Method for detecting metal objects

The invention relates to a method for detecting of metal objects; in particular it relates to a improved discrimination (distinction) in a Metal detector.

If a metal detector for inspection of large Persons employed, there is the perceiving werteste slowing factor of the process in the wrong alarms, arising from small harmless objects be called. If the cause of these false alarms wants to find out, this can be time consuming and lead to the formation of queues at the checkpoints. Simultaneously have firearms course an alarm cause. This ability of the metal detector to Distinguishing a dangerous, relatively large counter Prior under various small and innocuous objects is called discrimination.

An attempt was made, the discrimination of a metal detector using a variety of different improve processes, for. example by balancing the Sensitivity distribution by means of an improved Coil design. This is important because the Sensitivity of the device in accordance with general set the point with the lowest sensitivity is. Therefore, if the sensitivity distribution un is equal, can remarkably small items a cause alarm to the most sensitive points of the door. The sensitivity distribution was z. B. by means of a Two channel method improves such. B. U.S. Patent No. 4,605,898, where two sets of transmitter / receiver were used. You can also chose the Sensitive speed distribution through a computer-assisted Spulenaus improve interpretation.

Discrimination may also by adjusting the be strengthened term behavior of the metal detector. however is usually the machines used only one Measurement window used that behind the cut-off point of Transmission stream is arranged. It is therefore not possible with respect to the discrimination by means of this Method to achieve any substantial improvement.

Discrimination improving the coils of Metal detector be constructed so that a group of N similar and over a wide area distributed objects significantly lower response results as multiplied by N Answer of such an object. A corresponding system is z. B. U.S. Patent No. 4,605,898 described that from neighboring Loops is established, the in opposite directions are spooled.

From DE 37 27 416 A1 discloses a method for detecting known of metal objects, wherein the response from the Surface of two different measurement periods Abschaltpeaks at is scanned. This method is an improvement in terms of sensitivity and the material identification and can not for discrimination improvement of Metal detector are used, because according to this method only a slight exceeding of the alarm limit is observed, but not the correlation of the response from the surface of the Abschaltpeaks with the response from outside the area of Abschaltpeaks.

The object of the invention is, some of the to eliminate disadvantages of the prior art and a improved method for detecting of metal objects provide so unremarkable small items of can distinguish dangerous big objects (Discrimination).

This object is achieved by a method according to claim 1 and 11. Further developments of the inventive method described in the associated dependent claims.

According to the invention which is passing through the metal detector Material having at least two different delays analyzed for response, so that at least one intra-response half of the area of ​​Abschaltpeaks is defined or at least defines a response outside the area of ​​Abschaltpeaks , and that the correlation of the responses within a GE given period of time is examined. Thus behaves a reasonably large object, unlike many small objects, and Discrimination can be improved by the Kor relation between the comparing of the material with a long given delay response with the response of short Verzö delay. If one of the responses in the measurement channels and / or Frequencies in the same number used as answers yet are both the answers with a long as well as health zer delay essentially be determined simultaneously. If the measured responses with the help of a time variable sets at the same coordinates (time variable to answer) can be achieved at a reasonably large object that the maximum absolute values ​​of responses with different Setting repeated delays simultaneously, whereas in a Number of objects, when the absolute value of the response to health zer delay reaches its maximum, the answer with a long Delay is not far from zero.

With a metal detector with pulse technique is the About tragungs pulse sequence of a number of regularly repeatable ten identical pulses with alternating polarity with a Repetition frequency of 1 kHz, for example, or may after each sequence with alternating polarity are scanned.

Such a detector takes out disturbances within the repetition frequency lying or his odd represent harmonics. The computational Accuracy of the coefficients for the basis of the constitute discrimination, in a pulse-operated Device thereby be enhanced by the scanning of one so establishes that the responses with long and short Repeat using different recovery holungsfrequenzen be measured that are beneficial to each other in integer relationship. For example, you can the measurement with a short delay with the recovery holungsfrequenz of 500 Hz, and the measurement of the long Delay with a repetition rate of 1 kHz carry out. In the former measure are the hinge recessed disturbances at the frequencies 500 Hz, 1.5 kHz, 2.5 kHz, 3.5 kHz, etc., and in the second measurement at the Frequencies 1 kHz, 3 kHz, 5 kHz, etc. With the less than be different delays measured responses Thus disturbances of different frequencies connected, wherein the interference in the determination of the characterizing Parameters are weakened. To the maximum point of Absolute value of the short delay for inventive determine and avoid any measurement noise, the parameter a are as follows in accordance with equation (1) To be defined:

where A_S and A_L which in short and long delay represent measured responses. In equation (1) it stretches the integration over the time variable t while the measured material through the metal detector runs. The parameter represents a Korrelationskoeffi cient is, which is also comparable to the size the answer with a long delay. The absolute value of Parameter is high when the answers ge during entire measurement period in substantially the same sign have, ie if both the answers long as even with a short delay its maximum and / or their reach minimum substantially simultaneously. This is the case when the object to be detected is simple and relatively large. When a Koeinfluß various ferent objects is given, correlate with under ferent delays measured responses bad together, the absolute value of the parameter a small remains because the responses of the long and short Ver delay the maximum of their absolute values ​​not simultaneity reach tig. When applying the invention The method an alarm is given when the para meter a exceeds a preset alarm threshold.

