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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