An evaluation method of mutual inductance for buried RFID tags based on dumbbell-shaped magnetic cores

Abstract

The present invention discloses a method for evaluating the mutual inductance of buried RFID tags based on dumbbell-shaped magnetic cores, belonging to the field of RFID, and comprising the following steps: S1, establishing three-dimensional simulation models of dumbbell-shaped magnetic cores and cylindrical magnetic cores; S2, applying magnetic fields at the same positions on the three-dimensional simulation models of the dumbbell-shaped magnetic cores and the cylindrical magnetic cores respectively for numerical simulation to obtain the magnetic induction intensities of the dumbbell-shaped magnetic cores and the cylindrical magnetic cores respectively; S3, comparing the magnetic induction intensities of the dumbbell-shaped magnetic cores and the cylindrical magnetic cores and performing fitting; S4, establishing a mutual inductance evaluation model of the buried RFID tags of the dumbbell-shaped magnetic cores according to the fitting situation in step S3; S5, evaluating the mutual inductance situation of the buried RFID tags according to the mutual inductance evaluation model in step S4, which has stronger adaptability and higher accuracy for the mutual inductance prediction of dumbbell-shaped buried RFID tags.

Description

Technical Field

[0001] The present invention relates to the field of RFID, and particularly to a mutual inductance evaluation method for buried RFID tags based on dumbbell-shaped magnetic cores. Background Art

[0002] RFID (Radio Frequency Identification) technology is a wireless communication technology, and mutual inductance is an important parameter in the design and optimization of coupled antennas in RFID systems.

[0003] So far, exact semi-analytical evaluation expressions for the mutual inductance between two coupled air-core coils have been derived. Unfortunately, they involve complete elliptic integrals and do not have simple algebraic solutions, which is inconvenient for optimizing RFID systems. It should also be noted that when a ferrite magnetic core is inserted into an air-core tag antenna, a simple algebraic solution for evaluating the mutual inductance between two coupled antennas has been proposed. However, the current mutual inductance evaluation of this type of coil-shaped RFID tag antenna is only limited to traditional cylindrical magnetic cores and is not applicable to dumbbell-shaped magnetic cores. Currently, research on the mutual inductance evaluation of buried RFID tags with dumbbell-shaped magnetic cores is quite scarce. Summary of the Invention

[0004] In view of the above problems, the present invention provides a mutual inductance evaluation method for buried RFID tags based on dumbbell-shaped magnetic cores, which has stronger adaptability and higher accuracy.

[0005] The technical solution of the present invention is as follows:

[0006] A mutual inductance evaluation method for buried RFID tags based on dumbbell-shaped magnetic cores, comprising the following steps:

[0007] S1. Establish three-dimensional simulation models of dumbbell-shaped magnetic cores and cylindrical magnetic cores;

[0008] S2. Apply magnetic fields at the same positions on the three-dimensional simulation models of the dumbbell-shaped magnetic core and the cylindrical magnetic core respectively for numerical simulation, and obtain the magnetic induction intensities of the dumbbell-shaped magnetic core and the cylindrical magnetic core respectively;

[0009] S3. Compare the magnetic induction intensities of the dumbbell-shaped magnetic core and the cylindrical magnetic core and perform fitting;

[0010] S4. Establish a mutual inductance evaluation model for buried RFID tags of the dumbbell-shaped magnetic core according to the fitting situation in step S3;

[0011] S5. Evaluate the mutual inductance situation of the buried RFID tags according to the mutual inductance evaluation model in step S4.

[0012] In step S2, the position where the magnetic field is applied to the three-dimensional simulation model is the Z-axis of the simulation model.

[0013] In the step S3, the fitting algorithm is as follows:

[0014]

[0015] Where: B 哑铃 is the magnetic induction intensity of the dumbbell-shaped magnetic core, B 圆柱 is the magnetic induction intensity of the cylindrical magnetic core, r1 is the radius of the axis of the coil wound around the dumbbell-shaped magnetic core, and r2 is the radius of the large end of the dumbbell-shaped magnetic core and the cylindrical magnetic core.

[0016] In the step S4, the mutual inductance model establishment method is as follows:

[0017] 1) Equivalent the dumbbell-shaped magnetic core to a cylindrical magnetic core;

[0018] 2) Calculate the magnetic flux and mutual inductance of the air-core coil antenna;

[0019] 3) Calculate the mutual inductance of inserting the dumbbell-shaped magnetic core into the air-core coil.

