Post on 28-Nov-2014
description
Passive RFID sensors
Passive RFID sensor projectRF MEMS group, Li-Bachman labs
Michael Helmeste
Passive RFID basics
A short explanation of passive RFID
What is RFID?
➲ Radio Frequency IDentification➲ A very wide, encompassing term
● As simple as a 1 bit anti-theft device● As smart as a powered remote data logger or
processor● Most often means an “RF Barcode”
➲ RFID tags● “wireless barcode” labels applied to objects
➲ Interrogator● Queries RFID tags and receives a response
Why RFID?
➲ Inexpensive● Tags as cheap as $0.20 available in large quan-
tities● Push towards $0.05 tags
➲ Wireless● Does not require manual inspection or optical
scanning➲ Integrated
● Tags can be put under traditional barcodes or labels
● RFID interrogators can be integrated with ITinfrastructure (databases, etc.)
➲ Small● Implantable RFID chips and capsules used for
animal tracking
Example RFID implementations
➲ Octopus card for Hong Kong● Credit card sized RFID tag● Stores account balance● Put in your wallet and pass over reader to enter
subway● Purchase goods at supermarket, etc.
➲ RFID door locks● Pass RFID card near reader for room access,
seen in many buildings➲ RFID price tag
● Being pushed by Wal-Mart➲ RFID item management
● Used in libraries for books, CDs, etc.● Enables “self checkout” and integration with anti-
theft
How passive RFID works
Reader emits carrier
Tag charges up
How passive RFID works, contd.
Reader seesmodulated data (envelope detection)
Tag clocks out data (96-bit unique code)
RFID Internals
➲ Passive tags use “backscatter”● Tag does not emit its own RF carrier● Tag modulates existing carrier, usually using
ASK (amplitude shift keying)➲ ASK is detected with an “envelope detector”
● Detects sudden changes in amplitude● Outputs a digital waveform● Practical envelope detectors implement noise fil-
tering● Signal is often 60dB down from carrier
➲ Collision detection● Many RFID standards (ISO, EPC) implement
collision detection / resolution● Allows multiple tags to be queried in the same
area
Where the project is going
What we are doing with RFID
“Smart” RFID tags
➲ Combining sensors with RFID tags● Temperature● Chemical● Strain● Humidity
➲ Should be inexpensive and small, likeregular RFID tags
➲ Should be easy to implement➲ No power requirement
Existing products
➲ Sensor tags are not common➲ More expensive than regular tags
● Ranging from $2 to $195➲ Require power➲ No cheap, passive RFID tag with external
inputs● Microchip has “MCRF 202” RFID chip with 1 bit
sensor input, not available yet
Possible solutions
➲ Wait for RFID chips with external inputs➲ Change polarization of tag antenna (least practical)
● Requires two antennas at interrogator● Requires interrogator modifications and complex circuitry● Encodes 1 bit
➲ Change resonance of tag antenna● Requires modification of interrogator circuitry● Simple to encode 1 bit, more complex modifications
could allow for more➲ Use two tags
● Sensor breaks / activates one tag● One tag for reference● Simple solution – requires no modification to interrogator● Slightly more expensive● Encodes 1 bit per extra sensor
Two carrier solution
➲ Concept● Backscatter allows carrier to be off frequency● Part of the RFID tag's spiral antenna is shorted
out by a MEMS sensor when activated● Resonant frequency of tag becomes higher● Reflected power is higher when carrier is off-
resonance● By quickly switching between carriers, the sen-
sor data can be acquired at the envelope detec-tor, without complex interrogator modification
Two carrier solution, contd.
Carrier 1 Carrier 2
Resonant to carrier 2
1 0Output at envelope detector
Two tag solution
➲ Concept● Two tags, one with sensor, one regular● Activation of MEMS sensor breaks functionality
of one tag
X
Off
On
Implementation
The process of getting these ideas to work
RFID system
➲ Texas Instruments HF● Series 6000 S6350 midrange reader module● RI-I11-112A RFID tags (square inlay)● Operate in the HF band at 13.56 Mhz● HF tags have external spiral antenna● +20dBm (100mW) power
➲ Antennas● Self built antenna (first attempt)● Wire loop antenna with loop tuner (second at-
tempt)
RFID system, contd.
TI RFID reader
RFID tags
RFID system, contd.
➲ Original antenna design
➲ Low Q loop reso-nant at 13.56 MHz
➲ T match● Hard to match, didn't
work well
RFID system, contd.
➲ New loop antenna➲ Uses capacitive loop
tuner● MFJ-936B
➲ High Q➲ Much smaller than
wavelength (~22 m)➲ Hard to tune➲ Yields decent read
range➲ High Q creates a few
reliability problems
Reader modifications
➲ Two carrier idea requires modified reader➲ TI unwilling to provide “propriatery”
schematics or information➲ Reader must be reverse engineered➲ Goal is to cut path somewhere between os-
cillator and PA➲ Use a coax switch to choose between origi-
nal and new carrier
Reverse engineered reader
27.12 MHzOscillator
CPU
Divide by two
RS232
Envelope detector
Power amplifierDigital
RF
Modification board
Reader modification
Testing setup
Spectrumanalyzer
-20
tunercpl (-20 dB)
+12 VDCcarrier switch
+5VDC Reader
BiasT
+2VDC
Signalsource
BiasT
DMM
-20-20 -20-20 Term
Secondcarrier
Ant.
Testing setup, contd.
Tuning the antenna
Antenna is tuned for reflected power. Best tuning achieved, SWR 1.01:1
(not pictured)
Tuning the antenna, contd.
Antenna is tuned using MFJ-936B Loop Tuner●Wavelength (~22 m) is much larger than antenna length●Antenna can be detuned by moving, people walking nearby, etc.●Needs to be retuned often, sharp Q●Played around with widening Q, but read range was reduced (created loss)
● Tradeoff between read reliability and reading range
Completed system
➲ Modified RFID system complete➲ Problems with input for second carrier
● CMOS chip has high impedence input● Signal generator wants to see 50 ohms● Easily solved by making a simple CMOS oscillator
in place of signal generator➲ Problems with reader modifications
● Initially modified reader didn't work● Desoldered modifications and performed a second
time● Worked perfectly● Probably a short or something
➲ Problems with antenna● T match hard to achieve 50 ohms● Switched to the MFJ tuner in the interests of time
Modifying the tag
➲ Tag is harder to modify➲ Antenna pattern on tag is aluminum
● Can't solder➲ Conductive epoxy is too high resistance➲ Created PCB antenna to transplant tag chip
on to● Solderable● Can integrate sensors in more convenient ways● Still problems with binding the tag chip to the
PCB, initial method caused some reliability is-sues
➲ Possible to use alternative metals to bind sensors to the tag
Modifying the tag, contd.
Conclusions
The state of the project
State of the project
➲ Reader / system is set up, modified, and working
➲ MEMS sensors not available for testing at the time of writing (fabrication problems)
➲ Two tag approach demonstrated to work with a simple switch
● Convenient solution with no modification re-quired, easy to integrate
➲ Reader generates two carriers and can switch between them
● More work required to recognize “off resonance” state without test equipment
● Not ready yet, but will be with a tiny bit more work
People to thank
Professors
G.P. Liand
Mark Bachman
The Li-Bachman research group
People to thank, contd.
Harrison Chang
David Crosley
and the RF MEMS team
People to thank, contd.
Steve Long
Kevin Schmidt
People to thank, contd.
Said Shokairand the UROP office
The National Science Foundation
Goran Matijasevic