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Design of mechanical sensing system based on PVDF piezoelectric

sensor

Yan-Shen Wang*,Yu-Xian Gai, Shi-Ming LiDepartment of Mechanical Engineering, Harbin Institute of

Technology at Weihai, Weihai, 264209, Peoples Republic of China

E-mail: ysw710@tom.com

AbstractA mechanical sensing system based on PVDF

piezoelectric sensor was designed. Piezoelectric equation of

PVDF was studied and simplified for data analysis. The

algorithm of power frequency elimination was detailed

studied. 4-times sampling was adopted during A/D

transformation and the noise induced by 50Hz power

frequency was minimized. Butterworth low-pass filter and

trap filter was used for noise filtering. The measuring system

consisted of PVDF sensor, charge amplifier, signal filter,

A/D, MCU and LCD. The data acquisition and processing

program was also written. Simulation and experiments

proved the effective of this system.

Keywords-piezoelectric, PVDF, mechanical sensorI. INTRODUCTION

Sensing of rapid changing mechanical signals is ofgreat significance to monitoring the health status of high-speed running components used both in aircrafts,spacecraft and many types of civilian equipments. There

are many types of mechanical sensors, such aspiezoresistive, capacitive, fiber optical and piezoelectricones. Piezoresistive and capacitive sensors are slowresponding and have thermal drifts. While, fiber opticalsensors have low strength and its multi-peak phenomenoninterferes data acquisition. Piezoelectric sensors havemany virtues such as simple structure, good dynamiccharacteristics, which is suitable for broadband periodicforces and rapid changing impacts. Piezoelectric ceramicsand quartz are frequently used in mechanical sensing. Yet,they are hard and have poor impact resistance. It isdifficult to fit for rapid running components with varietiesof shapes. Polyvinylidene Fluoride (PVDF), as a new typeof piezoelectric material, is a polymer with advantages that

can overcome the above mentioned defects in piezoelectricceramics and quartz. Thus, PVDF is suitable for rapidmechanical sensing.

In 1969, Kawai found the piezoelectricity of PVDF,which can be got by polarizing organic fluorine polymermaterials.[1] Since then, researches in piezoelectric

polymer gradually flourished. In 1978, the ferroelectricityof PVDF was discovered by Kepler,[2] which pushforward the application of PVDF. F. Bauer does a lot ofwork in this field [3, 4] and he made high performancePVDF sensors.

In this paper, a mechanical sensing system based onPVDF piezoelectric sensor was designed. It can eliminate

the noise that took by city electricity.

II. MEASURING METHOD OF PVDFSENSOR

A. Piezoelectric equation of PVDFFor materials with piezoelectricity, the mechanical and

electrical behaviors couple together, i.e. one behavior canmotivate another. Piezoelectric equation is the formula thatdescribes relations between electricity amounts and

mechanical ones in piezoelectric crystals. Mechanicalsensor utilizes direct piezoelectric effect and can bedescribed by type 1 piezoelectric equation. The equation,as is shown in (1), used stress tensor and electric-fieldintensity vector as argument and used strain tensor andelectric displacement vector as dependent variable.

TD dT E= + (1)

In (1), D, d, T, T and E were electric displacementmatrix, piezoelectric constant matrix, stress tensor,dielectric constant matrix and electric-field intensityrespectively. When the piezoelectric material was not inelectric field, (1) can be simplified as

D dT= (2)

Where [ ]1 2 3 TD D D D= . Subscripts 1, 2 and 3represented three different directions, which was illustrated

in Fig.1. And [ ]1 2 3 4 5 6T

T T T T T T T = . Subscripts

1~6 represented different stress directions.1

T ,2

T and

3T represented normal stresses in three different directions,

and4

T ,5

T and6

T represented shear stresses in three

different directions.

