JP2003273718A - Proximity sensor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To realize high sensitivity of a proximity sensor. <P>SOLUTION: An MI (magnetic impedance) element 1 is arranged inside an exciting coil 2 and the magnetic flux T of an eddy current from a metallic body F is detected by the MI element 1. The oscillation frequency of an exciting oscillation circuit 5 is set so that eddy-current magnetic flux T of which the phase is delayed by 90° from exciting magnetic flux S. A timing control circuit 7 generates a sampling pulse in a prescribed period of which the center is a point of time that an exciting current from the circuit 5 becomes zero. Outputs from the MI element 1 are integrated in a pulse output period by the operation of a gate circuit 9 and a positive/negative sampling/holding circuit 10 corresponding to the sampling pulse. The integrated result is accumulated in a differential amplifier circuit 11 and compared with a prescribed threshold by a comparator 12. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a proximity sensor used for applications such as non-contact detection of a metal body to output an ON signal and detection of a distance to the metal body.

[0002]

2. Description of the Related Art Generally, a proximity sensor uses a high frequency oscillation circuit having an LC resonance circuit as a detection section to detect the presence or absence of a metal body and the distance to the metal body based on the change in the oscillation amplitude of the oscillation circuit. There is. Further increase the sensitivity,
A differential coil type proximity sensor is used when it is necessary to be able to detect a metal object that is farther away.

FIG. 12 shows the structure of the differential coil type proximity sensor. The detection unit of this sensor includes an oscillating circuit 33 that oscillates a high frequency, an exciting coil 30 that is connected to the oscillating circuit 33, and two detecting coils 31 that are arranged before and after the exciting coil 30 at equal distances. , 32. The two detection coils 31 and 32 are differentially connected, and the output ends of the coils 31 and 32 are input to the processing circuit 34. In addition, F in a figure shows the metal object of detection object.

In the above structure, each detection coil 31, 3
Since 2 is arranged at the same distance with respect to the exciting coil 30, the magnetic flux S generated from the exciting coil 30 (hereinafter, referred to as "exciting magnetic flux S") causes the detection coils 31 and 32 to have the same size. Voltage is induced. On the other hand, when the metal body F enters the detection area, the exciting magnetic flux S acts on the metal body F to generate an eddy current. The magnetic flux T due to this eddy current (hereinafter, referred to as “eddy current magnetic flux T”) also acts on each of the detection coils 31 and 32, but since each detection coil 31 and 32 has a different distance from the metal body F, The eddy current magnetic flux T causes a difference in the voltages induced in the detection coils 31 and 32.

The processing circuit 34 includes the detection coils 31,
It includes a detection circuit for detecting the differential voltage between 32 and a comparator for comparing the detected voltage with a predetermined threshold value, and outputs a signal indicating the presence or absence of a metal body based on the output from this comparator. . Further, depending on the type of sensor, a voltage signal of a predetermined level indicating the distance may be generated according to the differential voltage between the detection coils 31 and 32, and this may be output as a detection signal.

[0006]

In the structure of FIG. 12, the eddy current magnetic flux T generated from the metal body F is the exciting magnetic flux S.
It is much smaller than. Especially when the metal body F is far away, the eddy current magnetic flux T is detected by the detection coils 31, 32.
Therefore, the induced voltage due to the eddy current magnetic flux T becomes small. Therefore, it is necessary to minimize the potential difference between the detection coils 31 and 32 due to factors other than the eddy current magnetic flux T, balance the voltage between the detection coils 31 and 32, and sufficiently reduce the residual voltage.

However, since there is a slight difference in the inductance of the coils, even if the distance between the detecting coils 31 and 32 with respect to the exciting coil 30 is the same as described above, the exciting magnetic flux S induces in the respective detecting coils 31 and 32. There will be a difference in the actual voltage. In the processing circuit 34, in general,
Since the differential signals from the detection coils 31 and 32 are amplified and then processed, if the difference in induced voltage due to the exciting magnetic flux S becomes large, it may be difficult to detect the potential difference based on the eddy current magnetic flux T.

Further, in the above configuration, the detection coils 31, 3 are
Since the distance between 2 and the exciting coil 30 is short, a voltage is generated by electrostatic coupling between the coils. Therefore, the detection coil 3
It is difficult to balance the voltage between 1 and 32, and it is also difficult to reduce the residual voltage. Further, in order to detect the minute eddy current magnetic flux T, it is necessary to make the detection coils 31 and 32 large.

