Research Category: Bulk Material Handling
Researchers: Ohara, Hideyuki|
Location: Kyoto, Japan
The present invention relates to a distance measuring apparatus, a distance measuring method, a reflector, and a communication system, which perform wireless communication with a reflector via radio waves.
Recently, RFID (Radio Frequency Identification) tags (wireless tags) have been widely used. The RFID tags are used as a substitute for barcodes. The RFID tags are gathering popularity particularly in the field of distribution and are expected to spread widely in the near future.
Presently, as a frequency band exclusively for the RFID tags purpose, 13.56 MHz band, a so-called UHF band that is somewhere between 800 MHz and 950 MHz, and 2.45 GHz band are used. Among these bands, radio waves using the UHF band or the 2.45 GHz band are advantageous over radio waves using the 13.56 MHz in that it is easy to increase a communication range. In addition, the UHF band radio waves are advantageous over the 2.4 GHz band radio waves in that it is easy to go behind the object. For this reason, RFID tags and reader/writers using the UHF band radio waves are being developed.
In the case of using the UHF band radio waves, it is possible to increase the communicable distance between the reader/writer and a wireless tag from several ten centimeters to several meters, compared to the case of using the 13.55 MHz band radio waves which are widely used at present. Accordingly, by utilizing the UHF band radio waves, it becomes possible to relatively greatly increase a communication area which is a spatial area in which the reader/writer can communicate with the wireless tags.
Meanwhile, in order to estimate the location of the RFID tags, there has been suggested a technology for measuring the distance between the RFID tags and a communication station in communication with the RFID tags. As an example, a technology is known in which a plurality of base stations receive signals from an active IC tag as the RFID tag, and the distance to and location of the active IC tag are estimated on the basis of the reception signals from the active IC tag. In the example, the distance between each of the base stations and the active IC tag is estimated on the basis of the signal intensity of the reception signals from the active IC tag. That is, the distance estimation is performed by utilizing correlation between the reception signal intensity and the distance. Also is known a method in which by preparing access points of which the location is known, signals are received simultaneously from the active IC tag and the access points, and the delay amount between receiving timings for the respective signals, thereby estimating the distance to the active IC tag.
As shown in FIG. 26, Japanese Patent Publication No. 2004-507714T (published on Mar. 11, 2004; hereinafter will be referred to as Patent Document 1) discloses an RF communication system in which an interrogator 36 as a reader transmits signals 40 and 42 having different frequencies to an RF tag 38, and the number of null points in combined waves that are obtained by superimposing the two signals onto each other is counted, thereby estimating the distance between the interrogator 36 and the RF tag 38.
DISCLOSURE OF THE INVENTION
Problem to be Solved by the Invention
Although the RFID communication system using the UHF band radio waves enables a long distance communication, it becomes possible to uselessly communicate with an RFID tag that is located far away and does not need to be communicated with. Thus, there are problems in that useless processings are performed to the RFID tag, and the response performance may be deteriorated by being influenced by multiple paths. In order to solve such problems, a method can be conceived in which by allowing the reader/writer to calculate the distance to each RFID tag, the RFID tags that should be subjected to processes such as analysis or rewriting of the data section are chosen. The prior method of calculating the distance between the reader/writer and the RFID tag has the following problems.
First, the estimation of the distance based on the signal intensity of the reception signals from the RFID tag or the delay time results in a low measurement precision. The precision obtainable from the above distance estimation is actually about in the range of 1 meter to several meters. For example, when applied to a distribution management system, such precision is not practical, but a higher distance measurement precision is required.
In the technology disclosed in Patent Document 1, since the distance is estimated by counting the number of null points in the combined waves obtained by superimposing the two signal having different frequencies, the distance estimated has very poor precision. More specifically, according to the example described in Paragraph  of Patent Document 1, when a first signal of 880 MHz and a second signal of 884 MHz were used, a first null point was found at a position located about 37.5 meters from the interrogator 36 and additional null points were found every 75 meters from the first null point. For example, if three null points were found between the interrogator 36 and the RF tag 38, the estimated distance between the interrogator 36 and the RF tag 38 is in the range of 187.5 meter to 262.5 meter. Therefore, 75 meter is an error range. In the case of occurrence of multiple paths, radio signals may propagate to the longer distance, and thus the calculate distance may become different from the actual distance to the RFID tag.
As shown in FIG. 27, EP1239634A2 (hereinafter will be referred to as Patent Document 2) discloses a communication apparatus 138 that wirelessly communicates with an RFID tag. Reflection signals from the RFID tag are received through an antenna 140, reception signals inputted from a circulator 92 and carrier signals inputted from a circulator 90 and a splitter 98 are multiplied by mixers 100 and 102 to generate an I signal and an Q signal, and the amplitude and phase of the reflection signals from the RFID tag are calculated on the basis of the I and Q signals. Also is disclosed a method of calculating the distance to the RFID tag by using the phase difference between the carrier signals and the reflection signals from the RFID tag.
The present invention has been made in view of such problems, and an object of the invention is to provide a distance measuring apparatus, a distance measuring method, and a communication system capable of enabling to measure the distance to RFID tags with high precision. Another object of the invention is to provide a distance measuring apparatus, a distance measuring method, a reflector and a communication system capable of enabling to measure the distance to tags without using special RFID tags.
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