Research on Positioning Algorithm of Forklift-mounted RFID Reader

Transcription

1 Sensors & Trnsducers, Vol. 7, Issue 7, Jul 0, pp. 6-6 Sensors & Trnsducers 0 b IFSA Publshng, S. L. Reserch on Postonng Algorthm of Forklft-mounted RFID Reder Hongjn Wng, Tngtng Cheng, Zhn-Ao Wu School of Computer Scence nd Communcton Engneerng, Jngsu Unverst, Zhenjng, 03, Chn The 359 th Hosptl of PLA, Zhenjng, Jngsu, 000, Chn Tel.: , f: E-ml: DLhjwng@63.com Receved: 5 Mrch 0 /Accepted: 30 June 0 /Publshed: 3 Jul 0 Abstrct: To conduct rel-tme nd ccurte postonng of the forklfts (people) nd goods n the wrehouse s n effectve mens for mprovng wrehouse mngement effcenc. To ths end, ths rtcle puts forwrd the ctve postonng sstem wth the forklft nstlled wth RFID reder nd the ground pssgew embedded wth RFID tg. In the runnng process, the poston of forklft cn be determned through recognton of the reference tgs whch re embedded t both sdes of the pssgew bsed on the three-sde lout t rght ngle prncple nd ther RSSI vlue. The postonl ccurc cn be mproved b djustng the lout dstnce of those reference tgs. The epermentl results show tht ths sstem cn relze the postonng functon of forklft, nd t cn be used n prctcl stutons. Coprght 0 IFSA Publshng, S. L. Kewords: Forklft ctve postonng, RSSI rngng, Reflecton nterference, Reference tg, Postonl ccurc.. Introducton Wth the contnuous development of modern logstcs ndustr, the goods nd mterls tpes nd quntt re ncresng contnull for the wrehouse mngement. The wrehouse mngement operton s ver complcted nd dversfed. Owng to the low dsptchng mngement effcenc, the trdtonl wrehouse mngement whch s bsed on the pper documents nd s mplemented b mnul operton hs fled to stsf the requrements of speedness nd ccurc of lrge wrehouses nowds nd m serousl mpct the operton effcenc of logstcs ndustr nd becomes bg obstcle to restrct the development of logstcs ndustr. In order to mprove the effcenc of wrehouse mngement sstem, the people nd objects wthn the wrehouse should be postoned n rel tme nd ccurtel. As the prmr operton tool, the rel-tme postonng of forklft s of vtl mportnce. Currentl, the common ndoor postonng technologes nclude nfrred-r postonng, ultrsonc postonng, WLA (Wreless Locl Are etwork) ndoor postonng, Bluetooth, ZgBee, wreless sensor network etc. Among these postonng technologes, some cn onl crr out lne-of-sght propgton, some hve short trnsmsson dstnce, some hve hgh nvestment cost for bottom hrdwre fcltes nd some re es to be dsturbed b other sgnls, therefore, these technologes re not pplcble for postonng the motonl forklfts wthn the wrehouse. Due to the sgnfcnt dvntges of non-contct, hgh ccurc, good sensblt, low cost nd strong envronmentl 6

2 Sensors & Trnsducers, Vol. 7, Issue 7, Jul 0, pp. 6-6 dptblt, RFID (Rdo Frequenc Identfcton) technolog becomes the optmzton n the feld of ndoor postonng nd cn be combned wth WLA to ppl to poston the objects s well s to mprove the operton effcenc of logstcs nd wrehousng. The tpcl RFID Indoor Postonng Sstem bsed on RSSI (Receve Sgnl Strength Indctor) s consttuted b reder, reference tg, trget tg, dt processng unt nd postonng lgorthm whch re shown n Fg.. In the RSSI postonng method, through nlzng RSSI of reference tgs nd trget tgs red b multple reders nd usng the weght thought of the nerest neghbor nd postonng lgorthm to work out the coordnte of trget tg, the poston of trget tg cn be obtned, such s the sstems of SpotO [], LADMARC [], IMPRO_BVIRE [3], Scout [] etc. RSSI postonng method cn dopt the tgs wth low cost to replce the reders to reduce the cost for sstem constructon nd cheve hgh postonng ccurc, but ths method hs the defects such s the reference tg m mpct the postonng ccurc nd the lout denst of reference tgs m ffect the recept sgnl strength of reder nd the trcng pont clculton wll be omtted owng to lrge clculted mount for the processor when mn forklfts re opertng smultneousl wthn the wrehouse. z