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DEVELOPMENT AND EVALUATION OF A TOPOLOGY OF MEASUREMENT

INSTRUMENT BASED ON NIR DIFFUSE REFLECTION

Fabio Augusto Gentilin, Bruna Tiemi Kobo, Lucas de Souza Ribeiro, Jose Alexandre

de Franca, Ana Lucia de S. M. Felcio, Maria Bernadete de M. Franca, Dari de O.

Toginho Filho

Universidade Estadual de Londrina

Laboratorio de Automacao e Instrumentacao Inteligente

Caixa Postal 10039, Londrina, PR, 86057-970, Brazil

Emails: [email protected], [email protected], [email protected],[email protected], [email protected], [email protected], [email protected]

Abstract Generally, measurements of substances concentrations are realized by destructive and, sometimes,

slow methods. However, for quality control and time dependent experiments, it is desirable to perform non-

destructive online measurements. One of the techniques able to achieve these requirements is near-infrared

spectroscopy. Unfortunately, the development of an instrument based on this technology is challenging due to

the influence of external factors and low signal-to-noise ratio. This paper presents a hardware platform that

can be easily adapted to determine concentration of many substances. Therefore, an condenser optical system

was developed using fixed mirrors to increase signal-to-noise ratio. Also, EMI filters, active filters and EMI

shielding were developed to reduce signal noise. In addition, InGaAs sensor, which present rapid response andgood sensibility to the near-infrared spectrum, were used. The developed system was evaluated by the detection

of moisture in instant coffee samples. The result shows that the equipment is sensitive to variations of humidity,

but it can also be applied to detect concentrations of other substances.

Keywords InGaAs Sensors, NIR Spectrosmetry, Spectroscopy, Humidity Sensing, Temperature Control.

Resumo Geralmente, medidas de concentracao de substancias sao realizadas p or metodos destrutivos e, as

vezes, demorados. Porem, para controle de qualidade e experimentos dependentes do tempo, deseja-se a realizacao

de medidas online nao destrutivas. Uma das tecnicas capazes de atingir esses requerimentos e a espectroscopia de

infravermelho proximo. Infelizmente, o desenvolvimento de instrumentos baseados nesta tecnologia e desafiadora

devido a influencia de fatores externos e baixa relacao de sinal-rudo. Este artigo apresenta uma plataforma

em hardware que pode ser facilmente adaptada para determinar concentracao de varias substancias. Para isso,

foi desenvolvido um condensador optico com espelhos fixos para aumentar a relacao sinal-ruido. Alem disso,

filtros EMI, filtros ativos e blindagem EMI foram desenvolvidos para diminuir o ruido de sinal. Em adicao a

esses fatores, um sensor de InGaAs, que apresenta uma resposta rapida e boa sensibilidade para o espectro do

infravermelho proximo, foi utilizado. O sistema desenvolvido foi avaliado pela deteccao de umidade em amostras

de cafe soluvel. O resultado mostra que o equipamento e sensvel as variacoes de umidade, mas ele tambem pode

ser aplicado para detectar a concentracao de outra substancias.

Palavras-chave Sensores de InGaAs, Espectrometria NIR, Espectroscopia, Deteccao de umidade, Controle

de Temperatura.

1 Introduction

For decades, many techniques have been adoptedto determine specific concentrations in differenttypes of samples. Each approach has its limi-tations, ranging from problems associated withadopted mechanical model, such as the work ofKalamatianos et al. (2006), to changes in samplescharacteristics due to chemical reactions occurringduring the analysis. One of these techniques takesinto account the bonds between molecules of thesubstance of interest. This method is known asNIR (Near-Infrared) spectroscopy, and relates theamount of light applied to chemical bonds and theenergy absorbed by them. When these bonds havedipole moment, they are capable of absorbing in-frared vibrational energy at a characteristic wave-length. A major advantage of the infrared spec-troscopy is the possibility of performing measure-

ments using samples in their natural state. Thisanalysis method is nondestructive and allows sam-ples to be in almost any physical state. This fea-

ture gives it the ability to operate in environmentswhere sampling speed (real-time measurement) isan important factor.

