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22 Tag Movement Drecton Estmaton Methods n an RFID Gate System Yoshnor Okawa NEC TOKIN Corporaton Japan 1. Introducton An RFID system s desred to be ntroduced n large gate management systems because t can read the ID of a large number of target objects smultaneously n the feld of logstcs and retal busness. Especally, UHF RFID has gathered sgnfcant nterest snce t has the advantage of long dstance readng and low cost of tags. Customers usng an RFID gate system requre several convenent functons. One of them s to know the tag movement drecton for the purpose of recognton n warehousng or shpment for nventory management. Moreover t can check for undesrable objects or prevent theft. For ths purpose, some sensors are establshed at the entrance and the exst sde of the gate system n an exstng system. Therefore the drecton of movement of tags s judged by the tme dfference n the passng tme at these sensors. For example, the future store of the Metro group used ths gate system for ther stock management system of the backyard system [1]. However, n these systems t s necessary to use optonal expensve equpment such as several sensors. In ths chapter, an effectve tag movement drecton detecton method s proposed n whch an orgnal tag communcaton system s used as much as possble wthout usng optonal equpment. 2. Estmaton methods of the RF tag movement drecton It s bascally necessary for the judgment of tag movement to obtan two or more tme nformaton of an object. For obtanng that nformaton, t s common to use two sensors on both sdes of the gate. Ths method corresponds to the Range-based method, whch s a locaton allocaton system (LAS) method usng a fxed anchor [2][3][4][5]. A conventonal RFID gate system usng photoelectrc sensors s shown n Fg.1. Ths gate can detect the movement drecton of an RF tag by judgng the dfference between the two passng tmes at each sensor. For example, because the RF tag moves from the left sde to the rght sde n the case of Fg.1, sensor 1 detects t n advance of the detecton at sensor 2. Here, a new method of applyng the Range-free method to RF tag drecton detecton s proposed.
442 Current Trends and Challenges n RFID Tag Antenna Sensor1 Sensor2 Reader Fg. 1. Conventonal RFID gate system 3. Proposed methods 3.1 Basc prncple Detecton measures for measurng the tme dfference are consdered for the RF tag and reader antennas. A double antenna method usng two antennas s proposed. The confguraton of ths method s shown n Fg.2. The basc algorthm s that the tag movement drecton s estmated by measurng the tme dfference of two antennas. The mert of ths method s that the drecton of each tag can be estmated ndependently. The conventonal sensor system can detect only for the bulk n the case of many tags. The proposed method can estmate the movement drecton for each tag even f some tags move n the opposte drecton toward the other tags smultaneously. t c x=v t=t c + t v m/s xa t Target tag D d Antenna 1 Antenna 2 Reader Fg. 2. Double antenna method 3.2 Attrbutes for estmaton The types of nformaton obtaned from a tag s the read count, receved power and transmsson delay. In ths chapter, the former two types of nformaton are studed because they are smpler than the last one. Three methods are consdered for judgment of the detecton tme. They are (1) tag read tme, (2) the tme over the preset threshold, and (3) total judgment that consders the detecton pattern or weghted tme. In the case of usng the tme sequence pattern n the thrd method above, the processng functon s very heavy because of complcaton of ts algorthm. That does not match the phlosophy of Range-free. Therefore, n ths chapter, a weghted tme center method for the thrd method s proposed. Each method s shown n Table 1.
