JPS6048712B2 - Metal object detection method and device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for detecting a metal object that utilizes changes in self-inductance of a detection loop coil.

A detection loop coil is installed at the location to be detected, and an alternating current signal is electromagnetically induced into this loop coil, and the presence or absence of a metal object is detected by using the change in self-inductance of the loop coil as the metal object approaches the loop coil. The device is used as a vehicle detector in parking lots, a steel plate detector in steel mills, etc.

However, in this type of device, the rate of change in intagtance decreases rapidly when the approach distance M of the metal object to the loop coil and the ratio M2 of the width M2 of the loop coil to the width M2 of the loop coil is 1 or more. This type of device is configured to detect the phase of the induced voltage in the loop coil that changes according to changes in the loop coil's self-inductance, and process the detection signal in an analog manner to detect a metal object. Because of this, slight changes in intagtance caused by external disturbances such as temperature changes, deformation of the loop coil, and long-term changes in the permittivity of the ground can easily cause false detection, and the tuning frequency adjusts the sensitivity according to the inductance of the loop coil. I had to.

5 In addition, in order to prevent false detection due to external disturbances, conventional devices provide an automatic correction circuit in the detection signal processing circuit to stabilize the processing circuit to increase sensitivity, or increase the width of the loop coil. Sensitivity is increased by increasing the rate of change of inductance.

However, the former method is expensive because the detection signal processing circuit is complicated and there are various restrictions on the installation work of the loop coil to prevent disturbances. In the latter case, even if the inductance change rate is only a few percent, the width of the loop coil must be 1.5 TrL or more, for example in the case of automobile detection, and the installation work is proportional to the size of the loop coil. becomes complicated and expensive. This invention is
It is an object of the present invention to provide a method and device for detecting a metal object that processes signals digitally, is simple, has no detection errors, and can reduce the width of a loop coil. In this invention, a plurality of signals with different frequencies are repeatedly generated in sequence and supplied to a loop coil, and the phases of the sequentially generated signals and the signal induced in the loop coil are compared to determine whether the phase is positive or negative. A time-series digital signal is generated depending on whether the phase of the other signal leads or lags the phase of the other signal (referred to as a lag in this invention), and this digital signal is used to detect the metal object. The above objective is achieved by detecting the presence or absence of.

That is, in the present invention, a plurality of signals having a constant frequency around the tuning frequency of the loop coil and having different frequencies are sequentially and repeatedly supplied to the loop coil, and the phase of the signal and the induced signal is determined by a phase comparator. In comparison, if the frequency of the signal supplied to the loop coil to generate a digital signal (for example, a positive or negative signal) according to the phase lag is equal to or less than the tuning frequency, it becomes positive (or negative), A signal that becomes negative (or positive) if the above three conditions are met is generated in time series in synchronization with the signal supplied to the loop coil, and this signal is processed by a logic circuit to detect the presence or absence of a metal object. Detection of the presence or absence of metal objects in logic circuits is performed using Gold 3.
When a metallic object approaches the loop coil and the tuning frequency changes, the time-series digital signal generated at the output of the phase comparator changes. It can be detected by whether it is a logic signal ``H'' or 4''L'', and the time-series digital signal is a signal when a metal object approaches the loop coil. It can also be detected by decoding whether or not it is a sequence.

The embodiments shown in the drawings will be described below.

FIG. 1 is an example of using four types of signals. In the figure, 1 is a known voltage controlled oscillator used in a PLL (phase locked loop), 2 is a capacitor for the oscillation element, 3a, 3b, 3c, 3d are voltage controlled oscillators 1
The resistor 4 for the oscillation element is a known resistance switching circuit for switching the resistor connected to the voltage controlled oscillator 1. The oscillation frequency f of the voltage controlled oscillator 1 is generally expressed as follows: C) the capacitance of the capacitor, R) the value of the resistance, R) the power supply voltage, VS) the control voltage input from the voltage holding circuit 5, which will be described later, as Vc, and the coefficient k determined by the circuit. , can be expressed as .

6 is, for example, 1110 of the oscillation frequency of the voltage controlled oscillator 1.
An oscillator of a clock signal having a constant frequency of .about.1110 degrees, and 7 a 2-bit binary counter for counting the clock signal.

Although a binary counter is used as the counter 7 in this example, a quaternary ring counter may also be used. The count value of the counter 7 is supplied to the resistance switching circuit 4 as a switching signal, and is also supplied to a determination circuit 8, which will be described later, as a determination signal. The resistance switching circuit 4 sequentially and repeatedly connects the resistors 3a to 3d corresponding to the count value of the counter 7 to the voltage controlled oscillator 1, and as a result, the voltage controlled oscillator 1 sequentially and repeatedly oscillates four types of signals having different frequencies.

