WO1989007772A1

WO1989007772A1 - Device for the identification of objects

Info

Publication number: WO1989007772A1
Application number: PCT/SE1989/000051
Authority: WO
Inventors: Kurt Olof Edvardsson
Original assignee: Sweidco Ab
Priority date: 1988-02-10
Filing date: 1989-02-09
Publication date: 1989-08-24
Also published as: DE68926619D1; SE8800426D0; SE8800426L; EP0408573A1; SE501335C2; DE68926619T2; EP0408573B1

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

Device for the identification of objects

Background

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 en- tirely 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 con- figuration or construction. 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 emission of radio waves means that the radiated power must be kept low.

US 4 023 167 (Wahlstrom 1977) teaches the use of pulse

HF-power at varying frequency, for the purpose of reading a printed circuit board which incorporates resonators. As a result of this pulsation, the transmitter is switched off during the receiving interval. This pulsation, how- ever, also engenders large transients. An older patent related to printed circuits is US 3 671 721 (Hunn 1972), although this patent is less detailed with respect to signal processing. None of these two publications dis- cusses the difficulty of combining useful ranges for a broadband system with low radiation emission, and neither do the publications propose methods for increasing the pack density in a manner to enable large quantities of

5 information of practical interest to be accommodated

9 (>10 ) . US 4 209 783 (Ohyama et al, 1980) teaches the use of signal adapted filters which enable passive re¬ sonators to be packed closer together, although this patent relates to crystal resonators and no frequency

•JO modulation is applied. This latter feature, on the other hand, is employed in accordance with US 4 356 477 (Vande- bult 1982) to improve detection reliability, but since non-linear detection (amplitud or phase detection) is employed and since no filtering occurs between the de-

15 tectors. the resultant sensitivity is problably poor. US 4 251 808 (Lichtblau 1981) teaches the use of a counter- -connected coil, but this coil is intended primarily for mutually isolating the transmitter and receiver coils and for suppressing external disturbances.

20

Principle

The label includes a plurality of resonators and is in¬ fluenced by a magnetic field whose frequency is scanned continuously while simultaneously modulating frequency

25 and/or amplitude. This modulation spreads the power over substantially the same frequency band as that corre¬ sponding 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

30 responce can be caught by a receiver, via a receiver coil. Because the resonator has a narrow band, the various side¬ bands 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

35 disturbances. The scan causes each resonator to engender a pulse and the label as a whole will generate a 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 fre¬ quencies 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 simultan¬ eously being modulated sinusoidally with frequency modula¬ tion. 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 scan¬ ning 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 and its Q-value is Q. this can be ex¬ pressed such as to take at least Q/4 periods to change the carrier frequency f /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 re¬ ceiver 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 com¬ position 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 har- monies 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 leak¬ age 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 incorpo¬ rates 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 faci¬ litate 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 0-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 con¬ ceivable 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 illustra¬ tion, but will include, inter alia, a sensitivity adapta- tion for the purpose of enabling measurement at different distances. It is also suitable to limit the number of com¬ binations 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 resona¬ tors 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 trans¬ mitter coil has two or more counter-directed parts, so as to enable relatively large current to be used while keep¬ ing 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 necess¬ ary to eliminate this dependency, however. Greater in¬ dependency 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 band¬ width, so as to avoid generating disturbances. In accord¬ ance with the invention, the label resonators will have the highest Q-value with good margins.

Claims

1. Apparatus for identifying objects, comprising a trans¬ mitter which is intended to transmit, via a transmitter antenna, a high frequency magnetic field whose frequency is continuously scanned over a given bandwidth, while simultaneously modulating the phase or frequency and/or its amplitude to a narrow band, said object being provided with a label comprising a multiple of passive HF-resona- tors having resonance frequencies selected from a group of known frequencies, said label to be identified being located relatively close to said transmitter antenna, and further including a receiver, provided with a receiver antenna and detector means constructed to receive and to detect a signal produced by said HF-resonators, character¬ ized in that the receiver is constructed to mix the in- coming receiver signal with a mixing signal, such as a local oscillator signal, in a first linear mixer, and thereupon to filter the resultant signal in a first filter for the purpose of enhancing the desired signal components and suppressing disturbances, and in which the thus ob- tained signal is mixed in a second linear mixer with one or more multiples of the modulation frequency, and in which a second filter filters the thus obtained signal for the purpose of enhancing essentially pulse-like signal components.

2. Apparatus according to claim 1. characterized in that said modulation is a sinusoidal frequency modulation.

3. Apparatus according to claim 1, characterized in that said modulation is a triangular frequency modulation.

4. Apparatus according to claim 1, characterized by using a combination of frequency and amplitude modulation, such that only two or more carrier wave sidebands are present.

5. Apparatus according to any one of claims 1-4, charac¬ terized 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 detected separately with improved reliability against disturbancies and other imperfections.

6. Apparatus according to claim 1 in combination with any one of claims 2-5, characterized in that the frequency scan is controlled exponentially (a fixed number % per unit of time) and in that the bandwidth of the modulation is controlled in a corresponding manner.

7. 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 reso¬ nators will produce nominally the same amplitude, said apparatus including a test resonator for periodic and automatic calibration.

8. Apparatus according to claim 1 in combination with any one of claims 2-7. characterized in that at least one of the label resonators 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.

9. Apparatus according to claim 1 in combination with any one of claims 2-8, characterized in that one of the cali¬ brating resonators is much greater than the remainder and is used to initiate the sequence.

10. Apparatus according to claim 1 in combination with any one of claims 2-9. characterized in that the number of resonators is the same on all labels, so as to reduce the risk of reading errors and to adjust amplification such as to include all of the resonators.

11. Apparatus according to claim 1 in combination with any one of claims 2-10, characterized in that the transmitter antenna is counter-connected and in that current is re- stricted to comply with permitted radiation levels.

12. Apparatus according to claim 1 in combination with any one of claims 2-11, characterized in that two transmitter antennas are rotated through 90° in relation to one an- other and are also supplied 90° out of phase, whereby the field becomes more directional independent and tolerant to different directional positions of the label.