This paper was prepared by myself and Niraj Kumar , my friend who is in Computer Science & Engineering in this college in VI Semester itself.

1.     INTRODUCTION

1.1  What is the Electronic Nose?

     The "electronic nose" is a relatively new tool that may be used for safety, quality, or process monitoring, accomplishing in a few minutes procedures that may presently require days to complete. An electronic nose is not a replacement for people, it is a supplement.

          The electronic nose consists of two components, (1) an array of chemical sensors (usually gas sensors) and (2) a pattern-recognition system. The sensor array "sniffs" the vapors from a sample and provides a set of measurements; the pattern-recognizer compares the pattern of the measurements to stored patterns for known materials. Gas sensors tend to have very broad selectivity, responding to many different substances. This is a disadvantage in most applications, but in the electronic nose, it is a definite advantage. Although every sensor in an array may respond to a given chemical, these responses will usually be different.

 2.         In recent years, electronic noses have been sniffing out landmines, detecting contraband drugs, sensing for chemical and biological weapons, identifying batches of spoiled food, and even showing promise for aiding in the diagnosis of diseases like lung cancer and pneumonia. These interesting devices are designed to mimic the ability of the human nose to detect very small quantities of odorants. E-noses can also detect chemicals that have no odor, such as toxic carbon monoxide. Compared to the senses of sight, hearing, and touch, scientists know relatively little about how humans smell and taste. Designers of electronic noses have tried to mimic human noses by linking together sensors that detect a variety of volatile compounds.

3.  NEED FOR AN ELECTRONIC NOSE

          Human noses have always been the best odor receptors distinguishing between very similar ones. Contrary to physical senses (dealing for instance with acoustic or optic mechanisms), some aspects of the human taste and olfaction physiological working principle are still unclear. Because of these intrinsic difficulties toward the understanding of the nature of these senses, only sporadic research on the possibility of designing artificial olfactory systems was performed until the end of the eighties. But as good as human noses are for chemical detection, they have drawbacks. They "fatigue" if subjected to repeated smelling tasks or strong scents, as anyone knows who has initially been shocked by an overpowering smell and then has become acclimated to the odor. The exquisite sensitivity of the nose can be defeated by a common cold, and for obvious reasons, human noses have limitations on sniffing out highly toxic compounds.. Electronic nose is a new and promising technology which is rapidly becoming a valuable tool for the organoleptic evaluation of food parameters related to taste and smell and could replace human sensory panels in quality control applications, where the objective, rapid and synthetic evaluation of the aroma of many specimens is required. The electronic device offers major advantages over conventional tools and could eventually extend far beyond detecting pulmonary diseases. Rather than waiting two or three days for the results of a bacterial culture or relying on chest X-rays that may be inaccurate, doctors can almost instantaneously evaluate their patients for infection.E-noses, which can distinguish only a very small range of smells compared with the human nose but overcome a few of its shortcomings.

3.1  The advantages of ANN-containing electronic noses.

An Artificial Neural Network Structure performing odor identification

• Advantages over chemical sensors.

v      The `nose' is trained on examples rather than rules, negating the need for expert description of the domain.

v      The number of odours classified is greater than the number of sensors because the network can discriminate between patterns of activation across all the sensors. Fewer sensors are needed.

v      Thus one can use less selective (and less expensive) sensors.

v      Real-time odour identification. The time consuming part of the process is training of the network. Once trained the system's performance is governed by the speed of the chemical sensors.

v      It processes new smells, despite never having been trained on them.

• Advantages over human noses:-

v      Unbiased.

v      Not subject to interference by emotional states (e.g., tiredness, mood) or illness (e.g., allergies).

v      Use in dangerous situations (e.g., contamination testing).

v      Not subject to habituation.

4.  ANALOGY BETWEEN HUMAN & ELECTRONIC NOSES

               Human nose                                            Electronic nose

v     Nasal Receptors                                            Sensors          

v     Brain                                                               Microprocessors

v     Transmission through nerves                      Use of Artificial Neural Networks     

5.  DIFFERENT TYPES OF SENSORS

v     The sensors used in an electronic nose can be  mass transducers (such as Quartz Crystal Microbalance or QCM)

                        A smell sensor can be made from a quartz crystal with electrical connections and a special plastic coating. Quartz crystals are used in electronics because they can be made to vibrate at a precise frequency. A quartz crystal is what is used to control the speed of a processor in a PC. The frequency of vibration of the quartz crystal depends on its size, shape, stiffness and mass. The plastic coating on the crystal absorbs some chemicals so increases the crystals mass. The whole device is called a Quartz Crystal Microbalance (QCM).A quartz crystal can be thought of as mass on a spring.The frequency of oscillation of a mass on a spring is given by the formula:

                      Where k is the stiffness of the system in N/m, m is the mass of the system in Kg, f is the frequency of the system in Hertz.

v     Chemo resistors (based on metal-oxides )

v     Chemo resistors (based on conducting polymers) 

v     Some arrays comprise both types of sensors.

