RFID tag for rubber products

Abstract

Provided is an RFID tag for rubber products that is small and has excellent mass producibility and durability.　An RFID tag 1 for rubber products comprises a coupling transformer 40, an RF chip 10 connected to a secondary side of the coupling transformer, a printed circuit board 20 on which the RF chip is mounted, and an antenna 30. The antenna includes a coil part 31, a first element 32 extending from one end part of the coil part, and a second element 33 shorter than the first element and extending from the other end part parallel to the first element. The number of windings of the coil part is smaller than the number of windings on the secondary side of the coupling transformer, and the coil part constitutes a primary side of the coupling transformer by holding the printed circuit board in a wire-gap of the coil part. At least one of the first element and the second element is provided with an expansion / contraction part 31 that expands and contracts in the longitudinal direction of the element. Since the expansion / contraction part is appropriately deformed with respect to external force, the first element and the second element are less likely to be broken or deformed, so that the durability of the RFID tag is improved.

Description

RFID tags for rubber products

[0001] The present invention relates to an RFID tag for rubber products that is small, suitable for mass production, and has excellent durability.

[0002] RFID tags used in RFID (Radio Frequency Identification) systems contain an antenna and an RF chip, which receives carrier waves transmitted from the antenna of a reader / writer and transmits the identification data recorded on the RF chip on a reflected wave back to the reader / writer, thereby achieving contactless communication. By attaching or embedding RFID tags in the tires of vehicles such as automobiles, it is possible to manage the tire's unique information and its manufacturing, distribution, and maintenance history. Of course, RFID tags can also be attached to or embedded in rubber products other than tires to help manage those products.

[0003] Patent Document 1 discloses a tire with an RFID tag that includes a first antenna connected to an IC chip and a second antenna electromagnetically coupled to the first antenna, and discloses a technology for electromagnetically coupling the second antenna to a conductive carcass ply cord. Patent Document 2 discloses an RFID tag for rubber products developed by the present inventor. This RFID tag for rubber products includes an antenna consisting of a coil portion, a first element extending from one end of the coil portion, and a second element that is shorter than the first element and extends parallel to the first element from the other end of the coil portion, and holds a printed circuit board in the gaps between the wires of the coil portion. This RFID tag for rubber products forms a coupling transformer by holding the printed circuit board in the gaps of the coil portion. This allows for increased mass production of RFID tags for rubber products.

[0004] JP 2017-132291 A Patent No. 7016200 A

[0005] When attaching an RFID tag to a tire, there is a problem that the RFID tag may be damaged by deformation of the tire while in motion. Furthermore, when free electrons in the carbon black contained in the tire receive radio waves, charge transfers from negative to positive, causing current to flow from positive to negative, resulting in changes in impedance and relative dielectric constant. In Patent Document 1, the first antenna and the second antenna are electromagnetically coupled, and the second antenna has a high signal source impedance, which means that it is susceptible to changes in impedance and relative dielectric constant due to the influence of the carbon black. Furthermore, because the second antenna has a pair of extensions extending laterally from the electromagnetic field coupling portion, when the tire deforms, the left and right extensions are pulled laterally around the electromagnetic field coupling portion, which may result in damage.

[0006] Furthermore, RFID tags using conventional half-wave dipole antennas have the problem of being large in size, and even if meander line antennas, which are made smaller by bending the antenna elements, are used, the influence of carbon black makes it difficult to determine the effective meander length. In Patent Document 2, the first and second elements extend parallel to each other, making the RFID tag less susceptible to damage and more durable. Furthermore, RFID tags that are smaller than conventional half-wave dipole antennas are obtained. However, there is a demand for RFID tags with even greater durability. These problems can also occur when RFID tags are used in rubber products other than tires, especially rubber products that contain materials such as carbon black that affect the communication performance of RFID tags.

[0007] In consideration of these problems, the present invention aims to provide an RFID tag for rubber products that is small, suitable for mass production, and has excellent durability.

