JPH07218645A - Metal detection method - Google Patents

Abstract

(57) [Abstract] [Purpose] A metal detection method for non-contact detection of conductors such as metals. [Structure] An exciting coil connected to a pulse generator, a receiving coil, and a receiving coil voltage amplifier having a switch circuit. A pulse current is passed through the exciting coil.
An eddy current is caused to flow through the detection conductor by induction, and the terminal voltage of the detection coil is detected by the receiver that has a switch that operates when the energy remaining in the excitation coil is sufficiently attenuated, and only the eddy current flowing in the detection conductor is detected. Metal detection method using a metal detection device for selectively detecting

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is a highly sensitive metal detecting device and is used for detecting the position of a metal in a proximity switch or an invisible portion, and for determining the type of a detected metal.

[0002]

2. Description of the Related Art As a method for detecting metal, a change in magnetic field,
Some of them utilize electromagnetic waves, ultrasonic waves, X-rays, etc., and each has its own characteristics. The one using the electromagnetic wave is the one using the radar method and the change in the equivalent impedance of the detection coil due to the change in the eddy current and the magnetic permeability. The radar system has a drawback in that the cost of the device is high and that when detecting a buried object, it is affected by geology and water content. The method of detecting changes in eddy current and magnetic permeability is characterized by being relatively unaffected by geology when the signal frequency for detection is lowered. The method of utilizing the change of the equivalent impedance of the detection coil is generally a method of connecting two coils differentially or increasing the sensitivity by a bridge circuit, and the sensitivity and directivity were not sufficient. As a method for eliminating these drawbacks, Japanese Patent Laid-Open No. 5-232
The method of utilizing the change of the transient phenomenon due to the interaction between the exciting coil and the detecting conductor when the current of the magnetic field generating coil is cut off as in 245 can also use the exciting coil and the receiving coil relatively, It has become possible to improve directivity and sensitivity. However, this detection method has a feature that the detection signal contains many frequency components and contains a lot of information.
It was difficult to make full use of this information without using it with a frequency analyzer such as T. Further, if the comparator detects a coil voltage change due to a composite transient phenomenon of the exciting coil and the detection conductor, it has a drawback that it is very susceptible to noise. Further, the reflected waveform in the patent specification of JP-A-5-232245 is not a reflected wave from the detection conductor, but a transient phenomenon which is a combination of the excitation coil and the detection conductor is observed, and therefore it depends on the energy remaining in the excitation coil. When the voltage is applied during reception and the detection distance is large, even if the coil voltage is clamped with a diode, it is orders of magnitude greater than the received signal, and there is a limit to increasing the sensitivity of the receiver, or the constant change of the excitation coil. There was also a problem affected by.

[0003]

As described above, the conventional detection method is unavoidably affected by the constants of the exciting coil and the detecting coil, which has been a major obstacle in increasing the detection sensitivity. The present invention utilizes the difference between the time constant of the exciting coil and the time constant of the detecting conductor to detect only the signal propagating from the detecting conductor with high sensitivity and reduce the influence of the constant of the exciting coil, and at the same time, the detecting distance. Is intended to be significantly improved.

[0004]

When the exciting current is passed through the exciting coil to generate the pulse magnetic field, the current also flows through the detection conductor in the magnetic field by induction. During the period when the current is flowing in the exciting coil, a current called eddy current which is substantially proportional to the current flowing in the exciting coil also flows in the detection conductor. When the current flowing in the exciting coil is rapidly cut off, the current flowing in the detecting conductor is Te, where the equivalent inductance of the current loop flowing in the detecting conductor is Le and the equivalent resistance is Re, Te = Le / Re. Decays at. Therefore, if the current cutoff time of the exciting coil is cut off with a fall time sufficiently shorter than Te, and the energy accumulated in the exciting coil is sufficiently attenuated and then received by the detecting coil, the current flowing in the detecting conductor is caused. The presence of the detection conductor can be detected by selectively detecting only the generated magnetic field. Here, if the operation of the receiver is cut off by the switch circuit so that no signal is received while energy is being accumulated in the exciting coil, the only signal that enters the receiver is the weak signal from the detection conductor, Even a receiver with high sensitivity can receive without saturation. In this case, the input voltage of the receiver when the excitation coil and the detection coil are shared has a peak in the same direction as the direction of the current flowing in the excitation coil, and has a waveform that exponentially decays according to the equivalent time constant, Unlike the input voltage of the receiver described in the specification of Japanese Patent Laid-Open No. 5-232245, the waveform is not only reversed in polarity but is basically not affected by the constant of the exciting coil at all.

