JP2007292744A - Rfid tag distance measurement system and reader - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an RFID tag distance measurement system capable of measuring the distance to an RFID tag precisely under a multipath environment. <P>SOLUTION: The RFID tag distance measurement system includes a reader that outputs a prescribed carrier signal to a tag and receives the reflection signal of the carrier signal from the tag. The reader includes a signal output means for outputting signals having a plurality of mutually different frequencies as carrier signals; a transmission section for transmitting the signal outputted from the signal output means to the tag; a reception section for receiving a reflection signal to the plurality of mutually different frequencies from the tag; and an estimation means for estimating the distance between the tag and the reader, based on the amount of change in the phase between the reflection signal received by the reception section, and the carrier signal, and the frequency of the carrier signal. The transmission and reception sections include two pairs of transmission/reception antennas for obtaining a plurality of reception signals having different variations in the strength of the reflection signal when the reception section receives the reflection signal. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an RFID tag distance measurement system and a reader that can measure the distance between a reader and an RFID (Radio Frequency IDentification) tag, and in particular, an RFID tag distance measurement system that can reliably measure the distance between the reader and the RFID tag. Regarding the reader.

A conventional reader capable of measuring a distance from an RFID tag is disclosed in, for example, EP1233964 (Patent Document 1). According to Patent Document 1, using the fact that the phase of the reflected wave from the RFID tag with respect to the carrier signal from the reader changes depending on the distance between the two, the reflected wave has the same phase and a phase orthogonal to the carrier signal. It is disclosed that the signal is separated into a Q signal and the magnitude and phase of the reflected wave from the RFID tag is obtained, and the distance from the RFID tag is obtained from the phase.
EP1239634 (column 7 and FIG. 3 etc.)

  A conventional reader capable of measuring the distance from the RFID tag is configured as described above. When the reader receives only the direct reflected wave from the RFID tag, the distance between the reader and the RFID tag can be measured by the method described in Patent Document 1. However, actually, the reflected wave from the RFID tag is not limited to the direct wave, but may be a wave that reaches through a path reflected by a wall or a ceiling. That is, the reflected wave from the RFID tag may take many paths (this environment is referred to as a multipath environment). In such a multipath environment, even if signals having a plurality of different frequencies are transmitted to the RFID tag in order to obtain the distance from the RFID tag, a reflected signal from the RFID tag is often not received. Conventionally, however, such a point has not been considered.

  The present invention has been made paying attention to the above problems, and provides an RFID tag distance measuring system and a reader / writer that can accurately measure the distance to the RFID tag in a multipath environment. Objective.

  An RFID tag distance measuring system according to the present invention includes an RFID tag and a reader that outputs a predetermined carrier signal to the RFID tag and receives a reflected signal of the carrier signal from the RFID tag. Estimate distance. The reader includes a signal output means for outputting signals having a plurality of different frequencies as a carrier signal, a transmitter for transmitting the signal output from the signal output means to the RFID tag, and a plurality of different frequencies from the RFID tag. A receiving unit that receives a reflected signal from the receiver, and an estimation unit that estimates the distance between the RFID tag and the reader based on the amount of phase change between the reflected signal and the carrier signal received by the receiver and the frequency of the carrier signal The transmitting unit and the receiving unit include diversity means for obtaining a plurality of received signals having different ways of changing the intensity of the reflected signal when the receiving unit receives the reflected signal.

  When estimating the distance between the RFID tag and the reader, it is possible to selectively acquire a strong reflected signal by using diversity means for obtaining a plurality of received signals having different ways of changing the intensity of the reflected signal.

  As a result, field holes can be reduced in a multipath environment, and therefore an RFID tag distance measurement system that can reliably measure the distance to the RFID tag can be provided.

  Preferably, the plurality of different frequencies of the carrier signal are equally spaced from each other.

  More preferably, the diversity means includes a plurality of transmitting antennas connected to the transmitting unit and having different mounting positions, and a plurality of receiving antennas connected to the receiving unit having different mounting positions. Antenna selection means for selecting one set of transmission antennas and reception antennas for transmission and reception from the plurality of reception antennas.

  The antenna selecting means switches to another set of antennas when the receiving unit cannot receive a signal from the RFID tag for a part of signals of a plurality of frequencies by the selected set of transmitting antenna and receiving antenna. Thus, a plurality of frequency signals may be retransmitted to the RFID tag, and the antenna selecting means cannot receive a signal from the RFID tag with respect to a part of the plurality of frequency signals by one antenna. When switching to another antenna, a signal of a part of the frequency that could not be received may be transmitted to the RFID tag again.

  In one embodiment of the present invention, the diversity means is a scan antenna.

  Preferably, the apparatus includes a scan antenna control unit configured to control a reception unit to receive a reflected wave from the RFID tag for a plurality of mutually different signals transmitted using the scan antenna.

  When the receiving unit cannot receive a signal from the RFID tag for a part of a plurality of frequencies according to one scan pattern, the scan antenna control unit switches to another scan pattern to send the signals of the plurality of frequencies to the RFID tag. The scan antenna control means may transmit another scan pattern when the receiving unit cannot receive a signal from the RFID tag for a part of signals of a plurality of frequencies by one scan pattern. The signal of a part of the frequencies that could not be received may be transmitted again to the RFID tag.

  In another embodiment of the present invention, the diversity means includes polarization output means for outputting a plurality of different polarizations.

  Preferably, the polarization output unit includes a polarization output control unit configured to control the reception unit to receive a reflected wave from the RFID tag with respect to signals having a plurality of mutually different frequencies transmitted from the transmission unit.

  More preferably, the polarization output control means switches to another pattern of polarization when the receiver cannot receive a signal from the RFID tag for a part of the signals of a plurality of frequencies by the transmitter. The signal of the frequency is transmitted again to the RFID tag.

