564 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006 A Semicylinrical Capacitive Sensor With Interface Circuit Use for Flow Rate Measurement Cheng-Ta Chiang, Member, IEEE, an Yu-Chung Huang Abstract In this paper, a semicylinrical capacitive sensor with an interface circuit use for flow rate measurement is propose. The numerical analysis metho to calculate the capacitance of the semicylinrical capacitive sensor is analyze an iscusse. The picofara-range capacitive variation of the semicylinrical capacitive sensor can be etecte an converte into voltage variation by the interface circuit. Besies, the interface circuit is compact enough to simplify the circuit complexity an coul be easily implemente for flow rate measurement. All the functions of the capacitive sensing metho of the semicylinrical capacitive sensor use for flow rate measurement are prove successfully through HSPICE simulation. Measurement results have successfully confirme the correct functions an performance of the semicylinrical capacitive sensor with an interface circuit use for flow rate measurement, which ranges from 0.36 to 4.746 L/min, on the liqui crystal isplay panel coating machine. Inex Terms Capacitive sensor, flow rate measurement, HSPICE, interface circuit, liqui crystal isplay (LCD), switche capacitor. I. INTRODUCTION RECENTLY, flow etection for process control has mae important emans in the inustry such as panel coating techniques in the optoelectronics inustry an titrator evelopments in biotechnology. Generally, the methoologies for flow etection are istinguishe accoring to the material types (gas or liqui), properties (electric conuctivity, viscosity, an corrosiveness), an flow rate (slow or fast). Such traitional proucts to eal with flow etection such as ifferentialpressure flow meter [], rotameter, positive-isplacement (PD) flow meter, vortex flow meter [2], [3], electromagnetic flow meter [4], [5], ultrasonic flow meter [6] [8], etc., have been built. However, the manufacture techniques an the cost of the present flow meters are quite high. Besies, the congenital limitations of flow meters shoul be notice. For example, an electromagnetic flow meter is not suitable for use in nonelectric conuctive materials. Hence, to overcome the limitations of material properties an simplify complicate etecting flow systems to ecrease cost for the inustry, an innovative flow measuring using the sensing techniques of the capacitive sensor has been inspire. Manuscript receive January 4, 2006; revise July 20, 2006. This work was supporte by the National Science Council, Taiwan, uner Contract NSC92-228-E-009-0. The associate eitor coorinating the review of this paper an approving it for publication was Dr. Ai-Qun Liu. The authors are with the Measurement Techniques Laboratory, Department of Electronics Engineering an Institute of Electronics, National Chiao Tung University, Hsinchu 300, Taiwan, R.O.C. (e-mail: ctchiang23.ee90g@ nctu.eu.tw). Digital Object Ientifier 0.09/JSEN.2006.883847 Fig.. Architecture of the semicylinrical capacitive sensor. (a) Without ielectric flui. (b) With ielectric flui. Capacitive sensors have been use in various sensing applications, such as acceleration, force, pressure, ielectric properties, an isplacement sensors [9], [0]. Among them, in comparison with the researches of parallel-plate capacitors, research into cylinrical capacitive sensors have been mainly focuse on the analyses of properties, for example, the electrostatic forces of the charge coaxial cylinrical capacitor [], [2] an nonlinear analysis of the cylinrical capacitive sensor [3]. Until now, no stuy has analyze an iscusse semicylinrical capacitive sensors use in fluiic sensing applications. Hence, iffering from the structures of cylinrical capacitive sensors, a novel semicylinrical capacitive sensor is first investigate for fluiic measurement [4]. In this paper, a semicylinrical capacitive sensor with an interface circuit use for flow rate measurement has been propose. The numerical analysis metho use to calculate the capacitance of the semicylinrical capacitive sensor is analyze an iscusse. The picofara-range capacitive variation of the semicylinrical capacitive sensor can be etecte an converte into voltage variation by the interface circuit. Besies, the interface circuit is compact enough to simplify the circuit complexity an coul be easily implemente for flow rate measurement. All the functions of the capacitive sensing metho of the semicylinrical capacitive sensor use for flow rate measurement have been prove successfully through HSPICE simulation. Measurement results have successfully confirme the correct functions an performance of the semicylinrical capacitive sensor with an interface circuit use for flow rate measurement on the liqui-crystal-isplay (LCD) panel coating machine. Section II escribes the capacitive sensing metho to calculate the capacitance of the semicylinrical capacitive sensor. 530-437X/$20.00 2006 IEEE