In the following explanation, the frequency of the signal oscillated by the voltage controlled oscillator 1 (hereinafter simply referred to as the oscillation signal) is determined by ，
It is assumed that ``2'' is F., ``3'' is F3, and with respect to the center frequency f, each frequency is <F, <f'<F2<F3... This will be explained assuming that the relationship (2) exists.

Frequency F of the oscillation signal of the voltage controlled oscillator 1. ,f,,f.
, f3 are as follows: When the control voltage ■■c in the first equation changes,
Following this, the center frequency f' shifts, so it shifts in the same way. 9 is the amplification H for the signal oscillated by the voltage controlled oscillator 1, which is a commonly used sine wave amplifier, but it is a known AC amplifier of the constant current drive type whose output side is driven by a constant current. The amplified signal is transferred to transformer 1.
0 primary winding 11a.

The transformer 10 has three windings 11A, llb, and llc, and is provided with a tuning capacitor 12 on the secondary side. The secondary winding 11b of the transformer 10 is detected! The tertiary winding 11c is connected to one signal input terminal of the phase comparator 14. The phase of the signal induced in the tertiary winding 11c of the transformer 10 (hereinafter simply referred to as an induced signal) is determined by the amplifier 9.
Since it is a constant current drive type AC amplifier, the oscillation signal of the voltage controlled oscillator 1 changes as shown in FIG. 2 near the tuning frequency. In Fig. 2, the vertical axis is the phase, the horizontal axis is the frequency, and the solid curve indicates that the metal object is the loop coil 13.
The curve shown by the dotted line is the characteristic when it is not close to , and the curve shown by the dotted line is the characteristic when it is close to. is the frequency of

As is clear from FIG. 2, the phase of the induced signal of the tertiary winding 11c with respect to the oscillation signal of the voltage controlled oscillator 1 lags when the frequency is higher than the tuning frequency, and advances when the frequency is lower than the tuning frequency. Furthermore, when a metal object approaches the loop coil 13, the tuning frequency increases. Frequency F of the oscillation signal. ,f,,f. ,f. are selected such that they have a difference of about 0.5% of the tuning frequency of the loop coil 13 from each other. The phase comparator 14 is a known circuit that compares the phases of the oscillation signal of the voltage controlled oscillator 1 and the induced signal of the tertiary winding 11c and generates a digital signal corresponding to the positive or negative phase. If the phase of the induction signal is ahead of the phase of the oscillation signal, a positive signal (signal meaning logic signal ``1'') is output, and if it is delayed, a negative signal (logic signal ``o'' is output). outputs a signal that means

Such a phase comparator can be used, for example, by saturating a phase comparator used in a PLL. In this way, the voltage controlled oscillator 1, the resistance switching circuit 4, and the phase comparator 14 are combined as a set and are commercially available.
An integrated circuit for L can be used. The determination circuit 8 is a circuit that digitally processes the output signal of the phase comparator 14 to detect the presence or absence of a metal object, and also generates a correction signal for the voltage controlled oscillator 1, as shown in FIG. , AND circuits 5, 16, 17, 18, an inhibit circuit 19, inverters 20, 21, and a flip-flop 22.

AND circuits 15 and 16 indicate that the count value of counter 6 is "2"
When , the AND circuit 15 outputs the output signal of the phase comparator 14 to the set input terminal of the flip-flop 22, and the AND circuit 16 outputs the inverted signal of the output signal of the phase comparator 14 by the inverter 20 to the flip-flop. output to the reset input terminal. The AND circuits 17 and 18 are opened when the count value of the counter 6 is "zero", the AND circuit 17 outputs the Q output of the flip-flop 22 to the inhibit terminal of the inhibit circuit 19, and the AND circuit 18 outputs the Q output of the flip-flop 22 to the inhibit terminal of the inhibit circuit 19. The output signal is output to the voltage holding circuit 5. The inhibit circuit 19 has a signal input terminal connected to the output terminal of the phase comparator 14 , and an output terminal connected to the voltage holding circuit 5 via the inverter 21 . The voltage holding circuit 5 is a charging/discharging circuit that generates a control voltage for the voltage controlled oscillator 1 using the output signal of the determination circuit 8, and as shown in FIG. It includes a diode 25 and various resistors 26, 27, 28, 29, and 30.

The transistor 24 is a depletion type transistor, and supplies the voltage generated at its source electrode to the voltage controlled oscillator 1 as a control voltage Vc. Resistors 26 and 27 are selected such that the resistance value of resistor 26 is lower than that of resistor 27. Next, the operation of the above device will be explained in Figures 4 to 8.
This will be explained with reference to the figures.

