Radio frequency identification tag antenna for metal objects

Abstract

An RFID tag antenna for installation on a metal object includes a dielectric substrate having a first surface and a second surface opposite the first surface, a conductive layer disposed on the first surface of the dielectric substrate, the conductive layer configured to be attached to the surface of the metal object, an integrated circuit disposed on the second surface of the dielectric substrate, an antenna structure formed on the second surface of the dielectric substrate, a T-match structure formed on the second surface of the dielectric substrate, the T-match structure electrically coupling the antenna structure to the integrated circuit, a first via pin that passes through the dielectric substrate and electrically connects a first point on the antenna structure to a corresponding first point on the conductive layer, and a second via pin that passes through the dielectric substrate and electrically connects a second point on the antenna structure to a corresponding second point on the conductive layer.

Description

[Technical Field]

[0001] The present disclosure relates to radio frequency identification (RFID) tags, and more particularly to RFID tag antennas for installation on metal objects. [Background technology]

[0002] Nowadays, the rapid advancement of radio frequency identification (RFID) technology has attracted attention. Due to the long-distance identification capability of RFID tags in the ultra-high frequency (UHF) band, RFID tags have been widely implemented in supply chain management, inventory checks, asset tracking, gate automation, etc.

[0003] However, many applications require RFID tags to be placed on conductive metal objects such as automobiles, cylinders, containers, weapons, and equipment. When a regular UHF tag is placed on a metal surface, its read range is significantly reduced due to impedance mismatch, reduced radiation efficiency, and poor directivity. Therefore, so-called anti-metal tags must be specially designed to overcome these challenges.

[0004] Previously, tags for the UHF RFID band based on microstrip patch antennas with their own ground planes have been proposed. However, this resulted in very large antenna sizes, with resonant lengths approaching a quarter wavelength, making them difficult to install on small metal objects. Many dipole- and foldable-type tags have been designed to achieve miniaturization, but the tag antennas remain large and are constructed with multiple layers, resulting in fragile structures. For some specialized, compact metal platforms, such as metal cylinders and bearings, tag antennas must be designed conformally. Due to their large size and unstable structure, the above designs are difficult to implement using conformal designs. Summary of the Invention [Means for solving the problem]

[0005] According to one aspect of the present disclosure, there is provided an RFID tag antenna for installation on a metal object, the RFID tag antenna including: a dielectric substrate having a first surface and a second surface opposite the first surface; a conductive layer disposed on the first surface of the dielectric substrate, the conductive layer configured to be attached to the surface of the metal object; an integrated circuit disposed on the second surface of the dielectric substrate; an antenna structure formed on the second surface of the dielectric substrate; a T-match structure formed on the second surface of the dielectric substrate, the T-match structure electrically coupling the antenna structure to the integrated circuit; a first via pin extending through the dielectric substrate and electrically connecting a first point on the antenna structure to a corresponding first point on the conductive layer; and a second via pin extending through the dielectric substrate and electrically connecting a second point on the antenna structure to a corresponding second point on the conductive layer.

[0006] By providing a conductive layer on the first surface of the dielectric substrate, the RFID tag antenna can function when placed on a metal object.

[0007] In one embodiment, the dielectric substrate is formed from a flexible material, which allows the RFID tag antenna to be placed on both flat and curved metal objects, such as cylindrical bearings.

[0008] In one embodiment, the T-match structure is configured to conjugate match the impedance of the antenna structure with the impedance of the integrated circuit. The T-match structure may be formed in a serpentine pattern, which allows the length of the T-match structure to be varied without affecting the overall dimensions of the RFID tag antenna. The length of the serpentine structure may be selected to conjugate match the impedance of the antenna structure with the impedance of the integrated circuit.

[0009] In one embodiment, the antenna structure comprises two antenna sections arranged in a dipole configuration, each antenna section having a first leg portion and a second leg portion.

[0010] In one embodiment, the resonant length of each antenna section corresponds to the length from the end of the first leg section to the end of the second leg section, where the end of the first leg section of the first antenna section may correspond to a first point on the antenna structure and the end of the first leg section of the second antenna section may correspond to a second point on the antenna structure.

[0011] In one embodiment, the second leg portion of each antenna section has a serpentine configuration, which allows the length of the second leg portion to be increased without affecting the overall dimensions of the RFID tag antenna.

[0012] In one embodiment, the dielectric substrate is rectangular, and the first point on the antenna structure and the second point on the antenna structure may correspond to diagonally opposite corners of the dielectric substrate.

