JPH02262089A - Detection of metal - Google Patents

Abstract

PURPOSE:To detect the metallic object mixed in the product to be inspected with high sensitivity packed by a packing material having a membrane metal formed thereto or having a metal vapor-deposited thereon by markedly reducing the effect of the packing material by making the relative phase on a magnetic field generating side and a magnetic field detecting side controllable. CONSTITUTION:When a product W to be inspected packed by a packing material having an aluminum deposition layer is passed between the transmission coil 5 and receiving coils 7a, 7b of a metal detector, the level of a receiving signal is changed and the detection output from a detection circuit 11 is slightly changed by the content of the product W to be inspected and the effect of the packing material having the aluminum vapor deposition layer but the change quantity due to said packing material contained in this change becomes a limit value or less. When a metal is contained in the product W to be inspected, inspection output receives the effect of the mixed metal to largely rise and the level exceeding the limit value of a setting device 14 is held to a peak holding circuit 12 and, therefore, a judge signal informing the mixing-in of the metallic object is outputted from a comparing circuit 13.

Description

DETAILED DESCRIPTION OF THE INVENTION Industrial Application Field of the Present Invention The present invention relates to a metal detection method for detecting metals mixed into foods and the like.

<Prior art> This type of metal detection excavation involves passing the inspected item through a magnetic field.
It is configured to detect the influence of the inspected product on the magnetic field and inspect the presence or absence of mixed metals.

This detection method varies greatly depending on the product to be inspected, and if non-ferrous metals such as aluminum foil are used in the packaging of the product to be inspected, it is necessary to prevent eddy current from flowing through the non-ferrous metals and detect iron mixed in. , a detection method using a DC magnetic field is used.

<Problem to be solved by the invention> However, in the detection using a DC magnetic field as described above,
Although iron contamination can be detected satisfactorily, the detection sensitivity for nonferrous metal contamination is very poor, and there is a problem in that it cannot be detected unless the contaminant nonferrous metal is quite large in shape.

In particular, in recent years, many confectioneries and the like have been produced that are packaged with packaging materials coated with aluminum, making it even more difficult to detect metal contamination. Therefore, there is a strong desire for a highly sensitive detection fence that can accurately detect contamination even in items packaged with packaging materials coated with non-ferrous metals.

The present invention solves this problem and is a metal detector that can detect both ferrous and non-ferrous metals with high sensitivity, especially in inspected products packaged with non-ferrous metal evaporation materials such as aluminum evaporation materials. is intended to provide.

Means for Solving the Problems> In order to solve the above problems, the metal detection method of the present invention includes: A metal detection method that uses an alternating current magnetic field to detect the presence or absence of mixed metal in an inspected item wrapped in a packaging material. In this step, the phase difference between the signal that generates the alternating magnetic field and the received signal received by the magnetic field detection means placed at a position receiving the alternating magnetic field is set to a value that reduces the influence of the packaging material on the received signal. I try to set it in advance.

Therefore, if the phase is adjusted in advance to minimize the influence of the packaging material used to package the inspected item, the detection of metal mixed in the inspected item will be minimized due to the influence of the packaging material. It can be done without receiving any.

Embodiment of the present invention> (FIGS. 1 to 3) Hereinafter, an embodiment of the present invention will be described based on the drawings.

FIG. 1 is a block diagram showing the configuration of a metal detector n which is an embodiment of the present invention.

In the figure, a magnetic field generating unit 1 divides the frequency of a signal from an oscillator 2 with a frequency divider 3, converts this frequency-divided output into a sine wave with a filter 4, and sends this output as an excitation signal to a transmitting coil 5. An alternating current magnetic field is generated from the transmitting coil 5.

The frequency of the excitation signal of the transmitting coil 5 is a frequency (several 100 KH2) that is generally used in this type of device for metal detection of items packaged with packaging materials such as paper and vinyl with non-ferrous metals deposited on them. ) significantly lower frequency,
For example, it is set to several tens of kilohertz. This is because the influence of non-ferrous evaporated materials such as aluminum on the magnetic field frequency increases rapidly due to eddy current loss as the frequency increases, as shown in Figure 2. This is to enable good detection.

Reference numeral 6 denotes a magnetic field detection unit disposed at a position receiving the alternating current magnetic field from the transmitting coil 5, and a differentially connected receiving coil 7.
A, 7b1 variable resistor VR and capacitor Go form a resonant circuit. The resonant frequency is approximately tuned to the frequency of the excitation signal, and is adjusted so that the differential output is zero during normal times when there is no product W to be inspected.