  In another aspect of the present invention, a reader that outputs a predetermined carrier signal to the RFID tag and receives a reflected signal of the predetermined carrier signal from the RFID tag is a signal having a plurality of different frequencies as the carrier signal. The signal output means for outputting the signal, the transmission section for transmitting the signal output from the signal output means to the RFID tag, the reception section for receiving reflected signals from the RFID tag for a plurality of different frequencies, and the reception section Including an estimation means for estimating the distance between the RFID tag and the reader based on the amount of change in phase between the reflected signal and the carrier signal and the frequency of the carrier signal. The transmitter and the receiver are reflected by the receiver. Diversity means for obtaining a plurality of received signals having different ways of changing the intensity of the reflected signal when receiving the signal is included.

  Preferably, the frequencies of the plurality of different signals are equally spaced.

(1) First Embodiment (i) System Configuration Hereinafter, an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a diagram showing a configuration of a first embodiment of a reader / writer used in an RFID tag distance measuring system according to the present invention. This reader / writer outputs a predetermined carrier signal to the RFID tag in a multipath environment, and estimates the distance from the RFID tag.

  In this embodiment, in order to receive the reflected signal from the RFID tag reliably in a multipath environment, in this embodiment, the reader / writer receives a plurality of signals having different ways of changing the intensity of the reflected signal from the RFID tag. Diversity means. In this embodiment, a plurality of antennas are provided as diversity means. Here, a reader / writer is taken as an example, but a reader that does not have a writing function may be used.

Referring to FIG. 1, the reader / writer 10 controls the entire reader / writer 10, creates transmission data and controls communication, a transmission processing unit 12 connected to the control unit 11, and a transmission First and second transmission antennas 17 and 18 serving as transmission units connected to the processing unit 12, and first and second reception antennas 21 and 22 serving as reception units for receiving a reflected signal from the RFID tag, A reception processing unit 19 that performs predetermined processing on signals received by the first and second reception antennas 21 and 22 and a phase calculation comparison distance estimation unit (estimation unit) 25 connected to the reception processing unit 19 are included. . The transmission processing unit 12 is connected to a PLL (Phase Lock Loop) unit 13 that transmits a carrier signal according to an instruction from the control unit 11, and a modulation unit 14 that is connected to the PLL unit 13 and modulates transmission data generated by the control unit 11. And a power amplifying unit 15 connected to the modulating unit 14 and amplifying the modulated signal. In addition, a transmission antenna changeover switch 16 is provided for switching which of the first and second transmission antennas 17 and 18 transmits an antenna for transmitting the transmission data amplified by the power amplifier 15 to the RFID tag. Yes. Similarly, a reception antenna changeover switch 20 that switches between the first and second reception antennas 21 and 22 is provided.

  The reception processing unit 19 divides a reception signal from one of the reception antennas 21 and 22 into two parts and is connected to the amplification units 23a and 23b and the amplification units 23a and 23b, respectively. A frequency converter 24 (including mixers 24a and 24b and a 90 ° phase shift unit 24c, which will be described later) for converting the frequency of the reception signal amplified in 23b into a signal having a lower frequency. Including.

  The phase calculation distance estimation unit 25 includes a phase information acquisition unit 26 that receives a signal from the frequency conversion unit 24, and a distance calculation unit that calculates a distance from the RFID tag based on the phase information acquired by the phase information acquisition unit 26. 27.

  When the carrier wave is carried by the first transmitting antenna 17, the reflected signal is received by the first receiving antenna 21, and similarly, when the carrier wave is transmitted by the second transmitting antenna 18, the second receiving antenna is used. 22 is received. Further, the positions of the tip portions of the first and second transmitting and receiving antennas are different from each other in order to obtain a diversity effect.

  The control unit 11 constitutes signal output means for outputting signals having a plurality of different frequencies as carrier signals together with the PLL unit 13 and the modulation unit 14. In addition, the control unit 11 functions as an antenna selection unit that transmits an antenna designation signal 28 for switching the transmission antennas 17 and 18 and the reception antennas 21 and 22 to the respective antenna switching switches 16 and 20.

(Ii) Distance Measuring Method Next, a configuration for measuring the distance between the RFID tag and the reader / writer 10 in the reader / writer 10 will be described with reference to FIG.

In the transmission processing unit 12, the PLL unit 13 sets a carrier frequency of a transmission signal transmitted from the transmission antenna 17 or 18, and is configured by a PLL circuit. The modulation unit 14 performs a process of modulating the carrier signal generated by the PLL unit 13 and the oscillator and superimposing data on the transmission signal. In the present embodiment, the modulation unit 14 generates a transmission signal by ASK (Amplitude Shift Keying) modulation. The transmission signal modulation method is not limited to the above-described ASK modulation, but FSK (Frequency Shift Keying).
Other digital modulation methods such as modulation and PSK (Phase Shift Keying) modulation may be employed. The power amplifier 15 amplifies the transmission signal.

  In the phase calculation comparison distance estimation unit 25, the phase information acquisition unit 26 detects the amount of change in the phase of the received signal frequency-converted by the frequency conversion unit 24, and acquires this as phase information. Note that the amount of change in phase of the received signal indicates the amount of change in phase that occurs when the received signal propagates a predetermined distance.

More specifically, assuming that the carrier signal output from the PLL unit 13 is sin2πf1t, the frequency converter 24 receives the carrier signal sin2πf1t and the received signal D (t) Asin (2πf1 t + φ) input from the amplifiers 23a and 23b. The value obtained by multiplying (D (t) Acosφ) is sent to the phase information acquisition unit 26. The phase information acquisition unit 26 calculates the phase change amount φ based on the value sent from the frequency conversion unit 24. Here, t is time, D (t) is a baseband signal when ASK modulation is performed in the modulator 14, A is the amplitude of the carrier signal itself, and φ is a phase change caused by propagating a distance of 2r round-trip. Each amount is shown.

