JP2006311588A - Gate antenna, and rfid system with the gate antenna - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a gate antenna which can ensure the uniformity of a magnetic field that generates between antennas, in particular and in which antennas each composed of an figure eight-shaped antenna and a central loop are arranged facing each other, and to provide an RFID system provided with the gate antenna. <P>SOLUTION: A gate antenna 2, which is composed of: the figure eight-shaped loop which is driven by a high frequency power source; and a parasitic central loop 6a which is installed near the center of the figure eight-shaped loop and driven by an induced current that flows by magnetic coupling with at least one loop, or a feed central loop 6b which is driven by a different power source. Such gate antennas 2, are arranged facing each other. In each of the antennas, phases of figure eight-shaped upper and lower loops are shifted at 180°, the phase of the central loop 6 is shifted at 90° with respect to the figure eight-shaped upper and lower loops and the phases of the gate antennas 2 are locked or shifted at 90°, thereby improving the uniformity of magnetic field distribution between the gate antennas. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a reader / writer gate antenna used in an RFID (Radio Frequency Identification) system, and more particularly, to a gate antenna in which a loop antenna including an 8-shaped antenna is disposed oppositely.

  In recent years, RFID systems that perform data communication between a transponder including an IC chip and a reader / writer (or reader) have become widespread. Since this RFID system performs data communication using the antennas provided in each of the transponder and the reader / writer, communication is possible even if the transponder is several centimeters to several tens of centimeters away from the reader / writer. Due to its strength against static electricity, it has been used in various fields such as factory production management, logistics management, and entrance / exit management.

  Since this RFID system uses electromagnetic induction or electromagnetic coupling to exchange data, it is necessary to increase the magnetic field generated by the reader / writer antenna. On the other hand, in order to use the RFID system, It is necessary to set the electric field strength or antenna gain measured at a position 3 m away and the power supplied to the antenna (aerial) to a value determined by the Radio Law.

  As a method of weakening the electric field in the distance without weakening the magnetic field in the vicinity of the antenna, there is a method of using an 8-shaped antenna (or a double loop antenna). As shown in FIG. 16, this 8-character antenna is a device in which two loop antennas are connected in series or in parallel, and they are driven by the same high-frequency power source. The magnetic fluxes generated by the two loops (8-shaped antenna upper part 5a and 8-shaped antenna lower part 5b) are opposite to each other, and they appear to cancel each other at a distance. There is an advantage that an antenna having the same gain and a large size can be manufactured as compared with a normal single loop antenna.

  On the other hand, when the RFID system is used for entrance / exit management, a gate antenna in which two or more antennas are arranged to face each other is used. In such applications, in order to reliably communicate with the transponder passing between the gates, the gate antenna is required to have a uniform magnetic field strength at each position and in each direction component between the gates. For this reason, each antenna is not a single loop, but an 8-character shape or an applied shape thereof is adopted, thereby forming a complicated magnetic field distribution and maintaining a good communication state with the transponder passing between the gate antennas. be able to.

Japanese Patent Laid-Open No. 9-64633

  When each of the antennas constituting the gate antennas on both sides constituting the gate is driven by an individual reader / writer high-frequency power supply, the phase of the magnetic field radiated from each antenna is randomly determined by the power-on timing. Is done. As a result, the magnetic field between the gate antennas formed by the magnetic fields radiated from the two antennas changes over time due to beating, and it can be considered that the magnetic field is synergistic with the place and time, or is canceled. This adversely affects the generation of a uniform magnetic field.

  To solve the problem of magnetic field inhomogeneity in such a gate antenna, there is a method in which the positions of the antennas are shifted. For example, in Japanese Patent Laid-Open No. 9-64633, rectangular first and second loop antennas are provided substantially in parallel along the transponder moving direction, and the shape formed by the conductor of the first loop antenna and the second There is disclosed a method of mutually complementing the dead zone by shifting the center line with the shape formed by the conductor of the loop antenna up and down.

  However, the method described in the above publication is a method in the case where the gate antenna is composed of a single-loop antenna. Since the magnetic field distribution is relatively simple in the single-loop antenna, the magnetic field distribution is made uniform by shifting the positions of each other. However, the above-mentioned 8-shaped loop antenna has a complicated magnetic field distribution, and it is difficult to improve the uniformity of the magnetic field distribution by simply shifting the position of each other.

