Radio frequency identification tag antenna for metal object

Abstract

A radio frequency identification (RFID) tag antenna for mounting 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 a surface of the metal object; an integrated circuit mounted on the second surface of the dielectric substrate; an antenna structure formed on the second surface of the dielectric substrate; a T-shaped matching structure formed on the second surface of the dielectric substrate, the T-shaped matching structure electrically connecting the antenna structure to the integrated circuit; a first via pin penetrating the dielectric substrate, the first via pin electrically connecting a first point on the antenna structure with a corresponding first point on the conductive layer; and a second via pin passing through the dielectric substrate, the second via pin electrically connecting a second point on the antenna structure with 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 mounting on metal objects. Background Art

[0002] Nowadays, the rapid development of radio frequency identification (RFID) technology has attracted widespread attention. Due to its long-distance recognition capability in the ultra-high frequency (UHF) band, RFID tags are widely used in supply chain management, inventory inspection, asset tracking, access control automation and other purposes.

[0003] However, many applications require RFID tags to be mounted on conductive metal objects such as motor vehicles, cylinders, containers, weapons, and equipment. When ordinary UHF tags are placed on metal surfaces, their reading distance will be severely reduced due to impedance mismatch, reduced radiation efficiency, and poor directivity. Therefore, so-called anti-metal tags must be specially designed to overcome such problems.

[0004] In the prior art, for the UHF RFID band, tags based on microstrip patch antennas with built-in ground planes have been proposed. However, this will make the antenna extremely large, with a resonant length close to one-quarter of the wavelength, making it difficult to install on small metal objects. In order to achieve miniaturization, although a variety of dipole-like folding tags have been designed, the size of the tag antenna is still large, and because such antennas are composed of multiple layers, the structure is weak. In certain special compact metal platforms such as metal cylinders and bearings, conformal tag antennas are required. Since the above-mentioned designs are large in size and unstable in structure, it is difficult to achieve conformal design. Summary of the invention

[0005] According to one aspect of the present disclosure, an RFID tag antenna for mounting on a metal object is provided. The RFID tag antenna includes: a dielectric substrate having a first surface and a second surface opposite to the first surface; a conductive layer provided on the first surface of the dielectric substrate, the conductive layer being used to be attached to the surface of the metal object; an integrated circuit mounted on the second surface of the dielectric substrate; an antenna structure formed on the second surface of the dielectric substrate; a T-shaped matching structure formed on the second surface of the dielectric substrate, the T-shaped matching structure electrically connecting the antenna structure to the integrated circuit; a first via pin penetrating the dielectric substrate, the first via pin electrically connecting a first point on the antenna structure to a corresponding first point on the conductive layer; and a second via pin penetrating the dielectric substrate, the second via pin electrically connecting a second point on the antenna structure to a corresponding second point on the conductive layer.

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

[0007] In one embodiment, the dielectric substrate is made of a flexible material, so that the RFID tag antenna can be installed on both planar metal objects and curved metal objects such as cylindrical bearings.

[0008] In one embodiment, the T-shaped matching structure is used to conjugate match the impedance of the antenna structure with the impedance of the integrated circuit. The T-shaped matching structure can be formed into a meandering pattern. In this way, the overall size of the RFID tag antenna can be unchanged while the length of the T-shaped matching structure is changed. The length of the meandering structure can 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 parts arranged in a dipole structure. Each antenna part may comprise a first arm and a second arm.

[0010] In one embodiment, the resonant length of each antenna portion corresponds to the length from the end of the first arm to the end of the second arm. The end of the first arm of the first antenna portion may correspond to the first point on the antenna structure, and the end of the first arm of the second antenna portion may correspond to the second point on the antenna structure.

[0011] In one embodiment, the second arm portion of each antenna part has a meandering structure, so that the overall size of the RFID tag antenna is not affected while the length of the second arm portion is increased.