  The distance calculation unit 27 calculates the distance between the RFID tag and the reader / writer 10 based on the phase change information acquired by the phase information acquisition unit 26. Details of this distance calculation method will be described later.

  The control unit 11 controls the frequency of the carrier signal set by the PLL unit 13 and inputs data for modulating the transmission signal to the modulation unit 14.

  Next, details of the distance measurement method will be described. In the present embodiment, as will be described later, the reader / writer 10 transmits an R / W request signal (request signal) to the RFID tag, and the RFID tag returns a response signal (reflection signal) accordingly. It is like that.

  The reader / writer 10 always transmits a specific signal (a signal for supplying power to the RFID tag), while transmitting a response signal (hereinafter referred to as a tag response signal) to the RFID tag. When making a request, an R / W request signal requesting a reply of the tag response signal is transmitted. That is, the transmission processing unit 12 in the reader / writer 10 controls the modulation unit 14 to transmit data indicating the steady state in the steady state, and forms an R / W request signal when requesting a tag response signal. The modulation unit 14 is controlled to transmit data. The RFID tag always monitors a signal sent from the reader / writer 10, and when it detects that an R / W request signal has been received, it transmits a tag response signal in response to it.

More specifically, the reader / writer 10 transmits a signal of one frame composed of an R / W request signal and CW (Continuous Wave) described later. When the RFID tag receives the R / W request signal and CW from the reader / writer 10, the RFID tag transmits a tag response signal consisting of a carrier frequency f 1 corresponding to the frequency of the CW to the reader / writer 10. For example, the R / W request signal and CW (continuous carrier wave) are transmitted by the carrier frequency f1, and the tag response signal is transmitted by the carrier frequency f1 accordingly.

  The tag response signal is composed of a frame having a preamble part and a data part. The preamble portion indicates data indicating the start of a tag response signal, and is predetermined data common to all RFID tags within the same standard (for example, EPC). The data portion is transmitted subsequent to the preamble portion, and indicates data indicating substantial information transmitted from the RFID tag. The information included in the data part includes, for example, ID information unique to each RFID tag, but includes information to be transmitted from the RFID tag, such as various information stored in the storage part in the RFID tag. You may go out.

  The reader / writer 10 transmits the R / W request signal twice, and also makes the carrier frequencies different in transmission of each R / W request signal (more specifically, CW following the R / W request signal). . That is, when the first R / W request signal is transmitted, the control unit 11 in the reader / writer 10 controls the PLL unit 13 so as to output the carrier signal at the first frequency f1, and the second R / W request. At the time of signal transmission, the PLL unit 13 is controlled so as to output the carrier signal at a second frequency f2 different from the first frequency f1.

  That is, when the RFID tag receives the R / W request signal transmitted at the first frequency f1, the tag response signal is also returned at the first frequency f1. Then, the reader / writer 10 detects φ1 indicating the amount of change in the phase of the tag response signal by analyzing the preamble portion of the tag response signal received by the phase information acquisition unit 26. Similarly, when the RFID tag 10 receives the R / W request signal transmitted at the second frequency f2, the tag response signal is also returned at the second frequency f2. Then, the reader / writer 10 detects φ2 indicating the amount of change in the phase of the tag response signal by analyzing the preamble portion of the tag response signal received by the phase information acquisition unit 26. The analysis described here is processing for detecting the amount of change in the phase of the tag response signal.

  In the above example, the amount of change in the phase of the tag response signal is detected by analyzing the preamble part. However, the present invention is not limited to this, and the phase change including the data part is also included. The amount of change may be detected, or the amount of phase change may be detected in the data portion. However, when the modulation method is PSK, it is difficult to detect the amount of change in phase with distance based on the data part whose contents can change. Therefore, in the preamble part whose contents are fixed, It is preferable to detect the amount of change.

  As described above, when the phase information acquisition unit 26 detects the phase change amounts φ 1 and φ 2, information on the phase change amount is transmitted to the distance calculation unit 27. The distance calculation unit 27 calculates the distance between the RFID tag and the reader / writer 10 based on φ1 and φ2 as follows.

  First, it is assumed that the distance from any one of the transmission antennas 17 and 18 to the RFID tag and the distance from any one of the reception antennas 21 and 22 to the RFID tag are equal, and this is the distance r. Phase variations φ1 and φ2 caused by the signals carried by the first frequency f1 and the second frequency f2 propagating through the distance of the round trip 2r are expressed by the following equation (1).

  In the above formula, c represents the speed of light. Based on the above two formulas, the distance r is obtained by the following formula (2).

  As described above, the distance r from the transmission antenna 17 or 18 to the RFID tag can be obtained based on the phase change amounts φ1 and φ2. In the RFID tag, a phase shift is expected to occur between the reception of the R / W request signal and the transmission of the tag response signal. This phase shift is caused by the first frequency f1 and the first frequency shift. The same is true for both signals carried by the two frequencies f2. Therefore, a phase shift that occurs during transmission / reception of a signal in the RFID tag does not affect the calculation of the distance.

  In Equation (2), when φ2 is 2π or more, the distance r cannot be calculated accurately. That is, the maximum value rmax of the distance r that can be measured is when Δφ = 2π, and is represented by the following equation (3).

  Here, for example, when the difference between the first frequency f1 and the second frequency f2 is 5 MHz, the maximum distance rmax is 30 m from Equation 3. Similarly, when the difference between the first frequency f1 and the second frequency f2 is 2 MHz, the maximum distance rmax is 75 m from Equation (3). In the RFID communication system using the UHF band, since the assumed maximum communication distance is about 10 m, it can be understood that the above measurement has no practical problem.

