JP2001235549A - Active metal sensor and stabilizing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve sensitivity by stabilizing an active metal sensor, to enlarge an application range by preventing the influence of a jamming conductor, and to permit the recognition of approximate shape of an object conductor. SOLUTION: For the purpose of detecting an object conductive substance in a distant place where the magnetic field does not sufficiently get into the conductive substance, a current is made to flow to an exciting coil, and then the current is cut off. The transient voltage waveform of the exciting coil aftercutting off the current is monitored, and at a point of time when the transient voltage waveform is lowered to a specified level, coil voltage is sampled and made voltage A. Immediately after sampling the voltage A, coil voltage is further sampled in succession and made voltage B. Offset voltage is added to the voltage A or voltage B and differentially amplified, and the conductive substance in the magnetic field of the exciting coil is detected with high sensitivity by the output voltage.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention realizes high-sensitivity metal detection and can detect the position of a metal in an invisible portion, and is applied to a metal sensor, a distance sensor, a proximity switch, and the like. Is possible. Especially when trying to use metal sensors for landmine detection, recent plastic landmines have only small iron balls and small springs as metal.
This detection was extremely difficult. The present invention not only achieves this goal by increasing the sensitivity and stability of the detector, but also allows for general shape recognition.

[0002]

2. Description of the Related Art Changes in a magnetic field,
Some use electromagnetic waves, ultrasonic waves, X-rays, etc., and each has its own characteristics. Some use electromagnetic waves by a radar method, and others use a method in which induced current or magnetic permeability changes the equivalent impedance of a detection coil due to the presence of a target conductor. The radar system has a high equipment cost, and when detecting buried objects, it has the disadvantage of being affected by geology and water content and reacting to nonmetals, and it is difficult to detect at short distances of centimeters There were drawbacks.

[0003] Further, a method of detecting a detection conductor by utilizing the fact that the induced current or the magnetic permeability changes due to the detection conductor is as follows.
When the signal frequency for detection is lowered, there is a feature that the influence of geology is relatively low. The method of utilizing the change in the equivalent impedance of the detection coil is generally a method of connecting two coils differentially or increasing the sensitivity by a bridge circuit, and the sensitivity and directivity are not sufficient.

As a method for eliminating these drawbacks, there is an active type method utilizing a change in a transient phenomenon caused by an interaction between the exciting coil and the detection conductor when the current of the magnetic field generating coil is cut off. The coil can be shared, and the directivity and sensitivity can be relatively improved.

As an example of this, Japanese Patent Application Laid-Open No. Hei 2-500215
6 and JP-A-5-232245. However, in both methods, the transient voltage after the interruption of the exciting coil current is simply measured indiscriminately, and no attempt is made to increase the sensitivity or stabilize the detection level. In addition, the transient response of the coil in both the specifications is for a relatively short distance object, and corresponds only to a state where a magnetic field has sufficiently entered the target conductive material. If the distance to the target conductive material is large, the magnetic field cannot sufficiently enter the target conductive material, and the transient voltage change of the coil differs from the waveforms described in these specifications, effectively Since the measurement timing that can detect the target from the transient voltage has only a very limited time, it is difficult to obtain a sensor with very high sensitivity simply by measuring the transient voltage waveform after the current interruption of the exciting coil as it is It was. Further, in the conventional method, not only the sensitivity is not sufficiently obtained, but also there is a disadvantage that it is necessary to adjust the device again when the constant of the exciting coil changes. Further, if there is a conductive material for detecting interference in the vicinity of the excitation coil, the sensitivity may be significantly reduced, or the amplifier for amplifying the detection signal may be saturated and the target conductive material may not be detected.

[0006]

In the present invention, a method for further improving the sensitivity of the active metal sensor has been studied by many numerical calculations, experiments, and simulations. As a result, when detecting a metal in a distant place where a magnetic field does not sufficiently enter the target conductive material, the timing at which the presence or absence of metal can be detected by the voltage waveform after current interruption of the excitation coil is determined by the excitation coil. The best timing is when the point where the voltage level due to the transient caused by the stored energy and the transient caused by the induction from the target metal is at the same level in the receiving coil is the best timing, and this appropriate timing is automatically determined. The challenge is to prevent the deterioration of stability due to fluctuation factors such as changes in the constant of the exciting coil, changes in the parameters of the switching element that switches the current of the exciting coil, and changes in the voltage applied to the coil. is there.
Another object is to prevent the stability from being deteriorated due to external disturbance such as an external magnetic field such as an earth magnetic field. Furthermore, when a plurality of exciting coils and receiving coils are used at the same time, a difference in switching time between the exciting coils is a factor that hinders stable detection, and it is also an object to solve this problem.