Most of the researches in the NIR spec-troscopy area are developed using commercial in-struments (spectrometers). In these studies, theanalysis of the spectral response of each sampleis performed over a wide range of the electromag-netic spectrum in order to determine character-istics of interest (Riba Ruiz et al., 2012; Lianget al., 2011). However, some researchers have de-veloped new instruments, proposing cheaper al-ternatives, different acquisition methods or equip-ments for special conditions applications (Yanget al., 2008; Malinen et al., 1998; Brennan et al.,2003; Zhihai et al., 2011; Lauer et al., 2005; Lar-rain et al., 2008). Some of the developed equip-ments use light sources that are able to emit spe-cific wavelengths of the electromagnetic spectrum

onto the sample. In these studies, the emittedwavelength is set according to the types of molec-ular bonds in the sample, whose concentration is

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intended to be determined. (Kalamatianos et al.,2006; Yang et al., 2008; Malinen et al., 1998; Laueret al., 2005; Huimin et al., 2007; Filic et al., 2005).This was achieved by using LEDs and lasers asmonochromatic light sources. This kind of lightsource is more efficient than polychromatic ones,because each wavelength may be emitted sepa-rately. Furthermore, polychromatic light sources

usually have high emission of heat. One exam-ple is the tungsten halogen lamp, used in the re-searches of Brennan et al. (2003), Zhihai et al.(2011) and Larrain et al. (2008).

In the works made by Brennan et al. (2003)and Zhihai et al. (2011), optical systems were de-veloped using grating technology, with no movingparts, making it possible to analyze each wave-length. This kind of optical treatment is less sen-sible to mechanical noise. In another work, in or-der to determine ripeness in wine grapes, Larrainet al. (2008) designed an optical device suitable for

analysis in the field, to be used with a portablespectrometer and a light source. The developedoptical device was used to guide the light directlyto a wine grape berry and to collect the light thathas interacted with the fruit, what made it pos-sible to analyze the ripeness without altering theripping process.

This paper presents a hardware and softwareplatform that can be easily adapted to be usedin determining concentrations of various types ofsubstances. As shown above, there are many re-searchers developing new instruments based on in-

frared spectroscopy. However, most of them su-perficially discuss the electronics and optics in-volved. Since the developed prototype is an es-sential part for obtaining the expected results,superficial discussion about it tend to cause re-work and delays the progress of related researches.Such platform is sensitive to variations in molec-ular concentrations and is based on near-infrareddiffuse reflectance. Another important point, asthe developed equipment use only a few predeter-mined NIR wavelengths, based on LED technol-ogy, its not necessary the use of complex opti-cal system, dependent of active alignment, as pro-

posed by Kalamatianos et al. (2006). It was usedfixed mirrors, much more easier to align. Further-more, the noise found by Huimin et al. (2007) wasreduced through the use of EMI (ElectromagneticInterference) filters, electromagnetic shielding andactive filters, which caused the noise influence onthe output signal of the conditioning circuit to fallto negligible levels. The equipment also has a tem-perature control circuit, which acts on the surfaceof the photodetector to minimize the influence ofexternal thermal variation, since the dark resis-tance of that component changes with this varia-

tion.The objective of this research was the devel-

opment of an acquisition system capable of con-

Sample

LED drivercircuits

Temperaturesensor

Diffusereflectance

Opticalsystem

Receptor's signalconditioning circuit

Power supplyTemperature

control system

1,450 nm1,450 nm

1,200 nm

970 nm

Figure 1: Block diagram of the prototype.

ditioning the signals of optical systems (data col-lection) and delivering the resulting signals to acomputer (data processing). The evaluation ofthe developed system took into consideration the

sensitivity to moisture content variation of a setof instant coffee samples prepared by spray dry-ing process. The results showed that the systemis an option when signal conditioning for opticalsystems is required.

2 Development of the Prototype

The NIR spectroscopy allows to indirectly esti-mate the concentration of substances based ontheir spectral signature, e.g., covalent bonds ofH2O, which shows peaks of greater intensity of

energy absorption in the near infrared (NIR) spec-trum regions around 970 nm, 1,200 nm, 1,450 nmand 1,940 nm. According to this concept, themethod used by this research involves the emissionof light beams of three different wavelengths ontothe sample under test and the subsequent analy-sis of the amount of light reflected by it. This isdone by means of a lighting system based on GaAsLED operating in the NIR region, an InGaAs pho-todetector and an optical system. The developedacquisition system architecture is shown in Figure1.

