JP2008232999A - Loop coil type metal body detector - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enable exact detection even under a high temperature environment by arranging a reference loop coil in an installation zone of a measurement loop coil, and correcting the impedance measuring error by heat, based on the measured value of the reference loop coil. <P>SOLUTION: This loop coil type metal body detector comprises: the measurement loop coil 1 whose inductance is varied by approach of the metal body; the reference loop coil 1a whose inductance is varied in response to the temperature of the measurement loop coil 1 installation zone; an arithmetic circuit 6 that calculates the difference between the oscillation frequency corresponding to the impedance of the measurement loop coil 1 and the oscillation frequency corresponding to the impedance of the reference loop coil 1a, and corrects the impedance error by temperature variation from the calculation value; and a determining circuit 7 for determining whether the oscillation frequency detected by the arithmetic circuit 6 is within a defined range. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a loop coil type metal object detection device that detects a metal object under an environment where temperature changes.

In general, a machine-managed parking lot employs a method in which a loop coil type sensor 31 embedded in the ground detects the presence or absence of a parked vehicle 30 that is a metal object, as shown in FIG.
In the metal object detection device using the loop coil type sensor 31, an oscillation circuit including the loop coil as a part of the resonance circuit is configured. In a steady state, the oscillation operation is performed at a constant resonance frequency determined by the impedance including the inductance of the loop coil and the capacitance of the capacitor provided in the resonance circuit. When the metal object approaches the loop coil, the inductance of the loop coil changes, so the oscillation frequency and the crest voltage also change. Therefore, the presence or absence of a metal object, that is, a vehicle can be detected by detecting the change in the oscillation frequency or the change in the crest voltage.

In the metal object detection device using the loop coil type sensor, the loop coil itself that is a sensor can be embedded in the ground, so that it is not affected by the external environment and does not interfere with the movement of the vehicle. . Moreover, even when arrange | positioning on the ground, since the thickness of a loop coil is thin, the effect similar to the case where it embeds in the ground can be acquired.
A technique described in Japanese Patent Application Laid-Open No. 6-236498 is known as a technique related to such a loop coil type metal object detection device.
JP-A-6-236498

By the way, the impedance of the loop coil fluctuates due to a change in temperature such as the air temperature, and the frequency fluctuation accompanying such an environmental change causes variations in the reference value.
The difference in the amplitude of the output of the oscillation circuit, which should originally be constant, periodically fluctuates in a period corresponding to the frequency component. For this reason, it becomes impossible to accurately detect the oscillation frequency and the peak value, and it becomes impossible to accurately detect the position of the metal object.
On the other hand, in the manufacturing process of machine parts and the like, a heating furnace is used to perform heat treatment such as sintering on a metal material (object to be heated). In this heating furnace, while conveying the metal material (object to be heated) It is the structure which performs a sintering process.
In the heating furnace, the object to be heated is stopped at a specified position, and after a specified time has passed, sintering and cooling are performed while moving to the next position.
The position of the object to be heated in the heating furnace is confirmed by a photoelectric tube sensor, and the stop / movement of the object to be heated is automatically controlled. In order to prevent malfunction and damage due to heat, the photoelectric tube sensor Therefore, it is necessary to constantly cool the apparatus by injecting air, and there is a problem that the apparatus becomes complicated.
The present invention has been made to solve the conventional problems, and the object of the present invention is to place a reference loop coil in the installation area of the measurement loop coil, and based on the measured value of the reference loop coil. An object of the present invention is to provide a loop coil type metal object detection device that corrects an impedance measurement error due to heat and enables accurate detection even in a high heat environment.

  As a means for achieving the above object, in the loop coil type metal object detection device according to claim 1, the measurement loop coil whose inductance changes due to the approach of the metal object, and the temperature of the measurement loop coil installation area are supported. Calculate the difference between the reference loop coil whose impedance changes, the oscillation frequency corresponding to the impedance of the measurement loop coil and the oscillation frequency corresponding to the impedance of the reference loop coil, and correct the impedance error due to temperature change from the calculated value An arithmetic circuit and a determination circuit for determining whether or not the oscillation frequency detected by the arithmetic circuit is within a specified range are provided.

