Research Article Rain Sensor with Stacked Light Waveguide ...

Research ArticleRain Sensor with Stacked Light Waveguide HavingTilted Air Gap

Kyoo Nam Choi

Department of Information and Telecommunication Engineering, Incheon National University, Incheon 406-772, Republic of Korea

Correspondence should be addressed to Kyoo Nam Choi; [email protected]

Received 1 April 2014; Revised 4 June 2014; Accepted 5 June 2014; Published 26 June 2014

Academic Editor: Marco Consales

Copyright © 2014 Kyoo Nam Choi. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Vehicle sensor to detect rain drop on and above waveguide utilizing light deflection and scattering was realized, keeping widesensing coverage and sensitivity to detect mist accumulation. Proposed sensor structure under stacked light wave guide consistedof light blocking fixture surrounding photodetector and adjacent light source. Tilted air gap between stacked light waveguide andlight blocking fixture played major role to increase sensitivity and to enhance linearity. This sensor structure eliminated complexcollimating optics, while keeping wide sensing coverage using simple geometry. Detection algorithm based on time-to-intensitytransformation process was used to convert raining intensity into countable raining process. Experimental result inside simulatedrain chamber showed distinct different response between light rain and normal rain. Application as automobile rain sensor isexpected.

1. Introduction

Currently the portion by electronics in automobile has con-tinuously increasing tendency, and the rain sensor is one ofsuch electronic components in vehicle. Rain sensor in vehicleis used to actuate wiper automatically in case it senses rainon and above windshield glass. Numerous ideas and conceptsregarding windshield rain sensor have been proposed, suchas mechanical [1], acoustic [2], resistive [3–7], capacitive [8–16], optical imaging [17–26], microwave [27, 28], and optical[29–36] rain sensors; however, problems relating such as lin-earity, sensitivity, disturbance, malfunction, sensing area, andstructural complexity limited their application to vehicle.Thiswas due to the random nature of raining which is inherent toliquid. However, it is required to have adequate linearity inraining intensity for the range where driver wants to distin-guish towipe thewindshield for driving convenience. Amongconventional concepts regarding rain sensor, the mechani-cal sensor monitored the variation in vibration amplitudeand frequency by rain drops, and the acoustic method mon-itored sounds by rain drop using electret microphone. TheresistivemethodmeasuredDC resistance between the dot [3]or linear [4, 5] or interdigitated [6, 7] electrodes; however,

these electrodes were located outside windshield glass whichwere subject to abrasion and dust accumulation. The capac-itive detection method monitored the variation in capaci-tance between the electrodes inside windshield glass, andthis capacitance change by rain drops was converted toimpedance change [13] in capacitance bridge or frequencychange [14–16] in LC resonance circuit. In this method,if the interdigitated finger patterns were once wet, then itwas not able to sense the next rain drop, and it resulted inshifting the linear region to weaker raining intensity ratherthan practically usable intensity. Optical imaging method,using imaging array sensor [17–24] or camera [25, 26], hadthe advantage of sensing wider area using simple opticalcomponent; however, it was inherently susceptible to rushingscenery while driving. The other microwave radar detectionmethod was attempted by simulating scattering betweenmicrowave and rain interaction. Optoelectronic detectionmethod has been commonly used to detect the presence ofrain drops on windshield glass, because optically transparentnature of windshield glass, within limited optical spectrum,linked optical signal path between sensor located inside glassand rain drops located outside glass. Rain sensing methodusing direct attachment of light source and photodetector

Hindawi Publishing CorporationJournal of SensorsVolume 2014, Article ID 543530, 9 pageshttp://dx.doi.org/10.1155/2014/543530