[0008] The RFID tag for rubber products of the present invention includes a coupling transformer, an RF chip connected to the secondary side of the coupling transformer, a printed circuit board on which the RF chip is mounted, and an antenna. The antenna includes a coil portion, a first element extending from one end of the coil portion, and a second element extending from the other end of the coil portion, the second element being shorter than the first element and parallel to the first element. The number of windings in the coil portion is smaller than the number of windings in the secondary side of the coupling transformer. The coil portion forms the primary side of the coupling transformer by holding the printed circuit board with gaps between the wires of the coil portion. At least one of the first element and the second element has an expandable portion that expands and contracts in its longitudinal direction. The RFID tag is also characterized by being for use with rubber products containing carbon black. The tag is also characterized by having insulating layers on both the front and back sides of the printed circuit board. The tag is also characterized by the coil portion, the first element, and the second element being made of a single conductor. The tag is also characterized by the axis of the coil on the secondary side of the coupling transformer being aligned with the axis of the coil portion. Furthermore, when the wavelength of radio waves at a communication frequency is λ, the electrical length of the portion consisting of the first element, the second element, and the coil portion is λ / 4.

[0009] In the RFID tag for rubber products of the present invention, at least one of the first element and the second element has an elastic portion, which deforms appropriately in response to external forces. This makes the first element and the second element less susceptible to breakage or deformation, improving the durability of the RFID tag. In the RFID tag for rubber products of the present invention, a coupling transformer is formed by holding a printed circuit board in the gap between the coils. This improves the mass productivity of RFID tags for rubber products. Furthermore, in the present invention, the first element and the second element, which function as antennas, extend parallel to each other, making the RFID tag less susceptible to breakage and more durable, while also achieving a smaller RFID tag than conventional half-wavelength dipole antennas. Furthermore, in the present invention, the number of windings in the primary coil is less than the number of windings in the secondary coil, thereby lowering the input impedance of the primary side, and the coupling transformer converts the secondary side to a high impedance to match the input impedance of the RF chip. By lowering the impedance on the primary side, the effects of carbon black, etc., can be reduced, even when the RFID tag is used in rubber products containing carbon black, for example.

[0010] By providing insulating layers on both the front and back of the printed circuit board, it is possible to prevent the coil section from becoming electrically connected to the secondary coil. By forming the coil section, first element, and second element by bending a single conductor, it becomes easier to manufacture the antenna and the reliability of the electrical connection between the coil section, first element, and second element is increased. The efficiency of the coupling transformer can be increased by aligning the axis of the secondary coil of the coupling transformer with the axis of the coil section. Setting the electrical length of the section consisting of the first element, second element, and coil section to λ / 4 minimizes the length of the antenna, allowing for further miniaturization of RFID tags for rubber products.

[0011] Plan view (a), bottom view (b), side view (c), modified example (d), and modified example (e) of the antenna. Plan view (a) showing the state in which the RF chip and secondary coil are mounted on the printed circuit board. Plan view (b), bottom view (c), and side view (d) showing the state in which the RF chip and connection points are sealed. Plan view (a), bottom view (b), and side view (c) showing the state before the printed circuit board is inserted into the gap in the coil. Plan view (a), bottom view (b), and side view (c) showing the state after the printed circuit board has been inserted into the gap in the coil. Enlarged views (a) to (d) of the circled area in Figure 1(a). Equivalent circuit of the RFID tag for rubber products. Figures: Longitudinal cross-sectional view showing the state in which both the front and back surfaces of an RFID tag for rubber products are covered with rubber sheets; Longitudinal cross-sectional view showing the state in which an RFID tag for rubber products is attached to a tire; Plan views (a) to (c) showing a manufacturing method of an RFID tag for rubber products; Plan views (a) and (b) showing a manufacturing method of an RFID tag for rubber products; Plan views (a) and (b) showing modified antennas, and enlarged views (c) to (e) of the circled area in Figure 11(b); Plan views (a) showing modified antennas, and enlarged views (b) to (d) of the circled area in Figure 13(a)

[0012] The RFID tag for rubber products of the present invention will be described with reference to the drawings. As shown in Figures 1 to 4, the RFID tag for rubber products 1 comprises an RF chip 10, a printed circuit board 20, an antenna 30, and a coupling transformer 40. In the following description, the RFID tag for rubber products 1 may be simply referred to as "RFID tag 1."

