Patent ID: 7714773

Claim:
An RFID tag distance measuring system comprising: an RFID tag; a reader configured to output a predetermined carrier signal to an RFID tag and receive a reflected signal of the carrier signal from the RFID tag, wherein the system estimates the distance between the RFID tag and the reader, the reader comprising: a signal output unit for outputting signals at multiple different frequencies as the carrier signals; a transmitting section that transmits the carrier signals output by the signal output unit; a scan antenna section for transmitting the carrier signals, received from the transmission section, in multiple directions to the RFID tag, the carrier signals having a different electric field strength in each direction, and for receiving reflected signals from the RFID tag, the reflected signals having multiple different frequencies and strengths different from the outputted carrier signals and where the scan antenna section includes at least a plurality of antenna elements, and a divider/combiner for dividing the carrier signals for transmission to the antenna elements and combining the reflected signals received by the antenna elements, and the scan antenna section changes a directivity for transmission and reception of the signals supplied to the antenna elements and the reflected signals by controlling phase differences among the signals to be supplied to the antenna elements by using a phase shifter and a scan control section; a receiving section that receives the reflected signals from the scan antenna section, the reflected signals having multiple different frequencies and strengths different from the outputted carrier signals; and an estimating unit for estimating the distance between the RFID tag and the reader based on the amount of change in phase between the reflected signals received by the receiving section and the transmitted carrier signals and the frequencies of the transmitted carrier signals, wherein the scan control section controls the reception of reflected waves from the RFID tag of the signals at the multiple different frequencies, which are transmitted through the scan antenna, the scan control section switches from one scan pattern to another scan pattern if the reader does not receive signals from the RFID tag at a first frequency of the multiple different frequencies to the RFID tag, and the scan antenna control section switches from one scan pattern to another scan pattern if the reader does not receive signals from the RFID tag at a first frequency of the multiple different frequencies by using one scan pattern and retransmits signals at a third frequency of the multiple different frequencies to the RFID tag.