The detection of moisture depends on an ana-log conditioning circuit operating with a controlcircuit, which provides the measurements to acomputer and controls the temperature on thephotodetector used as sensitive element of the ac-quisition system. The activation of the NIR-LEDsoccurs sequentially, and a synchronous detectiontechnique is used. The light emitted by themhits the surface of the sample under test and theamount of light reflected reaches the photodetec-tor, causing the variation of current at its junc-tion. This current is conditioned by a condition-ing circuit and subsequently the resulting signal is

delivered to a microcontroller.In the following sections, the different parts

that compose the developed system are discussed



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Reflected light

(diffuse reflectance)

InGaAs PT511

Photodetector

Lighting system

with GaAs LEDs

Peltier element

Optical

systems

cylinder

Concave mirror

Plane mirror

Test sample in

the Petri dish

Heat

exchanger

Air flux

Temperature sensor

d

D

Ra

Rc

Figure 2: Sketch of the optical system. The lightreflected by the sample is directed to the windowof the photodetector.

in details.

2.1 Radiation emitter system

In this section, the main components of the opticalsystem used for this study are presented (Figure2).

2.1.1 Optical system

The optical system is composed of a cylinder

which confines the light beams of the diffuse re-flectance acquisition system and directs them tothe photodetector via a system of mirrors (planeand concave) aligned according to Figure 2.

The concave mirror is located at the upperpart of the optical system in order to reflect theascendant light (which comes from the portion re-flected by the sample), as shown in Figure 2. Thesurface of the plane mirror is aligned with theconcave mirror to reflect the concentrated lightthrough this window directly on photodetector(PT511), which is mounted in the upper centerof the concave mirror.

In the development of the optical system (Fig-ure 2), it was arbitrarily defined Rc = 5 cm as theradius of the circle formed by the cross sectionof the concave mirror (focus equal to f= 7 cm).Also, the radius of the flat mirror is 1.5 cm andthe distance between the mirrors (plane and con-cave) is equal to d = 4.5 cm. Thus, it was onlynecessary to calculate the distanceD between theconcave mirror and the sample to be examined,considering the application of as maximum rela-tive light intensity Ir as possible on the photode-tetors window. This should be done considering

two factors that act together: the focusing andthe amount of light collected by the surface of theconcave mirror. That is because, ifD is too large,

it will be obtained a small image which would be,therefore, compatible with the window of the pho-todetector, but with lower relative intensity. Onthe other hand, a very small D induces an imageof large relative intensity, but very sprawled out.Thus, a point of balance between area and relativelight intensity must be calculated. This was doneas follows.

In the case of curvature of a concave mirror,the equation

1

D+

1

di=

1

f (1)

associates the distance D, the distance di (be-tween the concave mirror and the image forma-tion point) and the focal length of the mirror (f).Thus, for a given fand assuming an initial valueforD, the above equation allows to determine thedistance di between the concave mirror and thepoint of its image formation. Moreover, since dis also known and GaAs LEDs are located justbelow the flat mirror (Figure 2), it is possible tocalculate the radius of the sample surface to beilluminated by the LEDs, Ra, i.e.,

Ra= (D d)sin , (2)

where a viewing angle of the LEDs equal to =15 was considered.

It is also necessary to know the magnificationfactor,A, which relates the radiusRa of the sam-ple surface illuminated by LEDs to the Ri of theimage formed by the concave mirror. As Ra is

proportional to D and Ri is proportional to di,then

A= diD

. (3)

Furthermore, ifA is known, it is possible to calcu-late the radius of the image formed by the concavemirror, i.e.,

Ri = Ra

A. (4)

At this point, it is important to note that the rel-ative light intensity reaching the surface of theconcave mirror decreases with the inverse square

of the distance between the sample and the con-cave mirror,D. This is because, after reaching thesample surface, the light is diffusely reflected, be-ing spread in shape of an approximately sphericalsurface. Therefore, it was defined as relative areaAr the ratio of the area of the circle formed by thecross-section of the concave mirror and the totalarea of the spherical surface generated from theilluminated region of the sample, which dependson the distance D,

Ar = R2c2D2

. (5)

Finally, the relative intensity Ir is obtained fromthe ratio between the relative area Ar, and the



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0 10 20 30 40 500

1

2

3

4

5

6

7

8x 10

4

Distance between the sample and the concave mirror (cm)

Rela

tiveIntensity(a.u.)