Reference numeral 8 denotes a phase adjustment circuit that changes the phase of the received signal by switching on a plurality of capacitors 01 to ON with a switch 9 and slightly varying the resonance frequency of the magnetic field detection section 6.

The phase adjustment circuit 8 is adjusted in advance so that the influence on the alternating current magnetic field of the aluminum vapor-deposited packaging material for packaging the inspected item W is minimized.

That is, the 11LI applied to the alternating current magnetic field of the aluminum vapor-deposited packaging material
is within a certain phase range (φ1 to φ2, φ3 as shown in Figure 3).
-φ4), and the range becomes wider as the excitation frequency is lower.Based on an experimental result, the phase adjustment circuit 8 is adjusted in advance to set the phase difference to be at its center φ0.

10 is a detection circuit 11 for detecting the received signal from the phase adjustment circuit 8.
The peak level of the detected output is held by the peak hold circuit 12, and the comparison circuit 1
3 is a discrimination circuit that discriminates the presence or absence of mixed metal by comparing this held output with a reference level (limit value) set in advance in the setting device 14.

When the inspected item W wrapped in aluminum vapor-deposited packaging material is passed between the transmitting coil 5 and the receiving coil 7a17b of the metal detector configured in this way, the level of the received signal changes and the detection circuit 11 As shown in Fig. 4(a), the detection output from the detector varies slightly depending on the contents of the inspected item W and the packaging material of the aluminum vapor deposition, but this change includes the amount of change due to the packaging material of the aluminum vapor deposition. is below the limit value as explained above.

Here, if metal is mixed in the inspected product W, as shown in FIG. 4(b), when the metal-containing part passes through, the detection output will greatly increase due to the influence of the mixed metal. Since the level exceeding the limit value of the setter 15 is held in the peak hold circuit 12, the comparison circuit 13 outputs a determination signal indicating the presence of metal contamination.

In addition, in the conventional detection method using a DC magnetic field, a stainless steel ball (SUS304) with a diameter of 6 mm is detected as a mixed metal.
), but the detection unit of this example was able to detect a stainless steel ball with a diameter of 3 mm.

<Other Embodiments of the Present Invention> In the above embodiments, the phase of the received signal is adjusted by slightly varying the resonance frequency of the receiving coils 7a and 7b as a phase adjustment means. Although the present invention is not limited to this, adjusting the phase of the excitation signal to the detection circuit 11 using a phase shift circuit is equivalent to changing the phase of the received signal, and the same effect can be obtained.

Further, in the above embodiment, the receiving coils 7a and 7b are differentially connected as means for detecting the magnetic field, and the output is adjusted using the variable resistor VR so that the output is zero during normal operation. The magnetic field detection means may be one in which a receiving coil is arranged along the passing direction and connected in parallel with a capacitor to form a resonant circuit, or another magnetic field detection sensor such as a Hall element may be used in addition to the coil. It's okay.

Effects of the Present Invention As explained above, the metal detection method of the present invention allows adjustment of the relative phase between the magnetic field generation side and the magnetic field detection side. However, the influence of the packaging material used to package the inspected item can be significantly reduced, and metals mixed in inspected items packaged with thin metal or metal-deposited packaging can be detected with high sensitivity. It can be detected by

[Brief explanation of drawings]

FIG. 1 is a block diagram showing the configuration of an embodiment of the present invention, and FIG. 2 is a characteristic diagram showing the degree of magnetic field influence on the frequency of non-ferrous metals. FIG. 3 is a characteristic diagram showing the degree of influence of a magnetic field on the phase of a non-ferrous evaporated material obtained through an experiment. FIGS. 4(a) and 4(b) are signal diagrams showing the operation of one embodiment. 1... Magnetic field generation section, 6... Magnetic field detection section, 8... Phase adjustment circuit, 10... Discrimination circuit.

Claims (1)

[Claims] A metal detection method for detecting the presence or absence of mixed metal in an inspected product wrapped in a packaging material using an alternating magnetic field, comprising: a signal that generates an alternating magnetic field; A method for detecting metal, characterized in that a phase difference between the received signal and the received signal received by the magnetic field detection means is set in advance to a value that minimizes the influence of the packaging material on the received signal.