Reliability of Passive RFID of Multiple Objects Using Folded Microstrip Patch-Type Tag Antenna Leena Ukkonen*(1), Daniel Engels(2), Lauri Sydänheimo(1), Markku Kivikoski(1) (1) Tampere University of Technology, Institute of Electronics, Rauma Research Unit, Kalliokatu 2, 26100 Rauma, FINLAND Tel. +358-44-534-1507, email:
[email protected] (2) Massachusetts Institute of Technology, Auto-ID Labs 400 Technology Square, 6th Floor, Cambridge, MA 02139, USA Tel. +1-617-253-5970, email:
[email protected] 1. Introduction The increasing use of and reliance upon Radio Frequency Identification (RFID) systems for completely automated object identification has created a set of challenging identification requirements that must be met across a broad array of environments and objects. The operating environments, business processes, packaging and product materials, and packaging configuration all impact the capabilities of an RFID system to identify every object in a particular scenario. This paper concentrates on challenges in identifying multiple objects containing conductive materials with passive UHF RFID technology. Passive UHF RFID systems are composed of three basic components: reader units located where objects are to be identified, tags affixed to the objects to be identified, and data processing systems to utilize the data collected by the readers. Tags, which are the data carrying components of the system, consist of a microchip attached to an antenna, all of which is contained within some packaging to protect the antenna and microchip. Passive UHF RFID tags harvest all of their operating power from the reader's communication signal, and they communicate to the reader by modulating their backscatter [1, 2]. There are three basic types of materials relevant to UHF RFID systems: radiolucent materials, attenuating materials, and reflecting materials. Electromagnetic waves penetrate easily with very marginal attenuation through radiolucent materials. These materials include thin layers of paper, cardboard and most fabrics. On the other hand, certain materials like liquids and moisture-containing materials attenuate the electromagnetic wave [3, 4]. Even bigger challenges for passive RFID at UHF spectrum are metallic objects and objects made of some other conductive material. Conventional tag antennas, such as folded dipoles, do not work well or at all attached to conductive objects. Performance degradations include lower radiation efficiency, shifted resonance frequency and degraded impedance matching. Conductive materials also reflect the electromagnetic wave radiated by the antenna and therefore affect the radiation pattern and radiation directions of the antenna [5, 6]. However, many objects contain or are totally made of conductive material. In this case, the vicinity of conductive materials has to be taken into account in tag antenna design. The negative effects of conductivity can be avoided for example by using tag antenna types that require a metallic ground plane to function or designing the antenna to exploit the highly conductive surface as a reflector. In addition, metallic surfaces can be used in energy harvesting for the tag’s microchip [7]. In this study, we have used a novel folded microstrip patch-type tag antenna in RFID of cigarette cartons, which contain highly conductive aluminium foil. Identification of multiple cartons and the effect of conductive material are studied with read range measurements and the results are compared and analyzed.
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2. The Antenna Design A microstrip patch-type antenna integrated on the cigarette carton was used in the study. This way exploiting the cigarette carton and also cigarette packs as substrate material becomes possible. Also, by using an antenna type that needs a ground plane to function the negative effects of the metallic foil can be minimized [7, 8]. The antenna is designed to resonate at 915 MHz which is the center frequency used in UHF RFID in North America. Figure 2.1 presents the structure and dimensions of the copper-fabricated patch antenna integrated on the cigarette carton. The width W of the radiating patch is 63 mm and its length L is 61 mm. The width of the microstrip feed line is 1.5 mm and the length of both of the two parts of the feed line is 15 mm. The IC chip is attached between the parts of the feed line and its input impedance is 1200 Ω. The ground plane of the antenna is folded over the edge of the carton to improve the energy supply for the microchip and the radiation properties of the antenna. Each cigarette carton contains ten packs of cigarettes in two layers. Cigarettes in a pack are completely wrapped around with aluminium foil that is commonly used also in many other products.