[0013] Embodiments of the invention will now be described, by way of non-limiting example, with reference to the accompanying drawings, in which: [Brief explanation of the drawings]

[0014] [Figure 1A] 1 is a cross-sectional view of an RFID tag antenna according to one embodiment of the present invention. [Figure 1B] 1 is a perspective view of an RFID tag antenna according to one embodiment of the present invention. [Figure 1C] FIG. 2 is a top view of an RFID tag antenna according to one embodiment of the present invention. [Figure 2] FIG. 2 is a diagram showing an equivalent circuit of an RFID tag antenna according to an embodiment of the present invention. [Figure 3] 3 shows the results of a comparison of input impedance between an RFID tag antenna according to an embodiment of the present invention and a simulation using High Frequency Simulation Software (HFSS) based on the equivalent circuit shown in FIG. 2. [Figure 4] FIG. 10 illustrates simulated and measured input impedance versus frequency for an RFID tag antenna according to one embodiment of the present invention. [Figure 5]FIG. 10 illustrates simulated and measured power reflection coefficients of an RFID tag antenna according to one embodiment of the present invention. [Figure 6A] FIG. 10 illustrates simulated and measured read patterns in the E-plane of an RFID tag antenna according to one embodiment of the present invention. [Figure 6B] 1A and 1B illustrate simulated and measured read patterns in the H-plane of an RFID tag antenna according to one embodiment of the present invention. [Figure 7] FIG. 10 illustrates simulated and measured maximum read distances versus frequency for an RFID tag antenna according to one embodiment of the present invention. [Figure 8A] FIG. 2 illustrates an RFID tag antenna according to one embodiment of the present invention installed on a metal object. [Figure 8B] FIG. 2 illustrates an RFID tag antenna according to one embodiment of the present invention installed on a metal object. [Figure 8C] FIG. 2 illustrates an RFID tag antenna according to one embodiment of the present invention installed on a metal object. [Figure 8D] FIG. 2 illustrates an RFID tag antenna according to one embodiment of the present invention installed on a metal object. [Figure 8E] FIG. 2 illustrates an RFID tag antenna according to one embodiment of the present invention installed on a metal object. [Figure 9] FIG. 2 is a diagram showing the resonant length of an RFID tag antenna according to an embodiment of the present invention. [Figure 10] FIG. 2 illustrates a T-matching impedance matching network used in an embodiment of the present invention. [Figure 11] FIG. 2 illustrates the impedance of the tag antenna and tag chip of an RFID tag antenna according to one embodiment of the present invention. [Figure 12] 1 is a flowchart illustrating a method for determining design parameters of an RFID tag antenna according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0015] The present disclosure provides an RFID tag antenna that can reside on metal platforms of different shapes and sizes, such as flat metal plates, metal cylinders, containers, bearings, and the like.

[0016] 1A to 1C show an RFID tag antenna according to one embodiment of the present invention, where Fig. 1A is a cross-sectional view, Fig. 1B is a perspective view, and Fig. 1C is a top view.

[0017] 1A-1C, RFID tag antenna 100 is designed to operate in the U.S. UHF RFID band applications (902-928 MHz). However, it will be appreciated that the dimensions of the RFID tag antenna can be adjusted to operate in other RFID frequency bands.

[0018] 1A, RFID tag antenna 100 includes dielectric substrate 102. To enable RFID tag antenna 100 to operate on both flat and curved metal platforms, dielectric substrate 102 is formed from a flexible material. For example, Arlon AD430 material may be selected. Alternatively, materials with similar or close dielectric constants and loss tangents may be selected, including, but not limited to, Arlon AD450 and Rogers TMM4. In one exemplary embodiment, dielectric substrate 102 has dimensions of 10 mm x 30 mm x 1.5 mm.

[0019] The RFID tag antenna 100 is fabricated using printed circuit board (PCB) technology. A bottom conductive coating is applied to the bottom surface of a dielectric substrate 102. A bottom conductive coating 104 covers the bottom surface of the dielectric substrate 102. A top conductive coating 106 is applied to the top surface of the dielectric substrate 102, and the top conductive coating forms the antenna structure and T-match structure, which will be described in detail below. Via pins 108 penetrate the dielectric substrate 102 and form conductive connections between the bottom conductive coating 104 and the top conductive coating 106. An integrated circuit chip 110 is mounted on the top surface of the dielectric substrate 102 and is connected to a portion of the top conductive coating 106. The integrated circuit 110 is an RFID tag integrated circuit, such as an Alien Higgs-9 (AH-9) tag chip.

