design and testing of a self-powered wireless hydrogen sensing platform university of florida jerry...

Design and Testing of a Self-Powered Wireless Hydrogen Sensing Platform

University of Florida

Jerry Chun-Pai Jun, Jenshan Lin, Hung-Tan Wang Fan Ren, Stephen Pearton and Toshikazu Nishida

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 Proton-Exchange Membrane (PEM) Fuel Cells

• Greater than 4% of hydrogen concentrations are explosive.

RFSOC

Long-lifetime operation in dangerous environments to keep equipment and humans out of harms waygrowing interest in h2 as a replenishable fuel source, but dangerous if not properly containedh2 used in spacecraftsand proton exchange membrane fuel cellsconcentrations higher than 4% become explosive

Limitations Of Sensor Development

• Limitations of Energy Harvesting Devices

• Limitations of Low-Power and Low- Voltage Commercial Components

• Limitations of a Wireless System

– Wireless Channel Estimation

– FCC Regulations

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limitations for sensor development include the limitations of the energy harvesting deviceslimitations of using low power and low voltage componentsand limitations of the wireless channel, including those restrictions set by the FCC

Energy Harvesting Techniques

Solar Energy Harvesting• Solar Cells are a mature

commercial Product• Dependent upon real-

time 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

RFSOC

Solar energy harvesting is a mature technique, but is dependent upon real time lighting and temperature.we used a pulse resonant power converter designed for this project to use to convert the solar cell's output of 0.8 - 1.2 V, to a stead 2V dc output for the systemVibration energy Harvesting is a newer less mature technique, and is nice in that the collection of energy is proportional to the volume of the device itself. however, it is limited to magnitude and frequency of vibrations.for proof of concept, used commercial PZT beams from PSI, and connected to a direct charging circuit

Energy Harvesting Techniques cont.

Solar Energy Harvesting Vibration Energy Harvesting

Pulse Resonant Power Converter Functional Block Diagram (a) Bare die

photo (b)

IXOLAR XOD17-04B Solar Cell

Four mounted PSI D220-A4-203YB Double Quick Mounted Y-Pole

Bender (a) Direct Charging Circuit (b)

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you can see the solar cells, functional block diagram of the power IC, the four mounted PSI PZT bi-morphs, and direct charging circuit used for powering up the system

ZnO Nano-Rods as a Sensing Mechanism

• 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

S D

ZnO M-NRs

Al2O3 Substrate

Al/Pt/Au

a) b)

S D

ZnO M-NRs

Al2O3 Substrate

Al/Pt/Au

a) b)

Schematic of Multiple ZnO Nano-Rods

Close-Up of Packaged ZnO Nano-Rod Sensor

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ZnO nano-rods are an attractive sensing mechanism for hydrogen detection, and currently used for humidity, UV light, and various gas detection. They're easy to synthesize on a variety of substrates, are bio-safe, and when put in an array, exhibit a large surface to volume ratio making them highly sensitive to hydrogen.additionally if coated with Pt or Pd, they can serve as a catalyst to increase a device's sensitivity to hydrogen.they also provide 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 hydrogen

• 90% recovery within 20 seconds upon removal of hydrogen from the ambient

Pt-coated ZnO Nano-Rod - Relative Resistance Change for Various

Hydrogen Concentrations

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Pt showed the best sensitivity to hydrogen, with greater than 8% change in relative resistance at 500 PPM.there exhibits a 2 % change for 10 PPM, and has a recovery of 90% within 20 seconds after removal of hydrogen from the ambient.this is a two fold increase as comapred to Pd-coated ZnO Nano-Rods 18 fold increase over uncovered.