Some Sensors

v     Nanomechanical Cantilevers

          The artificial nose demonstrator is based on micro fabricated nanomechanical cantilever sensors - thin silicon beams - a few hundred micrometers long and one micrometer thick. Eight cantilever sensors, each is coated with a different sensor layer, are integrated in an exchangeable array. On exposure to an analyte, the analyte molecules adsorb on the cantilever’s surface. This leads to formation of interfacial stress between sensor and adsorbing layer. The bending pattern is characteristic for each analyte.

Table 1. Sensors used by the electronic nose group to detect odors

6.   DESIGN & WORKING :

6.1  The Components of an electronic nose.

There are two main elements of an electronic nose:

1. The Sensing system can be an array of several different sensing elements (e.g., chemical sensors), or it can be a single sensing device (e.g., spectrometer) that produces an array of measurements for each chemical.
2. The Automated Pattern Recognition system. Here Artificial neural networks (ANNs) are showing promising results.
3. A microprocessor  to carry out the data interpretation.

          Current `noses' consist of approximately 20 sensors and 100 neurones. Although each chemical sensor is designed for a specific chemical, each actually responds to a wide variety of chemicals. Collectively the sensors respond with unique signatures to different chemicals. By presenting many different chemicals to the sensory array, a database of signatures is built up.

          This labelled database is then used to train the pattern recognition system, with the goal being to reconfigure the recognition system to produce unique classifications so that automated identification can take place. Once the ANN is trained operation consists of propagating test sensory data through the network. Since this is simply a series of vector-matrix multiplications, unknown chemicals can be rapidly identified in the field.

6.2              Electronic Nose Scheme

          The electronic systems work in the same way as human smell, with three components -signal generating, sampling and data processing. An electronic nose can be regarded as a modular system comprising a set of active materials which detect the odour, associated sensors which transduce the chemical quantity into electrical signals, followed by appropriate signal conditioning and processing to classify known odours or identify unknown odours, see Figure 1.

6.2.1  Data Processing Methods:      The signals generated by an array of odour sensors need to be processed in a sophisticated manner. The electronic nose research group has obtained considerable experience in the use of various parametric and non-parametric pattern analysis techniques. These include the use of linear and non-linear techniques, such as discriminant function analysis, cluster analysis, multi-layer perceptions, genetic algorithms, fuzzy logic, and adaptive models.

                       Basic schematic of an electronic nose

The chemical component is converted into electrical signals by the basic device (sensor adsorbent layer)

6.2.2  Pattern Recognition:

          A sensor comprises a material whose physical properties vary according to the concentration of some chemical species. These changes are then translated into an electrical or optical signal which is recorded by a device. The sensors are non-selective  A chemical compound is identified by a pattern of the outputs given by the different sensors, thanks to pattern recognition methods. There is an exhaustive database which contains the information about patterns of different chemicals. The pattern now generated by the sensors and the data processor is compared with every entity of the database. If a match occurs then the chemical is recognized by the system.

The signatures produced by the chemicals (Expt done at IIT)

6.2.3  Other techniques of operation

           Electronic odour sensing devices have arrays of sensors that detect the presence of vapors. In this way they act as volatile chemical detectors. The sensors respond by producing electrical signals that are passed on to an artificial intelligence system programmed to interpret them

6.2.4  An example on data processing about odors

A Prototype of E - Nose

          Considering a sponge as thin as human hair, if water is poured onto that sponge,it would swell up.Particles of carbon (similar to a pencil lead) are put into the sponge. As   the water makes the sponge expand, the distance between the carbon particles gets bigger. Carbon particles are good conductors of electricity , but the farther apart are the particles ,the harder it is to pass electric current from particle to particle.If the sponge is hooked upto a metre that measures electricity it can be said whether the sponge is swollen with water or not, by how easily the electricity passes through it. Thus this sponge can “smell” water. Similarly other substances’ smells can also be determined. Carbon particles are mixed into a collection of different sponges that swell up in the presence of different substances. There we have it – smelling through swelling. Each odor produces a different pattern, or smell print , on the collection of sponges. So, that’s how it can be known which smell is of what.In the best electronic noses, no two smells produce exactly the same smell prints. In reality instead of sponges the e-nose uses very thin films of different polymers painted on a hard ceramic plate.This way the entire device including a ciomputer to analyze the smell print can be quite small.

7.   APPLICATIONS

7.1   Environmental Monitoring.                                                                                                                             

          The development of portable electronic noses, along with ANN radiation and optical sensor systems, at the Pacific Northwest National Laboratory has produced promising results in the detection of contaminant chemicals.Further future applications include:

·        Monitoring of factory emissions, air quality and household odours.

·        Detection of oil leaks.

·        Analysis of toxic wastes and fuel mixtures.

