JP2008104094A - Radio tag of passive system, and radio tag system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To obtain a radio tag of a passive system (battery exchange unnecessary) with a detection distance of several meters (capable of remotely detecting a moving person or object) and weak radio wave use (without the need of legal utilization procedure and without influence upon human bodies and medial equipment). <P>SOLUTION: A radio tag 20 comprises a receiving antenna unit 21, an impedance conversion matching unit 22, a High-Q resonance circuit 23, a double wave rectifier circuit 24, an impedance adjusting unit 25, a smoothing circuit 26, an ID transmitting unit 27 and a transmitting antenna unit 28. In order to achieve the detection distance of 1m or longer, the transmitting antenna unit and the receiving antenna unit are separately structured and as regards the gain of the receiving antenna unit, by employing the receiving antenna unit of a type having a higher gain than that of the transmitting antenna unit, the detection distance can be made longer within the specific value of weak radio wave. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a passive wireless tag not equipped with a battery and a system using the wireless tag.

  FIG. 5 is a schematic configuration diagram of a conventional active wireless tag system. FIG. 6 is a schematic configuration diagram of a conventional passive wireless tag system.

  The conventional wireless tag 1 is provided with a battery 2, and the ID transmitter 3 is activated by the power supply of the battery 2 to emit an ID signal from the transmission antenna 4 (magnetic field loop antenna). This ID signal was received by the receiving antenna 5 (magnetic loop antenna), read by the wireless tag reader 6, and an ID response signal was emitted from the receiving antenna 5.

  That is, the passive wireless tag system transmits a radio wave for power supply from the wireless tag reader / writer 12 toward the passive wireless tag 10, and the passive wireless tag 10 receives the radio wave and converts it into DC power, The ID is transmitted to the reader / writer 12 as a power source.

  In other words, the active RFID tag system has a built-in battery inside the RFID tag, and transmits the ID signal by radio waves in one direction from the RFID tag to the RFID tag reader. The detection distance can be extended even with a small transmission output. There are advantages.

  Specifically, even a weak radio wave (as defined in the Radio Law Enforcement Regulation No. 6) is normally detected at about 10 m even with a transmission output value of about -43 dBm when assuming a 500 mV or less at a distance of 3 m and an antenna gain of 0 dBi Distance can be realized.

  On the other hand, since the battery is consumed, there is a drawback that the battery needs to be replaced periodically.

  The passive wireless tag system of FIG. 6 drives the transmission / reception antenna 11 (loop antenna) by the wireless tag reader / writer 12 to transmit a power supply transmission wave, and receives it by the transmission / reception antenna (loop antenna) of the passive wireless tag 10. The power source receiver 14 uses this received signal as a power source, and the ID transmitter 15 causes the transmission / reception antenna 13 to emit an ID response signal. That is, since the passive wireless tag system is not equipped with a battery, it is small and thin, and does not require battery replacement.

  The passive RFID tag system does not have a battery inside the RFID tag and supplies energy from the RFID tag reader / writer to the RFID tag. Therefore, the RFID tag reader / writer needs a large output. However, there is a drawback that the detection distance is short.

  Specifically, the detection distance of about several tens of centimeters is limited in the specific low power method (about -10 dBm output), and the maximum distance of several meters is also limited in the local wireless station method (about 1 W output).

  On the other hand, since the active method has a battery built in the wireless tag and transmits an ID signal by radio waves in one direction from the wireless tag to the wireless tag reader, the wireless tag itself has an energy source inside. Therefore, energy efficiency is high.

  Therefore, a relatively long detection distance can be secured even with weak radio wave output. However, the maximum is about 10m.

  If this is further extended, it is necessary to increase the transmission output from the wireless tag, and there is a problem that the battery does not fit within the size.

In addition, as described in Japanese Patent Application Laid-Open No. 2004-85277 (Patent Document 1), the above-described wireless tag is attached to clothing, and the detection device receives the radio wave from the wireless tag at the antenna portion to detect that the clothing is wet. There is also. The antenna unit has a transmission coil and a reception coil.
JP 2004-85277 A

  In general, the transmission output of a wireless tag reader / writer is limited by regulations of the Radio Law.

    There are roughly four types of wireless tags currently available in Japan.

(1) 124 KHz, 135 KHz band (2) 13.56 MHz band (wavelength of about 13 m)
(3) 900 MHz band (950 MHz band)
(4) 2.45 GHz band (5) 300 MHz band (weak radio wave)
Among these, (1) and (2) are about 10 cm in the passive method, and a detection distance of about several tens of centimeters is common even in the active method. Therefore, it is impossible to detect the tag from a distance of several meters or more.