Tag Movement Drecton Estmaton Methods n an RFID Gate System 443 Attrbute Method (a) Read count (n) (b) Recved power (P r ) (1) Tag read n 1 - (2) Threshold n Th1 P Th r 2 (3) Weghted center (n t ) n (P t ) r P r Table 1. Decson crtera of detecton tme 3.3 Basc model regardng receved power A basc model of an RFID system s shown n Fg. 3. The receved power of a tag (chp) P tr and the receved power of a reader P r are as follows usng Frss s formula [6]. P P G L G (1) tr t rt a tr P P G L G L G L G r t rt a tr m tt a rr P G G G G 2L L t rt rr tr tt a m (2) 4 L 2log d a Here, G rt and G rr s the transmsson gan and receved gan of the reader antenna, G tt and G tr s the transmsson gan and receved gan of the tag antenna, L m s the nternal loss of the tag, L a s the propagaton loss n the ar, d s the read dstance, λ s the wavelength. Generally, the antenna of an RFID system can be used for both transmsson and recepton. Therefore, let G r =G rt =G rr, G t =G tt =G tr, then eq. (1) and eq. (2) are P P G G L (4) tr t r t a r t r t a m P P 2 G G L L (5) Measurement results of P r n the case of P t =1W(3dBm), G r =6dBC(crcular polarzaton antenna), G t =dbl(lnear polarzaton antenna) are shown n Fg.4. Ths shows that the results are the same as the calculated values. Snce the tag nternal loss L m depends on vendor or nput level, the value of the actual used tag chp s appled. Fgure 4 shows that the dstance (read range) between the reader and the tag can be approxmately estmated by measurng P r. In eq. (4) and (5), P r s a maxmum when the tag s just n front of the reader antenna. However, P r decreases as the tag moves nto farther from the center of the antenna because of ts drectonal loss. Measurement results and (3)
444 Current Trends and Challenges n RFID calculated values of P r vs. the dstance x between the center of the reader and tag are shown n Fg.5. From Fg.5, the tag s nearest pont (x=) to the reader can be estmated. Reader Antenna Antenna Tag G rt G tr P t P tr Modulaton crcut L m P r G rr G tt P tt Read range: d Propagaton loss: L a Fg. 3. RFID system model 4 Receved power P r (nw) 35 3 25 2 15 1 5 M easured Calculated.2.4.6.8 1 1.2 1.4 Read range d P t =3dBm G r =6dBC G t =dbl (m) Fg. 4. Read range vs Receved power 3.4 Comparson of detecton methods 3.4.1 Method 1 In Method 1, the startng tme to read a tag s detected as shown n Table 1(1) even f read only one tme. In an actual RFID system, because tags are nventored n advance of readng the tag, the nventory tme can also be used. Ths method s so smple. However, t s hard to ncrease the decson accuracy snce t sometmes happens to nverse the sequence of the read tme of the two antennas.
Tag Movement Drecton Estmaton Methods n an RFID Gate System 445 8 P r (nw) 7 6 5 4 3 M easured Calculated 2 1-1 -.5.5 1 Dstance from the antenna x (m) -1.4-1 -.6 -.3.9.45.81 1.17 Fg. 5. Measurement result (x vs P r ) 3.4.2 Method 2 Incorrect judgment sometmes occurs due to a passng read for a reflected RF wave n the case of Method 1. Method 2 uses the threshold of detected values and judges the drecton usng the tme dfference between each tme when the detected value s over each threshold as shown n Table 1(2). Ths method s able to ncrease the accuracy of detecton. However, t s sometmes hard to decde the threshold because the read count depends on the speed of movement and the receved power depends on the dstance between the reader antenna and the tag. 3.4.3 Method 3 Method 3 s proposed for mprovement of the two methods,.e. preventon of tentatve read error caused by the nfluence of reflecton or null ponts. The prncpal of ths method s to estmate the tme of the tag s nearest poston from the reader antenna. Wlson has proposed the method for localzaton usng the passve tag count percentage [7]. In ths approach, tags can be estmated the closest poston by detectng the peak pont. However, t s dffcult to adopt ths method as RFID gate system because the varaton of detected value reaches up to several tens of meters and s equvalent to the dstance between two antennas. Therefore the algorthm we proposed s that each read tme s weghted by the read count n or receved power P r, and the tag drecton s estmated by the calculated dfference between two weghted centers of two antennas. Recently, RFID readers become to have hgh-performance receved power detecton functon [8]. Therefore, here, ths method wll be explaned usng the receved power as the tag attrbute. Fgure 6 shows the judgment procedure of the three methods. The detaled detecton method s explaned n Method 3. The receved power s a functon of tme actually because the tag goes through at a speed of v (m/s). Eq.(5) s shown as eq.(6) from Fg.2 and Fg.5. Δt n Fg.2 s the tme deference between the passng tme at the front of the reader antenna (t c ) and the present tme (t).
446 Current Trends and Challenges n RFID Method 1 Method 2 Method 3 6 5 Judge T2-T1>: ANT1ANT2 T2-T1<: ANT1ANT2 T1 T2 T1 T2 T1 T2 Decson of T1and T2 Read count (n)>1 Pr>Th (Pr. t) Pr ANT 1 ANT 2 Pr (nw) 4 3 2 Th 1.5 1. 1.5 2. 2.5 3. Relatve tme t (s) 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Fg. 6. Three methods n double antenna method P (t) P 2 G (t) G (t) L (t) L (6) r t r t a m The estmaton procedure s as follows. When the certan tme before the reader starts to read tags put t, weghted center of read tme t w1 (t k ) and t w2 (t k ) from tme t to tme t k of antenna 1 and antenna 2 are t (t ) k w1 k k P(t)t r1 P(t) r1 (7) t (t ) k w2 k k P (t ) t r2 P (t ) r2 (8) where P r1 (t) and P r2 (t) are the receved power of the two antennas at tme t. In the eq.(7) or (8), when t w2 (t k )-t w1 (t k )>, t s judged that the tag moved from antenna 1 to antenna 2, and when t w2 (t k )-t w1 (t k )<, t s judged that the tag moved from antenna 2 to antenna 1. The calculated results n the case of Fg.6 s shown n Fg.7. When t w1 and t w2 n the case of stable values after the elapse of a certan perod of tme put T1 and T2, respectvely, the tag drecton s fnally judged by T2-T1 as shown n Fg.6.
