EP0408573B1

EP0408573B1 - Device for the identification of objects

Info

Publication number: EP0408573B1
Application number: EP89902555A
Authority: EP
Inventors: Kurt Olof Edvardsson
Original assignee: REKONDO TEKNIK AB
Current assignee: REKONDO TEKNIK AB
Priority date: 1988-02-10
Filing date: 1989-02-09
Publication date: 1996-06-05
Anticipated expiration: 2009-02-09
Also published as: WO1989007772A1; DE68926619T2; SE8800426L; EP0408573A1; SE8800426D0; DE68926619D1; SE501335C2

Abstract

This invention relates to a method and apparatus for contactless identification of a label which incorporates passive resonators, with the aid of high-frequency magnetic fields. One significant feature of the inventive method is that large quantities of information of practical interest can be accommodated, despite the fact that the resonators can be given a Q-value which is so low as to enable the resonators to be given the form, e.g., of printed circuits mounted on plastic foil. The frequency stability requirement of the resonators is also low, which enables the labels to be produced very cheaply with the aid of suitable manufacturing techniques herefor, and also enables labels to be produced for one-time use only. The method is based on a high-frequency signal which is scanned over a broad frequency band and modulated at the same time, with appropriate detection, the modulation results in a response of narrower bandwidths than that obtained with earlier known methods, therewith enabling more resonators to be accommodated and to be separated within a given frequency range. The inventive apparatus with which the method is put into effect includes, inter alia, means for controlling the scan, modulation and amplification over the frequency band. One significant feature of the inventive apparatus resides in the provision of a coil arrangement which affords a comparatively large range while, at the same time, satisfying government regulations concerning maximum radiation powers.

Description

Although a wealth of contact-less identifying systems are known to the art, the following discussion will be limited to such assistance as those with which the label is entirely passive and incorporates no semiconductors or other surface mounted components. A principal advantage with such labels is that they can be made highly durable with regard to environmental influences, that they can be made cheaply, and that they obtain a smooth, mechanical configuration or contruction. For instance, the label can be given a sheet form and handled in a manner of a paper sheet (e.g. in a type writer) and may be used as a one-time use article. With appropriate selection of material, the labels may be used in extreme environments (e.g. in stoving furnaces) and conceivably may also be hidden in packages or the like. A principal problem with passive resonators, however, is that the information content is limited in a completely different manner to, e.g., an antenna-connected programmable memory. Another principal limitation of such broadband systems as this is that the range is restricted because government regulations on limited emssion of radio waves means that the radiated power must be kept low.
This invention regards apparatus for identification of objects provided with a multiple of passive HF resonators, of the type recited in the preamble of claim 1.
Apparatus of this kind is known from the patent document US-A-4 356 477. This document discloses such apparatus with a transmitter, comprising means for generating a modulated signal, wherein the modulating frequency is kept constant, the carrier frequency is scanned or swept over a frequency interval and the resonators are tuned to frequencies within the carrier sweep interval. The receiver is provided with an FM or an AM detector in which the received signal is mixed with a signal dependent on the sweep frequency and possibly with a signal dependent on the transmitted signal.
Another apparatus of this kind is disclosed in the patent document DE-C2-26 58 669. This apparatus uses a constant carrier frequency and a stepped change of the modulation frequency, which is further pulsed by being sent as damped oscillations, each directed to finding a resonator having a particular frequency.
A general problem in this art is to obtain dependable detection, one reason being that there are limits to the emission power, set by governmental regulations.
It is a general object of the invention to improve the detection and identification of labeled objects comprising passive radiation resonators, and a more specific object to be able to fit in and distinguish between a large number of resonators within a defined frequency interval. According to the invention, this is obtained by the features of the characterizing part of claim 1.
The label includes a plurality of resonators and is influenced by a magnetic field whose frequency is scanned continuously while simultaneously modulating frequency and/or amplitude. This modulation spreads the power over substantially the same frequency band as that corresponding to the bandwidth of a resonator. During those time intervals when the carrier wave frequency is close to the resonance frequency of one of the resonators, a response can be caught by a receiver, via a receiver coil. Because the resonator has narrow band, the various sidebands of the modulation will be influenced differently and signal processing by the receiver will induce a response from a resonator compared with internal and external disturbances. The scan causes each resonator to engender a pulse and the label as a whole will generate train of pulses during the scan period. Scan and modulation are controlled in a manner to "normalize" the pulse train and mutually different labels can be separated, one from the other, by different patterns taken by the pulse train. The method of applying a continuous scan renders the system tolerant to uniform displacement of the resonance frequencies of the resonators, e.g. displacements or shifts caused by variations in the properties of the material concerned. Because the system is a broadband system (> 1 octave), the configuration of the transmitter coil is important in achieving a sufficient range, despite the restrictions placed on current strength in the transmitter coil by government permitted radiation or emission levels. Various arrangements of contracting loops are employed for the purpose of providing strong magnetic field in relation to the radiation field.