  On the other hand, (3), (4), and (5) are suitable when searching at a long distance of several meters to several tens of meters.

  Among them, (3) is currently being developed exclusively for the passive system, and can detect a distance of several meters, so that the passive system has the longest detection distance.

  However, the reader / writer needs a large output of about 1 W, and the radio law corresponds to the local low power radio station system. For this reason, not only the manufacturer but also the user must follow the procedures for use with the supervisory authorities in accordance with Article 4 of the Radio Law.

  Further, (4) is a detection distance of about 1 m in the passive method, and a few meters in the active method.

  However, because of its strong radio wave propagation characteristics, it is difficult to detect objects and the like, and there is a drawback that it is also affected by radio wave absorption of a human body containing moisture as a conductor.

  Finally, (5) is generally the active method, and the detection distance is the longest, and depending on the method, it can be 10 m or more. Moreover, since it can be realized by weak radio waves (radio waves with electric field strengths below the provisions of Article 6 of the Radio Law Enforcement Regulations), it is not only subject to laws and regulations (Article 4 of the Radio Law), but it also takes into account the effects on the human body. There is no need to do it, and it is excellent to use.

  Considering these comprehensively, a blank area where a detection distance of several meters is secured, no battery is used, maintenance of battery replacement is unnecessary, and there is no system that can use weak radio waves. It turns out that it is.

  For example, there is a need to detect a wireless tag attached to a patient from a distance of several tens of centimeters to several meters at a medical site where use of radio waves is restricted. Examples include monitoring of epileptic patients and automatic identification of moving inpatients. Even if a hemorrhoid patient walks in the hospital, the doctor in charge of surgery or internal medicine does not know whether it is a hemorrhoid patient. In other words, even if a vaginal patient is seen, it can be understood only by the doctor or nurse in charge of the patient.

  This can be achieved by using the 300 MHz weak radio wave active tag (5) for such patients, but it is necessary to always check the remaining battery power inside the tag attached to the patient. The battery must be replaced and maintained regularly according to the situation, and there is a problem that it is very bad to use.

  In order to solve this problem of battery replacement, a passive method in which no battery is mounted on the tag may be employed.

  Therefore, when considering the passive method according to (1) to (4), only the 950 MHz passing method of (3) can realize the detection distance number m.

  However, the method (3) corresponds to a local low-power radio station with about 1 W output, which is the largest output in the wireless tag system.

  For this reason, procedures to the administrative authorities are necessary, and for use in hospitals or the like, the radio wave output is large, so it is necessary to fully consider the influence on medical devices and the like.

  Therefore, there is a problem that there are restrictions on the installation location, and it is necessary to conduct on-site investigations and individual considerations each time.

  Suppose now that the detection distance condition is relaxed and allowed to be shortened to about 1 m. In this case, the 2.45 GHz passing method (4) can be theoretically realized.

  However, since 2.45 GHz is a frequency belonging to the ISM band, there is a problem that it is exactly the same frequency as a medical device in a medical field and affects a medical device system.

  In addition, since the linearity is strong due to the nature of radio wave propagation, there is a problem that it is impossible to detect a tag in a position that becomes a shadow or a human shadow.

  In addition, since the absorption attenuation due to moisture is large, there is also a problem that radio waves are absorbed and attenuated by liquid drugs and the like.

  Furthermore, since the output corresponds to a specific low-power radio station of about 10 mW legally, the manufacturer is required to proceed to the competent authority in accordance with Article 4 of the Radio Law.

  The present invention is a passive method (no battery replacement required) with a detection distance of several meters (capable of detecting people and objects moving from a distance), weak radio waves (no need for legal use procedures, no impact on human body, medical equipment) ) Wireless tag.

The passive wireless tag of the present invention has an ID transmitter that receives a received radio wave at the receiving antenna unit and generates an ID signal of the wireless tag, and transmits from the transmitting antenna unit with a 300 MHz band radio wave. In the tag,
From the tank circuit, an impedance conversion matching unit that is matched to the link coil of the receiving antenna unit with low impedance and extracts a signal extracted by the link coil, a tank circuit that accumulates the signal extracted by the impedance conversion matching unit, and And a rectifying / smoothing circuit for supplying the smoothed DC power to the power supply terminal of the ID transmitter.

  The wireless tag system of the present invention includes a 300 MHz band wireless tag reader / writer and a passive wireless tag, and includes data including the wireless tag ID from the wireless tag reader / writer and the wireless tag reader / writer ID. Is a wireless tag system including a monitoring center server that receives a message via a communication network.