[0020] 1B, the integrated circuit chip 110 is mounted at the center of the top surface of the dielectric substrate 102. The RFID tag antenna 100 is rectangular, and the via pins 108 are located at diagonally opposite corners of the RFID tag antenna 100 (the upper left corner and the lower right corner in FIG. 1B). The via pins 108 connect the top surface conductive coating 106 to the bottom surface conductive coating 104.

[0021] 1C is a top view of the RFID tag antenna 100, showing the layout of the top surface conductive coating 106. The top surface conductive coating 106 forms a dipole-like structure that is rotationally symmetric around the integrated circuit 110 at the center of the top surface. The layout of the top surface conductive coating 106 can be considered to consist of a first radiating portion 120A, a second radiating portion 120B, and a T-match portion 130. The first radiating portion 120A and the second radiating portion 120B can be considered to be two antenna portions arranged in a dipole structure, with the first radiating portion corresponding to the first antenna portion and the second radiating portion corresponding to the second antenna portion. As described above, the first radiating portion 120A and the second radiating portion 120B are rotationally symmetric by 180 degrees around the integrated circuit 110 at the center of the top surface. As shown in FIG. 1C , the first radiating portion 120A includes a first leg portion 122A that extends from the upper left corner of the top surface, where it is connected to one of the via pins 108A, across the width of the top surface, and to the lower left corner of the top surface. The second leg portion 124A of the first radiating portion 120A is located near the right edge of the top surface but is separated from the right edge by the second radiating portion 120B. The second leg portion 124A has a serpentine pattern. This serpentine pattern allows the effective length of the second leg portion 124A to be increased without increasing the overall dimensions of the RFID tag antenna 100. The first leg portion 122A and the second leg portion 124A of the first radiating portion 120A are connected by a central portion 126A of the first radiating portion 120A that extends along the lower edge of the top surface. The coupling portion 128A of the first radiating portion 120A extends upward from the central portion 126A of the first radiating portion 120A and connects the first radiating portion 120A to the T-match portion .

[0022] Similarly, the second radiating portion 120B includes a first leg portion 122B extending from the lower right corner of the top surface, where it connects to the other via pin 108B. The first leg portion 122B of the second radiating portion 120B extends from the lower right corner to the upper right corner of the top surface. The second leg portion 124B of the second radiating portion 120B is located near the left edge of the top surface but is separated from the edge by the first leg portion 122A of the first radiating portion 120A. The second leg portion 124B has a serpentine pattern. This serpentine pattern allows the effective length of the second leg portion 124B to be increased without increasing the overall dimensions of the RFID tag antenna 100. The first leg portion 122B and the second leg portion 124B of the second radiating portion 120B are connected by a central portion 126B of the second radiating portion 120B, which extends along the upper edge of the top surface. The coupling portion 128B of the second radiating portion 120B extends downward from the central portion 126B of the second radiating portion 120B and connects the first radiating portion 120B to the T-match portion 130.

[0023] The T-match portion 130 comprises serpentine paths 130A and 130B connected in parallel between the coupling portion 128A of the first radiating portion 120A and the coupling portion 128B of the second radiating portion 120B. A conductive path 132 is connected in parallel to both of the serpentine paths 130A, 130B connecting the integrated circuit 110 to each of the coupling portion 128A of the first radiating portion 120A and the coupling portion 128B of the second radiating portion 120B.

[0024] Two shorting via pins 108 are employed to connect the ground plane, making this dipole tag a Planar Inverted-F Antenna (PIFA), thereby reducing the size of the tag. The serpentine line further reduces the size of the tag.

[0025] 2 shows an equivalent circuit of an RFID tag antenna according to one embodiment of the present invention. As shown in FIG. 2, the T-match section 130 can be considered as a double T-match circuit comprising four inductors L1, L2, L3, and L4, where two inductors L1 and L3 are connected in series to one of the inputs of the integrated circuit, and two inductors L2 and L4 are connected in parallel between the inputs of the integrated circuit. The tag antenna formed by the first radiating section 120A and the second radiating section 120B described above with reference to FIG. 1C can be considered as having three parallel connections: a capacitor C1 constituting a first connection, an inductor L5 constituting a second connection, and a capacitor C2 connected in series with an inductor L6 and a resistor R1 constituting a third connection.

[0026] FIG. 3 shows a comparison result of input impedance between an RFID tag antenna according to one embodiment of the present invention and a simulation using high frequency simulation software (HFSS) based on the equivalent circuit shown in FIG.

[0027] The detailed values of the equivalent circuit are as follows: L1 = L3 = 1.0nH, L2 = L4 = 12.85nH, L5 = 4.0nH, L6 = 12.9nH, C1 = 2.0pF, C2 = 1.9pF, R1 = 0.25ohm. As shown in Figure 3, the curves match very well with the simulation results. The impedance of the equivalent circuit is found to be 8.0 + j190.2 at the tag resonance of 912.5MHz, which is very close to the HFSS result (9.0 + j192.4) at 912.5MHz.