  3. The loop coil type metal object detection device according to claim 2, wherein the inductance of the metal object conveyed along the conveyance path in the heating furnace and the inductance changes depending on the approach of the metal object to a specific position of the conveyance path. And the difference between the oscillation frequency corresponding to the impedance of the measurement loop coil and the oscillation frequency corresponding to the impedance of the reference loop coil is calculated. And an arithmetic circuit that corrects an impedance error due to a temperature change from the calculated value, and a determination circuit that determines whether or not the oscillation frequency detected by the arithmetic circuit is within a specified range.

  In the loop coil type metal object detection device according to claim 3, in the loop coil type metal object detection device according to claim 1 or 2, measurement loop coils and reference loop coils are installed at a plurality of locations, and each of the loop coils is detected. A sequencer for selectively operating the oscillation sequentially is provided.

  The loop coil type metal object detection device according to claim 4, wherein the power failure detection device detects a power failure during operation of the loop coil type metal object detection device according to any one of claims 1 to 3, and the loop coil at the time of the power failure A loop coil constant storage circuit for storing the constants is provided, and the value stored in the loop coil constant storage circuit is collated when the power is restored to operate in the state immediately before the power failure.

In this invention, the following effects are acquired by employ | adopting the said structure.
The difference between the oscillation frequency corresponding to the impedance of the measurement loop coil and the oscillation frequency corresponding to the impedance of the reference loop coil is calculated, and the impedance error due to temperature change is corrected from the calculated value. Detection is possible.
In addition, since a metal object measurement loop coil and a reference loop coil are installed at a plurality of locations, and a sequencer that selectively operates the oscillation of each loop coil is provided, the resonance frequency of the resonance circuit including the loop coil being measured Is not affected by the adjacent loop coil. In addition, since a plurality of oscillation circuits do not operate simultaneously, a difference component between the oscillation frequencies of the oscillation circuits does not occur and the waveform of the oscillation output is not disturbed. Accordingly, the position of the metal object can be accurately detected by accurately detecting the oscillation frequency and the peak value.
Further, since the loop coil constant storage circuit for storing the constants of the loop coil at the time of power failure is provided, the value stored in the loop coil constant storage circuit at the time of power recovery can be collated to start the operation immediately before the power failure.

  The best mode for carrying out the present invention will be described.

First, the overall configuration will be described.
As shown in FIGS. 1 (a) and 1 (b), the loop coil type metal object detection device of the present invention is a heating furnace 20 for heat-treating a metal material, and a drive as a conveying means laid in the heating furnace 20. Type roller conveyor 21, heated object 23 placed on a metal storage box 22 on the conveyor 21 and conveyed, and disposed at a specific position in the heating furnace 20 between the rollers of the roller conveyor 21. A measurement loop coil 1 that detects the approach of the storage box 22 (metal object), and a reference loop that is disposed close to the lower side of the measurement loop coil 1 to detect impedance according to the temperature of the measurement loop coil installation area. A coil 1a is provided, and an electric circuit to be described later is connected to the measurement loop coil 1 and the reference loop coil 1a. The conveying means is not limited to the drive roller conveyor 21 but can be a hanging conveyor, a push conveyor, or the like.
The heating furnace 20 has a long passage separated by a heat insulating layer, heating means by a combustion burner using an electric heater, gas, liquid fuel, etc. in the furnace, and if necessary, the furnace temperature atmosphere is made uniform. A stirrer fan is installed.
In the heating furnace 20, processing spaces, standby spaces and the like partitioned by an open / close door are continuously arranged, and the roller conveyor 21 is laid through the spaces, and a metal storage box 22 is placed on the conveyor 21. The object to be heated 23 moves on the surface.
The object to be heated 23 is made of various steel materials or aluminum alloy, and the object to be heated 23 passes through the heating furnace 20 together with the metal storage box 22 and is subjected to heat treatment (quenching treatment, tempering treatment, etc.). .
In addition, although the said measurement loop coil 1 is detecting the metal storage box 22, it may replace with this and may detect the said to-be-heated object 23 made of metal.