Further, in the case of a proximity sensor of the type which detects with a single coil, by using a core having an E-shaped or T-shaped cross section for the coil, the exciting magnetic flux S is prevented from flowing laterally or rearward. Although it is possible to limit the directivity of the magnetic flux to the detection surface side by limiting, in the above-mentioned differential coil type sensor, since the detection coils 31 and 32 must be arranged in both the front and rear directions of the exciting coil 30, It is impossible to use such a core.

As described above, the conventional differential coil type sensor has drawbacks in that the size of the detecting portion is increased, the structure is restricted, and it is difficult to balance the voltage between the detecting coils. However, there is a problem that sufficient sensitivity cannot be obtained.

The present invention has been made in view of the above problem, and adjusts so as to detect an eddy current magnetic flux by using a magnetoresistive element and to detect the eddy current magnetic flux at a timing at which the influence of the exciting magnetic flux is less likely to occur. By doing so, it is an object to realize high sensitivity of the proximity sensor.

[0012]

A proximity sensor according to the present invention comprises an exciting coil for causing an exciting magnetic flux to act on a metal body to be detected, and a magnetic impedance element arranged in the exciting coil in a predetermined positional relationship. A high-frequency oscillation circuit connected to each of the exciting coil and the magnetic impedance element, and a signal processing for detecting a metal body by sampling an output from the magnetic impedance element with reference to a time point when a current flowing through the exciting coil is reversed. And a signal processing unit for executing.

The magneto-impedance element (hereinafter referred to as "MI element") has a characteristic that the magnetic permeability changes due to an external magnetic field under the condition that a high-frequency current is applied, and the impedance causes a large change. is there. In the proximity sensor according to the present invention, this MI element is used as an eddy current magnetic flux detecting means in place of the above-mentioned detection coil.

The MI element is a thin film element integrated on a non-magnetic substrate, and the element and the substrate can be made much smaller than the detection coil. Therefore, for example, the substrate of the MI element can be arranged inside the exciting coil. However, the arrangement position of the MI element is not limited to this, and the MI element can be arranged adjacent to the exciting coil or in front of the exciting coil.

The MI element has its longitudinal direction orthogonal to the detection surface of the proximity sensor (for example, the end surface of the case body constituting the proximity sensor body), in other words, a vortex from the metal body to be inspected. It is desirable to arrange the MI element so that the longitudinal direction of the MI element corresponds to the direction in which the current magnetic flux acts on the detection surface of the sensor. This is because the MI element has high sensitivity to magnetism in the longitudinal direction.

The high-frequency oscillator circuit is individually provided for the exciting coil and the MI element. These high-frequency oscillation circuits are for supplying a high-frequency current to the exciting coil and the MI element, respectively. The signal processing unit generates a sampling pulse at a predetermined timing while detecting a load resistance for extracting the impedance change of the MI element as a voltage, a detection circuit of this voltage signal, and a polarity change of the high frequency current given to the exciting coil. It may include a timing control circuit for controlling, a gate circuit for taking in the output from the detection circuit in response to the sampling pulse, and the like.
Further, the processing unit can include a circuit for amplifying or integrating the sampled signal, a comparator for comparing the signal processed by the circuit with a predetermined threshold value, and the like. Alternatively, a circuit for converting the amplified signal into a voltage signal indicating the distance and outputting the voltage signal may be provided. Therefore, the above-mentioned “signal processing for detecting a metal body” can be referred to as signal processing for executing at least one of the presence or absence of the metal body and the distance to the metal body.
Further, by incorporating a microprocessor in the signal processing unit, the main processing by each of the above circuits can be executed as digital signal processing.

According to the proximity sensor having the above structure, since a high-frequency current flows through both the exciting coil and the MI element, an exciting magnetic flux is generated, and then an eddy current from the metal body which is affected by the exciting magnetic flux. It is possible to cause the magnetic flux to act on the MI element and to cause the impedance to change according to the magnitude of the magnetic flux. However, since the exciting magnetic flux from the exciting coil also acts on the MI element, it is desirable to extract and process a signal that is less affected by the exciting magnetic flux. Since the signal processing unit of the present invention samples the output from the MI element with reference to the time point at which the current flowing through the exciting coil is reversed, the output from the MI element when the magnitude of the exciting magnetic flux is zero or close to zero is used. It is possible to detect the metal body. Therefore, it is possible to perform a highly accurate detection process by sampling a signal that is less affected by the exciting magnetic flux.