  Even when the measurement of the maximum distance rmax or more is necessary, it is possible to measure the distance r by using, for example, measurement of the reception intensity of the received signal. Specifically, when Δφ may be 2π or more, the candidate r ′ for the distance r is r ′ = r + n · rmax (n is an integer of 0 or more). Therefore, it is possible to specify the value of n by using the fact that the reception intensity of the reception signal decreases as the distance r increases.

  When an active type RFID tag is used, the distance is measured based on the tag response signal actively sent from the RFID tag side without transmitting the R / W request signal from the reader / writer 10 side. It may be like this.

  Next, a specific example of the reception processing unit 19 will be described. In the distance measurement described above, processing for detecting the amount of change in the phase of the received signal is performed. The detailed configuration of the reception processing unit 19 that enables detection of the amount of change in phase is illustrated in FIG. Description will be made with reference to FIG. In this specific example, the reception processing unit 19 separates the received signal into an I signal and a Q signal and inputs the I signal and the Q signal to the phase calculation comparison distance estimation unit 25 so that the phase change amount in the phase calculation comparison distance estimation unit 25 is calculated. The detection process is made possible. As shown in the figure, the reception processing unit 19 includes the two amplifying units 23a and 23b as the amplifying units, the mixers 24a and 24b as the frequency converting unit 24, and the 90 ° phase shifter 24c as described above. ing.

  A reception signal received by one of the reception antennas 21 and 22 is branched into two paths, one being input to the amplification unit 23a and the other being input to the amplification unit 23b. The amplifying unit 23a amplifies the input received signal and inputs it to the mixer 24a. The amplifying unit 23b amplifies the input received signal and inputs it to the mixer 24b.

  The mixer 24 a outputs an I signal by multiplying the reception signal input from the amplification unit 23 a and the carrier signal output from the PLL unit 13, and inputs this I signal to the phase information acquisition unit 26. The mixer 24b multiplies the received signal input from the amplifying unit 23b with the carrier signal output from the PLL unit 13 and whose phase is changed by 90 ° via the 90 ° phase shifter 24c. The Q signal is input to the phase information acquisition unit 26.

  Details of the reception process and the distance r calculation process performed in the above configuration will be described below. A signal that is propagated through the distance 2r and received by the reader / writer 10 is represented by the following equation (4), where the frequency of the carrier signal is f1.

  In the above equation, t is time, s1 (t) is the state of the signal transmitted by the carrier signal of frequency f1, D (t) is the baseband signal when ASK modulation is performed in the modulator 14, and A is the carrier The amplitude of the signal itself, φ1, indicates the amount of change in phase caused by propagating a distance of 2r round-trip. In this case, I1 (t) indicating the state of the I signal output by the mixer 24a and Q1 (t) indicating the state of the Q signal output by the mixer 24b are expressed by the following equations (5) and (6). It is represented by

  From the above, based on the I signal and the Q signal, the signal phase change φ1 due to the carrier signal having the frequency f1 is obtained by the following equation (7).

  Similarly, the signal phase change amount φ2 due to the carrier signal of frequency f2 is obtained by the following equation (8).

  As described above, the phase information acquisition unit 26 acquires the phase change amounts φ1 and φ2 based on the input I signal and Q signal. Then, the distance calculation unit 27 calculates the distance r by the following equation (9).

The above-described method for estimating the distance between the RFID tag and the reader / writer 10 is common to all the following embodiments.
(Iii) Specific System Operation Next, specific operations of the reader / writer 10 and an RFID tag (not shown) in this embodiment will be described. In the above description, the distance is calculated using the two frequencies f1 and f2. Here, not only f1 and f2 but also the distance between the two is measured using three frequencies including f3. explain. Note that the frequencies f1 to f3 are arbitrary different frequencies. In the following drawings, the reader / writer 10 is represented as R / W, and the RFID tag is represented as an IC tag.

  FIG. 2 is a diagram schematically showing the operation of the reader / writer 10 and an RFID tag (not shown). In FIG. 2, R / W → IC tag indicates transmission data from the reader / writer 10 to the RFID tag, and the arrow direction indicates the passage of time. Further, the upper part of the arrow indicates the content of the signal, and the lower parts f1, f2, etc. indicate the transmission frequency. On the other hand, IC tag → R / W indicates transmission data from the RFID tag to the reader / writer 10.

  Referring to FIG. 1 and FIG. 2A, the reader / writer 10 first uses the first transmission antenna 17 to read and write request signal (R / W request) at the frequency f1 to the RFID tag. Signal) and an unmodulated wave CW is transmitted. On the other hand, an unmodulated wave is returned from the RFID tag at the frequency f1, and the reader / writer 10 receives it. Next, the reader / writer 10 performs the same transmission at the frequency f2. However, there is no response from the RFID tag this time.

  Therefore, the reader / writer 10 switches the transmission antenna to the second transmission antenna 18 with the transmission antenna changeover switch 16, and this time, as shown in FIG. Send. This time, it is assumed that responses have been obtained for all three frequencies. Therefore, as shown in FIG. 2C, the reader / writer 10 creates an IQ demodulated signal by the frequency conversion unit 24, cuts it out at each frequency, sends it to the phase calculation comparison distance estimation unit 25, and calculates the above equation (9). ) To find the distance.