[0007]

[Means for Solving the Problems] When a pulse magnetic field is generated by supplying an exciting current to the exciting coil and the current is cut off, the transient voltage waveform of the exciting coil or the receiving coil is changed to a target conductive material. When the magnetic field sufficiently enters, the transient voltage waveform changes as shown in FIG. 1 when the target conductor exists as shown in FIG. 1 and when the target conductor does not exist, as shown in FIG. −2322
For example, a method such as 45 can achieve a certain purpose.

When a magnetic field cannot sufficiently enter the target conductive material and the target conductor is located at a distance, the transient phenomenon is as follows.
As shown in FIG. 2, the transient voltage value of the exciting coil is attenuated, and the effect of the transient voltage of a level which can be detected only at a point almost approaching zero appears. The voltage range of FIG. 2 is several thousand to several hundred thousand times smaller than that of FIG. This is because when the magnetic field does not enter the target conductive material sufficiently, the induced current flowing through the target conductive material is small, and the voltage appearing in the receiving coil due to the interaction with this current is the transient voltage due to the energy stored in the exciting coil itself. The size is so small that it can be ignored, and its detection is extremely difficult. In addition, it is possible to wait until the exciting current is sufficiently attenuated, wait for the energy of the exciting current to become substantially zero, and selectively observe the induced voltage from the target conductive material. Not only is the induced voltage from the target conductive material greatly attenuated and the measurement is difficult, but not only this value itself, but also the smaller the amount of the magnetic field entering the target conductive material, as shown in FIG. Not only the absolute value itself, but also the amount of change over time was extremely small, and it was concluded that this amount was lower than the level of many of the above-mentioned variables, making it extremely difficult to observe.

For the purpose of solving this problem, as a result of numerical analysis and many experiments and simulations, the optimal timing for sampling the transient voltage is determined by the transient voltage due to the energy stored in the exciting coil itself and the electromagnetic induction from the target conductive material. As a result, it has been found that the change of the transient phenomenon can be most easily detected only for a very short time when the voltage level generated in the exciting coil is at substantially the same level. From this result, if the time setting is set to an appropriate value in the circuit (86) in which the time setting values shown in FIG. 9 of Table 2 are different from each other, a certain purpose can be achieved. You. However, when a magnetic field does not sufficiently enter the target conductive material, not only the induced current flowing through the target conductive material is small, but also the timing conditions for sampling are extremely strict, and as shown in FIG. Even if you measure at the specified measurement time,
It became clear that the optimum timing was shifted and the detection output became unstable due to the time variation of the timing generation circuit, the variation of the characteristics of the switching element for switching the exciting coil current, and the variation of the exciting coil constant.

In this case, the accuracy of the timing for sampling data is improved by using an accurate time obtained by digitally dividing the oscillation frequency of an oscillator having a high frequency accuracy, such as a crystal oscillator, into a predetermined time setting. In addition, it is also possible to remove an unstable element due to a change in time to some extent.
However, even if the circuit becomes complicated or the method of keeping the sampling timing accurately by the above method is performed, the problem that the detection output becomes unstable due to the fluctuation factors such as the switching speed and the capacitance between the electrodes of the switching element is eliminated. I couldn't do that.

For the same reason, the method of measuring the duration of the transient phenomenon as disclosed in Japanese Patent Application Laid-Open No. Hei 5-232224, even if the time measurement is digitally measured with high precision, is particularly one of the above-mentioned fluctuation factors. There is a problem of unstable operation due to a change in switching time due to a temperature change and a change in capacitance between electrodes of the switching element, which has been a major obstacle to obtaining a stable and highly sensitive sensor.

