JPH06236498A - Loop coil type metallic object detector - Google Patents

Abstract

PURPOSE:To exactly detect a metallic object even when plural loop coils are closely arranged. CONSTITUTION:This device is equipped with plural loop coils 1, plural resonance circuits 2 including each loop coil 1 as one element of an resonance system, amplifier circuit 3 combined with the resonance circuits 2 constituting the oscillator circuits, plural switching circuits 4 connected with each resonance circuit 2, which switches whether or not each resonance circuit is functioned as the resonance circuit, sequencer 5 which selectively in turn operates the plural switching circuits 4, discriminating circuit 6 which discriminates whether or not the frequency or peak value of the output of the amplifier circuit 3 is within a normal range while being interlocked with the selecting operation of the sequencer 5, and control circuit 9 which controls a relay 7 according to the discriminated result.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a loop coil type metal object detecting device for detecting a metal object such as a vehicle by a loop coil arranged underground or on the ground.

[0002]

2. Description of the Related Art For example, in a parking lot, the presence or absence of a parked vehicle is detected by an optical sensor or an ultrasonic sensor,
The usage status is displayed. However, because of the operating principle of optical sensors and ultrasonic sensors, it is necessary to expose the sensors to the ground, so they are contaminated with dust or mud, malfunction due to contact with the vehicle, or obstacles to the movement of the vehicle. There is inconvenience such as becoming.

As the sensor, in addition to the above optical sensor and ultrasonic sensor, a loop coil type sensor utilizing magnetic induction is known. In a metal object detection device using this loop coil type sensor, an oscillation circuit including a loop coil as a part of a resonance circuit is configured. In the steady state, the oscillation operation is performed at a constant resonance frequency determined by the inductance 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 and Q of the loop coil change, so the oscillation frequency and peak voltage also change. Therefore, by detecting the change in the oscillation frequency or the change in the peak voltage, the presence or absence of the metal object, that is, the vehicle can be detected.

In the metal object detecting device using the loop coil type sensor, the loop coil itself, which is the sensor, can be embedded in the ground, so that it is affected by the external environment or interferes with the movement of the vehicle. Never. Further, even when it is arranged on the ground, since the thickness of the loop coil is thin, it is possible to obtain the same effect as when it is embedded in the ground.

[0005]

In a metal object detecting device using such a loop coil type sensor, a large number of loop coils are arranged at short intervals in order to detect the position and movement of the vehicle in detail. It is possible.
However, it has been found that when a large number of loop coils are actually arranged at short intervals, a phenomenon occurs that a metal object cannot be detected accurately. As a result of elucidating the cause of this phenomenon, the following facts have been revealed.

As described above, the loop coil constitutes a part of the resonance circuit, and when another loop coil exists in the vicinity of the loop coil, both loop coils are electromagnetically coupled. When a resonance circuit having a certain resonance frequency and a resonance circuit having a resonance frequency in the vicinity are coupled to each other, their resonance frequencies are affected by the resonance frequency of the other side and deviate from the original resonance frequencies. For this reason, the presence of a metal object is detected even though the metal object does not actually exist. Also, if not so extreme,
If the oscillation frequency deviates from the specified frequency, the detection sensitivity changes, and the position of the metal object cannot be detected accurately.

Further, independent oscillation circuits are connected to the respective loop coils, and the plurality of oscillation circuits oscillate at a frequency substantially equal to the specified frequency in a steady state. Since each oscillating circuit operates independently, the oscillating frequency of each oscillating circuit cannot steadily and completely match. That is, each oscillation circuit oscillates at a slightly different frequency. When each loop coil is arranged close to each other, a frequency component of the difference in the oscillation frequency of each oscillation circuit is generated, and the frequency component of this difference is mixed with the original oscillation frequency component in each oscillation circuit,
The amplitude of the output of the oscillation circuit, which should be originally constant, fluctuates periodically at the cycle corresponding to the difference frequency component. Therefore, the oscillation frequency and the peak value cannot be accurately detected, and the position of the metal object cannot be accurately detected.

Therefore, an object of the present invention is to enable a metal object to be accurately detected even when a plurality of loop coils are arranged close to each other. Another object of the present invention is to prevent unnecessary radiation from the oscillation circuit.

[0009]

In order to achieve the above-mentioned object, a loop coil type metal object detecting apparatus of the present invention is provided with a plurality of loop coils and a plurality of loop coils corresponding to the respective loop coils. A plurality of resonance circuits each including a loop coil as one element of a resonance system, an amplifier circuit that is combined with each resonance circuit of the plurality of resonance circuits to form an oscillation circuit, and each resonance circuit connected to each resonance circuit. A plurality of switching circuits that switch whether or not to function as a switch, a sequencer that sequentially and selectively operates the plurality of switching circuits, and a frequency or wave of the output of the amplifier circuit in conjunction with the selection operation by the plurality of control circuits. And a discriminating circuit for discriminating whether or not the high value is within a prescribed range.

