JP2007278719A - Metal detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the reduction in metal detection sensitivity of an object to be inspected itself, with respect to either iron or nonferrous metal. <P>SOLUTION: The metal detector comprises a driving coil 5 used for occurrence of a magnetic field, one pair of detecting coils for detecting the magnetic field varied by the metal in the object to be inspected, and a differential amplifying circuit for outputting a differential component by the detecting coils. An electric power conversion transformer 3, for supplying driving power to the drive coil 5, is provided with primary winding having an input tap, secondary winding for output, and winding connected to a tuning capacitor 9. For responding to two frequencies of high frequency and low frequency, the metal detector has an insulation gate drive circuit 4 and a power MOSFET switch 2 for changing the input tap and changing the capacity of the tuning capacitor, inputs the alternating current signals of high frequency or low frequency into the electric power conversion transformer 3, while repeating two frequency switching, and outputs them to the drive coil 5. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a metal detector that detects a metal foreign object in an object to be inspected, and more particularly to a metal detection apparatus suitable for an application for inspecting the presence or absence of metal foreign object while conveying the object to be inspected on an inspection line such as food.

  A conventional metal detector is shown in FIG. FIG. 4A is a schematic diagram showing the configuration, and FIG. 4B is an electric circuit diagram thereof. As shown in FIG. 4, the drive coil 7 is arranged at the center, the detection coils 6 and 8 are arranged on both sides thereof, and the search coil 41 is formed as a whole. The drive coil 7 generates an alternating magnetic field, and an electromotive force is generated in the two detection coils 6 and 8 by the alternating magnetic field, but the two detection coils 6 and 8 are differentially coupled and cancel each other in a no-signal state. ing.

  When the metal piece is passed through the search coil 41, the magnetic flux is changed by an induced magnetic field if it is an iron-based metal and by an eddy current magnetic field if it is a non-ferrous metal. Magnetic flux changes. The metal detector operates so as to output the electromotive force generated thereby from the differential amplifier 42.

  In addition, food as an object to be inspected is not a metal, and it is usually considered that no magnetic change occurs, but it becomes conductive due to moisture and salt contained in the raw materials, and the detection signal changes like metal, and foreign metal This hinders detection. For this reason, the metal detector uses a phase detection method. When the phase of the inspection object is detected in the X and Y axis directions, the influence of the inspection object can be detected with respect to the phase angle. There is a specific polar coordinate (phase angle θ) for each inspection object. By detecting at a phase angle orthogonal to the polar coordinates specific to the inspection object, the influence of the inspection object can be reduced, and the metallic foreign object Can be detected with high sensitivity (for example, Patent Document 1). FIG. 5 shows the iron sensitivity and the non-ferrous sensitivity in the direction orthogonal to the direction specific to the inspection object in phase detection.

JP-A-5-100047

  The influence of the object to be inspected on the detection of metallic foreign objects may be reduced by reducing the frequency of the drive coil. In this case, it is necessary to perform phase detection on the X and Y axes as described above at high and low frequencies, measure the influence of the object to be inspected and the metal sensitivity, and compare the measurement data. As a result of the measurement, the optimal value of the metal sensitivity should be either high frequency or low frequency. However, depending on the inspection object, the ratio of the metal sensitivity to the inspection object sensitivity (sensitivity of the inspection object itself) Sensitivity may be large at high frequencies and non-ferrous sensitivity (non-ferrous metal sensitivity) may be large at low frequencies.

  FIG. 6 shows an example of the ratio of the metal sensitivity to the inspection object sensitivity. FIG. 6A shows the iron sensitivity and the inspection object sensitivity, and FIG. 6B shows the non-ferrous sensitivity and the inspection object sensitivity. In this example, the ratio of the iron sensitivity to the inspection object sensitivity is large at high frequencies, and the ratio of the non-ferrous sensitivity to the inspection object sensitivity is large at low frequencies. In such a case, when detection is possible only at a fixed frequency, either the ferrous or non-ferrous detection sensitivity is given priority, and the other is sacrificed.

  Under such circumstances, an object of the present invention is to provide a metal detector that suppresses a decrease in metal detection sensitivity due to an object to be inspected for both ferrous and non-ferrous metals.

  The above-described problem is solved by simultaneously detecting two frequencies in the detection method in which one of the high frequency and the low frequency has been selected.

  As the means, switching of the input tap position of the power conversion transformer of the drive coil and switching of the capacitance of the tuning capacitor are configured by an insulated gate drive and a power MOSFET switch, and high-frequency and low-frequency two-frequency switching is performed at high speed. The AC power can be applied to the drive coil.