Fg.. Schemtc dgrm of RSSI postonng lgorthm. To solve the bove problems, ths rtcle proposes n ntellgent wrehouse mngement soluton whch cn offer the movng trjector of forklfts nd goods postonng wthn the wrehouse n rel tme. The rods nd goods shelves or storge rd for opertons of forklft wthn the wrehouse re embedded wth RFID tg (herenfter referred to s rod tg). When the forklfts equpped wth RFID reders nd dt processng module s movng n the wrehouse, the rod tgs nd the sgnl strength ndctor cn be red n rel tme nd the pssed rod tgs durng movng of the forklfts cn be obtned through processng the rod tg nformton nd sgnl strength ndctor nformton, thus gettng the movng trjector of forklft; The sme tgs red n perod of tme b the forklft cn help us deduce the forklft s lodng nd dschrgng the goods here, thus obtnng the poston of goods. The hndlng nformton bout the goods cn be gotten ccordng to ncrese nd decrese of goods tgs on the forklft, thus relzng ntellgent mngement of forklft nd goods wthn the wrehouse. In ths soluton, the dt processng unt cn utomtcll red the rod tgs durng movement of the forklft nd judge the poston of forklft, therefore fter upgrdng, ths soluton cn be ppled to the unmnned forklft to cheve unmnned operton of wrehouse lodng nd unlodng, thus further mprovng the lbor productvt. Ths soluton owns wde rnge of potentl pplctons.. Forklft Trjector Algorthm The clculton of forklft trjector lgorthm wthn the wrehouse s bsed on RSSI rngng lgorthm nd use ndoor sgnl propgton loss model to reflect the reltonshp of sgnl power loss nd trnsmsson dstnce [5]... Clculton of RSSI In the free spce envronment, the power of sgnl receved b the recever cn be epressed s PG t tgrλ Pr ( d) =, () ( π d) where P s the power, G s the gn of ntenn of recever nd trnsmtter, λ s the wve length of sgnl, d s the dstnce between the recever nd trnsmtter. If dbm s the unt, the receved sgnl strength cn be smplfed s ( ) = 0 ( ) +, () P dbm P R 0* n *lg d ζ where P 0 s the usull tken the mgntude of wreless sgnl power vlue receved b the recever when the dstnce between wreless recevng node nd trnsmttng node s d 0 m; n s pth loss nde; ζ s the rndom dsturbnce generted b brrers between recever nd trnsmtter whch s generll ssumed s the Gussn nose wth zero-men vrnce of σ. As the complcted envronmentl structure nd motonl objects ndoors m cuse sgnl reflecton, refrcton nd sctterng, resultng n the nondetermnc of the wreless sgnl propgton nd flng to perform quntttve epresson for the sgnl wth nlss formul. So the sgnl s usull mrked usng the numercl rnge of receved power,.e., receve sgnl strength ndctor, the formul () s rewrote to RSSI s follows: RSSI = A 0* n *lg( r), (3) A RSSI 0* n r = 0, () 65

3 Sensors & Trnsducers, Vol. 7, Issue 7, Jul 0, pp. 6-6 In the bove two formuls, both A nd n re emprcl vlue whch re relted closel to prctcll used reder, ntenn, reference tg nd the communcton envronment of wreless sgnl nd decdes the ccurc of RSSI postonng s well... Determnton of Envronmentl Prmeter Both A nd n relted to envronment cnnot be gven ccurtel, but the cn be vlued s the mens of multple epermentl vlues (the verge vlue of smples replces the lumped prmeter). The followng formul cn be obtned bsed on formul (3): RSSI RSSI n= r 0lg( ), r (5) A = RSSI 0nlg r n= n, A= A, (6) = =.3. Three-sde Lout t Rght Angle Prncple for Pssgew Sgn Reference Tgs Influenced b the ndoor envronment, there m be lrge error of dstnce between reder nd tgs obtned from formul (3). To ncrese the ccurc degree, the reference [6] proposes the RSSI rngng lgorthm bsed on the reference tg nd reference [7] ponts out tht when the postons of three reference nodes cn shpe up n equlterl trngle (equlterl trlterton for short), the postonng error wll be mnml. In the equlterl trlterton, the spce between two lnes of the lgs s tmes of the sde. When knowng the length of sde, t s necessr to clculte ths vlue. Under norml condtons, the