  The present invention has been made in view of the above problems, and its main purpose is to provide a gate antenna that can ensure the uniformity of the magnetic field generated between the antennas, particularly, an 8-shaped antenna and a central loop. And an RFID system provided with the gate antenna.

  In order to achieve the above object, a gate antenna according to the present invention includes a pair of antennas arranged opposite to each other, and one of the pair of antennas is driven by a high frequency power source and connected in parallel to a first loop and a first loop. A fourth loop connected in parallel to the other of the pair of antennas, driven by a high-frequency power source, and having a second loop and a third loop installed near the center of the first loop and the second loop; A loop and a fifth loop; and a sixth loop installed in the vicinity of the center of the fourth loop and the fifth loop, the first loop, the fourth loop, the second loop, and the fifth loop. In the gate antenna in which each of the third loop and the sixth loop is arranged to face each other, the phase is generated in the second loop when the phase of the magnetic field generated in the first loop is 0 °. The phase of the magnetic field is set to about + 180 °, the phase of the magnetic field generated in the third loop is set to about + 90 °, and the phase of the magnetic field generated in the fourth loop is set to about 0 ° by the phase control means. Control is performed so that the phase of the magnetic field generated in the five loops is approximately + 180 °, and the phase of the magnetic field generated in the sixth loop is approximately + 90 °.

  In the present invention, the third loop is fed by a parasitic loop driven by an induced current flowing by magnetic coupling with at least one of the first loop and the second loop, or by a power source different from the high frequency power source. The feeding loop is driven, and the sixth loop is different from a non-feeding loop driven by an induced current flowing by magnetic coupling with at least one of the fourth loop and the fifth loop, or the high-frequency power source. A feeding loop driven by a power source can be used.

  As described above, according to the gate antenna of the present invention, each of the two antennas constituting the gate antenna is configured by the 8-shaped loop and the central loop, and the carrier input to each antenna is synchronized by the dedicated synchronization signal. Furthermore, by causing an arbitrary delay time in the synchronization state, a phase difference is generated between the magnetic fields radiated from both antennas. As a result, it is possible to eliminate a minute point of magnetic flux density that is impossible to communicate with the conventional gate antenna, and to generate a uniform magnetic field between the gate antennas.

  According to the gate antenna of the present invention and the RFID system using the gate antenna, the magnetic field distribution between the gate antennas can be controlled, and the local non-uniformity of the magnetic field distribution which has been a problem of the conventional gate antenna can be reduced. Can be improved.

  The reason is that each gate antenna is composed of an 8-shaped antenna and a central loop antenna, and the phase of one of the gate antennas is 8-shaped with respect to the upper portion of the 8-shaped antenna as in the structure described in the previous application. By setting the lower part of the antenna to + 180 ° and the central loop antenna to + 90 ° and shifting each loop of the other gate antenna to 90 ° or the same phase with respect to the opposite loop, the combined magnetic field of both gate antennas is changed. This is because it can be made uniform.

  In a preferred embodiment, the gate antenna according to the present invention is installed in the vicinity of the center of the 8-shaped loop driven by a high-frequency power source and at least one loop, and is induced by magnetic coupling with at least one loop. An antenna composed of a parasitic loop driven by a current or a feeding loop driven by a different power source is arranged oppositely. In each antenna, the phases of the eight upper and lower loops are shifted by 180 °, The phase of the loop installed near the center is shifted by 90 ° with respect to the upper and lower loops of 8 characters, and the phase of each antenna is synchronized or shifted by 90 °, so that the magnetic field distribution between the gate antennas is uniform. Can be increased.

  In order to describe the above-described embodiment of the present invention in more detail, examples of the present invention will be described with reference to FIGS. FIG. 1 is a diagram illustrating an entire configuration of an RFID system according to an embodiment of the present invention, and FIG. 2 is a diagram illustrating a simulation model for calculating a magnetic field distribution between gate antennas. FIG. 3 is a diagram for explaining how to view the simulation results, and FIGS. 4 to 11 are graphs showing the simulation results. FIGS. 12 to 15 are views showing the structure of an 8-shaped antenna according to the prior application of the applicant of the present application.

