JPH0725735Y2 - Metal detector - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field of the Present Invention> The present invention relates to a metal detector for detecting a metal mixed in food or the like.

<Prior Art> This type of metal detector conveys an inspected product on a conveyor,
It is equipped with a magnetic field generation unit and a magnetic field detection unit that are placed across this conveyor, and is configured to detect the influence of the mixed metal of the conveyed inspected product on the magnetic field and inspect for the presence of mixed metal. There is. For general inspected products that are raw materials such as wheat flour, a metal detector using an alternating magnetic field is used because metal detection with high sensitivity can be performed by utilizing the resonance characteristics of the coil for magnetic field detection.

However, in the case of inspected items such as confectionery, the surface is often packaged with aluminum foil or aluminum vapor deposition material, and such packaging material greatly affects the AC magnetic field (causing eddy current loss).
Therefore, it is difficult to detect small mixed metals. For this reason, a metal detector for detecting a mixed metal with a DC magnetic field that is easy to remove the influence of the packaging material is used for the inspected product packaged with the non-ferrous metal material such as the aluminum vapor deposition material.

<Problems to be solved by the invention> However, when inspecting an inspected product packaged with a packaging material made of non-ferrous metal or a packaging material made of metal on the same line, it is most suitable for each product type change. Change to a characteristic metal detector, or two (for non-ferrous metal packaging materials and non-metal packaging materials) metal detectors must be installed in the line, and complicated replacement work and an increase in the area occupied by the inspection line There was a problem.

The present invention aims to provide a metal detector that solves this problem.

<Means for Solving the Problems> In order to solve the above problems, the metal detector of the present invention includes a magnetic field generating means (1) for generating an AC magnetic field in a conveyance path of an inspection object, and a magnetic field generating means for generating the AC magnetic field. Magnetic field frequency varying means (3) for varying the frequency of the alternating magnetic field to be generated, and change of the alternating magnetic field due to the inspected product which is arranged at a position for receiving the alternating magnetic field generated by the magnetic field generating means and which passes through the alternating magnetic field. A magnetic field detecting means (6) for detecting a magnetic field; a detecting means (11) for synchronously detecting an output signal of the magnetic field detecting means by a reference signal having the same frequency as the excitation signal generating the alternating magnetic field; Phase changing means (8) for relatively changing the phase difference between the output signal of the magnetic field detecting means and the reference signal, and the detection output of the detecting means are compared with a predetermined reference value to mix in the inspected product. With or without metal Constant determining means (13)
And a control means for driving the magnetic field frequency varying means and the phase varying means to variably control the frequency f of the alternating magnetic field and the phase difference φ between the output signal of the magnetic field detecting means and the reference signal.
0) and a metal detection device having: When the inspected product packaged with a non-metal material enters the alternating magnetic field, the control means sets the frequency f of the alternating magnetic field to a predetermined frequency f.
And the phase difference φ is set to a phase difference φw that minimizes the detection output when an article of the same quality as the inspected product and containing no mixed metal passes through the alternating magnetic field of the frequency f. , The presence of mixed metal in the inspected product packaged with the non-metal material is determined, and when the inspected product packaged with the non-ferrous metal material such as aluminum vapor deposition material enters the alternating magnetic field, The frequency f is set to a frequency f2 lower than the frequency f1, and the phase difference φ is such that the detection output becomes minimum when only the nonferrous metal packaging material passes through the alternating magnetic field of the frequency f2. By setting to φo, it is configured to determine the presence or absence of mixed metal in the inspected product packaged with the non-ferrous metal material.

<Operation> Therefore, in the metal detector of the present invention, the inspected product packaged with the non-metallic packaging material passes through the AC magnetic field of the predetermined frequency f1 and is compatible with the magnetic field change at that time. The signal is detected at the detection phase that minimizes the detection component due to its contents, the presence or absence of mixed metal is determined, and the inspected product packaged with a nonferrous metal packaging material such as aluminum vapor deposition material is f1 The presence or absence of mixed metal is determined by passing a signal in the AC magnetic field of lower frequency f2 and detecting the signal corresponding to the change in the magnetic field at the detection phase that minimizes the detection component of the packaging material.

<Embodiment of the Invention> (FIGS. 1 to 3) Hereinafter, an embodiment of the invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing the configuration of a metal detector of one embodiment.

In the figure, reference numeral 1 denotes a signal from an oscillator 2 which is frequency-divided by a frequency divider 3, the frequency-divided output is converted into a sine wave by a filter 4, and the output is sent to a transmission coil 5 as an excitation signal. Is a magnetic field generation unit that generates an alternating magnetic field from the.