[0013] The RF chip 10 is connected to the secondary side of a coupling transformer 40, which will be described later. A commercially available RF chip 10 can be used, but it is preferable to use one that can withstand a vulcanization temperature of about 120°C. As shown in Figure 2(a), the RF chip 10 is mounted on the surface of a printed circuit board 20 using an adhesive such as an epoxy die-bond material.

[0014] Furthermore, a secondary coil 41 of the coupling transformer 40 is formed on the surface of the printed circuit board 20. One of the two terminals of the secondary coil 41 is connected to a terminal of the RF chip 10 by wire bonding. The other terminal of the secondary coil 41 extends to the back surface of the printed circuit board 20 via a through-hole 21, as shown in FIGS. 2(a) and 2(c), and then extends to the front surface of the printed circuit board 20 via a through-hole 22, and is connected to a terminal of the RF chip 10 by wire bonding. In this embodiment, the number of turns of the secondary coil 41 is approximately four (actually, 3.96). The secondary coil 41 is not limited to a planar coil formed by vapor deposition or the like, but may also be a coil made of conductive wire. As shown in FIG. 2(b), the RF chip 10 and the connection points are preferably sealed with an insulating layer 27. As shown in FIG. 2(d), the surface of the printed circuit board 20, i.e., the surfaces of the RF chip 10 and the secondary coil 41, are sealed with an insulating layer 23. The back surface of the printed circuit board 20 is also sealed with an insulating layer 23. The insulating layer 23 can be made of insulating resin such as epoxy resin, acrylic resin (resin containing acrylic resin and derivatives as main components), urethane resin, or the like.

[0015] The antenna 30 is provided to receive a carrier wave from a reader / writer and to return a reflected wave to the reader / writer. The antenna 30 includes a coil portion 31, a first element 32, and a second element 33. The material of the wire of the antenna 30 may be a metal wire such as copper wire, iron wire, or brass wire.

[0016] The coil portion 31 functions as the primary side of the coupling transformer 40. The number of windings of the coil portion 31 must be smaller than the number of windings of the secondary side. In this embodiment, the number of windings of the wire constituting the coil portion 31 is 1.5, and gaps 31a are formed between the wires. Note that the wires may naturally be spaced apart when no external force is applied to the coil portion 31, and this space may serve as the gap 31a. Alternatively, as shown in FIG. 1(d), the wires may be in close contact when no external force is applied to the coil portion 31, and the wires may elastically open when an external force is applied to the coil portion 31 in the vertical (axial) direction, resulting in the gap 31a. The diameter of the coil portion 31 in plan view is preferably the same as or slightly larger than the outer diameter of the secondary coil 41.

[0017] The first element 32 is a member extending from one end of the coil portion 31. The second element 33 is a member extending from the other end of the coil portion 31, being shorter than the first element 32 and parallel to the first element 32. Both the first element 32 and the second element 33 have an expansion / contraction portion 34. The expansion / contraction portion 34 is a portion that expands and contracts in the longitudinal direction of the first element 32 and the second element 33 themselves. In this embodiment, the expansion / contraction portion 34 is made up of a compression coil spring, and extends over almost the entire longitudinal area of ​​the first element 32 and the second element 33. A tension coil spring may be used as the expansion / contraction portion 34 instead of a compression coil spring.

[0018] When the wavelength of radio waves at the communication frequency of RFID tag 1 is λ, the antenna 30 can be made approximately shortest by setting the electrical length of first element 32 to approximately 0.185λ of the wavelength, the electrical length of second element 33 to approximately 0.046λ of the wavelength, and setting the total length of first element 32, second element 33, and coil portion 31 to approximately λ / 4. Note that the electrical length of first element 32 refers to the linear distance from the start end of first element 32 (one end of coil portion 31) to the end end of first element 32, and the electrical length of second element 33 refers to the linear distance from the start end of second element 33 (the other end of coil portion 31) to the end end of second element 33, and the expanded length of the compression coil spring that constitutes expandable portion 34 is not taken into consideration.

[0019] The compression coil spring that constitutes the expandable portion 34 is a spring that utilizes a repulsive force when compressed, and gaps 35 are formed between adjacent wires as shown in Fig. 5(a). Therefore, the expandable portion 34 shortens when subjected to a compressive force as shown in Fig. 5(b), lengthens when subjected to a tensile force as shown in Fig. 5(c), and can bend when subjected to a lateral force as shown in Fig. 5(d). In this way, the first element 32 and the second element 33 are equipped with the expandable portion 34, and the expandable portion 34 deforms appropriately in response to an external force, making the first element 32 and the second element 33 less likely to break or deform, thereby improving the durability of the RFID tag 1.

[0020] In this embodiment, the coil portion 31, the first element 32, and the second element 33 are formed by bending a single conductor wire. This facilitates the manufacture of the antenna 30 and increases the reliability of the electrical connection between the coil portion 31, the first element 32, and the second element 33, but this is not limiting and the antenna 30 may also be formed by joining the ends of multiple conductor wires. If the ends of the first element 32 and the second element 33 are folded back as shown in Figure 1(e), when the RFID tag 1 is attached to a rubber product, it is possible to prevent the ends of both elements 32, 33 from piercing or tearing the rubber product, and further to prevent damage to the RFID tag 1.

[0021] 3 and 4, the printed circuit board 20 is held in place by inserting it into the gaps 31a between the wires of the coil section 31, and the coil section 31 forms the primary side of the coupling transformer 40. It is preferable to align the axis of the coil section 31 with the axis of the secondary coil 41. Both the front and back sides of the printed circuit board 20 are sealed with insulating layers 23, so that the coil section 31 and the secondary coil 41 are not electrically connected. Furthermore, the wires of the coil section 31 in the areas where the printed circuit board 20 is not inserted are naturally insulated to maintain electrical coil function.

[0022] FIG. 6 is an equivalent circuit diagram of an RFID tag 1 attached to a rubber product such as a tire. During reception, radio waves received by the first element 32 and the second element 33 are transmitted to the RF chip 10 via the coupling transformer 40. Specifically, currents with an inverted phase to the radio waves flow through the first element 32 and the second element 33, causing current to flow through the primary coil 31 and inducing an AC voltage in the secondary coil 41. While the impedance of the RF chip 10 is on the order of several kΩ to 10 kΩ, the impedance between the first element 32 and the second element 33 is small, at approximately 100 Ω. The low impedance between the first element 32 and the second element 33 is largely due to the impedance of the antenna 30 itself as well as the influence of resistive components such as carbon black contained in the rubber product. Some types of vulcanized rubber tires have a resistivity of several tens of kΩ-cm. If the RFID tag 1 of the present invention were embedded in a vulcanized rubber tire and both terminals of the RF chip 10 were directly connected to the first element 32 and the second element 33, the received radio waves could not be efficiently guided to the RF chip 10.

[0023] Therefore, in the RFID tag 1 of the present invention, the ratio n (n=N2 / N1) of the number of windings N2 on the secondary side to the number of windings N1 on the primary side of the coupling transformer 40 is adjusted. Specifically, when the RF chip 10 with input impedance Z is connected to the first element 32 and the second element 33 via the coupling transformer 40, the impedance on the primary side of the coupling transformer 40 is Z / n ２Therefore, by making the number of windings N1 of the primary coil 31 less than the number of windings N2 of the secondary coil 41, n is increased to lower the impedance of the primary input, and the coupling transformer 40 converts the secondary side to high impedance to match the input impedance Z of the RF chip 10. By lowering the impedance on the primary side, the effects of carbon black, etc. can be suppressed, even when the RFID tag 1 is used in a rubber product containing carbon black, etc. However, there are limitations, such as the area of ​​the coupling transformer 40, on increasing the number of windings N2 on the secondary side, and the ratio n of the number of windings on the secondary side to the number of windings on the primary side must be adjusted appropriately depending on the specifications of the RF chip 10, the material of the rubber product, the amount of carbon black, etc., etc.