In FIG. 1 (a), when the metal storage box 22 approaches the measurement loop coil 1 disposed between the rollers of the roller conveyor 21, the inductance of the loop coil 1 changes, and the oscillation frequency corresponds to this change. Changes. The presence or absence of the metal storage box 22 on the loop coil is detected based on the change in the oscillation frequency, and the driving of the roller conveyor 21 is controlled by the control means (not shown) based on the detection information, and the metal storage box 22 is stopped and moved. And so on.
The furnace temperature of the heating furnace 20 reaches 500 ° C. to 1000 ° C., the inductance of the loop coil changes due to heat, and the impedance also changes accordingly. For this reason, the oscillation frequency recognized by the discrimination circuit changes, and erroneous detection is made as to whether or not the metal storage box 22 exists on the loop coil.
Therefore, in the present invention, the reference loop coil 1a is set under the same temperature environment as the measurement loop coil 1, and the reference impedance and the corresponding oscillation frequency value are ensured under the same temperature environment.
Therefore, the difference between the oscillation frequency corresponding to the impedance of the measurement loop coil 1 and the oscillation frequency corresponding to the impedance of the reference loop coil 1a can be calculated, and the inductance error due to temperature change can be corrected from the calculated value. Yes.
This reference loop coil 1a is installed on the wall surface, ceiling surface, etc. of the same space in the heating furnace in which the measurement loop coil 1 is arranged to detect the temperature environment. Any position may be used as long as it does not cause an inductance change accompanying the approach of the object. As the arrangement position of the measurement loop coil 1 and the reference loop coil 1a, as shown in FIG. 1 (a), the measurement loop coil 1 is arranged horizontally between the rollers in the heating furnace 20, and the reference is located below the measurement loop coil 1. You may make it arrange | position the loop coil 1a. Further, as indicated by phantom lines in FIG. 1 (b), it may be arranged on the side or upper portion in the heating furnace 20. Further, as indicated by phantom lines in FIG. 1 (c), a reference loop coil 1 a having the same dimensions as the measurement loop coil 1 is provided together with the measurement loop coil 1 in order to make the impedance change conditions with temperature substantially the same. Is arranged at the lower part, the side part or the upper part of the heating furnace 20, and the reference loop coil 1a is opposed to the metal storage box 22 so that the inductance does not change as the metal storage box 22 (metal object) approaches. You may make it arrange | position the shielding board 24 which consists of a metal plate, a punching metal plate, or a metal lattice board so that the side to perform may be covered.

Next, FIG. 2 is an electric circuit diagram of the loop coil type metal object detection device.
The loop coil type metal object detection device of the present invention includes n loop coils constituting the metal storage box 22 measurement loop coil 1 and the reference loop coil 1a, and the inductance of these loop coils 1 and 1a as part of the circuit. N resonance circuits 2 included in the circuit, a common amplifier circuit 3 which constitutes an oscillation circuit in combination with any one of the n resonance circuits 2, and an amplification common to each resonance circuit. N switching circuits 4 provided between the circuit 3, a sequencer 5 for sequentially switching the switching states of the n switching circuits 4 in a predetermined cycle, and a measurement loop coil sent from the amplifier circuit 3 A calculation circuit 6 for calculating a difference between the oscillation frequency of 1 and the frequency of the reference loop coil 1a sent from the amplification circuit 3 and correcting an inductance error due to a temperature change; A discriminating circuit 7 that measures the oscillation frequency and detects whether or not the frequency is within a predetermined frequency range, and controls the n relays 8 based on the detection result in the discriminating circuit 7 and the detection result is serially digitalized. It is connected to a control circuit 10 that outputs to the output terminal 9 in the form of data, a power supply circuit 11 that supplies driving power for the entire apparatus, a power failure detection circuit 12 that detects a power failure of the driving power supply, and a discrimination circuit 7. And a loop coil constant storage circuit 13 for storing the loop coil constant (impedance) at the time of power failure.

  The loop coil 1, the resonance circuit 2 and the switching circuit 4 constitute one channel. In the embodiment of FIG. 2, there are channels # 1 to #n corresponding to n loop coils 1. The n loop coils 1 may be arranged close to each other in a high temperature furnace. In some cases, adjacent loop coils may be arranged in a state of being overlapped by half.

  The resonance circuit 2 includes a transformer 2a whose primary side is connected to the loop coil 1, and capacitors 2b and 2c connected to the secondary side of the transformer 2a. An LC resonance circuit is constituted by the inductance of the loop coil 1 and the capacitances of the capacitors 2b and 2c viewed through the transformer 2a. That is, the resonance circuit 2 includes the loop coil 1 as an element of the resonance system. The midpoint of connection between the capacitor 2b and the capacitor 2c is grounded, and a pair of signal lines L1 and L2 are derived from the connection point between the capacitors 2b and 2c and the transformer 2a. One signal line L1 is connected to the input side of the amplifier circuit 3 via the switching circuit 4, and the other signal line L2 is connected to the output side of the amplifier circuit 3 via the switching circuit 4.