rthmetc speed of the mcroprocessor served s the dt processng unt on forklft s slower thn PC. In order to sve clculton tme, when rrngng the reference tgs, three reference tgs should be rrnged to n sosceles rght trngle, nd b now the spce between the two sdes of tgs s equvlent to the rght-ngle sde length of the sosceles rght trngle, thus svng the computton tme. The followng pssgew sgns reference tgs (herenfter referred s pssgew tg) lout prncples re proposed combnng wth the specfc goods shelves szes nd pssgew bredth:. Arrngement of mn pssgew tgs. As shown n Fg., t the put re of goods shelves, when the sum of goods shelves wdth D nd the pssgew wdth between the shelves D s less thn the red rnge R 0 of reder,.e. D +D <R 0 (n most cses, ths condton cn be stsfed), the spce between the pssgew tgs on the mddle pssge s ( D+ D)/ nd the length of mdcourt lne of the sosceles rght trngle s ( D+ D)/; when the mn pssgew wdth D 3 s less thn (D +D ), the dstnce of pssgew tgs from the mn pssge s [ D3 ( D+ D)/]/; f the mn pssgew wdth D 3 s greter thn (D +D ), the rrngement of the pssgew tgs of the mn pssgew s crred out ccordng to shpe of sosceles rght trngle; l D D Fg.. Schemtc dgrm of rrngement of pssgew sgns reference tgs.. Arrngement of pssgew tgs between shelves. The tgs mrked wth shelf number re usull dopted for the pssgew between the shelves nd t s unnecessr to rrnge other pssgew tgs. But the mrkng tgs for shelves should be nterlced to form the sosceles trngles for ever three shelves. After rrngng the pssgew tgs bsed on the bove prncples n wrehouse, the forklft wll red t lest two tgs durng movng. Through the pssgew tgs nd ther poston nformton red b the reder, the locton of forklft cn be obtned quckl... Trjector Algorthm Bsed on Reference Tgs The common-used reder wth UHF usull dopts the ptch ntenn wth bem wdth of bout []. Therefore, durng movement of the forklft (ssume tht t s movng long the center lne of the pssgew nd movng long the rc when turnng the corner), the tg number red b t s mnml when ts ntenn bem just psses pssgew tg whch s the PA pont shown n Fg. 3. Assume the wdth of mddle pssgew D 3 s less thn (D +D ). When the forklft s t PA poston, corresponds to A pont n Fg., the tg t just psses loctes t A pont. As = ( )/, we cn know from the Fg. tht AA 3 = AA 3 = ( D+ D)/, = ( D + D)/ nd AAA 3 = AAA 3 = 5, so 3 6 l D 3 3 = = ( ), (7) 66

4 Sensors & Trnsducers, Vol. 7, Issue 7, Jul 0, pp = 6 + A6A7 = ( ), () 3. Epermentl Results nd Anlss 3.. Determnton of Envronmentl Prmeter PA d PB PC Fg. 3. Schemtc dgrm of tg number red b the forklft when t s movng. To confrm (A, n) of wrehouse envronment, the smulton envronment s constructed on the corrdor of epermentl buldng. Red the correspondng RSSI vlue (verge vlue) of tgs t 0. m, 0. m,, 0 m from the reder respectvel usng ALR-9900 pplcton-specfed reder of Amercn Alen Compn nd substtute nto formul () nd formul (5) to get the vlue of (A, n) nd estblsh the rngng model for postonng of forklft n the wrehouse RSSI = 3 0lg( r), () A A 7 A A 0 The comprson of RSSI vlue of tgs clculted ccordng to formul () nd the ctul mesured vlue s shown n Fg. 5. A 3 A 6 A 9 A A A 5 A RSSI mesured vlue A=-3,n= rngng model Fg.. Dstnce reltonshp of forklft wth the tgs when t loctes t PA. The formul (9) cn be obtned n smlr w: 6 = ( ), 50 = ( ) (9) The poston A of the forklft loctes t ( ) D + D bckwrd of poston of reference tg A,.e. PA = A ( ), (0) PA = A When pssgew tg of A 7 pont s wthn the coverge rnge of reder bem, the poston of the forklft s deemed to be t A pont, therefore the mmum vlue of postonng error s AA 5 D + D =, () Menwhle, when the reder on the forklft red the pssgew tgs of A, A 7, A successvel, we cn determne the forklft s movng long the center lne of pssgew. RSSI/dBm r/m Fg. 5. Comprson of RSSI vlue computed b model nd the ctul mesured vlue. We cn see from the fgure tht wthn rng less thn 6 m, the vlues re bscll dentcl, nd therefore ths rngng model cn determne the tgs whose dstnces from the reder re less thn 6 m. 3.. The Redble tg umber Durng Movement of the Forklft Bsed on the rrngement prncples for pssgew tgs n.3, ssume the onwrd forklft s used n the wrehouse s the mn tools for plng up the goods. Accordng to the dstnce requrement of pssge wdth whch should not be less thn 3 m, the mult-ler shelves re generll put wth the objects wth the wdth less thn m. The effectve red rnge of reder wth UHF s generll 6 m. Thus the prmeter vlues n.3 re: D = m, D =3 m, D 3 =3 m, R 0 =6 m, then 67

