JPH03125989A - Metal detecting machine - Google Patents

Abstract

PURPOSE:To prevent erroneous detection as caused by vibration accurately by providing a vibration signal discriminating means to identify a detection signal depending on whether a detection signal passing through a low-pass filter has polarity component positive or negative within a set time and a signal ignoring means. CONSTITUTION:A frequency low-pass filter 10 removes effect of a carrier with an upper limit frequency being 10Hz, for instance. A vibration signal discriminating means 11 discriminates a received signal leaving an amplifier 5 whether it is to be attributed to vibration or to a foreign metal. A control signal PC acts as signal ignoring means and disables judging circuits 8a and 8b for a specified time length to ignore an input signal. A curve A indicates a received signal generated by a foreign metal as caused by vibration. The curve B has bipolarity while the curve A has a unipolarity mostly. When a time from a positive peak to a negative peak value of the curve B is set at a specified time T1, no peak value appears beyond a negative detection limit LL in the curve A even if the time T1 passes from the positive peak value.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application J This invention relates to a metal detector for detecting the presence or absence of foreign metal in an object to be inspected.

[Conventional technology]

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, and 3a
, 3b are receiving coils arranged opposite to this transmitting coil 2;
They are placed in an alternating magnetic field and arranged so that their magnetic pole lines are equally interlinked. 4a and 4b are the receiving coils 3a
, 3b induced voltage e1. This shows a volume for adjusting the phase and amplitude of ez, which is normally set to 19+-8g=o by adjusting the volumes 4a and 4b. 5 is an amplifier that amplifies the differential induced voltage 6°62, 6a
, 6b are synchronous detectors for detecting ferrous and non-ferrous metals, respectively, and 7a.

7b is an analog filter of about 2 to 20Hz, 8a, 8
b is a discrimination circuit. Note that 9a and 9b are the first synchronous signals that are supplied to the synchronous detectors 6a and 6b. Second
shows a phase shifter. etc.

, are synchronous detection signals.

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

To explain this point using the vector diagrams in FIGS. 6(a) and 6(b), normally the induced voltages e+, ex
is only at the input side of amplifier 5.

・It is set so that -e z = O, but when the inspected object W containing iron passes from the direction of the arrow, first, as shown in FIG. 6(a), the receiving coil 3a is induced. The voltage 61 increases to m', and then the induced voltage of the receiving coil 3b increases to m'2. Therefore, e+−ez
= e. The differential induced voltage of , is input to the synchronous detector 6a,
It is detected only by the same-phase synchronous detection signal supplied to the synchronous detector 6a.

On the other hand, when the inspected object W containing non-ferrous metal (stainless steel, aluminum, etc.) passes through, an eddy current flows in the non-ferrous metal under the influence of the alternating current magnetic field of the oscillator 1. Then, due to the influence of this eddy current, the induced voltage e1. of the receiving coils 3a, 3b increases.
The phase of ez will change.

That is, as shown in FIG. 6(b), the receiving coil 3a
When the phase of the induced voltage changes to 6'''1, the differential induced voltage e''*e z = e. 8 occurs at a point out of phase with 90', as shown. So, look at this differential induced voltage. , non-ferrous metals can be detected by phase detection in the synchronous detector 6b supplied with the synchronous detection signal 63 which is almost in phase with .

[Problems to be Solved by the Invention] In the metal detector described above, when vibrations such as floor vibrations caused by the belt for conveying the object to be inspected are applied to the detection part, a detection signal is generated in the receiving coil, which causes an error. There was a problem with detection.

This invention was made to solve the above problems, and an object thereof is to provide a metal detector that does not malfunction due to vibration.

[Means for Solving the Problems J] The metal detector according to the present invention includes a high-frequency low-pass filter that inputs a detection signal obtained to a receiving coil, and a detection signal that passes through this low-pass filter within a predetermined time. vibration signal discriminating means for detecting whether or not the detection signal has positive and negative polarity components, and determining that the detection signal is a detection signal caused by vibration when the detection signal does not have positive and negative polarity components; The detection signal is provided with a signal ignoring means for ignoring a detection signal that is inputted over time.

[Operation J] In this invention, when a detection signal is received, if a peak value of the opposite polarity does not appear within a predetermined time from the first peak value, the detection signal is ignored for a certain period of time as a detection signal due to vibration.

〔Example〕

FIG. 1 is a block diagram showing one embodiment of the present invention.