  The switching circuit 4 includes a photoswitch 4a connected in series to the input-side signal line L1, a photoswitch 4b connected in series to the output-side signal line L2, and an input-side signal line L1 and an output-side signal. A photo switch 4c connected in parallel with the line L2. The photoswitches 4a to 4c are bidirectional switching elements whose on / off states are controlled in accordance with a control signal from the outside. For example, bidirectional optical MOSFETs are used. The photoswitches 4a and 4b are of a type that is off in a steady state and is on only when a control signal is applied. The photoswitch 4c is on in a steady state and has a control signal. Use a type that turns off only when is applied.

Next, FIG. 3 shows changes in the oscillation frequency recognized by the arithmetic circuit and the discrimination circuit.
3A, 3B, and 3C show the time-dependent changes in the oscillation frequency of the measurement loop coil and the reference loop coil, with the frequency (H _Z ) on the vertical axis and the time (t) on the horizontal axis. .

FIG. 3A shows changes with time in the oscillation frequencies of the measurement loop coil 1 and the reference loop coil 1a.
When the metal storage box 22 approaches the measurement loop coil 1, the oscillation frequency increases as the inductance changes, and the graph protrudes. When the metal storage box 22 moves away, the oscillation frequency returns to a steady state.
On the other hand, the reference loop coil 1a recognizes the inductance corresponding to the temperature in the measurement loop coil installation area and oscillates the frequency. In FIG. 3A, since the temperature environment is constant, the frequency of the reference loop coil 1a is constant. In the determination circuit 7, the value determination level K is set based on the reference loop coil frequency, and exceeds the determination level K. When the frequency is detected, it is determined that the metal object has approached. When the determination level K is exceeded, the control circuit 10 operates the relay 8 to control the stop / run of the metal storage box 22.
The value of the determination level K is appropriately set according to the installation environment of the loop coil type metal object detection device and the detection target.

FIG. 3B shows a change in frequency of the measurement loop coil and the reference loop coil set in the heating furnace.
In the heating furnace, the impedance of the loop coil varies due to heat, and the inductance also changes due to heat. As a result, the oscillation frequencies of the measurement loop coil and the reference loop coil also change as wavy as shown in FIG.
Here, since the measurement loop coil and the reference loop coil have the same shape, configuration, sensitivity, etc., the reference loop coil placed in the same temperature environment as the measurement loop coil installation area has an impedance corresponding to the temperature change. Recognize and detect the frequency corresponding to the temperature change.
Therefore, as shown in FIG. 3B, the detected value of the frequency fluctuates, but the frequency of the measurement loop coil and the reference loop coil shifts in synchronism, so the frequency of the reference loop coil is subtracted. Thus, as shown in FIG. 3C, a correction value that linearly changes on the graph is calculated.
This correction process is performed by the arithmetic circuit, and the determination after the calculation is performed by the determination circuit as in FIG.
Thus, since the impedance error due to the temperature change can be corrected from the calculated value of the reference loop coil, accurate detection can be performed even in a high heat environment.

The power supply circuit 11 supplies power to the apparatus body, and the power supply circuit 11 is connected to a power failure detection circuit 12 that detects a power failure.
When detecting a power failure, the power failure detection circuit 12 stores the constant (impedance) of each loop coil in the loop coil constant storage circuit.
Since the discriminating circuit recognizes the transmission frequency based on the impedance of each measurement loop coil or each reference loop coil, the loop coil constant storage circuit stores the impedance of each loop coil based on these values.
By storing these impedances, it is recorded whether or not each loop coil has recognized the metal storage box 22 at the time of a power failure.
Therefore, by checking the impedance of the loop coil constant storage circuit when the power is restored, it is possible to accurately determine whether or not the metal storage box 22 exists and to operate again in the state immediately before the power failure.

  The monitor circuit 14 is connected to the determination circuit 7 to monitor whether or not each loop coil is operating normally, and monitors whether control is performed according to the set value of the impedance of each loop coil.

  Next, the operation of the above-described loop coil type metal object detection device will be described.