The wireless tag reader / writer is
A power transmission unit that generates and outputs a signal to be a power source of the wireless tag, a first transmission antenna unit that transmits a signal from the power transmission unit as a radio wave, and higher efficiency than the first transmission antenna unit A first receiving antenna unit that receives radio waves from the wireless tag, and an ID signal of the wireless tag received by the first receiving antenna unit, and adds an ID of the reader to the ID signal A wireless tag reader for transmitting to the server.

The wireless tag is
A link between the second receiving antenna unit and the second receiving antenna unit that is highly efficient with respect to the first transmitting antenna unit of the wireless tag reader / writer and receives radio waves from the first transmitting antenna unit The impedance conversion matching unit that is matched with the coil at low impedance and extracts the signal extracted by the link coil, the tank circuit that accumulates the signal extracted by the impedance conversion matching unit, and the power from the tank circuit is rectified A rectifying / smoothing circuit for smoothing, an ID transmitting unit for generating and outputting an ID signal of the wireless tag by the DC power smoothed by the rectifying / smoothing circuit, and the second receiving antenna unit Accordingly, the magnetic field loop antenna is reduced in size, and the second transmission antenna transmits the ID signal from the ID transmission unit as a radio wave of 300 MHz band. And summarized in that with and.

  As described above, since the wireless tag of the present invention uses weak radio waves, there is no need for any procedures for using radio waves to the supervisory authorities based on Article 4 of the Radio Law, and the time, cost, and convenience are extremely excellent. . In addition, since the 300 MHz weak radio wave system is adopted, there is a maximum advantage that the detection distance can be increased as compared with a passive system in which a battery is not mounted on a tag using another frequency band.

  Furthermore, since the weak electric field has extremely low electric field strength, it does not affect the human body or precision electronic equipment. In addition, there is no restriction on the installation location, usage method, and the like, and it is possible to construct a system that can be applied to any environment with little awareness of using radio waves.

  In particular, it can be introduced without being restricted by electromagnetic field exposure safety in hospitals, nursing homes, etc., where radio wave usage is severely restricted.

  In addition, since the battery is not mounted inside the wireless tag, it is not necessary to manage the consumption of the battery during the operation of the system, and it is also necessary to periodically replace the battery.

  For this reason, when the battery is consumed, the reading rate of the wireless tag is lowered, and the reliability of the system is not lowered at all.

  Furthermore, since the RFID tag system of the present invention is a weak radio wave in the 300 MHz band, it is possible to improve the efficiency from the antenna size with respect to the wave power, and the radio wave propagation characteristics are further away in order to take advantage of the LHF band. Since detection from a distance is possible, it can be applied to applications that could not be applied by the conventional passive method, and the availability is dramatically improved. Weak radio waves have extremely low electric field strength, so there is no effect on the human body or precision electronic equipment. In addition, there is no restriction on the installation location, usage method, and the like, and it is possible to construct a system that can be applied to any environment with little awareness of using radio waves.

  In particular, it can be introduced without being restricted by electromagnetic field exposure safety in hospitals, nursing homes, etc., where radio wave usage is severely restricted.

<Embodiment 1>
FIG. 1 is a schematic configuration diagram of a wireless tag according to the present embodiment. This embodiment employs a passive system without a battery. The frequency band to be used is a 300 MHz band of 320 MHz or less, which allows a value of 500 μV / m or less as a legally regulated value of radio waves (Article 6).

  In this embodiment, the radio wave output is within the limits of weak radio waves and solves the problem of influence on the human body and medical equipment. In order to realize a detection distance of 1 m or more, the transmission antenna unit, the reception antenna unit, By adopting a type in which the efficiency of the receiving antenna unit is higher than that of the transmitting antenna unit, the detection distance can be made longer within the regulation value of the weak radio wave.

  As shown in FIG. 1, the wireless tag 20 of the present embodiment includes a receiving antenna unit 21, an impedance conversion matching unit 22, a High-Q resonance circuit 23, a two-wave rectifier circuit 24, and an impedance adjustment unit 25. , A smoothing circuit 26, an ID transmission unit 27, and a transmission antenna unit 28.

  The receiving antenna unit 21 includes a magnetic field loop coil 21a, a capacitor 21b (3PF to 5PF), and a link coil 21c that are small in size, high in efficiency, and hardly affected by electromagnetic influences from surrounding objects. The link coil 21c and the magnetic field loop coil 21a are fixed with glass epoxy or the like so as not to contact each other.

The length of the magnetic field loop coil 21a is
(Λ / 0.3)> circumferential length of magnetic field loop coil 21a
It is said.

  A 300 MHz band passive system is realized by the magnetic field loop coil 21a and the capacitor 21b.