Since the MI element and the substrate on which this element is mounted are thin, the sensor body can be miniaturized. Further, since it is not necessary to dispose another coil or the like behind the exciting coil, the ferrite core can be used to enhance the directivity of the exciting magnetic flux, and the sensitivity can be enhanced.

In a preferred mode of the proximity sensor, when an exciting magnetic flux is applied to a metal body, the difference between the peak of the eddy current magnetic flux generated in the metal body and the time when the current flowing through the exciting coil becomes zero is a predetermined time. The oscillating frequency of the high-frequency oscillating circuit connected to the exciting coil is adjusted so as to be inside.

Generally, the eddy current magnetic flux is
There is a characteristic that the phase is delayed by an angle corresponding to the frequency of the exciting magnetic flux. If the phase difference is adjusted to 90 degrees by adjusting the oscillation frequency, the time when the current flowing through the exciting coil becomes zero and the time when the peak appears in the eddy current magnetic flux. Therefore, by sampling the output from the MI element at this time, it is possible to detect the change in impedance due to the action of only the eddy current magnetic flux, which is not affected by the exciting magnetic flux at all. Further, the signal component due to the eddy current magnetic flux can be sampled most efficiently.

Therefore, according to the above aspect, the oscillation frequency for the exciting coil is adjusted so that a phase difference close to 90 degrees is obtained, and the output from the MI element is based on the time point when the current flowing through the exciting coil is reversed. Is sampled, the influence of the exciting magnetic flux can be largely removed. Further, the signal component due to the eddy current magnetic flux can be efficiently sampled to perform highly accurate detection processing. Since the relationship between the phases of the exciting magnetic flux and the eddy current magnetic flux also changes depending on the magnetic permeability and conductivity of the metal body, the proximity sensor of the above aspect further includes means for adjusting the oscillation frequency for the exciting coil. May be provided.

In a further preferred embodiment of the proximity sensor, a magnet or coil for setting a predetermined bias magnetic field with respect to this MI element is provided near the MI element. When an exciting magnetic flux is applied to the metal body, an eddy current magnetic flux having a constant polarity in a predetermined period centered around the time when the polarity of the current flowing through the exciting coil is reversed is generated from the metal body. The oscillation frequency of the high frequency oscillation circuit connected to the excitation coil is adjusted. Further, the signal processing unit is configured to perform the detection processing of the metal body by using the signal obtained by sampling and integrating the output signal from the MI element within the period.

According to the above aspect, when a bias magnetic field is applied to the MI element, when an eddy current magnetic flux acts on the MI element to change the impedance, a positive or negative voltage depending on the polarity of the applied magnetic flux. It becomes possible to take out the signal. According to the above aspect, in the period centered around the time when the exciting magnetic flux becomes zero, the eddy current magnetic flux has either positive or negative polarity, while the exciting magnetic flux is the same for both positive and negative polarities. The rate will change by the same amount. In this mode, since the output from the MI element within the above period is sampled and integrated, the positive and negative signal components of the excitation magnetic flux are canceled and the integration result becomes zero, and the eddy current magnetic flux is reduced within the period. A large value can be obtained by accumulating the signal components of. Therefore, by performing the detection processing of the metal body using the integration result, it is possible to perform the detection processing with high accuracy without being substantially affected by the exciting magnetic flux.

According to the above aspect, positive and negative polarities alternately appear in the integration result in each sampling period, but the polarity is not taken into consideration, and the presence or absence of the metal body and the distance are determined based on the size of the integration result. Should be determined. Further, if the integration result is negative, it may be inverted to be positive, and then the integration results in several sampling periods may be sequentially accumulated, and the above determination processing may be performed using the accumulated value.

Further, according to the present invention, an exciting coil for causing an exciting magnetic flux to act on the metal body to be detected, a pair of MI elements arranged symmetrically with the exciting coil sandwiched therebetween, and connected to the exciting coil. A first high-frequency oscillator circuit, and a second high-frequency oscillator circuit connected to each MI element,
There is provided a proximity sensor including: a signal processing unit that performs signal processing for detecting a metal body by sampling a differential signal of an output from each MI element with reference to a time point when a current flowing through the exciting coil is reversed.

In the above structure, each MI element is arranged at a position different in distance from the metal body to be detected, like the detection coil in the proximity sensor of the differential coil type. Therefore, by taking the difference between the outputs from these MI elements, it is possible to obtain a signal having a different level depending on the position of the metal body to be detected. Here, since the signal processing unit samples the differential signal of the output from each MI element with reference to the time point at which the current flowing through the exciting coil is reversed, it is possible to sample a signal that is less affected by the exciting magnetic flux.