2 Journal of Sensors

inside windshield glass [29, 30], in between barrier is placedto suppress primary parasitic radiation, was proposed; how-ever, this configuration had limitation to increase sensitivitydue to unwanted secondary parasitic radiation otherwise hadlow sensitivity. Two optoelectronic types, distinguishing byoptical paths, have been widely used in vehicle. Reflectiontype sensor utilized reflected light by rain drop on thesurface of windshield glass by using camera [31] or inclinedsensor attachment plane [32] or light beam [33, 34]. Amongthese, the method using collimated light beam [34] fromlight source was strong enough to reject ambient light effectwhile reaching to photodetector; however, it formed only asmall detection spot on windshield glass surface resulting ininsufficient sensing area and linearity to discriminate rainingintensity. To compensate this deficiency, the configurationhaving multiple optical paths was proposed [35] and used inthe automobile.Themost notable optical sensing scheme wasoptical waveguide type sensor [36], in which the collimatedinfrared light beam was coupled into windshield glass panelby using prism coupling method. In this scheme, a line ofchannel between light source and photodiode formed rainsensing lane, and the amount of leaked light by rain dropswas monitored. This method had limited sensing area buthad sensitive response to the presence of raindrop withreduced interference by ambient light. However, it was alsosusceptible to the shock and vibration imposed to the lightcoupling interface and required complex optical compo-nents. Most optical rain sensors require a rigid mountingapparatus [37–39]. Scattered light detection method is analternative to the previous arts, which can not only eliminatecomplex optical fixture but also extend detection area in3-dimensional spaces. The trade-off to these advantages isweak light response to raindrop. Therefore, there is a needfor rain sensor having wide sensing coverage and enhancedsensitivity without complex optical components. This paperis about a sensitive rain sensing method using both scatteredand deflected light detection without complicated opticalcollimation fixture.

2. Rain Detection Theory

2.1. Scattered Light Sensing. Conventional optoelectronic rainsensing method used collimated light beam to minimizeoptical power loss from infrared light source. Two kindsof rain sensing structures have been used. The concept ofreflection type rain sensor, which detects the degree of lightleaking on reflecting spot using collimating optical fixturebeneath windshield glass, is shown in Figure 1(a). And theconcept of optical waveguide type rain sensor utilizing lightchannel formed inside windshield glass connecting betweenlight source and photodetector is shown in Figure 1(b).Vehicle rain sensor, utilizing scattered light, places lightsource and photodetector underneath windshield glass. Lightshield is essential to block directly transmitted light fromlight source. In this case, transmitted light is reflected atthe interface of two transparent materials, if the refractiveindices are different. And the reflected optical power isproportionally related to index difference.The concept of rain

Rain drop

LEDPD

Lens

(a)

Rain drop

LED PD

Prism

Lens Lens

(b)

Figure 1: Optical rain sensors: (a) reflection type and (b) waveguidetype.

Rr

Rain drop

LED

Light block

PD

Glass

Figure 2: Concept of rain sensor using scattered light detection.

sensor using scattered light detection is shown in Figure 2.Actual light paths are various rather than the typical oneshown in Figure 2. The scattered light by rain drops whichare close to windshield glass is the major signal source fordetection. It is necessary to avoid direct light emission andreflection other than by rain drops on or above windshieldglass to avoid interference by stray light.

Transmitted light from light source travels the distance“𝑅” to rain drop. And it reflects back to photodiode aftertraveling the distance “𝑟.” The received light optical power atphotodetector location can be expressed as shown in (1) [40],and this received power has relationship with the rain dropmorphology and the distance between rain drop and LED. In(1), Φ(𝑟, 𝜃, 𝜙, 𝑡) is morphology of rain drop, 𝑆PD is photosen-sitive area of photodetector, 𝑃

𝑖(𝑡) is received intensity, 𝑃

𝑜(𝑡) is

emitted intensity, 𝑃𝑅1(𝑡) is reflection probability by rain drop,

and 𝑃𝑅2(𝑡) is signal reception probability at photodetector:

𝑃𝑖 (𝑡) = 𝑃𝑜 (𝑡) ⋅ 𝑃𝑅1 (𝑡) ⋅ 𝑃𝑅2 (𝑡) ,

𝑃𝑅1 (𝑡) =

𝜋𝑟2

(4𝜋𝑅2

1)⋅ Φ𝑅1(𝑟, 𝜃, 𝜙, 𝑡) ,

𝑃𝑅2 (𝑡) =

𝑆PD(4𝜋𝑅2

2)⋅ 𝑃𝑅1 (𝑡) .

(1)

Journal of Sensors 3

t

Glass

Minimum separation distance

Light block∼T2R

∼T2R3

∼T2R5

𝜃r

𝜃i

(a)

1st

t

Glass


Rain drop

2nd 3rdLight block

d∼T2R3

𝜃r

𝜃i

(b)

Figure 3: Concept of rain sensor using deflected light detection: (a) without rain drop and (b) with rain drop.