  Further, both ends of the link coil 21 c are connected to the impedance conversion matching unit 22. The impedance conversion matching unit 22 connects a link coil 21c having a number of turns of about 10 circumference of a magnetic field loop coil 21a (hereinafter simply referred to as a link coil) (output resistance is connected between the link coil 21c and the link coil 22a). It may be provided (if necessary), so that the output resistance is 50Ω. In the conventional passive wireless tag system, the impedance conversion matching unit 22 is provided with insufficient impedance matching between the antenna unit and the circuit unit, and energy cannot be efficiently transmitted from the antenna side to the circuit side. It was. In order to solve this problem, an impedance matching unit is provided between the antenna and the resonance circuit.

  The link coil 22 a is a link coil that is electrically coupled to the resonance coil 23 a of the High-Q resonance circuit 23.

  That is, the maximum energy obtained by the receiving antenna unit 21 by the impedance conversion matching unit 22 is extracted with low impedance.

  The High-Q resonance circuit 23 constitutes a resonance circuit (300 MHz band) including a resonance coil 23a, a capacitor 23b (3PF to 5PF), and the like so as to obtain a higher output voltage.

  The High-Q resonance circuit 23 is connected to the double-wave rectification circuit 24 (D1, D2, D3, D4).

  The two-wave rectifier circuit 24 is connected to an impedance adjustment unit 25 (for stepping down the impedance), and the impedance adjustment unit 25 is connected to a smoothing circuit 26 (a resistor 26a and a capacitor 26b).

  The smoothing circuit 26 is connected to the ID transmission unit 27. The ID transmission unit 27 includes a magnetic field loop antenna 28a of the transmission antenna unit 28 (having a shorter circumference than the loop coil of the reception antenna unit 21) and a capacitor 28b (3PF to 5PF), and emits a 300 MHz band radio wave.

  The transmission antenna unit and the reception antenna unit described above are built in a single casing (plastic case). Further, the impedance conversion matching unit 22, the High-Q resonance circuit 23, the both-wave rectification circuit 24, the impedance adjustment unit 24, and the ID transmission unit 27 are also housed in one casing.

  The operation of the wireless tag 20 configured as described above will be described below. In this embodiment mode, radio waves transmitted from the writer in order to supply power to the wireless tag are always transmitted without interruption.

  A radio wave (300 MHz band) from a reader / writer (not shown) is received by the loop antenna 21a of the receiving antenna unit 21, and received power is extracted by the link coil 21c and supplied to the impedance conversion matching unit 22.

  The received power via the impedance conversion matching unit 22 is transmitted to the High-Q resonance circuit 23 by the link coil 22a, and the high frequency component is resonated by a high Q value, and the received power is efficiently stored in the circuit.

  Next, the high-frequency energy accumulated in the High-Q resonance circuit 23 is rectified into a DC component by the double-wave rectification circuit 24. When the voltage of the DC component obtained by the two-wave rectifier circuit 24 is low, voltage doubler rectification may be used.

  Next, the DC component of the both-wave rectifier circuit 24 is adjusted in impedance by the impedance adjustment unit 25 (very low impedance: 50Ω), smoothed by the smoothing circuit 26, and an appropriate voltage is applied to the Vcc terminal of the ID transmission unit 27. DC power (power supply) having a current value is obtained.

  With this DC power, the ID transmission unit 27 is activated and outputs the ID signal of the wireless tag 20 to the transmission antenna unit 28.

  The transmitting antenna unit 28 emits a 300 MHz band radio wave by the capacitor 28b and the loop coil 28a.

  In other words, in this embodiment, a circuit system capable of obtaining high-efficiency energy at each stage enables energy transmission with high efficiency by obtaining optimum impedance matching between stages.

  FIG. 2 shows a result of studying a signal level diagram in the case where such a high-efficiency passive tag is configured and spatially propagated between a reader / writer and a wireless tag using weak radio waves.

  Since the minimum received power sensitivity of the RFID tag signal receiving unit (receiving antenna unit) used in the experiment was -138 dBm, the same transmission / reception antenna with the same 0 dB gain was used for the 300 MHz band weak radio wave as shown in FIG. If the distance between the reader / writer and the wireless tag is 2 m, the reception level becomes −152 dBm, which cannot be realized.

  Furthermore, only the reader / writer side and the receiving antenna have a gain of 6 dB. Even with an antenna having a practical size and a relatively high gain (for example, a three-element Yagi antenna or a two-element loop antenna), the received signal level only rises to about -146 dBm.

  Therefore, a system that can obtain a signal level of −138 dBm or more can be realized by using a reader / writer and a wireless tag that are more efficient than the transmitting antenna only on the receiving antenna side.

