Design and Testing of a Self-Powered
Wireless Hydrogen Sensing Platform
Jerry Chun-Pai Jun, Jenshan Lin, Hung-Tan Wang Fan Ren,
Stephen Pearton and Toshikazu Nishida
University of Florida
Motivation Behind a Self-Powered
Wireless Hydrogen Sensing Platform
• Popular topic due to need of inexpensive sensor devices
requiring minimal maintenance to monitor harsh and
dangerous environs.
• Growing interest in hydrogen as a fuel cell, which is
dangerous if not properly contained.
• Combustion gas detection in Spacecrafts and ProtonExchange Membrane (PEM) Fuel Cells
• Greater than 4% of hydrogen concentrations are explosive.
Limitations Of Sensor
Development
• Limitations of Energy Harvesting Devices
• Limitations of Low-Power and LowVoltage Commercial Components
• Limitations of a Wireless System
– Wireless Channel Estimation
– FCC Regulations
Energy Harvesting Techniques
Solar Energy Harvesting
• Solar Cells are a mature
commercial Product
• Dependent upon realtime lighting and
temperature conditions
• Pulse Resonant Power
Converter
– Self-powered and self
controlled
– Convert input voltage of
0.8-1.2V to steady 2V
output
Vibration Energy Harvesting
• Collection of energy
proportional to volume of
device
• Limited to magnitude and
frequency of vibrations
• For Proof of Concept
– PSI D220-A4-203YB Double
Quick Mounted Y-Pole PZT
Device
– Direct Charging Circuit
Energy Harvesting Techniques cont.
Solar Energy Harvesting
Vibration Energy Harvesting
IXOLAR XOD1704B Solar Cell
Pulse Resonant Power Converter
Functional Block Diagram (a) Bare die
photo (b)
Four mounted PSI D220-A4-203YB
Double Quick Mounted Y-Pole
Bender (a) Direct Charging Circuit
(b)
ZnO Nano-Rods as a Sensing Mechanism
D
S
Al/Pt/Au
ZnO M-NRs
Al2O3 Substrate
a)
b)
Schematic of Multiple ZnO Nano-Rods
Close-Up of Packaged ZnO NanoRod Sensor
• ZnO currently used for
detection of humidity, UV
light and gas detection
• Easy to synthesize on a
plethora of substrates
• Bio-safe characteristics
• Large chemically sensitive
surface to volume ratio
• If coated with Pt or Pd, can
increase device’s sensitivity
to hydrogen
• High compatibility to
microelectronic devices
Pt-ZnO Nano-Rod Sensors
• Sputtered with Pt coatings of
approximately 10 Å in
thickness
• Show no response to the
presence of O2 and N2 at room
temperature
• Pt increases conductivity of
Nano-Rods
• Up to 8% change in resistance
after 10 min. exposure to 500
PPM of hydrogen
• Greater than 2% change in
resistance after 10 min
exposure to 10 PPM of
Pt-coated ZnO Nano-Rod - Relative
hydrogen
Resistance Change for Various
• 90% recovery within 20
Hydrogen Concentrations
seconds upon removal of
hydrogen from the ambient
Comparison of ZnO Nano-Rods
Coated with Different Metals
|Δ R|/R (%)
8
500ppm H2
Air
Pt
Pd
Au
Ag
Ti
Ni
6
4
2
0
0
5
10
15
20
25
30
Time(min)
Relative Resistance Change for Various
Metal-coated ZnO Nano-Rods
Differential Measurement
R3
R1
V1
Vs
• Wheatstone Resistive
Bridge
Vg
V2
R4
R2
– Can limit current
consumption of resistive
bridge
– Best way to detect changes
in resistance
• Difference Amplifier
R2
R3
R2
R3
– Using differential
architecture of operational
amplifier to subtract
difference at input, and
apply gain
– Form of differential
measurement
Instrumentation Amplifier
V1
R2
V3
R3
R1
Rg
R1
V2
R2
V4
R3
 2  R1  R 3
VOUT  (V 2  V 1)  1 

Rg  R 2

• Provides High
Impedance Input
Buffers isolate V1 and
V2 from resistive
network of difference
VOUT
amplifier
• Buffers and provides
gain before difference
amplifier
• Gain can be easily
adjusted by varying a
single resistor, Rg.