Light block


Glasst

∼T2R ∼T2R3

∼T2R5

𝜃r

𝜃i

(a)


Glass

Light block

Rain drop

t

∼T2R3

𝜃r

𝜃i

(b)

Figure 4: Concept of rain sensor using deflected light detection method with tilted air gap: (a) effect of tilted air gap and (b) effect of raindrop with tilted air gap.

2.2. Deflected Light Sensing. If the collimated light is incidentinto glass medium, light is transmitted following trans-mission rate 𝑇 and reflected following reflection rate 𝑅 atinterface surface of two transparentmaterials having differentrefractive indices. For instance, transmission rate is around96% and reflection rate is around 4% at interface surfacebetween air and glass medium depending on incident angleand exact value of refractive indices. If light block, whichrejects direct transmission from light source to photodiode, isplaced to allow only signals after third reflection on top glasssurface as shown in Figure 3(a), then the signals after secondreflection can reach the photodetector because a rain drop onglass surface causes an increase in glass thickness, resulting inaltered light path as shown in Figure 3(b).

The difference in light intensity between second and thirdreflections is around 𝑅2 which is equivalent to 1.6 × 10−4 asshown in Figure 3. Thus, intensity difference shows almostdiscrete step and is suitable to set threshold point to deter-mine whether rain is dropping or not. In case the incidentlight is not a collimated light, intensity difference betweensecond and third reflections is close to discrete step due tothe presence of light block fixture, although the differencebecomes less than the case for collimated incident light. If theminimum separation distance, 𝑑, in Figure 3 approaches tothe point barely after second reflection, then the probability ofdetecting rain dropwill bemaximized. However, thismethodis expensive to realize, because it requires maintaining highprecision level in fixture dimension.

2.3. Deflected Light Sensing with Tilted Air Gap. Instead ofmaintaining high precision level in dimension to accuratelyadjust minimum separation distance by using light block-ing fixture, placing tilted air gap between windshield glassand light blocking fixture is a convenient way of continu-ously adjusting threshold point to maximize rain detectionsensitivity.

When the tilted air gap is not present, the second reflectedlight by upper glass surface is blocked by light blockingfixture. In this state, the small rain drop will not be sufficientto deflect light path of above the second reflected light out oflight blocking fixture. When the tilted air gap is present, thesecond reflected light by upper glass surface is barely blockedby light blocking fixture as shown in Figure 4(a). In this state,only small rain drop will be sufficient enough to deflect lightpath out of light blocking fixture and reach into the apertureof photodetector as is illustrated in Figure 4(b). By adjustingtilt angle between glass waveguide and light blocking fixture,the light deflection can be continuously adjusted by adjustingtilting angle between windshield glass and light blockingfixture, resulting in high sensitivity. The effect of tilting angleto the degree of deflection can be explained as shown inFigure 5. In case when light is incident to the bottom surfaceof windshield glass with incident angle 𝜃

𝑖, then this light will

be transmitted through bottom surface and again incident totop surface of windshield glass with incident angle 𝜃

𝑟. If we set

the tilting angle as 𝜃𝑡, then the reflection angle will be 𝜃

𝑟+2𝜃𝑡.

This deflection shifts light path as 𝑡 ⋅ (tan(𝜃𝑟+ 2𝜃𝑡) − tan(𝜃

𝑟))


Glass

Tilting angle

𝜃r

𝜃i

𝜃t

𝜃r + 2𝜃t t

Figure 5: Effect of tilting angle to light path shifting.

in horizontal direction, if we assume tilting angle is smallusually less than a few degrees.

2.4. Raining Intensity Quantization. It is important to deter-mine the intensity of raining to control wiper actuatinginterval in automobile. However, the signal response by lightscattering process [41–44] has random stochastic characteris-tics; thus, it is hard to configure linearly in relation to rainingintensity.Thus, it is necessary to quantize the raining intensityby transforming scattering process. Direct quantization ofanalog value is not feasible, because sensor output is digitalvalue. Thus the algorithm, converting digital value of rainingintensity to analog value, is necessary to utilize in rainsensor. The frequency of rain detection in predeterminedtime interval is used to determine raining intensity. And thefigure indicating the degree of continuous rain detection isused to determine rain drop size. One time frame, which ispredetermined time interval, is divided into multiple timeslots, and each time slot represents rain detection event;thus, it forms time-to-intensity transformation.Through thistransformation, the probability of raining is determined bycounting active time slots in predetermined time interval.The structure of one time frame having multiple time slotsis illustrated in Figure 6. In the case of drizzling rain, onlyone time slot represents active slot; however, more slots willbe active in more raining condition. In an extreme case asshower, all time slots will be active. We can extract the datafor raining intensity and rain drop size by analyzing activetime slot pattern in one frame time.

