multi-frequency temperature modulation for metal-oxide gas sensors

Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University

1

MULTIMULTI--FREQUENCY FREQUENCY TEMPERATURE MODULATION TEMPERATURE MODULATION

FOR FOR METALMETAL--OXIDE GAS SENSORSOXIDE GAS SENSORS

R. Gutierrez-Osuna1, S. Korah1 and A. Perera2

1Wright State University, Dayton, OH (USA)2Universitat de Barcelona (Spain)


2

Outlineg IntroductiongExperimental setup

n Analyte selection and exposuren Temperature modulation profiles

gData analysisn Selectivity improvementsn Pattern stability

gDiscussionn Conclusionsn Future work


3

INTRODUCTION


4

IntroductiongApproaches to improving the selectivity of

commercial MOS sensorsn Computational

g Transient response analysis

n Analyticalg Thermal desorption, chromatography, filters

n Instrumentationg Temperature modulation, AC impedance

gGoals of this studyn Study the effect of modulation frequencyn Analyze the stability of temperature-modulated

patterns


5

MOS transduction principleg In clean air

n Atmospheric oxygen chemisorbed on the surface

n Electronic carriers are tied, creating a potential barrier φb

n As a result, the conductivity of the MOS decreases

g In the presence of reactive gasesn Oxygen reacts and is removed from

the surfacen Electrons are freedn The conductivity of the MOS

increasesg Why temperature modulation?

n The stability of oxygen species (O-2,

O-, O2-) will depend on temperaturen Different gases have different optimal

reaction temperatures

O2 + e- O-2

½O2 + e- O-

½O2 + 2e- O2-

O-2

O-2

O-2

O-2

O-2

O-2 O-

2

O-2

O-2

O-2

O-2

Space chargeregion

e-e- e-O-

2

O2

O2

e-

e-

e-e-e-e-e-

e-e-

e- e-

e-e-

e-O-

2

SnO2grainsO2

O2

φb

O-2 + CO CO2 + e-

O- + CO CO2 + e-

O2- + CO CO2 + 2e-

e-

e-

e-

e-O-

2

e-e-

e-

e-

e-

e- e-e- e-e-

e-

e-

e-e-e-e-e-

e-e-

e- e-

e-e-

e-e-

φb

O2

O2

O2O2CO

CO

CO


6

Temperature modulation (?)g Isothermal operation

n Constant heater resistance n Constant heater voltage

gTemperature modulationn Operating temperature is

cycled during exposure toanalytes

R1

R3

R2

RH RS

R4

VH

RH RSVHRH RSVH

RH RSVHRH RSVH


7

Physical structure of the sensors

Gas sensitivematerial

Heatercoil

Lead wire

0.5 mm

0.3 mm

S

S

H

H

TGS 2610 FIS SB11A

Electrodes

Heater

(Reverse side)

GasSensitivematerial

1.5 mm

1.5

mm

Substrate


8

EXPERIMENTAL


9

Exposure to analytesgStatic headspace analysis

n 30ml glass vial with 10 ml analytesn Sensor inserted through a tight aperture on the capn This setup eliminates cooling effects by effluent flow

gAnalyte databasen Blank (air)n Vinegar (5% acetic acid)n Ammonian Isopropyl Alcoholn Acetone

gData collectionn 10 days, 30 samples/analyte

To electronics

analyte

sensorTo electronics

analyte

sensor


10

Analyte concentrationgHow to test for selectivity enhancements if

analytes can be discriminated isothermally?n Each analyte is serially diluted in water until the

sensor response is the same for all the analytesn Therefore, the concentration range is at or below the

isothermal discrimination threshold at nominal temperature

Analyte Dilution (v/v) Air - Vinegar 100% Ammonia 28% IPA 0.8% Acetone 0.08%


11

InstrumentationgData acquisition

n Personal computer with a data-acquisition cardn Data generation and acquisition at 100Hz

gMeasurementn Sensitive element placed

in a voltage dividergHeater excitation

n Analog output generates heater voltagen Current-boosting with a Darlington pair

RH RS

RL

5V

-

+

15V

VH

VOUT

RH RS

RL

5V

-

+

-

+

15V

VH

VOUT


12

Heater profileg Isothermally

n Heater voltage maintained at manufacturer’s nominal value

g Temperature-modulationn SIX segments at 0.125Hz, 0.25Hz, 0.5Hz, 1Hz, 2Hz and 4Hzn TEN cycles per frequency

2

4

6

0.2

0.4

0.6

time

3

4

5

VH

VTGS

VFIS

2

4

6

0.2

0.4

0.6

time

3

4

5

VH

VTGS

VFIS


13

ANALYSIS


14

Sensor response

ISOTHERMAL TEMPERATURE-MODULATION

0.125Hz

200 400 600 800

2

4

6

x 10-4

50 100 150 200

1

2

3

4

5

6

x 10-4

0.500Hz

AIR ACET AMM IPA VIN

1

2

3

4

5

Out

put (

V)

TGS2610

50 100 150 2000

1

2

3

4

x 10-4

0.500Hz

-4

0.125Hz

200 400 600 8000

1

2

3

4

5x 10

SB11A


1

2

3

Out

put (

V)