The frequency divider 3 and the filter 4 compare the frequency of the excitation signal with a relatively high frequency f conventionally used for detecting the metal contained in the inspected product that is packaged with a non-metal material such as paper or vinyl. ₁ (e.g., 100 KHz) and, aluminum in order to detect the metal contained in the inspected article packaged in a packaging material which is deposited on the paper or plastic or the like material, the frequency f ₂ lower than f ₁ (e.g., 25KHz and 50KHz ) And, the frequency division ratio and the pass band are switched by a switching signal sent from the control unit 20 composed of a CPU or the like.

This is because the packaging material deposited with non-ferrous metal such as aluminum tends to rapidly increase the detection output due to eddy current loss as the frequency increases, as shown in FIG.
This is to reduce this effect and to enable good detection of the mixed metal.

The magnetic field detecting means 6 is arranged at a position for receiving the AC magnetic field from the transmitting coil 5 and is differentially connected to the receiving coils 7a, 7b,
A resonance circuit is formed by a variable resistor VR for setting the differential output to a normally zero level, and tuning capacitors Ca and Cb.

The switch St, which is switched by the switching signal from the control unit 20, is configured so that its resonance frequency is substantially tuned to the frequency f ₁ or f ₂ of the excitation signal.

8 switches the switch Sp by a switching signal to switch the plurality of capacitors C _{1 to} C _n , and the magnetic field detector 6
It is a phase adjustment circuit that changes the phase of the received signal by slightly varying the resonance frequency of.

The phase adjustment circuit 8 is configured so that the contents of the inspected product packaged with a non-metallic material affect the detection output described later, or the aluminum-evaporated packaging material that wraps the inspected product changes the detection output. It is designed to minimize the impact.

That is, the influence of the content of the inspected product on the detection output changes gently with respect to the change in the phase difference between the excitation signal and the received signal, as shown in F of FIG. Is the smallest.

Further, the influence of the nonferrous metal packaging material such as aluminum vapor deposition material on the detection output has a certain phase range (φ _{1 to} φ ₂ , φ _{3 to} φ) as shown by G ₁ and G ₂ in FIG. ₄ ), the experimental result shows that the value becomes sharply smaller and the range becomes wider as the excitation frequency becomes smaller.

Therefore, select _one of the capacitors C _{1 to} C _n so that the phase difference with the received signal is φw when the frequency of the excitation signal is f ₁ and the phase difference is φo when the frequency of the excitation signal is f _2. For example, it is possible to detect a mixed metal with high sensitivity.

Reference numeral 10 is a detection circuit 11 for synchronously detecting a received signal from the phase adjustment circuit 8 by an excitation signal, holding a peak level of this detection output in a peak hold circuit 12, and a comparison circuit 13 for holding the held output and a presetting device. This is a discriminating circuit for discriminating the presence or absence of the mixed metal contained in the inspected product by comparing with the reference level (limit value) set in 14.

<Operation of the Embodiment> (FIG. 4) In the case of inspecting the inspection object Wa packaged with a non-metal material with the metal detector configured as described above, the frequency of the excitation signal is changed to f by the switching signal. _The value which is adjusted in advance to 1 by switching the switch St of the magnetic field detection unit 6 to the side of the capacitor Ca and the switch Sp of the phase adjusting circuit 8 to the phase difference φw that minimizes the influence of the contents of the inspected product Wa. Then, the inspection object Wa is passed between the transmission coil 5 and the reception coil 7.

At this time, the detection signal from the detection circuit 11 is slightly changed by the influence of the contents of the inspected product Wa as shown in FIG. 4 (a). Is very small below the limit value.

Here, in the case where metal is mixed in the inspected product Wa, when the metal mixed portion passes, as shown in FIG. 4 (b),
The detection output greatly rises under the influence of the mixed metal, and the signal of the level exceeding the limit value of the setter 15 is held in the peak hold circuit 12, so the metal is mixed into the inspected product from the comparison circuit 13. A determination signal indicating that there is a signal is output.

In addition, when detecting the mixed metal of the inspected product Wb packaged with packaging material such as aluminum vapor deposition, set the frequency of the excitation signal to
At f ₂ , the switch St is switched to the side of the capacitor Cb, and the switch Sp of the phase adjusting circuit 8 is connected to the capacitor that gives the phase difference φo where the influence of the packaging material is minimized. 5 and receiving coils 7a, 7b
Pass between and.

At this time, the detection signal from the detection circuit 11 changes due to the influence of the contents of the inspected product Wb and the aluminum-deposited packaging material (similar to FIG. 4 (a)). The influence of the vapor-deposited packaging material is slight.

When a metal is mixed in the inspected product Wb, the detection signal largely changes due to the mixed metal as described above, and the determination signal for notifying the metal mixing is output.