  As shown in FIG. 1, an RFID system 1 of the present invention includes a transponder 4 having an antenna and an IC therein, a power supply to the transponder 4 and a gate antenna 2 for transmitting and receiving signals, and a signal from the transponder 4. The reader / writer 3 further includes a phase control unit 3a for controlling the phase of power supplied to each gate antenna 2, a high-frequency power supply unit 3b, and a transmission / reception signal. Communication circuit section 3c and an arithmetic processing section 3d for decoding transmission / reception signals. The phase control unit 3a includes a synchronization unit that synchronizes the phase of power supplied to each gate antenna 2, and a delay unit that delays the phase of one gate antenna.

  First, before describing the phase control method of the gate antenna 2 of this embodiment, the characteristics of the 8-shaped antenna constituting the gate antenna 2 will be described. As shown in the conventional example, in the 8-shaped antenna 5, the magnetic fluxes generated by the two loops (8-shaped antenna upper part 5a and 8-shaped antenna lower part 5b) constituting the 8-characters are opposite to each other. Since these appear to cancel each other in the distance, there is an effect of weakening the electric field strength in the distance. However, the 8-shaped antenna 5 has a disadvantage that the magnetic field component in the antenna axial direction becomes 0 at the center of the 8-shaped antenna 5 at the same time. have.

  In order to solve this drawback, as shown in FIG. 17, by disposing another single-loop antenna (non-feed central loop antenna 6a) that is not fed at the center of the 8-shaped antenna 5, over time, By changing the magnetic field distribution, it is possible to eliminate the point that the magnetic field component, which is a drawback of the normal 8-shaped antenna 5, becomes zero (see US Pat. No. 6,166,706, etc.).

  However, in order for this parasitic central loop antenna 6a to function, it is necessary to intersect with one of the two loops constituting the 8-shaped antenna 5 connected in parallel, but it intersects with the parasitic central loop antenna 6a. In the other loop, the inductance of the antenna coil changes due to the mutual inductance, and the resonance point is detuned. As a result, only one of the two loops becomes capacitive or inductive, and a phase difference occurs between the applied voltage and the current value.

  Thus, in the 8-shaped antenna upper portion 5a and the 8-shaped antenna lower portion 5b, a current having a completely opposite phase does not flow with respect to an applied voltage having an opposite phase, and the symmetry that is a characteristic of the 8-shaped antenna 5 is lost. End up. The current phase shift increases the electric field strength at a distance and causes non-uniformity in the near magnetic field strength.

  Therefore, the applicant of the present application has proposed a reader / writer antenna structure that solves the above-mentioned problem in a prior application (Japanese Patent Application No. 2001-339806). The structure of the reader / writer antenna according to this prior application will be outlined with reference to FIGS.

  The prior application discloses a structure as shown in FIG. 12 as one of the reader / writer antennas. The reader / writer antenna includes an 8-shaped antenna 5 in which two symmetrical loop antennas (an 8-shaped antenna upper portion 5a and an 8-shaped antenna lower portion 5b) are connected, and the periphery of the 8-shaped antenna 5. And a parasitic central loop antenna 6a interlinking with at least one loop of the 8-shaped antenna, and further comprising an 8-shaped antenna upper portion 5a or an 8-shaped antenna lower portion 5b. One or both are provided with phase adjusting means such as a capacitor 9.

  In the above structure, by surrounding the 8-shaped antenna 5 and the parasitic central loop antenna 6a with the conductive wire 7, the distance between the 8-shaped antenna upper portion 5a or the 8-shaped antenna lower portion 5b and the floor surface or the metal body is increased. The difference in inductance due to the difference can be eliminated, and by connecting a phase adjusting means such as a capacitor 9 or a coil to the loop overlapping the parasitic central loop antenna 6a, the inductive component due to the influence of mutual inductance is canceled and generated. Current of the same phase as the measured voltage flows.

