Sensors andand Materials,Materials, Vol.
Vol. 17,17, No.
No. 77 (2005)(2005) 413– 421
Sensors
MYU Tokyo

413

S & M 0619

Odor Sensor System for Early Fire Detection
and Its Application to Utility Mobile Robot
Yoshinori Takei*, Tetsuro Tashiro, Tadanobu Misawa1, Hidehito Nanto,
Hiroshi Kasahara2 and Yukiyo Iwasaki3
Advanced Materials Science R&D Center, Kanazawa Institute of Technology,
3-1 Yatsukano, Hakusan, Ishikawa, 924-0838, Japan
1
Tokyo University of Science, 500 Shimokiyoku, Kuki, Saitama, 346-8512, Japan
2New Cosmos Electric Co., Ltd., 2-5-4 Mitsuyanaka, Yodogawa, Osaka, 532-0036, Japan
3tmsuk Co., Ltd., 1-7-8 Kimachi, Kokurakita, Kitakyusyu, Fukuoka, 803-0851, Japan(Received December 21, 2004; accepted May 9, 2005)
Key words:

Odor sensor system, mobile robot, early fire detection

In this paper, an odor sensing system for early fire detection is demonstrated. The odor
sensing system consists of a sensor array and a computer. The sensor array consists of fivemetal-oxide-semiconductor gas sensors. Sensing the generation of hydrogen duringincomplete combustion could be effectively utilized to detect the early stage of fire. As anapplication of the proposed sensor system, it was implemented into a utility robot named“Banryu” (tmsuk, Inc.) which is commercially available.

1.

Introduction

Odor sensing systems have attracted much attention in recent years, because developing an objective evaluation method for odors instead of relying on human sensory test isrequired. The odor sensing systems, called electronic nose (e-nose) systems, have been
applied to various areas (e.g. food processing, medical clinic, public health, etc.).(1) The
areas using e-nose systems have increasingly expanded and several applications of e-nose
systems can be found in the literature.(2,3)
In this paper, we show an odor sensing system for early fire detection. Fire alarmsystems generally sense smoke, heat and flame to detect fire. The detectors, however,cannot detect the initial stages of a fire. We have developed an odor sensing system forearly fire detection using commercially available metal oxide semiconductor (MOS) odor
sensors. Here we consider a house fire as the target to be detected by the sensors and we
discuss specifically fires caused by cigarettes. The proposed odor sensing system consistsof two different sensors: one with broad sensitivity for various odors, and the other withhigh selectivity for hydrogen. Sensing the generation of hydrogen during incomplete
combustion can be utilized to detect the early stages of a fire. The system developed has*Corresponding author, e-mail address: takei@neptune.kanazawa-it.ac.jp

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been attached to a commercial utility robot, which is used to detect the fire and warn ofchanges concerning odors in a room.

2.

Experimental

2.1 Odor sensor system
Odor sensors based on the change in the resistance in MOS have been studied for many
applications. In this experiment, commercial metal oxide odor sensors of two types, CHEz and CH-H (New Cosmos Electric, Inc.), were used. The CH-Ez-type odor sensor has
broad sensitivity for various odors, which can be used for detecting a change in theatmosphere of the room being measured. The CH-H-type has high selectivity for hydrogen, which can sense the generation of hydrogen in incomplete combustion. A sensor arraycombining these sensors was used for odor samples.
Figure 1 shows the odor sensing system. The sensor array is connected to the sensor
unit, which consists of circuit boards for each sensor and a power supply. Measuring achange in the resistance of the MOS sensor indicates the adsorption of odors on the surfaceof a sensor. The measured change in the resistance can be observed and visualized using
the software on the computer, which is connected through an A/D converter. We
considered two conditions for the experiment. First, we prepared an experiment in a sealed
chamber. Odors injected into the chamber using a syringe were measured by the two typesof MOS sensors located on the top of the chamber. In this experiment using the sealed
chamber, sensor response patterns were evaluated for the odors using the CH-Ez and theCH-H sensors. We next carried out an experiment in using the sensors in a room, which
was assumed to be a more practical environment.
Three locations of the sensor array relative to the odor source were considered as shownin Fig. 2 and the responses observed at these locations were compared.

3.

Results and Discussion

3.1 Evaluation of responses to burning cigarettes and disturbing odors
First, we target a fire caused by a cigarette; therefore, the odor of a burning cigarette isused as the sample. The odors assumed to exist normally in the house; that is, odors of hairmousse, hair spray, perfume and fragrances are used as the competing odors. Figures 3 and 4

Sensor Array

Fig. 1.