A metal storage box 22 loaded with an object to be heated 23 is conveyed on the roller conveyor 21 in the heating furnace 20. When the metal storage box 22 reaches a specified position, the inductance of the measurement loop coil 1 disposed between the rollers of the roller conveyor 21 changes to detect the approach of the metal storage box 22.
Here, although the measurement error of the impedance due to heat occurs in the heating furnace, the reference loop coil 1a detects the impedance corresponding to the temperature change in the installation area of the measurement loop coil 1, so that the oscillation frequency corresponding to the impedance of the measurement loop coil And the difference in oscillation frequency corresponding to the impedance of the reference loop coil can be calculated, and the impedance error due to temperature change can be corrected from the calculated value. Therefore, accurate detection is possible even in a high heat environment.
When the determination circuit 7 detects a frequency value greater than the determination level K, the determination circuit 7 determines that the metal storage box 22 has approached, and controls the metal storage box 22 to be stopped.

  Control signals S <b> 1 to Sn are sequentially generated from the sequencer 5 shown in FIG. 2 and supplied to each switching circuit 4. For example, when the control signal S1 is supplied to the switching circuit 4 of channel # 1, the photoswitches 4a and 4b of the switching circuit 4 are turned on and the photoswitch 4c is turned off. Therefore, the resonance circuit 2 is connected between the output side and the input side of the amplifier circuit 3 to constitute an oscillation circuit, and this oscillation circuit has a resonance frequency determined by the capacitance of the resonance circuit 2 and the inductance unique to the loop coil 1, For example, the oscillation operation is started at about 100 kHz and a predetermined peak voltage. When there is no metal object (metal storage box 22) in the vicinity of the loop coil 1, the resonance frequency is mainly determined by the inductance inherent to the loop coil 1 and the capacitances of the capacitors 2b and 2c of the resonance circuit 2. The oscillation output from the amplifying circuit 3 is supplied to the arithmetic circuit 6 and the discriminating circuit 7, and whether or not the oscillation frequency is within a predetermined specified frequency range or whether the crest voltage is within a specified range. Is detected. For example, when a metal object is present in the vicinity of the loop coil 1, the effective inductance and Q of the loop coil 1 change, the oscillation frequency becomes a frequency deviated from the specified frequency, and the peak voltage deviates from the specified range. Therefore, the presence / absence of a metal object can be determined by detecting this frequency shift or peak voltage shift. The specified frequency may be common to each channel, or may be set independently for each channel according to variations in characteristics of each channel.

  At this time, since the control signals S2 to Sn are not supplied to the switching circuits 4 of the other channels # 2 to #n, the photoswitches 4a and 4b of the switching circuits 4 of the other channels # 2 to #n are turned off. It has become. Therefore, with respect to the other channels # 2 to #n, the amplifier circuit 3 and the resonance circuit 2 are separated from each other, and the oscillation operation is not performed. In other words, since a plurality of oscillation circuits do not oscillate simultaneously, unnecessary signals such as beat signals are not generated.

  Further, since the photoswitch 4c is turned on simultaneously with the photoswitches 4a and 4b being turned off, both ends of the capacitors 2b and 2c of the resonance circuit 2 are short-circuited. Therefore, the resonance circuit 2 does not operate as an LC resonance circuit, and no resonance frequency exists. Therefore, it is not affected by the resonance frequency of the loop coil 1 of the adjacent channel # 2 during the measurement of the channel # 1, and accurate detection can be performed.

  The detection of the oscillation frequency in the discrimination circuit 6 is performed for each channel in synchronization with the synchronization signal SW from the sequencer 5, and only the resonance circuit 2 of the selected channel becomes operable, and the resonance circuits of other channels 2 are all inactive. Therefore, it is possible to accurately identify in which channel the metal object is present in the vicinity of the loop coil 1.

  The detection result in the discrimination circuit 7 is supplied to the control circuit 10 as, for example, n-bit parallel data, and the relay 8 of the channel in which the metal object is detected is turned on. The detection results for n channels are output to the data output terminal 9 in the form of serial digital data.