  Next, the control contents of the control unit 11 in this case will be described. FIGS. 3A and 3B are flowcharts showing the control contents of the control unit 11. Referring to FIG. 3A, the reader / writer 10 first transmits a read / write request signal and an unmodulated wave to the RFID tag using the first transmission antenna 17 at the frequency f1 (step). S11, hereinafter, steps are omitted). Next, it is determined whether or not a response signal of frequency f1 has been received (S12), and if received, the phase calculation distance estimation unit 25 demodulates the IQ orthogonal signal with the reference signal (carrier signal) of frequency f1, Data is taken in (S13). Then, after a predetermined time, the frequency is switched to the frequency f2, and similarly, the read / write request signal and the non-modulated wave are transmitted to the RFID tag at the frequency f2, and the response signal of the frequency f2 is received. Judgment is made (S14 and S15). If a response signal of frequency f2 can be received in S15, quadrature demodulation is performed using the reference signal of frequency f2, and data is captured (S16). Then, after a predetermined time, the frequency is switched to the frequency f3, and similarly, the read / write request signal and the non-modulated wave are transmitted to the RFID tag at the frequency f3, and the response signal of the frequency f3 is received. Judgment is made (S17 and S18). If a response signal of frequency f3 can be received in S18, quadrature demodulation is performed using the reference signal of frequency f3, and data is captured (S19). In this way, when reflected signals are obtained for all three frequencies as shown in FIG. 2B, the phase calculation distance estimation unit 25, as shown in FIG. The phase difference is calculated from the IQ orthogonal signals of .about.f3, and the distance from the RFID tag is calculated using the above equation (9) (S20, S21).

  As shown in FIG. 2A, when the response signal from the RFID tag at the frequency f1, f2 or f3 cannot be received at any of S12, S15 and S18, the second transmission antenna 18 is switched to S11. Returning to FIG. 5, the same processing is performed using the second transmitting antenna 18.

  As described above, according to this embodiment, when a reflected signal is received from an RFID tag using a plurality of transmission / reception antennas, a plurality of reception signals having different strength variations are obtained. The distance from the RFID tag can be known with certainty. Further, since the distance is estimated by obtaining the phase difference between the three frequencies, the accuracy of the measurement can be improved.

  Next, the simulation result in this case will be described. FIG. 4 and FIG. 5 are diagrams showing a simulation situation and the result under a multipath environment when the first and second plural transmitting and receiving antennas are used. As shown in FIG. 4, when transmission / reception is performed between two transmission / reception antennas (in the figure, only the transmission antenna 17 and the reception antenna 21 are shown) and the RFID tag 50, a direct wave ( reflected by the reflector (floor surface) 32 present on the lower surface, and the case of the wave reflected by the reflector (wall surface) 31 present on the side surface (denoted by a2 and b2). The simulation was performed using three waves in the case of the waves (shown by a3 and b3). The result is shown in FIG.

  FIG. 5A is a diagram showing the relationship between the distance between the reader / writer 10 and the RFID tag and the estimation error when the first transmission / reception antennas 17 and 21 are used, and FIG. It is a figure which shows the relationship between the true value and target value of the distance between the reader / writer 10 and RFID tag at the time of using the antennas 18 and 22, and an estimation error. As shown in FIGS. 5A and 5B, data having different characteristics can be obtained even when the same frequency is used. In any case, a similar estimated value close to the true value is obtained. I understand that. It can also be seen that the same result can be obtained even when switching to another antenna when some of the reflected signals cannot be received.

(2) Second Embodiment Next, a second embodiment of the present invention will be described. In this embodiment, the reader / writer has a scan antenna instead of a plurality of antennas as diversity means.

  FIG. 6 is a diagram for explaining the scan antenna. FIG. 6A is a diagram showing an outline of the scan antenna. As shown in FIG. 6A, the scan antenna has a plurality of antenna elements 41a to 41c, and the phase shifter 43 is controlled by the scan control unit 42 with respect to the phase difference between the signals fed to the antenna elements 41a to 41c. Thus, the directivity of the antenna can be changed.

  Next, the characteristics of the scan antenna will be described. FIG. 6B is a diagram showing the electric field strength distribution of a normal patch antenna, and FIG. 6C is a diagram showing the electric field strength distribution of the scan antenna. In the figure, a field hole 44 (area where communication is not possible) 44 is shown by hatching. In the patch antenna, a large number of field holes 44 that cannot communicate with the RFID tag are generated due to the influence of multipath. However, in the scan antenna, such a situation does not occur.

  A reader / writer in this embodiment is shown in FIG. Referring to FIG. 7, a reader / writer 50 includes a reader / writer main body 51 and an antenna unit 52 that operates as diversity means. The reader / writer main body 51 has a circular 53 in order to share one antenna unit 52 for transmission and reception, and the control unit 11 sends a scan control signal to the scan control unit in order to change the directivity of the antenna. Except for this point, the configuration is basically the same as that of the first embodiment shown in FIG. 1, and therefore, the same reference numerals are given to the same portions and the description thereof is omitted.

  The antenna unit 52 includes a scan control unit 42, a plurality of antenna elements 41 a to 41 c, and a phase shifter 43 that changes a phase difference between signals supplied to the plurality of antenna elements 41 a to 41 c by control from the scan control unit 42. And a distribution synthesizer 45 that is provided between the circular 53 and the phase shifter 43 and distributes the radio wave at the time of transmission to each antenna element 41 and synthesizes the radio wave received by each antenna element 41.

  Here, the control unit 11, the scan control unit 42, and the phase shifter 43 function as scan antenna control means.

  Next, operations of the reader / writer 50 and an RFID tag (not shown) in this case will be described. Here, as in the previous embodiment, a case where distance measurement is performed using three frequencies will be described. FIG. 8 is a diagram corresponding to FIG. 2 in the first embodiment in this embodiment.

  Referring to FIG. 8, also in this embodiment, the basic operation is the same as that in the first embodiment, and in the case of the first embodiment, a response from the RFID tag was not obtained. In this case, a difference is that the directivity of the scan antenna is changed in this embodiment, although a plurality of antennas are switched. That is, as shown in FIG. 8A, for example, when the reflected signal from the RFID tag is not obtained at the frequency f2, the directivity of the antenna unit 52 is changed. In this state, as shown in FIG. 8 (B), when reflected signals can be received for all frequencies, an IQ demodulated signal is obtained and a phase difference is calculated as shown in FIG. 8 (C). To find the distance.

