Project Title: Wireless Passive Sensors and Systems for

Florida Universities Hydrogen Review 2005Florida Solar Energy Center November 1-4, 2005

Task Title – PI – Organization 1

Project Title: Wireless PassiveSensors and Systems for Physical Sensors

and Hydrogen Sensing Applications

Principal Investigator: Donald (Don) C. Malocha

Organization: School of Electrical Engineering & Computer Science

University of Central Florida

Start Date = January 1, 2005 Planned Completion = May 30, 2007



Research Goals and Objectives1. Develop miniature, light weight, passive remote wireless sensors for

physical, chemical, gas and biological applications.2. Develop a device platform which operates over wide range of operating

environments and for various sensing applications.3. Theoretical and experimental studies on sensor device embodiments to

determine sensitivity and limits of operation.4. Verify SAW sensor can operate at and near liquid hydrogen

temperatures.5. By using multiple devices and/or temperature sensing, liquid level can

be determined.6. Develop a hydrogen gas sensitive film for the detection of hydrogen

gas.



Relevance to Current State-of-the-Art• New use of spread spectrum technique for surface acoustic wave (SAW) sensor• Orthogonal frequency coding allows lower spectrum emission, coding and

processing gain over conventional single carrier SAW devices• Initiate work on SAW sensor operation at cryogenic temperatures and hydrogen

sensing• Wireless-passive device platform can potentially be used for a wide range of

physical, gas, chemo and bio sensors with slight changes in embodiment

Relevance to NASA• Hydrogen gas sensor for leak detection• Hydrogen liquid level sensor by using multiple devices in a linear array-

sensor “dip stick” approach• Temperature sensor• Tire and other pressure sensor• Potential use in a wide range of NASA applications in future exploration

in remote vehicles and space craft requiring sensors



Budget, Schedule and Deliverables• Start date delay due to contract setup - 1st Year Funding• We are on schedule and within this year’s budget of $90K• Deliverables: Qtrly reports, 3 publications, devices fabricated and

tested

ID Task Name1 Contract Award

2 Project/contract initiation

3 Design Mask 1 Initial testing

4 Fabricate Device #1

5 Measure & Analysis Device #1

6 Design Mask #2 TCD Extraction


8 Measure Device & Analysis #2

9 Design Mask #3 Hydrogen Gas Sensor


11 Measure & Analysis Device #3

12 Antenna Designs and Measurenments

13 Final Report

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec



Anticipated Technology End Use

• Hydrogen gas sensor for leak detection• Hydrogen liquid level sensor by using multiple devices in a linear array-

sensor “dip stick” approach• Temperature sensor• Tire and other pressure sensor• Cement temperature curing monitor using a large array of coded SAW

sensors• Small wireless coded strain/ vibration sensor for moving systems• High temperature remote sensing for turbines or other harsh

environment applications• Bio- and Chemo- sensors can use the current and future progress of

other investigators working in the field since the device platform and interrogation system will be developed



Accomplishments and Results• Demonstrated low temperature operation of SAW OFC Sensor• Designed and fabricated a 500 MHz OFC SAW sensor with

approximately 1um lines using pattern generator• Designed and are attempting fabrication of .7 um, 1 GHz SAW OFC

sensor using ebeam writer– on third iteration of fabrication• Theory and experiments on SAW reflectivity to account for film

thickness and line width vs. period variation• Expanded interrogation system software and analysis for device

evaluation• Expand analysis and design software, including a 2-D coupling of

modes model which allows weighted reflector analysis• Built several OFC hydrogen gas sensors using palladium thin films and

begun characterization• Initial experiments using the FSEC color changing thin films for

integration into OFC sensor devices for detecting hydrogen• Initial cryogenic temperature testing of LiNbO3 substrates



Linear Stepped Chirp Time

Response with 7 Chips

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

0.2

0.4

0.6

0.8

Normalized Frequency

0 1 2 3 4 5 6 71

0.5

0

0.5

1

Normalized Time (Chip Lengths)

Basis of Operation



0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.80

0.2

0.4

0.6

0.8

Normalized Frequency

Schematic of 7chips/bit OFC SAW ID Tag

0 1 2 3 4 5 6 71

0.5

0

0.5

1

Normalized Time (Chip Lengths)

Piezoelectric Substrate

f1 f4 f6 f0f2 f5 f3



SAW Sensor Implementation

• OFC reflectors repeated on both sides of transducer• Transceiver yields two compressed pulses• Pulse separation proportional to sensed information• Different free space delays (τ1 ≠ τ2) yield temperature• For gas, chemo or bio sensing a sensitive film, such as palladium for

hydrogen gas, is placed in one delay path and a change in differential delay senses the gas (τ1 = τ2)


f1 f0f2 f3f1f0 f2f3

τ1 τ2



OFC SAW Temperature Sensor


f1 f4 f6 f0f2 f5 f3f1f4f6f0 f2f5f3

τ1 τ2

0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2 2.460

50

40

30

20

ExperimentalCOM Simulated

Time (us)

Mag

nitu

de (

dB)

•Seven chips / frequencies•YZ LiNbO3 substrate•250 MHz center frequency•25% fractional bandwidth•Free space delays of 1 µs and 2 µs•Simulation using coupling of modes (COM) model compared with experimental results



Uniform vs Weighted Input Transducer Results

9 10 11 12 13 14 15 16 1740

35

30

25

20

15Weighted IDTUnweighted IDT

Time (normalized to chip lengths)