Description

The signal processing principle is shown by the example in Figure 1, which illustrates a high frequency transmitter (e.g. 5-30 MHz) whose frequency is scanned while simultaneously being modulated sinusoidally with frequency modulation. The breadth of the modulation (frequency swing) is of the same order of magnitude as the bandwidth of a resonator, which may be 1%. The transmitter coil will create a magnetic field in its close proximity, but because of its two counter-directed halves, according to the Figure 1 embodiment, the radiated power will be relatively weak.
The label is excited by the magnetic field and the scanning speed is so low as to enable a resonator to begin to oscillate while the transmitter frequency is still close to the resonator frequency. When the frequency of the resonator is f_r and its Q-value is Q, this can be expressed such as to take at least Q/4 periods to change the carrier frequency f_r/Q. For instance, the modulation frequency can be selected so that a number of frequency periods are able to pass during the same periode. A receiver coil catches a response from an eventual resonator in the immediate proximity, and the response is mixed with a small part of the transmitted signal. Naturally, it is also possible to mix the response with a modified composition of the transmitted signal. Different leakage signals between transmitter and receiver will mean that quite some signal will be obtained from the mixer, even without a resonator. Among other things, there is present a slowly varying "direct current", and a signal which varies in keeping with the modulation frequency. However, if a resonator is present in the vicinity, higher harmonics can also be detected and both the second and the third harmonics are typical of a resonator when pure sinus modulation is employed. The harmonic or harmonics to be used are filtered out and amplified, and the fundamental harmonic of the modulation is preferably eliminated with the aid of a suppression filter in order to avoid over-modulation. Subsequent to passing through a coherent detector (ring modulator) the signal close to the desired harmonic can be filtered out and amplified. Certain leakage signals still remain in a form of a relatively slowly varying signal over the scan. It is known on the basis of the scanning speed and the Q-value of the resonators just how long the response pulse should be, and consequently a sufficiently wide bandpass filter will markedly enhance the response of the resonators. In order to improve detection reliability, several harmonics may be processed in parallel, and it is also possible to use a non-sinusoidal modulation.
Figure 2 is a block schematic which illustrates a more practical apparatus, in which, inter alia, the scan is effected proportionally (a given number of % per unit of time) so that all of the resonators will give responses of mutually equal time lengths, even when the scan incorporates a plurality of octaves. The illustrated apparatus includes a feedback which will ensure that the scan is accurately defined, in order to facilitate evaluation. Both modulation width and amplification are controlled during the scan, thereby enabling the pulse train to be normalized subsequent to running a test label, which is measured now and again. The modulation frequency is, on the other hand, constant in Figure 2, in order to facilitate filtration of the signals. An analysis of the responses obtained from the label illustrates that the response duration is about 2-3 times shorter when viewing the harmonics compared with the duration of the complete response. Consequently, the resonator frequencies can be packed more densely, to a corresponding degree, which is significant when resonators having a relatively low Q-value are used. A Q-value in the range of 50-100 is reasonable for a printed resonator in the high frequency range. Various filtering and modulating methods are conceivable for the purpose of optimizing band widths. Signal processing for converting the analogue pulse train to a binary pulse train is omitted in the Figure 2 illustration, but will include, inter alia, a sensitivity adaptation for the purpose of enabling measurement at different distances. It is also suitable to limit the number of combinations used in order to incorporate an error detection facility.
Figure 3 illustrates a further modulating method in which a combination of frequency and amplitude modulation is employed to restrict the power transmitted to three frequencies at a time (carrier frequency plus/minus modulation frequency) in order to minimize the bandwidth used and also to reduce disturbance from adjacent resonators during a reading period. In this case, the modulation frequency varies in proportion to the carrier frequency, so as to optimize the signal in relation to the bandwidth of the resonators. Figure 4 illustrates this function with the aid of a display diagram in which second harmonics are emphasized for narrow band resonators.
The term "frequency modulation" used here and in the aforegoing is meant also to include phase modulation, which in respect of signals are the same. Both the practical construction and the choice of parameters can vary, however.
The range is dependent on a number of parameters, of which some are independent of the signal processing process. However, in the described method there is used a narrow bandwidth (for instance compared with pulsated systems) resulting in the suppression of both external and internal disturbances. It is also important to the function that the resonators alone have a high Q-value (or have a narrow bandwidth on the high frequency side), so that both the transmitter coil and the receiver coil must be given a broad bandwidth and be free from parasite resonances to the greatest possible extent, both within the transmitter frequency band and the possibly occurring harmonics of the transmitter frequency. In the case of systems having a small number of resonator frequencies (type theft alarm), there is often used a tuned receiver coil in order to improve the sensitivity of the system, although such provision is normally not possible in the present case. Figures 5-6 illustrate two conceivable coil arrangements with built-in transmitter and receiver coil. The transmitter coil has two or more counter-directed parts, so as to enable relatively large current to be used while keeping radiation down. The magnetic field close to the coil will then cause acceptable excitation of the label. The size of the transmitter coil is not critical, but the receiver coil should be approximately the same dimension as the desired range. The orientation or positioning of the label is also of significance, in addition to the range. In many applications, the labels will "travel" in mutually the same manner, but it may sometimes be necessary to eliminate this dependency, however. Greater independency can be obtained by employing two perpendicular fields with a 90° phase offset, without needing to measure consecutively in said two directions. Since the geometry of the coils is dependent on use, this geometry will vary with different applications. One requirement of important practical significance is that the coils have a wide bandwidth, so as to avoid generating disturbances. In accordance with the invention, the label resonators will have the highest Q-value with good margins.