Figure 3: Relative intensity: a balance betweenthe amount of light radiation and the distance D.

image area formed by the concave mirror on thephotodetetors window, i.e.

Ir = ArR2i

. (6)

In the equation above, the term representedby Aris the factor that contributes to reducing theamount of light collected by the concave mirrorand Ai =R2i is the term that contributes to thevariation due to focusing.

Equations (1) to (6) relate D with Ir. Thismakes it possible, given a distance D, to deter-mine the relative intensity (Ir) analytically. Thus,it was possible to present this relationship graph-

ically in Figure 3. By analyzing this figure, it wasverified that Ir is maximum when D 11 cm.Therefore, D = 11 cm was adopted in the proto-type.

It is important to note thatD = 11 cm is theoptimal value to obtain the maximum relative in-tensity of light on the sensor. However, the higherRc (Figure 2), the greater the absolute intensityof light that reaches the sensor. As the results ofSection 3.1 show, Rc = 5 cm was chosen for thisapplication and led to satisfactory results.

2.1.2 Lighting system

The system has three driver circuits used to drivethree pair of LEDs. Each circuit has two inputs,a synchronization signal CLK, which imposes thefrequency of actuation (1 kHz), and the SX, whichenable the designated LEDs. When both signals(SX and CLK) have a high logic level (5 V), acurrent adjusted in the driver circuit flow throughthe selected pair of LEDs.

The board with the LEDs is mounted just be-low the flat mirror (Figure 2). Thus, the beamsemitted by the LEDs (viewing angle = 15)

reach the surface of the sample with the maxi-mum intensity and, at the same time, does notadversely affect the reflected light. The LEDs

used are LED970-06, LED1200-03 and LED1450-03, manufactured by Roithner. They were se-lected according to the spectral signature of water,which has higher absorption peaks of vibrationalenergy in the regions of the spectrum emitted bythese photoemitters (Li, 2007).

The technique used to establish tuning be-tween the activation of the LEDs and the condi-

tioning system is known as synchronous detection.In this case, the photoemitters are commuted ata frequency at which the photodetector condition-ing circuit is tuned. This was done using a band-pass filter. Thus, the influence of external lightsources that may introduce deviations in the out-put of the acquisition system is minimized. Theactivation pulse of the LEDs is repeated every 1ms.

During each cycle, a sequential activation ofthe pair of LEDs (LED1 to LED3) is done. First,the pair LED1 is turned on and remains in this

state for 500 s. This event occurs fifty times ina row every 1 ms, i.e., at a frequency of 1 kHz, forthe same LED. At the end of LED1s sequence,LED2 is driven with the same timing used forLED1. After that, the same cycle occurs withLED3. Then the complete light system activationcycle is completed. Thus, only one pair of LEDsis driven at a time, allowing the acquisition sys-tem to synchronize the output voltage measure-ment, which is proportional to the reflectance ofthe sample, and the emission of each separatedwavelength of the electromagnetic spectrum (970nm, 1,200 nm and 1,450 nm).

2.2 Conditioning Circuit

In this section, we present the electronic condi-tioning circuit. This circuits main objective is toconvert the light signal applied to the photode-tector into an electrical signal, so it may be digi-tized by a microcontroller. The conditioning cir-cuit is represented by the block diagram in Fig-ure 4. This circuit was adjusted to work with anoutput voltage value within a standard range of0 to 5 V. The operational amplifier OP07E was

used in the circuits. It was chosen due to thehigh common-mode rejection ratio (CMRR) of 106dB, high input impedance (160 G) and low offsetvoltage level (75V). In the following paragraphs,the conditioning circuit is described in details.