[0012] In one embodiment, the dielectric substrate is rectangular. 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. BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Hereinafter, various embodiments of the present invention will be described by way of non-limiting examples with reference to the accompanying drawings. In the drawings:

[0014] Figure 1A A cross-sectional view of an RFID tag antenna according to an embodiment of the present invention;

[0015] Figure 1B A three-dimensional diagram of an RFID tag antenna according to an embodiment of the present invention;

[0016] Figure 1C A top view of an RFID tag antenna according to an embodiment of the present invention;

[0017] Figure 2 is an equivalent circuit diagram of an RFID tag antenna according to an embodiment of the present invention;

[0018] Figure 3 The RFID tag antenna of the embodiment of the present invention is shown in FIG. 1 and FIG. 2 is a schematic diagram of the RFID tag antenna of the embodiment of the present invention. The RFID tag antenna of the embodiment of the present invention is shown in FIG. 2 based on the RFID tag antenna of the embodiment of the present invention. Figure 2 Input impedance comparison results between simulation tests performed with the equivalent circuit shown;

[0019] Figure 4 The figure is a graph showing the relationship between the simulated and measured input impedance and frequency of the RFID tag antenna according to the embodiment of the present invention.

[0020] Figure 5 Shown are the simulated and measured power reflection coefficients of the RFID tag antenna according to the embodiment of the present invention;

[0021] Fig. 6A Shown are the simulated and measured E-plane reading patterns of the RFID tag antenna according to the embodiment of the present invention;

[0022] Figure 6B Shown are the simulated and measured H-plane reading patterns of the RFID tag antenna according to the embodiment of the present invention;

[0023] Figure 7 The figure shows the relationship between the simulated and measured maximum reading distance and frequency of the RFID tag antenna according to the embodiment of the present invention;

[0024] FIG. 8A to FIG. 8E Shown is an embodiment of the RFID tag antenna of the present invention mounted on a metal object;

[0025] Fig. 9 The figure shows the resonant length of the RFID tag antenna according to the embodiment of the present invention;

[0026] Fig.10 The figure shows a T-shaped matching impedance matching network used in an embodiment of the present invention;

[0027] Fig.11 The figure shows the tag antenna impedance and tag chip impedance of the RFID tag antenna according to the embodiment of the present invention;

[0028] Fig.12 The present invention is a flowchart of a method for determining design parameters of an RFID tag antenna according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The present disclosure provides an RFID tag antenna, which can be arranged on metal platforms of different shapes and sizes, such as a flat metal plate, a metal cylinder, a container, a bearing and the like.

[0030] Figures 1A to 1C Shown is an RFID tag antenna according to an embodiment of the present invention. Figure 1A is a cross-sectional view, Figure 1B For a stereogram, Figure 1C This is a top view.

[0031] exist Figures 1A to 1C In the illustrated embodiment, the RFID tag antenna 100 is designed for use in the UHF RFID band (902-928 MHz) in the United States. However, it should be understood that the size of the RFID tag antenna can be adjusted to be used in other RFID frequency bands.

[0032] like Figure 1A As shown, the RFID tag antenna 100 includes a dielectric substrate 102. In order to enable the RFID tag antenna 100 to be used for both a flat metal platform and a curved metal platform, the dielectric substrate 102 is made of a flexible material. For example, the AD430 material of Arlon can be selected. Alternatively, a material with a similar or similar dielectric constant and loss tangent can be selected, including but not limited to Arlon's AD450 and Rogers' TMM4. In an exemplary embodiment, the size of the dielectric substrate 102 is 10 mm×30 mm×1.5 mm.

[0033] The RFID tag antenna 100 is manufactured by printed circuit board (PCB) technology. A lower surface conductive coating is provided on the lower surface of the dielectric substrate 102. The lower surface conductive coating 104 covers the lower surface of the dielectric substrate 102. An upper surface conductive coating 106 is provided on the upper surface of the dielectric substrate 102, and the upper surface conductive coating constitutes an antenna and a T-shaped matching structure described in further detail below. A via pin 108 passes through the dielectric substrate 102 and forms a conductive connection between the lower surface conductive coating 104 and the upper surface conductive coating 106. An integrated circuit chip 110 is mounted on the upper surface of the dielectric substrate 102, and the chip is connected to a portion of the upper surface conductive coating 106. The integrated circuit 110 is an RFID tag integrated circuit, such as the Higgs 9 (AH-9) tag chip of Alien.

[0034] like Figure 1B As shown, the integrated circuit chip 110 is mounted at the center of the upper surface of the dielectric substrate 102. The RFID tag antenna 100 is rectangular, and the via pins 108 are located at the corners opposite to the diagonal of the RFID tag antenna 100 (at Figure 1BThe via pins 108 connect the upper surface conductive coating 106 to the lower surface conductive coating 104.