A/D converter

PC

The odor sensor system.Sensors and Materials, Vol. 17, No. 7 (2005)

Fig. 2.

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Relative location of the sensor array and the odor source.

Fig. 3 (left). Response to the odor of a burning cigarette.
Fig. 4 (right). Response to a typical conflicting odor. (Response to the odor of hair spray.)show typical responses to the burning cigarette and the odor assumed to compete the sensor
response, which shows the response to the odor of the hair spray. These concentrations areset at 10,000 ppm.
In Fig. 3, as compared to the response of the CH-Ez sensor, we can see that the CH-H
type sensor has an adequate response to the target odor. It also shows that the generation ofhydrogen can be detected using the CH-H sensor. On the other hand, the response pattern
to the competing odor is different from the response to the odor of the burning cigarette; theCH-Ez sensor responds only to the odor of the hair spray and the generation of hydrogen
could not be detected. For the other odors assumed to compete the responses are similar tothese results as shown in Fig. 4.
Using these results of the responses we can discriminate the burning cigarette from theother odors, and the detection of the generation of hydrogen can be used for discrimination.
Figure 5 shows the discrimination of the odor of the burning cigarette from the odors
assumed to compete and influence the sensor responses.
Burning cigarettes are separated from other odors, and even cigarette brands can bedistinguished. However, the odors of burning cigarettes at low concentrations appear theplots near other odors. Here, the sensors directly sense the burning cigarettes that do not416Sensors and Materials, Vol. 17, No. 7 (2005)

denote the concentrations; therefore, their odors are injected at very high concentrations in
the experiment. From the result shown in Fig. 5, the change in the concentration of theodors influences the separation of the odors on the plots. Therefore, to avoid the influenceof the change in the concentration of the odors, we normalized the sensor output response.
Figure 6 shows the discrimination of the odors of burning cigarettes from the odors
assumed to compete with and influence the sensor responses using normalized sensor
outputs sampled at 240 s after starting the measurements. Each symbol indicates the sameodor in Fig. 6. The discrimination among the odors improved from that in Fig. 5, and theplots of the same odors at different concentrations are closer. Thus the proposed method
enables us to discriminate the odor of burning cigarettes.

Fig. 5. Discrimination of the odors of burning cigarettes, using the outputs of the sensors (CH-Ezand CH-H) at 240 s after starting the measurement.

Fig. 6. Discrimination of the odor of burning cigarettes, using the normalized outputs of the sensors(CH-Ez and CH-H) 240 s after starting the measurements (symbols are same as in Fig. 5).

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3.2 Experiments in a closed room
Assuming that the proposed odor sensing system will be used in a house, we consideredand evaluated the response to burning cigarettes and the competitions in a closed room asshown in the Fig. 2 as more representative environment. Here we choose the odor of hairspray as the competing odor, and we show only the results of a case in which the sensor
array is located over the odor source. Figure 7 shows the response to the odor of a burningcigarette in the room, where the sensor array is located over the odor source.
In Fig. 7, we can see that the CH-H type sensor responds sufficiently, similar to themeasurement in the sealed chamber. The generation of hydrogen is also detected in thiscase. Note that the output of the CH-Ez sensor correlates with that of the CH-H sensor.
Figure 8 shows the response to a typical competing odor, hair spray, when the sensor arrayis located over the odor source. The generation of hydrogen could not be detected, similar
to the response in Fig. 4. The response pattern obtained by the sensors is similar in theexperiment in the sealed chamber. Therefore, sensing the generation of hydrogen can beused to discriminate the odor of burning cigarettes.
It is remarkable that results for the other relative locations between the sensor and theodor source show the same result; the generation of hydrogen can be detected and theresponses between the CH-Ez and the CH-H sensors correlate to the odor of burningcigarettes. Since it is expected that the response to the odor generated in incomplete
combustion can be shown to be similar to burning cigarettes, the proposed system may be
useful for early detection of fires.
We next expand the target odorants. The responses to odors generated when othermaterials burn, which are generated by heating items with a soldering iron, are added andevaluated. The senor array used in the previous experiments is constructed using the sametwo types of sensors, CH-Ez and CH-H. The key to discriminating the odorants is thesensor response to hydrogen, which is obtained using the CH-H sensor. But hydrogen can
not be detected in some odorants obtained from burning materials, as shown in Fig. 9.
Therefore, we expand the sensor components on the array. Each sensor on the array isFig. 7 (left). Transient response to the odor of a burning cigarette in the room. (The sensor arrayis located over the odor source.)
Fig. 8 (right). Response to a typical competing odor (the response to the odor of hair spray), when
the sensor array is located over the odor source.