The inspected product Wb packaged with aluminum-deposited packaging material
The detection sensitivity to the above is that the conventional apparatus using the DC magnetic field can detect the stainless steel ball (SUS304) with a diameter of 6 mm and the metal having the size smaller than that can not be detected. It was possible to detect a stainless steel ball having a diameter of 3 mm or less, which is less than half.

<Other Embodiments of the Present Invention> In the above embodiments, the phase of the received signal was adjusted by slightly changing the resonance frequency of the receiving coil 7 as the phase adjusting means. However, even if the phase of the excitation signal input to the detection circuit 11 is adjusted by the phase shift circuit, it is equivalent to changing the phase of the received signal, and the same effect can be obtained.

Further, in the above-described embodiment, the receiving coils 7a and 7b are differentially connected as means for detecting an alternating magnetic field, and capacitors (Ca, C
Although the resonance circuit is formed with b), for example, it may be a magnetic field detecting means using one receiving coil arranged along the passage direction of the inspected product. Other magnetic field detection sensors such as elements may be used.

Further, as a method of changing the frequency of the AC magnetic field generated by the magnetic field generating means 1, two or more oscillators 2 or two or more transmitting coils 5 may be provided.

<Effects of the Present Invention> As described above, the metal detector of the present invention sets the frequency of the alternating magnetic field to a relatively high predetermined frequency f1 when inspecting an inspection object packaged with a non-metal material. ,
With the detection phase varied so that the detection output due to the contents of the inspected product is minimized, the presence or absence of metal mixed in the inspected product is determined, and a nonferrous metal material such as aluminum vapor deposition material is used. When inspecting the packaged inspection item, set the frequency of the AC magnetic field to f2, which is lower than f1, and
The presence or absence of metal mixed in the inspected product is determined with the detection phase varied so that the detection output when the nonferrous metal packaging material passes the AC magnetic field of frequency f2 is minimized. Is configured.

Therefore, not only for inspected products packaged with non-metallic materials, but also for inspected products packaged with non-ferrous metal materials,
Metals mixed in these inspected products can be continuously detected with a single unit with high sensitivity, significantly improving versatility.
It is possible to omit the complicated work involved in switching the inspected product and prevent an increase in the area occupied by the inspection line.

[Brief description 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 metal. FIG. 3 is a characteristic diagram showing the degree of influence on the detection output with respect to the detection phase of the contents of the packaging material and the inspected product vaporized with the non-ferrous metal obtained by the experiment. FIGS. 4A and 4B are signal diagrams showing the operation of the embodiment. 1 ... Magnetic field generator, 6 ... Magnetic field detector, 8 ... Phase adjustment circuit, 10 ... Discrimination circuit.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP 59-60274 (JP, A) JP 60-78378 (JP, A) JP 64-65485 (JP, A)

Claims (1)

[Scope of utility model registration request] 1. A magnetic field generating means (1) for generating an AC magnetic field in a conveyance path of an article to be inspected, and a magnetic field frequency varying means (3) for varying a frequency of an AC magnetic field generated by the magnetic field generating means, A magnetic field detecting means (6) arranged at a position for receiving the alternating magnetic field generated by the magnetic field generating means, for detecting a change in the alternating magnetic field due to an inspected product passing through the alternating magnetic field, and an output of the magnetic field detecting means. A detection means (11) for synchronously detecting a signal by a reference signal having the same frequency as the excitation signal generating the AC magnetic field, and the phase difference between the output signal of the magnetic field detection means and the reference signal is relatively variable. Phase changing means (8) for judging, and judging means (13) for judging the presence or absence of mixed metal in the inspected product by comparing the detection output of the detecting means with a predetermined reference value.
And a control means for driving the magnetic field frequency varying means and the phase varying means to variably control the frequency f of the alternating magnetic field and the phase difference φ between the output signal of the magnetic field detecting means and the reference signal.
0) and the metal detector having the above, the control means sets the frequency f of the alternating magnetic field to a predetermined frequency f1 when the inspection object packaged with the non-metallic material enters the alternating magnetic field.
And the phase difference φ is set to a phase difference φw that minimizes the detection output when an article that is of the same quality as the inspected product and does not contain mixed metals passes through the alternating magnetic field of the frequency f1. , The presence of mixed metal in the inspected product packaged with the non-metal material is determined, and when the inspected product packaged with the non-ferrous metal material such as aluminum vapor deposition material enters the alternating magnetic field, The frequency f is set to a frequency f2 lower than the frequency f1, and the phase difference φ is such that the detection output becomes minimum when only the nonferrous metal packaging material passes through the alternating magnetic field of the frequency f2. A metal detector characterized by being set to φo so as to determine the presence or absence of mixed metal in the inspected product packaged with the non-ferrous metal material.