Comparison of ZnO Nano-Rods Coated with Different Metals

0 5 10 15 20 25 30

0

2

4

6

8500ppm H

2 Air

Time(min)

|ΔR

|/R (

%)

Pt Pd Au Ag Ti Ni

Relative Resistance Change for Various Metal-coated ZnO Nano-Rods

Differential Measurement

• Wheatstone Resistive Bridge– Can limit current

consumption of resistive bridge

– Best way to detect changes in resistance

• Difference Amplifier– Using differential

architecture of operational amplifier to subtract difference at input, and apply gain

– Form of differential measurement

R3

R3

R2

R2

R3

R3

R2

R2

V2

V1

R3R1

R4R2

VgVs V2

V1

R3R1

R4R2

VgVs

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Good way to measure changes in resistance is by using a differential measurementwheatstone resistive bridge is a tried and true method for measuring unknown resistances.difference amplifier takes advantage of differential architecture of op amp to provide controlled gain, and use both inputs for a difference measurement

Instrumentation Amplifier

R3

R3

R2

R2

R1

R1

Rg

V1

V2

V3

V4

VOUT

R3

R3

R2

R2

R1

R1

Rg

V1

V2

V3

V4

VOUT

1 32 1

2

2( ) 1OUT

g

R RV V V

R R

• Provides High Impedance Input Buffers isolate V1 and V2 from resistive network of difference amplifier

• Buffers and provides gain before difference amplifier

• Gain can be easily adjusted by varying a single resistor, Rg.

RFSOC

intsrumentation amplifier is a improvement on difference amplifier by providing a high input impedance to isolate input from resistive bridge.this will be very important since we're using a wheatstone resistive bridge as the input.

Differential Detection Circuit• 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

+

-

-

+

-

+

VDD

GNDGNDE

xpos

ed Z

nO

Pas

siva

ted

ZnO

R B

ias

R B

ias

R1

R1

RG

R2

R2

R3

R3

VOUT

+

-

-

+

-

+

VDD

GNDGNDE

xpos

ed Z

nO

Pas

siva

ted

ZnO

R B

ias

R B

ias

R1

R1

RG

R2

R2

R3

R3

VOUT

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since ZnO nano-rod devices are sensitive to temperature and h2, by using a covered, passivated ZnO to reference against exposed, we can mitigate effects of temperature.Rbias, can be set to limit the current down each leg of the resistive bridgeand this maintains the concept of differential measurements.and instrumentation amplifier, can also help balance input offset voltages while providing gain.

Fabricated Pt-ZnO Nano-Rod for Use in Differential Detection

CircuitZnO with increase Pt catalyst

1400142014401460148015001520154015601580

time(min)

Resis

tan

ce(o

hm

s)

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these were fabricated for our use.nominal resistive span from 1565 (no h2) to 1461 after 500 PPM exposure

Fabricated Differential Detection Circuit

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pcb layout on top, and assembled device on the bottom

Fabricated Differential Detection Circuit

Output voltage vs sweep of exposed Pt-ZnO Nominal Resistance

0

100

200

300

400

1460 1480 1500 1520 1540 1560

Nominal Resistance (Ohms)

Ou

tpu

t V

olt

ag

e

(mV

)

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did a similar experiment as the simulation, and swept resistance from 1461 to 1560. can see an approximate linear correlation, of about -4mV/ohm, but without initial offset. when bridge is balanced, there is only a 30uV output, which is better than simulated. we don't need software compensation for this device.

Microcontroller Selection

• Low-Voltage• Low-Active Current• Low-Sleep Current• Onboard Memory• Onboard ADC• Serial Output• Reprogrammable

Type of Program Memory

Flash

Program Memory 8 kB

RAM 256 Bytes

I/O Pins 22 pins

ADC

10-bit SAR ( successive

approximation register )

Interface1 Hardware SPI or

UART, Timer UART

Supply Voltage Range

1.8 V – 3.6 V

Active Mode200uA @ 1 MHz, 2.2

Vsupply

Standby Mode 0.7 uA

# of Power Saving Modes

5

REQUIREMENTS

Features of Texas Instruments’ MSP430F1232IPW

RFSOC

in the selection of a microcontroller, these are what we were looking for.low voltagelow active currentlow sleep currentonboard memory to store runtime code and adc data,onboard adc,serial output for the transmitter,and a reprogrammable package.we found all this in the MSP430.

