Encoding of passive anticollision radio-frequency identification surface acoustic waves tags

Transcription

1 Encodng of passve antcollson rado-frequency dentfcaton surface acoustc waves tags Alexander Sorokn Major Problem-Orented Computer Complexes Department, State Unversty of Aerospace Instrumentaton (SUAI) St. Petersburg, Russa E-mal: Alexander Shepeta, Maurts Wattmena Major Problem-Orented Computer Complexes Department, State Unversty of Aerospace Instrumentaton (SUAI) St. Petersburg, Russa E-mal: Abstract Ths paper descrbes the encodng of antcollson rado-frequency dentfcaton tags based on surface acoustc waves. The study s based on the tag model wth specfc topology, whch allows us to receve a response sgnal wth tme frequency nformaton. Ths research consders the collson case for several passve tags. Therefore, the proposal s to analyze the possblty of usng several dstnctve sgns lke frequency and tme. We consder the model of passve surface acoustc wave tag, whch contans pezoelectrc substrate, nterdgtal transducer, and consecutve orthogonal-frequency-coded structures, whch are placed n tme slots. Smlar topology makes possble the relablty of ncreasng tag dentfcaton n the collson case. Durng our research on SAW RFID tags, we combned OFC codng and tme poston codng. II. TIME POSITION ENCODING FOR A PASSIVE SAW RFID TAG MODEL In general, passve SAW tags are represented as the delay lne blocks wth reflectors placed n tme slots [4, 5, 6]. The desgn of such tags s shown n Fg. 1. Keywords RFID, passve tag, antcollson, surface acoustc waves, OFC, SAW I. INTRODUCTION Rado-frequency dentfcaton (RFID) technology, surface acoustc waves (SAW) technology, and the ultra-hgh frequency of SAW RFID devces wth low power allow creatng devces and solvng ssues for dfferent applcatons. These systems allow remote control and detecton of dfferent objects and they can measure vehcle speed and temperature. At the base of RF nstruments s an ntegrated prncple of work, whch s based on pezoelectrc propertes of materals that provde hgh operatonal speed, a hgh level of envronmental frendlness, and safety n comparson wth the exstng systems of dentfcaton [1, 2, 3]. However, collson of the passve SAW RFID tags hnders the development of passve SAW RFID technology n ndustry. The collson problem for passve SAW tags leads to ncorrect dentfcaton and encodng of each tag. When several tags are placed at the same tme n the read feld, the response sgnals cover each other n the tme doman. Ths causes problems n dentfyng and encodng each tag. In ths paper, we suggest an approach for dentfcaton of several passve SAW tags n the collson case. In general, SAW RFID tag readers defne an dentfcaton code, whch depends on the tme delay. The tme delay depends on the tag topology and the reflector s placement on t. A varety of reflector placements n slots determne the number of unque dentfcaton codes. However, n the case when the reader smultaneously nterrogates several passve tags, t ncreases the probablty of concdence of the code groups, resultng n msdentfcaton. Ths case can occur, for example, wth freght cars and goods. Fg. 1. Smplfed model of a typcal SAW RFID passve tag. In ths model, the RFID reader nterrogates the passve SAW tag. In the nterdgtal transducer (IDT), the electromagnetc sgnal s transduced to an acoustc wave, whch propagates along the pezoelectrc substrate (LNbO 3 or LTaO 3). Thus, the response sgnal s represented by tmedelayed pulses. The tme delay between each pulse s determned by the SAW tag topology. The RFID reader processes the receved sgnal and provdes the possblty of gettng the dentfcaton code [7, 8, 9]. It should be noted that the descrbed codng s also called tme encodng for the SAW RFID tag. It relates to dstances between reflectors n the tag s topology. There are several approaches for SAW RFID tag codng: a) The bnary tag dentfcaton code s determned by the locaton of the reflector. In ths case, the presence or absence of a response sgnal determnes one bt of data. b) Tags reflectors are placed n tme slots. There are N slots for each tag total. For each tag, we have N reflectors and only one reflector n each slot. Specfc reflector s poston determnes N possble tme delay varants. Reflector s absence or presence n each slot poston determnes one bt of data. The total number of possble varants of dentfcaton codes s related to passve SAW tag topology lmtatons and tag s sze /17/$ IEEE 605.