5 Sensors & Trnsducers, Vol. 7, Issue 7, Jul 0, pp = ( ) =.9m 6, = ( D+ D) = 3.9 m< 6m 50 = ( ) =.m (3) We cn know from formul (3) tht f the red rnge s 6m, the reder s ble to red more thn 3 tgs ever tme Movng Trjector Descrpton To verf the bove trjector lgorthm, n meetng room wth bout 5 m *37 m, ppl the meetng room to smulte wrehouse pssgew to rrnge the tgs t ntervl of.5 m. A person holdng the reder wll replce the forklft. Accordng to the tg ID nd RSSI vlue red b the reder, the person s wlkng drecton cn be obtned nd the nerest tg ID cn be found s well. Adopt formul (0) to clculte person s poston, thus obtnng the movng trjector of the person whch s shown n Fg. 6. mportnce to the vlue of network mrketng. Fst development of the logstcs ndustr nd ncrese of the lrge wrehouses leds to the rse of goods enterng nd dschrgng from wrehouse surges. To gurntee the effcent operton of wrehouse, t s necessr to poston the people nd goods wthn the wrehouse n rel tme. To dpt to modern fst pce nd reduce errors nd worklod of mnul operton, ths rtcle proposes the ctve postonng sstem for forklft. When the forklft psses the reference tg embedded n the pssgew ground, the tgs nd RSSI vlue wll be red. Through the poston of reference tg nd clculton, the current poston of forklft wll be obtned, thus gettng forklft trjector n the wrehouse. The ctve postonng sstem of forklft cn provde the poston of goods lodng nd dschrgng, thus gettng the poston for puttng the goods. Then ccordng to RFID tgs of goods, the worklod of goods lodng nd dschrgng cn be obtned, relzng the utomtc sttstcs of quntt nd poston of the goods enterng nd dschrgng from the wrehouse, nd reducng lot of mnul sttstcs work nd the losses cused b humn errors, mkng t possble to mprove the work effcenc. References Fg. 6. Comprson of ctul wlkng pth nd the computed pth. The Fg. 6 s comprson drwng of the movng trjector of reder (people) clculted through lgorthm nd the ctul wlkng trjector. We cn see from the fgure tht the ll clculted nodes re wthn the pssgew rnge nd the error comprng wth the ctul wlkng pth s smll, mrkng t better to relze the postonng of people.. Conclusons Wth the speed development of nternet, the common people hs recognzed nd ttched []. Lu, H., Drb, H., Bnerjee, P, et l, Surve of Wreless Indoor Postonng Technques nd Sstems, IEEE Trnsctons on Sstems, Mn, nd Cbernetcs, Prt C: Applctons nd Revews, Vol. 6, Issue 37, 007, pp []. Lonel M., Abhshek Ptl, Yunho Lu, Yu Cho Lu, LADMARC: Indoor Locton Sensng Usng Actve RFID, Wreless etworks, 00, pp [3]. L Peng, M Lng, Yng Yongjun, Boundr vrtul reference rg mproved lgorthm bsed on RFID, Applcton Reserch of Computers, Vol. 30, Issue 03,pp []. Hung X, Jnswm R, Gng A, Scout: outdoor loclzton usng ctve RFID technolog, n BROADETS 006, n Proceedngs of the 3 rd Interntonl Conference on Brodbnd Communctons, etworks nd Sstems, Sn Jose, CA, -5 Oct, 006, pp [5]. Chen Yuq, The Reserch nd Applcton for RFID Locton, Gungdong Unverst of Technolog, 0. [6]. Wng Q, Reserch on n Indoor Postonng Technolog Bsed on RSSI Rngng, Electronc Sc. & Tech, Vol. 6, Issue 5, 0, pp [7]. Zho Juun, Reserch on ndoor locton lgorthm of rdo frequenc dentfcton bsed on RSSI, orth Unverst of Chn, 0. []. Alen Technolog, Gude, Hrdwre Setup, ALR pdf Coprght, Interntonl Frequenc Sensor Assocton (IFSA) Publshng, S. L. All rghts reserved. ( 6