Improvement of Cover Area in Ultrasonic Local Positioning System Using Cylindrical PVDF Transducer

Transcription

1 Improvement of Cover Area in Ultrasonic Local Positioning System Using Cylinrical PVDF Transucer J. M. Villaangos 1, J. Ureña, M. Mazo, Á. Hernánez, C. De Marziani, A. Jiménez, F. Álvarez* Electronics Dept., University of Alcalá. Polytecnics Scool, Ctra. Mari-Barcelona, km 33, Alcalá e Henares, Mari, Spain. Tel: / Fax: *Dept. of Electronics an Electromecanical Eng., University of Extremaura. Spain { 1 villa}{ urena }@epeca.ua.es Abstract Tis work escribes te esign of a reflector for a PVDF-base ultrasonic transucer (Piezofilm transucer to carry out a ultrasonic beacon system, were te covere area be improve an a 3D inoor positioning be guarantee. Te propose Local Positioning System (LPS for Mobile Robots (MR uses simultaneous emissions from ultrasonic beacons. In orer to solve te problem of simultaneous emissions from ultrasonic beacons, te well-known tecnique Direct Sequence Coe Multiple Division Access (DS-CDMA is use. Tis tecnique encoes te ultrasonic signal wit a 17-bit Gol coe for every beacon. It implies te emitte signal by every beacon to ave a wie banwit. PVDF-base transucers suitably guarantee tis requirement. Teir cylinrical or semi-cylinrical sape makes te emission pattern not suitable wen using tem as ultrasonic beacons, often locate in te ceiling of an inoor room. To aapt te emission pattern an to increase te covere area in te groun, te esign process of a conical reflector is escribe. I. INTRODUCTION 3D local positioning systems of mobile robots base on ultrasonic beacon systems are broaly well-known. Tey consist of using several beacons locate at known positions in te environment, were te robot is moving, an of measuring te Time-of-Arrival (TOA of te ultrasonic signal from te emission instant. To syncronize te emission instant of every beacon, a raio-frequency or a coe infrare signal is often use, by selecting also wic beacons are going to transmit [1. Te narrowban transucers use in most existing ultrasonic location systems consier piezoelectric ceramics as active elements. Tese kins of transucers are inexpensive, small, an tey ave a ig sensitivity. However, tey are igly resonant, an in most cases tey provie a usable banwit below 5kHz. Te way tey are built allows teir easy use, but it implies quite irective. Ten it is necessary to place a significant number of tem to ave a wie covere area. Many previous works ave use tis type of transucers wen beacons emit ultrasonic pulses witout encoing, not being necessary a large banwit in transucers. In te Bat system [, users wear small bages wic emit an ultrasonic pulse, raio-triggere by a central system. Te system etermines times-of-fligt for pulses from bages to a network of receivers in te ceiling, an it computes te 3D positions of bages, by using a multilateration algoritm. Te Cricket location system [3 consists of inepenent an non-connecte beacons istribute trougout a builing. Beacons sen an RF signal wile simultaneously sening an ultrasonic pulse. Tere exist several previous works tat ave use te encoing of te ultrasonic signal to implement avance sensors for te etection of obstacles in robotics, by using pseuo-ranom sequences [5 [6, Barker coes [7, or Golay coes [8. Hazas [9 as been te first one using Gol sequences [1. A novel polle location system employs transmitters, receivers an sprea spectrum signalling to allow simultaneous multiple accesses an to provie excellent performance in te presence of noise. Te location of mobile was estimate by measuring times-of-fligt, correlating te receive signals wit te expecte one (a 5kHz carrier moulate by Gol coes. Emitting beacons can be base on Piezofilm transucers. Tese are small, inexpensive, more rugge tan electrostatic transucers, an tey ave a wie frequency banwit. Neverteless, tey present low sensitivity. Tis means tat tey must be riven wit ig voltages as transmitters; an tey are particularly sensitive to noise as receivers. A piezofilm ultrasonic transucer for air ranging as been esigne by Fiorillo [11 [1, an furter caracterize by Wang an Toa [13. It consists of a small, rectangular piece of piezofilm mounte along two eges to form a alf-cyliner. Te emission pattern is not omniirectional enoug wen te beacon is place in te ceiling. Te main goal of tis work is to increase te area covere by a positioning system [14. Te main improvement of te escribe system involves not knowing te emission instant at te mobile robot. So, it is not necessary a syncronism trigger signal (RF, IR, etc. between mobile robot an beacons. To solve te problem of simultaneous emissions from beacons, te DS-CDMA tecnique is use, by moulating a carrier (5kHz /7/$. '7 IEEE 1473 Autorize license use limite to: IEEE Xplore. Downloae on April 9, 9 at 5:15 from IEEE Xplore. Restrictions apply.