Differential Detection Circuit
R2
+
R3
R1
R Bias
R Bias
VDD
GND
VOUT
+
R1
Passivated ZnO
Exposed ZnO
RG
-
+
R2
R3
GND
• Since Pt-ZnO Nano-Rod devices
react to both hydrogen and
temperature, the use of a
passivated ZnO as a reference
resistor can mitigate the
temperature dependency of the
differential Detection Circuit.
• Rbias used to limit current
flowing into both legs of resistive
bridge
• Maintains concept of a
differential measurement
• Instrumentation Amplifier helps
balance input offset voltages,
while providing gain, and
conditioning signal for ADC
Fabricated Pt-ZnO Nano-Rod for
Use in Differential Detection
Circuit
Resistance(ohms)
ZnO with increase Pt catalyst
1580
1560
1540
1520
1500
1480
1460
1440
1420
1400
00 .75 .50 .25 .00 .75 .50 .25 .00 .75 .50 .33 .00 .83 .50
.
0
1
3
5
7
8 10 12 14 15 17 19 21 22 24
time(min)
Fabricated Differential Detection
Circuit
Fabricated Differential Detection
Circuit
Output voltage vs sweep of exposed Pt-ZnO Nominal
Resistance
Output Voltage
(mV)
400
300
200
100
0
1460
1480
1500
1520
Nominal Resistance (Ohms)
1540
1560
Microcontroller Selection
Type of Program
Memory
Flash
Program Memory
8 kB
RAM
256 Bytes
I/O Pins
22 pins
ADC
10-bit SAR ( successive
approximation
register )
Interface
1 Hardware SPI or
UART, Timer UART
Supply Voltage
Range
1.8 V – 3.6 V
Active Mode
200uA @ 1 MHz, 2.2
Vsupply
Standby Mode
0.7 uA
# of Power Saving
Modes
5
Features of Texas Instruments’
MSP430F1232IPW
REQUIREMENTS
•
•
•
•
•
•
•
Low-Voltage
Low-Active Current
Low-Sleep Current
Onboard Memory
Onboard ADC
Serial Output
Reprogrammable
Microcontroller Operation
Level Monitoring State Machine
•
•
•
Runs through state until a discernable
presence of hydrogen is detected.
Once hydrogen is detected, microcontroller
forces RF front-end to transmit an
emergency pulse to the central monitoring
station before returning back to an idle
mode.
Hydrogen threshold level is at far less than
dangerous levels
Data Transmission State Machine
•
•
•
•
Runs through states until a discernable
presence of hydrogen is detected.
Once threshold is detected, the data
from the ADC is queued onto the serial
output port of the microcontroller to be
transmitted.
Once transmitted, state is reset to
sleep
For constant tracking of hydrogen
levels
Selection of a Modulation Technique

-DQPSK
4
OOK
Comparison of Complexity between π/4DQPSK and OOK
•
•
•
•
•
MODULATION
REQUIREMENTS
RF Power Amplifiers and
Oscillators have
efficiencies of 50% at
best
Low parts count
Low Duty-Cycle, Low
Data Rate.
