01-01-2013, 10:54 AM
SEMINAR REPORT ON ELECTRONIC NOSE
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ABSTRACT
The perception of volatile compounds by the human nose is of great importance
in evaluating the quality of foods. Therefore, it is not surprising that repeated
efforts have been made over the years to introduce instruments operating on a
similar principle as the human nose: the “electronic nose” is an instrument that
encloses the human sensitivity to the objectivity of the instrumental response
and supplies results similar to the human nose and in short time.
An electronic nose is “an instrument which comprises an array of electronic
chemical sensors with partial specificity and an appropriate pattern recognition
system capable of recognising simple or complex odours”. It 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.
INTRODUCTION
There have been many developments in electronics to create real life
environments with respect to sight and sound. But there are three other
significant senses- smells, touch and taste which have not been experimented
much with. The sensor technology of artificial olfaction had its beginnings with
the invention of the first gas multi sensor array in 1982. Advances in aromasensor
technology, electronics, biochemistry and artificial intelligence made it
possible to develop devices capable of measuring and characterizing volatile
aromas released from a multitude of sources for numerous applications. These
devices, known as electronic noses, were engineered to mimic the mammalian
olfactory system within an instrument designed to obtain repeatable
measurements, allowing identifications and classifications of aroma mixtures
while eliminating operator fatigue.
THE ELECTRONIC NOSE AND THE BIOLOGICAL NOSE
Biological Olfaction
The mammalian nose is a complex sensory system that can sense and distinguish
up to 10,000 different substances on the basis of odour even when there are only
minor differences in the chemical composition (e.g., Batches of perfumes, brands
of coffee). Current understanding of this system is based on a neural approach
which has the capacity to identify a large number of odours within a context of
background odours and varying environment conditions. It is made up of three
prominent parts: the olfactory mucosa with its own related odour presentation
system, the olfactory bulb and olfactory cortex.
Odours are transferred through various nasal passages and once in the central
one, odour molecules link with the chemically sensitive olfactory receptor cells of
the epithelium. Olfactory cells are specialized neurons that are replicated
approximately every 30 days. The sensory cells in the epithelium respond by
transmitting signals along neural "wires" called axons. Signals from neurons with
the same receptors converge on structures called glomeruli in the olfactory bulb.
The signals evoke unique pattern of activity (signature) in these glomeruli within
the olfactory bulb. The glomeruli form the first tier of information processing.
There then occurs a complex process of pattern recognition by the neural
network in the brain that learns to recognize various odour signatures through
training.
Electronic nose principles
The electronic nose is a system that consists of three functional components that
operate serially on an odorant sample-a sample handler, an array of gas sensors
and a signal processing system. The output of the electronic nose can be the
identity of the odorant, an estimate of the concentration of the odorant, or the
characteristics properties of the odour as might be perceived by a human.
ENS can be seen as arrays of nonspecific sensors able to generate electrical
signals in response to either simple or complex volatile compounds and give
through a suitable multi component analysis technique, the possibility of
discrimination, recognition and classification of odours. The target compound, in
gaseous form, is introduced into the sensing chamber where the sensors are
exposed to the vapour. A variety of basic sensors can be used according to the
nose strategy chosen. Some of them are sensitive to the mass of adsorbed
species, others show sensitivities to electric charges while others are based on
either surface or bulk conductivity changes due to chemically interactive
materials. These changes are dependent on a complex interaction between the
components of the vapour and the sensors, as each sensor responds to a number
of components in a unique manner. Each sensor in the array has different
characteristics (e.g., coatings, operating temperatures, etc.) and, hence, each
sensor will give a different electrical response (voltage output) for a particular
odour. The pattern of response across all the sensors in the array is used to
identify and/or characterise the odour.
Comparison of electronic nose with biological nose
Each and every part of the electronic nose is similar to human nose. The function
of inhaling is done by the pump which leads the gas to the sensors. The gas
inhaled by the pump is filtered which in the human is the mucus membrane. Next
comes the sensing of the filtered gas, which will be done by the sensors i.e.,
olfactory epithelium in human nose. Now in electronic nose the chemical retain
occurs which in human body is enzymal reaction. After this the cell membrane
gets depolarised which is similar to the electric signals in the electronic nose. This
gets transferred as nerve impulse through neurons i.e., neural network which is
analogous to the electronic circuitries in the electronic nose.