  Next, the control contents of the control unit 11 in this embodiment will be described. FIGS. 9A and 9B are flowcharts showing the control contents of the control unit 11 in the second embodiment. Also in this embodiment, the basic operation is the same as that in the first embodiment. In the case of the first embodiment, when a response from the RFID tag is not obtained, a plurality of antennas are connected. This embodiment is different from the first embodiment in that the field holes are reduced by changing the directivity of the scan antenna. Therefore, the contents indicated by S31 to S41 in FIG. 9A are the same as those in FIG. 3A. In FIG. 9A, the response signal from the RFID tag at the frequency f1, f2 or f3 cannot be received. In this case, as shown in S42 in FIG. 9B, the directivity of the scan antenna is changed. Therefore, the description of the details is omitted.

  As described above, according to this embodiment, when a reflected signal is received from an RFID tag using a scan antenna, a plurality of received signals having different strength variations are obtained. The distance from the RFID tag can be known. Further, since the distance is estimated by obtaining the phase difference between the three frequencies, the accuracy of the measurement can be improved.

  Next, the simulation result in this case will be described. FIG. 10 and FIG. 11 are diagrams showing a simulation situation and the result under a multipath environment when a scan antenna is used, and correspond to FIG. 4 and FIG. 5 in the first embodiment.

  Referring to FIG. 10, here, since it is a scan antenna, only one antenna portion 52 is displayed. Here, the simulation is also performed with a model of three waves of a direct wave between the antenna unit 52 and the RFID tag, a reflected wave from the floor surface 32 and a reflected wave from the wall surface 31.

  FIG. 11A shows a state before the directivity of the antenna unit 52 is changed, and FIG. 11B shows a state after the directivity of the antenna unit 52 is changed. Referring to FIGS. 11A and 11B, the amplitude of the estimated distance in (B) is smaller than the amplitude of the estimated distance in FIG. 11A, and the decrease in accuracy is improved. I understand that.

(3) Third Embodiment Next, still another embodiment of the present invention will be described. In the above embodiment, the scan antenna is used to control the response from the RFID tag at all frequencies when the antenna has a certain directivity. On the other hand, in this embodiment, when no response is obtained at some frequencies, the directivity of the antenna is changed. Here, the case where three frequencies are used will be described.

  The configuration of the reader / writer in this embodiment is the same as that shown in FIG.

  FIG. 12 is a diagram corresponding to FIG. 8 in this embodiment. Referring to FIG. 12A, in this embodiment, reflected signals are received from the RFID tag at frequencies f1 and f3, but no response from the RFID tag is obtained for frequency f2.

  Therefore, in this embodiment, as shown in (B), the RFID tag is retransmitted again only for the frequency f2 that has not been obtained. As shown in (C), after obtaining the reflection signals for all the frequencies in this way, this is demodulated to generate an IQ demodulated signal, and the phase difference is calculated.

  13 and 14 are flowcharts showing the operation of the reader / writer in this case. 13 and 14, the reader / writer 50 first sets a certain directivity and transmits a read / write request signal from the antenna unit 52 to the RFID tag at the frequency f1 (S51). In this embodiment, the frequency transmitted first is fixed at f1. Next, an unmodulated wave is transmitted (S52). Thereafter, it is determined whether or not a response signal having the frequency f1 has been received (S53). If the response signal can be received, the phase calculation comparison distance estimation unit 25 demodulates the IQ orthogonal signal using the reference signal having the frequency f1 and captures data ( S54). Then, after a predetermined time, the frequency is switched to the frequency f2, and similarly, the read / write request signal and the non-modulated wave are transmitted to the RFID tag at the frequency f2, and the response signal of the frequency f2 is received. Judgment is made (S55 and S56). If the response signal having the frequency f1 cannot be received in S53, the process proceeds to S55. If a response signal of frequency f2 can be received in S56, quadrature demodulation is performed using the reference signal of frequency f2, and data is captured (S57). Then, after a certain time, the frequency is switched to f3. Similarly, whether the read / write request signal and the unmodulated wave are transmitted to the RFID tag at the frequency f3 and the response signal of the frequency f3 is received. Is determined (S58 and S56). If the response signal having the frequency f2 cannot be received in S59, the process proceeds to S58.

  If a response signal of frequency f3 can be received in S59, quadrature demodulation is performed using the reference signal of frequency f3, and data is captured (S60). When IQ orthogonal signals of all frequencies f1 to f3 are obtained at this stage (YES in S61), the phase calculation comparison distance estimation unit 25 calculates a distance by calculating a phase difference from these data ( S62, S63). Note that the process also proceeds to S61 when the signal of the frequency f3 cannot be received in S59.

  If IQ data for all frequencies has not been acquired in S61, the process proceeds to S71 in FIG. 14 to change the directivity of the antenna unit. Then, it is determined which of the frequencies f1 to f3 has not been able to acquire IQ data (S72, S76, S80), and in the frequency that has not been acquired (NO in S72, S76, S80), to the RFID tag at that frequency It is transmitted (S73, 77, S81), and it is determined whether or not the response signal has been received (S74, S78, S82). Demodulate and take in data (S75, S79, S83). If a response signal cannot be received at that frequency (NO at S74, S78, S82), the process proceeds to acquisition of IQ data at another frequency.

  When IQ acquisition data is acquired for all frequencies (YES in S84), the process proceeds to S62, where the phase difference is calculated from the IQ data of each frequency, and the distance is calculated in the same manner as described above (S63). If IQ data has not been acquired at all frequencies in S84 (NO in S84), the process returns to S71.