2 For ths encodng method, the pulse wdth s approxmately doubled to ensure a clear separaton of adjacent postons that mpulses can occupy [7, 8, 9]. In ths case, each subsequent reflector decreases sgnal ampltude. Fg. 2. Tme encodng for passve SAW RFID tag. In ths case, a start pulse (reference pulse) s needed to provde tmng synchronzaton for the receved data pulses. Each mpulse occupes one of the N possble tme postons. Ths pulse encodes the correspondng data group of two bts. In addton, between data groups were added free of mpulse gaps. Usually, the reader emts an nterrogaton sgnal n the form of a chrp sgnal. The response sgnal and the nterrogaton sgnal are shown n Fg. 3. blocks. The receved response pulses are summed n the Sum block and go to the output. We suggest usng frequency and phase nformaton for SAW RFID tags encodng. It ncreases the unque dentfcaton code numbers and decreases the possblty of ncorrect tag dentfcaton. III. TIME FREQUENCY ENCODING OF PASSIVE SAW RFID TAG WITH OFС STRUCTURES PLACED IN TIME SLOTS The rado-frequency reader nterrogates tags usng the sgnal [10]: 1 2 π 1 N S(t) = h ) h( ) A sn 2 F t (, (1) where T 1 and T 2 are the ntal and fnte tmes of the nterrogaton pulse, A s the pulse ampltude, F s the frequency of the pulse, s the phase of the pulse, and N s the number of pulses for one perod of the nterrogaton sgnal. The tag s response s formed n the IDT n the form of a pulse sequence: 1 N S(t) = h ) h( ) A sn 2 F t (, (2) 1 2 π where T 1 and T 2 are the ntal and fnte tmes of the response pulse. Fg. 3. Interrogaton chrp sgnal and response sgnal (wthout loss): Bold nterrogaton sgnal. The tme codng s used for passve tags lmts capabltes of a SAW RFID system n the smultaneously tag readng case (collson). The topology that was descrbed above s based on the tme delays measurement. It sgnfcantly reduces the number of unque dentfcaton codes for passve SAW tags. The total number of dentfcaton codes s lmted by tag topology and reflectors placement varants n tme slots. In ths way, only tme encodng for passve SAW tags does not solve the collson problem. A model for a passve SAW tag wth tme encodng s represented n Fg. 4. Ths model s desgned n Smulnk and bult as a delay lne [10]. Ths model forms the response of the passve tag. Here, the nput sgnal goes to nput 1. The delay of the response pulses s proportonal to the dstance propagated by the nterrogaton sgnal of the reader to the th reflector, located on the passve tag. The model uses four reflectors. The delay tme of each pulse reflected from the th reflector s set n the Tme Delay Fg. 4. SAW RFID model as a tme delay n the Smulnk. Ths research takes a smple model wth four slots and one reference reflectve structure. Ths model uses tme slots T 1 T 12, as shown n Fg. 5. Fg. 5. Passve tag wth reflectve structures n the slots. In ths model, the poston of the reflectve structure n the tme slot s matched wth a certan frequency; t determnes the bnary code. The frst slot has duratons from T 1 to T 3, second from T 4 to T 6, thrd from T 7 to T 9, fourth from T 10 to T 12. Tme slots T 1 = T 4 T 2, T 2 = T 7 T 6, and T 3 = T 10 T 9, are set between slots contanng reflectve structures [11]. 606

3 The avalablty of tme frequency attrbutes allows the creaton of a tme frequency matrx for tag A, whch s shown n Fg. 6. Specfc passve SAW tag topology allows us to get tme frequency nformaton for codng, whch s represented n Equaton 2. It provdes the possblty to buld a tme frequency matrx n the reader. The reader processes the response sgnals and executes the antcollson algorthm. Fg. 6. Tme frequency matrx for tag A. Thus, usng nformaton about tme and frequency of the frst tag, the RFID reader forms the bnary code A 2 = Zeros fll spacng between slots. Placement of the reflectve structure n a certan poston denotes ether 0 or 1. The reflected sgnal from tag A n the tme doman s shown n Fg. 7. Fg. 9. A smplfed model of the passve SAW RFID tag wth tme frequency nformaton. We consder the collson case between two