[0010]

In the present invention, since the resonance circuit is not formed in the loop coil adjacent to the loop coil being measured, the resonance frequency of the resonance circuit including the loop coil being measured is affected by the adjacent loop coil. There is no change. Further, since the plurality of oscillation circuits do not operate simultaneously, the difference component of the oscillation frequency of each oscillation circuit does not occur.

[0011]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The features of the present invention will be specifically described below based on embodiments with reference to the drawings.

FIG. 1 is a circuit diagram showing an embodiment of a loop coil type metal object detecting apparatus of the present invention.

The metal object detecting apparatus shown in the figure has n loop coils 1, n resonance circuits 2 including the inductance of the loop coil 1 as a part of the circuit, and among the n resonance circuits 2. A common amplifier circuit 3 that is combined with any one of the resonance circuits to form an oscillation circuit; n switching circuits 4 provided between each resonance circuit 2 and the common amplification circuit 3; The sequencer 5 that sequentially switches the switching states of the switching circuits of the switching circuit 4 of FIG. 4 and the frequency of the oscillation output from the amplifier circuit 3 to detect whether or not the frequency is within a predetermined specified frequency range. Discriminating circuit 6 for
Based on the detection result of the discrimination circuit 6, n relays 7
And a control circuit 9 for outputting the detection result to the data output terminal 8 in the form of serial digital data. Each of the loop coils 1, the resonance circuit 2 and the switching circuit 4 constitutes one channel. In the embodiment of FIG. 1, there are channels # 1 to #n corresponding to the n loop coils 1. To do. The n loop coils 1 are arranged in the ground or on the ground, close to each other. Depending on the case, the adjacent loop coils may be arranged in a state of being overlapped by half.

FIG. 2 is a circuit diagram showing details of the resonance circuit 2 and the switching circuit 4.

The resonance circuit 2 includes a transformer 2a whose primary side is connected to the loop coil 1 and capacitors 2b and 2c which are connected to the secondary side of the transformer 2a. An LC resonance circuit is formed by the inductance of the loop coil 1 and the capacitance of the capacitors 2b and 2c viewed through the transformer 2a. That is, the resonance circuit 2 includes the loop coil 1 as one element of the resonance system. The midpoint of connection between the capacitors 2b and 2c is grounded, and the capacitors 2b,
2c and the transformer 2a from the connection point of the pair of signal lines L1,
L2 has been derived. The one signal line L1 is connected to the switching circuit 4
Is connected to the input side of the amplifier circuit 3 via the, 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 photo switch 4a connected in series to the signal line L1 on the input side and a signal line L on the output side.
2 has a photo switch 4b connected in series, and a photo switch 4c connected in parallel between the signal line L1 on the input side and the signal line L2 on the output side. The photoswitches 4a to 4c are bidirectional switching elements whose on / off states are controlled according to a control signal from the outside, and, for example, bidirectional optical MOSFETs are used.
The photo switches 4a and 4b are of a type that is off in a steady state and turns on only when a control signal is applied. The photo switch 4c is on in a steady state and has a control signal. Use a type that turns off only when is applied.

Next, the operation of the above-mentioned metal object detecting device will be described.

From the sequencer 5 shown in FIG.
Control signals S1 to Sn as shown in (a) to (c) are sequentially generated and supplied to the respective switching circuits 4. For example, when the control signal S1 is supplied to the switching circuit 4 of the channel # 1, the photoswitches 4a and 4b of the switching circuit 4 are switched.
Turns on (see (d) in the figure), and the photo switch 4c
Is turned off (see (e) in the figure). 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 capacitance of the resonance circuit 2 and the loop coil 1 as shown in FIG. Resonant frequency determined by intrinsic inductance, eg, about 100
The oscillation operation is started at a predetermined crest voltage at a frequency of kHz. When no metal object is present near the loop coil 1, the resonance frequency is mainly determined by the inductance of the loop coil 1 and the capacitances of the capacitors 2b and 2c of the resonance circuit 2. The oscillation output from the amplifier circuit 3 is supplied to the discriminating circuit 6 and it is detected whether the oscillation frequency is within a predetermined specified frequency range or whether the peak voltage is within a specified range. It For example, when a metal object exists near 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. Since the value is a value, the presence or absence of a metal object can be determined by detecting the deviation of the frequency or the deviation of the peak voltage. The specified frequency may be common to each channel, or may be set independently for each channel according to variations in the 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 supplied. Is off. Therefore, with respect to the other channels # 2 to #n, the amplification circuit 3 and the resonance circuit 2 are separated from each other, and the oscillation operation is not performed. That is, since a plurality of oscillation circuits do not simultaneously oscillate, unnecessary signals such as beat signals are not generated.

