IEEE SENSORS JOURNAL, VOL. 14, NO. 5, MAY 2014 1405

Data Fusion and Fault Diagnosis for FlexibleArrayed pH Sensor Measurement System

Based on LabVIEWJung-Chuan Chou, Member, IEEE, Chin-Yi Lin, Yi-Hung Liao, Member, IEEE, Jie-Ting Chen,

Ya-Li Tsai, Jia-Liang Chen, and Hsueh-Tao Chou

Abstract— This paper concerns the data fusion (DF) and faultdiagnosis (FD), which could increase the reliability of measuredresults when one sensing window was faulty. The flexible arrayedpH sensor was fabricated by radio frequency sputtering systemand screen-printed technology, which the ruthenium dioxidewas deposited on the polyethylene terephthalate substrate assensitive membranes and the miniature reference electrodes werefabricated by screen-printed technology. We measured pH buffersolutions for five times and the flexible arrayed pH sensor had awide sensing range of pH 1–13 solutions. The average sensitivityand linearity were 47.70 mV/pH and 0.839, respectively. We usedthe Laboratory Virtual Instrumentation Engineering Workbenchto do deficient diagnosis which could remove the fault sensingwindow. The average DF with FD, self-adaptive data fusion withFD and coefficient of variance data fusion with FD could increasesensitivity and linearity ∼22% and 0.14, respectively.

Index Terms— Data fusion, fault diagnosis, ruthenium dioxide,arrayed pH sensor, LabVIEW.

I. INTRODUCTION

THE measurement of pH value is an important parame-ter in clinical medical diagnosis, chemistry, agriculture,

waster resource management and environmental wastewatermonitoring. Accurate pH value is critical [1]. The rutheniumdioxide (RuO2) sensitive membrane was used to fabricatea hydrogen ion sensor [2]–[6], because the rutile crystalstructure easily forms hydrates. RuO2 sensitive membrane wasalso used to fabricate biosensor [7]–[9], gas sensor [10] andtemperature sensor [11]. Several methods were applied to pre-pare RuO2 sensitive membrane, such as screen-printing [12],chemical vapor deposition (CVD) [13], sputtering [14]sol-gel [15], etc.

Manuscript received September 11, 2013; accepted October 4, 2013. Date ofpublication December 23, 2013; date of current version March 11, 2014. Thiswork was supported by the National Science Council of Taiwan under ContractNSC 101-2221-E-224-046, Contract NSC 101-2221-E-265-001, and ContractNSC 102-2221-E-224-075. The associate editor coordinating the review ofthis paper and approving it for publication was Prof. Okyay Kaynak.

J.-C. Chou, C.-Y. Lin, J.-T. Chen, and H.-T. Chou are withthe Graduate School of Electronic and Optoelectronic Engineering,National Yunlin University of Science and Technology, Douliou 64002,Taiwan (e-mail: choujc@yuntech.edu.tw; m10113316@yuntech.edu.tw;m10013337@yuntech.edu.tw; chouht@yuntech.edu.tw).

Y. H. Liao is with the Department of Information Management, TransWorldUniversity, Douliou 64063, Taiwan (e-mail: liaoih@twu.edu.tw).

Y.-L. Tsai and J.-L. Chen are with the Department of Electronic Engineer-ing, National Yunlin University of Science and Technology, Douliou 64002,Taiwan (e-mail: u9813043@yuntech.edu.tw; u9913002@yuntech.edu.tw).

Digital Object Identifier 10.1109/JSEN.2013.2296148

Data fusion methods were used with measured data fromthe flexible arrayed pH sensor and improved accuracy ofa single sensor [16]. In last several years, the LaboratoryVirtual Instrumentation Engineering Workbench (LabVIEW)is gradually used as the built-in library and modules todesign program of virtual instrumentations and measurementsystem, especially for mechanical, biomedical engineering andelectronic. LabVIEW provided a relatively easy control ofthe hardware, and it had also provided methods to buildsimple graphical user interface (GUI) [17]. In the LabVIEWenvironment was developed a virtual instrument for monitoringthe analyzed gas mixtures [18].