Here, for the purpose of removing the problem of a shift in the timing of sampling data, Japanese Patent Application Laid-Open No. Hei 2-500
As shown in FIG. 9 of 2156, the previously set measurement time setting portion is abolished, and instead, the transient voltage waveform of the exciting coil is incorporated into a high-speed and high-precision comparator. An attempt was made to control the time to start the first sampling. That is, the level at which the comparator operates is set so that the voltage due to the energy stored in the exciting coil itself and the level of the voltage generated in the exciting coil induced from the target conductive material are substantially equal. As a result, even if the measurement points deviate in time due to the above-mentioned fluctuation factors, the point at which sampling is started is determined by the operating voltage level of the comparator.
Even if the optimal time to start sampling is shifted due to the above factors, voltage measurement is automatically started at the best sampling point regardless of this time shift, so that a stable and highly sensitive sensor can be obtained. Becomes possible. In addition, when the first sampling and the second sampling are performed and this voltage is differentially amplified, even if there is a disturbing conductor at a fixed distance from the exciting coil, the measurement of the induced voltage from this disturbing conductor is automatically performed. It also works to avoid the bird, so it is two birds with one stone.

[0013]

When a pulse magnetic field is generated by supplying an exciting current to the exciting coil, a current also flows through the detection conductor in the magnetic field by induction. While the current is flowing through the exciting coil, a current called an induced current that is substantially proportional to the current flowing through the exciting coil also flows through the detection conductor. When the current flowing through the exciting coil is rapidly cut off, the current flowing through the detection conductor becomes a time constant Te that satisfies Te = L / Rd, where L is the equivalent inductance of the current loop flowing through the detection conductor, and Rd is the equivalent resistance. Attenuate.

Therefore, the current interruption time of the exciting coil is interrupted in a sufficiently short time as compared with Te, and the energy remaining in the exciting coil is sufficiently attenuated before being received by the detection coil and flowing to the detection conductor. It is possible to selectively detect only the magnetic field generated by the current to detect the presence of the detection conductor. However, when the distance to the conductive material to be detected is large, the magnetic field cannot sufficiently enter the conductive material to be detected, and it is extremely difficult to detect the magnetic field due to the current flowing through the target conductor. Moreover, the equivalent time constant of the target conductor is extremely large as compared with the time constant of the excitation coil, and the time variation of the current flowing in the target conductor is extremely small after the current flowing in the excitation coil is attenuated, and this detection is difficult. It is.

Here, it is easy to detect the temporal change of the current flowing in the target conductor only when the current due to the energy remaining in the exciting coil and the current induced from the target conductor are at substantially the same level. I found it. In the numerical analysis, the absolute value change of the excitation coil voltage when the detection conductor is present and when it is not present is greatest immediately after the excitation coil is cut off, but this voltage is strongly affected by the speed at which the excitation coil is cut off, and As shown in FIG. 4, when the switching speed for switching the current of the exciting coil changes, the detected voltage does not change greatly, but the voltage level itself is large, and it is difficult to amplify with a high gain.

In the state shown in FIG. 4, the value differentiated with respect to the time of the voltage level is extremely large, and the detection voltage tends to be unstable with respect to the fluctuation of the switching time and the sampling time. Also, as the time after the current is interrupted, the change in the absolute value of the excitation coil voltage when the detection conductor is present and when the detection conductor is not present becomes smaller, but the differential value with respect to time becomes smaller by orders of magnitude. The influence on the axis fluctuation is reduced, and the level is easily added to a high gain amplifier. However, if the time has passed too much, as shown in FIG. 3, not only the absolute value of the coil voltage but also the change itself becomes small, and the measurement becomes difficult. Therefore, the optimal voltage sampling timing is the optimal point at which there is a trade-off between the rate affected by the jitter in the cutoff time and the rate at which the received signal attenuates too much over time and the sensitivity decreases. Will exist. By determining the timing based on the reception voltage level instead of the predetermined time, the optimum sampling timing can be easily obtained, and the influence of parameter fluctuation of the switching element can be prevented.

To explain the above with mathematical expressions, the voltage when the voltage is applied to the exciting coil and then the voltage is cut off becomes the value shown by the following equation (1). Here, looking at the value of the numerator on the right-hand side of Equation 1, one item has a small value itself, a large time constant, and the voltage slowly attenuates. The two items themselves have a large value, a small time constant, and rapidly attenuate. Therefore, immediately after the excitation coil is cut off, the values of the above two items are extremely large and one item is small enough to be ignored. Therefore, immediately after the current is cut off by the exciting coil, the difference between the relative values of the two is large, and it is difficult to detect a small value of one item whose value changes depending on the target conductor. It can be seen that the two items can be easily compared for the first time when the values of the two items attenuate after the time since the excitation coil current was cut off and become equal to the level of one item. This timing is the optimal voltage sampling timing, and since this timing varies depending on the parameters of the coil, it can be seen that it is more reasonable to determine the voltage level than to determine the time.

