JPS61501229A - Method and apparatus for detecting an indicating device - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed description of the invention] Methods and dosages for detecting indicating devices The present invention provides a method for detecting an indicating device within a limited inspection area and a method for implementing the method. It is related to the acquisition 1 for giving.

The invention is primarily used for monitoring substances in trade, especially in daily merchandise trade. It is intended to display the presence of other kinds of objects, e.g. the presence of humans. It can also be used to indicate the presence of

In Swedish publication 2... (patent application number 8400826-7), an indicating device An apparatus for detecting the presence of a location is disclosed. This manufacturing instruction device is an example of For example, a relatively long strip made of a material with high magnetic permeability, although it has a narrow and thin shape. It is shaped like a pu. This high permeability material is, for example, commercially available under the trade name Permalloy. It can be made of materials that have been The presence of such strips is due to the coil is detected. This coil emits a magnetic field, receives a magnetic field, and receives a magnetic field. This detection is performed using a device for detecting the detected signal.

If such a strip is placed in a relatively weak external magnetic field, its longitudinal A magnetic flux density is generated.

The magnitude of this magnetic flux density can be as much as 2000 times greater than the external magnetic field. this The presence of such strips is detected by a detection device. Inside this detection equipment , this detection is realized by detecting the change in the voltage induced in the receiving coil. will be carried out.

In that patent, a 21-turn coil is used to generate the alternating magnetic field. child The first of these coils generates an alternating magnetic field at a frequency of 1. this frequency The number f1 is different from the frequency f2 of the magnetic field generated by the second coil of these coils. Become. Difference frequency and sum frequency n”f1 caused by intermodulation by the indicating device 10 m-f2 is received by one or several coils. where n and m is a positive or negative integer. The fsM signal received in this way is inside the detection device. is processed to determine if there is any intermodulation, i.e. the It is determined whether a display device is present. The intermodulation is an indicator! magnetic properties of This is caused by the nonlinearity of

However, in reality, even objects made of steel are usually present within the inspection area, e.g. Intermodulation also occurs with contact rings, tinplate, etc. to other magnetic fields or objects made of iron. Therefore, it is possible that interference may occur, in which case the detector will not be able to receive it. This signal is interpreted as indicating that the pointing device has been detected.

For example, a tin plate can interact with each other if it is near a coil that generates a high frequency signal. It is possible to generate a received signal by modulation. The characteristics of this signal are A high magnetic permeability material located far away from the There is.

For this purpose, it is necessary to ensure that the received signal originates from an indicating device in the form of a strip. to identify whether the object is present or originates from other objects within the inspection area. For example, it is necessary to detect intermodulation generated signals of different magnitudes. It is necessary.

Such monitoring devices cannot be triggered by objects other than the specially designed indicating device to generate an alarm sound. It is extremely important to avoid this.

The present invention generates a magnetic field to generate a received signal, and then analyzes and directs this signal. A method and apparatus capable of easily distinguishing the apparatus from other objects within the inspection area It is related to

Therefore, the present invention allows the detection of objects that may normally be present within the inspection area. A method and apparatus is provided that virtually eliminates the possibility of such false alarms occurring. Ru.

Therefore, the present invention provides that the two coils radiate a high frequency alternating magnetic field and that the 1 is the frequency of the alternating current magnetic field radiated by the first coil among the coils. 2 in the frequency of the magnetic field emitted by the second coil, and n and 2 are positive. Or the frequency n41 + caused by intermodulation by the indicating device as a negative integer At least one difference frequency and or sum frequency of m°f2 is one or is received by several coils, and the magnetic field is emitted and received by a high permeability material. within a limited inspection area, comprising the step of detecting said indicating device made of material. The present invention relates to a method of detecting the presence of the indicating device. Features of this method The characteristic is that the frequency f1. A low frequency third wave with a frequency f3 that is significantly lower than f2. An alternating current magnetic field is radiated to the inspection area and the magnetic field strength of the low frequency magnetic field is sufficient. The viewing device is placed in front of the magnetic field independently of a magnetic field having frequencies f1 and f2. saturation with the frequency of the low frequency magnetic field and re-radiate from the indicating device. The generated magnetic field periodically changes between frequency 1 and frequency f2 with the frequency of the low-frequency magnetic field. This is detected by the detection device as intermodulation that occurs between the two.