[0024] The antenna 30 of the RFID tag 1 operates similarly to a monopole antenna 30, with the second element 33 serving as the ground plane and the first element 32 serving as the antenna wire. When the RFID tag 1 is attached to or embedded in a vulcanized rubber product, the second element 33 electrically connects to the vulcanized rubber product, further strengthening the ground plane. Conventional dipole antenna-type RFID tags have two antenna elements with an electrical length of λ / 4 extending laterally from a printed circuit board (PCB) equipped with an RF chip. This configuration can damage the connection between the antenna elements and the PCB if the rubber product deforms and forces are applied to the two antenna elements in opposite directions. In contrast, the RFID tag 1 of the present invention has the advantages of the first element 32 and the second element 33 extending parallel to the same direction, making the connection between the base of the first element 32 or the second element 33 and the PCB 20 less susceptible to damage and enabling miniaturization. Furthermore, vulcanized rubber enters the gaps 35 of the stretchable portion 34 and hardens, integrating the RFID tag 1 with the vulcanized rubber product such as a tire. Therefore, the stretchable portion 34 also deforms appropriately in response to deformation of the tire or the like, making the first element 32 and the second element 33 less likely to be damaged, and improving the durability of the RFID tag 1.

[0025] As shown in FIG. 7 , the front and back surfaces of an RFID tag 1 for use with rubber products may be covered with rubber sheets 50 and 51. Specifically, the RFID tag 1 is placed between a first rubber sheet 50 and a second rubber sheet 51, and the first rubber sheet 50 and the second rubber sheet 51 are then crimped together to produce a rubber-covered RFID tag 1. The rubber sheets 50 and 51 can be made of ordinary natural rubber or various synthetic rubbers, or they may be made of adhesive materials such as butyl rubber sheets. Covering the RFID tag 1 with the rubber sheets 50 and 51 prevents damage to the components that make up the RFID tag 1 and prevents misalignment of the components even if the rubber product is deformed, and also provides the RFID tag 1 with waterproof and dustproof properties.

[0026] FIG. 8 shows a state in which a rubber-covered RFID tag 1 is attached to the inside of a tire 60 as an example of a rubber product. The position at which the RFID tag 1 is attached is not particularly limited. The RFID tag 1 may be attached to the inside of the tire 60, or may be embedded in the rubber of the tire 60. Also, an RFID tag 1 that is not covered with rubber may be attached to or embedded in the inside of the tire 60. By attaching the RFID tag 1 to the tire 60, the second element 33 is electrically connected to the tire 60, and as a result, the tire 60 functions as the ground for the RFID tag 1. Therefore, the RFID tag 1 of the present invention can communicate with high sensitivity even when attached to a rubber product such as a tire 60. The connection between the second element 33 and the tire 60 may be capacitive or direct.

[0027] Next, a method for manufacturing the RFID tag 1 for rubber products will be described. First, as shown in FIG. 9(a), multiple secondary coils 41 of a coupling transformer 40 are arranged in the left-right direction on a printed circuit board 20 (Step A). ​​Next, as shown in FIG. 9(b), openings 24, 25 are provided on the front and rear sides of the multiple secondary coils 41 (Step B). The distance from the front opening 24 to the rear opening 25 must match the length of the RFID tag 1 in the front-to-rear direction. Next, as shown in FIG. 9(c), cuts 26 are made between the multiple secondary coils 41 (Step C). The spacing between the cuts 26 must match the length of the RFID tag 1 in the left-to-right direction. Steps A to C can be performed in any order. Furthermore, the RF chip 10 must be connected to the secondary coil 41 at the appropriate timing. After the RF chip 10 is connected to the secondary coil 41, its surface is preferably covered with an insulating layer 27.