Further, as described above, since the MI element is formed as a thin film element smaller than the coil, it can be arranged with a sufficient distance from the exciting coil, and the capacitance between the MI element and the coil can be increased. Can be made smaller. Also M
Since the I element is manufactured on the basis of a fixed specification, variations in output among individual MI elements are small. Therefore, the component of the individual difference in the difference signal can be reduced, and highly accurate detection processing can be performed.

[0028]

1 shows the structure of a proximity sensor according to an embodiment of the present invention. The proximity sensor includes a detection unit including an exciting coil 2 and an MI element 1. The excitation coil 2 and the MI element 1 are respectively composed of oscillation circuits 5 and 6 that oscillate high frequencies (in the figure, "excitation oscillation circuit 5" and "MI").
Driving oscillator circuit 6 ”. ) Is connected to. The exciting coil 2 is arranged so that the magnetic flux links in a direction perpendicular to the detection surface of the sensor (corresponding to the end surface of the case body that forms the sensor body (not shown)).

The MI element 1 is formed on a non-magnetic substrate 3 such as a glass plate. This substrate 3 is inserted inside the exciting coil 2 with the length direction of the MI element 1 aligned with the direction of the eddy current magnetic flux T to be detected.

The excitation oscillation circuit 5 and the MI driving oscillation circuit 6 are included in the signal processing circuit 4 of the proximity sensor.
In addition, the signal processing circuit 4 has a configuration for processing the output from the MI element 1 and outputting a signal indicating the presence / absence of the metal body F (hereinafter, referred to as “object presence signal”) as a timing control circuit 7. , A detection circuit 8, a gate circuit 9, a positive / negative sample hold circuit 10, a differential amplifier circuit 11, a comparator 12, an output circuit 13 and the like are incorporated.

The exciting oscillation circuit 5 includes an exciting coil 2
A resistor 14 for detecting the phase of a high-frequency current (hereinafter, referred to as “excitation current”) to the circuit as a voltage is connected. The voltage signal detected by the resistor 14 is input to the timing control circuit 7. In this timing control circuit 7,
A differentiation circuit for determining the polarity of the voltage signal, a timing generation circuit for generating a sampling pulse based on the change in the polarity, and the like are included.

The detection circuit 8 is connected to the output terminal of the MI element 1. A load resistance 15 corresponding to the reference impedance of the MI element 1 is applied to this connection path. A voltage corresponding to a change in impedance of the MI element 1 is applied to the detection circuit 8 by the effect of the voltage division by the resistor 15. The detection circuit 8 amplitude-modulates this voltage signal and outputs the processed signal to the gate circuit 9.

The timing control circuit 7 of this embodiment is centered around the time when the level of the exciting current becomes zero, and the polarity of this current is reversed within a predetermined period during which the polarity is reversed from positive to negative or from negative to positive. Generates and outputs a sampling pulse having a polarity opposite to the direction in which changes. That is, when the exciting current changes from positive to negative, a positive pulse is generated, and when the exciting current is changed from negative to positive, a negative pulse is generated, so positive and negative sampling pulses are output alternately. Will be done.

The sampling pulse is supplied to the gate circuit 9
And the positive and negative sample and hold circuit 10. The gate circuit 9 detects the detection circuit 8 according to the sampling pulse.
To pass to the positive / negative sample hold circuit 10. The positive / negative sample hold circuit 10 includes a circuit that operates according to a positive sampling pulse and a circuit that operates according to a negative sampling pulse. These circuits include an integrating circuit, and the gate circuit 9
The signal passed through is integrated according to the sampling pulse, and the integrated result is output to the differential amplifier circuit 11. The positive and negative sample hold circuit 10 of this embodiment outputs the held signal to the differential amplifier circuit 11 while holding the integration result until the next sampling pulse of the same polarity is given.

In the differential amplifier circuit 11, the signal corresponding to the positive sampling pulse is input to the + side input terminal and the signal corresponding to the negative sampling pulse is input to the negative side input terminal, respectively. Dynamic amplification processing is executed. The comparator 12 takes in the differential amplified signal and compares it with a predetermined threshold value. Furthermore, the output circuit 13
Takes in the comparison output of the comparator 12 and outputs it as an object presence / absence signal to the outside.