The invention further provides coils for radiating and receiving alternating magnetic fields and related thereto. a power supply connected to the coil, and a first film of the coil has a frequency of 1. a second coil of the coils; may emit a high frequency magnetic field having a frequency different from the first frequency. I can do it.

the two said coils are provided, and the detection device has a receiving coil and The cycle created by intermodulation by the indicating device, where y and ko are positive or negative integers. At least one turbidity wave number or sum frequency of wave number n-f 10 f2 can be detected and therefore preferably made of high permeability material. the indicating device within a limited monitoring area capable of detecting the new detecting device; It is a device for detecting the presence of The features of this device are The power supply device operates at the frequency f1. Low frequency with frequency f3 significantly smaller than f2 radiates a third alternating current magnetic field with a wave number and the magnetic field strength of the low frequency magnetic field is sufficiently large The indicating device is connected to the low frequency magnetic field independently of a magnetic field having a wave number of 1 and a frequency of 2. having a device capable of saturating the field with a frequency of The device is activated by the receiving coil periodically at a frequency of 3 at the frequency of said low frequency magnetic field. re-radiated by said indicating device performing intermodulation between frequency 1 and frequency f2; is capable of receiving a magnetic field and thereby detecting said magnetic field. It is possible to send a signal to an alarm device when such periodic mutual KU occurs. .

One problem solved by the patent is that the inspection area is limited in its spatial extent. It is clearly possible to limit the This means that outside the inspection area of interest The point that the alarm sound cannot be activated by the instruction f [or any other object] is important. For example, the inspection area may be a hallway between two adjacent checkout desks. Be able to do it. One coil is placed on each side of the inspection area, and these coils are Each wave produces a magnetic field with one of two different frequencies. This limitation can be effectively obtained by radiating.

Two external magnetic fields are generated by coils placed one on each side of the examination area. , the product of the magnetic field strengths is greater in the central part between these coils and outside the coils. becomes smaller rapidly. The magnitude of the intermodulation generated signal emitted by the indicating device is , when viewed roughly, it can be considered to represent the product of the magnetic field strengths of two external alternating magnetic fields 〇 This is true if one coil is placed on each side of the personal space. This means that the personal space of the person can be the inspection area. Therefore in this case , a relatively large magnetic field strength is accumulated inside the inspection area, and this occurs outside the inspection area. The product of is small.

For example, in the case of a cashier desk in a self-service store, the customer's aisle fortune should be 70an. products with strips and other goods carried on the conveyor belt of the cashier desk. Articles shall not be marked with such strips.

By arranging the coils on both sides of the monitoring area, the method and apparatus according to the invention In this case, this advantageous device is also used.

The present invention is based on the embodiments shown in the attachment 200, showing the basic appearance of the lower signal. Voltage-time diagram, Figure 3, after amplitude detection with bandpass filter and low-pass filter A voltage-time diagram showing the basic appearance of the received signal is shown here. The tag is generated by the indicating device and other ferromagnetic objects such as thin-walled tin plates. Enter. Figure 4 was received after performing the processing shown above on Figure 3. This is a voltage/time diagram showing the basic appearance of a signal, but in this case the signal is an indicating device or Only these are occurring. FIG. 5 is a schematic diagram of another detection device according to the invention.

One embodiment of the invention is shown in FIG. i [l in Figure 1 is coil 1, 2 These coils generate an alternating electromagnetic field on each side of the inspection area 3. Ru. One of the coils 1 includes a first oscillator 4 and a summing amplifier 5. An alternating current having a frequency f1 is supplied from a power supply device having the following. Of the said fill The second coil 2 includes a second oscillator 6 and the power supply! [1 and associated summing amplifier 7 and , an alternating current with a frequency of 2 is supplied. This power supply further includes a third oscillator 8. do.

This third oscillator 8 can generate an alternating current voltage of frequency f3. Said third shot The gain and loss are connected to the two summing amplifiers 5.7. These summing amplifiers are coil Generates an alternating current in which an alternating current with a frequency of 1 and an alternating current with a frequency of 3 are superimposed within 1. and an alternating current of frequency f2 and an alternating current of frequency f3 in coil 2. It is possible to generate an alternating current that is superimposed with the current.