Figure 2.1. Structure and dimensions of cigarette carton and the tag antenna. 3. Identification Tests of Multiple Tagged Cartons Antenna prototypes made of copper were fabricated to evaluate the performance of the antenna design. Antennas were integrated on the carton as shown in Figure 2.1. Figure 3.1 shows an example of an arrangement for identification of a case of cigarette cartons on a conveyor belt. Multiple reader antennas should be used for reliable identification. In Figure 3.1, four reader antennas are used in the identification of the case. The primary goals of the experimental measurements were to determine how many cigarette cartons could be identified, and the maximum range for their identification, using a single antenna. It was assumed that one case contains 60 cartons, and the configuration of the cartons is similar than in Figure 3.1. This kind of case is often used in the practical applications. In multiple object identification, a number of anti-collision algorithms are used in readers. These algorithms enable multiple objects to be read within a very short period of time. Therefore, identification can be seen as simultaneous [9, 10]. Read range measurements of one carton were performed using ThingMagic’s reader unit (Sensormatic, serial number 2001338) and a linearly polarized reader antenna. Testing showed that two tagged cartons could be identified on top of each other when the cigarette packs contained aluminium foil. To study the effect of the aluminium foil on the read range of the
tagged cartons, the measurements were carried out also when the foil was removed from the packs. Figure 3.2 presents the set up of ten tagged cartons in the identification measurements. First, the cartons were pushed by the reader antenna as shown in Figure 3.3 to demonstrate the function of a conveyor belt. The velocity of the cartons was approximately 1 m/s. The goal was to find a distance at which as many cartons as possible were identified when the tagged cartons were pushed by the reader antenna. When the aluminium foils were in place in the cigarette packs, 8 out of 10 cartons were identified at 0.25 m distance. The two missed tags were located in the inner row of cartons located furthest from the reader antenna. If the foils were removed, a third layer of tagged cartons could be added and thereby the number of cartons increased to 15. In this case, all the 15 cartons were identified at 0.17 m. Next, the tagged cartons were pushed at right angles towards the reader antenna to find a maximum reading distance where as many cartons as possible were read. To confirm reliable identification, the cartons had to be read in that distance continuously for one minute. In some cases, the energy supply for the microchip was slightly unstable. Therefore, some tags could not be read continuously at the specified distance. However, despite the short pauses in the identification, all the tags could be read frequently enough to guarantee the identification. When the foils were in place in the cigarette packs, 6-7 out of 10 tags could be read at 0.14 m. Third layer of cartons was again added when the aluminium foils were removed. In that case, 13-14 out of 15 tagged cartons were identified at 0.19 m. In both of these cases, the missed tags were in the row that was located furthest from the reader antenna. These results are achieved with the measurement set up presented previously and they are comparable with each other. However, they are not absolute values since the results may vary if a different set up is used.
Case of cigarette cartons
RFID port with four reader antennas
Conveyor belt Figure 3.1. Example of the case identification on conveyor belt.
Direction of pushing the case Read range
Figure 3.2. Measurement set up of ten cartons.
Figure 3.3. Measurement set up of moving objects.
4. Conclusions A folded microstrip patch-type tag antenna was shown to be capable of identifying objects containing high quantities of reflecting materials. Furthermore, tagged objects located behind large quantities of reflective materials were able to be identified when the tagged objects were configured in their typical case configuration. It was observed that read ranges are shorter when multiple objects are identified. For example, the maximum reliable read range of one tagged carton when the foils are in place in the cigarette packs is approximately 1.0 m when the set up described previously is used. Therefore, the read ranges drop about 80 % when 10 to 15 cartons are read together. However, two tagged cartons with foils inside the individual packs could be identified on top of each other. Also, five cartons side by side were read with no problems because in all the studied cases the first layer of cartons that was nearest to the reader antenna was identified. The tags that were not read were located in the second layer of cartons. If the conductive foils were removed from the cigarette packs, three tagged cartons on top of each other could be identified. This indicates that the tag antenna is very well suitable also for packages not containing any conductive material. The tests showed also that identification was more reliable when the cartons were in motion. This indicates that the tag antenna can be used in reading multiple objects on conveyor belt. Location and configuration of the tags have to be taken into account when multiple objects are identified. Reliable identification of multiple objects that are tagged with the microstrip patchtype tag antenna requires using suitable configuration of multiple reader antennas. References: [1] K. Finkenzeller, “RFID Handbook, 2nd Edition”, John Wiley & Sons, 2003, pp. 11-59 [2] K. V. S. Rao, “An Overview of Back Scattered Radio Frequency Identification System (RFID)”, 1999 Asia Pacific Microwave Conference, Nov.-Dec. 1999, Vol. 3, pp. 746-749 [3] J. R. Reitz, F. J. Milford, R. W. Christy, “Foundations of Electromagnetic Theory”, Addison Wesley 1993, pp. 441-469 [4] D. K. Cheng, “Fundamentals of Engineering Electromagnetics”, Prentice Hall, 1993, pp. 272-294 and 304-330 [5] P. Raumonen, L. Sydänheimo, L. Ukkonen, M. Keskilammi and M. Kivikoski, ”Folded Dipole Antenna Near Metal Plate”, 2003 IEEE International Antennas and Propagation Symposium, June 2003, Vol. 4, pp. 3808-3811 [6] B. R. Foster and R. A. Burberry, “Antenna Problems in RFID Systems”, IEE Colloquium on RFID Technology (Ref. No. 1999/123), October 1999, pp. 3/1-3/5 [7] L. Ukkonen, L. Sydänheimo, M. Keskilammi, M. Kivikoski, ”Development of Novel RFID Tags for Identification of Metallic Objects”, 11th IFAC Symposium on Information Control Problems in Manufacturing, INCOM 2004, April 2004 [8] M. Hirvonen, P. Pursula, K. Jaakkola, K. Laukkanen, ”Planar Inverted-F Antenna for Radio Frequency Identification”, IEE Electronics Letters, July 2004, Vol. 40, Issue 14, pp. 848-850 [9] Auto-ID Labs, Massachusetts Institute of Technology, Technical Report: 860 MHz – 930 MHz Class I Radio Frequency Identification Tag, Radio Frequency and Logical Communication Interface Specification, Version 1.01, 14th November 2002 [10] K. Penttilä, L. Sydänheimo, M. Kivikoski, ”Analysis of Multiple Object Identification with Passive RFID”, 5th International Conference on Machine Automation (ICMA 2004), November 2004, pp. 559-564.