  Further, the prior application discloses a structure as shown in FIG. 13 as another example of the reader / writer antenna. In this reader / writer antenna, an 8-shaped antenna upper portion 5a and an 8-shaped antenna lower portion 5b are connected in series with a twist, and by making the phases of currents flowing in the upper and lower loops substantially equal, The phase adjustment of the loop is unnecessary, and the distribution of the magnetic flux density in the height direction of the 8-shaped loop antenna 5 is made flat.

  As another example of the prior application description, there is a structure as shown in FIG. In this reader / writer antenna, the parasitic central loop antenna 6a is arranged so as to overlap both the upper part of the 8-shaped antenna 5a and the lower part of the 8-shaped antenna 5b, and the areas of the overlapping portions (region A and region B) are equal. In this way, the mutual inductance is made equal, and the antenna wires are crossed at an arbitrary point between the overlapping portions so that the interlinkage magnetic flux does not cancel out. Even with such a structure, the symmetry of the phases of both loops can be ensured, and the decrease of the magnetic flux in the central portion of the antenna can be suppressed by the parasitic central loop antenna 6a.

  Furthermore, as another example described in the prior application, there is a structure as shown in FIG. This reader / writer antenna is provided with a feeding central loop antenna 6b instead of the non-feeding central loop antenna 6a at the center of the 8-shaped antenna 5 for feeding a current adjusted in phase to the feeding central loop antenna 6b. A phase correction circuit 10 (for example, a circuit having the configuration shown in the figure) is connected.

  That is, the feeding central loop antenna 6b installed at the center is not operated by electromagnetic coupling with the 8-shaped antenna 5, but is operated by directly feeding power from the phase correction circuit 10. The phase of 6b is shifted from the 8-shaped antenna 5 by 90 °. Even with such a structure, a decrease in magnetic flux density at the center of the 8-shaped antenna 5 can be suppressed, and it is not necessary to superimpose the feeding central loop antenna 6b on the 8-shaped antenna 5. A shift in the phase of both loops of the antenna 5 can be suppressed.

  12 to 15, the symmetry of the magnetic field generated by the two loop antennas of the 8-shaped antenna can be ensured, and the generation of a region where the magnetic field is zero can be prevented. However, when the gate antennas 2 are configured by arranging the eight-shaped antennas having the above structure facing each other, when the gate antennas 2 are driven independently, the magnetic field is beaten due to the interaction between the gate antennas 2. In addition, the uniformity of the magnetic field between the gate antennas 2 is lost. Note that this beat is determined by the performance of the oscillators of both readers / writers. If an oscillator with the same oscillation frequency is used, the beat is eliminated, but the phase of the magnetic flux generated from both antennas is determined by the power-on timing. If each gate antenna 2 is driven independently, the uniformity will be lost.

  In order to suppress the beat of the magnetic field and improve the uniformity of the magnetic field between the gate antennas, it is effective to control the phase of each gate antenna 2, but by shifting the phase of each gate antenna. The generated magnetic field distribution is different. In particular, the gate antenna 2 of the present embodiment is composed of six loops in which a set of an 8-shaped antenna upper portion 5a, an 8-shaped antenna lower portion 5b, and a central loop antenna 6 are opposed to each other, and the phases of these six loops are independent. Therefore, it is difficult to obtain the optimum phase by setting. Therefore, the inventor of the present application uses the structure described in the prior application for the gate antenna 2 on one side described above, reveals the magnetic field distribution between the gate antennas when the phase of the opposing gate antenna 2 is changed, by simulation, and determines the optimum position. We decided to examine the phase difference.

  Specifically, as shown in FIG. 2, the structure of the gate antenna 2 composed of the 8-shaped antenna 5 and the central loop antenna 6 is simplified, and an 8-shaped antenna upper portion 5a and an 8-shaped antenna lower portion 5b ( Both have a structure in which a central loop antenna 6 (width 32 cm, height 38 cm) of the same width is provided between them as a separate loop with a width of 32 cm and a height of 42 cm). Simulation was performed using a model in which the same current (absolute value) was passed through the loop. The overlap between the upper portion of the 8-shaped antenna 5a and the central loop antenna 6 was 2 cm, and the distribution in the height direction of the magnetic field component in the center of the gate antennas 2a and 2b (z = 50 cm plane) was obtained by simulation.