When a strip of high permeability material is used, the length of this strip is its cross-sectional area , only a relatively weak magnetic field is required. The diameter of the coil is This diameter can vary widely, but this diameter should be as large as the distance between the coils. i.e., it should be 7oα in the case of a regular checkout desk in a self-service store. It is possible.

The alternating magnetic field generated by the two coils 1.2 has no effect at any point within the examination area 3. These coils are arranged so that they are substantially parallel.

One of the problems when performing an inspection using the device according to the present invention is that The device does not feed vertically oriented strips.

In order to perform a 100% complete test, three coil pairs were used, with each The magnetic fields generated by the coil pairs can be arranged to be substantially perpendicular to each other.

In this case, one illumination pair is activated at a time.

If the transition between the fill pairs can be made sufficiently rapid, the speed passing through the inspection area) The IJ tube will be inspected regardless of its orientation.

It is. This indicator device 31 is a strip made of high magnetic permeability material.

On the other hand, the magnetic field 5 re-radiated from this strike IJ knob (resulting from No. 1 received by the receiving coil 9 has sufficient energy. In order for the frequency to be 1 and the frequency f2 to be relatively high, A0 to the other In order for the magnetic field to not be shielded by objects within the examination area, , frequency 1 and frequency f2 must be selected low. Therefore, high The wavenumbers f1 and fl are lower than about 5 Kc/s and higher than about 1 KO/day. will be lost.

As mentioned above, the present invention uses the technology disclosed in the Swedish patent. Ru. This generates the sum and difference frequencies of 1 at frequency and 2 at frequency. This means using the nonlinear properties of the strip to

In one embodiment of the invention, at least the first order difference frequency and sum frequency are analyzed. It will be done. Preferably, these are of the same size.

Therefore, the frequency is 1 and the frequency f2 is fl is 1.5Kc/s and 1P2 is preferably 2.5 Kc/s. In this case, the sum frequency is 4Kc/s, and the difference frequency is 1KC/B.

Only frequency f1 and frequency f2 are radiated and their intermodulation generation No. 1 is created. When it is detected, as mentioned above, this mutual change v! 4 products are on the strip or a fairly large mass such as a thin-walled tin plate. generated by a ferromagnetic material with a slightly curved magnetization curve. It is not possible to directly determine whether In this case - strip and tin To distinguish between plates, it is necessary to compare the intermodulation generated signals of different magnitudes. Yes, this requires relatively complex and relatively expensive signal processing equipment. .

The magnitude of the magnetic field suitable for such a product inspection device is 1 mm. child The magnitude of the magnetic field is determined by the presence of ferromagnetic objects such as thin tin plates or ponds that often enter the inspection area. body, such as custom-made feelers, metal frames for glasses, and other personal items. Not enough to saturate. However, a magnetic field of such magnitude is is quite sufficient to saturate the top.

With the present invention, it is much simpler and much more efficient than using just two frequencies. It is possible to distinguish between strip and thin-walled tinplate in a safe manner. To the present invention As a result, the third 'ZR magnetic field is generated as described above.

This third alternating magnetic field has a considerably higher frequency f1 and frequency compared to 2, Fluctuations at significantly lower frequencies. The frequency of this low frequency magnetic field is about 100 c/s Low and good! It is preferable that the speed is lower than about 25 c/s. Suitable frequencies are e.g. 20 C76. The strength of this magnetic field is 1 at frequency and 2 at frequency a. independently, together with a quiet magnetic field, such as the earth's magnetic field, bring the strip into saturation in both directions. large enough to hold a The strength of the low frequency magnetic field is 2 of the total strength of the high frequency magnetic field. Preferably it is 6 times to 6 times. The preferred magnification in this case is 4 times.

When only 1 and fl are used at two frequencies, as in the said patent, f The magnetic field composed of l and 2 oscillates near the operating point on the magnetization curve of the 13 tube. do.

This causes the intermodulation generated signal to rise continuously.