RuO2 sensitive membrane was prepared by using radio fre-quency sputtering system in this study. The analyzed methodwas about response voltage that was measured on the flexiblearrayed sensor via voltage-time (V-T) measurement system,and the Origin drawing software was used as data analysis.The V-T measurement system was used LabVIEW graphicalprogramming language to measure the flexible arrayed pHsensor at the same time. The measurement results from theflexible arrayed pH sensor were computed via the averagedata fusion (ADF), self-adaptive data fusion (SADF), coef-ficient of variance data fusion (CVDF), average data fusionwith fault diagnosis (ADF_FD), self-adaptive data fusion withfault diagnosis (SADF_FD) and coefficient of variance datafusion with fault diagnosis (CVDF_FD) to obtain the optimalmeasured value. The fault diagnosis also used LabVIEW todetermine and removed sensing window, which the faultysensing window measured pH values was not correct.

II. EXPERIMENT

A. Materials

Ruthenium target was purchased from Ultimate MaterialsTechnology Co., Ltd. Taiwan. Polyethylene terephthalate(PET) was purchased from Zencatec Corporation, Ltd. Taiwan.Epoxy was purchased from Sil-More Industrial, Ltd.Taiwan. Silver paste was purchased from Advanced ElectronicMaterials Inc., Ltd. Taiwan. Data acquisition (DAQ) deviceand LabVIEW 2012 software were purchased from NationalInstrument (NI) Co., Ltd. Taiwan.

B. Fabrication of Flexible Arrayed pH Sensor andMeasurements System

The polyethylene terephthalate (PET) substrateswere cleaned in the ultrasonic bath with ethanol and

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1406 IEEE SENSORS JOURNAL, VOL. 14, NO. 5, MAY 2014

Fig. 1. Schematic structure of the flexible arrayed pH sensor.

Fig. 2. Practical size and the schematic diagram of flexible arrayed pH sensor.

deionized (D. I.) water for 10 minutes, respectively, whichcould remove adsorbed dust, grease and surface contamination.The flexible arrayed pH sensor included working electrode anddifferential reference electrode. The fabrication framework offlexible arrayed pH sensor was shown in Fig. 1.

In this study, the differential reference electrodes wereintegrated with the flexible arrayed pH sensor to form the2 × 3 working electrodes, as shown in Fig. 2. First, the ruthe-nium dioxide (RuO2) sensitive membranes were depositedon the PET substrate by radio frequency sputtering system.Then we prepared 2 × 3 working electrodes and silverreference electrodes by using a screen-printed method, andthe PET substrate was baked in the oven at 120 °C for30 minutes.

In this study, we used a real-time and programmable V-Tmeasurement system to analyze the 2 × 3 working electrodes(sensing windows). The measurement system was consisted offront part and back part devices. The front part represented theflexible arrayed sensor based on the PET substrate; the backpart contained a readout circuit (LT 1167), a DAQ device, andLabVIEW in personal computer. The measurement system wasshown in Fig. 3.

The output voltage of differential system was expressed asEq. (1).

Vout = Vout2 − Vout1 = (Vsen2−Vref)− (Vsen1−Vref)

= Vsen2−Vsen1 (1)

Where the Vout1 was the changed potential between work-ing electrode and reference electrode, Vout2 was changedpotential between contrast electrode and reference electrode,

Fig. 3. Schematic diagram of measurement system.

Fig. 4. Front panel window of LabVIEW for measurement data of sensingwindow 1 to sensing window 6.

Fig. 5. The values of standard deviation (σ ), mean (μ), variance (σ2), ADF,SADF and CVDF in the front panel window of LabVIEW.

Vsen1 was potential of working electrode, Vsen2 was potentialof contrast electrode, and Vref was potential of referenceelectrode.

The mean values of measured data were obtained for flexiblearrayed pH sensor in the ADF. The ADF was easy to becalculated. The SADF needed the mean and variance beforecalculating. CVDF needed to use the mean and standarddeviation [16].

We measured by immersing in different pH buffer solutionsfrom pH 1–13 to ensure the stability of sensitive membranes.The linearity and sensitivity were described by the softwareof Origin 7.0. The front panel windows of LabVIEW formeasurement system interface were shown in Figs. 4 and 5.The values of standard deviation (σ), mean (μ), variance (σ 2),

CHOU et al.: DF AND FD FOR FLEXIBLE ARRAYED pH SENSOR MEASUREMENT SYSTEM BASED ON LabVIEW 1407

Fig. 6. Front panel window of LabVIEW for measurement data of sensingwindow 1 to sensing window 6 after fault diagnosis.

Fig. 7. The values of standard deviation (σ ), mean (μ), variance (σ2), ADF,SADF and CVDF after fault diagnosis in the front panel window of LabVIEW.

ADF, SADF and CVDF in the front panel window ofLabVIEW were shown in Fig. 5.