  Here, the “directivity of the antenna” refers to the relationship between the radiation direction of the radio wave and the radiation intensity, and is represented by a diagram showing the relationship between the radiation angle and the radiation intensity in a radar chart. “Changing the directivity of the antenna” means changing the main lobe (direction in which radiation is maximized) of the antenna. Note that this antenna directivity change time pattern is referred to as a “scan pattern”, and switching the antenna directivity change time pattern is referred to as “switching to another scan pattern”.

  Therefore, the scan pattern for changing the directivity of the scan antenna in the above embodiment may be arbitrarily changed.

  That is, when the receiving unit cannot receive a signal from the RFID tag for a part of a plurality of frequencies by one scan pattern, the signal is switched to another scan pattern and the signals of the plurality of frequencies are transmitted to the RFID tag again. Alternatively, when the receiving unit cannot receive a signal from the RFID tag for a part of signals of a plurality of frequencies according to one scan pattern, it is switched to another scan pattern and cannot be received. The signal of the frequency of the part may be transmitted again to the RFID tag.

(4) Fourth Embodiment Next, still another embodiment of the present invention will be described. FIG. 15 is a block diagram showing a configuration of a reader / writer according to still another embodiment of the present invention. In this embodiment, the reader / writer includes a polarizer that outputs a plurality of different polarized waves as diversity means. That is, the reader / writer has one antenna, and outputs a signal having a different polarization pattern generated by the polarizer from the antenna to the RFID tag.

  By changing the polarization method, the influence of the reflected wave changes depending on the RFID tag. That is, by using a plurality of polarized waves, even if there is no response from the RFID tag by a certain polarization method, it is possible to acquire the response of the RFID tag by changing the polarization method. The distance to the RFID tag can be measured by combining the frequency responses acquired by the respective polarization methods.

  FIG. 15 is a block diagram showing the reader / writer 60 in this embodiment. Referring to FIG. 15, the reader / writer 60 in this embodiment is basically the same as that shown in FIG. In FIG. 1, a plurality of antennas are used, but here, the control unit is provided between the power amplifying unit 15 and the circular 62 between the point where the circular 62 is provided to transmit and receive only with the antenna 63. 11 is different from the case of FIG. 1 in that a polarizer 61 for polarizing a carrier wave is provided in accordance with a polarization control signal 64 from 11. The change of the polarization method by the polarizer 61 is performed by an instruction from the control unit 11. Therefore, the control unit 11 and the polarizer 61 function as a polarization output unit and a polarization output control unit.

  Next, the operation of the reader / writer in this embodiment will be described. FIG. 16 is a diagram corresponding to FIG. 2 and FIG. 8 showing the operation of the reader / writer in this embodiment. Referring to FIG. 16, in this embodiment, when there is a frequency that cannot be received among a plurality of frequencies, the polarization method is changed. Specifically, in the flowchart shown in FIG. 3, S22 in FIG. 3B only changes the polarization method, not the antenna switching, and thus the drawing and description thereof are omitted.

  As described above, according to this embodiment, when a reflected signal is received from an RFID tag using a polarizer, a plurality of received signals having different strength variations are obtained. It is possible to know the distance from the RFID tag. Further, since the distance is estimated by obtaining the phase difference between the three frequencies, the accuracy of the measurement can be improved.

  In the above-described embodiment, the specific frequency of the plurality of frequencies is not particularly limited. However, in order to increase the accuracy during distance measurement in a multipath environment, the phases of the plurality of frequencies having the same interval are used. It is preferable to use a high resolution algorithm such as the MUSIC (MUltiple SIgnal Classification) method using information.

  Therefore, it is preferable to use three frequencies having the same interval for the plurality of frequencies described above.

  Further, the plurality of frequencies is not limited to three, and four or more frequencies may be used. Further, if five or more frequencies f1 to f5 having an equal interval are used, for example, even if a reflected signal of the frequency of f2 and f4 cannot be obtained, the equal interval 3 of f1, f3, and f5 is obtained. Two response signals can be obtained.

  In the above embodiment, as an example of the diversity means, when a scan antenna is used, only the signals of some frequencies that cannot receive the reflected signal from the RFID tag are changed again and the directivity is changed again. Although the case of transmitting has been described, the present invention is not limited to this, and the same processing may be performed when a plurality of antennas are used or when a plurality of polarized waves are used.

  As mentioned above, although embodiment of this invention was described with reference to drawings, this invention is not limited to the thing of embodiment shown in figure. Various modifications and variations can be made to the illustrated embodiment within the same range or equivalent range as the present invention.

  Since the RFID tag distance measuring system according to the present invention reduces the number of field holes using diversity means in a multipath environment, the distance to the RFID tag can be reliably measured. Therefore, it is advantageously used as an RFID tag distance measuring system.

It is a block diagram which shows the principal part of the reader / writer which concerns on 1st Embodiment of this invention. It is a schematic diagram which shows the communication state of the reader / writer and RFID tag which concern on 1st Embodiment of this invention. It is a flowchart which shows operation | movement of the reader / writer based on 1st Embodiment of this invention. It is a figure which shows the simulation environment at the time of using the reader / writer based on 1st Embodiment of this invention. It is a figure which shows the estimation result of distance with an RFID tag at the time of using the reader / writer based on 1st Embodiment of this invention. It is a figure which shows the principal part of the reader / writer based on 2nd Embodiment of this invention, and the electric field strength distribution in that case. It is a block diagram which shows the principal part of the reader / writer which concerns on 2nd Embodiment of this invention. It is a schematic diagram which shows the communication state of the reader / writer and RFID tag which concern on 2nd Embodiment of this invention. It is a flowchart which shows operation | movement of the reader / writer based on 2nd Embodiment of this invention. It is a figure which shows the simulation environment at the time of using the reader / writer based on 2nd Embodiment of this invention. It is a figure which shows the estimation result of distance with an RFID tag at the time of using the reader / writer based on 2nd Embodiment of this invention. It is a schematic diagram which shows the communication state of the reader / writer and RFID tag in 3rd Embodiment of this invention. It is a flowchart which shows operation | movement of the reader / writer in 3rd Embodiment of this invention. It is a flowchart which shows operation | movement of the reader / writer in 3rd Embodiment of this invention. It is a block diagram which shows the principal part of the reader / writer which concerns on 4th Embodiment of this invention. It is a schematic diagram which shows the communication state of the reader / writer and RFID tag in 4th Embodiment of this invention.