In addition, in FIGS. 4 to 8 θ, A is the counter 6.
B is the output signal of the phase comparator 14, C is the output signal of the AND circuit, D is the output signal of the AND circuit 16, F is the output signal of the AND circuit 18, G is the output signal of the prohibition circuit 19, H respectively indicate the Q output of the flip-flop: 5. First, when the power is turned on, the capacitor 2 of the voltage holding circuit 5
3 is in a discharge state, and the control voltage Vc is applied to the resistors 28, 2.
9 and the pinch-off voltage of the transistor 24, the center frequency f' of the voltage control 10 oscillator 1 is higher than the tuning frequency of the loop coil 13.

Therefore, the output of the phase comparator 14 is a signal ``0'' as shown in FIG. 4B. Further, the AND circuit 18 is
As shown in FIG. 4F, each time the count value of the counter 6 becomes ``zero'', it is opened and outputs a signal ``1'', thereby charging the capacitor 23. Since the diode 25 is provided at the subsequent stage of the AND circuit 18, it will not discharge even if the output of the AND circuit 8 becomes the signal ``0'', and moreover, the output of the inhibit circuit 19 will not become the signal ``0'' as shown in FIG. 4G.
'o'', the output of the inverter 21 becomes the signal '1', charging progresses rapidly, and the center frequency f' of the voltage controlled oscillator 1 rapidly decreases.The Q output of the flip-flop 22 is Every time the count value of 6 becomes "2",
The AND circuit 16 opens and the signal '
Since it outputs ``1'', the signal ``o'' is output as shown in Fig. 4H.
''It is. The center frequency f' of the voltage controlled oscillator 1 decreases to around the tuning frequency of the loop coil 13, and the frequency F'
. When the oscillation signal becomes below the tuning frequency, the output of the phase avoider 14 becomes a signal "1" only when the count value of the counter 6 is "0", as shown in FIG. 5B. As a result, the output of the AND circuit 18 becomes the signal ``o'' as shown in FIG. 5F, and the output of the inhibit circuit 19 becomes the signal ``0'' as shown in FIG. The signal becomes “1” only when
As the charging speed becomes slower, the center frequency of the voltage controlled oscillator 1 gradually decreases. 2 Frequency F of voltage controlled oscillator 1.

When the oscillation signals of and F become lower than the tuning frequency of the loop coil 13, the output of the phase comparator 14 becomes as shown in FIG. When the signal is 441,, "2" and "3", the signal number is 0,,
Since the inhibition circuit 19 is in an open state, it outputs the same signal as the output of the phase comparator 14, as shown in FIG. 6G. Therefore, the capacitor 23 is charged and discharged when the count value of the counter 6 is "0" and "1", and when the count value is "2" and "1", the capacitor 23 is charged and discharged.
3'', charging occurs, and control 3: the voltage Vc is stabilized, and the voltage controlled oscillator 1 enters an equilibrium state because the center frequency is fixed, and maintains this state. In the above-mentioned equilibrium state, a metal object approaches the loop coil 13 and the tuning frequency becomes the frequency F of the voltage controlled oscillator 40 1.

between the oscillation signal of frequency F3 and the oscillation signal of frequency F3, the output of the phase comparator 14 becomes the signal "O'' only when the count value of the counter 6 is "3", as shown in FIG. 7B. As shown in FIG. 7c, the AND circuit 15 outputs a signal ``1'' to set the flip-flop 22 every time the count value becomes ``2''. Therefore, by setting the flip-flop 22, it is possible to detect that a metal object approaches the loop coil 13. When the flip-flop 22 is set, the AND circuit 17 outputs a signal ``1'' every time the count value of the counter 6 becomes zero, as shown in FIG. As shown in Figure 7G, the signal ``o'' is output when the count value is ``0'' and ``3'', and the signal ``'' is output when the count value is ``1'' and ``2''.
Outputs 1''.

Therefore, while the metal object approaches the loop coil 13, the capacitor 23 is charged and discharged in synchronization with the output signal of the inhibition circuit 19, so that the output of the phase comparator 14 is equal to the count value. The control voltage Vc is stabilized in a state where the signal becomes "0" only when the signal is "3" and the equilibrium state is maintained.The metal object moves away from the loop coil 13 and the tuning frequency changes to F and F.

When the count value of the counter 6 is "0" and "1", the output of the phase comparator 14 becomes a signal "1", "2" and "1", as shown in FIG. 8B. 3", signal ゛'o
'', and the AND circuit 16 outputs a signal ``1'' when the count value is ``2'' to reset the flip-flop 22, as shown in FIG. 8D. Therefore, by resetting the flip-flop 22, it is possible to detect that the metal object has separated from the loop coil 13. After the flip-flop 22 is reset, the inhibition circuit 19 remains open, so that when the count value of the counter 6 is "0" and "1", the signal ``1'' is generated, as shown in FIG. 8G. ', signal '゛0'' when '2' and '3'
is output, and the capacitor 23 is charged and discharged in synchronization with the output to maintain an equilibrium state.