The answers lead of the inventive method also to other parameters as in the equations (2) and (3) are shown.

The c parameters of the Equation (3) is a real Kor relations coefficient, the values ​​within the range [-1, +1] Contains. Because the parameter c with the size the answer is not comparable, it can not alone be used as a parameter characterizing, but the parameter c with parameter b to Be humor of the group are used by objects.

The parameter b of the equation (2) is similar to the Parameter b and the response with a long delay. This parameter c of the response differs with long delay to the part with the response correlated short delay. Because the parameter B are also with the signal size is comparable, the detection founded by metal objects on it.

In calculating the coefficients of the equations (1) to (3) a predetermined integration time is angewen det. The metal detector can with a photocell system be equipped, which emits a signal when the to be examined person enters the gate. The launch of Integration can be linked to that date. at an alternative system observes a response with short delay and it is the maximum point of its Absolute value determined. In this case, the integration carried out on both sides of this time point, wherein the recorded measurements used. If the time integration to the maximum point of the answer short delay is performed, the discrimin discrimination compared to the prior art by Using a short integration time (less than 100 ms) are improved because the maxima of long and short answers with a simple large object occur at the same time, at various However, smaller objects at different times.

In a preferred embodiment of the invention is the selected time period such that the observed time Range is the time when the through the metal detector moving material within the detection zone remains, wherein the length of 1 to 2 s varied.

With a device according to the invention can also different samples with different delays are scanned, wherein on the basis of these samples several characteristic parameters and / or on this Parameters constituting coefficients are defined , in order to detect the number of articles which contained in the material being studied. In the Education of the alarm can be a combination of several used parameters.

The invention is described in further detail with reference to the accompanying drawings he purifies; show it:

<b>Fig.</b> 1: a response obtained by using a preferred embodiment. the present Invention and by means of computer simulation as a function of time when a large metal object is passed through the metal detector; and

<b>Fig.</b> 2: a response, obtained by use of prior ferred embodiment according <b>Fig.</b> 1 and with Using computer simulation as a function of Time when a group of a plurality of small GE metal objects through the metal detector are run.

at <b>Fig.</b> 1 is in the through the Metal Detector overflowed object to a dangerous large metal object such. as a firearm. At the Pass the metal object through the metal detector was the answer given by him with two different repeated delays measured, ie 40 microseconds and 100 microseconds After switching off the current pulse. This falls short Delay (40 microseconds) in the face of the Abschaltpeaks Pulse. The Abschaltpeak takes about 50 to 60 microseconds. In order to is the second measured delay (100 microseconds) outside the surface of the Abschaltpeaks of the pulse. The computer simulated responses from a larger metal object be in accordance with the aid of a time variable. <b>Fig.</b> 1 in the same coordinate set. Because of the outside Surface of Abschaltpeaks the pulse formed Answer substantially smaller than the response with a short Delay, the answer with a long delay in will multiplies the coordinates as by a factor of 5 illustrated in order to improve the visibility. Out<b>Fig.</b> 1 shows that the different Delays responses received a similar shape have, and that their absolute values ​​maximum at one in substantially the same value of the time variable form.

<b>Fig.</b> 2 illustrates the results gem. <b>Fig.</b> 1 in a Case where a large group of several small and harmless, at least partially metallic objects by the Metal Detector walked. Such metal stands can be, for. example, be a set of keys, a calculator, a pocket knife, a lighter, a belt buckle, a small amount of change, a Glasses and footwear - ie items that by the Metal Detector border person may carry. To define the responses from <b>Fig.</b> 2 is to say that the articles with respect to the metal detector so are arranged to be in regard to its amount normal position to the human body are as z. B. a bunch of keys in a pocket. Out<b>Fig.</b> 2 It can be seen that the different Ver delays obtained answers in shape substantially different, the maxima of the absolute values ​​of the responses to significantly differing Ask occur.

From the in <b>Fig.</b> 1 and 2 responses shown can with the aid of equation (1) a distinctive Para meter a are formed when in equation (1) A_S replaced by the response of a delay 40 microseconds is obtained, and when A_L replaced by the Response obtained with the delay of 100 microseconds. The Integration of equation (1) can as a function of time carried out within the range of (-320 ms, 320 ms) will. Subsequently in relative units are the Maximum values ​​listed that are different Delays obtained, as well as characterizing Para meter gem. Equation (1):

The values ​​of the parameter a can be seen that during the passage of a larger metal article, such. B. a firearm through the metal detector, the absolute value the parameter a is large, while the absolute value of Parameter a is small when small objects through the run metal detector. It is thus for the parameter a a value can be predetermined, of an overrun in Alarm triggers (so-called. Alarm threshold). This is only the given the case of a larger single object alarm.

Although the invention above in a pulsbetrie surrounded metal detector has been used, it can also be in continuously operated metal detectors without wesent find Liche deterioration use. In this case, the A_S Measurement by a measurement in the frequency range of 200 to 500 Hz replaced (is advantageously a frequency of 300 Hz), and the A_L Measurement is replaced by a Measurement within the frequency range of 20 to 50 kHz (Advantageous is a measurement at 30 kHz). The above described characterizing parameters can then in similarly be used.