CHIANG AND HUANG: CAPACITIVE SENSOR WITH INTERFACE CIRCUIT USED FOR FLOW RATE MEASUREMENT 565 Fig. 2. Capacitive sensing metho for the semicylinrical capacitive sensor. Fig. 4. (a) Top view of the semicylinrical capacitive sensor with ielectric flui. (b) Approximate structure of the semicylinrical capacitive sensor for numerical analysis metho. (c) Equivalent capacitors between A an B terminals. Fig. 3. (a) Top view of the semicylinrical capacitive sensor without ielectric flui. (b) Approximate structure of the semicylinrical capacitive sensor for numerical analysis metho. (c) Equivalent capacitors between A an B terminals. Section III isplays the architecture of the interface circuit an simulation results. Section IV presents the measurement results. Section V gives the conclusion an future works. II. CAPACITIVE SENSING METHOD FOR SEMICYLINDRICAL CAPACITIVE SENSOR Generally, to calculate the capacitance of capacitive sensors, Maxwell s equations relating electric an magnetic fiels an charge ensity shoul be use. However, the analysis an calculation uner such fiels is quite ifficult an complicate. By simplifying the complexity, electrostatic analysis, which is use to reuce the iscovery of the complicate electric fiel prouce by various charge istributions of materials with various ielectric constants, is rather suitable in calculating the capacitance of capacitive sensors. Thus, base on the concept of electrostatic analysis, the capacitive sensing metho for this semicylinrical capacitive sensor is analyze an iscusse below. Fig. (a) an (b) shows the architecture of the semicylinrical capacitive sensor without an with ielectric flui. The semicylinrical capacitive sensor consists of two metal semi- Fig. 5. (a) Interface circuit. (b) Timing iagram of Vs an Vs2 nonoverlapping two-phase clock cycles. cyliners, which are separate by a gap istance. The ielectric material in Fig. (a) is air; thus, the ielectric constant is equal to one. In Fig. (b), the ielectric constant ε 2 of the specifie flui is more than two. First, the capacitance of the two unit metal plates shoul be analyze an iscusse. In Fig. 2, the capacitance is erive as C a 0 c a 0 ε 0 a x + xθ ε 0 A [ aθ ] where ε 0 is the permittivity of free space of magnitue 8.85 (pf/m), is the ielectric constant, A is the area of a ()
566 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006 Fig. 8. Calculate results from the output of the interface circuit by performing (7) to simulate the flow rate measurement uner varie flow rates, which ranges from 0.05 to, normalize with the maximum flow spee. Fig. 6. Capacitor ratio [(Ccyl Cref)/Ci] 0. (b) Capacitor ratio [(Ccyl Cref)/Ci] 0.0. (c) Capacitor ratio [(Ccyl Cref)/Ci] 0. uner Ci 000 pf, Cref 20pF, an Ccyl 20, 30, an 20 pf, respectively. following numerical analysis of the semicylinrical capacitive sensor. Fig. 3(a) isplays the top view of the semicylinrical capacitive sensor without ielectric flui. The two metal semicyliners have the raius R an a minimum gap istance. The numerical analysis metho is applie to approximate the capacitance of the semicylinrical capacitive sensor. The structure of the semicylinrical capacitive sensor for the numerical analysis metho is shown in Fig. 3(b). Base on the structure of Fig. 2, each iniviual capacitor within two metal semicyliners coul be moifie as each pairs of two unit metal plates with an increment istance. Hence, all the equivalent capacitors coul be parallelly structure between A an B terminals, as presente in Fig. 3(c). Following (2), the capacitance of two metal semicyliners without ielectric flui can be expresse as C 0 Ci( + i) i0 2 ε 0 A [ ] + + + +2 + + +(n ) Fig. 7. Calculate results from the output of the interface circuit by performing (7) uner Ci 000 pf, Cref 20pF, an Ccyl sweeping from 20 to 20 pf. unit metal plate, an is the minimum gap istance. Therefore, if the cutting number of the semicylinrical capacitive sensor is large ( aθ), the capacitance in () can be approximate as [ ] C ε0 A. (2) Hence, it can be notice that the minimum gap istance of two unit metal plates will be the most important factor in the + ε 0 A 2R 2 ε 0 A i [ ] +(i ) + ε 0 A (3) 2R where ε 0 is the permittivity of free space of magnitue 8.85 (pf/m), is the ielectric constant of air, n is the cutting number for numerical analysis, A is the unit area of metal semicyliners, is the minimum gap istance, an is an increment istance. Similarly, Fig. 4 isplays the structure of the semicylinrical capacitive sensor with ielectric flui for the numerical analysis metho. Base on (3), the