FIG. 2 shows the relationship between the exciting magnetic flux S generated from the exciting coil 2 and the eddy current magnetic flux T generated from the metal body F. Since the eddy current magnetic flux T is generated by the action of the exciting magnetic flux S, a phase delay of a predetermined time t occurs with respect to the exciting magnetic flux S.

FIG. 3 is a vector representation of the relationship between the exciting magnetic flux S, the eddy current magnetic flux T, and the phase difference generated between the two magnetic fluxes S, T by the electrical angle with the exciting magnetic flux S as a reference. The vector A in the drawing corresponds to the exciting magnetic flux S, and the vector B corresponds to the eddy current magnetic flux T. Also the vector C
Corresponds to a composite vector of these vectors A and B, that is, a magnetic flux acting on the detection surface of the proximity sensor.

The angle θ in the figure corresponds to the phase shift time t of the eddy current magnetic flux T with respect to the exciting magnetic flux S. The phase difference θ changes depending on the frequency of the exciting magnetic flux S (hereinafter, referred to as “exciting frequency”) and the magnetic permeability and conductivity of the metal F. If the excitation frequency f is changed from 0 to ∞, the tips of the vectors B and C change like the loci indicated by the broken lines X and Y in the figure. In the figure, X is a locus obtained for a magnetic metal and Y is a locus obtained for a non-magnetic metal. In any of the loci, the angle θ of the vector B with respect to the vector A, that is, the phase delay of the eddy current magnetic flux T with respect to the exciting magnetic flux S is 90.
There is a frequency f that is in degrees.

When the phase difference θ is 90 degrees,
The time when the exciting magnetic flux S becomes zero coincides with the time when the eddy current magnetic flux T reaches a peak. That is, the exciting magnetic flux S
At the time point when is zero, the exciting magnetic flux S does not act on the MI element 1, while the eddy current magnetic flux T having the highest level in the cycle acts. Therefore, MI at this point
From the element 1, it is possible to extract the impedance change due to the action of the eddy current magnetic flux T most efficiently and in a state where it is not affected by the exciting coil. Eddy current magnetic flux T
The relationship between the phase and the exciting magnetic flux S is constant as long as the type of the metal body F and the exciting frequency do not change. Therefore, if the excitation frequency is selected so that the phase difference θ becomes 90 degrees and the output from the MI element 1 is sampled at the timing when the polarity of the excitation magnetic flux S is reversed, the excitation magnetic flux S
It is possible to perform highly accurate signal processing without being affected by.

The proximity switch shown in FIG. 1 is made by applying the above-mentioned principle.
Based on the result of measurement in advance, it is set to oscillate at a frequency such that the phase difference θ with respect to the eddy current magnetic flux T becomes an angle close to 90 degrees. If the oscillation frequency can be adjusted in multiple stages, the excitation frequency can be adjusted according to the type of the metal body F to be detected.

FIG. 4 shows the operation of the proximity switch of FIG. In the figure, (a) shows the output timing of the sampling pulse from the timing control circuit 7. In this embodiment, as described above, a predetermined period centered on the time when the exciting magnetic flux S becomes zero is set as a sampling period, and a sample pulse having a polarity opposite to the direction in which the polarity changes during this period is output. In this sampling period, the exciting magnetic flux S changes by substantially the same amount for both positive and negative poles.
On the other hand, since the eddy current magnetic flux T in each sampling period has a phase delayed by about 90 degrees from the exciting magnetic flux S, it has the same polarity as the sampling pulse.

FIG. 4B shows the signal output timing of the positive / negative sample hold circuit 10. The integration result corresponding to the positive / negative sample pulse is maintained until the next same-polarity sampling pulse is given. While being output.

Since the positive / negative sample hold circuit 10 integrates the output from the MI element 1 during the sampling period, the exciting magnetic flux S changing by the same amount to the positive / negative poles is canceled, but the same polarity as the sampling pulse is applied. The levels of the eddy current magnetic flux T having the polarity of are accumulated. Therefore, the positive / negative sample-hold circuit 10 outputs a signal that highly accurately reflects the change in impedance due to the action of the eddy current magnetic flux T during the sampling period. Further, since the output from the MI element 1 in each sampling period has the same polarity as the sampling pulse, the signal indicating the integration processing result also has the same polarity.

