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Piezoelectric Pressure Sensors Based on

Flexible PZT Thick-film Composite Device

Swanson School of Engineering Department of Mechanical Engineering

and Material Science

Rongjie Liang

Committee members:Dr. Qing-Ming Wang (adviser)

Dr. Patrick SmolinskiDr. William S. Slaughter

Swanson School of Engineering

Overview• What is Piezoelectricity and Piezoelectric Material?

• Piezoelectric Pressure Sensor: a brief review

• PZT Thick Film Pressure Sensor• Design and Fabrication

• Theoretical Derivation of Sensitivity functions

• Experiment Design and Testing• Fluid Pulse Pressure

• Shock tube Blast Pressure

• Application: Human Blood Pulse Wave Testing

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What is Piezoelectricity?

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• Piezoelectricity is a property of certain materials that undergo a dipole deformation and electric charges accumulation in response to applied mechanical stress.

http://en.wikipedia.org/wiki/Piezoelectricity3


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In pressure sensor, direct piezoelectric effect is used

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Piezoelectric equations in matrix form:


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𝐷𝐷3=𝑑𝑑31𝑇𝑇1

𝐷𝐷3=𝑑𝑑33𝑇𝑇3

Figure 3. Force/Charge relationship on piezoelectric element:(a) d33 mode; (b)d31 mode [3]

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Piezoelectric Pressure Sensor----a brief review

• High modulus of elasticity • High dynamic amplitude range• Fast response to highly dynamic load

• Self-powering-active sensor• Insensitive to electromagnetic fields and

radiation• Relatively simple circuit readout

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Table 1. comparison table of characteristics of piezo sensor materials vs other types

Gautschi, G. (2002). Piezoelectric sensorics. Springer Berlin, Heidelberg, New York. p. 3 – via Google Books.

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General Structures of Piezoelectric Sensor

Compression Bending Shear

Compression: highly rigid, useful for high frequency pressure and force sensing;Bending: Simple, flexible and sensitive to low pressure and force loading;Shear: wide frequency range, low sensitive to base strain and thermal inputs.

http://www.pcb.com/TechSupport/tech_gen

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Common Piezoelectric Materials for Sensor

http://www.wtec.org/loyola/mems/c4_s3.htm

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• PZT ceramicsPros:

– Highest Piezoelectric Coefficient

– Highest Electrical-mechanical Coupling Coefficient

– Simple fabrication process, less expensive

Cons:

– Rigid and Brittle

– Difficult to obtain film structure, which is key to diaphragm pressure sensor

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Research Motivation

• What if using PZT to make a diaphragm type pressure sensor?

The Answer is PZT Thick Film:

• Highly Sensitive

• Relatively Flexible

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Tap-Casting Process:

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The thickness of PZT films can be controlled in the range about tens of micrometers, making it possible for the design and fabrication of flexible PZT diaphragm pressure sensor.

Qin, Lifeng, et al. "Fabrication and characterization of thick-film piezoelectric lead zirconate titanate ceramic resonators by tape-casting." Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on 59.12 (2012): 2803-2812.

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Design and Analysis of Sensor Structure• Design 1: Unimorph Structure

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Working under d31 bending mode, the PZT film is stretched or bended due onormal pressure, and the stress/strain along radial and transverse directionare much large than those in normal direction, which results in highsensitivity

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• Design 2: Sandwiched Structure

Sandwiched structure is applied if used in harsh environment such as fluctuated flow or curved surface such as human skin

• Electrodes and PZT layer can be protected• Non-contact with sensed surface such as water or human skin• Further enhance flexibility by applying pre-compressed thermal stress

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Analytical Analysis

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Assumptions:1. No shear strain along the plate2. No stress along thickness direction3. The deflection is much smaller than the thickness of the plate4. Effects of electrodes and bonding layer are neglected5. For sandwiched structure, the inner composite is regarded as

equivalent piezoelectric layer (Eequ and vequ)6. For simplicity, Poisson’s ratio is unified.

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Vinson JR (1974) Structural Mechanics: The Behavior of Plates and Shells. John Wiley & Sons New York.

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Vinson JR (1974) Structural Mechanics: The Behavior of Plates and Shells. John Wiley & Sons New York.Mo, Changki, Leon J. Radziemski, and William W. Clark. "Analysis of piezoelectric circular diaphragm energy harvesters for use in a pressure fluctuating system." Smart Materials and Structures 19.2 (2010): 025016.