Impu

lse

Res

pons

e (d

B)

120 140 160 180 200 220 240 260 280 300 320 340 360 3803

2.5

2

1.5

1

0.5

0

Weighted IDTUnweighted IDT

Frequency (MHz)

S11

(dB

)

Purpose is to obtain maximum uniform transducer bandwidth

Weighted transducer

Time Domain

Frequency Domain



Chirp Interrogation-Transceiver Concept

SAWsensor

RF Oscillator

Digital control and analysis circuitry

SAW up-chirp filter

SAW down-chirp filter

IF Oscillator

A / D

IF Filter

Currently software simulated



Bit, PN, OFC Signal Comparison

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2-40

-35

-30

-25

-20

-15

-10

-5

0

Normalized FrequencyNormalized to Peak of Single Carrier (dB)

7 chips/bit PN-OFC7 chips/bit PN Single CarrierBPSK

0 1 2 3 4 5 6 70

0.2

0.4

0.6

0.8

1

Time Normalized to a Chip Length

Normalized A

mplitude

7 chips/bit PN-OFC7 chips/bit PN Single CarrierBPSK

Matched Filter Correlated Response

BPSK –no coding

PN Code

OFC Code



Verification of Predicted Results- Time Correlated Output

4 6 8 10 12 14 16 18-50

-45

-40

-35

-30

-25

-20

← →

↔ 0.28⋅τc

Mag

nitu

de (d

B)

Ideal Compressed Pulses

4 6 8 10 12 14 16 18-50

-45

-40

-35

-30

-25

-20

Mag

nitu

de (d

B)

COM Simulated Compressed Pulses

4 6 8 10 12 14 16 18-50

-45

-40

-35

-30

-25

-20

Time Normalized to a Chip Length

Mag

nitu

de (d

B)

Experimental Compressed Pulses

Ideal

Predicted w/SAW COM

Experimental



Measured Time Correlated Output Compressed Pulse Responses

versus Temperature

Single temperature measurement showing the 2 compressed pulses and differential delay



• 250 MHz LiNbO3 OFC SAW sensor tested using temperature controlled RF probe station

• Results applied to simulated transceiver and compared with thermocouple measurements

0 20 40 60 80 100 120 140 160 180 2000

20

40

60

80

100

120

140

160

180

200Temperature Sensor Results

Time (min)

Tem

pera

ture

( °C

)

LiNbO3 SAW SensorThermocouple

Temperature Sensor Results



Cryogenic Sensor Results

• SAW device feasibility is established• Initial device design, fabrication and

testing completed at liquid nitrogen temperatures

– Device performed over multiple temperature cycling

– Initial temperature coefficient of delay (TCD) obtained

– Using TCD, temperature is predicted and compared to measurement (graph)

0 5 10 15 20 25-200

-150

-100

-50

0

50

Time (min)

Temperature (°C)

ThermocoupleLiNbO3 SAW SensorScale

Vertical: +90 to -200 oCHorizontal: Relative

time (min)

Measurement system with liquid nitrogen Dewar and vacuum chamber with DUT



Device Fabrication Progresspush to higher frequencies to reduce

antenna size and improve performance

1GHz (0.75 um lines)

1.27 um line resolved in positive PR on mask plate using PG shows good line definition

1.5GHz (.5um lines)

1 um lines after lift off using +ve PR (S1813) by direct on wafer write using Image Repeater, (lines are jagged, lift off needs optimization)

E-beamPattern generator should take us to .7um lines while Ebeam should yield .5 um lines

Lines look like “spaghetti” due to poor adhesion in liftoff processing



Wireless Testing of a 500 MHz Sensor

• Simple dipole antenna for wireless transmission• Demonstrated Tx/Rx at a distance of 2 feet

350 400 450 500 550 600 650-1.6

-1.4

-1.2

-1

-0.8

-0.6

-0.4

-0.2

0

Frequency (MHz)

dB(S

11)

predictedexprimental

Experimental and COM Predicted 500 MHz sensors

0.2 0.4 0.6 0.8 1 1.2

-200

-180

-160

-140

-120

-100

-80

S11 Time Domain Response

Time (µs)

S11

(Im

puls

e R

espo

nse)

predictedexprimental



Pd Hydrogen Gas Sensor – Initial Results for 3 films


f1 f0f2 f3f1f0 f2f3

τ1 τ2

•For palladium hydrogen gas sensor, Pd film is in only in one

delay path, a change in differential delay senses the gas (τ1 = τ2)

Gas/probe chamber

Schematic Actual device with probe

Initial results demonstrate device sensitivity to hydrogen gas.

Green is delay w/o film

Blue is delay with film 3% H2+N2 mixture



Future Plans• Fabricate and build a 1 GHz SAW OFC sensor to reduce size and

optimize performance• Build or purchase a cryostat for sensor testing at liquid hydrogen

temperatures and build a cryogenic data acquisition system for device testing

• Continue hydrogen gas sensor development using metal and/or oxide thin films and continue application of the FSEC films. Materials evaluation work will need to be conducted to understand the film and SAW interaction

• Analysis, design and fabrication of an interrogation/receiver system for “pinging” the OFC sensor and extracting the received data

• Design antenna and device to optimize performance• Investigate parallel track embodiments to relax fractional bandwidth

requirements of the antenna• Goal is to have a fully operational proof-of-concept, multi-sensor SAW

OFC system within approximately the next 18 months, for hydrogenliquid and gas sensing

Documents

Project Title: Wireless Passive Sensors and Systems for