CHIANG AND HUANG: CAPACITIVE SENSOR WITH INTERFACE CIRCUIT USED FOR FLOW RATE MEASUREMENT 567 Fig. 9. Manufacture structure of the semicylinrical capacitive sensor. The material of the flui is acrylonitrile butaiene styrene (ABS). Two metal semicyliners are covere on the insie bakelite. The maximum raius of the flui is 5 mm, an the minimum gap istance of two metal semicyliners is 0.2 mm. capacitance of two metal semicyliners with ielectric flui can be written as C Cj(+j) j0 2 [ ε 0 A + + ε 0 A + + + 2R 2 + [ 2 ε 0 A j ε 0 A + 2[R (+n)] + + + + + + +(j ) + +(n ) where ε 2 is the ielectric constant of the specifie flui. Therefore, as analyze in (3) an (4), the capacitance of the semicylinrical capacitive sensor varies when the ielectric flui flows through these two metal semicyliners. Moreover, accoring to the efinition of the flow rate Q, Q can be expresse by ] ] (4) Flow rate (Q) (L/ min) V t π h r2 t (5) where r an h are the fluiic raius an the fluiic length flowing through the two metal semicyliners, respectively. Combine with (4) an (5), the flow rate Q is moifie as Flow rate (Q) (L/ min) V t h n j0 Cj( + j). (6) t Fig. 0. Photographs of (a) semicylinrical capacitive sensor with an interface circuit applie to the LCD panel coating machine. (b) Uner the ABS flow rate of.434 L/min. (c) Uner a flow rate of 4.34 L/min. Therefore, the capacitive variation n j0 Cj( + j) has a proportional relationship with the flow rate Q. All the simulations use to verify the capacitive sensing metho of the
568 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006 Fig.. Measure results of the interface circuit uner the ABS flow rates. (a) 4.746 L/min. (b) 3.338 L/min. (c).434 L/min. () 0.36 L/min. semicylinrical capacitive sensor for flow rate measurement are analyze in Section III. III. INTERFACE CIRCUIT AND SIMULATION RESULTS Following the switche-capacitor techniques, the compact interface circuit an timing iagram are shown in Fig. 5(a) an (b). This circuit applies the charge reistribution metho. The signals Vs an Vs2 are nonoverlapping two-phase clock cycles. When Vs signal is logic high, the total charge is store on the semicylinrical capacitor Ccyl. The output voltage of operational amplifier is fixe at controlling voltage Vb. In another phase, the same reference c voltage Vr charges the reference capacitor Cref. Besies, the ifference in charge between two phases will be store on the capacitor Ci. Hence, the capacitor ratio (Ccyl Cref)/Ci is erive as Ccyl Cref Ci Vout Vb Vr Vb. (7) All simulation results are base upon the evice parameters of 0.35-µm 2P4M CMOS technology with a 3-V power supply. The two-phase clock cycles Vs an Vs2 are both operate at 50 khz to perform the functions of an interface circuit. Then, the output of the interface circuit is use to calculate the capacitor ratio. As shown in Fig. 6(a) (c), this interface circuit has performs well in converting picofara-range capacitive variation into voltage variation. These output voltages are.5,.53, an.67 V, respectively. All the calculate results, by performing (7), are plotte in Fig. 7. MATLAB an HSPICE software are performe (3) (7) to simulate the varie flow rate measurement. Taking the simulation uner the maximum flow rate as an example, the capacitors C 0 an C, by performing the MATLAB, are calculate as Fig. 2. Calculate results from the measurement output of the interface circuit by performing (7) uner varie ABS flow rates, which ranges from 0.36 to 4.746 L/min. The bigger capacitance is expecte to increase the ABS flow rate, an measure results are observe. 3.7382 an 24.256 pf. Then, to verify through HSPICE, these two capacitors C 0 an C are replace as capacitor Cref an Ccyl, respectively. After simulation, the output voltage of the interface circuit is.52 V, an the calculate result, by performing (7), is 0.033. Similarly, uner other flow rates, simulations are also performe in the same way. Finally, all the calculate results uner varie flow rates, which ranges from 0.05 to, normalize with the maximum flow spee, are shown in Fig. 8. These simulation results above have successfully confirme the correct functions of the capacitive sensing metho of the semicylinrical capacitive sensor an performance of the semicylinrical capacitive sensor with an interface circuit use for flow rate measurement.