Tag Movement Drecton Estmaton Methods n an RFID Gate System 447 Weghted center (t w1, t w2 ) (s) 3. 3 2.5 25 2. 2 1.5 15 1. 1.5 5 t w1 (t) t w2 (t) t w2 (t) - t w1 (t) 2.5 4 6 8 1. 1 12 14 1.516 18 2. 2 22 242.526 28 3. 3 Relatve tme t (s) (T2) (T1) Fg. 7. Shft wth tme of t w1 and t w2 n Method 3 Measurement results and the expermental envronment usng 1 dense tags are shown n Fg.8 and Fg.9. Measurement condtons are shown below. P t =3 dbm, G r =6 dbc, G t = dbl, D=9 cm, xa=6 cm, v=1 m/s, heght of antenna=1.3 m, data rate=8 kbps, Reader: NEC TOKIN (Speedway) Tags: UPM Raflatac ShortDpole movement drecton: from antenna 1 to antenna 2 (T2-T1>) Because the dstance between two antennas that are the same type s 6 cm, T2-T1 becomes.6 seconds n theory. There are occasonal erroneous decsons because of reflecton or nterference n severe measurement envronment, whch causes undesrable readng n method 1, and tags placed n the mddle (e.g. tag #3, #4, #7 and #8 n Fg.8) are hard to read n method 2. On the other hand, method 3 s very stable because t s not msjudged, has low devaton and a desrable average. Fgure 1 shows the tme transton of the dfference t w2 -t w1 n method 3. We can see ths method can obtan a stable and correct result (expectant value n the case of Fg.1 s.6s) even n the case of msjudgments caused by reflecton and nterference n the measurement stage. 4. Measurement results n Method 3 4.1 Detecton of the tag drecton The detal performance of Method 3 was measured. Fgure 11 shows the tag read counts and tme dfference T2-T1 n the case of two methods. Though the devaton s wder n the case of a low read count, the judgment result s plus n pattern 1, and mnus n pattern 2. Therefore t has enough stablty for use as an actual tag drecton decson tool. Pattern 3 shows the results n the llegal case assumng turnng back n the center of the antenna. In ths case, the expectaton value s. Fgure 12 shows the summary of means m and devaton σ of the measurement results of Fg.11. By the way, an RFID system needs ant-collson technology that prevents no-read stuatons caused by collson when many tags are read smultaneously. The sequence to read tags s
448 Current Trends and Challenges n RFID 15cm Tag array #5 #4 #3 #2 #1 #6 #7 #8 #9 #1 4cm 1.5 1..5. m:.54 :.32 -.5 -.5 1 2 3 4 5 6 7 8 9 1 11 1.5 1..5. 1 2 3 4 5 6 7 8 9 1 (1) Method 1 m:.67 :.31 -.5 -.5 1 2 3 4 5 6 7 8 9 1 11 1.5 1.5 1. 1..5.5. 1 2 3 4 5 6 7 8 9 1 (2) Method 2 m:.63 :.1 -.5 Fg. 8. Dfferent tme between two antennas -.5 1 2 3 4 5 6 7 8 9 1 11 1 2 3 4 5 6 7 8 9 1 (3) Method 3 Calculated Sample number: 1@tag
Tag Movement Drecton Estmaton Methods n an RFID Gate System 449 Tag array Antenna Reader 15cm #5 #4 #3 #2 #1 #6 #7 #8 #9 #1 4cm Dsplay Tag dentfcaton (EPC code) Tag movng drecton Tag count per each antenna Fg. 9. Photograph of expermental envronment 1. t w2 -t w1 (s) Calculated.5. -.5-1...3.5.8 1.1 1.4 1.6 1.9 2.2 2.5 Relatve tme t (s) Fg. 1. Relatve tme vs (t w2 -t w1 ) n method 3 1 2 3 4 5 6 7 8 9 1
45 Current Trends and Challenges n RFID Movng pattern: 1 2 3 ANT1 ANT2 8 7 Count number 6 5 4 3 2 1 + 2 1 1 2 3 4 5 6 7 8 9 1 1 2 3 4 5 6 7 8 9 1 (1) 8kbps (2) 64kbps 1.5 1..5 -.5-1. 1 3 2-1.5 1 2 3 4 5 6 7 8 9 1 1 2 3 4 5 6 7 8 9 1 (3) 8kbps (4) 64kbps Sample number: 1@tag Fg. 11. Relatve tme vs (t w2 -t w1 ) n method 3 random because a typcal ant-collson system s used for usng the probablstc approach [9]. A varaton of tag read sequence drectly becomes a valdaton of detecton tme dfference. Therefore, when a weghted center s normally-dstrbuted, the tme dfference T2-T1 s also ndependent and dentcally dstrbuted because of ts reproducng property. From Fg.12, t s assumed that the crtera of detecton precsely s 3σ or less, and the tag drecton can be judged correctly from the data of pattern 1 and pattern 2. However, a data rate up to round 64kbps s necessary when the dfference from abnormal acton such as turnng needs to be detected (pattern 3 n Fg.12).