Microcontroller Operation

• 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

• 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

Data Transmission State Machine Level Monitoring State Machine

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there are two modes of operation.one mode is for the transmission of a pulse once a threshold has been detected, and another is for the constant transmissin of the data from the adc, once a threshold has been detected.

Selection of a Modulation Technique

• 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

4

-DQPSK

OOK

4

-DQPSK

OOK

4

4

-DQPSK

OOK

-DQPSK

OOK

Comparison of Complexity between π/4- DQPSK and OOK

MODULATION REQUIREMENTS

RFSOC

for a modulation technique, since RF components are of low efficiencies, we want less components, equalling a lower parts count, and lower complexity.since our device is of low duty-cycle and data rate, we can get by with a modulation technique that only expend energy on the transmission of data.for an example, shown here is the complexity comparison between ook and a form of QPSK modulation.standards such as zigbee 802.15.4 use a OQPSK (offset quadrature phase shift keying), and would require an architecture similar to this.

Selection of RF Transmitter (1)

300 MHz Ming TX-99• Onboard antenna• OOK Modulation• Low Part Count• Low Complexity• Tunable Frequency• Colpitts Oscillator

VDD

GND

VDD

GND

Ming TX-99 Transmitter in OOK Mode

Ming TX-99 Transmitter

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This is the ming transmitter. it offers a tunable frequency, and low parts count which is nice. it also offers the least power requirement compared to other rf transmitters of this type.

Selection of RF Receiver (1)

300 MHz Ming RE-99• Onboard antenna• External Antenna Tap• Low Part Count• Low Complexity• Tunable Frequency• Envelope Detection• Little Documentation

Ming RE-99 Receiver Schematic

Ming RE-99 Receiver

RFSOC

this is the matching ming receiver. it uses an envelope detection scheme to demodualte the data. unfortunately, there is little documentation on this component.

Distance Measurements

Received Power at 1m

Test Setup

Layout of Testing Room Maximum Transmission Distances

Received Power at 8m

19.4 mTransmitter & Receiver

16.8 mTransmitter Only

14.5 mReceiver Only

Maximum DistanceAntenna Locations

19.4 mTransmitter & Receiver

16.8 mTransmitter Only

14.5 mReceiver Only

Maximum DistanceAntenna Locations

AtriumHallway HallwayAtrium

3.5 m 10 m 20 m0 m

Tra

nsm

itte

r

0.4

5 m

0.5

5 m

Distance (m)

Transmitter

Receiver

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

-75

-65

-55

-45

-35

0 5 10 15 20

Distance (m)

Rec

eive

d Po

wer

(dB

m)

RFSOC

A distance measurement was performed to see both the maximum distance, and to see the sensitivity of the receiver.with antennas on both tx and receiver, a maximum distance of 19.4 m was acheived.the received power at this distance was approximately -70 dBm, so can assume that the receiver sensitivity.this was performed in the atrium of NEB, and we can see that the received power changes with the room shape.

Central Monitoring Station

• 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

Labview Block Diagram Code and Labview Front Panel Gui

Moving Average Filter Example

RFSOC

at the time, used a ming-RE-99connected it to a Daq device for power, and data capture, and used labview 7.1 to analyze data.since using envelope detection, saw quite a bit of noise on the channel. differentiated between pulse and noise through the use of a moving average filter with a window slightly larger than the pulse itself.

Full System Integration and Testing

Schematic of Hydrogen Chamber

Schematic of Hydrogen Chamber

or

Future Work: New Receiver

Linx Technologies RXM-315-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

Pin-Out of RXM-315-LR receiver, and receiver test board, shown with

SPLATCH antenna

System Level Architecture for RXM-315-LR

RFSOC

this is the matching receiver for the linx transmitter. It uses a single conversion super heterodyne architecture, rather than just an envelope detector.this is so that it can acheive an outstanding performance of -112 dBm sensitivity.

