JP2010021617A - Electromagnetic wave output circuit and communication device for radio tag - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To generate an electric field of fixed strength to a fluctuation in a frequency under a fixed voltage, relating to the reader/writer device of a contactless IC tag utilizing electromagnetic waves. <P>SOLUTION: On a circuit between a variable frequency constant voltage oscillator 3 for generating an AC current which changes in a frequency range including a first frequency and a second frequency at a fixed voltage and an antenna 5 for generating the electromagnetic waves by the input of the AC current, a resistive element 4 is provided for making the electric field intensities of the output electromagnetic waves of the antenna equal or nearly equal to each other between at the first frequency and at the second frequency. A constant voltage control circuit 2 includes a level setting reference voltage source 21, a level detection circuit 22 and a comparator 23. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a reader / writer device for a non-contact IC tag using electromagnetic waves, and relates to a technique for generating an electric field having a constant intensity with respect to frequency fluctuations under a constant voltage.

  A non-contact IC tag using electromagnetic waves has an IC chip and an antenna coil that operate at a predetermined resonance frequency (for example, 13.56 MHz), and the reader / writer device is configured to transmit electromagnetic waves of the frequency. Has been.

  Depending on how the tags are used, it may be necessary to perform a reading operation in a state where a plurality of non-contact IC tags are close to each other or overlapped. In the non-contact IC tag in the close state or the overlapped state as described above, the inductance changes due to the mutual induction action between the antenna coils, and the resonance frequency decreases. For this reason, even if an electromagnetic wave having a predetermined frequency is received, the power required to operate as an IC tag cannot be obtained, and communication may be disabled.

  In order to read a non-contact IC tag having a lowered resonance frequency, a method in which the reader / writer device repeatedly changes (sweeps) the transmission frequency including a frequency range lower than a predetermined frequency is conceivable. In this way, if the transmission frequency is changed, sufficient power can be supplied even at a lowered frequency as usual.

  However, it is not permitted to output an output level permitted at a predetermined frequency (for example, 13.56 MHz) defined by the Radio Law in a range other than the predetermined frequency. Therefore, it is legally not allowed for the reader / writer device to change the transmission frequency with the conventional output level.

  In order to perform communication in a range including a frequency lower than a predetermined frequency, the output level of the reader / writer device must be suppressed to a range of electric field strength allowed as a weak radio wave. As shown in FIG. 6, in the frequency range of 10 kHz to 322 MHz, only a low output of up to 500 μV / m is allowed at a distance of 3 m, so the output of the reader / writer device is close to the upper limit of the allowable range. It is desirable to control to maintain the output level.

  However, since the antenna used in the reader / writer device is often a coil antenna of several turns, according to the characteristics of such an antenna, the frequency of electromagnetic waves output at a constant voltage with a simple structure is changed. In addition, the output level fluctuates accordingly. Specifically, the electric field strength is lower in the high frequency range than in the low frequency range. Therefore, if the voltage is set to obtain the upper limit output level of the allowable range in the low frequency range in consideration of the reading performance in the lowered resonance frequency range, when the frequency is increased to the high frequency range while maintaining the same voltage Thus, only a low level output that does not reach the upper limit of the allowable range can be obtained, and there is a possibility that power necessary for the operation of the IC tag cannot be supplied during normal communication at a predetermined resonance frequency. On the other hand, if the voltage is set so that the output level at the predetermined resonance frequency matches the upper limit of the allowable range, the legal output standard will be exceeded in the low frequency range.

As a method of adjusting the fluctuation of the electric field strength when the frequency is changed, it is conceivable to adjust the voltage according to the frequency. In other words, the electric field strength can be kept constant by controlling the voltage to be higher in the high frequency range. However, in order to control the voltage according to the frequency, a complicated and expensive configuration including a level monitor and a variable gain amplifier is required. FIG. 7 shows a comparative example in which the level monitor is monitored by the CPU and the variable gain amplifier is controlled.
JP 2007-193785 A Japanese Patent Laid-Open No. 10-145987

  Therefore, the present invention mainly aims to solve such problems, and in a reader / writer device that communicates with an IC tag while changing the frequency of an electromagnetic wave to be output, regardless of the frequency range, even with a constant voltage power source. It is an object of the present invention to realize control for obtaining a constant electric field intensity with a simple and inexpensive circuit configuration.

The electromagnetic wave output circuit according to the present invention has the following elements.
(1) A variable frequency constant voltage oscillator that generates an alternating current that varies in a frequency range including the first frequency and the second frequency at a constant voltage. (2) An antenna that generates an electromagnetic wave by the input of the alternating current. (3) Variable. A resistive element provided on the circuit between the frequency constant voltage oscillator and the antenna.

