Interrogating Passive, Wireless SAW RFID Sensors With the USRP
James ‘Trip’ HumphriesUniversity of Central Florida
Cyberspectrum
2016-02-03
Outline
• Research Overview
• Passive, Wireless SAW Sensors
• USRP SAW Sensor Interrogator
• System Demo w/ SAW Sensors
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CAAT @ UCF
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Motivation
• Multiplexed, Wireless, Passive Sensors
• Originated ~2002 by NASA request for space shuttle sensors
• SAW Sensors meet needs
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USRP
TX
RXGNU Radio /
Post Processing
Passive
Sensor Tags
SAW Devices
• Solid state devices– Converts electrical energy into mechanical wave (and vice versa)
on piezo-electric substrate– Very complex signal processing in small size (Spatial mapping of
sampled time function)
• 4-5 Billion SAW devices produced each year– Filters, Delay Lines, Resonators– Sensors, RFID
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SAW Sensors
• One Port Device– Response is reflected signal from sensor (S11)– Stimulus affects frequency, phase, delay, or amplitude (or combination)– SAW Device modulates interrogation pulse and reflects (encode measurand, ID, etc)
• Very similar to RADAR
• Operate from 10Mhz-3GHz– Fabrication tolerances limit
• Passive Operation– No Batteries– No Energy Harvesting
• Radiation Hard• Operate Over Large Temperature Range
– Cryogenic - +1000C (Depending on material)
• Variety of Measurands– Temperature, Strain, Gas, etc
• Variety of Device Embodiments– Resonant, CDMA (Narrowband), Orthogonal Frequency Coded (Wideband)
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OFC Overview
• Spread spectrum coding technique
• High Processing Gain• Frequency and Time
Diversity• Nj = τc∙fj
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SAW Substrate
f1f2 f4 f5 f3
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1-1.5
-1
-0.5
0
0.5
1
1.5
1.2 1.3 1.4 1.5 1.6 1.7 1.8-0.01
-0.008
-0.006
-0.004
-0.002
0
0.002
0.004
0.006
0.008
0.01
Time (s)
Mag
nitu
de (d
B)
Magnitude S11 Time Response
2012-03-07_BF401.txt
Measurement Extraction–Matched Filter Correlator
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Temperature Sensors
• SAW substrate is temperature sensitive
• Temperature causes SAW velocity shift
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1
( ) (1 )
( , ) cos 2 ( )c
i
i i
ND
i chip Dchipi
T TCD T
th t a f t rect
High Temperature Sensors
• SAW device on Langatate (LGT)
– Stable up to melting point of ~1450°C
• Integrated On-Wafer Antennas
– Electroplating or direct-write
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Hydrogen Sensor
• Room temperature, reversible gas sensor
• Nano-Cluster Thin Film Interaction with H2
• Film resistivity changes with exposure to H2 causing propagation loss
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Piezoelectric Substrate
Film Sensitivities:Temperature, Chemical,Gas, Pressure, Humidity,
Magnetic Field, etc.
ReflectorTransducerReflector
RF Energy
Strain Sensor
• Strain on SAW die also causes velocity shift
• Structural Health Monitoring (SHM) and Rotating (Torque) Parts
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( ) (1 · )oV S V S
UCF Interrogation System History
• 250 MHz Prototypes (2008-2009)
• 915MHz Pulsed (2011)• 915MHz Noise Coherent
(2012-2014)• 915MHz Software Radio
(2015)
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Interrogator Requirements – SAW Sensors
• High Bandwidth– 915MHz ISM Band 26MHz (or more)
– Improves processing gain (Increase OFC Chips)
– Better SAW device performance (Optimize reflectors)
• Tx/Rx Synchronization– Define precise t=0
– Define precise listen window (SAW Responses)• Correlation software expects SAW responses at specific
location in time
– Averaging multiple sweeps to improve SNR
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Design Challenges
• Need Max BW– Difficult to design OFC sensors as bandwidth becomes
small (Long reflectors -> Ringing, less #Codes -> Less PG )
• Synchronize Tx/Rx– Timing latency over USB is much greater than sensor
response time (<10µs)– Need to trigger Tx/Rx states with very high accuracy
(start/stop, RF switches..)
• Micro-second TX Pulses• “Active RADAR” illuminates passive sensors
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Solutions
• Use on-board FPGA to implement custom functionality to USRP
• Transmit– Interrogation Signal Generator– Remove transmit streaming; saves CPU and USB resources– Control External RF Switch Timing
• Receive– Synchronize to interrogation pulse (latency reduced to
single clock cycle)– Define listen window– Buffer samples in RAM (No longer real time streaming of
data) and read-back at slower rate
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FPGA Block Diagram
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Transmit – Pulsed Chirp
Time (Generated Samples) Frequency (Predicted vs USRP)
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Transmit – Noise Pulse
• Linear feedback shift register (LFSR) Network– Generates pseudo-random
bits– Each LFSR seeded with
different random value
• 1µs noise pulse
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Transmitted Pulse - Time
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External Components
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System – Internal
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USRP B200
RF Switch TX Amp Power Distribution
(SW, Amp, Fans)
System - Exterior
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Host Software
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*Malocha, D.C.; Gallagher, M., "Coherent correlator multi-sensor receiver," Frequency Control Symposium (FCS), 2012 IEEE International , vol., no., pp.1,5, 21-24 May 2012
*
USRP SAW Sensor Interrogation Systems
• Integrated into Custom Enclosures– 10”x8”x2” (25x20x5cm)– 2-3 lbs (~1kg)
• Two Embodiments Developed– USRP w/ External Host CPU– USRP w/ Embedded Host CPU
• Output Power: > +20dBm• Range: Up to 7m Tested
– Use higher gain antenna
• Reading Time:– 1 Average: ~40-80ms– 1000 Average: ~400ms
• Component Cost: <$1100
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USRP SAW Sensors
Sensor Delay and Code Sequence OFC Reflectors
Device
Name
Code
Sequence
Delay to
First Chip
(µs)
Bit # Frequency
(MHz)
Number of
Reflector
Strips
usrp-m2-
d1
3,1,2,4 2.57 1 909.91 127
usrp-m2-
d2
1,4,3,2 3.25 2 916.99 128
usrp-m2-
d3
2,1,4,3 3.92 3 924.07 129
Usrp-m2-
d4
4,2,3,1 4.60 4 931.15 130
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• CF = 915MHz; BW=28MHz
• 4 Sensors
• 4 Chips Each
• Withdrawal Weighted
Reflectors
Single Sensor Temperature Run
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Averaging Multiple Sweeps
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Multiple SAW Sensors
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Current Work
• Adding support for different SAW sensor embodiments– Temperature (Demonstrated), Strain, &
Gas (Hydrogen, Methane and Other)
• Enhanced User Interface– Better facilitate testing
• Enhanced System Integration– Light and Rugged (And small!)– B200mini
• Improved reading accuracy/precision and speed– Currently ~40-80ms per reading with
Desktop host PC, aiming for 1ms– Off-load more tasks to FPGA!
• Real time statistics for sensors
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Acknowledgements
• CAAT Group Members (Current & Former)– Luis Rodriguez, Jose Figueroa, Scott Smith, Chris Carmichael,
Roman Grigorev– Mark Gallagher, Dan Gallagher, Nancy Saldanha, Nick Kozlovski,
Brian Fisher
• External Encouragement & Support– Dr. Robert Youngquist (NASA-KSC), Dr. Cy Wilson (NASA-LaRC)
• Advisors– Dr. Donald Malocha– Dr. Arthur Weeks
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Demo
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