[0035] Figure 1C is a top view of the RFID tag antenna 100, which shows the layout of the upper surface conductive coating 106. The upper surface conductive coating 106 forms a structure similar to a dipole, which is rotationally symmetrical around the integrated circuit 110 located at the center of the upper surface. The layout of the surface conductive coating 106 can be regarded as consisting of a first radiating portion 120A; a second radiating portion 120B; and a T-shaped matching portion 130. The first radiating portion 120A and the second radiating portion 120B can be regarded as two antenna portions located in the dipole structure, wherein the first radiating portion corresponds to the first antenna portion, and the second radiating portion corresponds to the second antenna portion. As described above, the first radiating portion 120A and the second radiating portion 120B are rotationally symmetrical 180 degrees around the integrated circuit 110 located at the center of the upper surface. The first radiating portion 120A includes a first arm portion 122A, as shown in FIG. Figure 1C As shown, the arm extends from the upper left corner of the upper surface connected to one of the via pins 108A to the lower left corner of the upper surface over the entire width of the upper surface. The second arm 124A of the first radiating portion 120A is close to the right edge of the upper surface and is separated from the right edge of the upper surface by the second radiating portion 120B. The second arm 124A has a meandering pattern. This meandering pattern effectively increases the length of the second arm 124A without increasing the overall size of the RFID tag antenna 100. The first arm 122A and the second arm 124A of the first radiating portion 120A are connected by the middle portion 126A of the first radiating portion 120A extending along the bottom edge of the upper surface. The connecting portion 128A of the first radiating portion 120A extends upward from the middle portion 126A of the first radiating portion 120A to connect the first radiating portion 120A to the T-shaped matching portion 130.

[0036] Similarly, the second radiating portion 120B includes a first arm 122B extending from the lower right corner of the upper surface connected to another via pin 108A. The first arm 122B extends from the lower right corner of the upper surface to the upper right corner. The second arm 124B of the second radiating portion 120B is close to the left edge of the upper surface and is separated from the left edge of the upper surface by the first arm 122A of the first radiating portion 120A. The second arm 124B has a serpentine pattern. This serpentine pattern makes it possible to effectively increase the length of the second arm 124B without increasing the overall size of the RFID tag antenna 100. The first arm 122B and the second arm 124B of the second radiating portion 120B are connected by the middle portion 126B of the second radiating portion 120B extending along the top edge of the upper surface. The connecting portion 128B of the second radiating portion 120B extends downward from the middle portion 126B of the second radiating portion 120B to connect the first radiating portion 120B to the T-shaped matching portion 130 .

[0037] The T-shaped matching section 130 includes serpentine paths 130A and 130B connected in parallel between the connecting portion 128A of the first radiating portion 120A and the connecting portion 128B of the second radiating portion 120B. In addition, the conductive path 132 is connected in parallel with the two serpentine paths 130A, 130B connecting the integrated circuit 110 to the connecting portion 128A of the first radiating portion 120A and the connecting portion 128B of the second radiating portion 120B, respectively.

[0038] The two short-circuit via pins 108 are used to connect to the ground plane, so that the dipole-type tag becomes a planar inverted F antenna (PIFA), thereby reducing the tag size. In addition, the meandering line can further reduce the tag size.

[0039] Figure 2 FIG. 1 is an equivalent circuit diagram of an RFID tag antenna according to an embodiment of the present invention. Figure 2 As shown, the T-shaped matching part 130 can be regarded as a double T-shaped matching circuit, which includes four inductors L1, L2, L3, and L4, wherein two inductors L1 and L3 are connected in series with one of the input terminals of the integrated circuit, and the other two inductors L2 and L4 are connected in parallel between the two input terminals of the integrated circuit. Figure 1C The tag antenna composed of the first radiating part 120A and the second radiating part 120B described can be regarded as having three parallel connections: a first parallel connection consisting of a capacitor C1; a second parallel connection consisting of an inductor L5; and a third parallel connection consisting of a series-connected inductor L6, a resistor R1 and a capacitor C2.

[0040] Figure 3 The RFID tag antenna according to the embodiment of the present invention is shown in FIG. Figure 2Input impedance comparison results between simulation experiments performed using the equivalent circuit shown.

[0041] The specific 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 = 0.25Ω. Figure 3 As shown in the figure, each curve is very consistent with its simulation results. As can be seen from the figure, when the resonant frequency of the tag antenna is 912.5MHz, the impedance of the equivalent circuit is 8.0+j190.2, which is very close to the HFSS simulation result (9.0+j192.4) at 912.5MHz.

