JPH0229996B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a metal detector that detects whether metal is mixed in an object to be inspected (particularly food) being conveyed by a conveyor or the like.

First, an overview of a conventionally used metal detector will be explained with reference to FIG.

In this figure, 1 is an oscillator, 2 is a transmitting coil connected to the oscillator 1, 3a and 3b are receiving coils arranged opposite to this transmitting coil 2, and these receiving coils 3a and 3b are It is placed in the alternating magnetic field of the coil 2 and arranged so that the lines of magnetic force intersect equally.

4a and 4b indicate adjustment volumes for the phase and amplitude of the induced voltages e〓 ₁ and e〓 ₂ of the receiving coils 3a and 3b, and by adjusting the adjustment volumes 4a and 4b, the differential voltage of the receiving coils 3a and 3b is The induced voltage is e〓 ₁ −
It is set so that e〓 ₂ =0. 5 is an amplifier that amplifies the differential induced voltage e〓 ₁ −e〓 ₂ ; 6a and 6b are synchronous detectors that detect ferrous and nonferrous metals, respectively; 7a,
7b is a low-pass filter, and 8a and 8b are discrimination circuits. Note that 9a and 9b are the synchronous detectors 6
Fig. 6 shows first and second phase shifters that form synchronization signals to be supplied to terminals a and 6b.

In the metal detector having such a configuration, when an object W to be inspected passes between the transmitter coil 2 and the receiver coils 3a and 3b and metal is mixed in the object W,
A detection signal is generated in the discrimination circuits 8a and 8b depending on the type of metal (ferrous or non-ferrous).

To explain this point using the vector diagrams in Fig. 2 a and b, normally, the induced voltages e〓 ₁ ,
e〓 ₂ is set so that e〓 ₁ −e〓 ₂ = 0 on the input side of the amplifier 5, but the test object W containing iron
passes in the direction of the arrow, first the induced voltage e〓 ₁ of the receiving coil 3a increases to e〓 ₁ ' as shown in Fig. 2a, and then the induced voltage e〓 ₂ of the receiving coil 3b increases to e〓 ₂ ′ increases. Therefore, the differential induced voltage of e〓 ₁ ′−e〓 ₂ = e〓 _Df is input to the synchronous detector 6a, and the signal for synchronous detection of the same phase is supplied to the synchronous detector 6a.
e〓 Detected by _F.

On the other hand, when the inspected object W containing non-ferrous metals (stainless steel, aluminum, etc.) passes through, eddy currents flow in the non-ferrous metals under the influence of the alternating current magnetic field of the oscillator 1. Then, due to the influence of this eddy current, the phases of the induced voltages e〓 ₁ , e〓 ₂ of the receiving coils 3a, 3b change.

That is, as shown in Fig. 2b, when the phase of the induced voltage e〓 ₁ of the receiving coil 3a changes to e〓 1 '', the differential induced voltage e〓 _{1 '} ' - e 〓 ₂ = e〓 _DS becomes Almost like I did
Occurs at a point with a 90° phase shift. Therefore, non-ferrous metals can be detected by performing phase detection in the synchronous detector 6b, which is supplied with a signal for synchronous detection indicated by e〓 _S , which is approximately the same as the differential induced voltage e〓 _DS . (When the induced voltage e〓 ₂ changes to e〓 ₂ ″, it can be detected for the same reason.) As mentioned above, the conventional metal detector uses the transmitter coil 2 and the receiver coils 3a and 3b to detect It was used to form a transformer and detect ferrous or non-ferrous materials, but
The receiving coils 3a and 3b need to have an opening area that can cover the detection range of the object W to be inspected.

For example, if the object to be inspected is ham containing salt, a signal similar to non-ferrous metal may be output due to its physical properties, and the degree to which it is susceptible to this physical property depends on the opening of the coil. Since it is proportional to the area occupied by the object to be inspected within the plane, the detection ability is limited, and there is a problem that the detection sensitivity is reduced.

In addition, when a plurality of such metal detectors are arranged in parallel, or depending on the packaging state of the object to be inspected,
It is necessary to change the frequency of the alternating magnetic field applied to the object to be inspected.

However, when the frequency of the alternating magnetic field is changed, it is necessary to readjust the number of windings of the receiving coil and the resonance frequency to minimize the detection error, but such adjustment is a tedious task and at the same time Since the detection characteristics change due to the influence of the stray capacity of the coil, there has been a problem in that adjustment work to obtain accurate detection signals becomes extremely difficult.