  In this case, since the tag side does not need to be downsized, the transmission antenna efficiency is −10 dB and only the reception side is 0 dB (for example, a small magnetic field loop antenna).

  In the first embodiment, since the receiving antenna unit 21 has a high gain, even when the wireless tag 20 is far away from the wireless tag reader / writer and the received radio wave is weak, reception is possible by the high gain receiving antenna unit 21. It becomes. Further, the circumferential length of the loop coil 28 a of the transmission antenna unit 28 is made shorter than the loop antenna of the reception antenna unit 21.

    Therefore, since the wireless tag 20 of the present embodiment uses weak radio waves, there is no need for any procedures for using radio waves to the supervisory authorities based on Article 4 of the Radio Law, and the time, cost, and convenience are extremely excellent. Yes. In addition, since the 300 MHz weak radio wave system is adopted, there is a maximum advantage that the detection distance can be increased as compared with a passive system in which a battery is not mounted on a tag using another frequency band.

  Furthermore, since the weak electric field has extremely low electric field strength, it does not affect the human body or precision electronic equipment. In addition, there is no restriction on the installation location, usage method, and the like, and it is possible to construct a system that can be applied to any environment with little awareness of using radio waves.

  In particular, it can be introduced without being restricted by electromagnetic field exposure safety in hospitals, nursing homes, etc., where radio wave usage is severely restricted.

  In addition, since the battery is not mounted inside the wireless tag, it is not necessary to manage the consumption of the battery during the operation of the system, and it is also necessary to periodically replace the battery.

  For this reason, when the battery is consumed, the reading rate of the wireless tag is lowered, and the reliability of the system is not lowered at all.

  When the frequency of the radio wave from the wireless tag reader / writer and the frequency of the radio wave from the wireless tag 20 are the same, the reception antenna unit 21 of the wireless tag 20 is set to 300 MHz, and the frequency of the radio wave from the transmission antenna unit 28 is set to 303 MHz. Different from the above, it may be possible to prevent interference.

<Embodiment 2>
The second embodiment employs a 300 MHz band passive tag system, does not mount a battery on the tag, and the radio wave output is within the regulation of weak radio waves and considers the influence on the human body and medical equipment. In this embodiment, a case where the present system is applied in a hospital will be described.

  FIG. 3 is a schematic configuration diagram of the wireless tag system according to the second embodiment.

  As shown in FIG. 3, in the second embodiment, a wireless tag reader / writer 31 is provided in a corridor, door, entrance, or room within a hospital, and the wireless tag reader / writer 31 is connected to a system server 38 via a communication network 37. is doing. The patient is provided with the same wireless tag 20 as in the first embodiment. Since the configuration of the wireless tag 20 is the same as that of the first embodiment, description thereof is omitted.

  The wireless tag reader / writer 31 includes a power source transmission unit 32, a transmission antenna 33, a wireless tag reader 35, and a reception antenna unit 34. The power source transmission unit 32 sends a signal in the 300 MHz band to the transmission antenna unit 33. This transmission timing is shifted from the transmission timing of radio waves from the transmission antenna portion of the wireless tag 20 described later.

  The loop coil of the transmission antenna unit 33 has a circumference of about 5 cm (−10 dBi). A capacitor is connected to the loop coil so that a radio wave of about 301 MHz can be emitted.

  The receiving antenna unit 34 includes a loop coil larger than the transmitting antenna unit 33 (circumferential length is about 30 cm).

  The wireless tag reader 35 decodes the signal received by the receiving antenna unit 34 and transmits it to the system server 38 of the monitoring center via the communication network 37.

  That is, in the second embodiment, the circumference of the loop antenna of the transmission antenna unit 33 on the transmission side (wireless tag reader / writer) is reduced, and the loop antenna of the reception antenna unit 21 on the reception side (patient side wireless tag) is reduced. By enlarging the circumference, reception is possible even if the radio wave on the transmission side is weak. These are built in a housing (plastic case).

  As in the first embodiment, the wireless tag 20 includes a receiving antenna unit 21, an impedance conversion matching unit 22, a High-Q resonance circuit 23, a both-wave rectifier circuit 24, an impedance adjustment unit 25, a smoothing Since the circuit 26, the ID transmission unit 27, and the transmission antenna unit 28 are provided, the energy of the radio wave received by the reception unit antenna unit 21 is stored in the high-resonance circuit 23 via the impedance conversion matching unit 22, The DC power rectified by the both-wave rectifier circuit 24 and rectified by the smoothing circuit 26 serves as a power source for the ID transmitter 27.

  The ID transmission unit 27 is activated by this power source and sends an ID signal to the transmission antenna unit 28. The transmission antenna unit 28 is slightly different from the frequency on the transmission side, for example. For example, radio waves of 303 MHz to 315 MHz are emitted.

