Communications: electrical – Selective – Interrogation response
Reexamination Certificate
1999-01-12
2001-11-13
Holloway, III, Edwin C. (Department: 2735)
Communications: electrical
Selective
Interrogation response
C340S010300, C340S572400
Reexamination Certificate
active
06317027
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of radio-frequency identification systems (RFID) and in particular concerns improvements in RFID proximity readers.
2. State of the Prior Art
Radio-frequency identification (RFID) systems include passive systems in which the ID tag circuits are powered by energy radiated by the reader, and active tags which carry a battery. Passive ID systems tend to have a shorter operating range because actuation of the ID tags requires a sufficient RF field strength from the reader. Typical passive proximity readers have a relatively short range of operation, about two feet or less between reader and ID tag. Operating range is often an important criterion in the selection of a proximity system, and it is generally desirable to detect ID tags at somewhat greater range, such as five feet away, for greater flexibility in positioning of the reader unit or to cover a wider area with a single reader.
The reader in a passive ID system has an RF frequency generator and a tuned antenna circuit which establish a radio frequency field near the reader unit. The passive ID tags lack a frequency generator and instead communicate with the reader by loading down the reader's radiated field in a pattern which is detected by the reader and decoded as ID tag data. The ID tag includes a transponder integrated circuit which is powered by energy derived from the reader's RF field. The energy required by the tag transponder is received by means of a tag antenna circuit tuned to peak resonance at the reader's transmission frequency. The effective operating range of the reader/ID tag system is determined in part by the efficiency with which the RF energy is radiated by the reader and received by the tag. This calls for accurate tuning of the resonant antenna circuits of both tag and reader. In practice, however, environmental factors and manufacturing tolerances result in a departure from this ideal.
In existing passive proximity identification systems both reader and identification tags are intended to operate on a single common radio frequency, typically 125 kHz. Metallic and dielectric materials in the vicinity of the reader, and changes in temperature and humidity may all affect the tuning of the antenna circuits in the reader, resulting in less than optimal radiation of the RF field and reduced power transfer from the reader to the tags. That is, for I.D. tags of given sensitivity, the tag must be brought closer to the reader before the tag transponder will be activated by the reader's weakened transmission. The tuned antenna circuit of the tags is similarly affected by environmental factors and also by manufacturing tolerances, both of which may degrade tag performance by shifting the peak resonance of the tag away from the reader's operating frequency. If the tag tuning is off frequency, the operating range of the proximity system is again reduced because of diminished tag sensitivity and reduced loading of the reader's RF field. Tag performance can be improved by using high precision components, but such precision is costly. Five percent tolerance parts are much cheaper than one percent components, and ten percent tolerance parts are cheaper still. Since ID tags are often used in large numbers, it is desirable to keep the unit cost of the tags as low as possible.
In order to accommodate off-frequency drift in both the proximity reader and the ID tags, current practice is to use low-Q antenna circuits in the reader. Low-Q resonant tank circuits have a broader frequency response but at the expense of lowered sensitivity at the center frequency of the antenna circuits. The broader response allows the reader to detect off-frequency tags but at a lower level of system performance, that is, with lower sensitivity so that the tags must be closer to the reader before being detected.
A continuing need exists for more effective proximity readers capable of detecting passive ID tags at greater range and in particular more reliably detecting off-frequency ID tags.
SUMMARY OF THE INVENTION
This invention addresses the aforementioned need by providing an improved proximity reader with auto-tuning capability for negating environmental influences on the tuning of the reader antenna circuit, and frequency scanning capability for improved detection of off-frequency ID tags. The improved reader may use high-Q antenna circuits for greater sensitivity without excluding off-frequency tags. A relatively wide range of frequencies can be scanned at higher sensitivity over the scanned range than was possible with broadband low-Q antenna circuits, resulting in markedly improved overall system performance.
According to this invention a proximity reader for a radio frequency identification system has a frequency generator for generating a center frequency and a number of side frequencies, an antenna circuit connected for radiating the center frequency and side frequencies to interrogate transponder tags, a microprocessor connected to the antenna circuit for receiving and decoding transponder tag information, an antenna resonance tuner circuit including a selectable tuning impedance in the antenna circuit, and a tuner program operative for selecting values of the tuning impedance to substantially optimize the reactance of the antenna circuit at each of the center and side frequencies. The selected values are stored in memory for reference by a frequency scanning program. The selectable impedance may include a number of binarily weighted capacitances addressable by the microprocessor, and the impedance values may be stored as a table of addresses accessible by the microprocessor under control of the scanner program for switching the selected capacitances into the antenna circuit. Preferably the tuner program is executed upon initial power-up of the reader and periodically thereafter to regularly sense and compensate for any changes in peak resonance of the antenna circuit due to environmental factors.
The frequency scanning program is operative for executing a frequency scanning sequence including sequentially switching the frequency generator through each of the side and center frequencies thereby to improve interrogation range of off-frequency transponder tags. The side frequencies are selected on either side of the center frequency over a range of frequencies sufficient to include off-frequency ID tags targeted by the proximity identification system. The scanning program is further operative for interrupting the frequency scanning sequence upon detection by the microprocessor of a response from a transponder tag and for resuming the scanning sequence upon completion of the response.
In one form of the invention the frequency generator is a frequency synthesizer based on the crystal controlled clock internal to the microprocessor and including a clock frequency divider controlled by the tuner program means for setting the desired center and side frequencies. In this embodiment the tuner program and scanning program set the desired frequencies of operation by control of frequency synthesis by the microprocessor. Impedance of the antenna circuit is adjusted so as to obtain a resonant peak at each desired frequency of operation, and each desired frequency is stored in memory in association with the antenna impedance value required to achieve peak resonance.
In another form of the invention the antenna resonance tuning circuit comprises a peak detector circuit operative for modifying an output frequency of the frequency generator to obtain a peak resonance of the antenna circuit, and the tuner program is operative for selecting a value of the selectable impedance effective for bringing the output frequency to a desired frequency such that the reactance of the antenna circuit is optimized at the desired frequency. In this embodiment, the microprocessor and tuner program are operative for measuring the output frequency and for storing a value of impedance associated with the desired frequency. The desired frequency may inc
Beehler & Pavitt
Epstein Natan
Holloway III Edwin C.
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