  Further, the resistance element includes an electric field strength of an electromagnetic wave output from the antenna when the variable frequency constant voltage oscillator generates an alternating current of the first frequency at the constant voltage, and the variable frequency constant voltage oscillator It has a resistance value in which the electric field strength of the electromagnetic wave output from the antenna when the alternating current of the second frequency is generated at a constant voltage is the same or substantially the same.

  Furthermore, the frequency range is 2 to 14 MHz, and the antenna is a looped coil antenna.

  And a constant voltage control circuit for controlling a voltage generated from the variable frequency constant voltage oscillator, a level detection circuit for detecting the voltage level by the constant voltage control circuit, and a level for generating a reference voltage. It comprises a set reference voltage source and a comparator for comparing voltage levels.

  Furthermore, the wireless tag communication device according to the present invention includes an electromagnetic wave output circuit, an arithmetic device for inputting information to be transmitted to the wireless tag to the electromagnetic wave output circuit, and a receiving circuit for receiving information returned from the wireless tag. It is characterized by.

  According to the present invention, by providing a resistor in series with the antenna, it is possible to make the electric field output constant even in driving with a constant voltage source by suppressing an increase in current in a low frequency region.

  In particular, as a wireless tag communication device, a radio wave having a constant electric field strength of a plurality of swept frequencies (for example, a frequency range of 2 to 14 MHz) can be transmitted, so that the resonance frequency is lowered in a superimposed state. A plurality of wireless tags can be read. In addition, since the electric field intensity of the output radio wave can be controlled to a desired value, it can be operated so as to exhibit the maximum effect within the range defined by the Radio Law.

Embodiment 1 FIG.
FIG. 1 is a diagram showing an example of the configuration of an electromagnetic wave output circuit of the present invention. The electromagnetic wave output circuit includes a variable frequency constant voltage oscillator 3, a resistance element 4, and an antenna 5, and further preferably includes an arithmetic unit (CPU) 1 and a constant voltage control circuit 2. In the present invention, a resistance element 4 having a predetermined resistance value is provided between the antenna 5 and the variable frequency constant voltage oscillator 3. Thus, by providing the resistance element 4 in series with the antenna 5, even if the constant voltage alternating current changes in a wide frequency range, the electric field intensity output from the antenna 5 can be kept constant.

  The variable frequency constant voltage oscillator 3 is an oscillator that generates an alternating current having a constant voltage even when the frequency changes.

  The antenna 5 is an antenna that generates an electromagnetic wave by an input current, and examples thereof include a Yagi antenna, a dipole antenna, and a looped coil antenna. Among these, a coil antenna is preferable, and it is configured in a loop shape in which a conductive material is spirally wound, and the number of windings may be appropriately selected depending on the frequency of the output electromagnetic wave. Examples of the conductive material constituting the coil antenna include a conductive wire made of a metal such as gold, silver, copper, aluminum, and nickel, a foil film, and a conductive paste containing these metal particles. As a method of creating a coil antenna using the conductive material, it is only necessary to wrap around a loop when using a conductive wire, and when using a foil film, an extra foil film is formed so as to form a desired loop. May be removed by etching, and when a conductive paste is used, it may be printed in a loop by gravure printing, screen printing, flexographic printing, ink jet printing, or the like.

  The arithmetic unit (CPU) 1 gives a frequency value to be swept to the variable frequency constant voltage oscillator 3. The constant voltage control circuit 2 is a constant voltage generation circuit for keeping the AC voltage output from the variable frequency constant voltage oscillator 3 constant, and includes a level setting reference voltage source 21, a level detection circuit 22, and a comparator 23. The level setting reference voltage source 21 generates a voltage serving as a reference when setting the voltage level. The level detection circuit 22 is a circuit that detects the voltage level of the alternating current output from the variable frequency constant voltage oscillator 3. The comparator 23 receives the reference voltage level from the level setting reference voltage source 21, receives the detection voltage level from the level detection circuit 22, compares both voltage levels, and the voltage output from the variable frequency constant voltage oscillator 3 is obtained. The variable frequency constant voltage oscillator 3 is controlled so as to coincide with the reference voltage.

  Here, a theory will be described in which the resistance element 4 is provided on a circuit between the variable frequency constant voltage oscillator 3 and the antenna 5 so that the electric field strength is kept constant with respect to a change in frequency under a constant voltage.

  FIG. 2 is a diagram showing the positional relationship of the electromagnetic field. The center of the coil antenna is the origin, the z-axis is provided in the axial direction perpendicular to the coil antenna, and the x-axis and the y-axis are perpendicular to each other on a plane orthogonal to the z-axis.

The loop cross-sectional area of the coil antenna is indicated by S [m ² ], and the antenna current is indicated by I [A].

  A reference point indicating an electric field and a magnetic field has a distance r [m] from the origin, an angle θ formed by a vector from the origin to the reference point and the z-axis, a perpendicular leg of the vector to the xy plane, and a line segment of the origin at y It is specified by the angle φ formed with the axis.