According to the present invention, the operating point is moved at a frequency f3 by a low frequency magnetic field. Therefore, many intermodulation generated signals are seen every half cycle of f3. therefore, Theoretically, the mutual change occurs at times when the strip has not yet reached saturation due to the low-frequency magnetic field. The modulated signal will be created and re-radiated by the strip - thin-walled strip. A ferromagnetic object such as a lithium plate certainly produces intermodulation between the frequency 1 and the frequency f2. These generated signals are generated continuously. The strength of the low frequency magnetic field is , for example, is significantly less than the strength required to saturate a thin-walled tin plate. this Due to the low frequency Virtually independent of magnetic fields. The reason is that this follows the magnetization curve of the thin tin plate. This is because the operating point does not change much.

The patent ensures that the resultant magnetic field moves through the nonlinear portion of the magnetization curve of the indicating device. For example, a suitable value for the strength of the emitted magnetic field is 5 Gauss.

According to the invention, approximately 1. has the lowest intensity of fus and has 2DC/a By applying a magnetic field that reliably moves through the nonlinear part of the magnetization curve 40 times/second. As a result, high frequency It is sufficient that the strength of the magnetic field is about 1 of the strength of the low frequency magnetic field. Low frequency magnetic field from both sides compared to emitting a low-frequency magnetic field into the inspection area from only one side. Therefore, the magnetic field generating coil can be designed at low cost.

Therefore, in order to distinguish between thin-walled tin plates and strips, a low-frequency magnetic field is applied to the Periodic changes in the received signal caused by the effect on the lips, i.e. It is therefore necessary to investigate the periodic changes in the pulses.Therefore, one aspect of the present invention is to In a simple embodiment, one frequency, For example, detect the amplitude of the signal received at f1+ at 2, thereby detecting the detected There is a pulse in the signal with double the frequency and a repetition frequency corresponding to 3. It is sufficient to decide whether

The detection of the amplitude is performed by an amplitude detector. This amplitude detector itself is a conventional design. It can be of a meter.

For example, the device shown in FIG. 1 has a detection H'fL with a bandpass filter 0. are doing. This bandpass filter has a coil and a capacitor, and this filter For example, pass 4 KC/Il+ at the above frequency, and its bandwidth is, for example, For example, it is 500C/B. An amplitude detector 11 is connected after this bandpass filter 10. Ru. This wave detector is composed of, for example, a diode. of the amplitude detector 11 A low-pass filter 12 is connected afterwards. This low-pass filter is made up of, for example, 80 circuits. This low-pass filter filters out frequency components of 4 Kc/e and higher. minutes are removed. For example, the intermodulation generated signal produced by a thin tinplate plate has an oscillation After the waveform and low-pass filter, the low frequency, mainly 40 C/B, with small amplitude This produces a virtually DC-like voltage in numbers. Intermodulation generated signals generated by strips occurs as a pulse.

The signal produced by the tin plate and the strip is filtered after the bandpass filter 10. , has the appearance shown in the principle diagram of FIG. 2, and this signal has pulses 25. low range 2 after the filter 12, the signal produced by the tin plate and the strip is It has the appearance shown in the principle diagram of the figure, in which the signal has pulses 25. vinegar The basic shape seen after the low-pass filter 12 for signals resulting from trips only is the fourth As shown in the figure.

Therefore, if both the thin-walled tin plate and the strip are within the inspection area, Fig. After the low-pass filter, the above-mentioned results from the strip are shown in A signal is returned that is above the signal level where the pulse originates from the thin-walled tin plate.

Therefore, as mentioned above, the simplest embodiment of the invention The key is to sufficiently detect the appearance of a pulse in the pulse detector 13. As a result, it is possible to detect that an IJ lump has appeared in the inspection area.

At that time, pulses are applied 40 times in order to avoid interference effects in the test results. It is preferable to wake up/second and be detected. When such a pulse is detected, the indication The device is assumed to be within the inspection area. This pulse detector 13 is appropriately designed. A signal can be sent to the alarm device 24 that has been activated.

