multi-frequency temperature modulation for metal-oxide gas sensors
TRANSCRIPT
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)
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
2
Outlineg IntroductiongExperimental setup
n Analyte selection and exposuren Temperature modulation profiles
gData analysisn Selectivity improvementsn Pattern stability
gDiscussionn Conclusionsn Future work
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
3
INTRODUCTION
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
8
EXPERIMENTAL
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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%
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
13
ANALYSIS
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
AIR ACET AMM IPA VIN
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
AIR ACET AMM IPA VIN
1
2
3
4
5
Out
put (
V)
TGS2610
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
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
AIR ACET AMM IPA VIN
1
2
3
Out
put (
V)
SB11A
AIR ACET AMM IPA VIN
1
2
3
Out
put (
V)
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Raw TGS-vinegar Raw FIS-ammonia
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Raw TGS-vinegar Raw FIS-ammonia
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
Raw TGS-vinegar Raw FIS-ammonia
Norm TGS-vinegar Norm FIS-ammonia
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
21
DISCUSSION
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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
Multi-frequency Temperature Modulation for Metal-oxide Gas SensorsRicardo Gutierrez-OsunaWright State University
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