  In the simulation, the gate antenna 2a is set so that the upper part of the 8-shaped antenna 5a is a reference (0 °), the phase of the lower part of the 8-shaped antenna 5b is set to + 180 °, the phase of the central loop antenna 6 is set to + 90 °, The phase of each loop of 2b was classified into eight types (types a, b, c, d, aa, bb, cc, dd) as shown in Table 1. That is, the phase difference of the loop corresponding to types a to d is the same phase or opposite phase, and types aa to dd are obtained by adding a phase difference of + 90 ° to the gate antenna B of types a to d. As parameters, the number of turns in each loop was 1 and the current flowing through the loop was 1A.

  In the simulation procedure, first, the magnetic flux density generated by each loop of the gate antennas 2a and 2b is calculated for each component in the plane parallel to the antenna surface at the center (Z = 50 cm) of both gate antennas, and the result is calculated for each loop. The maximum magnetic flux that is generated is multiplied by a sine wave and changed over time. Then, the time maximum value of each point was calculated, and the maximum value in the x direction (transponder traveling direction) was calculated.

  FIG. 3 shows a positional relationship between an example of a result of simulation by the above method and the gate antenna 2. The right side of the figure is the floor surface (GND8), the horizontal axis of the figure represents the position in the height direction (y direction), and the vertical axis represents the magnetic flux density. For example, in FIG. 3, the magnetic flux density has a substantially symmetric and flat distribution centered on the middle (position where the height direction is 0) between the 8-shaped antenna upper portion 5a and the 8-shaped antenna lower portion 5b.

  The results of a similar simulation for the eight types shown in Table 1 are shown in FIGS. In each figure, (a) shows the x component, (b) shows the y component, and (c) shows the z component.

  Here, the gate antenna 2 is required to have two performances: (1) a high magnetic flux density and (2) a uniform magnetic flux density. Considering the simulation results from the above viewpoint, the type b (FIG. 5), the type c (FIG. 6), the type bb (FIG. 9), and the type cc (FIG. 10) are different from each other in terms of the magnetic flux density. In comparison, the peak value of the magnetic flux density is large, but on the other hand, the distribution of the magnetic flux density is not uniform, and in particular, there is a point where the magnetic flux density is almost zero. In the distribution of the magnetic flux density, there is a possibility that an area that cannot communicate with the transponder 4 may be generated.

  On the other hand, although type aa (FIG. 8) and type dd (FIG. 11) have a peak value of magnetic flux density that is not larger than that of the above type, there are points where the magnetic flux density is zero for each of the x, y, and z components. It does not exist and its distribution is relatively flat. In type a (FIG. 4) and type d (FIG. 7), since the measurement point is an intermediate point of the gate antenna 2, the magnetic flux density of the z, x, and y components is almost zero. Is relatively flat in shape.

  Thus, although the magnitude and uniformity of the magnetic flux density are different for each type, the peak value of the magnetic flux density is assumed on the assumption that the communication with the transponder 4 passing between the gate antennas in parallel at the same angle is stable. A shape that is large and has no minimum value is preferred. Therefore, judging comprehensively, the combination of phases where a relatively uniform magnetic field strength is obtained at each position and each magnetic field component between the two gate antennas 2 is set to 0 on the 8-shaped antenna upper portion 5a of the gate antenna 2a on one side. ° When the lower part of the 8-shaped antenna is + 180 ° and the center loop antenna 6 is + 90 °, each of the loops of the gate antenna 2b on the other side is shifted by 90 ° with respect to the opposite loop (type aa or Type dd) is considered preferred.

  Further, the magnetic flux density is smaller than that of type aa or type dd, but the combination of phases for obtaining a relatively flat distribution is that the loops of the gate antenna 2b on the other side have the same phase or opposite phase with respect to the opposite loops. This is a combination of opposite phases (type a or type d), and even this combination enables stable communication with the transponder 4.

  As described above, in the gate antenna 2 in which the 8-shaped antenna 5 and the central loop antenna 6 are arranged to face each other, one of the gate antennas 2a is configured to have an 8-character shape on the basis of the upper portion of the 8-shaped antenna as in the structure described in the previous application. Magnetic flux density between the gate antennas by setting the lower part 2b of the mold antenna to + 180 °, the central loop antenna to + 90 °, and shifting the loops of the other gate antenna 2b by 90 ° with respect to the opposite loops or the same phase. Can be made flat, and it is possible to communicate with the transponder 4 stably regardless of the position between the gate antennas 2.