In the embodiment described above, a common amplifier is used for each channel. However, the present invention is not limited to this, and an amplifier 3 is provided for each channel # 1 to #n, and the output of each amplifier 3 is connected via a buffer circuit. It may be combined and supplied to the determination circuit 7. In this configuration, the switching circuit 4 connected between the resonance circuit 2 and the amplifier 3 is provided with a photoswitch connected in parallel between the input-side signal line and the output-side signal line. The photo switch is turned on in a steady state and turned off only when a control signal is applied.
Even in this configuration, the control signals S1 to Sn from the sequencer 5 are supplied to the switching circuit 4 of each channel, as in the embodiment shown in FIG. For example, when the control signal S1 is supplied to the switching circuit 11 of the channel # 1, the photo switch 4a of the switching circuit 4 is turned off. Therefore, the resonance circuit 2 is connected between the output side and the input side of the amplifier circuit 3 to form an oscillation circuit. This oscillation circuit has a resonance frequency determined by the capacitance of the resonance circuit 2 and the inductance unique to the loop coil 1. Starts oscillating operation.

  At this time, since the control signals S2 to Sn are not supplied to the switching circuits 11 of the other channels # 2 to #n, the photoswitch 4a of the switching circuit 4 of the other channels # 2 to #n is turned on. Yes. Therefore, for the other channels # 2 to #n, both ends of the capacitors 2b and 2c of the resonance circuit 2 are short-circuited. Therefore, the resonance circuit 2 does not operate as an LC resonance circuit and no oscillation operation is performed.

Although the embodiments have been described above, the specific configuration of the present invention is not limited to the above-described embodiments, and design changes and the like within a scope not departing from the gist of the invention are included in the present invention.
For example, in the above-described embodiment, the oscillation of the measurement loop coil and the reference loop coil is selectively switched sequentially. However, this switching can be omitted in an installation situation where the oscillation frequency of each loop coil does not interfere. .

(A) It is a schematic block diagram (side view) of a loop coil type metal object detection apparatus. (B) It is AA sectional drawing in Fig.1 (a). (C) It is AA sectional drawing which shows the other example of coil arrangement | positioning. It is an electric circuit diagram of a loop coil type metal object detection device. It is a graph which shows transition of the change of the frequency of a measurement loop coil and a reference loop coil. It is a schematic block diagram of the loop coil type metal object detection apparatus which concerns on a prior art example.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Measurement loop coil 1a Reference | standard loop coil 2 Resonance circuit 2a Transformer 2b, 2c Capacitor 3 Amplifying circuit 4 Switching circuit 4a-4c Photoswitch 5 Sequencer 6 Arithmetic circuit 7 Discriminating circuit 8 Relay 9 Output terminal 10 Control circuit 11 Power supply circuit 12 Power failure detection Device 13 Loop coil constant memory circuit 14 Monitor circuit 20 Heating furnace 21 Roller conveyor 22 Metal storage box (metal object)
23 Heated object 24 Shield plate 30 Parked vehicle 31 Sensor

Claims (4)

A measurement loop coil whose inductance changes as a metal object approaches,
A reference loop coil whose impedance changes corresponding to the temperature of the measurement loop coil installation area;
An arithmetic circuit that calculates a difference between an oscillation frequency corresponding to the impedance of the measurement loop coil and an oscillation frequency corresponding to the impedance of the reference loop coil, and corrects an impedance error due to a temperature change from the calculated value;
A loop coil type metal object detection device comprising a determination circuit for determining whether or not an oscillation frequency detected by the arithmetic circuit is within a specified range. A metal object transported along a transport path in the heating furnace;
A measurement loop coil whose inductance changes due to the approach of a metal object to a specific position in the conveyance path;
A reference loop coil whose impedance changes corresponding to the temperature of the measurement loop coil installation area;
An arithmetic circuit that calculates a difference between an oscillation frequency corresponding to the impedance of the measurement loop coil and an oscillation frequency corresponding to the impedance of the reference loop coil, and corrects an impedance error due to a temperature change from the calculated value;
A loop coil type metal object detection device comprising a determination circuit for determining whether or not an oscillation frequency detected by the arithmetic circuit is within a specified range.   The loop coil type metal object detection according to claim 1 or 2, further comprising a sequencer that sets measurement loop coils and reference loop coils at a plurality of locations and selectively operates the oscillation of each loop coil sequentially. apparatus.   A power failure detection device that detects a power failure while the device is in operation and a loop coil constant memory circuit that stores the constants of the loop coil at the time of power failure are collated, and the values stored in the loop coil constant memory circuit are collated when power is restored. The loop coil type metal object detection device according to any one of claims 1 to 3, wherein the loop coil type metal object detection device is configured to operate in a state immediately before.

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