  This radio wave is received by the receiving antenna unit 34 of the wireless tag reader / writer 31, and the ID code is decoded by the wireless tag reader 35 and transmitted to the server 38.

  As a result, if the position of the RFID tag reader / writer in the hospital (door, corridor number, etc.) and the number of the RFID tag reader / writer correspond to each other on the monitoring center side, where the patient is currently passing Can be grasped.

The effects of the second embodiment will be further described.

    In the second embodiment, when the frequency at which energy is transmitted from the writer to the wireless tag and the frequency at which the ID signal is returned from the wireless tag to the reader is the same, interference occurs, so energy supply and ID reply are alternately performed in time division. There is a need to do. That is, it results in supplying energy intermittently.

  Therefore, the wireless tag reader / writer used in the second embodiment makes the ID response from the wireless tag 20 to the wireless tag reader / writer side so that the frequency of the transmitting antenna unit 33 and the frequency of the receiving antenna unit 34 are different. Interference is prevented by using a bi-directional duplex system that varies the frequency of the radio waves to be returned.

  In addition, energy efficiency is increased by continuously supplying power energy (radio waves) from the transmitting antenna unit 33 to the wireless tag 20.

  Further, the transmission and reception antennas of the wireless tag reader / writer 31 are separated, and the gain of the reception antenna unit 34 is made larger than the gain of the transmission antenna unit 33. If the gain of the transmission antenna unit 33 is high, the standard value of weak radio waves in the Radio Law is exceeded.

  For this reason, the gain is increased only by the receiving antenna unit 33 so that the radio antenna 20 can be received by the high gain receiving antenna unit 34 even when the radio tag 20 is farther away and the received radio wave is weak.

  For the same reason, if the wireless tag 20 side separates the transmission / reception antenna and raises the antenna gain only on the reception side for the same reason, the same effect is synergized.

  Therefore, since the wireless tag system of the present embodiment uses weak radio waves, there is no need for any procedures related to radio wave use by supervisory authorities based on Article 4 of the Radio Law, and time, cost, and convenience are extremely high. Are better.

  Furthermore, since the weak electric field has extremely low electric field strength, it does not affect the human body or precision electronic equipment. In addition, there is no restriction on the installation location, usage method, etc., and it is possible to construct a system that can be applied to any environment with little awareness of using radio waves.

  In particular, it can be introduced without being restricted by electromagnetic field exposure safety in hospitals, nursing homes, etc., where radio wave usage is severely restricted.

  As described above, since this system can be manufactured, sold, and used without requiring radio wave administrative procedures, anyone can use the RFID tag system widely.

  Furthermore, it is not conscious of restrictions on electromagnetic field exposure, can be installed anywhere, because it can be installed on any object that is sensitive to radio waves, such as the human body and precision electronic equipment.

  Moreover, because it is a 300 MHz band weak radio wave, the detection distance can be made longer than that of a normal passive tag, so detection from a greater distance is possible, making it possible to apply to applications that could not be applied with the conventional passive method, Availability improves dramatically.

  Next, the description of the receiving antenna unit and the transmitting antenna unit will be supplemented using FIG. 4 and the following formula.

  In order for such a 300 MHz band antenna to exhibit optimum performance in a building or the like, it is necessary to define optimum design parameters.

    The design parameters (including capacitor capacity, loop coil length, width, etc.) will be described below.

    First, the relationship between the distance from the source of the electromagnetic wave radiated from the antenna and the intensity of the electromagnetic wave will be shown below.

Assuming that the amount of charge at a place P that is a distance r away from an electric dipole having a length of 1 at the origin of a certain space is q (t) (t is time), the rate of change in the amount of charge is current.

  And can be expressed in the form of differentiation.

When there is a minute dipole facing the z-axis direction at the origin, the electric field E (t) and magnetic field H (t) of the minute dipole at the point P shown in FIG.

  It becomes. These equations are expressed based on polar coordinates, and er, eθ, and eΦ are e-direction, θ-direction, and Φ-direction unit vectors, respectively. C is the propagation speed of electromagnetic waves in space.

    In these equations, the term proportional to r-3 is a term that creates an electrostatic magnetic field, and in the case of an electric dipole, it exists only in the electric field.

The term r-2 is a term that generates an induction electromagnetic field. The r-1 term is a term that creates a radiation field. If it is far enough, the electrostatic term and the induction term are much smaller than the radiation term. Therefore, the electric field / magnetic field (far field) in a place sufficiently away from the dipole is

  It becomes. Since the direction unit vectors are orthogonal, the electric field and the magnetic field are orthogonal in the far field, and there is no electric / magnetic field component in the wave traveling direction.