The redox potential was determined by [19]

E = E0 − 2.303RT

FpH = E0 − 0.05916 pH (2)

where E0 is standard electrode potential, R is gas constant,T is absolute temperature and F is Faraday’s constant. Thus,the ideal value is 59.16 mV/pH at room temperature, whichis called the Nernstian response.

In Figs. 4 and 5, the measured value of sensing window 1was smaller than sensing window 2 to sensing window 6.We used SADF value to determine which the sensing windowwas faulty. As shown in Fig. 5, sensing window 1 of CVDFcoefficient was −0.00368157 that because the measured valuebetween sensing window 1 and sensing windows 2–6 wastoo large. So the resulted in coefficient became negative andthe lamp became red. The judgment different values were±30 mV, which Nernstian response was divided by two. If theLabVIEW determined the sensing window was breakdownthen would remove the fault sensing window and do datafusion again, as shown in Figs. 6 and 7.

III. RESULTS AND DISCUSSION

A. Measurement of the Flexible Arrayed pH Sensor WithAg Reference Electrodes

In this study, we measured the pH 1–13 buffer solu-tions with the flexible arrayed pH sensor based on Agreference electrodes for five times. The response voltages

Fig. 8. Response voltages of flexible arrayed pH sensor were measured bydifferent pH solutions from pH 1 to pH 13 based on Ag reference electrodes.

TABLE I

SENSITIVITIES AND LINEARITIES OF EACH SENSOR, DATA FUSION AND

FAULT DIAGNOSIS FOR FLEXIBLE ARRAYED pH SENSOR

of the flexible arrayed pH sensor in pH 1–13 buffer solu-tions were shown in Fig. 8. The average sensitivity andlinearity of sensing were 47.70 mV/pH and 0.839, respec-tively, as shown in Table I. The measurement data ofsensing windows 1–6 were calculated with ADF, SADFand CVDF, respectively, and obtained the sensitivities were47.96 mV/pH, 58.25 mV/pH, and 64.54 mV/pH, respectively.The linearities were also calculated with ADF, SADF, andCVDF, respectively, and obtained 0.978, 0.980 and 0.451,respectively.

In Fig. 9, the sensitivities of sensing window 2 to sens-ing window 6 of standard deviations were all smaller than3 mV/pH, which showed that the flexible arrayed pH sen-sor had good stability characteristics. However the sensing

1408 IEEE SENSORS JOURNAL, VOL. 14, NO. 5, MAY 2014

Fig. 9. Sensitivities of sensing windows from window 1 to window 6 basedon Ag reference electrodes.

Fig. 10. Linearities of sensing windows from window 1 to window 6 basedon Ag reference electrodes.

window 1 was faulty so that the standard deviation was94.95 mV/pH.

According to the experimental results, CVDF had the bestsensitivity but the linearity was not good, which caused bysensing window 1 was breakdown and measured pH val-ues was not correct. In Figs. 8–10, the sensing window 1was more unstable than other sensing windows. We usedLabVIEW to do deficient diagnosis which could removethe faulty sensing window. The result of comparing sens-ing window 1 to sensing window 6 with the ADF, SADF,CVDF, ADF_FD, SADF_FD and CVDF_FD were shown inTable I. After fault diagnosis the sensitivity and linearity ofADF_FD, SADF_FD and CVDF_FD were mutually and thesensitivity and linearity were close to 58 mV/pH and 0.98,respectively.

Sensitivity after computing with the ADF_FD thenincreased about 22.47% from 47.70 mV/pH to 58.42 mV/pH.Sensitivity after computing with the SADF_FD then increaseabout 22.24% from 47.70 mV/pH to 58.31 mV/pH. Andthe CVDF_FD increased about 22.49%, which was betterthan ADF_FD and SADF_FD. Furthermore, the data fusionresults were obviously superior to original data which without

TABLE II

RESULTS OF COMPARING SENSING WINDOW 1 TO SENSING WINDOW 6

WITH ADF, SADF, CVDF, ADF_FD, SADF_FD AND CVDF_FD

Fig. 11. Response voltages of flexible arrayed pH sensor were measuredby different pH solutions from pH 1 to pH 13 based on Ag/AgCl referenceelectrodes.

processing via data fusion methods. The results of sensitivityand linearity comparing sensing window 1 to sensing win-dow 6 with ADF, SADF, CVDF, ADF_FD, SADFFD andCVDF_FD were shown in Table II.

