Improved traceability solutions for cheese sector
4. Hybrid traceability solution based on RFID and barcode
4.1. Design technique
A traditional barcode is composed of several parallel bars with varied widths and gaps. A small modification of a barcode can transform it to an RFID tag antenna just by connecting its vertical bars with very thin horizontal stubs as shown in figure 11. By connecting the bars to each other’s, the barcode readability would not be modified. However, in order to have a good RFID read range, impedance matching with the RFID chip and antenna directivity should be optimized.
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Fig. 11. Transforming a barcode to Meander RFID tag by connecting its bars a) Code 128 b) EAN13 code
Impedance matching can be achieved by the optimization of both bars lengths and widths as illustrated in figure 12. While changing the barcode length does not affect its optical readability, the optimization of bars width should be realized carefully else wise the barcode cannot be scanned correctly. The optimization of bars width can be achieved merely by scaling the entire structure in the direction of the width axis and thus the width of all bars and gaps are scaled with the same factor, and the optical readability is maintained.
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Fig. 12. Impedance matching based on optimization of barcode dimensions: Bar length (L) and Bar width (W)
Matching the IC impedance to the antenna is not the only parameter for designing RFID tags. Previous work [5] managed to match a QR code to RFID chip. However, the overall radiation efficiency and directivity of the structure were very low which decreased the read distance to the range of dozens of centimeters. Compared to conventional RFID tags, linear barcodes will have similar omnidirectional radiation pattern as well as a good directivity. Figure 13 shows a comparison between the simulated directivities of a meander tag and barcode tags of the same size using the 3-D full-wave electromagnetic simulation tool HFSS.
The meander antenna has a maximum directivity (at Phi 90 deg) of 1.98 dB whereas the barcode tag has a lightly lower directivity of 1.82 dB.
Fig. 13. Comparison between the simulated directivities of a conventional meander tag and proposed barcode tag design.
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EAN-13, and Code 128 barcodes have been considered to design smart labels as they are the most used barcode standards to label consumer goods as well as in logistics and transportation industries for ordering and distribution worldwide [6]. Barcodes were generated using Microsoft® Office Barcode Add-In where all barcodes are pre-configured according to industry standards. Barcodes are then exported under .dxf format for simulation and optimization with ANSYS HFSS. Analysis frequency band goes from 820 MHz to 920 MHz where reflection coefficient, gain, and read-range are the main characteristics of interest.
Optical barcodes have standard sizes, and thus the tag design should not exceed these limits [7][8]. For EAN-13 barcodes, the recommended standard total width is 31.35 mm with narrow bar width = 0.33 mm. The EAN13 width can be reduced to 80% and can be increased up to 200% without affecting reliable scanning. For Code 128, the narrow bar width can vary from 0.13 mm to 1.02 mm. Code 128 total width is limited by the scanner resolution. Practically one should restrict the number of characters to whatever fits comfortably in a scanner's field of view [9]. The barcode height is recommended not to be less than 13 mm. All tag designs presented in the article respected these limits.
Fig. 14. The realized prototypes on Rogers RO4003™ substrate, Code128 (right) two different sizes of EAN (left).
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Two tags with different sizes based on EAN-13 barcode and a tag based on Code 128 are designed without exceeding the limits of barcodes standard sizes. They were fabricated on Rogers RO4003 substrate of 1.52 mm thickness as shown in figure 14. The dielectric constant and loss tangent of Rogers RO4003 are 3.55 and 0.0027, respectively, at the European UHF RFID operation frequency of 867 MHz.
Figure 15 shows a comparison between the gain of the proposed structure. As the tags are all realized on the same substrate, the main parameter which affects the tag directivity is the size of the antenna. Therefore, Code 128 tag and EAN (large) tag realized relatively higher gain (≈ 0 dB) compared to EAN (Small) tag (≈ - 3 dB) as it has the smallest dimensions.
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All realized prototypes were successfully read using a handheld optical barcode reader. The UHF read range performance of the realized tags was then measured using the Voyantic Tagformance measurement system. This system can provide the backscattered signal strength from the tag under test and thus estimates its read range. Figure 16 presents a comparison between the read ranges measured and simulated for each of the three realized smart labels. A good agreement between measurements and HFSS simulations is achieved.
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EAN Large HFSS EAN Large Measured EAN Small HFSS EAN Small Measured Code128 HFSS Code128 MeasuredFig. 16 Comparison between the read ranges of realized prototypes: HFSS simulations vs. Measurements
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Code128 & EAN-13 (large) achieved a read range of 12.5 and 11 meters respectively. The small configuration design of EAN-13 achieved a lower performance of 8 m. This lack of performance is expected because a tag with smaller size presents a lower directivity reducing thus its read range. These results demonstrate the feasibility of smart labels combining both barcodes and RFID technologies.