In the frequency domain display, the far-field electric and magnetic field components are

It becomes.

Here, the ratio ξ between the far electric field and the magnetic field is called wave impedance, and is expressed by the following equation.

    From [Equation 6] and [Equation 7], since the wave impedance ξ increases as the distance r from the wave field decreases in the near field region, the magnetic field component of the electromagnetic field is strong in the near field region (Fresnel region). It can be said that the electric field component becomes stronger in the far field region (Fraun for Far region).

    From the above, in order to detect only the magnetic field component near the antenna, the antenna itself is operated in the current mode, generating a large current to the antenna conductor (loop coil), and the magnetic field component is first radiated into the space from the current. The physical configuration of the antenna may be determined as described.

    In order to realize this, the antenna itself may be formed as a circuit that is in series resonance in the 300 MHz band that is the use frequency so that a large series resonance current flows through the antenna conductor and the structure including the antenna conductor.

    Specifically, since the inductance is constituted by the antenna conductive wire, the value of the capacitor that resonates with the inductance is determined using itself as a closed loop, and they are electrically connected in series.

    As a method for transmitting and receiving energy to the resonance loop and the coaxial cable for feeding, any method such as a link coil method and an impedance tap method may be adopted.

    By doing so, a magnetic loop antenna is formed, and its operating impedance becomes the resonant impedance of the series resonant circuit.

    That is, the inductive reactance due to the inductance of the conductor and the electrostatic reactance due to the capacitance of the capacitor cancel each other out to zero, and the closed-loop impedance is theoretically very low, less than 1Ω, which is only the net resistance of the conductor. Value.

    Therefore, a large current flows in the closed loop. And it reacts only with the magnetic field component of the area | region of the vicinity of an antenna, The energy can be efficiently transmitted / received between space and an antenna. In other words, since the antenna operates in a current mode with a very low impedance, even if a substance such as metal or concrete that disturbs the electric field component of the electromagnetic wave is present in the immediate vicinity of the antenna, the characteristics of the magnetic loop antenna are almost affected. There is nothing.

      A quantitative numerical example is shown below.

About the size of conductors close to each other N: Number of turns of loop conductor (times)
W: Length of one side of the conductor (m) (assuming a square shape)
a: Radius of conductor (m) (assuming conductor is linear)
Assuming μs: relative permeability, this inductance is defined by the following equation.

    In consideration of various wireless tag sizes applied to the actual system, an appropriate value may be substituted into this equation to determine the conductor size and the capacitor value.

For example,
L = about 40 (nH)
If a conductor structure of the size is given, if a capacitor of C = about 7 (PF) is connected,
f = 1 / 2π√LC
Therefore, resonance occurs at about 300 MHZ.

    As to how efficiently the electromagnetic wave energy in space is transmitted to and received from the antenna, the gain of the antenna increases as the loop area increases.

That is,
h: Relative gain of antenna (effective height)
A: loop area N: number of loop turns
h = (2π / λ) NA
It becomes.

    Therefore, if the loop is made larger and larger in order to increase the gain of the antenna, the inductance of the loop will also increase. Therefore, in order to cancel the large inductance and cause the loop to resonate in the 300 MHz band, the value of the capacitor must be made smaller and smaller.

  According to calculations and experiments, when copper is used as the material for the loop, which is cheap and easy to obtain and easy to process, the total length of the loop is about 20 cm (loop diameter is about 5 to 6 cm), and the value of the resonance capacitor is several PF. It is. The antenna gain in this case is about −several to −10 dB with respect to a normal half-wave dipole antenna, and a sufficiently practical gain can be obtained.

    If the loop length is increased to further increase the antenna gain, the value of the capacitor becomes 1PF or less in the calculation, and even if an antenna is actually manufactured, the capacitance value of the capacitor that cannot be physically realized. End up.

    Furthermore, the theoretical limit for the loop to operate in the magnetic field mode is 0.5 wavelength or less in the entire circumference (50 cm or less in the case of 300 MHZ band), and if the length is longer than this, the current distribution on the loop conductor is not constant, Standing waves are generated.

    This standing wave inevitably changes the mode to operate as an electric field antenna, and is no longer a magnetic field loop antenna.

    On the other hand, if the value of the capacitor is made larger than about 10 PF so as to facilitate manufacture and adjustment, the loop length (loop area) becomes smaller and the antenna gain decreases.

    Moreover, the smaller the loop length, the lower the conductor impedance of the loop, and the DC resistance of the material forming the loop (in this case copper) cannot be ignored, combined with the skin effect due to the high frequency of 300 MHZ, The decrease in antenna efficiency due to conductor resistance loss becomes significant.