Tag Movement Drecton Estmaton Methods n an RFID Gate System 451 : Average (m) : m-(t2-t1) < 3.9.6.3 -.3 -.6 -.9 Movng pattern Data rate 1 2 3 1 2 3 8kbps 64kbps Fg. 12. Measurement results of (T2-T1) 4.2 Estmaton of the tag movng speed The detal performance of Method Moreover, the speed of movement can be also estmated by measurng the tme dfference T2-T1 because the dstance between two antennas s fxed. Fgure 13 shows the measurement results of the movement speed. Varaton of measurement results n the case of v=2m/s s larger than n other cases because the precse speed s nversely proportonal to the speed of movement of the measurer. Fgure 13 shows that ths method can estmate not only the tag drecton but also the speed of movement. It s very useful to set the threshold Th of movement speed as the decson crtera n order to ncrease the accuracy. For example, when the threshold Th1 and Th2 are set to -3.5 and 3.5 respectvely, t s possble to elmnate abnormal movement such as turnng n Fg.13. Movng speed (m/s) >5 5 4 3 2 1-1 -2-3 -4 <-5 m:.59 :.8 m:-.51 :.8 m:.93 :.8 m:-.94 :.13 Fg. 13. Measurement results of movng speed m:2.9 :.4 m:-2.2 :.39 (1).5m/s (2) 1m/s (3) 2m/s (4) U turn - -
452 Current Trends and Challenges n RFID 4.3 Effect of the orentaton of the tag Generally, a tag are used a lner polarzed dpole antenna n consderaton of read range and cost. In ths case, the read performance n reader depends on the orentaton of the tag. The tag movement detecton results of the tme dfference T2-T1 n three cases s shown n Fg.14. Angle of tags (1) (2) (3) 1.5 ANT 1. 1..5.5. -.5 -.5 1 2 3 4 5 6 7 8 9 1 11 1.5 1 2 3 4 5 6 7 8 9 1 (1) Angle of degrees 1..5. -.5 -.5 1 2 3 4 5 6 7 8 9 1 11 1.5 1 2 3 4 5 6 7 8 9 1 (2) Angle of 45 degrees 1..5. -.5 Fg. 14. Dfferent tme between two antennas -.5 1 2 3 4 5 6 7 8 9 1 11 1 2 3 4 5 6 7 8 9 1 (3) Angle of 9 degrees Calculated Sample number: 1@tag
Tag Movement Drecton Estmaton Methods n an RFID Gate System 453 T2-T1 of the tags that have 9 degrees angle aganst the reader antenna ((3) n Fg.14) vares wdely because they are hard to be read. The percentage of read n ths case was 79% and the accuracy among tags to be read was 95%. However, when tags set 45 degrees angle, the movement drecton of tags can be detected wth as hgh accuracy as a parallel case ((1) n Fg.14). In other words, t s useful to tlt two antennas of the reader n place of tags. 4.4 Effect of the ntersecton of the tags In actual cases, t may happen that two tag groups pass through n the opposte drecton ndvdually and smultaneously. The measurement results n that case are shown n Fg.15. 1 2 ANT1 Smultaneously Reader 9cm ANT2 15cm Movng drecton: Both way (cross n the center) 1 2 #5 #4 #3 #2 #1 #15 #14 #13 #12 #11 Tag array #6 #7 #8 #9 #1 #16 #17 #18 #19 #2 Count number 8 7 6 5 4 3 2 1 1.5 1..5 -.5-1. -1.5 1 2 1 2 12345678911112131415161718192 12345678911112131415161718192 12345678911112131415161718192 12345678911112131415161718192 (1) Data rate: 8kbps (2) Data rate: 64kbps Sample number: 1@tag Fg. 15. Measurement results n smultaneous cross movng
454 Current Trends and Challenges n RFID One tag group (#1-#1) passed through from antenna 1 to antenna 2, and the other group (#11-#2) passed through n the opposte drecton behnd the former group. Tag group (#1- #1) has the same characterstcs as n Fg.11. However, tag group (#11-#2) s strewn wdely because the rado wave s blocked by the other tag group n passng n front of the reader antenna. In the case of 8kbps data rate, 14% of ths tag group could not be read and around 5% among all the read tags made an error (that s, the accuracy was about 95%). However, when the data rate s 64kbps, both of the read rate and the accuracy are 1%. Therefore, ths method s useful because the tag movng drecton can be detected correctly by ncreasng the data rate even f the most severe case lke ntersecton n front of the antenna. 