EP0565481A1 - Label detection apparatus - Google Patents

Abstract

The apparatus for detecting labels which are used for protecting goods against theft and are provided with an electrical resonant circuit (21) having a resonant frequency (fR) in the MHz range, comprises a transmitter (1) and a receiver (2). The transmitter (1) generates a transmitted signal sequence, the frequency of which is swept in sweep cycles over the predetermined resonant frequency of the labels and is radiated via a transmitting antenna (11). The signal is generated digitally and with corresponding phase angle with respect to each sweep cycle in the transmitter (1). In the receiver (2), the signals received by means of a receiving antenna (12) are sampled synchronously with the signal generation in the transmitter (1) and digitised. From the digitised signal values of n sweep cycles, a background is formed and subtracted from the digitised signal values of the last sweep cycle in each case. A long-time and a short-time background can also be formed and the latter can be subtracted from the former. Due to the background subtraction, in particular, interference which is relatively constant with respect to time can be discriminated such as is caused mainly by diffraction and reflections of the transmitted signal on objects existing in the environment of the transmitting and receiving antenna. <IMAGE>

Description

Technical field

The present invention relates to a device for the detection of labels which are used for theft protection of goods and are provided with an electrical resonance circuit with a resonance frequency (f _R ) in the MHz range, the device comprising a transmitter and a receiver, of which the transmitter a transmission signal sequence is generated and emitted via a transmission antenna, the frequency of which is wobbled in wobble cycles over the predetermined resonance frequency of the labels and the presence of a label in the area between the transmitting and the receiving antenna is detected in the receiver by evaluating signals received via a receiving antenna becomes.

State of the art

Devices of this type are known in several versions and in use. Common to all devices is the problem of discriminating the relatively weak label signals from the interference background which is always present and avoiding false alarms. In particular, there are particular difficulties in the form of disturbances which are caused by diffractions and reflections of the high-frequency electromagnetic waves emitted by the transmitting antenna on all possible objects in the vicinity, such as walls or objects. Such disturbances can namely be very similar to the label signals sought, in which case they cannot be easily eliminated by filtering.

In the device known from US-A-4,531,117, the problem mentioned is taken into account by intermittent transmission and reception. It is only received and detected during pauses in transmission of the transmitter and after the transmission signal and the interference have decayed (echo method). This takes advantage of the fact that the label signals generally decay more slowly than the interference.

Presentation of the invention

The present invention takes a different approach to solving the same problem in which there is no need for intermittent transmission and reception. In the device according to the invention, as is characterized in claim 1, the transmission signal sequence in the transmitter is generated digitally and with a corresponding phase position with respect to each wobble cycle. After demodulation, the signals received by means of the receiving antenna are sampled and digitized synchronously in the receiver with the signal generation in the transmitter. A background is formed from the digitized signal values of n wobble cycles. This is subtracted from the digitized signal values of the last wobble cycle or from a foreground which is formed like the background, but only using the digitized signal values of the last m wobble cycles. The number n is considerably larger (preferably by one to two orders of magnitude) than the number m.

According to the invention, use is made of a background subtraction to eliminate the interference critical for label discrimination. A prerequisite for this, however, is that the disturbances critical for label discrimination are practically stationary in comparison to the label signals, i.e. always appear in the same way in a large number of successive wobble cycles. This is actually the case as far as the disturbances caused by the aforementioned diffractions and reflections are concerned, but only if exactly the same signal sequence with exactly the same phase position is emitted via the transmitting antenna in each wobble cycle. The label signals, on the other hand, are subject to a temporal change with regard to their occurrence, if only because the labels are inevitably moved when they pass through the antenna arrangement.

In order to be able to meet this requirement, the signal sequence emitted via the transmitting antenna is generated digitally in the transmitter. With digital signal generation, the very critical phase condition can be met relatively simply and with sufficient precision. For the same reason, the sampling in the receiver is carried out synchronously with the signal generation in the transmitter.

Advantageous and preferred refinements or developments of the present invention are characterized in the dependent claims.

The invention is explained in more detail below with reference to the accompanying drawings, for example.

Brief description of the drawings

• Fig. 1 schematisch eine Vorrichtung nach der Erfindung, und
• Fig. 2 unter a), b) und c) Zeitdiagramme zur Erläuterung der Funktion der Vorrichtung von Fig. 1.