[0020] In the step 2), the calculation formulas for the magnetic flux and mutual inductance are as follows:

[0021]

[0022] Where: ψ 空 is the magnetic flux of the air-core coil; M 空 is the mutual inductance of the air-core coil; I is the current of the reader coil antenna; μ0 is the magnetic permeability of free space; n t is the number of turns of the reader antenna; nc is the number of turns of the coil antenna of the dumbbell-shaped buried RFID tag; l t is the height of the reader antenna; l is the height of the coil antenna of the dumbbell-shaped buried RFID tag; h is the vertical distance from the bottom of the reader antenna to the center of the dumbbell-shaped buried RFID tag; r t is the radius of the reader coil; S is the simplified expression of the integral.

[0023] In the step 3), the mutual inductance M algorithm for inserting the dumbbell-shaped magnetic core is as follows:

[0024]

[0025] Where: μ e is the effective magnetic permeability equivalent to the cylindrical magnetic core; is the introduced correction term related to the aspect ratio, M is the mutual inductance of inserting the dumbbell-shaped magnetic core, r1 is the radius of the axis of the coil wound around the dumbbell-shaped magnetic core, r2 is the radius of the large end of the dumbbell-shaped magnetic core and the cylindrical magnetic core, and l is the height of the coil antenna of the dumbbell-shaped buried RFID tag.

[0026] In step S5, the mutual inductance evaluation index is as follows: when the distance between the center of the ferrite core and the reader antenna is h, if the mutual inductance value between the two is greater than a specific value Q, within the range of this distance h, the dumbbell-shaped core antenna can achieve effective reading and has good performance, where the value of Q is the minimum value required for the mutual inductance of the antenna determined in advance according to different frequencies.

[0027] The beneficial effects of the present invention are as follows:

[0028] 1. The mutual inductance prediction model and method proposed by the present invention are for the mutual inductance prediction of dumbbell-shaped buried RFID tags, which can avoid the dilemma that in the traditional mutual inductance calculation of dumbbell shapes, the calculation error climbs synchronously due to the amplification of the large-end angle, and has more excellent adaptability and higher accuracy.

[0029] 2. The mutual inductance prediction model and method proposed by the present invention are more suitable for the performance optimization of dumbbell-shaped ferrite RFID antennas.

[0030] 3. The mutual inductance prediction model proposed by the present invention avoids the cumbersome elliptic integral calculation and gives a very simple algebraic analytical formula. Description of the Drawings

[0031] Figure 1 is a flowchart of a method for evaluating the mutual inductance of a buried RFID tag based on a dumbbell-shaped core according to an embodiment of the present invention;

[0032] Figure 2 is a size diagram of a dumbbell-shaped core of a method for evaluating the mutual inductance of a buried RFID tag based on a dumbbell-shaped core according to an embodiment of the present invention;

[0033] Figure 3 is a size diagram of a cylindrical core of a method for evaluating the mutual inductance of a buried RFID tag based on a dumbbell-shaped core according to an embodiment of the present invention;

[0034] Figure 4 is a schematic diagram of a dumbbell-shaped buried RFID antenna and a reader antenna of a method for evaluating the mutual inductance of a buried RFID tag based on a dumbbell-shaped core according to an embodiment of the present invention;

[0035] Figure 5 is a fitting diagram of the coil segment l wound around a cylindrical core and the magnetic induction intensity B of the ferrite core l segment of a method for evaluating the mutual inductance of a buried RFID tag based on a dumbbell-shaped core according to an embodiment of the present invention;

[0036] Figure 6 is a comparison diagram of the accuracy of the mutual inductance evaluation model and the existing model of a method for evaluating the mutual inductance of a buried RFID tag based on a dumbbell-shaped core according to an embodiment of the present invention. Detailed Embodiments

[0037] The embodiments of the present invention will be further described below with reference to the accompanying drawings.

[0038] Embodiment:

[0039] As Figure 1 - Figure 6 shown, a mutual inductance evaluation method for buried RFID tags based on dumbbell-shaped magnetic cores includes the following steps:

[0040] S1. Establish three-dimensional simulation models of dumbbell-shaped magnetic cores and cylindrical magnetic cores;

[0041] In the Maxwell modeling space, with O as the origin, dumbbell-shaped and cylindrical magnetic cores with their central axes coinciding with the Z-axis are established. The overall height L of the dumbbell-shaped and cylindrical magnetic cores is 170 mm, and the radius of the large end of the dumbbell-shaped magnetic core and the cylindrical magnetic core is r2 = 40 mm; the height of the central axis of the coil wound around the dumbbell-shaped magnetic core is l = 140 mm, and the radius is r1 = 20 mm; the magnetic core material is set as ferrite.

