Download Bio/Gas Sensor Characterizer

Characterization of a Bio/Gas Sensor
By Charles Guthy and Angel Madera
16.541 Introduction to Biosensors
5/9/2007
I.
Overview
This is the new Bio/Gas Sensor Characterizer. Used for the
characterization and qualification of resistive gas sensors, it measures the
changing electrical resistance of a gas sensor by passing different gases and
concentrations and mixtures thereof and measuring the resultant voltage
changes across the sensor.
The design of the Characterizer is quite straightforward, consisting of an
airtight chamber for gas, necessary pipes and gauges, and test circuitry. The
user first puts the gas sensor, along with its wiring, into the glass tube. Being
sure that the wiring has been brought outside the tube, the user then puts the
rubber stopper on the tube’s open end. The user attaches two gas canisters to
the system, turns on a DC power supply (not included), sets it to the desired
1
voltage (generally +5VDC, although other voltages can be used), and turns
each canister’s spigot.
The test circuitry is also very simple in design. It is made of a Wheatstone
Bridge (for proper measurement of changing voltage) and an instrumentation
amplifier op-amp topology for data collection and analysis. When gas flows
over a test sensor, the Wheatstone Bridge detects the voltage change and
sends it to the instrumentation amplifier. The amplifier topology, in turn,
magnifies the signal from the Bridge and outputs a signal at least 3 times
stronger than the input signal voltage, limited only by the supply voltage. The
factor that the instrumentation amplifier multiplies in the input signal by can
be changed to the user’s preference by adjusting R8.
2
II.
Product Pictures
3
4
III.
AutoCAD Drawings
5
IV.
Operating Circuit
VCC
R1
RESISTOR
R2
RESISTOR
R(sensor)
RESISTOR
R3
RESISTOR
4
U2A
LT1014
3 +
R7
R8
RESISTOR
RESISTOR
1
2 -
11
11
R5
RESISTOR
3
1
+
R11
LT1014
-
2
4
POT
U1A
11
1
RESISTOR
R10
RESISTOR
4
+
3
-
2
R9
R6
LT1014RESISTOR
V.
Specification Sheet
Abs. Max. Supply Voltage: ±22VDC (±5VDC or ±12VDC recommended)
Operating Temperature Range: 0ºC to +70ºC
Gain Range: 2 to 10 V/V
Measurable Sensor Resistance Range: 0 to 400K ohms
Slew Rate: 400000 V/s
Noise: 0.55 uVp-p
6
U3A
VI.
Graphs
This graph shows the results of the first experiment with the test circuit to prove its
potential worth. As you can see, the output voltage (Gain x v2-v2) has a curve that
eventually falls to ground. This experiment was done with a 220 kilo-ohm resistor within
the Wheatstone Bridge connected in parallel with a decade box (used to simulate a
resistive gas sensor)
Voltage (VDC)
Resistance Change vs. Input and Output Voltage
5.000
4.500
4.000
3.500
3.000
2.500
2.000
1.500
1.000
0.500
0.000
Vin (VDC)
Vo (VDC)
0
100000
200000
300000
400000
500000
600000
Resistance (Ohms)
We later did more testing, removing the 220 kilo-ohm resistor and placing a 5 kiloohm resistor in series with a 20 kilo-ohm potentiometer in between the outputs of the
first two op-amps (together known as a gain resistor). We performed two tests, one
with the pot set all the way counter clockwise (Rg=26.1 kilo-ohms) and the other set
all the way clockwise (Rg=5.001 kilo-ohms). The curves generated were quite
interesting. Both were much sharper than the curve above, with the clockwise curve
beginning a steep fall when the decade box was set at 100 kilo-ohms and stopping at
ground at around 400 kilo-ohms, and the counter-clockwise curve beginning an even
steeper fall at 200 kilo-ohms and stopping ar about 400 kilo-ohms. It can be inferred
that, had the op-amps’ power supply been +/- 15VDC instead of simply +VDC, we
would have seen a much more complete curve that dropped sharply from a little less
that +15VDC to a little more than -15VDC.