At low frequency, 3 is selected as a sufficiently low frequency. Therefore, the bandpass filter 10 The pulses that can be transmitted through the It would be a minute shorter. Therefore, although the bandpass filter pans, at low frequencies 3 is , so that a sufficient number of pulses can be generated in a reasonable amount of time to enable their detection and evaluation. and must be selected to be sufficiently large. The appropriate number of pulses for detection is It can be 4. 3 to scan the inspection area in three directions perpendicular to each other. Conversion between different transmitters and receivers when multiple transmitters and receivers are used can be executed, for example, 5 times/second. Therefore, in this case, It is appropriate that the wave number f3 is 20 c/s. However, at frequencies 1 and The same is true for frequency 2, but for frequency 3 other than the above Appropriate frequencies can be selected.

However, according to the preferred embodiment, compared to just detecting the number of pulses, , the signal is analyzed more accurately. In this example, the sum frequency and or difference frequency The wave number is analyzed both in terms of phase position and amplitude relative to the initially emitted magnetic field. be done.

However, when only the presence of pulses is detected, more pulses per second are detected. frequency in order to obtain a loss of noise and thereby be safer against interference. Number f1. f2 and f3 must be large compared to the 1″X mentioned above.

According to this embodiment, the measurement signal received by the receiving coil 9 is filtered through a bandpass filter. Therefore, at least the difference frequency or the sum frequency created by the mutual V key is One is also filtered. Reference signals with the same frequency radiate a high frequency magnetic field. It is created by mixing the f2 signal with the high frequency generated for the f2 signal. measurement The signal and the reference signal thus created are sent to a phase and amplitude detector. . Preferably, the phase and amplitude detector is a quadrature detector. this right angle A phase detector detects the amplitude of the pulse @A and the phase difference between the measurement signal and the reference signal. ψ and b″− by generating a signal corresponding to ψ in a manner well known per se. Wear.

A detection device for carrying out the method according to the second embodiment is illustrated in FIG.

The detection device detects at least one difference frequency or difference between frequency 1 and frequency 2. has a device capable of creating a reference signal that falls from the sum frequency. This device is an example For example, it can be mixer 17. This mixer has a conductor 1 from the first oscillator 4. 8 and from the second oscillator 6 through the conductor 19 respectively. It will be done.

A bandpass filter 20 is provided, and the difference selected by this bandpass filter is Only frequencies or sum frequencies are selected. This selected frequency is, for example, 4 x c/s. This reference signal is one input 2 of the quadrature detector 15. Begged to be supplied to 6.

The signal 14 received by the receiving coil, that is, the measurement signal, is sent to the bandpass filter 16. The sum frequency is then filtered from the difference frequency fs'. its frequency is equal to the frequency of the reference signal. Therefore, the bandpass filter 4 is, for example, 4KC/ [1, and its bandwidth is 500 c/s. pass through a bandpass filter After that, the measurement signal is fed to the second input 27 of the quadrature detector.

The quadrature detector is designed in a conventional manner and has two mixers. child These mixers mix the stationary signal and the reference signal.

In one mixer, the reference signal with a phase angle of mixed with No. Before the second mixer, the phase of the measurement signal and/or reference signal is changed. and the phase difference between these signals is 9) R-9 + M +90 or ψ, -9 It will be made into M-90. The quadrature detector 15 also has a frequency of 4Kc/a and higher. It has a low pass filter to remove the numbers. obtained from the quadrature phase detector 15 The low frequency pulse contains information about the phase and amplitude of the pulse of the intermodulation generated signal. Ru.

The quadrature detector 15 can supply latent signals to two channels 21 and 22. Ru. The signal of the first channel corresponds to A·sinψ. Here, A is the measurement signal is the amplitude, and ψ[E9R−ψ]. The second channel signal goes to A. handle. Microprocessor Cogψ 23, two or more occur mutually in channels 21 and 22. pulses are compared with respect to A and ψ. This microprocessor is a suitable type It can be something. of two or more subsequent pulses When there is a predetermined degree of similarity between A and ψ, It is assumed that there is a lip. The microprocessor 23 sends a signal to the appropriate alarm device 24. You can send the number.