CHIANG AND HUANG: CAPACITIVE SENSOR WITH INTERFACE CIRCUIT USED FOR FLOW RATE MEASUREMENT 569 TABLE I SUMMARY OF THE CHARACTERISTICS OF THE SEMICYLINDRICAL CAPACITIVE SENSOR WITH AN INTERFACE CIRCUIT USED FOR FLOW RATE MEASUREMENT IV. MEASUREMENT RESULTS Fig. 9 shows a photograph of the manufacture structure of the semicylinrical capacitive sensor. The material of flui is acrylonitrile butaiene styrene (ABS). Two metal semicyliners are covere on the insie bakelite. The maximum raius of the flui is 5 mm, an the minimum gap istance of two metal semicyliners is 0.2 mm. Moreover, a prototype base on the circuit shown in Fig. 5 has been built. The switches an an opamp are implemente with analog bilateral switches CD4066 an LF4, respectively. The two nonoverlapping phase clock cycles are implemente with some gates. Fig. 0 shows the photographs of a set of the semicylinrical capacitive sensor with an interface circuit applie to the LCD panel coating machine. The hea of the ABS container is connecte to the air compressor through a soft pipe. The air compressor an the movement of the LCD panel coating machine are precisely controlle by the mainframe of the factory. Moreover, the ABS flow rate can be known from the exclusive lookup table recore on the mainframe, which is liste in the scale of pouns per square inch for the air compressor versus the scale of liter per minute for the ABS flowing through the container. Therefore, accoring to the lookup table, the ABS flow rate of the LCD panel coating machine coul be precisely ajuste by the mainframe. Besies, by controlling the air compressor cooperating with the mainframe, the ABS can uniformly flow on the LCD panel, an the viscosity property of the ABS can also be overcome, as shown in Fig. 0(b) an (c). Fig. isplays the oscilloscope traces of the output voltage of the interface circuit uner the ABS flow rates: Fig. (a) at 4.746 L/min; Fig. (b) at 3.338 L/min; Fig. (c) at.434 L/min; an Fig. () at 0.36 L/min. Hence, accoring to the measurement output of the interface circuit, all the calculate results, by performing (7), are plotte in Fig. 2. The varie ABS flow rates range from 0.36 to 4.746 L/min. As iscusse in Section II, the bigger capacitance is expecte to increase the ABS flow rate, an measure results are observe. Hence, by using the novel flow measuring techniques on the LCD panel coating machine, the mainframe can perioically monitor an immeiately ajust the ABS flow rate accoring to the calculate results. Measurement results have successfully confirme the correct functions an performance of the semicylinrical capacitive sensor with an interface circuit use for flow rate measurement on the LCD panel coating machine. The characteristics of the semicylinrical capacitive sensor with an interface circuit use for flow rate measurement are summarize in Table I. V. C ONCLUSION A semicylinrical capacitive sensor with an interface circuit use for flow rate measurement is newly propose. The numerical analysis metho use to calculate the capacitance of the semicylinrical capacitive sensor is analyze an iscusse. The picofara-range capacitive variation of the semicylinrical capacitive sensor can be etecte an converte into voltage variation by the interface circuit. Besies, the interface circuit is compact enough to simplify the circuit complexity an coul easily be implemente for flow rate measurement. All the functions of the capacitive sensing metho of the semicylinrical capacitive sensor an performance of the semicylinrical capacitive sensor with an