This invention was made in view of the above circumstances, and it simplifies the detection circuit by employing a Hall element for detecting ferrous and non-ferrous materials, and also simplifies the adjustment work even when the frequency of the alternating magnetic field is changed. The aim is to provide a metal detector that is rarely needed.

An embodiment of the present invention will be described below with reference to the schematic diagram of FIG.

In this figure, 10 is an alternating magnetic field generator formed by a transmitting coil that generates an alternating magnetic field;
Hall elements a, 11b, 12a, and 12b are provided to detect the strength of the magnetic field. 12a and 12b are assumed to detect non-ferrous metals (non-magnetic metals). Note that one pair of Hall elements 11a and 11b (first Hall elements) and the other pair of Hall elements 12
As will be described later, a and 12b (second Hall elements) can also be constituted by a Hall element group in which a plurality of Hall elements are arranged in parallel or in series.

13 is an AC source that supplies an alternating current to the alternating magnetic field generator 10 and the Hall elements 11a, 11b, 12a, 12b;
Phase shifters 14 and 15 are provided to 2a and 12b so that bias currents having a constant phase relationship with respect to the alternating magnetic field are supplied as will be described later.

16A and 16B are a pair of Hall elements 11a and 1
Output voltage of 1b, 12a, 12b (Hall voltage)
17A and 17B are low-pass filters, 18
Comparators A and 18B have reference power supplies V _r1 and V _r2 as input signals, and control circuits 19A and 19B output control signals.

Note that the Hall elements 11a, 11b, 12
As is well known, a and 12b are sensors in which a Ho voltage V _H is induced depending on the strength of the magnetic field B, and as shown in FIG. 4, the bias current i(t)
When a magnetic field _B (t ₎ is applied when _{. H} is generated.

Next, the metal detection operation in this embodiment will be explained with reference to the waveform diagrams in FIGS. 5a and 5b.

Magnetic field B ₀ (t) generated from the alternating magnetic field generator 10
On the other hand, the bias current of the pair of Hall elements 11a and 11b is I ₀ (t), which is in phase with the magnetic field B ₀ (t),
If the configuration is such that V _H0 is supplied from each Hall element 11a and 11b, V _H0
An output voltage (Hall voltage) with a waveform (solid line) of _H0 is output. Therefore, operational amplifier 1
The output of 6B is 0.

In this state, when the inspected object W containing iron advances in the direction of the arrow, the lines of magnetic force generated from the alternating magnetic field generator 10 concentrate on the iron mixed in the inspected object W. First, the number of lines of magnetic force extending to the Hall element 11a increases, and then the number of lines of magnetic force extending to the Hall element 11b increases. Then, since the magnetic field B ₀ (t) applied to each Hall element 11a, 11b changes as shown by the dashed line and the dotted line, the output voltages V _H0 and _H0 have a time lag from each other. If the signal output from the operational amplifier 16B is filtered by the low-pass filter 17B, the iron detection signal will be increased or decreased.
D _f is obtained.

Therefore, this detection signal D _f is compared with the reference power supply V _r2 by the comparator 18B, and if the value is above a certain value, the control circuit 19B is operated to detect the object W to be inspected.
The control signal may be used as the control signal.

Above, we have explained the case where the mixed metal is iron, but when the inspected object W contains non-ferrous metal, 1
It is detected by a pair of Hall elements 12a and 12b.

Detection of non-ferrous metals will be described below with reference to the waveform diagram in FIG. 5b.

It is generally known that when non-ferrous metal passes through an alternating magnetic field, the phase of the magnetic field changes due to eddy currents generated within the non-ferrous metal. Therefore, the magnetic field B ₀ (t)
Non-ferrous metals can be detected by detecting the phase change.

Therefore, in order to detect the phase of the magnetic field B ₀ (t), the Hall element 12a is equipped with the phase of the magnetic field B ₀ (t).
supplying a bias current i ₉₀ (t) with a 90° phase shift;
A current of ₉₀ () of opposite phase is supplied to the Hall element 12b.

At this time, the output voltages V _H90 and _H90 of the respective Hall elements 12a and 12b are as shown in the waveform diagram, so the average value of the summed outputs is zero.