In time-to-intensity transformation, the raining proba-bility, 𝑃(𝑡), in one frame time is evaluated by combiningthe probability for size of rain drop and the probability forraining time as shown in (2), where 𝑁 represents the timeslot number, 𝐴

𝑖represents the weighing factor of rain drop

size, 𝐵 represents the weighing factor to active time slots,𝑆(𝑡) represents continuous slot times, and 𝑛ts(𝑡) represents thenumber of time slots detecting rain drop:

𝑃 (𝑡) =

𝑁

∑

𝑖=1

𝐴𝑖𝑆𝑖 (𝑡) + 𝐵 ⋅

𝑛ts (𝑡)

𝑁. (2)

Thus, this time-to-intensity transformation process uti-lizes data for rain drop size,𝐴 code, and raining time, 𝐵 code,to determine wiper actuating interval. In the case of showerraining, 𝐴 code sends signal to actuate with high speed, and𝐵 code sends signal to actuate windshield wiper. In case oflight raining, 𝐴 code sends signal to actuate with low speed,and 𝐵 code sends signal to actuate windshield wiper. While

Slot1 Slot2 Slot3 Slot20

Frame time

Rain drop Slot time

· · ·

Figure 6: Frame time and slot time in time-to-intensity transforma-tion.

LEDLight block

S

Rain drop

Windshield glassPhotodetector

d

PCB

Tilt angle

W

𝜃

Figure 7: Rain sensor structure.

only with mist condition, 𝐵 code only sends signal to actuatewiper motor when the rain drop is detected.

3. Experiment

3.1. Rain Sensor Structure. The windshield glass of auto-mobile has the laminated glass structure in which thinpolyvinyl butyral resin film is inserted as impact absorber.The refractive index of above resin film is in the rangebetween 1.6 and 1.8, which is close to refractive index of glass,and the thickness of film is negligible in comparison to glassthickness. The rain sensor was fabricated using commercialwindshield glass having 2 stacked light waveguides, 2mmthickness each, as shown in Figure 7. The tilt angle ofwindshield glass was adjusted so that the second reflectionfrom top glass surface is barely blocked by light blockingfixture. In case rain drop is present on or above the top glasssurface, then new deflected light path is created by reflectionon rain drop surface.This deflection causes reduced reflectionstep, resulting in order of magnitude higher optical powerinput to photodetector than the state without rain drop.

The deflected light path outside windshield glass is theprimary signal source for wiper actuation.The photodetectorsenses two signal paths, reflected light by rain drop onwindshield glass and scattered light by falling rain drop. Thereflected optical power from the rain drop on windshieldglass is adjusted close to scattered optical power by falling raindrop above windshield glass, by keeping adequate distance 𝑆to equalize signal intensity by rain drops. The incident anglefrom light source is controlled by adjusting the distance 𝑊between light block and light source. Directly propagatinglight from light source was blocked by light blocking fixturebefore reaching to photodetector as shown in Figure 8.Signal amplification rate for deflected and scattered lightswas limited by the amount of stray light even while recessingphotodetector inside light block.


Light

PhotodetectorLight

block

source

Figure 8: Top view of fabricated rain sensor.

3.2. Signal Processing. The emission angle of light source iscritical for rain detection. The wide angle provides widerdetection area, while the narrow angle expands detectionresolution within limited detection coverage. The LED withflat top window having half power angle of 90 degreewas chosen to expand detectable area. The higher periodicinstantaneous drive current of 60mA was injected to LEDby keeping 25% of duty cycle ratio to prevent overheating,in comparison to normal continuous drive current of 20mA.The light modulation frequency was chosen to 73 kHz toreject unwanted ambient light effect. The higher frequencyis advantageous to increase detection probability because ofhigher sampling rate; however, from practical view point,this also increases chance for oscillation due to high receivergain and complexity in band-pass filter of receiver circuit.Alternating light radiation was used to trace fast fallingrain drops in wide area. The center wavelength of infraredlight source was chosen to 875 nm, which was close to thepeak response of silicon photodiode by using Ga

1−𝑥Al𝑥As

emitter with proper composition in III–V ternary compoundsemiconductor. Also the emission wavelength of light emitterneeds to match with transmission spectral range of pho-todiode with daylight filter to provide additional rejectionto sun light. The block diagram of transmitter is shown inFigure 9, and measured transmitter waveform is shown inFigure 10.