2 wit a 17-bit Gol coe, ifferent for every beacon, an by transmitting it in a perioic way. A receiver on-boar te robot carries out simultaneous correlations wit coes assigne to eac beacon, in orer to etect te Time-Differences-of-Arrival (TDOAs among a reference beacon (te nearest an te oters. Tis meto also avois te robot to know te emission instant, because it is only necessary a common syncronism among all te beacons. Te use transucer is Murata Super Tweeter Driver (ESTD1, wic as a wie banwit (1kHz. Altoug it presents an open emission pattern, it is not enoug to ave available a large covere area (see Fig. 1. propose setup; Section IV sow te experimental results, an finally, conclusions are iscusse in Section V. Fig.. Horizontal an vertical beam pattern of PVDF transucer (MSI corporation. II. DESIGN OF A CONIC REFLECTOR A isposition of te transucer as sown in Fig. 3 as been consiere for te esign of te conic reflector. Furtermore, te vertical emission pattern given by te manufacturer as been taken into account. Fig.1. Directivity of Super Tweeter Driver (Murata ESTD1 [15. Te new transucer is manufacture by MSI an it is PVDF type. It as been alreay use in [15, were a new ultrasonic sensor for 3D coorinate estimation as been especially esigne to localize an sketc finings after tey are extracte by arcaeologists. Te system contains two ultrasonic emitters an measures times-of-fligt (TOF: te ultrasonic signal reaces several fixe receivers, an a robust trilateration algoritm to etermine te position of a ro tip wit 1mm accuracy. Te MSI transucer (see Fig. presents an emission pattern completely omniirectional in te orizontal plane. Tis feature makes te transucer optimal for tis application, since all te beacons are place at te same eigt. In orer to use te transucer as an ultrasonic beacon in a LPS, it is possible to couple a conical reflector to obtain a maximum raiation reflecte over te floor. Te work escribes te esign process of te reflector, caracterizing it at pysical level, an analysing te most remarkable features contributions to te loss of signal intensity wit te istance (assuming movements on te surface of te floor. It is organise as follows: Section II proposes te esign of a conic reflector, wereas Section III analyses effects an losses in te 8º Fig. 3. Position of te transucer insie te conic reflector an vertical beam irectivity of te transucer. Regaring te parameters remarke in Fig. 3, is te vertical beam angle of te transucer an is te aperture angle of te conic reflector, were to avoi reflections. If te istance to te vertex of te cone soul be reuce to consier punctual te transucer wit te reflector, te structure can be transforme as sown in Fig. 4. Here, te conical surface beaves as a specular surface for te transucer locate insie. Taking into account tat te new transucer moel is place at a etermine eigt from te floor, te overlapping area appearing in te floor soul be avoie. For tat, it is 1474 Autorize license use limite to: IEEE Xplore. Downloae on April 9, 9 at 5:15 from IEEE Xplore. Restrictions apply.