Figure 1. Direction illustrator of PVDF polymer

Suppose direction 3 in Fig.1 was the polarizeddirection, the polarized PVDF material belongs to C6V(6mm point group) symmetry. Then, the piezoelectricconstant matrix

3

1

2PVDF polymer

978-1-4577-0860-2/11/$26.00 2011 IEEE

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15

15

31 31 33

0 0 0 0 0

0 0 0 0 0

0 0 0

d

d d

d d d

=

(3)

Here,15

d ,13

d and33

d were three independent

variables. The first number in the subscripts signified the

direction of electric effect, while the second one stood forthe direction of mechanical effect.

The charge density deposited on both sides of PVDF

film was3

D . According to (2),

3 31 1 31 2 33 3D d T d T d T= + + (4)

Equation(4) means the normal stress in the threedirections can all deposit charges in both sides. When thefilm received pressures along polarized directions, (4) canbe simplified as

3 33 3D d T= (5)

PVDF pressure gauge was developed according to theabove algorithm.

B. Method of power frequency eliminationNoises from power frequency interference can

influence the measurement accuracy greatly. Althoughfilter circuit were used, there were still many interferencefrom industrial power. Moreover, high harmonic signalsproduced during power rectifying can also influence thesignal. The following will brought out a method foreliminating such noises.

Suppose the input signal of A/D isi

u , which consists

of the real signalo

u and the interfere signale

u , i.e.

i o eu u u= + (6)

In (6),

1 2 3sin sin 2 sin 3eu U t U t U t+ += + (7)In (7), was angular power frequency. 1U , 2U ,

were the amplitude of all the harmonics, which attenuatewith the increase of frequency. To simplify the followinganalysis, only harmonics before 8th harmonic amount wereadopted.

Given Twas the power frequency cycle, take sampling

cycle 0 4 2T T = = , i.e. sampling four times per

power frequency cycle. The sampling time was

1

( 1)( 1, 2, 3, 4)

2i

i t t i

= + = (8)

Substituted (8) into (7), interference signals in the foursampling time were listed as

18171615

141312114

18171615

141312113

18171615

14131211

1813

12112

181312111

8sin7sin6sin5sin

4sin3sin2sinsin

8sin7sin6sin5sin

4sin3sin2sinsin

8sin7sin6sin5sin

4sin3sin2sinsin

28sin

23sin

22sin

2sin

8sin3sin2sinsin

tU

tU

tU

tU

tUtUtUtUu

tUtUtUtU

tUtUtUtUu

tUtUtUtU

tUtUtUtU

U

U

U

Uu

tUtUtUtUu

e

e

e

e

++

++=

++

++=

+

++=

++++

++++=

++++=

Took the mean value of the four sampling data, used(6), we can got

1 2 3 44 1 8 1sin 4 sin 8

4

o o o oi

U U U U u U t U t=

+ + ++ + (9)

In (9), 1oU , 2oU , 3oU and 4oU were real values of eachsampling. And the interference signal only left the 4th andthe 8th harmonic parts. The 1st, 2nd and 3rd harmonicparts that have strong interference didnt exist. In fact, if

sampling n times from the original signal in a powerfrequency cycle, the mean value of the n samplingamounts only left n times harmonics and other harmonicswill be eliminated. In this paper, 4 times sampling wasadopted during A/D transformation and noises induced bypower frequency were restrained heavily.

C. Sensor design and measuring schemeCommercial PVDF piezoelectric film in 30m width

was used. The film was packaged by polyethylene.Electrodes were made from aluminum alloy, and copperlines were used as wires. The sensor was slim, soft andwas good in conductivity and fatigue resistance.

Charge mode and current mode are two types of

measuring methods in PVDF sensing. In charge mode, thesignal was sent to oscilloscope through charge integrator.By measuring the voltage that is proportional to exertedpressure, time dependent variations of pressure can be got.In current mode, the sensor discharges through a resistorlinked between the two electrodes. Measuring the resistorvoltage, the current in the circuit can be got, whichreflected the charge strength. In this mode, the datameasured directly is the derivative of pressure. Pressuresignals can be acquired by time integration. Due to thelinear relations between pressure induced strains in PVDFand charges caused by polarization, charge mode wasadopted in this paper.

