlled by the sensorDischarge Time Constant (DTC)‒ Factory set and cannot be changed‒ Can be changed at time of manufacture• High frequency response is generally a function ofsensor resonance (natural) frequency• Filtering in the sensor or signal conditioning can alsoeffect the sensor frequency response• High sensitivity sensors generally have lowresonance frequency• Low sensitivity sensors generally have highresonance frequencyDytran Instruments, Inc.31

Frequency Response of PiezoelectricAccelerometersDytran Instruments, Inc.32

Panel Meter ExamplesTrouble Shooting Aide010BATT.O.K.2010BATT.O.K.0 20010BATT.O.K.20NORMALNORMALNORMALSHORTOPENSENSOR BIASDC VOLTSSHORTOPENSENSOR BIASDC VOLTSSHORTOPENSENSOR BIASDC VOLTSShortedNormalOpenDytran Instruments, Inc.33

Discharge Time Constant (DTC)• Discharge Time Constant – the time required for theoutput voltage from a sensor to discharge to 37% ofits original value in response to a zero rise time stepfunction input‒ Determines sensor low frequency response• Low frequency response is approximately equal to:-3dB Point = .16/DTC-5% = -3dB Point x 3Dytran Instruments, Inc.34

Charge Mode Transducers• Usually ceramic based• No built-in electronics• Good for high temperature (+500°F)• Expensive signal conditioning• Low noise cable required for most applications• Charge amps are generally more difficult to use (i.e.setting scale factors, etc.)Dytran Instruments, Inc.35

Mounting Options• Stud mounting‒ Optimum frequency response• Epoxy adapter, stud mount sensor‒ Good frequency response• Magnetic mount‒ Convenient, limited frequency response• Magnetic mount to epoxied target‒ Preferable method for magnetic mountingDytran Instruments, Inc.36

Frequency Response EffectsDytran Instruments, Inc.37

Transducer Selection Criteria• How much space is available for mounting thesensor (i.e. footprint)?• What max mass can be used (beware of massloading)• What is the vibration level?• What are the frequencies of interest (max and min)?• What is the temperature range required?• Are corrosive chemicals present (industrial)?• Is physical clearance a problem?• Is the environment wet or dry?• How will the sensor be mounted (stud, wax, glue)?• What cable length will be required?• What cable jacket material will be adequate?Dytran Instruments, Inc.38

Dynamic Range• Select proper full scale amplitude from transducerspecifications• Full scale ranges normally listed for ±5 Volts out• Over range capability usually set by current sourceopen circuit output voltage (not always true)• If P.C. based data acquisition make sure board isconfigured properly for your specific measurementDytran Instruments, Inc.39

Impulse Hammers• Provide a known force input (mV/Lb) to a structure• Many different physical sizes are available• For industrial use the 1Lb, 3Lb, or 12Lb sledgehammers are most popular• Four different tip materials are provided for pulsetailoring• Transfer function measurements consist of aminimum of two channels• Impulse hammers are used to help determine thetransfer function (output / input) of a given structureDytran Instruments, Inc.40

Typical Multi-Channel Modal SetupDytran Instruments, Inc.41

• Do notGeneral care and handling of yourpiezoelectric device‒ Drop your sensor on a concrete floor‒ Connect a bench power supply to your sensor‒ Remove you sensor with a hammer‒ Use un-calibrated accelerometers‒ Rub your feet across the floor and touch the connector (staticelectricity)Dytran Instruments, Inc. 42

General care continued• Do‒ If possible, store your sensor in the box it came in‒ Connect a constant current supply‒ Remove your sensor with a solvent and proper tool‒ Re-calibrate your sensor on a yearly basis‒ Discharge yourself before handling the sensorDytran Instruments, Inc. 43

TEDS- Transducer Electronic Data Sheet

MEMS AccelerometersVariable Capacitance MEMS accelerometers