B. Measurement of the Flexible Arrayed pH Sensor WithAg/AgCl Reference Electrodes

We also used silver/silver chloride (Ag/AgCl) referenceelectrodes to confirm sensing window 1 was faulty andmeasured pH buffer solutions for five times. The averagesensitivity and linearity were 46.34 mV/pH and 0.907, respec-tively, as shown in Figs. 11–13. The sensitivities after com-puting the measurement results of the flexible arrayed pHsensor with the ADF, SADF and CVDF were 44.25 mV/pH,51.87 mV/pH, and 128.18 mV/pH, respectively. As well asthe linearities after computing with the ADF, SADF andCVDF were obtained 0.966, 0.998 and 0.832, respectively.The CVDF had a high sensitivity and the linearity was non-ideal. Due to sensing window 1 failed and reduced ADFvalue. The sensitivity and linearity of each sensor window,data fusion and fault diagnosis for the flexible arrayed pHsensor based on Ag/AgCl reference electrodes were shown inTable III.

CHOU et al.: DF AND FD FOR FLEXIBLE ARRAYED pH SENSOR MEASUREMENT SYSTEM BASED ON LabVIEW 1409

Fig. 12. Sensitivities of sensing windows from window 1 to window 6 basedon Ag/AgCl reference electrodes.

Fig. 13. Linearities of sensing windows from window 1 to window 6 basedon Ag/AgCl reference electrodes.

Sensitivities after computing with ADF_FD SADF_FDand CVDF_FD were increased about 12.11%, 12.30%, and12.41%, respectively. CVDF_FD was better than ADF_FDand SADF_FD but the sensitivity and linearity were mutu-ally close, which removed the faulty sensing window andshowed good stability and repeatability, sensitivity and lin-earity were close to 52 mV/pH and 0.998, respectively.The results of sensitivity and linearity comparing sens-ing window 1 to sensing window 6 based on Ag/AgClreference electrodes with ADF, SADF, CVDF, ADF_FD,SADF_FD and CVDF_FD were shown in Table IV. Linearityof CVDF increased about −0.075% that because CVDF wassmaller than the average of sensing window 1 to sensingwindow 6.

According to previous researches [8], [19]–[25], the com-parisons of sensing characteristics of pH sensors based onvarious sensing films were shown in Table V. The sensingwindow 4 had optimal sensitivity and linearity therefore weused sensing window 4 to compare with other literatures.In this study, the sensitivity of RuO2 sensitive membranebased on Ag reference electrodes was 57.1 mV/pH, which was

TABLE III

SENSITIVITIES AND LINEARITIES OF EACH SENSOR, DATA FUSION AND

FAULT DIAGNOSIS FOR FLEXIBLE ARRAYED pH SENSOR BASED

ON Ag/AgCl REFERENCE ELECTRODES

TABLE IV

RESULTS OF COMPARING SENSING WINDOW 1 TO SENSING WINDOW 6

BASED ON Ag/AgCl REFERENCE ELECTRODES WITH ADF, SADF,

CVDF, ADF_FD, SADF_FD AND CVDF_FD

better than other previous researches. Furthermore the flexiblearrayed pH sensor had wide sensing range from pH 1 to pH 13solutions.

The measurement results showed that the linearity of flexi-ble arrayed pH sensor with Ag reference electrode was lowerthan Ag/AgCl reference electrode, but the sensitivity of theflexible arrayed pH sensor with Ag reference electrode washigher than Ag/AgCl reference electrode. The Ag referenceelectrode could be stored in dry environment and the volumewas small. Therefore, the Ag reference electrode was moresuitable for flexible arrayed pH sensor than Ag/AgCl referenceelectrode.

1410 IEEE SENSORS JOURNAL, VOL. 14, NO. 5, MAY 2014

TABLE V

COMPARISONS OF PH SENSORS WITH THE PREVIOUS PAPERS [8], [19]–[25]

IV. CONCLUSION

This study used the flexible arrayed pH sensor to measurepH buffer solutions for five times with a V-T measurementsystem. Response voltages were used for ADF, SADF andCVDF to obtain the optimal sensitivity. Although the datafusion has been studied in previous researches [16] but wecould use LabVIEW to remove fault sensing window whichmeasured pH values was not correct. According to the exper-imental results, CVDF had the best sensitivity and linearitythan other data fusion methods, and the sensitivity and linearitywere 58.43 mV/pH and 0.981, respectively. But SADF couldimmediate to be used, which one sensing window was faultybefore without removing faulty sensing window. The datafusion results were obviously superior to original data whichwithout processing via data fusion method. Furthermore whenwe used fault diagnosis method, then the sensitivity andlinearity could reach to be achieved 58 mV/pH and 0.98,respectively. The experimental results would certainly surpassonly data fusion and the sensitivity and linearity increasedabout 22% and 0.14, respectively.