[0042] S2. Apply magnetic fields at the same positions on the three-dimensional simulation models of the dumbbell-shaped magnetic core and the cylindrical magnetic core respectively, conduct numerical simulations, and obtain the magnetic induction intensities of the dumbbell-shaped magnetic core and the cylindrical magnetic core respectively;

[0043] The position where the magnetic field is applied to the three-dimensional simulation model is the Z-axis of the simulation model. A simulation area is defined outside the established three-dimensional simulation model, and a tangential magnetic field of U(x) = 1 A / m is applied on one plane parallel to the Z-axis, and the magnetic field direction is parallel to the Z-axis; the two planes perpendicular to the Z-axis are zero tangential magnetic fields; the solution accuracy is 0.1 and the maximum number of passes is 50, and the simulation settings are checked for simulation calculation.

[0044] S3. Compare the magnetic induction intensities of the dumbbell-shaped magnetic core and the cylindrical magnetic core and perform fitting;

[0045] The fitting algorithm is as follows:

[0046]

[0047] where: B 哑铃 is the magnetic induction intensity of the dumbbell-shaped magnetic core, B 圆柱 is the magnetic induction intensity of the cylindrical magnetic core, r1 is the radius of the central axis of the coil wound around the dumbbell-shaped magnetic core, and r2 is the radius of the large end of the dumbbell-shaped magnetic core and the cylindrical magnetic core.

[0048] As Figure 6The fitting curve shown. Since the coil is wound around the central axis l, the magnetic induction intensity of the cylindrical magnetic core multiplied by the fitting coefficient can be successfully fitted with the magnetic induction intensity of the central axis l section of the dumbbell-shaped magnetic core. Only the simulation and fitting of the magnetic induction intensity of the ferrite type and cylindrical magnetic core with L = 170mm, r2 = 40mm, r1 = 20mm, and l = 140mm are shown here. The magnetic cores with other dimensions can be obtained by repeating steps S2 and S3.

[0049] S4. According to the fitting situation in step S3, establish the mutual inductance model of the buried RFID tag of the dumbbell-shaped magnetic core;

[0050] The method for establishing the mutual inductance model is as follows:

[0051] 1) Equivalent the dumbbell-shaped magnetic core to a cylindrical magnetic core;

[0052] 2) Calculate the magnetic flux and mutual inductance of the air-core coil antenna;

[0053] The calculation formulas for the magnetic flux and mutual inductance are as follows:

[0054]

[0055] Among them: ψ 空 is the magnetic flux of the air-core coil; M 空 is the mutual inductance of the air-core coil; I is the current of the reader coil antenna; μ0 is the magnetic permeability of free space; n t is the number of turns of the reader antenna; nc is the number of turns of the coil antenna of the buried RFID tag of the dumbbell shape; l t is the height of the reader antenna; l is the height of the coil antenna of the buried RFID tag of the dumbbell shape; h is the vertical distance from the bottom of the reader antenna to the center of the buried RFID tag of the dumbbell shape; r t is the radius of the reader coil; S is the simplified expression of the integral.

[0056] 3) Calculate the mutual inductance of inserting the dumbbell-shaped magnetic core into the air-core coil.

[0057] The algorithm for the mutual inductance M of inserting the dumbbell-shaped magnetic core is as follows:

[0058]

[0059] Among them: μ e is the effective magnetic permeability equivalent to the cylindrical magnetic core; is the introduced correction term related to the aspect ratio, M is the mutual inductance of inserting the dumbbell-shaped magnetic core, r1 is the radius of the central axis of the coil wound on the dumbbell-shaped magnetic core, r2 is the radius of the large end of the dumbbell-shaped magnetic core and the cylindrical magnetic core, and l is the height of the coil antenna of the buried RFID tag of the dumbbell shape.

[0060] S5. Evaluate the mutual inductance of the buried RFID tag according to the mutual inductance model in step S4.

[0061] The mutual inductance evaluation index is as follows: When the distance between the center of the ferrite core and the reader antenna is h, if the mutual inductance value between them is greater than a specific value Q (50 nH), then within the range of this distance h, the dumbbell-shaped core antenna can achieve effective reading and has good performance. Among them, the value of Q (50 nH) is the minimum value required for the mutual inductance of the antenna determined in advance according to different frequencies.

[0062] The traditional complete elliptic integral is to divide the reader antenna and the RFID tag coil antenna into (2K + 1)×(2S + 1) units, and the corresponding formula for calculating the mutual inductance is:

[0063]

[0064]

[0065] k = -K,..., K; s = -S,..., S

[0066]

[0067] m = l f / r1,

[0068] In the formula, M′ f is the mutual inductance of the antenna calculated by the complete elliptic integral, l f is the height of the antenna core, l c is the height of the antenna coil, μ r is the relative permeability, D fc is the effective demagnetization coefficient, n t is the number of turns of the reader antenna, n c is the number of turns of the antenna coil, μ0 is the magnetic permeability in free space, r t is the radius of the reader antenna coil, r c is the radius of the tag antenna coil, l t is the height of the reader antenna coil, r1 is the radius of the tag antenna core, and h is the distance between the tag core and the reader.