However, as described above, when the inspected object W containing non-ferrous metal passes through an alternating magnetic field, the magnetic field B ₀ (t)
As shown by the dotted line, the phase of the magnetic field B ₀ (t) that affects the Hall element 12a changes in the first half, and then the phase of the magnetic field B 0 (t) that affects the Hall element 12b changes in the _second half. The phase of t) changes. Therefore, the output voltage of each Hall element 12a, 12b
Since the phase of the detection signal V _H(90- 〓 ₎ and _H(90- 〓 ₎ changes in the first half and second half as shown by the dotted line, the output of the low-pass filter 17A outputs the average value from the signal obtained by adding these signals. is output as a detection signal D _S.

This detection signal D _S is compared with the reference power supply V _r1 , and if the value is above a certain value, it is supplied to the control circuit 19A as a signal indicating non-ferrous metal, and becomes a control signal for the object W to be inspected.

Note that the detection sensitivity of non-ferrous materials improves as the frequency of the alternating magnetic field increases, but in the case of this invention, the Hall element itself has the function of detecting the phase change of the magnetic field at the same time as detecting the magnetic field, so the detection sensitivity of the alternating magnetic field increases. Even if the frequency is changed, almost no adjustment is required, and there is no difference in detection sensitivity even if the frequency is changed.

The Hall elements 11a, 11b for detecting iron and the Hall elements 12a, 12b for detecting non-ferrous metals may be used separately, but as shown in FIG. S and the non-ferrous Hall elements F may be arranged sequentially or may be arranged alternately at the same position.

In addition, since it is easy to form a plurality of such Hall element groups on the same semiconductor substrate using IC technology, their circuit wiring is easy, especially when a thin film of magnetic material is formed on the top or back surface of each Hall element. Forming a magnetic field, etc. has the effect of increasing the focusing effect of magnetic lines of force and increasing the detection sensitivity.

Furthermore, since the shape of the Hall element itself is small, there is an advantage that the influence of the physical properties of the object W to be inspected and the influence of a small metal mixed therein can be distinguished with high sensitivity.

As explained above, the metal detector of the present invention includes an alternating magnetic field generator that generates an alternating magnetic field, and two or more pairs to which a bias current having the same frequency as the alternating magnetic field generator is supplied. The Hall element is configured to detect iron and non-ferrous metals contained in the non-inspection object.
This has the advantage that a detection function is added to the Hall element itself, eliminating the need for a synchronous detector. Therefore, even if the frequency of the alternating magnetic field changes, the same sensitivity characteristics can be obtained, making adjustment operations easier. Furthermore, since it becomes easy to arrange a plurality of Hall elements corresponding to a small area of the object to be inspected, the effect of the physical properties of the object to be inspected can be reduced and the detection sensitivity can be increased.

[Brief explanation of drawings]

Figure 1 is a schematic diagram of the configuration of a conventional metal detector, Figure 2
3 is a block diagram of the metal detector according to the present invention, FIG. 4 is a diagram illustrating the operation of the Hall element, and FIG. 5 is a waveform explaining the operation of the metal detector according to the present invention. FIG. 6 is a diagram showing an example of the arrangement of Hall elements. In the figure, 10 is an alternating magnetic field generator, 11a, 11
b, 12a, 12b are Hall elements, 13 is an AC source, 16A, 16B are operational amplifiers, 17A, 17
B indicates a low-pass filter, and 18A and 18B indicate comparison circuits.

Claims (1)

[Claims] 1. In a metal detector that uses an alternating magnetic field to detect metal mixed in an object to be inspected, generating an alternating magnetic field that generates a required alternating magnetic field; at least a pair of first Hall elements and a second Hall element arranged sequentially in the traveling direction of the inspection object; supplying an alternating signal having the same frequency as the alternating magnetic field to the pair of first Hall elements; an AC signal source that supplies the second Hall element with an alternating signal that is shifted by a predetermined phase with respect to the alternating signal applied to the first Hall element; a first operational amplifier that outputs a difference signal between the detection signals obtained from the pair of second Hall elements; and a second operational amplifier that outputs a difference signal between the detection signals obtained from the pair of second Hall elements; first and second filters that smooth the output of the operational amplifier, respectively; first and second filters that compare the outputs of the first and second filters with a reference value, respectively, to detect ferrous and non-ferrous metals; A metal detector characterized in that it is equipped with a comparison circuit.