[0028] Figure 4 shows the simulated and measured input impedances versus frequency for an RFID tag antenna according to one embodiment of the present invention. A differential probe method was used to measure the input impedance of the proposed tag antenna. The measured input impedance (real and imaginary parts) of this tag shown in Figure 4 is slightly shifted to lower frequencies. The measured real part is larger than the simulated value. As shown in Figure 4, the value of the imaginary part does not change significantly between the measurement and simulation.

[0029] 5 shows the simulated and measured power reflection coefficients of an RFID tag antenna according to one embodiment of the present invention. As can be seen from FIG. 5, the final measured center operating frequency of the power reflection coefficient (PRC) shifts to 911.5 MHz.

[0030] Figure 6A shows simulated and measured read patterns in the E-plane of an RFID tag antenna according to one embodiment of the present invention, and Figure 6B shows simulated and measured read patterns in the H-plane of an RFID tag antenna according to one embodiment of the present invention.

[0031] To obtain the results shown in Figures 6A and 6B, the tags were 15 x 15 cm 2 The proposed tag was placed on a metal plate. We found that the maximum read distances on both the E and H planes were shorter than the simulations, which is mainly due to poor matching. In fact, the maximum simulated gain of the proposed tag is -7.7 dB.

[0032] 7 shows the simulated and measured maximum read distances versus frequency for an RFID tag antenna according to an embodiment of the present invention. As shown in FIG. 7, the operating frequency is shifted to a lower frequency by approximately 1 MHz in the measured results compared to the simulation results.

[0033] 8A-8E show an RFID tag antenna according to one embodiment of the present invention installed on a metal object.

[0034] Figure 8A shows an RFID tag antenna placed on a 100mm x 100mm flat metal object. As shown in Figure 8A, the dimensions of the RFID tag antenna are 10mm x 30mm x 1.5mm.

[0035] FIG. 8B shows an RFID tag antenna mounted on a cylindrical bearing with a diameter of 30 mm.

[0036] FIG. 8C shows an RFID tag antenna mounted on a cylindrical bearing with a diameter of 50 mm.

[0037] FIG. 8D shows an RFID tag antenna mounted on a cylindrical bearing with a diameter of 100 mm.

[0038] Figure 8E shows an RFID tag antenna mounted on a cylindrical bearing with a diameter of 200 mm.

[0039] As shown in Figures 8A to 8E, the dielectric substrate is made of a flexible material, so the RFID tag antenna itself is flexible and can be conformally placed on bearings and flat metal objects of different diameters. The designed RFID tag antenna is so miniaturized that it can still function well despite the small size of various conformal platforms.

[0040] Figure 9 shows the resonant length of an RFID tag antenna according to one embodiment of the present invention. As shown in Figure 9, the resonant length is the length of the first radiating portion 120A (which is equal to the length of the second radiating portion). The length extends from the end of the first leg portion 122A corresponding to one of the via pins 108 to the end of the second leg portion 124A.

[0041] Typically, the resonant length of an antenna is approximately half a wavelength. Short pin loading means that adding via pins 108 to a PIFA antenna structure effectively shortens the effective length of the antenna. By shortening the resonant length to approximately one-quarter wavelength, the antenna can achieve resonance at UHF frequencies. This approach helps reduce the overall tag size while maintaining proper resonance. Thus, the resonant length 900 shown in FIG. 9 corresponds to one-quarter of a resonant wavelength. Note that the serpentine pattern of the second leg portion 124A further miniaturizes the RFID tag antenna.

[0042] Figure 10 shows a T-match impedance matching network. As shown in Figure 10, the RFID tag antenna can be considered to be an integrated circuit 110 chip connected to a radiator (composed of a first radiating portion 120A and a second radiating portion 120B) by a T-match network 130.

[0043] The T-match is an impedance matching network that comprises a T-shaped structure connected to the feed line of the tag antenna. The component values can be determined using the following equations:

[0044]

number

[0045]

number

[0046] L is the inductor value, C is the capacitor value, C in and C out are the input and output capacitances of the tag antenna, respectively. Z in is the desired input impedance of the tag antenna, and f is the operating frequency. Actual component values may need to be adjusted or fine-tuned via simulation or experimental iterations.