In order to further increase the safety of this detection device, the conventional ah The time course of the signal can be scanned and sent to the microprocessor 23. Can be sent. The microprocessor 23 has a stage with a 20 c/s signal. It is possible to evaluate whether a pulse is occurring during the Due to this , other possible external fluctuating magnetic field influences and other disturbances are suppressed. .

said microprocessor 23 for analyzing the amplitude A and the phase angle ψ; The circuitry associated with is any suitable conventional device, and this device itself is not encompassed within the present invention. It is not included. Therefore, we will not explain them in detail here. stomach.

Sum frequency quadrature detection was described above.

Of course, instead of that, we can handle a difference frequency of 1 Kc/s, and Both sum frequencies can be treated in the same way. Of course, in the latter case, The analysis of the received signal becomes even more secure.

In addition to the sum frequency and difference frequency, a large number of frequencies f1+m are used during intermodulation. -f2 occurs. Here, n and m are positive or negative integers. of course, By analyzing one or more of these frequencies, The sensitivity of ill is further increased.

All frequencies of f, , f2 and f3 are connected to a common oscillator (not shown). ) can be obtained by frequency division. In that case, the quadrature detection Another advantage is that it is easy to create a four-phase signal used in

It is immediately obvious that the invention obviates the drawbacks mentioned in the first part of the specification. Therefore, according to the present invention, the indicating device can easily and Safely remove, for example, thin-walled tin plates or other materials made of magnetic material within the inspection area. The main advantage is that it can be distinguished from objects. As mentioned above, the low frequency magnetic field The high frequency magnetic field can be made weaker than in the case of conventional technology that does not use additional I’C. can. By being able to reduce the strength of the high-frequency magnetic field, The customization of the issue will improve. By combining this and applying a third magnetic field, the strip This combination of improved al for distinguishing thin-walled tin plates from Can be designed smaller. The indicating device, i.e. the strip The fact that the strip can also be designed to be small is particularly important when the strip is integrated with the price tag. There are advantages.

Therefore, the present invention is particularly advantageous when applied to such fields.

Various embodiments have been described. However, there are various It is possible to change. For example, to generate high-frequency magnetic fields or low-frequency magnetic fields. The number of coils can be changed and also the one for generating the required magnetic field Laws and equipment can also be changed. It is also possible to have multiple coils. It is Noh.

However, the present invention should not be considered limited to the above embodiments, but rather It is to be understood that modifications may be made within the scope of the claims.

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Claims (1)