  In the above description, the case where two gate antennas 2 are arranged opposite to each other has been described. However, the present invention can also be applied to a configuration in which three or more gate antennas are installed. What is necessary is just to adjust a phase so that a phase may satisfy | fill the relationship mentioned above. In the above-described embodiments, the case where the gate antenna is used as the reader / writer antenna of the RFID system has been described. However, the present invention is not limited to the reader / writer antenna, and other communication devices and communication apparatuses. It can be applied to the antenna used in the above.

1 is a diagram illustrating an overall configuration of an RFID system according to an embodiment of the present invention. It is a figure which shows the simulation model of the gate antenna which concerns on one Example of this invention. It is a figure which shows the relationship between the result of the simulation which concerns on one Example of this invention, and the position of a gate antenna. It is a figure which shows the result (type a) of the simulation which concerns on one Example of this invention. It is a figure which shows the result (type b) of the simulation which concerns on one Example of this invention. It is a figure which shows the result (type c) of the simulation which concerns on one Example of this invention. It is a figure which shows the result (type d) of the simulation which concerns on one Example of this invention. It is a figure which shows the result (type aa) of the simulation which concerns on one Example of this invention. It is a figure which shows the result (type bb) of the simulation which concerns on one Example of this invention. It is a figure which shows the result (type cc) of the simulation which concerns on one Example of this invention. It is a figure which shows the result (type dd) of the simulation which concerns on one Example of this invention. It is a figure which shows the structure of the antenna for reader / writers concerning the prior application of this invention. It is a figure which shows the structure of the antenna for reader / writers concerning the prior application of this invention. It is a figure which shows the structure of the antenna for reader / writers concerning the prior application of this invention. It is a figure which shows the structure of the antenna for reader / writers concerning the prior application of this invention. It is a figure which shows the structure of the conventional antenna for reader / writers. It is a figure which shows the structure of the conventional antenna for reader / writers.

Explanation of symbols

1 RFID system 2 Gate antenna 2a Gate antenna A
2b Gate antenna B
3 reader / writer 3a phase control unit 3b high frequency power supply unit 3c communication circuit unit 3d arithmetic processing unit 4 transponder 5 8-shaped antenna 5a 8-shaped antenna upper portion 5b 8-shaped antenna lower portion 6 central loop antenna 6a parasitic central loop antenna 6b Feed central loop antenna 7 Conductor 8 GND
9 Capacitance 10 Phase correction circuit

Claims (2)

It is composed of a pair of antennas arranged opposite to each other,
A first loop and a second loop connected in parallel and driven by a high-frequency power source on one of the pair of antennas; and a third loop installed near the center of the first loop and the second loop; With
A fourth loop and a fifth loop connected in parallel and driven by a high-frequency power source on the other of the pair of antennas, and a sixth loop installed near the center of the fourth loop and the fifth loop With
In the gate antenna in which each of the first loop and the fourth loop, the second loop and the fifth loop, and the third loop and the sixth loop are opposed to each other,
When the phase of the magnetic field generated in the first loop is 0 °, the phase of the magnetic field generated in the second loop is set to about + 180 °, and the phase of the magnetic field generated in the third loop is set to about + 90 °. ,
In addition, by the phase control means, the phase of the magnetic field generated in the fourth loop is approximately 0 °, the phase of the magnetic field generated in the fifth loop is approximately + 180 °, and the phase of the magnetic field generated in the sixth loop is approximately +90. A gate antenna characterized by being controlled to be °. The third loop is a non-powered loop driven by an induced current flowing by magnetic coupling with at least one of the first loop and the second loop, or a power feeding loop driven by a power source different from the high frequency power source And
The sixth loop is a parasitic loop driven by an induced current flowing by magnetic coupling with at least one of the fourth loop and the fifth loop, or a feeding loop driven by a power source different from the high frequency power source. The gate antenna according to claim 1, wherein:

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