[0028] Next, as shown in FIG. 10(a), the front opening 24 is used to insert the secondary coil 41 into the gaps 31a between the wires of the coil section 31. This causes the coil section 31 to function as the primary side of the coupling transformer 40. At this time, the first element 32 and the second element 33 are placed on the side edge 28 of the front opening 24, and the coil section 31 can be moved toward the secondary coil 41 by sliding it along the surface of the side edge 28. Furthermore, with the secondary coil 41 inserted into the gaps 31a between the wires, the first element 32 is placed on the side edge 28, so there is no risk of the antenna 30 falling off the printed circuit board 20. If necessary, the coil section 31 and the printed circuit board 20 may be glued together. Finally, as shown in FIG. 10(b), the printed circuit board 20 is cut along the notches 26 to complete the RFID tag 1 for rubber products.

[0029] Next, modified examples of antennas are shown. Antenna 70 shown in Figures 11(a) and (b) has an expandable section 36 formed by bending portions of the first element 32 and the second element 33. In this case, the expandable section 36 shortens when subjected to a compressive force as shown in Figure 11(c), lengthens when subjected to a tensile force as shown in Figure 11(d), and can bend when subjected to a lateral force as shown in Figure 11(e). Antenna 71 shown in Figure 12 has an expandable section 37 formed by forming a compression coil spring in portions of the first element 32 and the second element 33. In this case, the expandable section 37 deforms when subjected to an external force as shown in Figures 5(b) to (d).

[0030] The antenna 72 shown in Figure 13(a) has an expandable portion 38 formed throughout the first element 32 and the second element 33 by twisting multiple wires (for example, three wires). In this case, the expandable portion 38 shortens slightly when subjected to a compressive force as shown in Figure 13(b), lengthens slightly when subjected to a tensile force as shown in Figure 13(c), and can bend when subjected to a lateral force as shown in Figure 13(d). Note that the expandable portions 36, 37 do not necessarily need to be provided on both the first element 32 and the second element 33; they may be provided only on the second element 33, which is more susceptible to external forces, as shown in Figure 14.

[0031] The present invention is an RFID tag for rubber products that is small, suitable for mass production, and has excellent durability, and has industrial applicability.

[0032] REFERENCE SIGNS LIST 1 RFID tag for rubber products 10 RF chip 20 Printed circuit board 21 Through hole 22 Through hole 23 Insulating layer 24 Front opening 25 Rear opening 26 Notch 27 Insulating layer 28 Side edge 30 Antenna 31 Coil portion 31a Gap 32 First element 33 Second element 34 Stretchable portion 35 Gap 36 Stretchable portion 37 Stretchable portion 40 Coupling transformer 41 Secondary coil 50 First rubber sheet 51 Second rubber sheet 60 Tire

Claims

1. An RFID tag for rubber products comprising: a coupling transformer, an RF chip connected to the secondary side of the coupling transformer, a printed circuit board on which the RF chip is mounted, and an antenna, wherein the antenna comprises a coil portion, a first element extending from one end of the coil portion, and a second element which is shorter than the first element and extends parallel to the first element from the other end of the coil portion, the number of windings of the coil portion is less than the number of windings of the secondary side of the coupling transformer, the coil portion forms the primary side of the coupling transformer by holding the printed circuit board in the gaps between the wires of the coil portion, and at least one of the first element and the second element has an expansion portion that expands and contracts in its longitudinal direction.

2. The RFID tag for rubber products according to claim 1, characterized in that it is for rubber products containing carbon black.

3. An RFID tag for rubber products as described in claim 1 or 2, characterized in that the printed circuit board has insulating layers on both the front and back sides.

4. The RFID tag for rubber products according to claim 1, characterized in that the coil portion, the first element and the second element are made of a single conducting wire.

5. The RFID tag for rubber products according to claim 1, characterized in that the axial center of the secondary coil of the coupling transformer coincides with the axial center of the coil portion.

6. An RFID tag for rubber products as described in claim 1, characterized in that when the wavelength of radio waves at the communication frequency is λ, the electrical length of the portion consisting of the first element, the second element and the coil portion is λ / 4.

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