7
VII. Possibilities for Expansion
The Bio/Gas Sensor Characterizer provides the user a limited ability to
expand or otherwise change the setup to suit test requirements, localized entirely
within the test circuit. Signal gain may be adjusted (3.6x to 10x) by setting the 20
kilo-ohm pot at R8. In the future, when hardy (projected lifetime of over 10
million cycles), adequately sized potentiometers can be located, the user will be
able to adjust the voltage across both nodes of the Wheatstone Bridge.
In addition, op-amp buffers and passive, low-pass filters (with a cutoff
frequency of at least 30Hz) may be installed to improve signal quality. A CERDIP
op-amp, such as Analog Devices’ AD620, or Texas Instruments’ INA126,
specifically designed as an instrumentation amplifier, may also be obtained and
installed. This would improve performance, reduce the amount of necessary
wiring, and effectively “ruggedize” the circuit. It is our group’s determination to
bring this already fruitful project to a successful conclusion.
8
VIII. Appendix
Original Schematic
9
Raw Data from the First Experiment with the Test Circuit (220 kilo-ohms in
parallel with the decade box)
Resistance
10000
20000
30000
40000
50000
55000
60000
65000
70000
75000
80000
85000
90000
95000
100000
105000
110000
115000
120000
125000
130000
135000
140000
145000
150000
155000
160000
165000
170000
175000
180000
185000
190000
195000
200000
210000
220000
230000
240000
250000
260000
270000
280000
290000
Vin
(VDC)
0.123
0.232
0.332
0.424
0.510
0.549
0.587
0.624
0.657
0.692
0.726
0.758
0.789
0.818
0.849
0.876
0.904
0.930
0.954
0.979
1.003
1.026
1.048
1.068
1.088
1.112
1.131
1.152
1.168
1.188
1.207
1.224
1.238
1.254
1.267
1.296
1.329
1.357
1.383
1.408
1.432
1.454
1.482
1.501
10
Vo
(VDC)
4.350
4.350
4.350
4.340
4.230
4.150
4.060
3.980
3.900
3.840
3.780
3.710
3.650
3.590
3.530
3.470
3.410
3.360
3.310
3.250
3.210
3.160
3.110
3.070
3.020
2.980
2.940
2.900
2.860
2.830
2.800
2.750
2.720
2.680
2.650
2.590
2.530
2.480
2.410
2.360
2.310
2.260
2.210
2.170
300000
310000
320000
330000
340000
350000
360000
370000
380000
390000
400000
410000
420000
430000
440000
450000
460000
470000
480000
490000
500000
1.522
1.537
1.557
1.572
1.587
1.608
1.625
1.641
1.654
1.663
1.683
1.698
1.712
1.724
1.735
1.747
1.759
1.772
1.785
1.794
1.810
2.120
2.080
2.040
1.990
1.960
1.920
1.890
1.860
1.820
1.792
1.766
1.738
1.711
1.682
1.654
1.629
1.605
1.583
1.560
1.539
1.514
List of Materials
Item #
38105K51
38105K51
5079K62
5449K133
8729K51
51025K178
G1168275
G240175
3550-170
Description
Miniature Gauge - 5% Full-Scale Accuracy 29/32" Dial, 1/8" NPT Center Back
Miniature Gauge - 5% Full-Scale Accuracy 29/32" Dial, 1/8" NPT Center Back
Polycarbonate Panel-Mount Flowmeter .1 to 1 Scfh, with Valve
Metric Acetal Instant Tube Fitting Tee for 6mm Tube OD
Borosilicate Glass Tube 1" OD X .812" ID, 12" Length
Brass Instant Tube Fitting Male Straight Adapter for 1/4" OD, 1/4" NPTF
2 ft^3 N2
0.5 lb. CO2
Gas Regulators
11
Vendor
McMaster-Carr
McMaster-Carr
McMaster-Carr
McMaster-Carr
McMaster-Carr
McMaster-Carr
Matheson-Trigas
Matheson-Trigas
Matheson-Trigas