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Dec 16, 2009 - An Electronic-Nose Sensor Node Based on Polymer. Coated Surface Acoustic Wave Array for. Environmental Monitoring. Kea-Tiong Tang\ ...
An Electronic-Nose Sensor Node Based on Polymer­ Coated Surface Acoustic Wave Array for Environmental Monitoring Kea-Tiong Tang\ Shih-Wen Chiu\ Hsu-Chao Hao2, Shang-Chia Wee, Tai-Hsuan Lin2, Chia-Min Yang4, Da-Jeng Yao2 and Wei-Chang Yeh3

IOepartment of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan 2Institute of NanoEngineering and MicroSystems, National Tsing Hua University, Hsinchu 30013, Taiwan 3 0epartment of Industrial Engineering and Engineering Management, National Tsing Hua University, Hsinchu 30013, Taiwan 40epartment of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan Email: [email protected] D

frequency, a SAW-based E-Nose is composed with large equipments such as spectrum analyzer or frequency counter. In environmental monitoring applications, it is impossible to equip every sensor node to have these large volume and high cost instruments. Therefore, developing a frequency readout circuitry to replace these instruments[12, 13] is the key to this application.

Abstract-We report an electronic-nose sensor node based on polymer-coated surface acoustic wave (SAW) sensor array for environmental

monitoring

applications.

The

sensor

node

consists of a SAW sensor array, its readout circuitry and a Wireless Sensor Network (WSN) platform. The 2 X 2 non­ continuous gas SAW sensor array is coated with different polymer composite materials for different gas detection. The frequency signals from the SAW array are processed by the frequency

readout

circuitry

to

obtain

frequency

In this paper, we report an E-Nose sensor node based on a 2x2 non-continuously chemical SAW sensor array chip[14] by using MEMS technology with polymers coated on the active sensing region by a self-assembled[15, 16] process. A small­ volume low-cost frequency readout circuit has been developed. In addition, placement of the sensor nodes in order to form an efficient sensing network is also considered. The system diagram of the proposed environmental monitoring network is shown in Fig. 1.

shift

information. The sensor data is transmitted from sensor nodes by Octopus II WSN platform. Experimental results have shown good performance of gas detection and recognition. In order to achieve network,

a

high the

efficient

decision

environmental

supporting

monitoring

system

by

sensing

implementing

weighted voting system (WVS) has also been considered.

I.

INTRODUCTION

Environmental monitoring attracts consideration because the global warning and climate change have profound impacts on our life. Environmental monitoring system is established to provide comparable, reliable, multi-disciplinary, long-term runs of data of environmental change and to widespread the use of Earth Observation (EO) [1-3]. Electronic nose (E-Nose) have found many applications such as food product quality control[4], indoor air quality monitoring[5], automotive industry[6], clinical diagnosis[7], and environmental monitoring[8-1O]. If the E-Nose system could be integrated into sensor nodes, an environmental sensing network which could monitor regional global warning and climate change may be established. Polymer-coated surface acoustic wave (SAW) devices have been used in E-Nose because of their high sensitivity to change physical and chemical properties at the surface of the transducer system[ll]. Traditionally, in order to read out the

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118

II.

THE ELECTRONIC-NoSE SENSOR NODE

The basic structure for each sensor node, which is shown in Fig. 2, includes a 2 X 2 non-continuous gas surface acoustic wave (SAW) sensor array and its readout circuit. The output of the readout circuit is sent to a Wireless Sensor Network (WSN) platform such as Octopus II (described in later section) to transmit the sensor node data to the server.

Sensing Network

Figure l.

System diagram of the proposed network.

Exposed 1

between the molecules and the film[18]. To improve the selectivity and sensitivity, six polymers such as polyvinyl acetate (PVAc), poly4-vinylphenol (P4VP), polystyrene (PS), polystyrene-co-maleic anhydride (PSMA), polysulfone (PSu) and poly ethylene glycol (PEG) were selected as the sensitive film materials as shown in Fig. 4 and were coated on the surface of different resonators of the array by spin-coating method respectively. By coating different polymers on the surface of different resonators, a SAW array was constructed. The thickness of the film was about 10 �m.

i Readout Circuitry

WSN

Platform (Octopusll)

Non-Continuous SAW Sensor Ana

Figure 2.

A.

Basic structure of sensor node.