[0042] Figure 4 The figure is a graph showing the relationship between the simulated and measured input impedance and frequency of the RFID tag antenna according to the embodiment of the present invention. The input impedance of the tag antenna according to the present invention is measured using a differential probe method. Figure 4 As shown in , the measured input impedance (real and imaginary parts) of the tag is slightly shifted toward the low frequency direction, and the measured value of the real part is greater than the simulated value. Figure 4 As shown, there is little difference between the measured and simulated values ​​of the imaginary part.

[0043] Figure 5 The figure shows the simulated and measured power reflection coefficient (PRC) of the RFID tag antenna according to the embodiment of the present invention. Figure 5 It can be seen that the measured center operating frequency of the power reflection coefficient finally moves to 911.5MHz.

[0044] Fig. 6A Shown are the simulated and measured E-plane reading patterns of the RFID tag antenna according to the embodiment of the present invention. Figure 6B Shown are the simulated and measured H-plane reading direction diagrams of the RFID tag antenna according to the embodiment of the present invention.

[0045] Fig. 6A and Figure 6B The results shown are obtained by placing the label on a 15×15 cm 2 It can be seen that the maximum reading distance of the E plane and the H plane are both smaller than the simulation results, mainly due to the poor matching. In fact, the maximum gain simulation value of the tag of the present invention is -7.7dB.

[0046] Figure 7 The figure shows the relationship between the simulated and measured maximum reading distance and frequency of the RFID tag antenna according to the embodiment of the present invention. Figure 7 As shown, the operating frequency of the measured results is shifted toward the low frequency direction by about 1MHz compared with the simulation results.

[0047] FIG. 8A to FIG. 8EShown is an RFID tag antenna according to an embodiment of the present invention mounted on a metal object.

[0048] Fig. 8A The figure shows an RFID tag antenna mounted on a 100mm×100mm flat metal object. Fig. 8A As shown, the size of the RFID tag antenna is 10 mm×30 mm×1.5 mm.

[0049] Figure 8B Shown is an RFID tag antenna mounted on a 30mm diameter cylindrical bearing.

[0050] Figure 8C Shown is an RFID tag antenna mounted on a 50mm diameter cylindrical bearing.

[0051] Fig.8D Shown is an RFID tag antenna mounted on a 100mm diameter cylindrical bearing.

[0052] Fig. 8E Shown is an RFID tag antenna mounted on a 200mm diameter cylindrical bearing.

[0053] like FIG. 8A to FIG. 8E As shown, since the dielectric substrate is made of flexible material, the RFID tag antenna itself is a flexible antenna, which can be conformally arranged on partial bearings of different diameters and flat metal objects. Since the RFID tag antenna using this design is greatly miniaturized, it can also be well applied on various small-sized conformal platforms.

[0054] Fig. 9 FIG. 1 shows the resonant length of the RFID tag antenna according to an embodiment of the present invention. Fig. 9 As shown, the resonant length is the length of the first radiating portion 120A (equal to the length of the second radiating portion), which is the length from the end of the first arm portion 122A corresponding to one of the via pins 108 to the end of the second arm portion 124A.

[0055] Generally speaking, the resonant length of the antenna is about half the wavelength. "Short pin loading" means that by adding via pins 108 to the PIFA antenna structure, the effective length of the antenna can be effectively reduced. By reducing the resonant length to about a quarter of the wavelength, the resonant frequency of the antenna can reach the UHF frequency. This approach helps to maintain a suitable resonant frequency while reducing the overall size of the tag antenna. Accordingly, Fig. 9 The resonant length 900 shown corresponds to one quarter of the resonant wavelength. Note that the meandering pattern of the second arm 124A can further miniaturize the RFID tag antenna.

[0056] Fig.10 The figure shows a T-shaped impedance matching network. Fig.10 As shown, the RFID tag antenna can be considered to be obtained by connecting the integrated circuit 110 chip and the radiation structure (composed of the first radiation part 120A and the second radiation part 120B) through a T-shaped matching network 130.

[0057] The T-shaped matching network is an impedance matching network that includes a T-shaped structure connected to the feed line of the tag antenna. The values ​​of each component can be determined by the following equation:

[0058]

[0059]

[0060] Where L is the value of the inductor, C is the value of the capacitor, Cin and Cout are the input and output capacitances of the tag antenna, Zin is the required input impedance of the tag antenna, and f is the operating frequency. The actual values ​​of each component may need to be further adjusted or fine-tuned through simulation or experimental iteration.