Further, since the photo switches 4a and 4b are turned off and the photo switch 4c is turned on at the same time,
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 the resonance frequency does not exist. Therefore, during the measurement of the channel # 1, there is no influence of the resonance frequency of the loop coil 1 of the adjacent channel # 2, and accurate detection can be performed.

The detection of the oscillation frequency in the discriminating 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 other channels are operated. All the resonance circuits 2 of 1 are made inoperative. Therefore, it is possible to accurately identify which channel the loop coil 1 is near and the metal object is present.

The detection result of the discrimination circuit 6 is, for example, n
It is supplied to the control circuit 9 as bit parallel data, and the relay 7 of the channel in which the metal object is detected is turned on. Further, the detection result for n channels is output to the data output terminal 8 in the form of serial digital data.

In the above-described embodiment, the common amplifier is used for each channel. However, as in the other embodiment shown in FIG. 4, each channel # 1 to #n is provided with the amplifier 3 and each amplifier is provided. The output of the amplifier 3 may be combined via the buffer circuit 10 and supplied to the discrimination circuit 9.
In the case of the embodiment shown in FIG. 4, the switching circuit 11 connected between the resonance circuit 2 and the amplifier 3 is connected in parallel between the input side signal line L1 and the output side signal line L2. A photo switch 11a is provided. Photo switch 11a
Is a type that is on in a steady state and is off only when a control signal is applied. In the embodiment shown in FIG. 4, the parts corresponding to those of the embodiment shown in FIG.

Also in the embodiment shown in FIG. 4, as in the embodiment shown in FIG. 1, the control signals S1 to S from the sequencer 5 are transmitted.
n is supplied to the switching circuit 11 of each channel. For example, the control signal S1 is the switching circuit 11 of the channel # 1.
Is supplied to the photo switch 11a of the switching circuit 11.
Turns off. Therefore, the resonance circuit 2 is connected between the output side and the input side of the amplification 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. Start the oscillation 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 photo switch 11a of the switching circuits 11 of the other channels # 2 to #n is turned on. Has become. 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 the oscillation operation is not performed.

Therefore, also in the embodiment shown in FIG. 4, a plurality of oscillator circuits do not simultaneously perform an oscillating operation, and unnecessary signals such as beat signals are not generated. Further, during measurement of a certain channel, there is no influence of the resonance frequency of the loop coil 1 of the adjacent channel,
Accurate detection can be performed.

[0027]

As described above, in the present invention, the resonance frequency of the resonance circuit including the loop coil being measured is not affected by the adjacent loop coil, so that the metal object can be accurately detected. be able to. Further, since the plurality of oscillation circuits do not operate simultaneously, the difference component of the oscillation frequency of each oscillation circuit does not occur and the waveform of the oscillation output is not disturbed. Therefore, the position of the metal object can be accurately detected by accurately detecting the oscillation frequency and the peak value.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of a loop coil type metal object detection device of the present invention.

FIG. 2 is a circuit diagram showing details of a resonance circuit and a switching circuit used in the metal object detection device shown in FIG.

FIG. 3 is a timing chart for explaining the operation of the metal object detection device shown in FIG.

FIG. 4 is a circuit diagram showing another embodiment of the loop coil type metal object detecting device of the present invention.

[Explanation of symbols]

1 ... Loop coil, 2 ... Resonance circuit, 2a ... Transformer, 2
b, 2c ... Capacitor, 3 ... Amplifier circuit, 4 ... Switching circuit,
4a to 4c ... Photo switch, 5 ... Sequencer, 6 ... Discrimination circuit, 7 ... Relay, 8 ... Output terminal, 9 ... Control circuit, 1
0 ... Buffer circuit, 11 ... Switching circuit, 11a ... Photo switch

Claims (1)

[Claims] 1. A plurality of loop coils, a plurality of resonance circuits provided corresponding to the respective loop coils of the plurality of loop coils and including the respective loop coils as one element of a resonance system, and a plurality of the resonance circuits. An amplifier circuit that is combined with a resonance circuit to form an oscillation circuit, a plurality of switching circuits that are connected to each resonance circuit and switch whether or not to function each resonance circuit as a resonance circuit, and the plurality of switching circuits are sequentially and selectively selected. And a determination circuit that determines whether or not the frequency or crest value of the output of the amplification circuit is within a specified range in conjunction with the selection operation by the plurality of control circuits. Loop coil type metal object detection device.