This point has the same meaning as the point at which the level of the transient voltage due to the energy stored in the exciting coil itself and the level of the voltage induced from the target conductive material and induced in the exciting coil become substantially equal, as described above. Have. It is understood that this timing is not a fixed time after the excitation coil is cut off, but can be more accurately determined by determining the voltage level of the excitation coil. To achieve this, add the received voltage of the coil to the comparator,
When this voltage becomes an appropriate value, the threshold of the comparison voltage of the comparator is set in advance so that the comparator operates,
The first sampling time is determined by the operation time of this comparator, and the second sampling time may be determined by the output of a similar comparator whose level is slightly lowered, or by using a timer, The result is almost the same even when the second sample is performed after a certain time has elapsed from the first sample time.

[0019]

FIG. 5 shows an embodiment of the present invention.
The switching element 4 is driven by the output of the driving pulse generator 2, and the voltage of the power supply 3 is applied to the excitation coil 1 in a pulsed manner.
When the switching element 4 is shut off, most of the energy stored in the drive coil is absorbed by the equivalent resistance of the damping circuit 5. The excitation coil voltage generates a large voltage when the switching element 4 is cut off. The voltage value is determined by the current flowing through the excitation coil immediately before the voltage of the excitation coil is cut off and the equivalent resistance of the damping circuit. The time constant at which the voltage attenuates is given by the equivalent resistance value Rd of the dubbing resistance and the inductance L of the exciting coil.
It is determined by L / Rd. If this voltage value is too high, the voltage is attenuated by a voltage divider or a voltage clamp circuit, and if necessary, converted to a voltage that can be easily processed through a buffer amplifier or the like. FIG. 5 shows an example in which the exciting coil and the receiving coil are shared, but each of the coils may be separately provided.

The coil voltage converted into a voltage which can be easily processed is applied to an input of a comparator 17 and voltage sampling switches 6 and 7. One input of the comparator 17 is applied with a voltage obtained by dividing a stable voltage of the reference voltage 12 by a voltage divider using resistors 13 and 14. This voltage value is the value of comparator 1
7 is an important voltage that determines the threshold at which it operates. If the operation delay of the comparator cannot be ignored, the voltage is determined in consideration of the delay.

The output of the comparator 17 is the synchronous pulse generator 1
8 is added. The output of pulse generator 18 is applied to a flip-flop or counter of pulse distributor 19. The function of the pulse distributor 19 is to immediately turn on the voltage sampling switch 6 by the pulse width determined by the oscillation period Ts of the synchronous pulse generator when the output of the comparator 17 is present. In the next cycle of the synchronous pulse generator, the voltage sampling switch 6 is turned off, and the voltage sampling switch 7 is turned on for the oscillation cycle Ts.
Here, when the pulse distributor 19 is a flip-flop, a function of stopping the output of the pulse distributor until the current of the exciting coil is interrupted in the next cycle is necessary, but is omitted in the figure. When the pulse distributor is composed of a counter, as shown in FIG.
It is also possible to make the pressure sampling switch 7 ON only for the oscillation period Ts after an appropriate delay time after N is set. Also, as described above, the switch 7 is set to O
The same result can be obtained by using a comparator whose output is slightly lower in threshold than the former comparator without using a pulse divider for the timing of N.

The delay time Dt shown in FIG. 5 may be determined digitally as shown in FIG. 5, but may be arbitrarily varied by providing another timer. The smaller the delay time Dt is, the more the characteristics of the differential amplifier are exhibited.
The effect of suppressing disturbance is enhanced, and high-order noise can be significantly reduced, but the sensitivity is reduced. If Dt is increased, the sensitivity is increased but the effect of suppressing disturbance is reduced.

