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Clemson C3B, Clemson University, Clemson South Carolina 29634  College of Engineering & Science  C3B Advisory Board
The e-Nose
Of the body's five senses, the sense of smell is the most mysterious. The human nose has become an exciting new frontier as scientists and engineers who try to better understand how the nose functions. An electronic nose is a device used to analyze the content of air through the classification of odors. Although the electronic noses in use today are far from replacing the human olfactory system, the possible uses for this technology are endless. Human noses are employed all over the world to test many different products. The odors of food products such as grains, wines, cheeses, whiskey, and fish are all examined by human noses to determine their quality and freshness. Perfumes and deodorants are also tested to see if they are appealing to the nose. The sense of smell is even used by doctors to help classify common disorders. Certain problems such as pneumonia or diabetes give the patient's breath or bodily fluids characteristic odors that can be noticed by a trained nose. If these odors could be classified by a machine, then an electronic nose could be employed to do the same job with more possibilities. Another problem electronic noses could solve is the health risk associated with smelling certain chemicals. The toxicity of certain chemicals could prove harmful to someone who was trying to determine an odor using their nose. Even grain samples can give off mold spores which can cause allergic reactions or other disease symptoms. In addition, electronic noses could be taken places the human nose could not, for example into areas of extreme temperatures, inside the body, inside oil rigs or gasoline tanks, sewer systems, hog farms, or even onto another planet.

Electronic noses are generally made up of two main parts: a sensing system and a pattern recognition system. In the past, gas chromatography and mass spectrometry have been used as the sensing systems, although these are usually expensive and time consuming. Today, the use of chemical sensors has been established to analyze odors. Essentially, each odor leaves a characteristic pattern or fingerprint of certain compounds. Known odors can be used to build up a database to train a pattern recognition system. One possibility is to have a sensor for every chemical, though this would be costly since there are so many different chemicals. The answer is in artificial neural networks (ANNs). ANNs are able to detect more chemicals than the number of sensors it is utilizing. ANNs also allow for less selective and therefore less expensive chemical sensors. The artificial neural networks are trained to distinguish certain odors from certain chemical combinations. Pattern recognition is gained though giving the network known odors and classifying them with a signature.
Then the nose is tested to see how well the ANN has learned. The results can be adjusted through experimentation The sensors basically measure the change in voltage due to the presence of certain chemicals. The chemicals in the air change the oxygen content over the sensors, which are electronic circuits. By changing the oxygen content, the resistance across the sensor is changed which can be measured as a voltage drop from the normal or standardized conditions. This analog signal must then be translated into a digital signal by an A/D converter in order for the computer to understand the information. The number of odor signatures the system can recognize depends on the number of sensors used and the number of grey levels in the convertor. The maximum signature number is given by gn, where n is the number of sensors and g is the number of grey levels. A 10-bit converter has a grey level value of 1024, so an array of three sensors could yield over a billion different signatures. Unfortunately, the actual number is far below this value.

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Rev 2 Dec 2007