It shows:

• Fig. 1 shows schematically a device according to the invention, and
• 2 under a), b) and c) timing diagrams to explain the function of the device of FIG. 1st

In Fig. 1 only the device components essential for understanding the inventive idea are shown. As far as in Fig. 1 it concerns multi-bit data connections (e.g. with 16 bits), these are drawn widely. In Fig. 2, the actually very high-frequency processes are shown at low frequency for graphic reasons.

Way of carrying out the invention

1 comprises a transmitter 1 and a receiver 2. Both the transmitter 1 and the receiver 2 are clocked by a common clock generator 3. The clock generator generates a clock signal sequence with a clock frequency of approximately 48 MHz. This is therefore several times higher than the resonance frequency of the resonance circuits on the labels to be detected, which is usually predetermined at about 8.2 MHz, or that of the signal sequence to be generated by the transmitter 1 for this purpose.

The clock signal is reduced in transmitter 1 in a frequency reducer 4 (e.g. by a factor of 9) and then fed to a modulo-m counter 5. At its data output, this supplies the numbers from 0 to m as input variables for a downstream function generator 6 in the sequence of the reduced clock rate. The wobble cycles are determined by the cycle of the modulo-m counter 5. Their frequency is usually chosen between 80 and 85 Hz.

The function generator 6 is preferably a PROM (with a storage capacity of e.g. 1 megabyte), in which a function table is stored. The stored function values correspond to the phase angle increment Δα for the signal sequence to be generated digitally in the transmitter 1. 2, the phase angle increment as a function of time t is shown schematically under a). Because of its discrete generation, the temporal course of the phase angle increment is step-shaped, the function value changing in the rhythm of the step-down clock. This is shown in FIG. 2 a) only on the left at the beginning of the time scale, whereas the stair curve is extended continuously at longer times to make the course of the function easier to recognize. The course of the function is sinusoidal around a phase angle increment value Au, and not, as is usually the case with wobbling, selected as a sawtooth. The sinusoidal function curve advantageously results in fewer harmonics.

In an accumulator 7, the phase angle increment values supplied by the function generator 6 are in the rhythm of those supplied directly by the clock generator, i.e. clock signal, which is not reduced, is accumulated by binary addition in order to form the values for the phase angle a therefrom. The phase angle values a are fed as input variables to a further function generator 8. The value is automatically limited by the predetermined number of bits of the accumulator 7. The further function generator delivers a fixed amplitude value of the signal sequence to be generated for each phase angle value. The amplitude values of the signal sequence to be generated in turn correspond to values of the sine function.

The digital-to-analog converter 9 is used to convert the signal values supplied by the further function generator 8 into an analog voltage signal. Its output signal is smoothed by means of the low-pass filter 10 before it is fed to the transmission antenna 11 as a transmission signal. 2 shows under b) on the left at the beginning of the time scale the discrete voltage values supplied by the digital-to-analog converter 9 and, extended over the entire time scale shown, the transmission signal curve resulting after the filtering by means of the low-pass filter 10. 2 b) clearly shows how the frequency of this curve increases or decreases over the wobble cycle shown in FIG. 2 a) (usually about +/- 10%).

In the receiver 2, the signal sequence received via the receiving antenna is first demodulated by combining it with the pure, i.e. undisturbed transmission signal is multiplied. For this purpose, the transmission signal must be transmitted from the transmitter 1 not only by air but also directly to the receiver 2 via a line connection. The demodulation in the mixer 13 using the pure, undisturbed transmission signal instead of the otherwise usually customary demodulation by multiplying the interference-affected reception signal by itself contributes significantly to the interference reduction.

The result of the demodulation results in a "DC voltage component" and a component with twice the transmission signal frequency. The latter is eliminated in the downstream low-pass filter 14 (cut-off frequency at approx. 7 kHz). The remaining "DC component" is shown schematically in Fig. 2 c). Approximately in the middle of the time scale, it contains a small “interference” labeled “S”, as is typically caused by a resonance circuit 21 located between the transmission antenna 11 and the reception antenna 12, but possibly also due to diffractions and reflections of the transmission signal. For further evaluation and discrimination, the "DC voltage component" is sampled (e.g. 128 times per wobble cycle) and digitized. In Fig. 1, only an analog-to-digital converter 15 is shown for performing these functions. For the purpose of synchronizing the scanning with the wobble cycles determined by the transmitter 1, the clock signal of the clock generator 3, but appropriately reduced in the frequency reducer 20, is fed to the latter.