Future Work: Low-Profile Antenna

• Linx Technologies ANT-315-SP ‘SPLATCH’ Style Antenna

• Grounded Line, Microstrip Monopole Antenna

• After matching, -9dB gain, trade off for low-profile antenna

• 5 MHz -10 dB BW, Center Frequency = 315 MHz

‘SPLATCH’ dimensions, matched S-parameters

Antenna Test Board w/ Matching Circuit

RFSOC

used a low profile antenna, 50 ohms matched to 315 MHz. has a 5 MHz -10 dB BW, centered at 315 MHz.low gain of about -9 dB, but is a trade off for low-profile antenna.this can be due to a poor rad. efficiency and low directivity since we're using a grounded line microstrip monopole antenna

• 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:

• 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.

# #

#

| |% 100

avgsourcehighbits avgcodedhighbitsReduced

avgsourcehighbitsPower

g

Future Work: Minimum Redundancy Minimum Energy Coding

1100011000000000000001111

10100010100000000000001110

011000010010000000000001101

0011000010001000000000001100

10010000010000100000000001011

100010000010000010000000001010

0100100000010000001000000001001

01000000000010000000100000001000

010100000000010000000010000000111

0010100000000010000000001000000110

00011000000000010000000000100000101

001000000000000010000000000010000100

0100000000000000010000000000001000011

00010000000000000010000000000000100010

000010000000000000010000000000000010001

000000000000000000000000000000000000000

CODED -2 “high”

CODED – 1“high-delay”

CODED – 1 “high”(Previous Work)

Source

1100011000000000000001111

10100010100000000000001110

011000010010000000000001101

0011000010001000000000001100

10010000010000100000000001011

100010000010000010000000001010

0100100000010000001000000001001

01000000000010000000100000001000

010100000000010000000010000000111

0010100000000010000000001000000110

00011000000000010000000000100000101

001000000000000010000000000010000100

0100000000000000010000000000001000011

00010000000000000010000000000000100010

000010000000000000010000000000000010001

000000000000000000000000000000000000000

CODED -2 “high”

CODED – 1“high-delay”

CODED – 1 “high”(Previous Work)

Source

Proposed Source Coding Technique

RFSOC

because ook expends power only on transmission of a "high" bit, and no power on the transmission of a "low" bit, a minimum energy coding scheme can be proposedif we can reduce the number of "high" bits sent per message, then can reduce the energy required to send that message. other schemes used only 1 high bit, but if you take an original source message with 3 bits, and try to code to a message with only 1 high bit, you need a message of length 7 to do this. for 4 bits, its 15, and so on and so forth.this introduces a lot of overhead at the receiver side, and increase the time a transmitter occupies the carrier, increasing likely-hood of collison.what is proposed here is a minimum energy coding scheme, with minimum redundancy.there are 2 techniques.one is using a "single high" delay based, which means that only a single "high" is sent, but there is a delay between "high"s which correlates to a bit. another method is using a maximum of 2 or 3 "high" bits.for example, SHOW EXAMPLE.given a 6 source bit to 7 bit coded message with a maximum of 3 highs, by adding a single redundant bit, the 7 bit coded message can represent all 64 symbols of the 6 source bit message with only 3 high bits per 7 bit length message.using this technique, can expect a 20 to 40 percent reduction in energy required for message transmission without cost to message length.

Minimum Redundancy Minimum Energy Coding (cont.)

Power Consumption Reduction per Additional Redundant Bit

0

10

20

30

40

50

60

70

80

90

3 4 5 6 7 8 9 10

Original Source Bit Length

Pe

rce

nta

ge

of

Po

we

r R

ed

uc

ed

p

er

ad

dit

ion

al R

ed

un

da

nt

Bit

3 high

2 high

1 high

1 delay

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.

design and testing of a self-powered wireless hydrogen sensing platform university of florida jerry...

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