Electronic Nose

Presentation by:Ameer Iqbal

Roll No.- 10EE64R02

M.Tech. (Instrumentation)

ContentsIntroductionComponentsSensors Pattern recognitionWireless electronic noseAdvantages & limitationsApplicationsFuture & conclusion

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Biological NoseDetection and identification of odourQuantifying smells are useful in gas

chromatographyHuman nose is very sensitiveSubject to fatigue, inconsistencies,

adaptation etc.Smelling toxic gases may involve risk

Fig. Conduction route diagram of animal olfactory system 3

Electronic NoseInstrument intended to mimic the human

sense of smellCombines human sensitivity & instrument’s

objectiveConsists of:

Sample handling systemSensing systemPattern recognition system

4Fig. Schematic diagram of an electronic nose

Electronic NoseCorrespondence of electronic nose with

biological noseBiological nose Electronic nose

Lungs Pump

Mucus, Hair, Membrane Inlet Sampling System

Olfactory cells Sensors

Olfactory vesicle Data pre-processing module

Olfactory centre Pattern recognition module

Nerve Impulses Electrical signal

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ComponentsSample handling system

Generates the headspace of sample to be analyzedExposes the odorant to the sensors

Sensing systemArray of different sensors Each sensor has different sensitivity to different

gasesProduces a pattern characteristic of the odour

Fig. Response of sensor array to different pure chemicals

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Sensing systemQuantity & complexity of the data collected

can make analysis of data in an automated system difficult.

Using array of sensors, each sensor designed to respond to a specific chemicalNumber of unique sensors must be at least as

great as the number of chemicals being monitored

Difficult to build highly selective chemical sensors

Expensive also7

Sensing systemUse of Artificial neural networks (ANN)ANN combined with a sensor array

Number of detectable chemicals is greater than that of sensors

Less selective sensors can be usedLess expensive too

Electronic noses incorporating ANNs have been demonstrated in various applications.

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Electronic nose sensorsConductivity Sensors(a) Metal Oxide Sensor

Oxides of tin, zinc, titanium etc. doped with platinum – Active material

Doped material deposited between two metal contacts over a resistive heating element

Operating temp.: 200°C-400°CAs VOC passes over the active material,

resistance changesResistance changes in proportion to the

concentration of the VOC.9

Conductivity Sensors(b) Polymer Sensor

Active material is a conducting polymere.g. Polypyroles , thiophenes , indoles etcWhen exposed, chemicals forms bond with

polymersBonding may be ionic or covalentTransfer of electrons along polymer chain is

affected, i.e. conductivity changesOperate at ambient temperature, no heater

required

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Conductivity Sensors

Metal Oxide Sensor Polymer Sensor

Susceptible to poisoning by sulphur compounds present in the odorant mixture

Difficult and time consuming to electropolymerize the active material

Wide availability Susceptible to humidity & can mask the responses of VOC

Relatively low cost, hence widely used

Electronic interface is simple, suitable for portable instruments

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Fig. Comparison between metal oxide & polymer sensors

Piezoelectric Sensors(a) Quartz crystal microbalance (QCM)

Consists of a resonating disk with metal electrodes on each side connected to lead wire

Resonates at a characteristic frequency (10-30 MHz) when excited with an oscillating signal

Polymer coating serves as sensing materialGas adsorbed at the surface of the polymer

increases the mass, reduces resonance frequency

Reduction is inversely proportional to mass adsorbed by the polymer

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Piezoelectric Sensors(b) Surface acoustic-wave (SAW)

An ac signal is applied across the input metal transducer

Fingers of this gas sensor creates an acoustic wave that "surfs" the piezoelectric substrate

When the wave reaches the metal fingers of the output transducer, ac voltage is recreated

Voltage is shifted in phase as a result of the distance travelled.

Phase shift depends on the mass & the absorption properties of the sensing polymer layer

SAW devices are less sensitive than QCMs

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Piezoelectric SensorsLimitations:

More complex electronics are needed by these sensors than conductivity ones

Resonant frequencies can drift due to the active membrane ageing

Requires frequency detectors

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MOSFET SensorsGate is covered by noble metal catalyst, e.g.

platinum, palladium, or iridiumCharge applied to the gate leads to current

flow from source to drainVOCs sweeping over the catalyst forms

products that alter the sensor's gate chargeChannel conductivity varies

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MOSFET SensorsAdvantage:

Can be made with IC fabrication processes, batch to batch variation is minimized

Disadvantage :Reaction products should penetrate the

catalytic metal layer in order to influence the charge

Hermetic seal for the chip’s electrical connections in harsh environments

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Optical SensorsGlass fibre coated on its sides & ends

with a thin active material containing fluorescent dyes

Pulse of light from an external source propagates along the fibre

VOCs can alter the polarity of the dyesDyes responds by shifting fluorescent

spectrum of the lightSimple fabrication- Fluorescent dyes

can easily be coupled

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Signal processing & pattern recognitionMain sequential steps:

Pre-processingFeature extractionClassification and Decision making

Data base of the expected odorant should be compiled

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Signal processing & pattern recognitionPre-processing

Compensates for sensor driftCompress the transient response of the sensor arrayReduces sample to sample variations

Feature extractionReduce the dimensionality of the measurement space

Can be more readily inspected visuallyExtract information relevant for pattern recognitionPerformed with linear transformations e.g. PCA &

LDANonlinear transforms, e.g. Sammon nonlinear maps

and Kohonen self organizing maps 19

Signal processing & pattern recognitionClassification

Bayesian classifiers, Artificial Neural Network(ANN) etc are used

Trained to identify the patterns that are representative of each odour

Identify the odorant by comparing it with trained ones

Decision MakingUsed for application specific knowledgeCan determine that given sample does not belong

to any one in database

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Wireless Electronic NoseDeveloped in 2010Can perform remote multipoint odour

monitoringSignal from isolated locations can be

combined and processed at a database serverData measured are delivered via ZigBee

wireless network

Fig. ZigBee node of wireless electronic nose network.

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Wireless Electronic Nose(a)Electronic Hardware

MCU acquires gas sensor data through ADC interface & sends the data to ZigBee wireless network

Real time clock in MCU stamps date and time of the transmitted data

Fig. Block diagram of wireless electronics nose. 22

Wireless Electronic NoseSensors were designed to be particularly

sensitive to different gasesTemp. & humidity sensors for environmental

conditions.Name of sensors Compound to be detected

TGS3870 Carbon monoxide

TGS4161 Carbon dioxide

TGS825 Hydrogen sulfide

TGS826 Ammonia

KE-25 Oxygen

SHT15 Temp. sensor

SHT15 Humidity sensorTable- Sensors used for the developed electronic nose

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Wireless Electronic Nose(b)Gas Flow System

Two solenoid valves control the flow of reference air & air sample

Reference air - air filtered by activated carbon (valve 1)

Valve 1 open- valve 1 closed-valve 2 open- valve 2 closed

Fig. Gas Flow System

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Wireless Electronic Nose(c) Principal Component Analysis (PCA)

Data from ADC is stored in 2-D array versus sampling time

Data for each sensor are subtracted by their mean values

Covariance matrix of the subtracted data is computed

Eigenvectors & Eigenvalues of the covariance matrix are then calculated

Then principal components are chosen and featuring vectors are formed

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(d) ZigBee TechnologyZigBee is famous for its low cost, low power

consumption & miniaturizationTree topology, with benefits of star & mesh, was

usedMostly operates in sleep mode, low power

consumptionEnd nodes acquire e-nose data and send them to the

router nodesRouter nodes combines its own data & send to base

nodesData was sent to database server

Wireless Electronic Nose

Fig. Tree Topology 26

Wireless Electronic Nose

Fig. Normalized data set from wireless electronic

nose.

Fig. PCA plot between PC1 & PC2

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Electronic NoseAdvantages

Detection of poisonous gas is possibleCan be done in real time for long periodsCheaper than Trained human sniffersIndividuals vary, e-nose don’t

LimitationsTime delay between successive testsInsensitivity to some speciesAccording to application, e-nose has to be

changed

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ApplicationsEnvironmental control (air quality, gas

emission levels of factories, chemical plant monitoring etc.)

Medical applications (urine, skin, breathe odour analysis, ulcer monitoring etc)

Food industry (coffee, fermentation process, identification of bacteria etc.)

Defence and security industries (detecting land mines)

Pharmaceutics, chemical industry (odour, quality control of pharmaceutical compounds etc.)

Semiconductor industrial process

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Future WorkResearch is being done on IC E-NosesMiniaturizing current TechnologyImprovement in sensitivity for lower levels of

organisms or smaller samplesMinimizing cost

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Conclusion Humans are not well suited for repetitive

tasks. Electronic nose has the potential to become standard tool for smelling. Researches are still going on to make electronic nose much more compact than the present one and to make e-nose ICs.

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References [1] T.Pogfay, N.Watthanawisuth, W.Pimpao, A.Wisitsoraat, S.

Mongpraneet, T.Lomas & M.Sangworasil: “Development of Wireless Electronic Nose for Environment Quality Classification”, International conference 0n Electrical Engineering/Electronics, Computer, Telecommunications and Information technology, 19-21 May, 2010

[2] S.H. Saeed, Z. Abbas, B. Gopal: “Experimental use of electronic nose for analysis of volatile organic compound”, Multimedia, Signal Processing and Communication Technologies, 2009. IMPACT '09, 14-16 March 2009

[3]Nagle H T, Gutierrez-Osuna R, Schiffman: “The how and why of electronic nose”, IEEE Spectrum, Sep 1998

[4] Lars J. Kangas, Lars H. Liden, Sherif Hashem, Richard T. Kouzes: “Electronic noses & their applications”, IEEE Technical  Applications Conference and Workshops, 1995

[5] http://en.wikipedia.org/wiki/Electronic_nose

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Thank you

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