In this manner, the voltage of the receiving coil is alternately captured by the voltage sampling switch 6 and the voltage sampling switch 7, and the captured signal is added to the integrator or the peak is held, and then the differential signal is obtained. Add to amplifier 9. The output of the differential amplifier 9 is applied to a DC amplifier to obtain an output 11. The feedback resistor 8 has a part of the output 9
And use it for the purpose of improving stability by applying negative feedback. Here, the differential amplifier 9 functions as a pre-amplifier and converts the voltage of the feedback resistor 8 and the offset voltage obtained by dividing the reference voltage 12 by the voltage dividers 12 and 13 into necessary polarities to obtain a sampled signal. It also has the function of adding or subtracting the difference value. The resistance 8
It is also possible to reduce the long-term drift by putting an integrator after and applying negative feedback to the input. These operations are not limited to the analog circuit shown in FIG. 5, but can convert the sampled voltage into a digital signal, perform an operation by the CPU or process the data by a DSP, and provide an operation of integration or difference.

FIG. 5 shows an example in which the exciting coil and the detecting coil are shared. However, the present invention can be similarly implemented even if the receiving coil and the exciting coil are separated. Also, it is possible to provide a plurality of receiving coils for one excitation coil, detect a signal for each of them with a similar receiving device, and estimate the position and approximate dimensions of the detection conductor. Further, if an iron core or a ferrite core with little loss is used for the excitation coil and the receiving coil, a detector having sharp directivity and high sensitivity can be obtained.

Further, the excitation signal is two-dimensionally detected by a plurality of excitation coils synchronized with each other or by a large number of reception coils arranged in an array, and the output is used as it is, or the difference between adjacent coil outputs is determined by two. It is possible to display the detection conductor image more realistically by displaying it in a dimension. Of course, it is also possible to use a small detection coil and physically scan this coil to display the conductivity distribution in two or three dimensions of the detection conductor, making it a non-destructive inspection device. Is also possible. FIG. 7 illustrates this case, in which a pulse magnetic field is generated by a single rectangular excitation coil 1 and reception coils having 2 to 6 or the same sign are arranged in an array. is there. In FIG. 7, the rectangular single coil 1 can be removed, and the receiving coil can be shared with the exciting coil, which can make the distribution of the magnetic field uniform.

The present system has a feature that a plurality of exciting coils and receiving coils can be arranged without affecting each other, and when a rod-shaped core is inserted into the exciting coil or receiving coil, strong directivity can be provided. In this case, when the target conductor is rod-shaped or one side is long, the sensitivity is highest when the axis of the rod-shaped core is aligned in the length direction. Utilizing this characteristic, it is possible to know in which direction the target conductor is longer based on the output ratio with respect to the axial direction by using an exciting coil or a receiving coil containing a rod-shaped core arranged at right angles to each other. In this case, the receiving coils of each axis may be excited simultaneously or used with time sharing with each other.

The return signal from the detection conductor has an attenuated waveform that varies depending on the conductivity of the detection conductor. Therefore, the delay time Td in FIG. 6 is changed, and the change in output with respect to this time change indicates the equivalent resistance of the detection conductor. It is also possible to detect the difference and estimate the type of the detection conductor.

In order to prevent induction of an external high-frequency signal, if a shield having a higher equivalent series resistance than the detection conductor and a large time constant is used, the induction can be effectively prevented without lowering the detection sensitivity. It is possible. As a specific material, a wire mesh made of a metal having a relatively high specific resistance such as iron, a wire mesh made of an insulated thin wire, and the like are effective, and a similar purpose can be obtained with many shield materials whose loss is intentionally increased. be able to. Although this method seems simple, it is a method that cannot be applied because the balance is lost with the conventional detection method of the comparison type. The present invention effectively utilizes two features that can be selectively detected by the equivalent time constant of the detection conductor and that the zero point of the detector does not shift even if the constant of the detector changes.

In this embodiment, an example is shown in which the magnetic field returned from the detection conductor is detected by a coil. However, the detection of the magnetic field is not limited to the coil, but may be performed by a magnetic element such as a Hall element, a magnetoresistive element, an orthogonal flux gate sensor, or a SQUID. It can also be implemented using a detection element. In an element that directly detects a magnetic field, if the output of this element is differentiated with respect to time, it can be treated as equivalent to a coil voltage, and the present invention can be easily implemented. There is also an advantage that the present invention can be used as it is even when the detecting element outputs an absolute value with respect to the polarity of the magnetic field and the direction cannot be detected. When a magnetic detection element is used as the magnetic field detection element, electrical feedback is also possible.However, an auxiliary feedback coil is added, a feedback current flows through this coil, and negative feedback is applied by the magnetic field generated by this current. To stabilize. If an AC bias is required, the frequency of this bias
It is convenient to use the frequency of the driving pulse generator as shown in the above as it is or to make the ratio of each be an integral multiple.