7.2   Medicine.                                                                                                                                                        

          Sense of smell has some role in diagnosis in medicine. Sensitive `noses' may help clinicians by examining:

v      Breath odours. Abnormal breath can be indicative of gastrointestinal problems, sinus problems, infections, diabetes and liver problems. The Highland Psychiatric Research Group is pioneering a breath odour analyser for the prediction of acute schizophrenic illness in vulnerable patients; normally an extremely complicated procedure. This is based on findings that lipid metabolism in the brain is disturbed some days before behaviour is detected.

v      Body fluids. The smell of urine and blood can help in the diagnosis of liver and bladder problems.

v      Wounds. Following surgery, early diagnosis of wound infection considerably improves healing rate, and presently it takes greater than two days to isolate the pathogens involved.

          Additionally, these instruments may help in Telemedicine, the practice of medicine (in particular surgery) when patient and doctor are in different locations (e.g., battlefields, remote locations). Smell can be an important indicator that the operation is not going well (e.g., the bowel has been perforated), and so a remote electronic nose coupled with a local odour generator would help in the transmission of olfactory information for medicine

7.3 Food industry applications.                                                                          

This is currently the biggest market for electronic noses:

v     Inspection of food to test for ripening/rotting.

v     Testing of packaging materials for odour containment.

v     Microwave oven cooking control.

v     Verifying if orange juice is natural.

v     Grading whiskey and controlling fermentation.

7.4 Some other applications

v     Discrimination between single volatile compounds

v     Tracking of the aroma evolution of ice stored fish or meat

v     Tracking of the evolution of cheese aroma during the aging process

v     Classification of wines produced under the same denomination (in order to find out those productions not fitting with the standard)

v     Determination of androsterone in pork fat

v     Identification of the presence of blood in urine samples

v     Classify different varieties of peaches

v     Classify different ripeness degrees of apples

v     Classify olive oils according to their production country

7.5  E-nose could sniff out time of death.

          A device, which when waved over a corpse, can identify the vapors and odours present, as well as the relative quantities of chemicals and related compounds. The readings of the electronic nose would then be compared to the compared to the team's collated results, thus identifying the stage of decomposition.

7.6 Army applications.

          Inspecting the presence of landmines, detecting contraband drugs, sensing for chemical and biological weapons is generally done by an e-nose. Normally landmine detection instruments use the electromagnetic properties of the mine. But now some landmines exist whose electromagnetic characteristics are same as that of sand, hence problem generally arises in their detection. Caltech team has developed special olfactory sensors which discriminate odors by measuring how different chemical vapors change the polymers electrical resistance. A mobile device would guide odor detectors to location that ultrasound devices suggest presence of mines. The ultrasound device would be intense enough to stir up under ground chemical particles for identification. This involves the uses of MEMS e- noses (Micro Electro Mechanical System).

           Sensors pick up subtle changes in the dielectric, optical or thermal properties of the soil in which the landmine is buried. The army grade TNT is also contaminated with MNT(mononitrotoluene), DNT(trinitrotoluene) ,MNB(mononitrobenzene) , DNB( dinitrobenzene) ,the equilibrium vapor pressures of which are much higher than TNT itself ,hence they can be detected by examining the vapors escaping from the landmine.

Various prototypes of electronic noses .                                  A sensor

8.  CONCLUSION

          Thus we see that no one expects e-noses to duplicate all the capabilities of the human nose anytime soon. But they can deliver substantial benefits in situations where, given the choice, we'd prefer not to use our own sniffers. No instrument is complete without its shortcomings and an electronic nose is no exception.

Its drawbacks include:

v     Difficulty in maintaining an exhaustive database of different fingerprints of chemicals.

v     It may be very difficult to analyze a complex mixture of different chemicals.

v     The precision of the device while analyzing similar smell is controversial.

          So it can be inferred that inspite of the above shortcomings the electronic nose is a blessing in this era. There are numerous potential applications of electronic noses from the product and process control through to the environmental monitoring of pollutants and diagnosis of medical complaints. However, this requires the developments of application-specific electronic nose technology that is electronic noses that have been designed for a particular application. This usually involves the selection of the appropriate active material, sensor type and pattern recognition scheme. The work of the group has led to several commercial instruments, one employing commercial tin oxide sensors (Fox 2000, Alpha MOS, France) and another employing conducting polymer sensors (NOSE, Marconi Applied Technology, UK). Collaborations also exist with Osmetech (UK) and Cyrano Sciences (USA) Future developments in the use of hybrid micro sensor arrays and the development of adaptive artificial neural networking techniques will lead to superior electronic noses.

9.  REFERENCES

v     http://www.inapg.inra.fr/ens_rech/siab/asteq/elba/sommelen.htm

v     http://www.cogs.sussex.ac.uk/lab/nlp/gazdar/teach/atc/1998/web/sloss/index.html

v     Danny Kingsley – ABC Science Online

v     Gardner J W and Bartlett P N 1999 Electronic Noses ( OUP Press, Oxford); Gardner J W and Bartlett P N (Eds) 1992 Sensors & Sensory Systems for an Electronic Nose (Dordrecht: Kluwer Academic Publishers) NATO ASI Series:
Applied Science Vol. 212 pp.327

v     Gardner J W and Bartlett P N 1994 Sensors and Actuators B 18 211-220 "A
brief history of electronic noses"'