Expend energy only for
transmission of Data
Low complexity
Selection of RF Transmitter (1)
VDD
GND
Ming TX-99 Transmitter in OOK Mode
Ming TX-99 Transmitter
300 MHz Ming TX-99
• Onboard antenna
• OOK Modulation
• Low Part Count
• Low Complexity
• Tunable Frequency
• Colpitts Oscillator
Selection of RF Receiver (1)
Ming RE-99 Receiver Schematic
Ming RE-99 Receiver
300 MHz Ming RE-99
• Onboard antenna
• External Antenna Tap
• Low Part Count
• Low Complexity
• Tunable Frequency
• Envelope Detection
• Little Documentation
Receiver
Atrium
Atrium
Hallway
Transmitter
Hallway
0.45 m
0.55 m
Transmitter
Distance Measurements
Distance (m)
0m
3.5 m
Layout of Testing Room
Maximum Transmission Distances
Distance (m)
Received Power (dBm)
0
5
10
Test Setup
20 m
10 m
15
20
-35
Antenna Locations
Maximum Distance
Receiver Only
14.5 m
-45
Transmitter Only
16.8 m
-55
Transmitter & Receiver
19.4 m
-65
-75
Received Power vs. Distance With
Reference to Room Shape
•
•
•
•
Shape of room resulted in a wave-guide effect at 10
meters
Last successful data transfer occurred at 19.4 m
Received power at this distance was approximately -70
dBm
Can assume Ming RE-99 Receiver sensitivity is
approximately -70 dBm
Received Power at 1m
Received Power at 8m
Central Monitoring Station
Moving Average Filter Example
Labview Block Diagram Code and
Labview Front Panel Gui
• At the time, used Ming
RE-99 Receiver
• NI USB-6008 DAQ device
for power to Receiver,
and ADC to capture data
• Powered from HP
Laptop’s USB Port
Running LabVIEW 7.1
• Moving Average Filter to
differentiate data “pulse”
from noise
Full System Integration and
Testing
or
Schematic of
Hydrogen Chamber
Schematic of Hydrogen Chamber
Future Work: New Receiver
System Level Architecture for RXM-315-LR
Pin-Out of RXM-315-LR receiver, and
receiver test board, shown with
SPLATCH antenna
Linx Technologies RXM315-LR
• Replacement for Ming
RE-99 since Rayming
Corp. went out of
business
• OOK Modulation
• Low Part Count
• Low Complexity
• RSSI/PDN
• -112 dBm Sensitivity
Future Work: Low-Profile Antenna
‘SPLATCH’ dimensions, matched Sparameters
Antenna Test Board w/ Matching
Circuit
• Linx Technologies ANT315-SP ‘SPLATCH’ Style
Antenna
• Grounded Line,
Microstrip Monopole
Antenna
• After matching, -9dB
gain, trade off for lowprofile antenna
• 5 MHz -10 dB BW,
Center Frequency = 315
MHz
Future Work: Minimum Redundancy
Minimum Energy Coding
•
Source
CODED – 1 “high”
(Previous Work)
CODED – 1
“high-delay”
CODED -2
“high”
0000
000000000000000
000000000000000
00000
0001
000000000000001
000000000000001
00001
0010
000000000000010
00000000000001
00010
0011
000000000000100
0000000000001
01000
0100
000000000001000
000000000001
00100
0101
000000000010000
00000000001
00011
0110
000000000100000
0000000001
00101
0111
000000001000000
000000001
01010
1000
000000010000000
00000001
01000
1001
000000100000000
0000001
01001
1010
000001000000000
000001
10001
1011
000010000000000
00001
10010
1100
000100000000000
0001
00110
1101
001000000000000
001
01100
1110
010000000000000
01
10100
1111
100000000000000
1
11000
Proposed Source Coding Technique
•
Mapping (n) source bits to message with
a maximum of 2, or 3 “high” bits
–
Example: 6 source bits 6 source bits = 64
messages (symbols)Find Codeword of
length (m) that allow
for 64 symbols, with a maximum of 3
high bits.
–
64 = mC3 + mC2 + mC1 + mC0 ; m = 7
Power Reduction
–
Assumptions:
for now, all source code symbols
have equal probability of
occurrences, and power is only
consumed with the transmission of a
high bit.
–
So, Power Consumption Reduction is:
% PowerReduced 
•
|  # avgsourcehighbits   # avgcodedhighbits |
 # avgsourcehighbits
By using a minimum energy coding
technique, we can expect to reduce the
power required to transmit an un-coded
message by 20 to 40 percent.
g100
Minimum Redundancy Minimum
Energy Coding (cont.)
90
Power Consumption Reduction per Additional
Redundant Bit
3 high
Percentage of Power Reduced
per additional Redundant Bit
80
2 high
70
1 high
60
1 delay
50
40
30
20
10
0
3
4
5
6
7
8
Original Source Bit Length
9
10
Conclusions
• Successfully designed a low-power sensor
interface for the Pt-ZnO Nano-Rod hydrogen
sensing mechanism
• In conjunction with the microcontroller, RF
transmitter, and separate energy harvesting
techniques, were successful in detecting and
reporting the presence of 500 PPM of H2 in N2.
(.05%) using Pt-ZnO Nano-rods as our sensing
mechanism
• Energy harvesting techniques include solar and
vibration energy devices.