  The detection signals are input to four phase detectors and processed. Each phase detector is capable of phase detection with the logic H of the control signal. At this time, the control signal corresponding to the high frequency magnetic field is The high-frequency X component or the Y-component phase detection clock whose phase is 90 ° different from that of the high-frequency X component and the two-frequency switching clock are generated by AND operation. Similarly, for a low-frequency magnetic field, a control signal is generated by an AND operation of a low-frequency X component or a Y-component phase detection clock whose phase is 90 ° different from that of the low-frequency magnetic field and the inversion of the two-frequency switching clock. During the logic L period, the detection clock is in an operating state. In this way, the high frequency and low frequency phase detection signals are separated so that the four components of the high frequency X and Y components and the low frequency X and Y components can be detected simultaneously.

  As described above, the metal detector according to the present invention can detect a high-frequency magnetic field and a low-frequency magnetic field almost simultaneously at two frequencies, thereby suppressing a decrease in metal detection sensitivity due to the inspected object itself and improving the metal detection sensitivity. It becomes possible.

  FIG. 1 is a drive circuit diagram of a metal detector according to an embodiment of the present invention. FIG. 3 is a timing diagram showing voltage waveforms at various parts of the metal detector according to one embodiment of the present invention. FIG. 3 (a) is a frequency switching clock, FIG. 3 (b) is a drive waveform of a drive coil, FIG. 3C shows a high frequency detection clock, and FIG. 3D shows a low frequency detection clock.

  As shown in FIG. 1, high frequency and low frequency sine wave signals are input to a frequency selector, and the frequency selector is high at a logic H (H level) of a two-frequency switching clock and low at a logic L (L level). Switch to frequency. The sine wave (FIG. 3B) switched at the logic level of the two-frequency switching clock is input to the power amplifier 1 and drives the power conversion transformer 3.

  At this time, the input tap of the power conversion transformer 3 is switched to the high frequency tap by the logic H of the two-frequency switching clock and to the low frequency tap by the logic L. Similarly, the tuning capacitor capacity is switched to a high frequency tuning capacitor with a logic H of a two-frequency switching clock, and a low frequency tuning capacitor is switched with a logic L to obtain an optimum input tap and tuning, and a drive coil with low power consumption. AC power can be applied to 5.

  The insulated gate drive circuit 4 is used to separate the control system power supply and the high breakdown voltage drive system power supply, and the power MOSFET switch 2 is turned ON by the input logic H of the insulated gate drive circuit 4. Accordingly, the high frequency input tap of the power conversion transformer 3 is turned ON by the logic H of the two frequency switching clock, the switches at both ends of the low frequency tuning capacitor are turned on (shorted), and the switches at both ends of the high frequency tuning capacitor are switched on. Is in an OFF (open) state.

  It is generally considered that the tuning capacitor is normally connected in parallel and a power MOSFET switch is used on one side of the capacitor. However, the power MOSFET has a large capacitance of several thousand pF when it is off, and it can be tuned even when the switch is off. The capacitor is connected in series and the capacitor capacity cannot be ignored. Accordingly, if the capacitance of the tuning capacitor connected to the OFF side is changed even though the switch is in the OFF state, the tuning on the ON side is shifted. In other words, if one tuning capacitor capacity is adjusted and the other is adjusted, the value of the previously adjusted capacitor shifts the tuning. Since the capacity adjustment is complicated as described above, capacitors are connected in series as shown in FIG. The high frequency is about several hundred kHz to 1 MHz, and the low frequency is about several tens kHz to several hundred kHz.

  FIG. 2 is a detection circuit diagram in an embodiment of the present invention. 21 is a high frequency X component phase detector, 22 is a high frequency Y component phase detector, 23 is a low frequency X component phase detector, 24 is a low frequency Y component phase detector, and 25, 26, 27 and 28 are AND circuits. .

  The drive coil and the two detection coils that generate the high-frequency magnetic field and the low-frequency magnetic field in the present embodiment are the same as the configuration of FIG. 4A described as the conventional example. As shown in FIG. 2, the detection signals from the two differentially coupled detection coils are divided into four phase detectors (high frequency X component phase detector 21, high frequency Y component phase detector 22, low frequency X component phase detector 23). The low frequency Y component phase detector 24). At this time, the phase detector is a circuit capable of phase detection with the logic H (H level) of each control signal. This control signal is obtained by inputting a phase detection clock of a high frequency X component or a Y component orthogonal thereto and a two frequency switching clock (frequency switching clock) to the AND circuit. This control signal is in an operating state during the logic H time of the two-frequency switching clock (frequency switching clock), as shown as the high-frequency detection clock in FIG.

  Similarly, the control signal obtained by inputting the low frequency X component or Y component phase detection clock and the inversion of the two frequency switching clock to the AND circuit is as shown in FIG. 3D as the low frequency detection clock. The operation state is entered during the logic L (L level) time of the two-frequency switching clock (frequency switching clock).