[0030]

The most significant feature of the present invention is that the point at which the voltage of the receiving coil is sampled is determined not by a predetermined time but by the voltage level of the receiving coil.
When this voltage level is set to an optimum level, it is possible to remove the influence of jitter generated due to fluctuation in the cutoff time due to a change in the parameter of the switching element that cuts off the drive current of the exciting coil, and realizes a stable detector. It becomes possible. Moreover, even if the constants of the exciting coil and receiving coil change and the receiving level changes significantly, the voltage sampling point of the receiving coil can always be kept at the optimum level, and the sensitivity and directivity can be changed according to the purpose. However, it is practical because there is no need to set the difficult voltage sampling timing each time.

When detecting a conductive material at a remote location where a magnetic field does not sufficiently enter the target conductive material, the receiving voltage of the receiving coil is extremely low, and a comparative voltage for determining the threshold value of the comparator, The fluctuation of the offset voltage applied to the first stage of the amplifier greatly affects the detection output. However, when both are applied from the same reference voltage 12 as shown in FIG. 5, the fluctuations act in a direction to cancel each other out, so that the stability can be further improved.

Furthermore, conventionally, when two-dimensional detection is performed by arranging excitation coils and receiving coils in an array, it is necessary to individually set the voltage sampling point and sensitivity of each receiver. In the case of the present invention, the voltage sampling points of all the receivers are set to the optimum points only by setting the threshold value of the comparator, and there is an effect that the adjustment is extremely simple. Conventionally, when there are a plurality of exciting coils, even if synchronization is performed so that the shut-off times of the exciting coils are the same, there is a large difference in characteristics due to variations in switching time of switching elements that drive the respective exciting coils. Although there is a drawback, in the present invention, this time lag is automatically corrected, so that the effect is particularly large when a plurality of receivers are used at the same time.

Further, the voltage waveform of the receiving coil is determined by the equivalent time constant of the detection conductor to be detected.
It is also possible to estimate the type of the detection conductor by capturing the change in the detection output in this case by changing the delay time shown in FIG.
For example, iron and stainless steel can be detected most easily, and can be effectively used for sorting coins. If the excitation pulse width is further narrowed and the repetition frequency is increased, the conductivity of not only metal but also the electrolyte solution and the molten substance can be measured in a non-contact manner.

[0034]

(Equation 1) R _d is the value of the damping resistor inserted in parallel with the exciting coil

[0035]

(Equation 2) L ₁ is the equivalent inductance of the exciting coil, L ₂ is the target conductor equivalent inductance, R ₂ is the equivalent resistance of the target conductor k is the equivalent coupling coefficient between the excitation coil and the target conductor, and M is the mutual inductance ρ ₁ is the exciting coil current T _on immediately before interrupting the exciting coil current, and T _on is the time when the voltage E is applied to the exciting coil L _1. ρ ₂ is the equivalent current flowing in the target conductor immediately before the current of the exciting coil was cut off

[0036]

[Brief description of the drawings]

FIG. 1 Transient voltage waveform of a receiving coil when a magnetic field sufficiently enters a target conductive material

FIG. 2 is a transient voltage waveform of a receiving coil showing the vicinity of an optimum sampling timing when a magnetic field does not sufficiently enter a target conductive material. (The voltage range is significantly smaller than that of Fig. 1.)

FIG. 3 is a transient voltage waveform in a case where a magnetic field does not sufficiently enter a conductive material of interest and a time has elapsed more than an optimum sampling time. (The voltage range is significantly smaller than that of FIG. 1.)

FIG. 4 shows a coil voltage waveform at a timing when the coil voltage changes greatly with respect to a time differential value.

FIG. 5 is a block diagram showing an embodiment of the present invention.

FIG. 6 shows an operating point, a sampling point A, and a sampling point B of a comparator when the present invention is implemented.