The non-continuous SAW sensors as high-resolution mass-sensitive transducers are composed of piezoelectric crystal plus at least one layer of chemically interactive material film deposited on one of their surfaces in order to infer a given chemical sensitivity. The sensors are driven one at a time by the multiplexer (MUX) to establish operating power. This saves the total power and reduces frequency interference between different components. 1)

1)

Frequency Readout Circuit

Figure S shows the block diagram of the frequency readout circuitry. Counterl is the main element to calculate the SAW output frequency. Due to the stabilization time of the SAW sensor, the sampling starting time is set at 0.5 seconds after switching. Theoretically speaking, the more bits of Counter1, the more accurate the reset. But considering the tradeoff of sampling time and circuit area, Counterl is selected to be 24bits with a total sampling time 0.12 seconds.

SAW Device

Interdigital transducers (lOTs) have been widely used for electric signal excitation and detection of surface acoustic waves[17]. Each period of lOTs consists of multiple strips aligned and connected to the bus-bars periodically. Different center frequencies and phase responses, based on the design of lOTs, have been measured by network analyzer. The electric characteristics of lOT have been determined by the fmger geometry during the period, the number of fmger-pairs, and the substrate material. For simplicity, a simple velocity equation, as in (1), is used to design the center frequency of SAW device,

V=f'A

(1)

where v is the velocity of the surface wave for the chosen substrate, f is the center frequency of the designed SAW device, and A is the wavelength of the surface wave. The wavelength of the surface wave is a function of the spacing between fmgers and the width of the fmgers. The spacing of a single-electrode-type lOT is determined to be one-fourth of the wavelength (IJ4), which gives the required defmition for photolithography. According to the design parameters from the simulation data of delta function model and cross field model, the center frequency was designed at 99.8 MHz.

:= w:c5r!� .� .

Iit,.

.

_

(e)

Figure 3. (a) Ihe fabrication process of the SAW substrate with !DIs includes four steps: l. photoresist coating,2. photolithography process,3. E­ beam evaporator, 4. lift-off process. (b) Optical micrograph of fabricated !DIs,and (c) photograph of a SAW sensor on the PCB.

P4VP

PS

PEG

t-o-fO-.-I-o-t �.

.

PSMA

Figure 4. Schematic of the chemical structural formula of six polymer in the study: poly4-vinylphenol (P4VP),polyvinyl acetate (PVAc), polystyrene (PS),poly ethylene glycol (PEG), polystyrene-co-maleic anhydride (PSMA) and polysulfone (PSu).

The SAW device was fabricated using standard photolithography technology as shown in Fig. 3. First, a thin layer of photoresist (AZS214) was spin-coated on a Y+1280 X-propagation lithium niobate (LiNb03) wafer, patterned with a UV light source, and developed in a photoresist developer (AZ400K). A two metal layer (Cr/Au, 20nmllS0nm) was subsequently deposited on the wafer using an e-beam evaporator, and followed by a lift-off process. Finally, the SAW substrate integrated lOTs were obtained.

2)

Sensor Electronics

B.

SAW Sensor Array

Composited Polymers

The film is like a smart skin of sensor and is responsible for generating the chemical signals from the interactions

Figure 5.

119

Block diagram of the frequency readout circuitry.

Counter2 controls the D flip-flop and temporarily stores data in CounterI for the WSN platform to collect. CLKI enables CounterI and Counter2 at the same time resets the previous data. CLK2 controls Counter2 to trigger the DFF after CounterI starts action for a fixed period of time. CLKI and CLK2 are generated by connecting the counters with oscillators with fixed frequencies. The WSN platform collects data from the DFF, and then transmits the real SAW sensor frequency to the server.

D(I) means that the decision (output) of the entire WVS resulted from input I. The corrective judgment model fails if D(I) i- I. Hence, the reliability of corrective judgment model can be defmed as R=Pr{D(I)=I} and estimated based on universal generating function methodology (UGFM) [20]. In order to maximizing the reliability of corrective judgment model, we have to optimize wjand T based on discrete particle swarm optimization (DPSO) [21], as the following equation (3):

2) Octopus II The Octopus II-A WSN platform is selected for our sensor node to transmit sensing data. Because the output of the frequency readout circuitry is not a fast-changing signal, it can be directly connected to the Octopus 50-pin extension connector. Octopus II-A includes MSP430F 161 I, USB Interface, Inverted F and SMA Type Antenna, as shown in Fig. 6. The simple feature of Octopus II-A is as follows: RF range is about 450 meter, board size is 80mmx3 I mm, maximum output power is about 10dBm, compatible with IEEE 802.15.4(ZigBee), and operating with 2 x AA batteries (3.3 V 2700 mAh). Detailed information can be referenced from the website (http://www.wsnc.ntu.edu.tw/Files/Octopus­ - 0913 VI 2%20[_m).pdf). C.

Environmental Monitoring Sensing Network

if

x,

if

d/nt