Figure 2. Equivalent circuit in actual measurement of PVDF polymer

in charge mode

In charge mode, when charges accumulated in the twopolar plates to some extent, the piezoelectric componentcan be looked on as a charge source. So, the circuit modelconsisted of a charge source Qthat parallel connected with

a capacitor aC . Due to the existence of charge leak both in

the electronic components and other media, an equivalentresistor Ra should also be connected parallelly. In actual

measurement, the capacitors iC and resistors iR in

external circuit should also be considered. Fig.2 gave theequivalent circuit scheme in actual measurement of PVDFpolymer in charge mode. Because of energy losses, theoutput voltage was lower that its theoretical values.

III. DESIGN OF MEASUREMENT CIRCUIT

The measuring system used in this paper consisted ofPVDF sensor, charge amplifier, signal filter, A/D, MCUand LCD. The sensor transformed mechanical signals intocharge signals that cannot be measured directly. Then,

charge signals were converted into voltage signals. Toeliminate interference from 50Hz power frequency,voltage signals were processed by filter circuit. Then, the

Q

Ca CcRa Ri Ci

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signals were transformed into digital through A/D andwere sent to MCU for processing and display.

A. Charge amplification circuitFig.3 schematically showed the charge amplifier,

which was a secondary amplification unit consisting ofintegrated operational amplifiers. It had two functions: thefirst is to match with the impedance of the piezoelectricthin film sensor; and the second is to transform andamplify the weak charge signals.

3

2

6

7

4 5 1 8

U?

CA3140

3

2

1

4

11

U?A

LM324

0.1uF

1K

10M

2k

2k

+5

+5

-5-5

Figure 3. Charge amplifier schematic

Due to the leakage of the charge on PVDF film throughdischarge circuit, errors emerged. So, discharge timeconstant in the circuit should be increased in the amplifier,so as to minimize the error. Increasing the capacity andresistance in the circuit are two methods to increase timeconstant. Yet, the sensitivity of PVDF film will decreasewith the increasing of circuit capacity. So, in this paper,preamplifier with high input impedance was utilized,which can improve time constant.

High input impedance operational amplifier CA3140was used here. Its feedback capacitance, feedbackresistance, time constant and cutoff frequency were 0.1F,10M, 1s and 0.16Hz respectively. Charge amplifyingcomponents can produce 500mV voltage signal. Then, thevoltage was amplified by a standard amplifier LM324, asis shown in Fig.3.

B. Signal filter circuitNoises in the system came from components, 50Hz

frequency interference, electromagnetic interference andthermoelectric effects. Low noise preamplifier and high-precision metal film resistors and tantalum capacitors were

used to inhabit noises from components. Through trapfilter and MCU processing, 50Hz frequency interferencewere well inhabited. Fig.4 showed the schematic offiltering unit, which was consisted of Butterworth low-passfilter with 500Hz cutoff frequency. Fig.5 showed 50Hzdouble T trap circuit.

R1

10K

R2

10K

R3

10K

R4

10K

R5

10K3

2

1

4

11

U1A

LM324

3

2

1

4

11

U2A

LM324

C1

0.1uF

C2

0.022uF

C4

0.1uF

C5

0.047uF

C3

0.033uF

Figure 4. Low-pass filter with 500Hz cutoff frequency

Figure 5. 50Hz double T trap circuit

C. Data processing and displayA/D transform circuit used ADC0832 (National

Semiconductor, US), which was a chip with 8-digitalresolution and double channels. MCU and display used inthis paper chose AT89C51 and LCD1602 respectively.Fig.6 showed the scheme of interface circuit between A/D

transform chip ADC0832 and MCU. Fig.7 was theinterface circuit scheme between LCD and MCU.

CH02

CH13

CS1

CLK7

DI5

VREF8

DO 6

U?

ADC0832

P101

P112

P123

P134

P145

P156

P167

P178

U?