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Fig. 9.

Response to odorants generated by burning cigarette and several materials.

characterized by its sensitivity to hydrogen, ammonia, hydrogen sulfide, volatile organiccompounds (VOC) and various other odors. The sensor array is reconstructed with thesesensors.
First, we evaluate the previous odorants again with a sensor array consisting of fivesensors. The odors are generated in two ways. One method generates a single odor
separately. The other generates two odors sequentially at an interval of several minutes.
The sensor array generates a response pattern to the odors; typical response patterns areshown in Figs. 10 and 11, when the responses to the target odorants can be characterized by
the response of the CH-H type sensor primarily selective for hydrogen.
Next, we consider the influence of the rest of the competing odorants by a principal
component analysis (PCA) plot using the array output at each time. Figure 12 shows theresult of PCA using the sensor response pattern constructed by the sensor array output
sampled 420 s after stating the experiments. The data were obtained by experiments in theroom. The measurements using two odors were obtained by generating a second odor 60 s
after generating the first odor. The result shows that the target odor, cigarette, can bediscriminated from the competitive odors. The response patterns to the target odor, which
was characterized by the response to hydrogen, were different from the responses to thecompetitive odors. Although the concentration of the target odor increased, responses tothe competing odors were constant or decreased. Thus, the response pattern to the mixtures
was similar to the response to the target. Therefore, the mixtures containing the target odor
can also be isolated from the other odorants. The detection of the generation of hydrogen
could be utilized to discriminate the odors of fire from that of the competing odors.

3.3 Discrimination of the odorants generated by burning materials
We also applied the proposed system to detect the odorants generated by burningmaterials such as cellulose, polyethylene and nylon. Figure 13 shows the typical transient
response to the odor obtained by burning nylon. It shows that the CH-H type sensor can notSensors and Materials, Vol. 17, No. 7 (2005)

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Fig. 10 (left). Response to the odor of burning cigarette in the chamber.
Fig. 11 (right). Response to the odor of perfume in the chamber.

Fig. 12. PCA plot of the data measured in the room. The odors are generated in two ways. One
method generates a single odor separately. The other method generates two odors sequentially at aninterval of several minutes (shown as A→B).

Fig. 13.

Response to the odor of the burning nylon.

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respond to the odor sufficiently. Therefore, as an indicator, the generation of hydrogencannot be used to detect the fire. Therefore we consider the response pattern of theodorants obtained by the sensor array output. The response patterns are utilized toconstruct data vectors for PCA. We applied PCA to the discrimination of the sensor array
responses to the odorants generated by burning materials.
The pattern vectors corresponding to the odors were normalized to reduce the influencecaused by changes in the concentration of the odorants. In Fig. 14, PCA is performed onunit gases with changing concentration. The result shows the method can discriminate thegases even if concentrations change. We next apply the method to the odorants shown inFig. 15. Figure 15 shows the proposed system can also discriminate the odorants generated
by burning materials from the competing odorants. The proposed sensor array system canbe utilized to detect fires.

Fig. 14.

PCA plot of the data from responses to the gases at different concentrations.

Fig. 15. PCA plot of the data from the chamber.

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Fig. 16. The utility robot “Banryu” (tmsuk, Inc.).

4.

Conclusions

An odor sensing system for early fire detection was developed and demonstrated. We
have shown that sensing the generation of hydrogen in incomplete combustion could be
utilized to detect the early stages of fire effectively. Some odorants generated by burningmaterials can be isolated from the competitors using the sensor array system with PCA. As
an application of the proposed system, the sensing system has been attached to thecommercial utility robot “Banryu” (tmsuk, Inc.) shown in Fig. 16.

Acknowledgments
This research was partially supported by the Ministry of Education, Science, Sports andCulture, Grants-in-Aid for No.15760328, 16360208 and 16656031, and by the Fire andDisaster Management Agency.

References
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2 K. Toko: Kansei Biosensor (Asakura shoten, Tokyo, 2001) (in Japanese).
3 T. C. Pearce, S. S. Schiffman, H. T. Nagle and J. W. Gardner: Handbook of Machine Olfaction,(Willey-VCH, Weinheim, 2003).
4 tmsuk, Inc., http://www.banryu.jp/.