passve tags A and B, wth the same number of reflectng structures and dfferent dentfcaton codes. Fg. 7. Response sgnal of the passve SAW tag A. Here, T d s the pulse length, T slot s the tme slot boundary, T s s the shft tme of the reflectve structure. Usng tme ntervals and frequences of pulses at the collson stuaton moment allows buldng the tme frequency matrx. Then, shftng both matrces n the tme doman wth a gven step allows them to be separated. Fg. 8. Tme frequency matrx of tags A and B. Smlarly, we obtaned the code of tag B 2 = Formng two tme frequency arrays A and B allows us to create a model of the tags collson. A shft of matrces A and B relatve to each other wth the ptch T slot creates the A and B tag codes. The resultng matrx C that was shfted by one step s shown n Fg. 8. The code bts that trapped the separaton ntervals T 1 = T 4 T 3, T 2 = T 7 T 6, T 3 = T 10 T 9, T 4 = T 12 T 13, determne the place of the reflectve structure of tag B. The presence of a reference pulse makes t possble to separate tags n the gven frequency range from F 1 to F 2. Fg. 10. Collson case for two SAW tags. As s shown n Fg. 10, the frst and thrd structures of tags A and B are placed n the same tme slot places. In ths way, these structures have dfferent central frequences. For the frst tag, we have F 1, F 2, F 3, F 4 and for the second tag F 2, F 3, F 4, F 1. The reader receves reflected sgnals from the passve tags, whch contan nformaton about frequences and tme delays. As a result, the reader creates a matrx, whch s shown n Fg. 11. The reader uses ths nformaton and makes a shft of columns n the tme doman. Ths shft s equal to the T 12 T 1 nterval. Thus, nformaton about frequency and tme delays allows hghlghtng the dentfcaton code of each tag n the collson case. For tag А, the code s For tag B, the code s

4 IV. RESULTS As s shown n Fg. 11, the sgnal receved from the SAW passve tag wth four reflectors comes to the RFID reader. The sgnal comes to the nputs of four parallel-connected band-pass flters. If one of the response pulses has ts frequency matched wth the frequency range of the bandwdth, t comes to the frequency counter and then to the delay detecton unt. Each delay detecton unt determnes the delay between pulses and compares t wth the gven delays matrx n the reader s memory. The presence or absence of a sgnal n the specfc nterm means that the reader wrtes ths nformaton n the matrx n Boolean data format. antenna s nstalled n the lne of sght from the sensors to provde wreless communcaton wth the temperature sensor. The temperature reader s placed n the compartment of the automatc devces. If the sensor temperature exceeds the specfed threshold level, the reader ndcates the event on the LED panel and sends a message to the automaton system by the commutaton relays. The reader transmts the temperature data to the ndustral computer by an RS-485 nterface. The ndustral computer s connected to a local network. In ths case, the operator can vew and analyze the temperature data by web access. Fg. 13. Example of applcaton n a multsensor system for the control of electrcal equpment s temperature. Fg. 11. Model of the SAW RFID system n Smulnk. After shftng of columns, we get two dentfcaton codes. Thus, we can separate the tags from each other usng tme frequency nformaton. Fg. 12. Matrx for tags A and B n the collson case. Ths antcollson dentfcaton approach can also be appled for temperature measurement n the swtchgears and other hghvoltage devces. SAW RFID based on devces allows us to carry out montorng of temperature. Temperature alters correlaton propertes of the tag response. As a rule, response sgnal modulaton allows us to get a temperature value from each tag. As an example, we consder the nnovatve technology of wreless passve temperature sensors, whch have already been used for a real-tme onlne montorng system. Passve wreless temperature sensors are nstalled n the contact pont of the dstrbuton equpment to measure the contact temperature. An In general, such systems use