0.125Hz

200 400 600 800

2

4

6

x 10-4

0.125Hz

200 400 600 800

2

4

6

x 10-4

50 100 150 200

1

2

3

4

5

6

x 10-4

0.500Hz

50 100 150 200

1

2

3

4

5

6

x 10-4

0.500Hz


1

2

3

4

5

Out

put (

V)

TGS2610


1

2

3

4

5

Out

put (

V)

TGS2610

50 100 150 2000

1

2

3

4

x 10-4

0.500Hz

50 100 150 2000

1

2

3

4

x 10-4

0.500Hz

-4

0.125Hz

200 400 600 8000

1

2

3

4

5x 10

-4

0.125Hz

200 400 600 8000

1

2

3

4

5x 10

0.125Hz

200 400 600 8000

1

2

3

4

5x 10

SB11A


1

2

3

Out

put (

V)

SB11A


1

2

3

Out

put (

V)


15

Pattern analysisgFeature selection

n Sub-sampling (down to 25 features per TM pattern)gDimensionality reduction

n Linear Discriminants Analysis (down to 4 dimensions)gClassification

n K Nearest Neighbors (k=N/NC/2)gValidation

n 10-fold cross-validation (1 fold = 1 day)

Freq (Hz) DC 0.125 0.25 0.5 1 2 4 TGS2610 51 100 97 85 85 75 67 SB11A 47 99 99 97 96 87 73


16

Pattern stabilityg How to measure the stability of sensor patterns over

time?n Increasing training data (N) allows the pattern-classifier to filter

out the driftn Larger time-stamp differences (D) between training and test data

are likely to reduce pattern-classification rate

g Worst-case scenarion Set N=1 ⇒ pattern-classifier is trained on data from a single day

Day: 1 2 i j k M

N D

Training set Test set


17

Predictive accuracy over time

2 4 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

D

Pred

ictiv

e A

ccur

acy

D

A

B

C

D

E

FA

B

C

D

F

E

(a) Raw TGS (b) Raw FIS

1 3 5 7 2 4 61 3 5 72 4 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

D

Pred

ictiv

e A

ccur

acy

D

A

B

C

D

E

FA

B

C

D

F

E

(a) Raw TGS (b) Raw FIS

1 3 5 7 2 4 61 3 5 7

A 0.125 HzB 0.250 HzC 0.500 HzD 1.000 HzE 2.000 HzF 4.000 Hz


18

Drift compensationg Drift behavior

n Mostly multiplicative (gain)n Also additive (offset)

g Compensation by normalization( )

( ) ( )TT

TTT GminGmax

GminGG−

−=

200 400 600 800

2

4

6

x 10-4

200 400 600 8000

5

10

15

x 104

Raw TGS-vinegar Raw FIS-ammonia

200 400 600 800

2

4

6

x 10-4

200 400 600 8000

5

10

15

x 104



19

Normalized patterns

200 400 600 8000

0.2

0.4

0.6

0.8

1

200 400 600 8000

0.2

0.4

0.6

0.8

1

200 400 600 800

2

4

6

x 10-4

200 400 600 8000

5

10

15

x 104


Norm TGS-vinegar Norm FIS-ammonia

200 400 600 8000

0.2

0.4

0.6

0.8

1

200 400 600 8000

0.2

0.4

0.6

0.8

1

200 400 600 800

2

4

6

x 10-4

200 400 600 8000

5

10

15

x 104


Norm TGS-vinegar Norm FIS-ammonia


20

Predictive accuracy over time

2 4 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

D

Pre

dict

ive

Acc

urac

y

DD D

AB

C

D

EF

A

B

C

D

EF

A

B

C

D

E

FA

B

C

D

F

E

Raw TGS Raw FISNorm TGS Norm FIS

1 3 5 7 2 4 61 3 5 72 4 61 3 5 7 2 4 61 3 5 72 4 60

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

D

Pre

dict

ive

Acc

urac

y

DD D

AB

C

D

EF

A

B

C

D

EF

A

B

C

D

E

FA

B

C

D

F

E

Raw TGS Raw FISNorm TGS Norm FIS

1 3 5 7 2 4 61 3 5 72 4 61 3 5 7 2 4 61 3 5 7


21

DISCUSSION


22

ConclusionsgSelectivity

n Temperature modulation increases the selectivity below the isothermal discrimination threshold

g Information contentn At low frequencies: in the shapen At high frequencies: in the DC offset

gSpeedn FIS allows for faster frequencies than TGS due to

physical dimensionsgStability

n Both sensors are, unfortunately, also affected by driftn TGS appears to be more stable than FIS


23

Future workg Study pattern stability over longer periods of time (weeks,

months)g Study pattern repeatability across nominally identical

sensorsg Drift compensation by normalization with respect to a

reference gasg Merging information from multiple frequenciesg Heater resistance control as opposed to heater voltage

controlg Discrimination performance with mixtures and complex

odors

multi-frequency temperature modulation for metal-oxide gas sensors

Documents