A background is formed in the background generator 16 from the digitized signal values of the n (e.g. 800) wobble cycles preceding the respective last wobble cycle and subtracted in the difference generator 17 from the digitized signal values of the respective last wobble cycle. However, preferably m (e.g. 16) wobble cycles are combined to form a "foreground" and the background is then formed from n '(e.g. 50) such "foregrounds". The background is then correspondingly not only subtracted from the digitized signal values of the last wobble cycle but from the last "foreground" formed.

The background formation and subtraction serves to distinguish interference "S" in the "DC voltage component", which is caused in particular by reflections, diffractions and superimpositions of the transmitted signal and is practically permanently present, from interference "S" caused by a resonance circuit will.

So that the permanently present "disturbances" in the background subtraction can drop out, as already mentioned above, the conditions in each wobble cycle must correspond as closely as possible, in particular with regard to the phase of the generated signal sequence. In each sweep cycle, the sum taken over all phase angle increments must be an integer multiple of 2. At the points in time denoted by t _o in FIG. 2, there must be no phase jump. However, as already mentioned, this very strict "phase condition" can be very easily fulfilled by the digital transmission signal generation. The synchronization of the received signal sampling using the clock signal with which the signal generation in the transmitter is also clocked also serves to establish exactly the same conditions in successive wobble cycles.

It goes without saying that for background formation in the background former 16 storage means must be provided, in which the digitized signal values determined by scanning can be stored cycle by cycle. The signal values belonging to a wobble cycle each form a data record. The background formation is carried out by adding the signal values determined at the same sampling time within the wobble cycles used for background formation and then dividing the total values obtained by the number of wobble cycles used for background formation, i.e. be standardized to this number. The formation of the “foreground” mentioned is basically done in the same way. The background subtraction in the difference former 17 is also carried out separately with respect to the individual sampling times.

The data record that results after the background subtraction is finally fed to an evaluation unit 18, in which it is further evaluated. The evaluation unit can be of a known type and is therefore not further described here. In particular, on the basis of the data records supplied to it, it makes the decision about the presence of a resonance circuit in the area between the transmission antenna 11 and the reception antenna 12. On a line 19 it generates an alarm signal in the event that the presence of a resonance circuit is determined.

Claims (4)

1. Device for the detection of labels, which are used for the theft protection of goods and are provided with an electrical resonance circuit (21) with a resonance frequency (f _R ) in the MHz range, the device comprising a transmitter (1) and a receiver (2) The transmitter generates a transmission signal sequence and emits it via a transmission antenna (11), the frequency of which is wobbled in wobble cycles over the predetermined resonance frequency of the labels, and the presence in the receiver by evaluating signals received via a reception antenna (12) a label is detected in the area between the transmitting and receiving antennas, characterized in that the transmission signal sequence in the transmitter is generated digitally and with respect to each wobble cycle with a matching phase position, that the signals received by means of the receiving antenna in the receiver after demodulation with the signal generation in the transmitter synchronously scanned and digit alized that a background is formed from the digitized signal values of n wobble cycles and that this background is subtracted from the digitized signal values of the last wobble cycle or from a foreground, which like the background, but only using the digitized signal values of the last m Wobbeizkien is formed, where n is significantly larger than m. 2. Device according to claim 1, characterized in that the transmitter (1) and receiver (2) are clocked by the same clock generator (3) or by synchronized clock generators, the clock generator (3) generating a clock signal with a clock frequency around a Multiple, preferably about is a factor of 5-6 higher than the predetermined resonance frequency of the resonance circuits (21) on the labels. 3. Device according to claim 1 or 2, characterized in that the amplitude values of the transmission signal sequence in each wobble cycle in the same way in the transmitter discretely digitally generated and subsequently converted into voltage values by digital-analog conversion. 4. Device according to one of claims 1 to 3, characterized in that the demodulation of the signals received by means of the receiving antenna is carried out in the receiver by multiplying these signals by the direct signal from the transmitter, also supplied to the transmitting antenna.

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