FIG. 7 is an explanatory drawing in a case where a receiving coil or a transmitting coil is arranged in an array or a plurality of receiving coils are arranged in a single rectangular coil.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Exciting coil (The case where it is shared with a receiving coil is shown.) 2 Drive pulse generator 3 Exciting coil driving power supply 4 Siege coil driving switching element 5 Damping circuit (including voltage dividing and clamping circuit) 6 Voltage sampling switch 7 Voltage sampling Switch 8 Negative feedback resistor 9 Differential amplifier (including peak hold or integrator, adder / subtractor) 10 High gain DC amplifier 11 Detector output 12 Reference voltage 13 Dividing resistor 14 Dividing resistor 15 Dividing resistor 16 Voltage divider 17 Comparator 18 Synchronous pulse generator 19 Pulse distributor (flip-flop or counter)

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[Procedure amendment]

[Submission Date] April 7, 2000 (200.4.7)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] Explanation of sign

[Correction method] Change

[Correction contents]

[Explanation of Symbols] 1 Exciting coil (shown in the case of sharing with receiving coil) 2 Drive pulse generator 3 Exciting coil driving power supply 4 Exciting coil driving switching element 5 Damping coil circuit (including voltage dividing and clamping circuit) 6 Voltage sampling switch 7 Voltage sampling switch 8 Negative feedback resistor 9 Differential amplifier (including peak hold or integrator, adder / subtractor) 10 High gain DC amplifier 11 Detector output 12 Reference voltage 13 Resistor for voltage divider 14 minutes Voltage resistor 15 Voltage divider 16 Voltage divider 17 Comparator 18 Synchronous pulse generator 19 Pulse distributor (flip-flop or counter)

Claims (7)

[Claims] 1. To detect a conductive material at a remote location where a magnetic field does not sufficiently enter the conductive material of interest,
After a current is passed through an excitation coil having an air core or a magnetic core, the current is cut off. After the current is interrupted, the voltage of the exciting coil is received by the exciting coil itself that is shared with the receiving coil or another receiving coil that is coupled with the exciting coil by mutual inductance. The voltage is sampled for the first time to obtain a voltage A. After the first sampling, the voltage of the exciting coil is sampled again as the second sampling as the second sampling. A method for stabilizing an active metal sensor in which an offset voltage is added to a voltage A or a voltage B and differential amplification is performed, and a conductive material is stably detected at an output of the amplifier. 2. In order to detect a conductive material at a remote place where a magnetic field does not sufficiently enter the conductive material of interest,
After a current is passed through an excitation coil having an air core or a magnetic core, the current is cut off. After the current is cut off, the voltage of the exciting coil is received by the exciting coil itself shared with the receiving coil or another receiving coil coupled with the exciting coil by mutual inductance, and when the received voltage reaches a specified level Esr, the exciting coil Is the voltage A as the first sampling. After the first sampling, the voltage of the exciting coil is sampled again as the second sampling as the second sampling. An active metal sensor that differentially amplifies the voltage A or the voltage B after adding the offset voltage Eoff and then stably detects a conductive material at the output of the amplifier, and determines a threshold value for sampling the voltage A. An active metal sensor in which the level Esr is proportional to the offset voltage Eoff. 3. In order to detect a conductive material in a remote place where a magnetic field does not sufficiently enter the conductive material of interest,
After a current is passed through an excitation coil having an air core or a magnetic core, the current is cut off. After the current is cut off, the voltage of the exciting coil is received by the exciting coil itself shared with the receiving coil or another receiving coil coupled with the exciting coil by mutual inductance, and when the received voltage reaches a specified level Esr, the exciting coil Is the voltage A as the first sampling. After the first sampling, the voltage of the exciting coil is sampled again as the second sampling as the second sampling. An active metal sensor that differentially amplifies the voltage A or the voltage B after adding the offset voltage Eoff and stably detects a conductive material at the output of the amplifier, samples the voltage A, and then samples the voltage B. A metal sensor that changes the delay time Td up to and detects the change in the detected voltage with respect to the delay time Td, and knows the difference in the conductivity of the target conductor. 4. A metal sensor in which a plurality of metal sensors according to the method of claim 1 or 2 are arranged and synchronized so that the cutoff times of the respective exciting coils coincide. 5. A method of arranging receiving coils according to the method of claim 3 in an array and knowing an outline of a target conductor by a voltage level detected by each receiving coil. 6. A metal detecting method according to claim 1, wherein a single exciting coil is used and a plurality of receiving coils are arranged in an array. 7. A metal sensor according to claim 1 or 2, wherein a plurality of exciting coils or receiving coils having different directivities are arranged, and currents for driving the exciting coils are simultaneously or temporally shifted to excite. Then, the method of knowing the shape of the target conductor from the difference between the amplifier output voltages.