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124 IEEETRANSACTIONSONULTRASONICS,FERROELECTRICS,ANDFREQUENCYCONTROL, VOL. UFFC-34,NO. 2 , MARCH 198

A Sensor Classification SchemeRICHARD M . WHITE,F ELLOW, I E E E

Abstract-We discus s lexible an d comprehen sive ategorizingscheme that is useful fo r describing and comparing sensors.

I virtually every field of application we find sensorsthat transform real-world data into (usually) electricalform. Today many groups around the world are investi-gating advanced sensors capab le of responding to a widevariety of measurands. In an attempt to facilitate compar-ing sensors and obtaining a compre hensive overview ofthem, we present here a scheme for categorizing sensors.Sensor classification schemes range from the very sim-ple to the complex. Extremes are the often-seen divisioninto just three categories (physical , chemical , and biolog-ical) and the finely subdivided hierarchical categories usedby abstracting journals. The scheme to be described hereis flexible, ntermediate in complexity,andsuitable oruse by individuals working with computer-ba sed storageand etrievalsystems. t is derived romaHitachiRe-search Laboratory communication.Tables I-VI, containing possible sensor characteristics,appea r in order of degree of importance for the typicaluser. If we take for illustration a surface acoustic-waveoscil lator accelerometer, hese entries mightbe as fol-lows:hemeasurand-acceleration;echnological as-pects-sensitivity in frequency shift per g of acceleration,short- and long-term stability in hertz per unit time, et c.;detectionmeans-mechanical; senso r onve rsion he-nomena-elastoelectric; senso r mater ials-the key mate-rial is likely an norganic nsulator; and jie lds of appli-cation-many, including automotive and other means oftransportation; marine, military, and space; and scientificmeasurement.Tab le I lists alphabetically most measura nds for whichsensors may be needed unde r the head ings: acous tic, bi-ological, chemical, electric, magnetic, mechanical, opti-cal,adiationparticle),nd therm al. A conventionadopted to limit the number of Tab le I entries is that anyentry may represent not only the measurand itself but alsoits temporal or spatial distribution. Thus, the entry Am-plitude under he heading Optical could apply o adevice that measures the intensity of steady infrared ra-diation a t a point, a fast photodiode detecting time-vary-ing optical flux, o r a cam era for v isible l ight imaging.

Manuscript received August 29, 1986; revised October 27, 1986.The autho r is with the Department of Electrical Engineering and COm-puter Sciences and the Electronics Research L aboratory,University Of Gal-ifornia, Berkeley, CA 94720.IEEE Lo g Number 8612468.

With a particular measurand, ones primarily interestein sensorcharacteristicssuchassensitivity,selectivity,and speed of response. The se are terme d techn ologica laspe cts and listed in Tab le 11. Ta ble I11 lists the detec -tion means used in the sensor.Tables IV and V are of interest primarily to technologists involved in sensor design an d fabrication. Entries iTable IV are intended to indicate the primary phenomeused to convert he measuran d nto a form suitable forproducing he sensor output. The entries under Physi-calarederived rom he nteractionsamong physicavariablesdiagrammed in Fig. 1 . This s amodificationand simplification of the diagrams used by Nye [ l ] anMason [2 ] to show binary relations among he commonphysical variables.Most sensors con tain a variety of materials (for exam-ple, almost all contain some metal). The entries in TablV sho uld be understood to refer to the materials chieflresponsible for sensor operation. Finally, an alphabeticalist of fields of application comprises Table VI.

USESFOR THE CLASSIFICATION SCHEM EA useful scheme should facilitate comparing senso rcommunicatingwithotherworkersabout ensors,an dkeeping rack of sensor progress and availability. Cate-gorizing might help one think about new sensing principles that could be explor ed, and Table I1 might serve a

a checklist to consult when considering comm ercial sen-sors .All the entries in the tables have been given un ique alphanumeric identifiers to facilitate use with computerizedfile systems such as electronic spreadsheets and databasused for storing information about sensors. The identifcan be used a s well in the keyword field of the esserknown bibliographic utility refer, a part of the Unix operating system package, that enables a user to create andeasily retrieve entries from a personalized database of ctations to journal articles, books, and reports.S ENS OR EXAMP LES

We consider several examples to l lustrate how ermsin the tables can be used to ch aracterize sensors.Diap hragm P ressur e Sensor: Differential pressure dis-torts a hin silicon diaphragm in which he deflection inferred from the change of the values of resistorsiffuseinto the diaphragm. The measurand s pressure, A6.5; thprimarydetectionmeans is mechanical, C 5 ; thesensorconversionphenom enon piezoresistance) is elastoelec0885-3010/87/0300-0124$01 O O @ 1987 I E E E

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WHITE:SENSORCLASSIFICATION SCHEME

TABLE I125

TABLE 111A.MEASURANDSAl . Acoust i cA l. 1 Wave ampl i tude, phase, polarizat ion, spect rumA1.2 W ave veloci tyA1.3 Other (specify)AZ. BiologicalA2.1 Biomass (ident i t ies, concentrat ions, states)A2.2 Other (specify)