Optical interference filter was placed on photodetectorwindow of optical receiver to reject unwanted wavelengthand electrical demodulation enhanced signal-to-noise ratio.Recovered response was boosted to the usable signal levelfor analog-to-digital conversion with enhanced SNR. Blockdiagram of optical receiver is shown in Figure 11, andmeasured receiver waveform is shown in Figure 12.

It is needed in automobile to have multiple operatingsteps by monitoring rain intensity. Rain detection algorithmbased on time-to-intensity transformation process is usedto have rain data such as rain drop size and rain dropcount and further to closely match sensor output to actualraining condition. In Figure 13, the flow chart of the aboverain detection algorithm is shown. And actual LCD windowviewing the result of rain detection algorithm is shown inFigure 14, as instantaneous rain count is 50% in one frametime, average rain count is 45%, instantaneous rain drop sizeis 30% in one frame time, and average rain drop size is 30%.

Signalf = 2kHz

Carrierf = 73kHz

Pulse width = 7𝜇s

Pulse width =

High speedswitch

LED

LED

Driver

Driver

0.25ms

Figure 9: Block diagram of transmitter.

Further it shows that wiper starts to operate because presetthreshold is 20%.

3.3. Laboratory Measurement. Rain sensor was mountedunder windshield glass by using tilt mechanism as shown inFigure 15(a), and this experimental setupwas used tomonitormaximum sensitivity as mist accumulation on the surface ofwindshield glass. The output of rain sensor showed sufficientsensitivity to detect mist on glass surface as shown in Fig-ure 16. The shown waveform was modified after convertingto digital signal by applying threshold voltage.

Rain sensor was situated inside simulated rain chamberas shown in Figure 15(b), and this experimental setup wasused to monitor sensor output against light rain, spraying400 cc/min, and normal rain, spraying 900 cc/min.The watersprayers inside simulated rain chamber were installed abovewindshield glass, and two different sizes of nozzle in watersprayer gun were used to simulate lightly raining conditionand normal raining condition. The output signal waveformof rain sensor after digitizing and time expansion (×140) forcontrol circuitry is demonstrated in Figure 17. The waveformin Figure 17(a) shows the response for light rain, whichis equivalent to 45 rounds/min of wiper actuation, and thewaveform in Figure 17(b) shows the response for normal rain,which is equivalent to 70 rounds/min of wiper actuation.

Experimental result inside simulated rain chambershowed distinct difference between the cases for light rainand normal rain. For the raining intensity below light rainlevel, the intermittent wiper actuation was used wheneversensor detected rain drops. In case when averaging functionto count raining time slots is used, the sensor responseshowed tendency of a little weaker response than the casewithout averaging. In the proposed rain sensor structure, thetilting angle of rain sensor against windshield glass is keyparameter to determine sensitivity. Also this angle is subjectto adjustment depending on the thickness of windshieldglass and distance to windshield glass. The enclosure forrain sensor was securely attached to windshield glass byusing adhesive. The entire optical structure of rain sensorwas situated close to windshield glass surface within 0.5mmclearance inside enclosure. And one end of sensor was fixedto the enclosure by using hinge joint and the other endwas tilted by using adjustment bolt with fine thread. Thefreedom to tilting angle was found to be within 6 degrees


13.7 𝜇s

250𝜇s

13.7 𝜇s

250𝜇s

T = 500𝜇s

T = 500𝜇s

Channel 1

Channel 2

(a)

Channel 1

Channel 2

(b)

Figure 10: Transmitter signal: (a) signal format and (b) measured waveform, 𝑋 = 0.1ms/div 𝑌 = 2V/div.