[0047] Figure 11 shows the impedance of the tag antenna and tag chip. As shown in Figure 11, the tag antenna has an impedance Z a and Z a =R a +jX a The tag chip has impedance Z c It has. Z c =R c +jX c

[0048] For maximum power transfer, the complex conjugates of the impedances need to be matched. Z a =Z c *

[0049] FIG. 12 is a flowchart illustrating a method for determining design parameters for an RFID tag antenna according to one embodiment of the present invention.

[0050] First, a tag chip is selected, from which the input impedance of the tag chip is known, in step 1202. In step 1204, an antenna type and substrate material are selected.

[0051] Then, in step 1206, the parameters are investigated and optimized. This step may involve adjusting the length of the T-match structure to adjust the antenna's input impedance to match that of the integrated circuit chip. Conjugate matching is employed to achieve maximum energy transfer between the tag and the chip. Therefore, when changing the chip, the antenna's input impedance must also be adjusted. As shown in FIG. 12, the parameters are optimized and a check is made in step 1208 to see if the impedance matching requirements are met. If the requirements are met, the design is finalized in step 1210. If not, the method returns to step 1206 and further optimization is performed.

[0052] As described above, the present disclosure provides a highly miniaturized UHF RFID tag antenna with a serpentine structure for tagging small metal objects. In one exemplary embodiment, the total volume is only 10 mm x 30 mm x 1.5 mm. This tag can be used on flat platforms as well as conformal platforms such as metal cylinders and bearings.

[0053] Furthermore, this disclosure provides a very simple and compact foldable dipole structure designed for a metal-resistant UHF tag antenna. The proposed tag can achieve a large read distance of over 4 m. The proposed tag antenna is very thin and can be easily made to flexibly conform to some curved surfaces.

[0054] While the foregoing description describes exemplary embodiments, those skilled in the art will recognize that many variations thereon are possible within the scope and spirit of the invention. [Explanation of symbols]

[0055] 100 RFID tag antennas 102 Dielectric substrate 104 Underside conductive coating 106 Top surface conductive coating 108 Beer Pin 110 Integrated Circuit Chips 120A First Radiating Part 120B Second radiating part 122A First leg section 122B First leg section 124A Second leg section 124B Second leg section 126A central part 126B Central part 128A joint part 128B Joining part 130 T-match part, T-match network 130A Serpentine Route 130B Serpentine Route 132 Conductive Path 900 resonance length

Claims

1. 1. An RFID tag antenna for installation on a metal object, comprising: a dielectric substrate having a first surface and a second surface opposite the first surface; a conductive layer disposed on the first surface of the dielectric substrate, the conductive layer being configured to be attached to a surface of the metal object; an integrated circuit disposed on the second surface of the dielectric substrate; an antenna structure formed on the second surface of the dielectric substrate; a T-match structure formed on the second surface of the dielectric substrate, the T-match structure electrically coupling the antenna structure to the integrated circuit; a first via pin that penetrates the dielectric substrate and electrically connects a first point on the antenna structure to a corresponding first point on the conductive layer; a second via pin that penetrates the dielectric substrate and electrically connects a second point on the antenna structure to a corresponding second point on the conductive layer; An RFID tag antenna comprising:

2. The RFID tag antenna of claim 1 , wherein the dielectric substrate is formed from a flexible material.

3. 3. The RFID tag antenna of claim 1, wherein the T-match structure is configured to conjugately match the impedance of the antenna structure with the impedance of the integrated circuit.

4. The RFID tag antenna of claim 3 , wherein the T-match structure is formed in a serpentine pattern.

5. 5. The RFID tag antenna of claim 4, wherein a length of the serpentine structure is selected to conjugate match the impedance of the antenna structure with the impedance of the integrated circuit.

6. The RFID tag antenna of claim 1 , wherein the antenna structure comprises two antenna sections arranged in a dipole configuration.

7. The RFID tag antenna of claim 6 , wherein each antenna section comprises a first leg portion and a second leg portion.

8. 8. The RFID tag antenna of claim 7, wherein the resonant length of each antenna section corresponds to the length from an end of the first leg portion to an end of the second leg portion.

9. 9. The RFID tag antenna of claim 8, wherein the end of the first leg portion of a first antenna portion corresponds to the first point on the antenna structure, and the end of the first leg portion of a second antenna portion corresponds to the second point on the antenna structure.

10. The RFID tag antenna of claim 7 , wherein the second leg portion of each antenna section has a serpentine configuration.

11. The RFID tag antenna of claim 1 , wherein the dielectric substrate is rectangular.

12. The RFID tag antenna of claim 11 , wherein the first point on the antenna structure and the second point on the antenna structure correspond to diagonally opposite corners of the dielectric substrate.

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