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Jung-Chuan Chou (M’01) was born in Tainan,Taiwan, on July 13, 1954. He received the B.S.degree in physics from the Kaohisung Normal Col-lege, Kaohsiung, Taiwan, in 1976, the M.S. degreein applied physics from Chung Yuan Christian Uni-versity, Chung-Li, Taiwan, in 1979, and the Ph.D.degree in electronics from National Chiao-TungUniversity, Hsinchu, Taiwan, in 1988. He taught atChung Yuan Christian University from 1979 to 1991.Since 1991, he has been an Associate Professor withthe Department of Electronic Engineering, National

Yunlin University of Science and Technology. Since 2010, he has been aProfessor with the Department of Electronic Engineering, National YunlinUniversity of Science and Technology. From 1997 to 2002, he was the Dean,Office of Technology Cooperation with the National Yunlin University ofScience and Technology. From 2002 to 2009, he was a Chief Secretary withthe National Yunlin University of Science and Technology; from 2009 to2010, he was the Director of the library with the National Yunlin Universityof Science and Technology. From 2010 to 2011, he was the Director of Officeof Research and Development with the National Yunlin University of Scienceand Technology. Since 2011, he has been a Distinguished Professor with theDepartment of Electronic Engineering, National Yunlin University of Scienceand Technology. His research interests are in the areas of sensor materialand device, biosensor and system, microelectronic engineering, optoelectronicengineering, solar cell, and solid state electronics.

Chin-Yi Lin was born in Taichung, Taiwan, onFebruary 2, 1990. He received the bachelor’sdegree from the Department of Electronic Engi-neering, National Chin-Yi University of Technology,Taichung, in 2012. He is currently pursuing the mas-ter’s degree at the Graduate School of Electronic andOptoelectronic Engineering, National Yunlin Univer-sity of Science and Technology, Yunlin, Taiwan. Hisresearch interests are LabVIEW design, biosensors,and their applications.

Yi-Hung Liao (M’12) was born in Yunlin,Taiwan, on January 10, 1963. He received the bach-elor’s in electronic engineering from the NationalTaiwan Institute of Technology, Taipei, Taiwan, in1990, the M.S. degree in electronic engineeringfrom the National Yunlin Institute of Technology,Yunlin, in 1997, and the Ph.D. degree from theGraduate School of Engineering Science and Tech-nology, National Yunlin University of Science andTechnology in 2010. His research interests includechemical sensors and their applications, the array

sensors and multisensors for biosensing, the characterization of biosensors,and implement home care system with PIC microprocessor.

Jie-Ting Chen was born in Taichung, Taiwan, onJanuary 20, 1989. He received the bachelor’s degreefrom the Department of Electrical Engineering,Da-Yeh University, Changhua, Taiwan, in 2011.He has been pursuing the master’s degree at theGraduate School of Electronic and OptoelectronicEngineering, National Yunlin University of Scienceand Technology, Douliou, Taiwan, since 2011. Hisresearch interests circuit system of biosensors andintegrates with wireless sensor network.

Ya-Li Tsai was born in Tainan, Taiwan, on Novem-ber 1, 1990. He received the bachelor’s degreefrom the Department of Electronic Engineering,National Yunlin University of Science and Tech-nology, Douliou, Taiwan, in 2013. His researchinterests are in MEMS technology, biosensor, andtheir applications.

Jia-Liang Chen was born in Taipei, Taiwan, onAugust 3, 1992. He is currently pursuing the bache-lor’s at the Department of Electronic Engineering,National United University, Douliou, Taiwan. Hisresearch interests include remote real-time monitor-ing, biosensor, and their applications.

Hsueh-Tao Chou was born in Taipei, Taiwan, onSeptember 8, 1958. He received the B.S. degree inphysics from National Central University, Taoyuan,Taiwan, in 1981, the M.S. degree in physicsand astronomy from National Central University,Taoyuan, Taiwan, in 1983, and the Ph.D. degreein physics from the University of Texas at Austin,Austin, TX, USA, in 1990. Since 1990, he hasbeen an Associate Professor with the Department ofElectronic Engineering, National Yunlin Universityof Science and Technology. His research interests

include semiconductor materials and devices, surface science and applications,optoelectronic engineering, and sensor devices.