  The electric field calculation formula is shown below.

Here, E [V / m] is an electric field obtained by multiplying the r-direction magnetic field by Z ₀ .
H _r [A / m] is a magnetic field in the r direction.
f [Hz] is the frequency of the alternating current.
Z ₀ is a free space characteristic impedance.

  The magnetic field calculation formula is shown below.

  Here, k (f) is a propagation coefficient, and j is an imaginary number.

Here, λ ₀ (f) is the wavelength [m].

Here, c ₀ is the speed of light in vacuum.

  Focusing only on the change in frequency with the reference point fixed, the electric field can be regarded as E (f, I) with the frequency f and current I as variables.

  Assuming that the antenna current I is constant even when the frequency changes, ignoring the coil characteristics, the electric field E tends to increase as the frequency f becomes higher than Equation 1 (the tendency to higher magnetic field, FIG. )reference).

  However, in the conventional technique in which no resistor is provided, the antenna current I actually decreases as the frequency f increases, as shown in the following antenna current calculation formula (no resistor).

Here, V _rms is the effective voltage of the alternating current, Z _L (f) is the impedance of the antenna, and L is the inductance of the antenna.

  Further, according to the electric field calculation formula (1), when the voltage is constant, the electric field E tends to decrease as the antenna current I decreases. That is, if Equation 5 is substituted into Equation 1, the electric field tends to decrease as the frequency range increases (the tendency to lower the magnetic field, see FIG. 3B).

  In a circuit without a resistor, the tendency to increase the magnetic field in the high frequency region and the tendency to decrease the magnetic field cancel each other, but the tendency to decrease the magnetic field as a whole is exhibited due to the influence of the decrease in magnetic field.

  In the present invention, the tendency to lower the magnetic field is weakened, and the magnetic field is made constant by offsetting the tendency to increase the magnetic field. The resistor R provided in the present invention acts to weaken the tendency to lower the magnetic field.

  An antenna current calculation formula (with resistance) in the circuit having the configuration of the present invention is shown below.

  According to this equation, as the frequency f increases, the antenna current I decreases, and the tendency to decrease current in the high frequency region is weakened. As a result, the tendency to lower the magnetic field in the high frequency range is also weakened.

Verify the above trends. Loop cross-sectional area S [m ² ] = 0.000624, distance r [m] = 3, angle θ [deg] = 0, frequency to sweep f [MHz] = 1.0, 1.1, 1.2,. .., 20.0, free space characteristic impedance Z ₀ [Ω] = 376.73, light velocity in vacuum C ₀ [m / sec] = 2.998 × 10 ⁸ , antenna voltage V _rms [V] = 2 .263, the change in the electric field intensity E calculated with the antenna inductance L [μH] = 1 is shown in FIG. AC current at f _s = 1 MHz (example in low frequency range), electric field strength E _s by a circuit without a resistor is 6236 [μV / m], and f _s = 20 MHz (example in high frequency range) Electric field strength E _e is 500 [μV / m].

In the present invention, a resistor is provided so that the same electric field strength is obtained in a low frequency region and a high frequency region under a constant voltage. Hereinafter, a procedure for calculating the resistance value will be described.
(1) Based on the electric field calculation formula, calculate the antenna current value in each frequency range in order to obtain the same electric field strength from a circuit that does not provide resistance with alternating currents in the low frequency range and high frequency range under a constant voltage. Then, the change ratio of the antenna current when shifting from the low frequency range to the high frequency range is obtained.
(2) The ratio of the total impedance of the circuit provided with the resistance, which is the change ratio of the antenna current under a constant voltage, is calculated.
(3) A resistance value R that realizes the impedance ratio is calculated.

An example of actual calculation under the above conditions will be shown.
(1) An electric field calculation formula and an antenna current calculation formula under the condition that the electric field strength E _s is 500 [μV / m] by an AC current when f _s = 1 MHz (an example in a low frequency range) without a resistor. If the antenna current value is calculated by the above, I _s = −0.36i. Further, when the antenna current value is calculated under the condition that the electric field intensity E _e is 500 [μV / m] with an alternating current at f _e = 20 MHz (example in a high frequency range), I _e = −0.225i It becomes. Therefore, when the frequency is changed from 1 MHz to 20 MHz, the change ratio of the antenna current is I _s / I _e = 1.603.
(2) according to the following formula, the impedance ratio _{K = Z e / Z s =} 1.603.

(3) Each impedance is calculated by the following formula.

According to the following formula, the resistance value R = 99.89. At this time, Z _s = 100.178 and Z _e = 160.585.