3 necessary to rotate te vertical emission pattern by an aperture angle, so te extreme axes become parallel. Te new transformation of te structure is sown in Fig. 5, were it can be obtaine (1 an (: beam coming from a circular ring cut by te angle from te vertical emission pattern. Te aspect of a first prototype is epicte in Fig ( º ( / / Fig. 6. Aspect of te prototype: cylinrical PVDF transucer wit conic reflector. Cover Area Superposition Area Fig. 4. Transformation of te transucer consiering te conic reflector as a specular surface. III. LOSS IN THE TRANSDUCER WITH CONIC REFLECTOR Different loss effects ave been analyze wen moving away from te vertical of te sensor on te floor, in orer to evaluate global loss an to etermine ow te receive energy ecreases moving away from te sensor. Te first effect to be consiere is te absorption loss relate to te istance D from te beacon, as sown in Fig. 7. Consiering a typical value of B/m at ambient temperature, te istance attenuation because of absorption is given by (3: / / 8º [ B A 1+ tg (3 Anoter effect is ivergence, wose loss is prouce by two effects: attenuation from te emission pattern H(, an te effect of sperical ivergence A(r ue to te istance D from te surface points of te floor to te sensor transucer: 6 H( (8 6 [ B (4 16 Cover Area Fig. 5. Final result after eliminating te overlapping area. Assuming cm an a eigt from te floor 3m, an optimal cover is practically assure up to te central axis from te vertical emission pattern central axis. Tis cover area, consiere on te surface of te floor, correspons to a circle wit a raius of.5m wose centre is at te vertical of te sensor. Notice tat te raiation on te floor as become a r cos (5 A( r 1log cos ( [ B Were 4º is te axial axis; an H( is parabolically approximate an referre to tis axis. Te losses A(r consiers te incience angle of te beam wit te floor (see Fig Autorize license use limite to: IEEE Xplore. Downloae on April 9, 9 at 5:15 from IEEE Xplore. Restrictions apply.

4 3 m 8º been excite wit a voltage of 4V pp. Te use receiver is a conenser micropone capsule CM16 by Avisoft Bioacoustics. Fig. 1 sows te losses obtaine, referre to vertical axis of te sensor. Te sape of tis experimental attenuation is similar to te teoretical one, existing some ifferences in magnitue ue to efects in te manufacturing of te reflector (aperture angle an size an losses from te material use. It is important to note tat it as been possible to etect te emission up to 5m far away from te vertical axis, remarkably increasing te cover area compare to te isolate PVDF transucer. D 1+ tan 8º º r.5 m (4º 4º Losses in acoustic pression relative to vertical axis of te sensor (B -1 - A ( D 1 + tg 8º [ B Fig. 7. Loss effect by absorption. Te aition of bot effects (Fig. 9 provies tat, up to 6º, a covere area aroun a circle of 5m raius can be guarantee on te surface of te floor wit attenuation levels below 8B, for a eigt of te transucer of 3m from te floor. Taking into account te correlation tecniques an te encoing of te beacon signal, an acceptable ynamic range is available in te receiver to assure a wie covere area wit te esigne reflector. IV. RESULTS Te prototype of te reflector transucer as been place at te ceiling of wie enoug room (wit a eigt of 3m, were some experimental tests ave been carrie out to measure te real covere area by te assembly. Te PVDF transucer as Losses in acoustic pression relative to vertical axis of te sensor (B m r cos r º 6 H ( (8 16 A ( r log [ B ( cos [ B 4º H(+A(r Fig. 8. Loss effect by ivergence Autorize license use limite to: IEEE Xplore. Downloae on April 9, 9 at 5:15 from IEEE Xplore. Restrictions apply.