The input gain stage is divided into two parts:a current-to-voltage converter and a gain circuit.The photodiode produce current when receiveslight. So, the current must be converted to volt-age. The photodetector PT511 was chosen be-cause its spectral operational range complies withthe three wavelengths (970, 1,200 and 1,450 nm)

emitted by the LEDs. Then, the resultant volt-age output of the current-to-voltage converter isamplified by a inverter amplifier.



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Light (NIR)reflectedby thesample

Peak-detector

nput ga nstage Band-pass filter

Output gainand offset

Conditioned

outputsignal

Figure 4: Conditioning circuit block diagram.

The signal from the input gain stage is appliedto a bandpass filter with a center frequency off0= 1 kHz 1%. This accuracy was achieved byimplementing the filter using the integrated circuitUAF42AP, manufactured by Texas Instruments,which has capacitors and resistors with precision

of 0.5 %. The type of filter used was a second-order Butterworth.The gain and offset stage is responsible for re-

ceiving the output signal from the bandpass filter(BP OUT) and adapting it to the next stage. So,the circuit includes a precision voltage reference of5.000 V (0,04%) called V STAB, provided by theintegrated circuit LT1461CCS8-5. This stage alsoincludes a summing amplifier circuit. The voltageoutput Vc of this circuit is given by

Vc = R14

V S T A B

P OT4 +

BP OUT

P OT3

, (7)

whereP OT4 and P OT3 are resistances.The next circuit is a peak detector (Figure 5).

The information related to theH2Oconcentrationin the sample is the amplitude of the band passfilters output signal. Therefore, the purpose ofthis detector circuit is to retain information con-cerning the maximum of that signal.

In the proposed system, for a sample with lowmoisture content, little light is absorbed by cova-lent bonds ofH2O. Then, more light emitted bythe illumination system is reflected to the opticalsystem. This causes an increase in the signal am-plitude of the peak detector stage. On the otherhand, a sample with high concentration of mois-ture tends to absorb more light, resulting in a lowsignal amplitude at the output of this stage.

The signal generated by this stage,Vd, corre-sponds to the output of the conditioning circuit.It is controlled by the control system (discussedin Section 2.3) via signal CT1 (Figure 5), syn-chronized with the LEDs activation. The signalgenerated by this stage is digitized by a micro-controller and depends on the synchronism withthe command CT1 (Figure 5). When a LED is

pulsed, the capacitor of the peak detector circuitstarts charging. After three cycles of the signalVc, it is considered that the output signal of the

CI5

R10

R12

R11

270

Vd

CI6

Vc

CT1

D3

D4C3

Q3

OP07E

OP07E

1N4148

1N4148

3,3 k

1 M

100 nF

Figure 5: Peak detector circuit and conditioningcircuit output.

peak detector reaches its maximum level. Then,during two cycles ofVc, the voltage measurementof this stages output is performed. Finally, CT1is triggered to discharge the capacitor of the peakdetector, which is required to perform a new mea-surement.

2.3 Control System

The Control System (CS) is an independent mod-ule of the acquisition system. This is based onthe 8-bit microcontroller MC9S08JM32, of HCS08family, manufactured by Freescale, which presentsan A/D converter of 10 bits resolution. It is usedto digitize the conditioning system output signal.The CS performs the functions of triggering theLEDs in a pre-defined sequence, controlling theCT1 signal, controlling the temperature of thephotodetector, processing the measurement dataand communicating with a computer (via USBcommunication port).

The InGaAs photodetectors are known to bequite sensitive to temperature variations. There-fore, the temperature of the sensor and all theconditioning circuit of Figure 4 is constantly main-tained at 25 C, by means of a digital controllerwith proportional and integral actions. This con-troller is also based on the MC9S08JM32 mi-crocontroller, and uses a thermoelectric cell withPeltier effect as the actuator and an integrated cir-cuit LM92, which has a resolution of only 0.0625

C, as the temperature sensor. The Peltier chip ispowered by an H bridge excited by PWM modu-lation, whose duty cycle depends on the action ofthe digital controller.