[0005]

EXAMPLE An exciting coil is formed by using a thin wire so as to reduce the eddy current flowing on the surface of the winding, or by using a stranded wire in which thin wires are bundled when the energized current is large. When the number of windings is large, a winding method such as honeycomb winding is used to avoid unnecessary resonance due to the distributed capacitance. A pulse voltage is applied to this winding by a switch such as a semiconductor switch. The pulse width depends on the desired exciting current and voltage source, and the pulse width may be narrow if the voltage of the voltage source is high. A resistor for rapidly releasing the energy accumulated in the exciting coil when the switch for applying the exciting voltage is released is connected in parallel to the exciting coil. The shorter the time constant of the parallel resistance and the exciting coil is, the better, and the equivalent time constant Te of the detection conductor is on the order of 100 μsec, although it varies depending on the type of the detection conductor, so in practice μsec
To 10 μsec is desirable. The current flowing in this parallel resistor decreases exponentially, but better results are obtained by using non-linear components to decay the exciting current more rapidly. If a current switch that uses a semiconductor is used instead of applying a pulse voltage with a switch, the parallel resistance of the exciting coil is unnecessary, and an exciting current waveform with ideal current attenuation characteristics can be obtained, and the internal resistance of the exciting coil can be obtained. It is possible to flow an accurate exciting current that is not affected by the change of. Also, a switch circuit is inserted between the receiving coil and the receiver, and the switch is released or the input of the receiver is short-circuited only while the current is flowing in the exciting coil. Of course, the present invention can be implemented by inserting a buffer or an amplifier while the receiving coil is connected to the switch, or by using an amplifier having a switch function. Depending on the amount of switching noise generated by the switching, the amplifier may be provided in the preceding stage of the switch. May also produce good results. This switch is conveniently a semiconductor switch, and if the coil voltage is high, an appropriate protection circuit is also required. The input impedance of the receiver is set to a high value so as not to interfere with the attenuation of the exciting coil current. When using a semiconductor switch, etc., if there is leakage in the switch and the release is not sufficient, short the input of the receiver only while the switch is released, or connect a circuit in cascade to the exciting coil. The point is to completely cut off the signal during the current flow. Masking unwanted signals in this manner allows the receiver to utilize high gain amplifiers. The detection conductor can be detected by directly monitoring the output of the amplifier directly with an oscilloscope or the like, and the material of the detection conductor can be known from the attenuation characteristic of the detected voltage. It is also possible to repeatedly generate a current flowing through the exciting coil and obtain a DC output according to the detection distance by a synchronous rectification method synchronized with the signal of the exciting coil. The amplifier may be amplified by an ordinary amplifier, but if an amplifier having a logarithmic characteristic is used, it is possible to perform measurement for a distance from a short distance to a long distance without saturating the amplifier. It is also possible to share the excitation coil and the receiving coil, but it is also possible to estimate the position and size of the detection conductor by providing the receiving coil independently or by providing multiple receiving coils in an array and connecting similar receivers to each. Become. When an iron core or a ferrite core with little loss is used for the exciting coil and the receiving coil, it is possible to obtain a detector having sharp directivity and high sensitivity.

[0006]

The greatest feature of the effect of the present invention is that, as described in the operation, the receiver is operated by the switch after the energy remaining in the exciting coil is sufficiently attenuated, unlike in JP-A-5-232245. As a result, the only signal that enters the receiver is the weak signal generated by the eddy current of the detection conductor that is being detected. Therefore, since the signal due to excitation is completely masked, the sensitivity of the receiver can be greatly increased, and the sensitivity of the excitation circuit is extremely insensitive, so the detection sensitivity can be greatly increased. is there. Also, the signal due to excitation is masked,
Since the sensitivity of the receiver and the exciting power can be set independently, it is possible to increase the exciting power to increase the detection distance. When the exciting coil and the receiving coil are the same, the peak value of the detection voltage of the receiving coil is proportional to the squared value of the equivalent coupling coefficient of the exciting coil and the detecting conductor, so if the material and shape of the detecting conductor are constant, Also, it becomes possible to measure the distance from the exciting coil to the detecting conductor by the received voltage. Furthermore, since the received voltage waveform is determined by the equivalent time constant of the detection conductor to be detected, it is possible to easily determine the material of the detection conductor. The present invention can realize an extremely high-sensitivity metal detector which has not been obtained in the past, and can accurately detect a metal separated by 30 cm or more even with a small exciting coil having a diameter of several centimeters without an iron core. If a coil is used or a large exciting coil is used, metal detection of 1 m or more is possible, and many applications are possible.

Claims (1)

[Claims] 1. A pulsed magnetic field is generated by passing a pulsed current through an exciting coil, and an induced current is caused to flow through the detecting conductor through mutual inductance with the detecting conductor to be detected by the pulsed magnetic field, and the induced current is received. In the metal detection device that receives by, the exciting circuit that cuts off the exciting current with a fall time that is significantly smaller than the equivalent time constant of the detecting conductor, and the voltage of the detecting coil is changed only while the energy remaining in the exciting coil is sufficiently attenuated. A metal detection method that selectively detects only signals due to eddy currents flowing in conductors by a receiver circuit that has a switch for detection.