[Claims] 1. Two coils (1, 2) radiate a high frequency alternating magnetic field, and at that time the coils The frequency (f1) of the alternating current magnetic field radiated by the first coil (1) of the coil is It is different from the frequency (f2) of the alternating current magnetic field emitted by the second coil (2), and , n and m are positive or negative integers by mutual modulation by the indicating device (31). The difference frequency of at least one of the frequencies n・f1+m・f2 generated by or sum frequency is received by one or several coils (9) and the radiation of said magnetic field is a stage for detecting the indicating device (31) made of high transmittance material by emitting and receiving; Method for detecting the presence of the indicating device within a limited inspection area having a floor and has a frequency (f3) significantly lower than the frequency (f1, f2). a third alternating current magnetic field of low frequency is radiated to the region; and a magnetic field of the low frequency magnetic field. the finger independently of the magnetic field having a sufficiently large intensity and frequencies f1 and f2; bringing the indicating device (31) into a saturated state at the frequency of the low frequency magnetic field; The magnetic field re-radiated from the indicating device (31) periodically has the frequency of a low-frequency magnetic field. Detected by the detector as intermodulation occurring between frequency f1 and frequency f2. The indicating device is present within a limited inspection area characterized by How to detect. 2. In claim 1, each of the coils ( 1 and 2) are arranged one each on one side of the inspection area. Law. 3. Claim 1 or 2, wherein the low frequency alternating magnetic field is applied to the coil. A method characterized in that radiation is emitted from either of (1, 2). 4. In claim 1, 2 or 3, the frequency of the high frequency magnetic field (f1, f2) is lower than about 5 Kc/s and higher than about 1 Kc/s. how to. 5. In claim 1, 2, 3 or 4, the low frequency magnetic field The frequency (f3) of is lower than about 100 c/s, preferably about 25 c/s. A method characterized by 6. In claim 1, 2, 3, 4 or 5, the low The magnetic field strength of the frequency magnetic field is 2 to 6 times the magnetic field strength of the high frequency magnetic field, A method characterized in that it is preferably 4 times as large. 7. In claim 1, 2, 3, 4, 5 or 6, , the difference frequency caused by intermodulation of the signals received by the receiving coil (9) or bandpass filtered around at least one frequency of the sum frequencies. The amplitude is detected by the amplitude detection signal, which is twice the frequency of the low-frequency magnetic field of the amplitude-detected signal ( f3) indicating device (31) when a pulse appears with a repetition frequency corresponding to A method characterized in that the presence of is detected. 8. In any of claims 1 to 7, the received signal or That is, the measurement signal is the lesser of the difference frequency or the sum frequency caused by intermodulation. Both are band-pass filtered around one frequency and emit a high-frequency magnetic field. By mixing the high frequency (f1, f2) signals generated for A reference signal with a The reference signal is applied to a phase and amplitude detector (15) to determine the amplitude (A) of the pulse. ) and the phase difference (■) between the measurement signal and the reference signal. A method characterized by: 9. Coils (1, 2) for radiating and receiving alternating magnetic fields and associated therewith; a first coil of the two coils (1, 2); (1) can emit a high frequency alternating magnetic field with frequency (f1) and A second coil (2) of the coils has a frequency (f2) different from the first frequency. and the detection device has a receiving coil. and by mutual modulation by the indicating device (31), where n and m are positive or negative integers. difference frequency or at least one of the frequencies n・f1+m・f2 created by or the appearance of a sum frequency, thereby making it possible to detect limited detection capable of detecting said indicating device (31), which is preferably A device for detecting the presence of the indicating device within the scanning area, the device The source device (4-8) has a frequency (f3) that is significantly lower than the frequency (f1.f2). radiates a low-frequency third alternating current magnetic field with a sufficient magnetic field strength; The indicating device ( 31) can be brought into a saturated state even at the frequency (f3) of the low-frequency magnetic field. The detection device (10-13; 15-23) is connected to the receiving coil (9). Therefore, the frequency f1 that occurs periodically at the frequency (f3) of the low frequency magnetic field and re-radiated by said indicating device (31) with intermodulation between frequency f2 is capable of receiving a magnetic field and thereby detecting said magnetic field and be able to send a signal to an alarm device when periodic intermodulation occurs; for detecting the presence of the indicating device within a limited inspection area characterized by equipment. 10. In claim 9, the two coils (1, 2) are arranged in the inspection area. A device characterized in that one device is placed on one side of the device. 11. In claim 9 or 10, the power supply device (4-8) comprises two A device characterized in that it can radiate a low frequency alternating current magnetic field by means of two coils (1, 2). Place. 12. In claim 9, 10 or 11, the high frequency f1 and and f2 are lower than about 5 Kc/s and higher than about 1 Kc/s. Device. 13. In claim 9, 10, 11 or 12, low frequency The frequency f3 of the magnetic field is lower than about 100 c/s, preferably about 25 c/s. A device characterized by new features. 14. In claim 9, 10, 11, 12 or 13, In this case, the power supply device (4-8) has a power of 2 to 6 times, preferably 2 times to 6 times, the entire strength of the high frequency magnetic field. It is characterized by being able to emit a low frequency magnetic field with an intensity of 4 times as much. device to do. 15. Claims 9, 10, 11, 12, 13 or 1 In clause 4, the detection equipment (10-13) is the difference frequency or or a bandpass filter (16) for filtering the sum frequency; The appearance of pulses that occur with a repetition frequency corresponding to the double frequency is detected by amplitude detection. and an amplitude detector (11, 12) capable of scanning in the detected signal. A device characterized by: 16. In any one of claims 9 to 15, the detection device ( 15-23) is at least one difference frequency between the frequency f1 and the frequency f2 Or a mixer (17) that can create a reference frequency composed of sum frequencies. and for filtering said difference frequency or sum frequency of the received signal. a bandpass filter (16) in which the filtered signal is the measurement signal; a filter (16) and a phase and amplitude detector (15) a signal and the measurement signal and an amplitude (A) of the pulse; and generating a signal corresponding to a phase difference (■) between the measurement signal and the reference signal; A device characterized by being able to.