    In order to prevent this, it is necessary to take measures such as applying gold plating, which is a material having a low high-frequency resistance, to the surface of copper to reduce the ohmic loss due to the skin effect, resulting in a very high cost.

    Therefore, the practical size of the magnetic loop magnetic field loop antenna for the 300 MHZ band is 0.5 wavelength or less on the entire circumference, and the value of the resonance capacitor is 10 PF or less.

1 is a schematic configuration diagram of a wireless tag according to a first embodiment. It is explanatory drawing of the level diagram of this Embodiment. 3 is a schematic configuration diagram of a wireless tag system according to a second embodiment. FIG. It is explanatory drawing for supplementing the characteristic of an antenna part. It is a schematic block diagram of the conventional active wireless tag system. It is a schematic block diagram of the conventional passive wireless tag system.

Explanation of symbols

DESCRIPTION OF SYMBOLS 20 Radio tag 21 Reception antenna part 22 Impedance conversion matching part 23 High-Q resonance circuit 24 Both-wave rectification circuit 25 Impedance adjustment part 26 Smoothing circuit 27 ID transmission part 28 Transmission antenna part

Claims (8)

In a passive type wireless tag that has an ID transmission unit that receives a reception radio wave at the reception antenna unit and generates an ID signal of the radio tag and transmits from the transmission antenna unit with a radio wave of 300 MHz band,
An impedance conversion matching unit that is matched with a low impedance to the link coil of the receiving antenna unit and extracts a signal extracted by the link coil;
A tank circuit for accumulating signals taken out by the impedance conversion matching unit;
A passive wireless tag, comprising: a rectifying / smoothing circuit that rectifies and smoothes high-frequency energy from the tank circuit and supplies the smoothed DC power to a power supply terminal of the ID transmitter.   The passive radio tag according to claim 1, wherein the magnetic field loop antenna efficiency of the receiving antenna unit has a high gain characteristic of about 1 dB or more with respect to the magnetic field loop antenna efficiency of the transmitting antenna.   3. The passive radio tag according to claim 1, wherein the frequency of the radio wave received by the reception antenna unit and the frequency of the radio wave emitted from the transmission antenna unit are different from each other in a 300 MHz band. A monitoring center comprising a wireless tag reader / writer of 300 MHz band and a passive wireless tag, and receiving data including the wireless tag ID and the wireless tag reader / writer ID from the wireless tag reader / writer via a communication network A wireless tag system comprising:
The wireless tag reader / writer is
A power transmitter that generates and outputs a signal to be a power source of the wireless tag;
A first transmission antenna unit that transmits a signal from the power transmission unit as a radio wave;
A first receiving antenna unit that receives radio waves from the wireless tag that is more efficient than the first transmitting antenna unit;
A wireless tag reader that decodes the ID signal of the wireless tag received by the first receiving antenna unit, adds the ID of the reader to the ID signal, and transmits the ID signal to the server;
The wireless tag is
A second receiving antenna unit that is highly efficient with respect to the first transmitting antenna unit of the wireless tag reader / writer and receives radio waves from the first transmitting antenna unit;
An impedance conversion matching unit that is matched with a low impedance to the link coil of the second receiving antenna unit and extracts a signal extracted by the link coil;
A tank circuit for accumulating signals taken out by the impedance conversion matching unit;
A rectifying / smoothing circuit that rectifies and smoothes the electric power from the tank circuit;
An ID transmitter that uses DC power smoothed by the rectifying / smoothing circuit as a power source, and generates and outputs an ID signal of the wireless tag by the power source;
A radio tag comprising: a second transmission antenna unit configured such that a magnetic field loop antenna is made smaller than the second reception antenna unit, and an ID signal from the ID transmission unit is transmitted as a 300 MHz band radio wave. system.   The magnetic field loop coil of the second receiving antenna portion of the wireless tag has a longer circumferential length and a larger effective area than the magnetic field loop coil of the first transmission antenna portion of the wireless tag reader / writer. 5. The RFID tag system according to claim 4, wherein the RFID tag system is efficient.   6. The transmission of radio waves from the wireless tag reader / writer and the transmission of radio waves from the wireless tag to the wireless tag reader / writer are performed at different timings, respectively. Wireless tag system.   7. The frequency of the radio wave from the wireless tag reader / writer and the frequency of the radio wave from the wireless tag to the wireless tag reader / writer are different frequencies in the 300 MHz band, respectively. Wireless tag system.   The radio tag system according to claim 4, 5, 6, or 7, wherein a radio wave for supplying power to the radio tag is continuously supplied as a continuous carrier wave to the radio tag.

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