only one sensor for wreless montorng. Our soluton expands the possbltes for RFID wreless temperature montorng systems based on the passve SAW sensors [12]. SAW RFID tags wth tme frequency nformaton provde the possblty of ncreasng number of passve temperature sensors for one swtchgear. In ths case, we could potentally use at least seven tags. Thus, the temperature measurement example explans realworld measurement results. In addton, t llustrates the basc dea of how to realze our soluton n practce. V. CONCLUSION In ths paper, we propose a model that allows solvng the collson problem usng the frequency tme separaton of passve SAW tags. Specfc reflectve OFC structures, whch are placed n tme slots on the surface of the pezoelectrc substrate, make t possble to get a unque dentfcaton code, whch contans tme frequency nformaton. Reflectvty structure postonng and ts placement n the tme slots determne the total number of codes and decrease the possblty of ncorrect dentfcaton. Our approach s based on a combned codng method ncludng orthogonal-frequency codng and encodng of the pulse tme poston. The combned codng method allows us to ncrease the number of tags that are nterrogated smultaneously. Ant-collson algorthm potentalty ncreases the number of dentfcaton tags and any passve wreless smart sensors. The number of varants for placng reflectve structures n tme slots, the number of slots, as well as the technologcal 608

5 capabltes of producng SAW devces and the dmensons of the pezoelectrc materal substrate determne the number of dentfed tags. Our research covers the applcaton of the SAW RFID tag n temperature measurement n the electrcal ndustry. As s shown above, we ncreased the number of possble sensors. ACKNOWLEDGMENT The authors wsh to acknowledge contnung support from RF SAW R&D Center n St. Petersburg, Russa, whch operates wth SAW RFID technology n the smart sensors doman. Ths company s developng smart wreless passve temperature sensors based on SAW RFID technology. The authors express thanks to all the current and past colleagues for ther vared contrbutons to ths research. REFERENCES [1] Jongho Park, Mn Young Chung. Identfcaton of RFID Tags n Framed- Slotted ALOHA wth Robust Estmaton and Bnary Selecton, IEEE Communcatons Letters, 11(5), (2007). [2] EPCglobal, EPC rado-frequency dentty protocols Generaton-2 UHF RFID Specfcaton for RFID ar nterface protocol for communcatons at 860 MHz 960 MHz, verson ratfed (2015). Avalable [3] Jalal, A. S. A. Passve RFID Tags, Wulfena Journal, 22(12), (2015). [4] Harma, S., Plessky, V. P. Surface Acoustc Wave RFID Tags, Development and Implementaton of RFID Technology 1(1), (2009). [5] Plessky, V. P., Rendl, L. M. Revew on SAW RFID tags, Proc. IEEE Trans Ultrason Ferroelectr Freq Control. 57(3), (2010). [6] Harma, S., Arthur, W. G., Hartmann, C. S., Maev, R. G., Plessky, V. P. Inlne SAW RFID Tag Usng Tme Poston and Phase Encodng, Proc. IEEE Transactons on Ultrasoncs, Ferroelectrcs and Frequency Control 55(8), (2008). [7] Bnder, G. SAW Transponder RFID for Extreme Condtons, Deployng RFID Challenges, Solutons and Open Issues, 1(1), (2011). [8] Stelzer, A., Scheblhofer, S., Schuster, S., Brandl, M. Mult- Reader/Mult-Tag SAW RFID Systems combnng Taggng, Sensng, and Rangng for Industral Applcatons, IEEE Frequency Control Symposum, 1(2), (2008). [9] Koygerov, A. S., Dmtrev, V. F. Radomarker on surface acoustc waves wth nose-proof frequency-manpulated code, Proc. Modelng of processes and systems Informaton management systems [10] Sorokn, A. V., Shepeta, A. P., Smrnov, Y. G. Passve antcollson radofrequency dentfcaton tag on surface acoustc waves wth frequency tme dfference, Patent , publshed: [11] Podoplekn, Y. F., Sorokn A. V., Shepeta, A. P. Antcollson passve rado frequency label wth tme frequency nformaton sgns, Marne Rado Electroncs 1(59), 41 (2017). [12] Sorokn, A., Shepeta, A. P. Antcollson rado-frequency dentfcaton system usng passve SAW tags, Proc. SPIE 10246, Smart Sensors, Actuators, and MEMS VIII, (June 2, 2017); do: /