interface circuit use for flow rate measurement are successfully verifie. In future research, this semicylinrical capacitive sensor with an interface circuit will be aaptively ajuste accoring to the esire applications, for example, applie in the meical instrumentation systems of the intravenous rip. ACKNOWLEDGMENT The authors woul like to thank Chi-Mei Corporation for their support in this paper. REFERENCES [] J. Q. Zhang an Y. Yan, A self-valiating ifferential-pressure flow sensor, in Proc. IEEE IMTC, May 200, vol. 2, pp. 227 230. [2] C. Filips an V. H. Hans, Comparison of analogous an igital emoulation methos of moulate ultrasonic signals in vortex flow metering, in Proc. IEEE IMTC, May 2002, vol. 2, pp. 657 660. [3] V. H. Hans, New aspects of the arrangement an geometry of bluff boies in ultrasonic vortex flow meters, in Proc. IEEE IMTC, May 2002, vol. 2, pp. 66 664. [4] A. Michalski an S. Wincenciak, Metho of optimization of primary transucer for electromagnetic flow meter, in Proc. IEEE IMTC, Jun. 996, vol. 2, pp. 350 353. [5] A. Michalski, J. 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570 IEEE SENSORS JOURNAL, VOL. 6, NO. 6, DECEMBER 2006 [8] T. Vontz an V. Magori, An ultrasonic flow meter for inustrial applications using a helical soun path, in Proc. IEEE Ultrason. Symp., Nov. 996, vol. 2, pp. 047 050. [9] T. N. Toth an G. C. M. Meijer, A low-cost smart capacitive position sensor, IEEE Trans. Instrum. Meas., vol. 4, no. 6, pp. 04 044, Dec. 992. [0] L. K. Baxter, Capacitive Sensors Design an Applications. New York: IEEE Press, 997. [] Y. S. Ku, Developing an axisymmetric parametric moel for an electrostatic force balance, Meas. Sci. Technol., vol. 5, no., pp. 267 270, Jan. 2004. [2], Optimization of axisymmetric parametric moel for electrostatic force balance, Meas. Sci. Technol., vol. 5, no. 0, pp. 208 22, Oct. 2004. [3] H. J. Ahn, I. H. Kim, an D. C. Han, Nonlinear analysis of cylinrical capacitive sensor, Meas. Sci. Technol., vol. 6, no. 3, pp. 699 706, Mar. 2005. [4] C.-T. Chiang an Y.-C. Huang, A semi-cylinrical capacitive sensor with interface circuit using for fluiic measuring, in Proc. IEEE IMTC, Apr. 2006, pp. 238 2322. Yu-Chung Huang receive the M.S. egree in electrical engineering an the Ph.D. egree in process engineering from Technology University of Berlin, Berlin, Germany, in 982 an 985, respectively. Since 985, he has been a Professor with the Department of Electronics, National Chiao Tung University, Hsinchu, Taiwan, R.O.C. His research interests are sensors an measuring technologies. Prof. Huang is a member of the Committee of the Chinese Metrology Society an the Micromechanical Science Institute, China. Cheng-Ta Chiang (S 00 M 05) was born in Taiwan, R.O.C., in 977. He receive the B.S. egree in electronics engineering from Chung-Yuan Christian University, Chung-Li, Taiwan, in 999 an the M.S. egree in biomeical engineering from National Cheng-Kung University, Tainan, Taiwan, in 200. He is currently working towar the Ph.D. egree at the Institute of Electronics, National Chiao-Tung University, Hsinchu, Taiwan. His main research interests have been with analog integrate circuits, biomeical electronics, neuromorphic vision chips, an sensor signal conitioning. Mr. Chiang receive the visiting scholar appointment for the perio October, 2004 November 30, 2005, with the Department of Electrical an Computer Engineering, The Johns Hopkins University, Baltimore, MD. He has been selecte to be inclue in the Marquis Who s Who in Science an Engineering, 9th e., 2006 2007.