  Details will be described with reference to FIG. (1) The high frequency X component detection clock and the frequency switching clock that takes the H level and the L level are ANDed by the AND circuit 25 to become a control signal for the high frequency X component phase detector 21. (2) The high frequency Y component detection clock (with a phase difference of 90 ° from the X component) and the frequency switching clock are subjected to AND operation by the AND circuit 26 and become a control signal for the high frequency Y component phase detector 22. Further, (3) the low frequency X component detection clock and the inverted signal of the frequency switching clock are subjected to an AND operation by the AND circuit 27 and become a control signal for the low frequency X component phase detector 23. (4) The AND circuit 28 performs AND operation on the low frequency Y component detection clock (the phase is 90 ° different from the X component) and the inverted signal of the frequency switching clock, and the low frequency Y component phase detector 24 is controlled. Signal. As described above, the high frequency X component phase detector and the high frequency Y component phase detector operate at the H level of the frequency switching clock (two frequency switching clock) that takes the H level and the L level, and low at the L level of the two frequency switching clock. The frequency X component phase detector and the low frequency Y component phase detector operate.

  In this way, high-frequency and low-frequency phase detection signals can be separated and X and Y phase detection at two frequencies can be performed. The obtained four detection outputs can pass through an integrator and an effective value circuit, respectively, and be converted into a DC output.

  As described above, the high frequency X and Y outputs and the low frequency X and Y outputs can be taken out simultaneously in parallel, and simultaneous dual frequency detection can be performed.

  Note that detection by switching between two frequencies generates switching noise in the detection signal, so it is necessary to cut the noise with a low-pass filter (LPF), but the noise band is set to a two-frequency switching clock frequency that can be sufficiently attenuated by the LPF. Then, detection becomes possible without any problem. Therefore, the frequency of the two-frequency switching clock is about several hundred Hz to several kHz.

  As described above, by detecting the differential signal by the detection coil that responds to the high frequency magnetic field and the low frequency magnetic field in two frequencies in parallel, the metal detection sensitivity is prevented from being lowered by the object to be inspected, and the metal detection sensitivity is improved. can do.

The drive circuit diagram of the metal detector in one embodiment of this invention. The detection circuit diagram in one embodiment of this invention. FIG. 3 is a timing chart showing voltage waveforms at various parts of the metal detector in one embodiment of the present invention, FIG. 3A is a frequency switching clock, FIG. 3B is a drive waveform of a drive coil, and FIG. FIG. 3C shows a high frequency detection clock, and FIG. 3D shows a low frequency detection clock. The metal detector of a prior art example is shown, Fig.4 (a) is a schematic diagram which shows the structure, FIG.4 (b) is the electrical circuit diagram. The figure which shows the iron sensitivity and non-ferrous sensitivity of a phase detection in the direction orthogonal to the direction intrinsic | native to a to-be-inspected object. FIG. 6A shows an example of the ratio of metal sensitivity to inspection object sensitivity, FIG. 6A shows iron sensitivity and inspection object sensitivity, and FIG. 6B shows non-ferrous sensitivity and inspection object sensitivity.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Power amplifier 2 Power MOSFET switch 3 Power conversion transformer 4 Insulated gate drive circuit 5, 7 Drive coil 6, 8 Detection coil 9 Tuning capacitor 21 High frequency X component phase detector 22 High frequency Y component phase detector 23 Low frequency X component phase detection 24 Low frequency Y component phase detector 25, 26, 27, 28 AND circuit 41 Search coil 42 Differential amplifier

Claims (2)

Metal detection having a drive coil used for generating a magnetic field, a pair of detection coils for detecting the magnetic field changed by the metal in the inspection object, and a differential amplifier circuit for outputting a differential component by the detection coil A vessel,
The power conversion transformer for supplying drive power to the drive coil is provided with a primary winding having an input tap, a secondary winding for output, and a winding connected to a tuning capacitor,
An insulated gate drive circuit and a MOSFET switch for switching the input tap and switching the capacitance of the tuning capacitor are provided to correspond to two frequencies of high frequency and low frequency,
A metal detector, wherein a high frequency or low frequency AC signal is input to the power conversion transformer and output to the drive coil while repeating two-frequency switching. For the differential component signal by the detection coil in response to the high frequency magnetic field and low frequency magnetic field generated by the drive coil,
The duration of the H level by a control signal generated by AND operation of each of the high frequency X component detection clock and the high frequency Y component detection clock whose phase is 90 ° different from the high frequency X component detection clock and the frequency switching clock taking H level and L level. A high-frequency X-component phase detector and a high-frequency Y-component phase detector that perform phase detection therein,
The phase of the low frequency X component detection clock and the low frequency Y component detection clock whose phase is 90 ° different from that of the low frequency X component detection clock and the inverted frequency switching clock during the duration of the L level by the control signal generated by the AND operation. By providing a low frequency X component phase detector and a low frequency Y component phase detector that perform detection,
The phase detection signal in the high frequency magnetic field and the low frequency magnetic field is separated, and the four components of the high frequency X component, the high frequency Y component, the low frequency X component, and the low frequency Y component are detected almost simultaneously in parallel. Item 1. A metal detector according to Item 1.

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