Communications: electrical – Condition responsive indicating system – Specific condition
Reexamination Certificate
2002-02-25
2003-10-28
Wu, Daniel J. (Department: 2632)
Communications: electrical
Condition responsive indicating system
Specific condition
C340S572100, C340S010100, C235S385000
Reexamination Certificate
active
06639514
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for reading out and writing to RFID-transponders with an inductive coupling, using a read/write unit, wherein the transponders operate at a resonance frequency. The invention further relates to a system for reading out and writing to RFID-transponders, wherein the system comprises a transponder, which is disposed on a carrier and has a resonant circuit with a resonance frequency, a receiving unit and a read/write unit with a transmitter/receiver.
RFID-systems are becoming increasingly more frequently used for means of contact-free, automatic identification purposes. Approx. 90% of all sold RFID-systems are nowadays inductively-coupled systems with an inductive coupling between the reading device and the transponder. Such systems, so-called remote-coupling systems generally function in the ranges of up to 1m in the read/write operation.
The transmission frequencies used are frequencies below 135 kHz or the frequencies 6.78 MHz, 13.56 MHz and 27.125 MHz, i.e. the ISM frequency ranges which are held free especially for industrial, scientific or medical purposes. Depending upon the frequencies used, differences occur in the data transmission rates, clock frequencies, output, etc.
It is possible nowadays to use RFID-labels to identify goods and other objects as these labels can now be produced practically as thin as conventional adhesive labels and thus are not generally recognised by the user as being RFID-labels. For example, RFID-labels can be adhered or laminated to books, periodicals or similar documents.
RFID-systems which function in the radio frequency range (3 MHz to 30 MHz) operate with LC-resonant circuits at a resonance frequency f
R
. If a magnetic alternating field acts with a frequency f
s
on the transponder of an RFID-label, then the resonant circuit of the transponder starts to respond and is excited to resonance oscillation. In so doing it takes energy from the magnetic alternating field, which for example can be acquired by increasing the coil current or the voltage drop at the internal resistor in the transmitter circuit. In this manner the operating voltage for the transponder chip can also be produced.
In the case of EAS-systems, i.e. electronic article security, a warble frequency is used. The transmitter frequency continuously scans a frequency range. This can be recognised by the energy absorption which occurs at an unknown resonance frequency of a transponder. DE 195 14 601 A1 describes an EAS-system of this type which has a broadband pre-amplifier which for example in the case of two transponder types passes through their two frequency ranges one after the other.
The drop in voltage in the transmitter circuit as a consequence of the receiving transponder being excited to oscillate is exploited during load modulation, when by means of switching on and off the load resistor of the transponder voltage changes are caused at the antenna of the transmitter and thus amplitude modulation of the antenna voltage is effected.
If two RFID-labels are located in close proximity to each other, possibly stacked one on top of the other in a document file or adjacent to each other on a bookshelf, then these have a mutual effect on each other during reception, i.e. they receive approximately with identical strength an in-phase signal from the transmitter and coupling effects occur. If they lie precisely one above the other, then there is practically a common coil, wherein the two capacitors are connected in parallel. Thus a frequency displacement occurs, i.e. a change in the resonance frequency. This leads to the relevant read device only being able to receive the data in a limited form or not being able to receive the data at all.
Tests have shown that the resonance frequency of an RFID-label always displaces downwards when a second RFID-label comes into the coupling range of the first. In the extreme case, a rigid coupling can occur, wherein the resonance frequency of the two RFID-labels then amounts to
f
R
/{square root over (2)}
Even when using high field strengths, i.e. high transmitter output, it is not always possible to communicate at the transmission frequency f
s
. Depending upon the coupling level and resonant frequency of the transponder circuits, zero settings can lie above the resonance frequency which has been displaced downwards. If such a zero setting occurs particularly where the transmission frequency f
s
is used, then the chip no longer functions.
This is illustrated with reference to the replacement circuit diagrams in FIG.
4
. There is located in the transmitter branch of the transmitter/receiver A an oscillator
2
with a frequency f
s
, whose output signal is transmitted, where appropriate after modulation, to an output end phase
4
. The receiving branch which commences immediately at the antenna bush is provided with a demodulator
6
and a band pass filter
8
or a different filter. The antenna is a coil
10
with inductivity L
s
. The replacement circuit diagrams of two RFID-labels a,b are also illustrated. These include in each case a coil
12
with inductivity L
1
and L
2
and a capacitance
14
parallel to the transponder chip
16
. The coupling ratios are illustrated by lines, wherein k
s
represents the coupling transmitter/receiver and the transponder and k represents the coupling between the two transponders. If there are differences in size (and thus coupling) or frequency of the RFID-labels, then the mentioned deletions occur.
FIGS. 5 and 6
illustrate simulations of this process of the undesired coupling between two adjacent RFID-labels with like and unlike resonance frequency.
In the first example (
FIG. 5
) the frequency responses for transponders with identical resonance frequency are illustrated, where there is no coupling (k=0) at 13.56 MHz and total coupling (k=1) at approx. 10 MHz. Thus, as the coupling increases the occurring resonance frequency drops to lower frequencies.
In the case of different resonance frequencies as shown by the second example illustrated in
FIG. 6
, there is in fact again no coupling (k=0) at 13.56 MHz and the coupling increases [sic] to lower frequencies, wherein again total coupling (k=1) occurs at approx. 10 MHz. However, a zero setting x has formed just above 13.56 MHz, so that under certain conditions the chip can fail.
It is known to use RFID-systems which operate in the microwave range to eliminate randomly occurring interference signals, where the transponders operate at several resonance frequencies. This is the case with an RFID-system described in U.S. Pat. No. 5,446,447 for reducing the read time.
Furthermore, RFID-systems with transmission frequencies in the microwave range have been used, where the transmission signal is modulated with a signal of e.g. 1 kHz and in addition to the resonance frequency the second harmonic of the transponder is also detected. Following demodulation and the passing through of a 1 kHz detector it is possible in a reliable manner to distinguish between the received transponder signals and interference signals and thus false alarms are avoided. However, disadvantages of these RFID-systems are the influences exerted by the multi-path and running-time effects.
SUMMARY OF THE INVENTION
The object of the invention is to provide a method which renders it possible to perform a reliable operation, in particular communication and antenna feeding, even if several transponders are located over a large spatial area.
Thus, in the case of the method in accordance with the invention for reading out and writing to RFID-transponders with inductive coupling using a read/write unit, the transponders operate at a fixed resonance frequency. The transmission frequency is reduced from a transmission basic frequency according to this resonance frequency for operating conditions at a high range to a fixed alternative value of the transmission frequency for operating conditions with a high recognition rate, so that a reliable communication between transponder and read/write uni
Dennison, Schultz & Dougherty
Lai Anne V.
Lucatron AG
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