5. Concluson In ths chapter, a method for precsely estmatng the tag movement drecton n an RFID gate system was proposed. Ths method uses the tme dfference between two antennas of the reader. Ths method has the advantage of beng able to judge tag drecton ndvdually even when there are some tags movng to the reverse drecton. Especally, when t uses the proposed algorthm of the weghted center of passng tme, the precson of the estmaton can be ncreased. Fnally, the feasblty of the method was proved by measurement results. 6. References [1] Metro Future store http://www.rfdjournal.com/artcle/vew/889 [2] H. Och, S. Tagashra and S. Futa, A localzaton system for wreless sensor networks, IPSJ SIG Tech. Rep., ARC-16, pp.17 22, Dec.24 [3] T. He, C. Huang, B. John, A. Stankovc and T. Abdelzaher, Range-free localzaton schemes for large scale sensor networks. Mobcom, September 23, pp.81 93 [4] J. Hghtower, G. Borrello and R. Want, SpotON: an ndoor 3D locaton sensng technology based on RF sgnal strength UW CSE Tech. Report #2-2-2, February 2 [5] L. N, Y. Lu, Y. Lau and A. Patl, LANDMARC: ndoor locaton sensng usng actve RFID Wreless Networks 1, pp.71 71, Kluwer Academc Publshers, Netherlands, 24 [6] T. Yoshkawa, Rado engneerng B, Tokyo Denk Unversty [7] P. Wlson, D. Prashanth and H. Aghajan, Utlzng RFID sgnalng scheme for localzaton of statonary objects and speed estmaton of moble objects Internatonal conference on RFID, pp.94 99, March 27 [8] Y. Okawa, UHF IC tag and reader/wrter products NEC Tech. Journal, vol.2, No.4, pp.76 8, December 27 [9] Y. Kawakta, J. Mtsug, O. Nakamura and J. Mura, Acceleraton of UHF-band RFID nventory leveragng capture effect. IEICE, vol.j91-b, No.1, pp.1279 1286, October, 28
Current Trends and Challenges n RFID Edted by Prof. Cornel Turcu ISBN 978-953-37-356-9 Hard cover, 52 pages Publsher InTech Publshed onlne 2, July, 211 Publshed n prnt edton July, 211 Wth the ncreased adopton of RFID (Rado Frequency Identfcaton) across multple ndustres, new research opportuntes have arsen among many academc and engneerng communtes who are currently nterested n maxmzng the practce potental of ths technology and n mnmzng all ts potental rsks. Amng at provdng an outstandng survey of recent advances n RFID technology, ths book brngs together nterestng research results and nnovatve deas from scholars and researchers worldwde. Current Trends and Challenges n RFID offers mportant nsghts nto: RF/RFID Background, RFID Tag/Antennas, RFID Readers, RFID Protocols and Algorthms, RFID Applcatons and Solutons. Comprehensve enough, the present book s nvaluable to engneers, scholars, graduate students, ndustral and technology nsders, as well as engneerng and technology afconados. How to reference In order to correctly reference ths scholarly work, feel free to copy and paste the followng: Yoshnor Okawa (211). Tag Movement Drecton Estmaton Methods n an RFID Gate System, Current Trends and Challenges n RFID, Prof. Cornel Turcu (Ed.), ISBN: 978-953-37-356-9, InTech, Avalable from: http:///books/current-trends-and-challenges-n-rfd/tag-movement-drecton-estmatonmethods-n-an-rfd-gate-system InTech Europe Unversty Campus STeP R Slavka Krautzeka 83/A 51 Rjeka, Croata Phone: +385 (51) 77 447 Fax: +385 (51) 686 166 InTech Chna Unt 45, Offce Block, Hotel Equatoral Shangha No.65, Yan An Road (West), Shangha, 24, Chna Phone: +86-21-6248982 Fax: +86-21-62489821