This document summarizes research on a passive, wireless SAW OFC strain sensor. It presents the motivation for such a sensor, an overview of OFC coding, the theory behind using SAW devices for strain sensing, methodology for extracting strain from sensor readings, experimental results demonstrating strain measurement and magnetic field sensing, and plans for future work.

1. Passive, Wireless SAW OFC
Stain Sensor
J.R. Humphries and D.C. Malocha
Department of Electrical Engineering and Computer Science
University of Central Florida
IFCS 2012 – Baltimore, MD
James Humphries
May 22, 2012 1

2. Outline
• Research Motivation
• Orthogonal Frequency Coding (OFC) Overview
• SAW Strain Sensor Theory
• Stain Extraction Methodology (Software)
• Experiments and Results
• Application – Magnetic Field Sensing
• Conclusion and Future Work
James Humphries
May 22, 2012 2

3. Research Motivation
• Passive and Wireless Operation (915MHz)
• Cantilever Sensing Approach
• OFC for Sensor Identification and Strain
Extraction
OFC #1: OFC #2: Force
Reference Strain
Package SAW Substrate
James Humphries
May 22, 2012 3

4. OFC Overview
• Spread spectrum coding
technique
• High Processing Gain
• Frequency and Time
Diversity N = # electrodes
(integer)
• Nj = τc∙fj τ = chip length
f = chip freq
• Possible to measure sub-
nanosecond time delay
changes
James Humphries
May 22, 2012 4

5. Strain Sensor Theory
• Isotropic approximations
made for simplicity and Max Strain
ease of calculations
– Strain only in Z direction Force
(YZ-LiNbO3) , linear
– Ignore small strain Y
components in Y (Consider X Z
surface only)
– Effective stiffness constant
instead of tensors
• co = (Vsaw)2*ρ
James Humphries
May 22, 2012 5

6. Velocity / Strain Relationship
• Previous work* showed that: V(S) = SAW
velocity due
V (S )  Vo (1   ·S ) to strain
• Strain coefficient – γ Vo=
unstrained
– Dependent on material properties SAW velocity
(3488 m/s)
– Need to express γ in terms of time delay change
* Roller, M.; Malocha, D.C.; Vaidyanathan, R.; , "SAW OFC strain sensor," Ultrasonics
Symposium (IUS), 2009 IEEE International , vol., no., pp.2515-2518, 20-23 Sept. 2009
James Humphries
May 22, 2012 6

7. Strain Coefficient
• Using relationships:

V (S )· (S )  d d = length of
cantilever *2
 (S ) 1
   (S ) τ = SAW time delay
 (0) (1   S ) S = strain
T = stress
T (S )  c(S )·S V = SAW velocity
c( S )  co ·(1   S )
James Humphries
May 22, 2012 7

8. Strain Coefficient – cont.
 ( S )  1 1 
 · ·   ·(1  4 S )
T co (1   S ) (1  2 S )
2
co
• Assume 4γS << 1 ( S → micro-strains)
 ( S )  ( S )
  co ·  co ·A·
T F
James Humphries
May 22, 2012 8

9. Strain Coefficient Extraction
• Test setup to apply force at end of cantliver
• Cylinder ≈ 20 grams
Hollow
Cylinder
Sensor (Wired Version
Connected to Network
Analyzer)
James Humphries
May 22, 2012 9

10. Strain Coefficient Extraction
• Found experimentally
• 250MHz (Wired) strain
sensor fabricated
γ = -52.2
(YZ-LiNbO3)
James Humphries
May 22, 2012 10

11. Strain / Force Equations
• Rearrange and integrate:
 ( S )  ( S )
  co ·  co ·A·
T F
• To give strain or force equation:
1   (S ) 1   ( S ) 
S Beam  FBeam  co ·A· 
   
James Humphries
May 22, 2012 11

12. Strain Extraction Methodology
• Expanded correlator
software to extract time 0
delay changes
-5
• Process OFC #1
Magnitude (dB)
(Unstrained) -10
– Get frequency and -15
temperature info
• Process OFC #2 -20 Unstrained Correlation
Strained Correlation
– Use info from OFC #1 to -25
-0.1 -0.05 0 0.05 0.1 0.15
get strained and unstrained Normalized Time Delay
time delay
James Humphries
May 22, 2012 12

13. Wireless Test Setup
Hollow
Dipole Cylinder
Antennas
Sensor +
Antenna
Radio Specifications
1’ (0.3m) Peak Power 28 dBm
Output 700ns, stepped chirp
Bandwidth 60MHz
James Humphries
May 22, 2012 13

14. Sensor
Dipole Antenna (~2dBi)
OFC #1 OFC #2
Package Transducer
915 MHz (~0.8μm linewidth)
James Humphries
May 22, 2012 14

15. Results
Measured Mass
• Placed 20 gram 30
mass on end of 25
cantilever 20
Mass (Grams)
• ~10mm 15
cantilever 10
5
0
-5
50 100 150
Reading Number
James Humphries
May 22, 2012 15

16. Results – cont.
Variable Mass Measurements
• Varied mass to 40
observe tracking 35
30
• ~6mm cantilever
Mass (Grams)
25
Measured 20
15
10
Actual 5
0
-5
0 50 100 150
Reading Number
James Humphries
May 22, 2012 16

17. Magnetic Field Sensor
• Bond small magnet to end of cantilever
• Magnetic field in close proximity to sensor
causes cantilever displacement
Magnets
James Humphries
May 22, 2012 17

18. Magnetic Field Sensor Test
Magnetic Closure Sensor Test
• Magnet brought close
to end of cantilever
• Force changes
Force
observed
Mass
• Many Applications
– Door Closure
– Proximity
Reading Number
– Magnetic Field
Strength Magnet Close Magnet Far
James Humphries
May 22, 2012 18

19. Conclusion
• First demonstration of passive, wireless SAW OFC
strain sensor
– 1 foot range (0.3m)
• Developed model to relate time delay change to
strain on device
• Extracted strain coefficient (γ) experimentally
– -52.2 for YZ-LiNbO3
• Developed software method to extract strain
• Magnetic field sensor demonstrated
James Humphries
May 22, 2012 19

20. Future Work
• Test multiple strain sensors concurrently
• Improved strain extraction software
• Increased wireless range
• Investigate more substrate adhesives
James Humphries
May 22, 2012 20

21. Acknowledgements
• Dr. Robert Youngquist (NASA-KSC) for his
continued support through the Graduate
Student Researches Program (GSRP)
Fellowship
• Mark Gallagher for Correlation Software
• Brian Fisher for Device Photo-mask
James Humphries
May 22, 2012 21