|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.
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.