VCC

Figure 6. Interface scheme between A/D transform chip ADC0832 andMCU

EA/VP31

X119

X218

RESET9

RD17

WR16

INT012

INT113

T014

T115

P101

P112

P123

P134

P145

P156

P167

P178

P00 39

P01 38

P02 37

P03 36

P04 35

P05 34

P06 33

P07 32

P20 21

P21 22

P22 23

P23 24

P24 25

P25 26

P26 27

P27 28

PSEN 29

ALE/P 30

TXD 11

RXD 10

U?

8031

Y?

12.000MHZ

C?30pF

C?30pF

12345678910111213141516

RP?16PIN

VCC

R?10K

VCC

VCC

1 2 3 4 5 6 7 8

16

15

14

13

12

11

10

9

RP110K

VCC

R2

10K

Figure 7. Interface scheme between LCD and MCU

Using the two circuits in Fig.6 and Fig.7, the acquireddata can be turned into digital signals and displayeddirectly in LCD, which facilitated the succeedingprocessing and analysis.

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IV. PROGRAM DESIGN AND SIMULATION RESULTS

The program was written by assembly language. Themain program consisted of initial, read and displayfunctions, which completed the initialization of LCD, dataacquisition and processing and data display.

Eliminating power frequency interference is a key part

of the program. Through effective algorithms, theinterference was minimized. A/D transformation is anotherkey part in the program. Using this part, data can beacquired at 5ms intervals. Take mean values of every 4sequent measured amounts. And, then take the maximumdata of 50 such mean values and saved it to MCU. Afterthat, the data was sent to LCD. Thus, the process of PVDFmechanical sensing was finished.

0 1 2 3 4 5

0

50

100

150

200

250

DisplayValue(a.u.)

Voltage (V)

Sin50Hz

DC

Figure 8. Simulation graph of A/D and display circuit by Proteus

Using Proteus software, the A/D and display circuit canbe simulated. Fig.8 showed the outcome of Proteussimulation, which illustrated linear relations between theinput voltage and displayed value. When inputting 50Hzsinusoidal and DC signals, the values displayed in LCDkept stable, which proved that the 50Hz notch filter raneffectively. By debuging, the final output voltage of thesensing circuit was 0~1.5V.

V. CONCLUSION

In this paper, a mechanical sensing system based onPVDF piezoelectric sensor was designed. Firstly, usingPVDF piezoelectric film, a mechanical sensor wasproduced. And the piezoelectric equation of PVDF wasstudied and simplified for the following analysis in this

paper. Moreover, the algorithm of power frequencyelimination was detailed studied. 4-times sampling wasadopted during A/D transformation and the noise inducedby 50Hz power frequency was restrained heavily.Butterworth low-pass filter and trap filter was used fornoise filtering. The measuring system used in this paperconsisted of PVDF sensor, charge amplifier, signal filter,A/D, MCU and LCD. The data acquisition and processingprogram was also written. Simulation and experimentsproved the effective of this system. Further study will becarried out in sensing ultra-fast and ultra-high frequencymechanical signals.

ACKNOWLEDGMENT

This work is supported by Natural Science Foundationof China (Grant No. 51005062), Natural ScienceFoundation of Shandong Province (Grant No.ZR2009FL001) and Science Foundation of HarbinInstitute of Technology at Weihai (Grant No.HIT(WH)XB200805).

REFERENCES

[1] Kawai, H., The piezoelectricity of poly (vinylidene fluoride).Japanese Journal of Applied Physics, 1969. 8: p. 975.

[2] Kepler, R., Piezoelectricity, Pyroelectricity, and Ferroelectricity inOrganic Materials. Annual Review of Physical Chemistry, 1978.29(1): p. 497-518.

[3] Bauer, F., PVDF shock sensors: applications to polar materials andhigh explosives. Ultrasonics, Ferroelectrics and Frequency Control,IEEE Transactions on, 2002. 47(6): p. 1448-1454.

[4] Klein, R.J., et al., Influence of composition on relaxor ferroelectricand electromechanical properties of poly (vinylidene fluoride-trifluoroethylene-chlorofluoroethylene). Journal of AppliedPhysics, 2009. 97(9): p. 094105.