In this figure, numerals 1 to 9a and 9b are the same as those in FIG. 5, and 10 is a frequency low-pass filter whose upper limit frequency is, for example, 10 Hz, and is provided to remove the influence of the carrier wave. Reference numeral 11 denotes vibration signal discriminating means, which determines whether the received signal output from the amplifier 5 is caused by vibration or foreign metal. The PC is a control signal and serves as a signal ignoring means, which causes the discrimination circuits 8a and 8b to be inactive for a predetermined period of time to ignore the input signal.

The operation of the embodiment shown in FIG. 1 will be explained with reference to FIGS. 2 to 4.

Curve A shown in the second factor is the received signal (
curve B is the received signal generated by the foreign metal. Curve B has bipolar characteristics (bilateral characteristics), whereas curve A shows almost unipolarity (bilateral characteristics).

As shown in FIG. 2, the time from the positive peak value to the negative peak value of curve B is determined in advance by a predetermined time T.

As shown in FIG. 3, in the case of curve A, no peak value exceeding the negative detection limit LL appears even after a predetermined time T has elapsed since the positive peak value. Note that LH indicates a positive detection limit, and L indicates a reference level. Furthermore, the detection limit LL may be determined by having the opposite polarity of the percentage of the peak value of the curve A.

From the above, it is possible to distinguish between the detection signal due to vibration and the detection signal due to foreign metal. The vibration signal discrimination means 11 in FIG. 1 has the above-mentioned functions, but in reality, it is activated by a peak detection circuit for an input signal, and a reverse polarity signal of a certain value or more is input within a predetermined time T. It can be configured with a timer circuit that is reset if the control signal PC is input, and outputs the control signal PC if the control signal PC is not input.

Alternatively, the determination may be made by the CPU.

When it is determined that the detection signal is caused by vibration, the vibration signal discrimination means 11 issues a control signal PC, which is applied to the discrimination circuits 8a and 8b to ignore the detection signal for a certain period of time. That is, this fixed time T, (see Figure 4)
Even if a large detection signal is detected, it will not be considered as a detection signal of foreign metal. The fixed time T (see FIG. 4) is, for example, 100 m5. Further, the predetermined time T is, for example, 50 m5.

FIG. 4 shows a waveform diagram when the signal is ignored.

Curve C is the result of ignoring the detection signal due to vibration. In these processes, the sampled value of the detection signal is stored in memory and processed, and the detection signal caused by vibration and the detection signal caused by foreign metal are distinguished.The detection signal shown in curve C is ignored to prevent malfunction. To prevent.

In the above embodiment, the control signal Pc is used as the signal ignoring means.
In the above example, a gate is separately provided, and this gate may be closed by the control signal Pc to prevent the detection signal from passing through.

[Effects of the Invention] As explained in detail above, the present invention includes a high-frequency low-pass filter that inputs a detection signal obtained to a receiving coil, and a detection signal that has passed through this low-pass filter and changes the polarity between positive and negative within a predetermined time. vibration signal discriminating means, which detects whether the detection signal has a positive or negative polarity component, and determines that the detection signal is a detection signal due to vibration when it does not have a positive or negative polarity component; Since the present invention is provided with signal ignoring means for ignoring detection signals caused by vibrations, false detection due to vibration can be reliably prevented and metal detection can be performed with high precision, which is an excellent effect.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a block diagram showing an embodiment of the present invention, FIGS. 2, 3, and 4 are waveform diagrams for explaining the operation of the embodiment of FIG. FIG. 5 is a block diagram showing an example of a conventional metal detector, and FIGS. 6(a) to 6(b) are block diagrams for explaining the detection operation of FIG. 5. In the figure, 1 is an oscillator, 2 is a transmitting coil, and 3a. 3b is a receiving coil, 4a, 4b are volumes, 5 is an amplifier, 6a, 6b are synchronous detectors, 7a, 7b are analog filters, 8a, 8b are discrimination circuits, 9a, 9b are first... Second
10 is a frequency low-pass filter, 11 is a vibration signal discrimination means, Pc 1 (- 5 (a) (b)

Claims (1)

[Claims] The object to be inspected is passed between a transmitting coil that generates an alternating magnetic field and a receiving coil that receives this alternating magnetic field, and a discrimination circuit discriminates the magnitude of the detection signal obtained from the receiving coil to detect foreign metal. The metal detector includes a high-frequency low-pass filter that inputs the detection signal obtained to the receiving coil, and detects whether or not the detection signal that has passed through the low-pass filter has positive and negative polarity components within a predetermined time; vibration signal discrimination means for determining that the detection signal is a detection signal due to vibration when it does not have positive or negative polarity components; and signal ignoring means for ignoring the detection signal input for a certain period of time when the detection signal due to vibration is detected. A metal detector characterized by comprising: