RSM2013 Proc. 2013, Langkawi, Malaysia

A Novel of Fluidic Based One Side Electrode Type Pressure Sensor

Mohd Norzaidi Mat Nawi1, Asrulnizam Abd Manaf1, Mohamad Faizal Abd Rahman1 , Mohd Rizal Arshad1,

Othman Sidek2 1Underwater Robotics Research Group (URRG), School of Electrical and Electronic Engineering,

2Collaborative MicroElectronic Design Excellence Centre (CEDEC), Universiti Sains Malaysia Engineering Campus, 14300 Nibong Tebal, Pulau Pinang, Malaysia.

E-mail: norzaidiurrg@gmail.com Abstract—This paper presents the development of the novel fluidic based one side electrode pressure sensor. The structure of the sensor consists of container and an electrode. For the container, the membrane and microchannel were combined in one structure. Meanwhile, the design of the electrode was extended from the pattern of coplanar electrode based on the previous study. The microfluidic technology by using a small amount of liquid as a sensing element was proposed to measure the external pressure. When the external pressure was applied to the sensor, the membrane was deflected and the liquid displaced inside the microchannel. The sensing electrode measured the changes using electrical double layer concepts and gave the output in capacitance. The sensor was successfully fabricated for electrode using printed circuit board (PCB) process and the container using polymer fabrication process. The polymer such as PDMS was chosen because it was suitable to be implemented as a membrane. The methanol was used as a liquid due to its dielectric constant. The data experiment was recorded and measured using LCR meter for different applied pressure. The capacitance response was demonstrated for pressure 2kPa until 30kPa. The sensitivity of the sensor was 0.06pF/kPa. Also, the standard deviation of output capacitance equaled to 0.04.

Keywords-microfluidic; pressure sensor; capacitive; electrical double layer

I. INTRODUCTION Capacitive type sensors have found wide applications in

areas such as automotive, biomedical and industrial. By utilizing the capacitive effect, the capacitive type sensors measure the external types of force such as flow rate, pressure and other physical quantities. It has been developed by researchers for many years ago due to the advantages of the capacitive type sensor that has high resolution, high robustness and lower power consumption compared to the sensor using piezoresistive effects [1-3]. The example of sensor using the capacitive effect is pressure sensor where it converts the pressure change into capacitance variation. Generally, the capacitive pressure sensor consists of two electrodes parallel opposite which having a suspended structure and it measures the change in capacitance between this two electrodes. When the external pressure imparts the sensor, the electrode attached at the membrane or diaphragm is displaced and then gives the change in capacitance [4]. The micro machined capacitive pressure sensor has been developed using MEMS fabrication

technology and it allows the researcher to integrate with CMOS sensing circuits [5]. Another type of electrode structure is coplanar where it usually used dielectric material to form a capacitance [6].

Nowadays, the implementations of the microfluidic technology to the sensors continue to grow because it offers advantages in term of damping characteristic where it greatly enhances the external forces. The commonly used material in microfluidic technologies is PDMS polymer where it is more compatible which it provides greater advantages especially in mechanical yield strain than commonly used material such as silicon [7]. By using a simple fabrication process such as casting and molding, the PDMS can easily create any type of geometry for many applications. Polymers are significantly have lower cost to acquire and the fabrication process can be accomplished outside the cleanroom confinement and ease to handle [8]. The microfluidic based sensor offers a great enhancement to the sensor development where it only requires handling and processing of the small amount of liquid. Also, it can improve the resolution and reduce the size of the sensor [9].

In our work, we proposed the novel fluidic based one side electrode pressure sensor by implementing the microfluidic structure which used electrical double layer concept (EDLC) and PDMS material. The EDLC concept was already being used for some other application such as an inclinometer sensor [10], acoustic sensor [11] and detection of microdoplet [6]. In micro scale of the channel, the effect of liquid properties such as kinematic viscosity is dominated and cannot be ignored. Previously, the modeling of the sensor using same electrode pattern was done and it shows the suitable electrolyte is methanol [12]. The main objective of this study is to develop the sensor using the fluid technology by implementing the liquid and integrated with one side electrode. The membrane and microchannel are important parts in this sensing mechanism because it will transfer the external pressure based on membrane deflection to the liquid displaced inside microchannel.

In this paper, we discussed in details for the principle and sensor design. The experimental result was also demonstrated and discussed for input pressure within the range of 2kPa to 30kPa and lastly the conclusions were made.

37 978-1-4799-1183-7/13/$31.00 ©2013IEEE

RSM2013 Proc. 2013, Langkawi, Malaysia

II. SENSOR DESIGN AND PRINCIPLE As mentioned before, the fluidic pressure sensor consists of

the PDMS container and sensing electrode. The sensing electrode has a circular shape at the middle membrane and rod line along the microchannel meanwhile the common electrode was located around the circular electrode on one side with the sensing electrode. An electrolyte such as methanol was used as a liquid to fill in the cavity to form the ionic layer between electrodes on the substrate and behaved as a conductor [10]. The membrane was deflected when the external pressure applied to the sensor and the liquid inside microchannel moved at certain distances, l. By implementing the ionic layer concept, the capacitance was formed and capacitance changes depended on the surface area of electrode facing the electrolyte. Fig. 1(a) shows the sensor pattern for no load and applied pressure condition. Schematic diagram of the fluidic based capacitive pressure sensor was shown in Fig. 1(b), it consisted of container which composed of the membrane and microchannel, and also two electrodes which were common electrode and sensing electrode. Given the basic equation of the capacitance for initial condition based on the initial liquid displaced inside microchannel in (1). The relationship between change in capacitance C and change of liquid displaced l is given in (2)

dA

C oroo

εε= (1)

( ) lwd

AAd

CCC roo

roo Δ=−=−=Δ

εεεε (2)

where Co, Ao, o, r, d, and w are the initial capacitance, initial surface area of liquid, vacuum permittivity, dielectric constant, width and change of the length of the electrolyte, respectively.

III. EXPERIMENTAL

A. Sensor Fabrication For sensor fabrication, there were three major stages

including fabrication of electrode, container and sealing process. The copper electrode was fabricated on the top of composite material (Flame Retardant, FR-4) using printed circuit board (PCB) process. Then, the container was fabricated using polymer fabrication technology such as molding and casting. The mold was placed on the top of the petri dish (see Fig. 2a) and the PDMS solvent was poured into it. Before that, the solvent was mixed with PDMS base and its catalyst with ratio of 10:1. The bubble was removed by locating the mixed PDMS into the vacuum about 1 hour. The hardened PDMS was peeled off from the mold (see Fig. 2b). The PDMS solvent also was used as a layer on the top of electrode to act as an insulator. Finally, the container and electrode were sealed using PDMS. The Fig. 3 shows the fabricated sensor.

B. Experiment Setup The fluid based pressure sensor was tested by applying

varying compressive force which perpendicular to the membrane of the sensor. The sensor was connected to the LCR meter (GW Instek LCR-821) and computer as shown in Fig. 4. The RS232 cable was used to enable the LCR meter to communicate with the computer. The capacitance behavior for each measurement was recorded using provided custom GUI. The operating frequency was set 1.2 kHz to measure the capacitance response.

(b) Fig. 1 Fluid based capacitive pressure sensor (a) sensing principle;

(b) the schematic of the sensor

(a)

Fig. 3 The fabricated fluidic based capacitive pressure sensor

(a) (b) Fig. 2 (a) The mold placed on the top of Petri dish; (b) the container after

peeled off process

38

RSM2013 Proc. 2013, Langkawi, Malaysia

TABLE I

THE PROPERTIES OF METHANOL

Liquid Types of Properties

Surface tension (mN/m)

Dielectric constant

Kinematic Viscosity (kg/ms)

Methanol 791.8 32.6 0.7

IV. RESULT AND DISCUSSION The selection of liquid is also important and for this

experiment, the methanol was chosen. The methanol properties such as surface tension, dielectric constant and kinematic viscosity were given in Table 1. The methanol was injected inside the container until the initial length of liquid is 5mm. The pressure was applied straight on the top of the membrane which was perpendicular to the sensor. The applied pressure gave the membrane a deflection where high pressure will give high deflection. From that, the liquid inside microchannel was displaced at a certain distance depended on the membrane deflection. The change of liquid displacement was observed before and after applying the pressure as shown in Fig. 5. The liquid distance (blue color) inside the channel will displace when the pressure applied and it will return to the original distance once the pressure is released.

The liquid displaced by pressure was measured and recorded for several times. Pressure within the range of 2kPa until 30kPa was varied. Fig. 6 shows the experimental result for liquid displaced based on the different applied pressure. The lowest pressure 2kPa showed that the liquid displaced about 0.1mm and the maximum pressure 30kPa was about 13mm. This sensor was able to measure the small range of pressure input which below than 40kPa. However, for high pressure application which is more than 100kP, this sensor was not able to perform because it depended on the liquid displaced inside microchannel. Therefore, the modification of the microchannel and membrane need to be studied and discussed to achieve the sensor for high pressure application.

The resulting change of liquid displacement in microchannel converted the change in capacitance. Therefore, the capacitance increased as the pressure applied increased. For the capacitance measurement, we also varied the applied pressure and the response of capacitance was recorded using a computer. Fig. 7 shows the capacitance value for each pressure versus time. The increasing of pressure increased the capacitance. The initial capacitance was obtained when no load was applied and the change in capacitance was calculated between the current capacitance and initial capacitance. Then, the change in capacitance for each pressure applied was plotted in Fig. 8. The linear fit curve form the output showed that the result was linear. Given the relationship between input and output obtained from the graph is

PC 057.0037.0 +=Δ (3)

where C and P are changes in capacitance and applied pressure, respectively. The smallest pressure gave change in capacitance of 0.1 pF, meanwhile the largest pressure gave capacitance of 1.8 pF for 30kPa. The experimental result showed a standard deviation of capacitance was 0.04 for applied pressure at that range. The result showed that the stability and capability of the fluidic based one side electrode pressure sensor where we obtained the sensitivity of sensor was about 0.06 pF/kPa. This pressure sensor was suitable to be used in pneumatic and hydraulic applications to sense the low pressure below than 100kPa [12].