AmpxVar. Demod. BPF TIA

Photodetector

Output signal

Pulse width = 250𝜇s

Pulse width = 7𝜇sPulse width = 250𝜇s

Pulse width =

ComparatorVth = 100∼500mV

Time exp.x250max

Processor(S&H) Interface

35ms

fc = 73kHz

Figure 11: Block diagram of receiver.

Light raining

Baseline

Signal

(a)

Heavy raining

Baseline

Signal

(b)

Figure 12: Inverted receiver signal before digital conversion: (a) light raining and (b) heavy raining, 𝑋 = 0.5ms/div 𝑌 = 1V/div.

in the experiment. After adjustment, adjustment bolt wasfixed by using thread locking compound. However if wetry to integrate this rain sensor to the windshield glassof automobile outside laboratory, further consideration forrigid mounting methodology needs to be derived againstmechanical displacement due to vibration while driving.Also simple calibration procedure for the specified sensitivityis necessary to suit mass production environment throughfurther research.

4. Conclusion

Optoelectronic rain sensor with stacked light waveguidehaving tilted air gap was investigated towards the applicationfor automotive rain sensor. This open type without opticalcollimation fixture had the advantage of wide sensing cover-age and simple structure, yet having enhanced sensitivity andlinearity. Proposed rain sensor detected deflected light pathby rain drop on top surface of waveguide glass and scattered


Decide slot time

Input signal S/H Menu setup

Menu up/down Select up/down

Decide signal value LCD displayDecide frame time

Threshold voltage

Averagingnumber

Select max oraverage value

Calculate maximumcontinuous raining

slotsAdd raining slots

Communication Interface circuitInterface circuit

Analog signal Digital signal Analog signal Digital signalCommunication signal

Determine wiper speed Determine wiper operationRS232 or CAN bus

Trigger generator Trigger signal polarity

Trigger signal

Figure 13: Rain detection algorithm.

Signal polarityControl menu selection

Wiper operation

Size of rain drop(by averaging)

Size of rain drop(by peak)

Rain probability(by averaging)

Rain probability(by peak)

Fixed analog voltage Analog output voltage

Figure 14: LCD window viewing rain detection algorithm.

Rain sensor withmounting frame Windshield glass

(a)

Rain sensor withmounting frame

Windshield glass

Sprayer

(b)

Figure 15: Rain sensor under measurement: (a) mounted on jig and (b) encased in simulated rain chamber.


Mist

Baseline

Signal

Figure 16: Inverted sensor response against mist accumulation on glass surface just after converting to digital signal, 𝑋 = 0.5ms/div 𝑌 =1V/div.

Light rain

Signal

Baseline

(a)

Normal rainBaseline

Signal

(b)

Figure 17: Rain sensor output after digital conversion inside simulated rain chamber: (a) light rain 400 cc/min and (b) normal rain 900 cc/min,𝑋 = 400ms/div 𝑌 = 1V/div.

light reflection by rain drop above thewaveguide glass; thus, itreplaced complex guided wave optics in previous rain sensor.The omnidirectional light emitting pattern from light sourceexpanded sensing coverage, and tilting air gap between lightblocking fixture and stacked glass waveguide played majorrole not only to increase sensitivity but also to enhancelinearity against raining intensity. The sensing area of rainsensor covers the adjacent area connecting light source andphotodetector, which is comparable to guided wave opticalsensor having sensitivity only in the line connecting lightsource and photodetector. Adequate signal-to-noise ratio wasmaintained by using optical interference filter and electricalsignal modulation to have noise rejection capability, whilekeeping high signal amplification rate. Detection algorithmbased on time-to-intensity transformation process was usedto convert raining intensity into countable raining processby using time division detection slots. Rain sensor mountedon laboratory jig showed high sensitivity to detect mistaccumulation on the surface of windshield glass. Experi-mental result, which was measured inside simulated rainingchamber, showed distinct difference between the cases forlight rain, spraying 400 cc/min, and normal rain, spraying900 cc/min. This test result using simulated rain chamberindicated prospect as rain sensor for automotive application.However, natural raining condition is random; thus, the testresult outside laboratory may differ from the result inside

laboratory using water spray guns. In this case, extendedexperiment in outside raining environment will be helpful tooptimize.

Conflict of Interests

The author declares that there is no conflict of interestsregarding the publication of this paper.

Acknowledgment

This work was supported by Incheon National UniversityResearch Grant in 2013.

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