When the antenna voltage is obtained so that the electric field strength approaches 500 [μV / m] using the resistance value obtained in the above procedure, V _rms [V] = 36.06. When the electric field strength due to this antenna voltage is calculated again, the electric field strength E _s by the circuit provided with the resistance becomes 499.992384 with an alternating current of 1 MHz, and the electric field strength E _s by the circuit provided with the resistance with an alternating current of 20 MHz. Was 500.07463 and proved to be almost constant. The electric field strength in this frequency range is shown in FIG. It can be seen that the change in electric field strength is very small.

  FIG. 5 shows an example of a wireless tag communication device of the present invention. The wireless tag communication device of the present invention includes an electromagnetic wave output circuit having a variable frequency constant voltage oscillator 3, a resistance element 4, and an antenna 5, an arithmetic unit (CPU) 1, and a receiving circuit 6. Furthermore, it is preferable to provide a constant voltage control circuit 2.

  Next, the operation of the wireless tag communication device of the present invention will be described. The voltage to be output, the frequency, and the information to be transmitted to the wireless tag are input from the arithmetic unit (CPU) 1 to the variable frequency constant voltage oscillator 3. The variable frequency constant voltage oscillator 3 generates a current having an input voltage and frequency accompanied by information to be transmitted to the wireless tag, and the current is sent to the antenna 5 through the resistance element 4. When a current flows through the antenna 5, a radio wave having a certain electric field strength accompanied by information to be transmitted to the wireless tag is transmitted toward the wireless tag. A radio wave accompanied by information returned from the wireless tag is received by the antenna 5 and received by the receiving circuit 6. Information received from the receiving circuit 6 is input to the arithmetic unit (CPU) 1, and the information returned from the wireless tag is judged by the arithmetic unit (CPU) 1, recorded and managed. Next, the same operation is performed by sweeping the frequency input to the variable frequency constant voltage oscillator 3. Even in this case, the electric field intensity oscillated from the antenna can be suppressed to substantially the same intensity as before the frequency is swept.

  The wireless tag communication device of the present invention can transmit a radio wave having a constant electric field strength at a plurality of swept frequencies (for example, a frequency range of 2 to 14 MHz). A plurality of lowered wireless tags can be read. In addition, since the electric field intensity of the output radio wave can be controlled to a desired value, it can be operated so as to exhibit the maximum effect within the range defined by the Radio Law.

It is a figure which shows the structure of the electromagnetic wave output circuit of this invention. It is a figure which shows the positional relationship of an electromagnetic field. (B) Changes in the electric field strength when the frequency of the alternating current with a constant current is changed in a circuit without a resistor, and (b) When the frequency of the alternating current with a constant voltage is changed in a circuit without a resistor It is a figure which shows the change of the electric field strength. It is a figure which shows the change of the electric field strength at the time of changing the frequency of the alternating current of a fixed voltage in the electromagnetic wave output circuit of this invention. It is a figure which shows an example of the communication apparatus for radio | wireless tags of this invention. It is a figure which shows the standard of the weak radio wave in communication distance 3m. It is a figure which shows the comparative structural example.

Explanation of symbols

  1 arithmetic unit (CPU), 2 constant voltage control circuit, 3 variable frequency constant voltage oscillator, 4 resistance element, 5 coil antenna, 6 receiving circuit, 21 level setting reference voltage source, 22 level detection circuit, 23 comparator.

Claims (5)

An electromagnetic wave output circuit characterized by having the following elements: (1) A variable frequency constant voltage oscillator that generates an alternating current changing at a constant voltage in a frequency range including the first frequency and the second frequency (2) an alternating current (3) A resistance element provided on a circuit between the variable frequency constant voltage oscillator and the antenna.   The resistance element includes an electric field strength of an output electromagnetic wave of the antenna when the variable frequency constant voltage oscillator generates an alternating current of the first frequency at the constant voltage, and the variable frequency constant voltage oscillator includes the constant voltage. 2. The electromagnetic wave output circuit according to claim 1, wherein the electromagnetic wave output circuit has a resistance value that is the same or substantially the same as an electric field intensity of the output electromagnetic wave of the antenna when the alternating current of the second frequency is generated.   The electromagnetic wave output circuit according to claim 1, wherein the frequency range is 2 to 14 MHz, and the antenna is a loop-shaped coil antenna.   A constant voltage control circuit for controlling a voltage generated from the variable frequency constant voltage oscillator; a level detection circuit for detecting a voltage level by the constant voltage control circuit; and a level setting reference for generating a reference voltage 4. The electromagnetic wave output circuit according to claim 1, comprising a voltage source and a comparator for comparing voltage levels. 5.   5. A wireless circuit comprising: the electromagnetic wave output circuit according to claim 1; an arithmetic unit that inputs information to be transmitted to the wireless tag to the electromagnetic wave output circuit; and a receiving circuit that receives information returned from the wireless tag. Tag communication device.

Images