Explanation of symbols

  10, 50, 60 Reader / writer, 11 Control unit, 12 PLL unit, 13 Modulation unit, 14 Power amplification unit, 15 Transmit antenna switch, 16 First transmit antenna, 17 Second transmit antenna, 18 Receive antenna switch, 19 1st receiving antenna, 20 2nd receiving antenna, 21 BPF, 22 90 ° phase shift unit, 23, 24 mixer, 25 phase calculation distance estimation unit, 42 scan control unit, 43 phase shifter, 45 distribution synthesis unit, 51 reader Writer body, 52 antenna section, 53, 62 circular.

Claims (14)

An RFID tag that includes an RFID tag and a reader that outputs a predetermined carrier signal to the RFID tag and receives a reflected signal of the carrier signal from the RFID tag, and estimates a distance between the RFID tag and the reader A distance measuring system,
The reader, as the carrier signal, signal output means for outputting signals of a plurality of different frequencies,
A transmitter for transmitting the signal output from the signal output means to the RFID tag;
A receiving unit that receives reflected signals from the RFID tag for the plurality of different frequencies;
An estimation unit that estimates a distance between the RFID tag and the reader based on a phase change amount between the reflected signal and the carrier signal received by the receiver and the frequency of the carrier signal;
The RFID tag distance measuring system, wherein the transmitting unit and the receiving unit include diversity means for obtaining a plurality of received signals having different ways of fluctuation of the intensity of the reflected signal when the receiving unit receives the reflected signal. The RFID tag distance measurement system according to claim 1, wherein the plurality of different frequencies of the carrier signal are equally spaced from each other. A plurality of transmitting antennas connected to the transmitting unit and having different mounting positions; and a plurality of receiving antennas connected to the receiving unit having different mounting positions;
3. The diversity means according to claim 1, wherein the diversity means includes antenna selection means for selecting a set of transmission antennas and reception antennas that perform transmission and reception from the plurality of transmission antennas and the plurality of reception antennas. RFID tag distance measurement system. The antenna selecting means may select another set when the receiving unit cannot receive a signal from the RFID tag for a part of the signals of the plurality of frequencies by the selected one set of transmitting antenna and receiving antenna. The RFID tag distance measurement system according to claim 3, wherein the antenna is switched to another antenna and signals of the plurality of frequencies are transmitted again to the RFID tag. When the receiving unit cannot receive a signal from the RFID tag for a part of the signals of the plurality of frequencies by one antenna, the antenna selecting unit is switched to another antenna and cannot receive the signal. The RFID tag distance measurement system according to claim 3, wherein a signal having a certain frequency is transmitted to the RFID tag again. The RFID tag distance measurement system according to claim 1 or 2, wherein the diversity means is a scan antenna. The scan antenna control unit according to claim 6, further comprising: a scan antenna control unit configured to control the reception unit to receive a reflected wave from the RFID tag for the signals having a plurality of different frequencies transmitted using the scan antenna. RFID tag distance measurement system. The scan antenna control means switches to another scan pattern when the receiving unit cannot receive a signal from the RFID tag for a part of the plurality of frequencies by one scan pattern, and the plurality of frequencies The RFID tag distance measurement system according to claim 7, wherein the signal is transmitted again to the RFID tag. The scan antenna control means switches to another scan pattern when the receiver cannot receive a signal from the RFID tag for a part of the signals of the plurality of frequencies by one scan pattern, and receives the signal The RFID tag distance measurement system according to claim 7, wherein a signal of a part of the frequencies that could not be transmitted is transmitted again to the RFID tag. The RFID tag distance measuring system according to claim 1 or 2, wherein the diversity means includes polarization output means for outputting a plurality of different polarized waves. The polarization output means includes polarization output control means for controlling the reception unit to receive a reflected wave from the RFID tag with respect to the signals having a plurality of different frequencies transmitted from the transmission unit, The RFID tag distance measuring system according to claim 10. The polarization output control means, when the reception unit is unable to receive a signal from the RFID tag for a part of the signals of the plurality of frequencies by the transmission unit, switch to the polarization of another pattern, The RFID tag distance measurement system according to claim 11, wherein the plurality of frequency signals are transmitted again to the RFID tag. A reader that outputs a predetermined carrier signal to the RFID tag and receives a reflected signal of the predetermined carrier signal from the RFID tag,
As the carrier signal, signal output means for outputting signals having different frequencies from each other;
A transmitter for transmitting the signal output from the signal output means to the RFID tag;
A receiving unit that receives reflected signals from the RFID tag for the plurality of different frequencies;
An estimation unit that estimates a distance between the RFID tag and the reader based on a phase change amount between the reflected signal and the carrier signal received by the receiver and the frequency of the carrier signal;
The transmitting unit and the receiving unit are readers including diversity means for obtaining a plurality of received signals having different ways of changing the intensity of the reflected signal when the receiving unit receives the reflected signal. The reader according to claim 13, wherein the frequencies of the plurality of different signals are equally spaced.