That is, when the flip-flop 22 is reset,
The device returns to its initial equilibrium state. The tuning frequency is F due to the metal object approaching the loop coil 13. or more, the output of the phase ratio converter 14 is all the signal ``1''
'', but prohibited] The output of the path 19 becomes the signal ``o'' only when the count value of the counter 6 is ``zero''), and the discharge time of the capacitor 23) becomes longer, thereby controlling the Since the voltage 'c decreases and the center frequency of the voltage controlled oscillator 1 increases, the control voltage becomes stable when the oscillation signal of frequency F becomes equal to or higher than the tuning frequency, and the above-mentioned equilibrium state is reached. The invention is not limited to the embodiments described above; for example, instead of repeatedly sequentially switching the resistors of the voltage controlled oscillator 1 to generate a plurality of signals sequentially, the capacitors may be sequentially and repeatedly switched; It is also possible to switch both the resistor and the resistor.

As described above, the present invention sequentially and repeatedly generates a plurality of signals with different frequencies and supplies them to a loop coil, and compares the phase of the signal and the induction signal of the loop coil to cope with a phase delay. Since a series of digital signals are generated and these signals are used to detect metal objects, the signals can be processed digitally, which simplifies the digital signal processing circuit and makes it less expensive, and reduces detection errors. can be reduced.

Furthermore, since each signal supplied to the loop coil has a constant frequency around the tuning frequency of the loop coil, a slight change in inductance due to the approach of a metal object to the loop coil will cause a change in the digital signal. It has high sensitivity and can accurately detect even if the loop coil is small.Furthermore, the installation work of the loop coil is simple and inexpensive. According to the second invention, a signal to be supplied to the loop coil is generated by a voltage controlled oscillator,
In addition, since the center frequency of the voltage-controlled oscillator is controlled using a time-series digital signal, the frequency of the signal supplied to the loop coil is automatically adjusted to the optimum value depending on whether a metal object is detected or not detected, thus eliminating the possibility of artificial No adjustment is required,
The stability of the device is significantly improved. In addition, by switching the resistor and/or capacitor that determines the oscillation frequency of the voltage controlled oscillator, multiple signals are repeatedly oscillated in sequence, so the stability of the oscillation signal is uniquely determined by the ratio of resistance value to capacitance. , is not affected by other elements and is extremely high.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of an electric circuit showing an example of a metal object detection device, Fig. 2 is a diagram showing the relationship between signal frequency and phase,
Fig. 3 is a block diagram showing an example of the judgment circuit and voltage holding circuit, Fig. 4 is a diagram explaining the operation when the power is turned on, Fig. 5 is a diagram explaining the operation when transitioning to the stable state, and Fig. 6 is the stable state. 7 is an explanatory diagram of the operation at the time of detection, and FIG. 8 is an explanatory diagram of the operation J at the time of non-detection. 1: Voltage controlled oscillator, 2: Capacitor, 3a to 3d: Resistor, 4: Resistance switching circuit, 5: Control voltage holding circuit, 7:
Counter, 8: Judgment circuit, 10: Transformer, 13: Loop coil, 14: Phase comparator.

Claims (1)

[Claims] 1. In a metal object detection device that detects a metal object using a change in intagtance caused by a loop coil installed at a location to be detected and a metal object approaching the loop coil, the loop coil A plurality of signals around the tuning frequency are sequentially and repeatedly generated and supplied to the loop coil, and the phases of the signals and the induction signal of the loop coil are compared to generate a time-series digital signal corresponding to the phase delay. death,
A metal object detection method characterized by detecting a metal object using this signal. 2. In a metal object detection device that detects a metal object by installing a loop coil at a location to be detected and utilizing a change in intagtance caused by a metal object approaching the loop coil; a voltage controlled oscillator; and the voltage control means for sequentially and repeatedly switching the resistor and/or capacitor of an oscillator to sequentially and repeatedly generate a plurality of signals around the tuning frequency of the loop coil; means for guiding the output signal of the voltage controlled oscillator to the loop coil; a phase comparator that compares the phase with the induction signal of the coil and generates a time-series digital signal according to the phase delay; the digital signal is used to determine the presence or absence of a metal object, and a correction signal is generated; A metal object detection device comprising: a logic circuit for outputting; and means for generating a control voltage for controlling the center frequency of the voltage controlled oscillator using the correction signal.