Interrogating Passive, Wireless SAW RFID Sensors With the...
Transcript of Interrogating Passive, Wireless SAW RFID Sensors With the...
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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