Passive RFID sensors. Passive RFID sensor project RF MEMS group, Li-Bachman labs. Michael Helmeste

Transcription

1 Passive RFID sensors Passive RFID sensor project RF MEMS group, Li-Bachman labs Michael Helmeste

2 Passive RFID basics A short explanation of passive RFID

3 What is RFID? Radio Frequency IDentification A very wide, encompassing term RFID tags 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 wireless barcode labels applied to objects Interrogator Queries RFID tags and receives a response

4 Why RFID? Inexpensive Wireless Does not require manual inspection or optical scanning Integrated Tags as cheap as $0.20 available in large quantities Push towards $0.05 tags Tags can be put under traditional barcodes or labels RFID interrogators can be integrated with IT infrastructure (databases, etc.) Small Implantable RFID chips and capsules used for animal tracking

5 Example RFID implementations Octopus card for Hong Kong RFID door locks Pass RFID card near reader for room access, seen in many buildings RFID price tag Credit card sized RFID tag Stores account balance Put in your wallet and pass over reader to enter subway Purchase goods at supermarket, etc. Being pushed by Wal-Mart RFID item management Used in libraries for books, CDs, etc. Enables self checkout and integration with antitheft

6 How passive RFID works Tag charges up Reader emits carrier

7 How passive RFID works, contd. Tag clocks out data (96-bit unique code) Reader sees modulated data (envelope detection)

8 RFID Internals Passive tags use backscatter ASK is detected with an envelope detector Tag does not emit its own RF carrier Tag modulates existing carrier, usually using ASK (amplitude shift keying) Detects sudden changes in amplitude Outputs a digital waveform Practical envelope detectors implement noise filtering 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

9 Where the project is going What we are doing with RFID

10 Smart RFID tags Combining sensors with RFID tags Temperature Chemical Strain Humidity Should be inexpensive and small, like regular RFID tags Should be easy to implement No power requirement

11 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

12 Possible solutions Wait for RFID chips with external inputs Change polarization of tag antenna (least practical) Change resonance of tag antenna Requires two antennas at interrogator Requires interrogator modifications and complex circuitry Encodes 1 bit 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

13 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 offresonance By quickly switching between carriers, the sensor data can be acquired at the envelope detector, without complex interrogator modification

14 Two carrier solution, contd. Carrier 1 1 Resonant to carrier 2 Carrier 2 0 Output at envelope detector

15 Two tag solution Concept Two tags, one with sensor, one regular Activation of MEMS sensor breaks functionality of one tag Off X On

16 Implementation The process of getting these ideas to work

17 RFID system Texas Instruments HF Series 6000 S6350 midrange reader module RI-I11-112A RFID tags (square inlay) Operate in the HF band at Mhz HF tags have external spiral antenna +20dBm (100mW) power Antennas Self built antenna (first attempt) Wire loop antenna with loop tuner (second attempt)

18 RFID system, contd. RFID tags TI RFID reader

19 RFID system, contd. Original antenna design Low Q loop resonant at MHz T match Hard to match, didn't work well

20 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

21 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 oscillator and PA Use a coax switch to choose between original and new carrier

22 Reverse engineered reader MHz Oscillator CPU Divide by two RS232 Envelope detector Digital RF Power amplifier

23 Modification board

24 Reader modification

25 Testing setup +12 VDC carrier switch +5VDC Reader Second +2VDC carrier Signal source Bias T Spectrum analyzer Ant. -20 cpl (-20 db) tuner DMM Bias Term T

26 Testing setup, contd.

27 Tuning the antenna Antenna is tuned for reflected power. Best tuning achieved, SWR 1.01:1 (not pictured)

28 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

29 Completed system Modified RFID system complete Problems with input for second carrier Problems with reader modifications 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 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

30 Modifying the tag Tag is harder to modify Antenna pattern on tag is aluminum Conductive epoxy is too high resistance Created PCB antenna to transplant tag chip on to Can't solder Solderable Can integrate sensors in more convenient ways Still problems with binding the tag chip to the PCB, initial method caused some reliability issues Possible to use alternative metals to bind sensors to the tag

31 Modifying the tag, contd.

32 Conclusions The state of the project

33 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 required, 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

34 People to thank Professors G.P. Li and Mark Bachman The Li-Bachman research group

35 People to thank, contd. Harrison Chang David Crosley and the RF MEMS team

36 People to thank, contd. Steve Long Kevin Schmidt

37 People to thank, contd. Said Shokair and the UROP office Goran Matijasevic The National Science Foundation