Fig. 5 The electrolyte movement (blue color) for certain applied

pressure.

Fig. 6 liquid displacement- pressure of the sensor performance

Fig 4. The experiment setup for the capacitive pressure sensor

39

RSM2013 Proc. 2013, Langkawi, Malaysia

V. CONCLUSION The fluidic based one side electrode pressure sensor has been fabricated by implementing the electrical double layer concept using two electrodes. The container which consisted of membrane and microchannel was fabricated using casting and molding fabrication process. The sensor also was tested for different pressure within the range of 2kPa to 30kPa. We successfully obtained the relation between the liquid displaced inside microchannel and applied pressure where we obtained the linear output. The experiments result also showed that the capacitance increased as the applied pressure increased. We were able to measure the external pressure for pressure lower than 30kPa. The sensitivity was calculated from the output capacitance response which equaled to 0.06pFa/kPa. This sensing method can be implemented for various types of application including the flow sensor for underwater application [13].

ACKNOWLEDGMENT The author is a USM fellowship holder. This study was

supported by internal grant 304/PELECT/60311037 from Universiti Sains Malaysia, Penang, Malaysia. The authors also would like to thank all members of Underwater Robotic Research Group (URRG), USM and School of Electrical and Electronics Engineering for their support and assistance throughout the work.

REFERENCES [1] R. C. Luo. “Sensor technologies and microsensor issues for

mechatronics systems”.,IEEE/ASME Transactions on Mechatronics, vol. 1, 1996, pp. 39-49.

[2] P. Eswaran, and S. Malarvizhi. "Sensitivity analysis on MEMS capacitive differential pressure sensor with bossed diaphragm membrane." IEEE International Conference on Devices, Circuits and Systems (ICDCS), 2012, pp. 704-707.

[3] W. P. Eaton and J.H. Smith, “Micromachined pressure sensors: review and recent developments” Proc. SPIE 3046, 1997, pp. 30– 41

[4] C. S Tsui. “Performance analysis in modeling micro capacitive pressure sensor”. IEEE 4th International In Microsystems, Packaging, Assembly and Circuits Technology Conference, 2009. IMPACT 2009., pp. 39-42.

[5] H. Y. Yu, M. Qin, J. Q. Huang, & Q. A. Huang. “A MEMS capacitive pressure sensor compatible with CMOS process”. In IEEE Sensors, 2012 pp. 1-4.

[6] C. Elbuken, T. Glawdel, D. Chan, & C. L. Ren, “Detection of microdroplet size and speed using capacitive sensors”. Sensors and Actuators A: Physical, vol. 171, 2011, pp. 55-62.

[7] W. N. Jr. Sharpe, K. M. Jackson, K. J. Hemker, & Z. Xie, “Effect of specimen size on Young's modulus and fracture strength of polysilicon”. Journal of Microelectromechanical Systems, vol. 10, 2001, pp. 317-326.

[8] C. Liu. “Recent developments in polymer MEMS”. Advanced Materials, vol. 19, 2007, pp. 3783-3790.

[9] Chou, Jung-Chuan, Wu, Da-Gong, Tseng, Shi-chang, Chen, Shien-Cheng, Ye, Guan-Chen. "Application of Microfluidic Device for Lactic Biosensor. IEEE Sensors Journal, vol. 13, no. 4, 2013, pp. 1363-1370.

[10] A. A. Manaf, K. Nakamura, & Y. Matsumoto. "Characterization of miniaturized one-side-electrode-type fluid-based inclinometer." Sensors and Actuators A: Physical, vol. 144, 2008,pp. 74-82.

[11] M. F. A. Rahman, M. R. Arshad, A. A. Manaf, M. I. H. Yaacob. "Modelling of a novel design of microfluidic based acoustic sensor, IEEE Regional Symposium on Micro and Nanoelectronics 2011 (RSM), 2011, pp. 56-59.

[12] B. P. Mahale, D. Bodas & S. A. Gangal. Development of PVdF based pressure sensor for low pressure application. In IEEE International Conference on Nano/Micro Engineered and Molecular Systems (NEMS), 2011 pp. 658-661.

[13] M. N. M. Nawi, A. A. Manaf, M. R. Arshad, O. Sidek. "Modeling of Novel Microfluidic based Flow Sensor Inspired from Fish Canal Neuromast for Underwater Sensing." Jurnal Teknologi vol. 62, 2013, pp. 33-38

Fig. 7 The output response for change in capacitance

Fig. 8 capacitance change- pressure of the sensor performance

40