3 Experimental Results

Aiming to test the proposed technique, a proto-type was built. The acquisition system developedhas two overlapping enclosures: the upper one,made of metal which houses the electronics mod-ules; and the lower one, made of wood, where the

sample accesses the optical system, as shown inFigure 6. The sample placed in a Petri dish is putat the bottom of the optical system and receives



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DoorDoor

Sample compartmentSample compartment ComputerComputer

Metal enclosureMetal enclosureOpticalSystemOpticalSystem

Figure 6: he sample placed in the Petri dish isintroduced at the bottom of the optical system.The computer is used as a data acquisition system.

the light emitted by the NIR LEDs.

In Figure 7, it is possible to verify the distribu-tion of the electronic circuitry housed in the metalenclosure, corresponding to the blocks shown in

Figure 1. The LED driver circuits and condition-ing circuit is present in the analog signal condi-tioning board. The conditioning circuit containsthe blocks shown in Figure 4: input gain stage,band-pass filter, output gain and offset and peak-detector. The light and temperature sensors areplaced in the photodetector board. The opti-cal system, the NIR LEDs and the sample com-partment are assembled below the photodetectorboard. Furthermore, the temperature control sys-tem is composed by the H bridge, heat exchangerand the peltier chip. The control system board

contains the CPU and the A/D converter.The H bridge (used to drive the Peltierchip) was assembled in a metallic box to reducethe spread of electromagnetic interference (EMI).Also, we used two power supplies. One of themis dedicated to instrumentation circuits, and theother one is used to drive the Peltier chip.

The signal output provided by the band-pass filter is shown in Figure 8 (referred to asBP OUT). Also, as discussed above, the signalBP OUTis conditioned as indicated in Equation(7), and then its peak value is detected by the cir-cuit of Figure 5, whose output signal is shown in

Figure 8 and referred to as Peak Output. Onecan see even the discharge of the capacitor C3caused by signal CT1 (Figure 5). In addition,in the same figure, it is possible to observe themoment when the signal Peak Output is digi-tized by the SC, after three periods of the signalBP OUT. After two more cycles, a new pulseoccurs at CT1, what discharges the capacitor C3and drop Peak Output to zero volts.

3.1 Set of samples used in the tests

In the experimental tests of the prototype, we used19 samples of instant coffee with moisture concen-trations between 1.55% and 2.53%. The moisture

Heat

exchanger

Control system

board (microcontroller)

Photodetector

board

Heat

exchanger

Control system

board (microcontroller)

Photodetector

board

Analog signal

conditioning board

Analog signal

conditioning board

H bridgeH bridge

Power

supply

Power

supply

Peltierchip

Figure 7: Top view of the enclosure of the devel-oped prototype. One can observe the grouping

of the electronic modules and the support of theoptical system.

10 15 20 251

0

1

2

3

4

5

Time (ms)

Output(V)

BP_OUT

Peak Output

ADConversion

Figure 8: Output signals from the conditioningcircuit. The measurement depends on the com-mand CT1.

of the samples were obtained by the Karl Fischermethod, and are presented in Table 1. Also, in theselection of these samples, we used the triplicatemethod, i.e., three of each sample with the sameamount of moisture have been (non-sequentially)inserted in the prototype. This method was ap-plied to a group of 19 samples, totaling 57 anal-ysis. Finally, it is important to notice that, ineach of the 57 trials, the samples had a standardvolume of 40.2 103 m3 and, before being in-

serted in the prototype, they were homogenizedin the Petri dish in order to make the surface ofthe sample uniform.



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Table 1: Samples used in tests with the prototype.Sample: 1 2 3 4 5 6 7 8 9 10

Moisture(%):1.551.671.781.821.861.901.911.931.981.98Sample: 11 12 13 14 15 16 17 18 19 -

Moisture(%):2.002.002.012.042.122.172.262.402.53 -

0 2 4 6 8 10 12 14 1622.5

23

23.5

24

24.5

25

25.5

Time (minutes)

Temperature(Celsius)

Figure 9: Temperature inside the prototype in re-lation to time.

