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