1.The general solution of w:

8 Constants C1-C8 can be determined by 8 boundary and continuity conditions:• Clamped edge condition : At r=a, w=0, dw / dr=0• Finite value condition: At r=0, dw/dr<∞, thus terms with natural logarithm

should be eliminated. (C5,C7=0)• Continuity conditions at interface: At r=b, wc=wm, dwc / dr=dwm / dr ;

QC=Qm, Mrc=Mrm,

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2. Derive Moment Functions from w functions:

Mrc, Mrm, Mθc, Mθm thus can be expressed as functions of pressure P.

3. Derive Moment Functions from stress integration

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4. Expressions of stress and strain:

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Piezoelectric Constitutive Equations

Inner Piezo layer

Outer substrate layer

Therefore, stresses and strains are functions of pressure P and electric field E

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5. Energy Method: Find P-Q, P-V relationship

The energy density can be expressed as

For inner piezo part:

For inner and outer metal part

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The integration will result in three terms of U, that is,

The Charge Generation is

Substituting E3 with V/hp.

The capacitance is

The Voltage Generation is

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Sensitivity performance with different thickness ratios and substrate materials.(b/a=0.6)

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Charge-generating performance with different radius ratios and substrate materials.(hp/hm=1.5)

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Experiment A: Pulsed Fluid Pressure Testing

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Sandwiched PZT sensors

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Flow Chamber DesignHow to get the pressure magnitude applied on the sensor?

---Measuring the center deflection of a metal disk close to sensor port

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Waterproofed O-ring


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(a) Measurement setup block diagram; (b) picture of experimental setup

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Recorded Waveforms

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Sensitivity curvatures

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Conclusion• Exhibit good linearity of voltage-pressure relationship at certain pulse frequencies• Agree well with theoretical value• Able to sensing weak pressure as low as several kPa• Some limitations of setup: turbulence flow in chamber, inaccuracy of the metal

disk vibration measurement

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Experiment B: Blast Air Pressure Testing

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Unimorph Structure Sensor

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Using different membranes to get various blast pressure magnitudes

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Experiment Results

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Two Methods for sensor calibration

1. Using Standard Pressure Sensor

2. Shock tube wave calculation

shock wave equation

P : absolute shock pressureP0: standard atmosphere pressurek : a constant (=1.4 for air)M: the Mach number of shock wave

M=v/C0 C0: Sound velocity in air

If the wave velocity v can be measured, the blast pressure can be calculated

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Discussion: Inconsistence of Sample 1

One of the most possible factor that can significantly influence thesensitivity performance is the edge condition. Compared Sample 1with Sample 2 and 3, it is noted that the first sample uses moreepoxy for bonding onto the sensor housing, of which the actual edgecondition may be considered as elastically supported due to theeffect of epoxy

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Corrections Factor: related to torsional stiffness Kt (Leissa 1969).

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Conclusion

• High linearity in the measured range up to 55 kPa• Fast response to dynamic blast pressure• Higher sensitivity than commercial quartz pressure sensor• Agree well with theoretical value, if minimizing the edge effect

Future Work• Investigate the sensitivity performance under simple support condition,

and how epoxy effects the torsional stiffness Kt• Optimize the dimensions and materials of sensor structure to further

enhance the sensor performance

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Application: Human Blood Pulse Wave Measurement

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Pulse Wave Velocity measurements: Left: Arterial regions in human body. Right: measure PWV between brachial and radical arteries.

McLaughlin, J., et al. "Piezoelectric sensor determination of arterial pulse wave velocity." Physiological measurement 24.3 (2003): 693.32. Tadinada, Akhila. Piezo film sensor for capture of arterial wave pulse - Implementation. OpenStax-CNX. 19 Dec. 2007 <http://cnx.org/content/m15664/1.1/>. 43


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Pulse Wave Velocity Measurement

/ 0.25 / (0.065 0.0005) 3.6 ~ 4.2 /PWV m sD t= ∆ = ± =

The PZT pressure sensor is flexible enough to be bonded onto the various skin surfaces such as mid arm region and wrist region. It is sensitive to low frequent radical and brachial arterial pulses, and the estimated PWV value obtained by the sensor was reliable.

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Summary• The feasibility of PZT thick film used as pressure sensor was

investigated ;

• The Charge and Voltage sensitivity Equations were derived using energy method;

• Two Types of Pressure loading were measured using different setups and sensor structures, both exhibiting good linearity and satisfied sensitivity in the measuring range;

• The application in human Pulse Wave detection was studied.

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Thank you!

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rongjie defense ppt

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