Robotics Reference
In-Depth Information
Smell Recognition
Smells are caused by chemical molecules that are small enough and light
enough to become vapour. A smell might be just one type of molecule or
it could be a mixture of many different types. As an example, more than
600 different types of molecule waft into your nose when you smell the
aroma of fresh coffee.
Humans detect smells by taking into their noses samples of air. Any
dust particles are swept out of the way by a mucus layer on the inner
surfaces of the nostrils together with a forest of sticky hairs. The filtered
air then passes on to a collection of about five million smell sensor cells
that comprise what is called the nasal epithelium. 20
The design of most electronic noses is based on that of the human
nose. There is a device, replacing our nostrils, for sampling the air, and
an array of chemical sensors to mimic the function of the epithelium.
One of the most common types of sensor for this purpose is called a
conductivity sensor, often made of tin dioxide, which exhibits a change
in resistance when exposed to molecules of various substances.
The two main components of an electronic nose are the sensing sys-
tem and the automated pattern recognition system. The sensing system
sometimes consists of several elements, each of which measures a differ-
ent property of the chemical causing the smell, while others are made
of a single sensing device that produces several measurements for each
chemical. Each chemical vapour savoured by the sensor system produces
its own “signature”, a pattern that is characteristic of the vapour. By
presenting many different chemicals to the sensor system, a database of
signatures can be built up and employed to train a pattern recognition
system, such as an artificial neural network, so that it produces a unique
classification of each chemical. In this way the identification of a smell
can be automated.
There are many important uses for our sense of smell, outside of the
pleasures of wining and dining. Doctors can diagnose certain diseases
from their smell alone; for example, some electronic noses are being used
to aid the rapid diagnosis of lung cancer, others to diagnose gastroin-
testinal problems by monitoring a patient's breath. People can detect the
smell of smoke and thereby warn of a fire in the home. Environmen-
tal applications of electronic noses include the detection of oil leaks and
toxic wastes, the identification of household odours, and the quality of
20 Dogs have 100 million of these cells, hence the greater sensitivity of their sense of smell.
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