[0069] This calculation method is complex. Compared with this method, the present technology can obtain the same result, can significantly simplify the calculation method, reduce the evaluation difficulty, and the mutual inductance curve calculated by the calculation method proposed by the present invention basically coincides with the accurate calculation of the complete elliptic integral.

[0070] The above-described embodiments merely represent specific implementation manners of the present invention. The description is relatively specific and detailed, but it should not be construed as a limitation on the scope of the patent for the present invention. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made, and these all fall within the protection scope of the present invention.

Claims

1. An evaluation method for mutual inductance of buried RFID tags based on dumbbell-shaped magnetic cores, characterized in that, It includes the following steps: S1. Establish three-dimensional simulation models of dumbbell-shaped magnetic cores and cylindrical magnetic cores; S2. Apply magnetic fields at the same positions on the three-dimensional simulation models of the dumbbell-shaped magnetic cores and the cylindrical magnetic cores respectively for numerical simulation, and obtain the magnetic induction intensities of the dumbbell-shaped magnetic cores and the cylindrical magnetic cores respectively; S3. Compare the magnetic induction intensities of the dumbbell-shaped magnetic cores and the cylindrical magnetic cores and perform fitting; S4. According to the fitting situation in step S3, establish a mutual inductance evaluation model for the buried RFID tag of the dumbbell-shaped magnetic core; the method for establishing the mutual inductance evaluation model is as follows: 1) Equivalent the dumbbell-shaped magnetic core to a cylindrical magnetic core; 2) Calculate the magnetic flux and mutual inductance of the air-core coil antenna, and the calculation formulas for the magnetic flux and the mutual inductance are as follows: Where: ψ 空 is the magnetic flux of the air-core coil; M 空 is the mutual inductance of the air-core coil; I is the current of the reader coil antenna; μ0 is the magnetic permeability of free space; n t is the number of turns of the reader antenna; nc is the number of turns of the tag coil antenna of the dumbbell-shaped buried RFID tag; l t is the height of the reader antenna; l is the height of the tag coil antenna of the dumbbell-shaped buried RFID tag; h is the vertical distance from the bottom of the reader antenna to the center of the dumbbell-shaped buried RFID tag; r t is the radius of the reader coil; S is the simplified expression of the integral; 3) Calculate the mutual inductance of inserting the dumbbell-shaped magnetic core into the air-core coil, and the mutual inductance algorithm for inserting the dumbbell-shaped magnetic core is as follows: Where: μ e is the effective permeability equivalent to a cylindrical core; is a correction term related to the aspect ratio introduced, M is the mutual inductance of the inserted dumbbell-shaped core, r1 is the radius of the axis of the coil wound around the dumbbell-shaped core, r2 is the radius of the large end of the dumbbell-shaped core and the cylindrical core, and l is the height of the coil antenna of the buried dumbbell-shaped RFID tag; S5. Evaluate the mutual inductance situation of the buried RFID tag according to the mutual inductance evaluation model in step S4.

2. The mutual inductance evaluation method of the buried RFID tag based on a dumbbell-shaped magnetic core according to claim 1, wherein In step S2, the position where the magnetic field is applied to the three-dimensional simulation model is the Z-axis of the simulation model.

3. A mutual inductance evaluation method for buried RFID tags based on dumbbell-shaped magnetic cores according to claim 1, characterized in that In step S3, the fitting algorithm is as follows: Where: B 哑铃 is the magnetic induction intensity of the dumbbell-shaped magnetic core, and B 圆柱 is the magnetic induction intensity of the cylindrical magnetic core, r1 is the radius of the axis of the coil wound around the dumbbell-shaped magnetic core, and r2 is the radius of the large end of the dumbbell-shaped magnetic core and the cylindrical magnetic core.

4. A mutual inductance evaluation method for buried RFID tags based on dumbbell-shaped magnetic cores according to claim 1, characterized in that In step S5, the mutual inductance evaluation index is: when the distance between the center of the ferrite magnetic core and the reader antenna is h, if the mutual inductance value between the two is greater than a specific value Q, then within the range of this distance h, the dumbbell-shaped magnetic core antenna can achieve effective reading and has good performance, where the Q value is the minimum value required for the mutual inductance of the antenna determined in advance according to different frequencies.

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