Claims

Apparatus for identifying objects, each such object being provided with a label (2) comprising a multiple of passive HF-resonators (4) having resonance frequencies selected from a group of known frequencies; the apparatus comprising a transmitter (8) for transmitting, via a transmitter antenna (6), a high frequency magnetic field whose frequency is continuously scanned over a given bandwidth by means of scan control signal generating means (22) while by means of modulating control signal generating means (24) simultaneously modulating the phase or frequency and/or its amplitude with a breadth of modulation of an order of magnitude equal to that of the bandwith of said HF-resonators (4), said label (2) to be identified, being detectable when it is located relatively close to said transmitter antenna (6); the apparatus further comprising a receiver (10) provided with a receiver antenna (12) and detector means devised to receive and to detect a signal generated by said HF-resonators (4), the detector means comprising a first detector (14), coupled to the receiver antenna (6) and devised to mix the incoming receiver signal with a mixing signal composed of the transmitted signal, a second detector (16), devised to mix a signal dependent on the signal obtained from the first detector (14) with one or more harmonics of the modulation frequency, and a first filter (20) for filtering the signal obtained from the second detector (14) for the purpose of enhancing essentially pulse-like signal components, characterized in that the first detector (14) is a linear mixer; an output signal of the first detector (14) is filtered in a second filter (18) for the purpose of enhancing harmonic signal components and suppressing disturbances, an output of the second filter (18) being coupled to the second detector (16); and in that the second detector is a linear mixer.
Apparatus according to claim 1, characterized in that said modulation control signal generating means (24) is devised to generate a sinusoidal modulation signal.
Apparatus according to claim 1, characterized in that said modulation control generating means is devised to generate a triangular modulation signal.
Apparatus according to claim 1, characterized in that the modulation control signal generating means (24) is devised to generate a combination of frequency and amplitude modulation signals, achieving a modulation such that only two or more carrier wave sidebands are present.
Apparatus according to any one of claims 1-4, characterized in that more than one channel is filtered out downstream of the first mixer; and in that said channels are detected a second time each with a respective harmonic of the modulation frequency, or each with a respective combination of harmonics of said modulation frequency, said channels then being reliably detected separately against disturbancies and other imperfections.
Apparatus according to claim 1 in combination with any one of claims 2-5, characterized in that the frequency scan is controlled proportionally with a given percentage of frequency per unit of time.
Apparatus according to claim 1 in combination with any one of claims 2-6, characterized in that amplification is controlled during the frequency scan, such that all resonators will produce nominally the same amplitude, said apparatus including a test resonator for periodic and automatic calibration.
Apparatus according to claim 1 in combination with any one of claims 2-7, characterized in that at least one of the label resonators (4) has a fixed nominal frequency and is used for calibration purposes, such as to enable a uniform change in resonator frequency to be compensated for, the highest and the lowest frequency being preferably used.
Apparatus according to claim 1 in combination with any one of claims 2-8, characterized in that one of the calibrating resonators is much greater than the remainder and is used to initiate the sequence.
Apparatus according to claim 1 in combination with any one of claims 2-9, characterized in that the number of resonators (4) is the same on all labels (2), so as to reduce the risk of reading errors and to adjust amplification such as to include all of the resonators (4).
Apparatus according to claim 1 in combination with any one of claims 2-10, characterized in that the transmitter antenna (6) is counter-connected and in that current is restricted to comply with permitted radiation levels.
Apparatus according to claim 1 in combination with any one of claims 2-11, characterized in that two transmitter antennas (6) are rotated through 90° in relation to one another and are also supplied 90° out of phase, whereby the field becomes more directional independent and tolerant to different directional positions of the label.