How Does an Electronic Nose Work?

Introduction:

The electronic nose is a device that detects the smell more effectively then the human sense of smell. An electronic nose consists of a mechanism for chemical detection. The electronic nose is an intelligent sensing device that uses an array of gas sensors which are overlapping selectively along with a pattern reorganization component.   Now a day the electronic noses have provided external benefits to a verity of commercial industries, agriculture, biomedical, cosmetics, environmental, food, water and various scientific research fields. The electronic nose detects the hazardous or poisonous gas which is not possible to human sniffers.

The smells are composed of molecules, which has a specific size and shape. Each of these molecules has a corresponding sized and shaped receptor in the human nose. When a specific receptor receives a molecule it sends a signal to the brain and brain identifies the smell associated with the particular molecule. The electronic noses work in a similar manner of human. The electronic nose uses sensors as the receptor. When a specific sensor receives the molecules, it transmits the signal to a program for processing, rather than to the brain.

Electronic Nose Working Principle:

The electronic nose was developed in order to mimic human olfaction whose functions are non separate mechanism , i.e. the smell or flavor is perceived as a global finger print. Essentially the instrument consists of sensor array, pattern reorganization modules, and headspace sampling, to generate signal pattern that are used for characterizing smells. The electronic nose consists of three major parts which are detecting system, computing system, sample delivery system.

The sample delivery system: The sample delivery system enables the generation of headspace of sample or volatile compounds which is a fraction analyzed. The system then sends this head space into the detection system of the electronic nose.

The detection system: The detection system which consists of a group of sensors is the reactive part of the instrument. When in contact with volatile compounds at that time the sensors reacts causing changes in electrical characteristics.

The Computing system: In most electronic noses each sensor is sensitive to all molecules in their specific way. However in bioelectric noses the receptor proteins which respond to specific smell molecules are used. Most of electronic noses use sensor arrays that react to volatile compounds. Whenever the sensors sense any smell , a specific response is recorded that signal is transmitted into the digital value.

The more commonly used sensors in electronic nose

Metal oxide semiconductor (MOSFET)

Conducting polymers

Quartz crystal microbalance

Piezoelectric sensors

Metal Oxide sensors

Metal Oxide semiconductor sensor:

This is used for switching or amplifying electronic signals. The Working principle of MOSFET is that molecules entering into the sensor area will be charged positively or negatively which have directly effect on the electric field inside MOSFET.

Metal Oxide sensors: (MOS)

This sensor is based on adsorption of gas molecules to provoke change in conductivity. This conductivity change is the measure of the amount of volatile organic compounds adsorbed.

Piezoelectric sensors:

The adsorption of gas onto the surface of the polymer leads to change in mass on the sensor surface. This is turn produce a change in the resonant frequency of the crystal.

Quartz crystal microbalance:

This is a way of measuring mass per unit area by measuring the change in frequency of crystal resonator. This can be stored in a data base.

Conducting polymers:

Conductive polymer gas sensors operate based on changed in electrical resistance caused by adsorption of gases onto the sensor surface.

Data Analysis for Electronic Nose:

The digital output generated by electronic nose sensors has to be analyzed and interpreted in order to provide. There are three main types of commercially available techniques.

• Graphical analysis
• Multivariate data analysis
• Network analysis

The choice of method utilized depends on available input data from sensors.

The simplest form of a data reduction is a graphical analysis useful for comparing samples or comparing smells identification elements of unknown analysts relative to those of known sources in reference libraries.

The multivariate data analysis generates a set of techniques for the analysis of data that is trained or untrained technique. The untrained techniques are used when a data base of known samples has not been built previously. The simplest and most widely used untrained MDA technique is a principle component analysis. The electronic nose data analysis MDA is a very useful when sensors have partially coverage sensitivities to individual compounds present in a sample mixer. The PCA is a most useful when no known sample is available.

The neural network is the best known and most derived analysis techniques utilized in a statistical software packages for commercially available electronic nose.

For examples electronic nose system for the fruit smell detection:

The proposed electronic nose system was tested with the smells of three fruits namely, leman, banana, litchi. The smells were prepared by placing a sample of fruits in the breakers sealed with a cover. The 8051 was set in to testing or training mode. If the system is in training mode, sensor value is shown on the LCD. If the system is in testing mode, classification result of the target fruit is shown on the LCD. The sensor array gets the gas through Valve1, which is normally closed. The vacuum pump is turned on for 20 sec to  pump the gas out of the sensor array.

 The value1 was closed and the sensor resistance was given 60 sec to reach a study state mode. The classification result of sensors characteristic value appeared on the LCD. The sensor array chamber was disconnected from the fruit sample breaker and the valve1 was opened to turn fresh air, the valve 2 was opened so that the smells were pumped out. The chamber was aired out with fresh air for two minutes.

Application of Electronic nose:

• Medical diagnostics and health monitoring
• Environmental monitoring
• Application in food industry
• Detection of explosive
• Space applications (NASA)
• Research and development industries
• Quality control laboratories
• The process and production department
• Detection of drug smells
• Detection of harmful bacteria

I hope now you have got an idea about the how electronic nose work. if any queries on this concept or on electrical and electronic project please leave the comments section below.

Photo Credit:

• Electronic nose by sciencedialy
• Electronic nose block diagram by origin-ars.els-cdn
• Electronic nose system by mdpi