[0061] Fig.11 The following table shows the tag antenna impedance and tag chip impedance. Fig.11 As shown, the tag antenna impedance Z a for:

[0062] Z a =R a +jX a

[0063] Tag chip impedance Z c for:

[0064] Z c =R c +jX c

[0065] To achieve maximum power transfer, the complex conjugates of these two impedances should be matched:

[0066] Z a =Z c *

[0067] Fig.12 The present invention is a flowchart of a method for determining design parameters of an RFID tag antenna according to an embodiment of the present invention.

[0068] First, in step 1202, a tag chip is selected and the input impedance of the tag chip is obtained. In step 1204, an antenna type and a substrate material are selected.

[0069] Then, in step 1206, the parameters are optimized through research. This step may include: adjusting the length of the T-shaped matching structure to adjust the input impedance of the antenna to match the impedance of the integrated circuit chip. In order to achieve maximum energy transmission between the tag and the chip, conjugate matching is used. Accordingly, when replacing the chip, the input impedance of the antenna needs to be adjusted at the same time. Fig.12 As shown, in step 1208, it is checked whether the optimized parameters meet the impedance matching requirements. If the requirements are met, the design is completed in step 1210. If the requirements are not met, the method returns to step 1206 for further optimization.

[0070] As described above, the present disclosure provides a highly miniaturized UHF RFID tag antenna with a meandering line structure for mounting on a small metal object. In an exemplary embodiment, its total volume is only 10mm×30mmX1.5mm. The tag antenna can be used on both planar platforms and platforms that require conformal properties, such as metal cylinders and bearings.

[0071] The present disclosure also provides an extremely simple and compact folded dipole structure, which is designed for metal-resistant UHF tag antennas. The tag antenna of the present invention can achieve long-distance reading greater than 4 meters. The thickness of the tag antenna of the present invention is extremely thin, and it can be easily configured in a flexible and conformal manner on a partial curved surface.

[0072] Although exemplary embodiments have been described above, it will be appreciated by those skilled in the art that various modifications may be made to these embodiments within the scope and spirit of the present invention.

Claims

1. A radio frequency identification tag antenna for mounting on a metal object, characterized in that: The radio frequency identification tag antenna comprises: 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, wherein the conductive layer is used to be attached to the surface of the metal object; an integrated circuit mounted on the second surface of the dielectric substrate; an antenna structure formed on the second surface of the dielectric substrate; a T-shaped matching structure formed on the second surface of the dielectric substrate, wherein the T-shaped matching structure electrically connects the antenna structure to the integrated circuit; a first via pin extending through the dielectric substrate, wherein the first via pin electrically connects a first point on the antenna structure to a corresponding first point on the conductive layer; and A second via pin extends through the dielectric substrate, wherein the second via pin electrically connects a second point on the antenna structure to a corresponding second point on the conductive layer.

2. The RFID tag antenna according to claim 1, characterized in that: The dielectric substrate is made of flexible material.

3. The RFID tag antenna according to any one of the preceding claims, characterized in that: The T-shaped matching structure is used to conjugate match the impedance of the antenna structure with the impedance of the integrated circuit.

4. The RFID tag antenna according to claim 3, characterized in that: The T-shaped matching structure is formed in a serpentine pattern.

5. The RFID tag antenna according to claim 4, characterized in that: The length of the meander structure is selected to conjugate match the impedance of the antenna structure to the impedance of the integrated circuit.

6. The RFID tag antenna according to any one of the preceding claims, characterized in that: The antenna structure comprises two antenna parts arranged in a dipole structure.

7. The RFID tag antenna according to claim 6, characterized in that: Each antenna portion includes a first arm and a second arm.

8. The RFID tag antenna according to claim 7, characterized in that: The resonant length of each antenna portion corresponds to the length from the end of the first arm portion to the end of the second arm portion.

9. The radio frequency identification tag antenna according to claim 8, characterized in that: The end of the first arm of the first antenna portion corresponds to the first point on the antenna structure, and the end of the first arm of the second antenna portion corresponds to the second point on the antenna structure.

10. The radio frequency identification tag antenna according to any one of claims 7 to 9, characterized in that: The second arm portion of each antenna portion has a meandering structure.

11. The RFID tag antenna according to any one of the preceding claims, characterized in that: The dielectric substrate is rectangular.

12. The radio frequency identification tag antenna according to claim 11, characterized in that: 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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