5 V. CONCLUSIONS A conic reflector as been esigne for a cylinrical PVDFbase transucer tat allows to use tis type of transucers for beacons in ultrasonic LPS. Absorption an ivergence effects ave been stuie for te new resulting transucer moel to obtain te attenuation of te ultrasonic signal on te surface of te floor, aroun te vertical of te transucer. Te reflector provies an increase in te covere area, assuming beacons locate in te ceiling. In tis way, for attenuation below 8B obtaine wit a beam inclination of 6º an a eigt of 3m, te covere area is 85m on te surface of te floor, given by only one transucer. Losses in acoustic pression relative to vertical axis of te sensor (B Fig. 9. Loss effect by sum of absorption an ivergence influence. Attenuation referre to te vertical axis of te sensor REFERENCES [1 S. Aoyagi, M. Takano an H. Noto. Development of Inoor Mobile Robot Navigation System Using Ultrasonic Sensors. Proc. ICV'98, pp.: ,1998. [ A. War, A. Jones an A. Hopper. A new location tecnique for te active office. IEEE Personal Communications, 4(5, pp.: 4-47, [3 N. B. Priyanta, A. Cakraborty an H. Balakrisnan. Te Cricket location support system. Proc. Sixt International Conference on Mobile Computing an Networking (ACM MobiCom,. [4 C. Ranell an H. Muller. Low cost inoor positioning system. Proc. Ubicomp 1 Ubiquitous Computing, pp.: 4-48, 1. [5 K. W. Jörg an M. Berg. Using Pseuo-ranom Coes for Mobile Robot Sonar Sensing. Proc. IAV'98. pp.: 31-36, [6 A. Heale an L. Kleeman. A Sonar Sensing wit Ranom Double Pulse Coing. Proc. Australian Conference on Robotics an Automation, pp.: 81-86,. [7 J. Ureña. Contribución al Diseño e Implementación e un Sistema Sonar para la Automatización e una Carretilla Inustrial. PD. Tesis, Electronics Department, University of Alcalá, [8 A. Hernánez, J. Ureña, J. J. García, V. Díaz, M. Mazo, D. Hernanz, J. P. Dérutin an J. Serot. Ultrasonic signal processing using configurable computing. Proc. 15t Triennial Worl Congress of te International Feeration of Automatic Control (IFAC',. [9 M. Hazas an A. War. A novel broaban ultrasonic location system. Proc. UbiComp : Ubiquitous Computing, pp.: 64-8,. [1 R. Gol. Optimal binary sequences for sprea spectrum multiplexing. IEEE Trans. on Information Teory, IT-13(4, pp.: , [11 A. S. Fiorillo. Design an caracterization of a PVDF ultrasonic range sensor. IEEE Trans. on Ultrasonics, Ferroelectrics, an Frequency Control, 39(6, pp.: , 199. [1 A. S. Fiorillo. PVDF ultrasonic sensors for location of small objects. Sensors an Actuators A- Pysical, 4(1-3, pp.: 46-49, [13 H. Wang an M. Toa. Curve PVDF airborne transucer. IEEE Trans. on Ultrasonics, Ferroelectrics, an Frequency Control, 46(6, pp.: , [14 J. M. Villaangos, J. Ureña, M. Mazo, A. Hernánez, F. J. Álvarez, J. J. García, C. De Marziani an D. Alonso. Improvement of Ultrasonic beacon-base Local Position System Using Multi-Access Tecniques. Proc. IEEE International Symposium on Intelligent Signal Processing (WISP 5, 5. [15 A. R. Jiménez an F. Seco. Precise localisation of arcaeological finings wit a new ultrasonic 3D positioning sensor. Sensors an Actuators A, 13-14, pp.: 4-33, Fig. 1. Attenuation referre to te vertical axis of te sensor, etermine by experimental measurements. ACKNOWLEDGMENTS Tis work as been possible tanks to te Spanis Ministry of Science an Tecnology (RESELAI: ref. TIN CO-1 an Spanis Ministry of Fomento (VIATOR: ref. 75-T Autorize license use limite to: IEEE Xplore. Downloae on April 9, 9 at 5:15 from IEEE Xplore. Restrictions apply.