3.2 Evaluation of the prototype regarding the in-

fluence of temperature

Experimentally, it was verified that, without thetemperature control, for a 5 C variation in roomtemperature, there was a variation in the responseof the photodetector of 3.63%, 3.62% and 3.61%for the bands centered at 970 nm, 1,200 nm and

1,450 nm, respectively. This result shows theadopted sensors dependence of the temperature.On the other hand, by using the digital controllerwith proportional and integral gains equal to, re-spectively, Kp = 2 and Ki = 1/32 (empiricallydetermined), the temperature of the photodetec-tor in steady state remains practically constant,as discussed below.

Figure 9 shows the variation of the temper-ature of the conditioning circuit over time rightafter switching on the equipment. One can ob-serve that the temperature becomes stable at 25C, after five minutes, when the steady state RMSerror is equal to ERMS = 0.0391

C, while themaximum error was 0.1 C.

3.3 Evaluation of the prototype concerning the

influence of external light sources

In order to assess the influence of the externallight on the prototype response, 57 trials were per-formed as described in Section 3.1, this time witha 60 W incandescent light bulb switched on insidethe cabinet where the samples are placed. Despitethis interference, it was observed that the presence

of a continuous spectrum of the light emitted bythe incandescent lamp did not significantly influ-ence the system output. In fact, considering the

measurements taken in the presence of externallight and those ones taken under normal condi-tions, the average differences between them were0.22% at 970 nm, 0.31% at 1,200 nm e de 0.28%at 1,450 nm.

3.4 Response concerning the moisture detection

With the prototypes temperature control activeand considering the system immune to externallight sources, the 57 tests described in Section 3.1were repeated. The results are shown in Figure10. Looking at this figure, its possible to observethat there is a significant variation in the proto-type output depending on the amount of moisturepresent in the coffee samples. In addition, thegraph of the curves corresponding to the spectralbands of 970 nm and 1,200 nm shows a step in2.0% of moisture. This occurs due to the fact thatthese bands are not characteristic of the water,

but complement bands, which are needed to makeit possible the multivariate calibration (Li, 2007).Thus, it is natural that these curves have a non-linear tendency since these wavelengths are alsoabsorbed by other molecular bonds (other thanOH) in the chemical composition of the coffeesamples (Johnson and Wichern, 2007). Instead,the band centered at 1,450 nm is characteristic ofwater and is not absorbed by any other type ofchemical bond. Therefore, this presents a muchmore linear behavior as a function of the coffeesamples moisture.

The multivariate calibration of the prototypeis not covered in this article. However, it is wellknown that, to make it possible to perform sucha calibration, those curves corresponding to eachof the wavelengths used in the system should fitto a linear model with a coefficient of determina-tion (R2) greater than 0.80 (Ewing, 1975). For thespecific case of this prototype, the calculated R2

was 0.83, 0.93 and 0.98 for the wavelength bands970, 1,200 e 1,450 nm, respectively. These resultsindicate that the proposed system is very promis-ing.

4 Conclusions

This paper discussed the process of construction,adjustment and testing of an optoelectronic sys-tem prototype, which can be used for quantify-ing substances in solid materials. This equipmentis based on NIR spectroscopy, wherein moleculesof the substance to be measured absorb part ofthe incident radiation. Thus, by observing the re-flected radiation, it is possible to quantify the sub-stance of interest in the material under analysis.The proposed technique uses a set of fixed mirrors,

light emitters of GaAs and an InGaAs photode-tector. Electromagnetic interferences were mini-mized using EMI filters, metal shielding and active



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1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.40

1

2

3

4

5

6

Moisture (%)

Output(V)

970 nm

1200 nm

1450 nm

Figure 10: Response of the prototype as a functionof moisture present in the coffee powder, in a 62%air relative humidity and 25 C.

filters. Besides, the prototype has a temperaturecontrol system which acts on the surface of thephotodetector in order to minimize the influenceof the environment temperature variation. Theexperimental results show that the technique ispromising. Finally, it is important to emphasizethat, although the system has been assessed byquantifying the moisture in samples of coffee pow-der, it can be easily modified to quantify othertypes of substances. For that, it is necessary justto change the wavelength of the LED emitters.

Agradecimentos

The authors would like to thank CAPES, CNPqand Fundacao Araucaria for the financial supportfor the research.

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