A3.1 Compo nents (identities, concentrations, states)A3.2 Other (specify)A4.1Charge,currentA4.2 Potential, potential differenceA4.3 Electric field (am plitude, phase, polarization, spectrum)A4.4 ConductivityA4.5PermittivityA4.6 Other (specify)A5.1 Magne tic field (am plitude, phase, polarization, spectrum)A5.2Magnetic fluxAS.3PermeabilityAS.4 Other (specify)A6.1Position linear, ngu lar)A6.2 Veloci tyA6.3ccelerationA6.4orceA6.5 tress, ressureA6.6trainA6.7Mass,ensityA6.8Moment ,orqueA6 .9 Speed of flow, rate of mass ransportA6.10 Shape, roughness , orientationA6.11Stiffness,complianceA6.12ViscosityA6.13 Crystallinity, structural ntegrityA6.14 Other (specify)A7.1 Wav e amplitude, phase, polarization, spectrumA7.2 Wave velocityA7.3 Other (specify)A8. RadiationA8.1TypeA8.2EnergyA8.3 ntensityA8.4 Other (specify)A9.1TemperatureA9.2FluxA9.3 Specific heatA9.4 Thermal conductivityA9.5 Other (specify)

A3 .Chemical

A4 .Electric

A5.Magnetic

A6.Mechanical

A7 .Optical

A9.Thermal

A10. Other specify)

TABLE I1B. TECHNOLO GICAL ASPECTS OF SENSORSB1 SensitivityB2 Measurand rangeB3 Stability short-term ,ong-term)B4 ResolutionB5 SelectivityB6Speedof esponseB7 Ambient onditions llowedB8 Overload haracteristicsB9 Operatingif eB10 Output formatB1 1 Cost, size, weightB12 Other (speci fy)

C. DETECTION MEANS USED IN SENSORSC l BiologicalC2ChemicalC3 Elect ric, Magnet ic, or Elect romagnet ic WaveC4 Heat , TemperatureC5 Mechanical Displacement or WaveC6 Radioactivity, RadiationC7 Other (speci fy)

TABLE IVD. SENSOR CONVERSION PHENOME NAD l. BiologicalD 1.1 Biochemical transformationD1.2 Physical ransformationD1.3 Effect on test organismD l 4 Spect roscopyD1.5 Other (specify)

D2.1Chemical ransformationD2.2 Physical ransformationD2.3Electrochemical processD2.4 Spect roscopyD2.5 Other (speci fy)D 3. I ThermoelectricD3.2hotoelectricD3.3hotomagneticD3.4agnetoelectricD3.SlastomagneticD3.6hermoelasticD3.7lastoelectricD3.8hermomagnet icD3.9hermoopticD3.10PhotoelasticD3.11 Other (specify)

D2.Chemical

D3 .Physical

TABLE V

E . SENSOR MATERIALSE l Inorgan icE2OrganicE3ConductorE4 nsulatorE5SemiconductorE6 Liquid, gas or plasmaE7 BiologicalsubstanceE8 Other (specify)

TABLE VIF. FIELDS OF APPLICATIONFAgricultureF 2 Automot iveF 3 Civi l ngineering, onst ructionF 4 Dist ribut ion, ommerce, inanceF5DomesticappliancesF 6 Energy , owerF7 Environment ,meteorology, ecurityF 8 Hea lt h,medicineF 9 In formation,elecommunicationsF 10 ManufacturingF11 Mar ineF 12 M ilitaryF 13 Scientific measurementF 1 4 S p a c eF 15 Transportation (excluding automotive)F 16 Other (specify)

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ELECTROMAGNETIC Fiberpticagnetic Field Pro be: A magnetostrictivnickel film deposited on an optical fiber in an nterfer-ome ter is d istorted by an external magnetic field, causinga photoelectrically detected change of ight evel at thinterferometer. The primary detection means is mechan-ical , C 5 , and secondarily electromagnetic waves are in-volved, C 3. The fundamental conversion phenomenon elastomagnetic, D 3.5, nvolving primarily a metallic filmE 3 , and an insulating fiber, E 4 . Since fiber optic sensorsconstitute an important identifiable class, one might keTHERMAL ELASTIC alluc hensorsimilarly, for example by specifyingn-

ELECTRIC MAGNETIC

Fig. 1. Physical henomenaepresented by lines onnecting nodes that derOther in DetectionMeans acategory C7.1representhysical fields. fiber-optic.tric, D3.7; and he key sensor material s an inorganicsemiconductor, E 1 a nd E 5 .SAW Vapor Sensor: A polymethylmethacrylate(PMM A) polymer coating n he propagation path of asurface acoustic wave delay-line oscillator absorbs vapor,causing mass loading and hence a change of wave veloc-ity and oscillator frequency. The measurand is chemicalconcentration, A3.1; the primary detection means is me-chanical , C 5; the sensor conversion phenomenon s phys-ical ransformation (a vapor becomes an absorbed con-stituent), D2 .2 ; and the key sensor material is the organicpolymer, E2. If , for greaterelectivity, the polymer werealtered so that it reacted chemically with only one type fvapor, chemical transformation, D 2. 1, would be the pri-mary conversion phenom enon. If the polymer were re-placed with an im mobilized an tibody to d etect a pa rticularantigen, biochemical transformation would be involved,D 1 . l .

A C K N O W L E D G M E N TResearch supported by the Berkeley Integrated SensoCenter,nNSF/Industry/UniversityCooperativeRe-searchCenter.Discussions withLeslieFieldandTedZellers are also gratefully acknowledged.

REFERENCES[ I ] J . F . N y e , PhysicalProper t ies of Crystals. Ox fo rd :Ox fo rdUniv . .[2] W. . Mason, Crys talPhys ics of InteractionProcesses. New York1957.Academic. 1966, see Figs . 1 . 1 an d 1.2.

Richard M. White (63-F 72). fo r a photograph and biography pleasesee page 123 of the March issue of this T R A N S A C T I O N S .