FIG. 4C shows the timing of signal output from the differential amplifier circuit 11, and FIG. 4D shows the state of change of the object presence / absence signal from the output circuit 13. As described above, since the differential amplifier circuit 11 inputs the signal corresponding to the positive sampling pulse to the + side input terminal and the signal corresponding to the negative sampling pulse to the-side input terminal, respectively, The negative integration result corresponding to the sampling pulse is inverted to the positive polarity and accumulated in the positive integration result. Therefore, when the eddy current magnetic flux T increases due to the approach of the metal body F, the differential amplification output also takes a large value. When the level of the differential amplification output exceeds the comparison level of the comparator 12, the output level of the comparator 12 is turned on. Corresponding to this, output circuit 1
The object presence / absence signal from 3 is also set to the ON state indicating “with metal body”.

In the example of FIG. 4, the eddy current magnetic flux T is gradually increased, and a signal centered on each of positive and negative peaks in each cycle is sampled, and the integration process and the differential amplification process are performed. As a result, the differential amplifier circuit 11 outputs a signal having a level higher than the comparison level of the comparator 12 when the sample-hold output of the positive polarity signal in the third cycle is performed in the figure. The object presence / absence signal is turned on according to the change in the signal level.

As described above, in the embodiment shown in FIG. 1, since the thin and small MI element 1 is used in place of the detection coil in the conventional differential coil type sensor, the sensor can be miniaturized. Further, according to this embodiment, since it is not necessary to provide another coil or the like behind the exciting coil 2, it is possible to limit the flow of magnetic flux to the side or the rear of the exciting coil 2 by using the ferrite core. It is possible to improve the directivity of the exciting magnetic flux S and improve the sensitivity. Moreover, by the above-described adjustment of the excitation frequency and signal processing, it is possible to efficiently sample a signal that is not easily affected by the excitation magnetic flux S and perform a highly accurate detection process.

Although the MI element 1 is arranged inside the exciting coil 2 in the above embodiment, the arrangement is not limited to this, and the arrangement relations shown in FIGS. 5 and 6 may be set.

In the examples of FIGS. 5 and 6, the MI element 1 and the exciting coil 2 are arranged in parallel. In the example of FIG. 5, the orientation relationship between the MI element 1 and the exciting coil 2 is the same as in the first embodiment. On the other hand, in the example of FIG.
The exciting coil 2 is made long in the width direction and its end face is
It is arranged so as to face the I element 1. With such an arrangement, the exciting magnetic flux S with respect to the MI element 1
It works from the direction along the width direction of the. Since the sensitivity of the MI element 1 to the magnetism in the width direction is much weaker than the sensitivity in the longitudinal direction, the effect of the action of the exciting magnetic flux S on the MI element 1 can be reduced. When setting the arrangement relationship as shown in FIG. 6, if the core 16 having an I-shaped cross section is used for the exciting coil 2 as shown in FIG. 7, the influence of the exciting magnetic flux S on the MI element 1 is further reduced. can do.

In the examples of FIGS. 5 and 6 as well, the signal processing unit 4 can be configured similarly to that of FIG. By the way, the impedance of the MI element 1 with respect to the external magnetic field has a characteristic that, as shown in FIGS. 8A and 8B, the maximum is when the external magnetic field is zero or close to zero, and the impedance decreases as the magnetic field becomes stronger. In the configuration shown in FIG. 1, it is necessary to apply a predetermined bias magnetic field to the MI element 1 in order to input signals having positive and negative polarities to the positive and negative sample hold circuit 10. That is, FIG. 8 (1)
In (2), the magnetic field near the midpoint (indicated by point p in the figure) of the region r in which the impedance changes substantially linearly is set as the bias magnetic field, and the impedance corresponding to this bias magnetic field is set to the resistance 15 If it is provided, the impedance change of the MI element 1 can be detected as a voltage signal touching both positive and negative polarities.

FIG. 9 shows an example in which a coil 17 for a bias magnetic field is provided in the detection section of the proximity sensor having the structure shown in FIG. In the example of FIG. 9 (1), inside the exciting coil 2,
A bias magnetic field coil 17 is provided in the same direction as that of the exciting coil 2, and the MI element 1 is provided inside the coil 17.

In the example of FIG. 9B as well, the coil 17 for the bias magnetic field is provided inside the exciting coil 2, but the coil 17 is oriented in the direction orthogonal to the exciting coil 2. The MI element 1 is arranged on one side of the coil 17.

9 (1) and 9 (2), the bias magnetic field coil 17 is connected to a DC power source (not shown), and a positive bias magnetic field is applied to the MI element 1. The means for applying the bias magnetic field is not limited to the coil, and a permanent magnet may be arranged near the MI element 1.

10 and 11 show the structure of another proximity sensor according to the present invention. In the proximity sensor of this embodiment,
MI elements 1a and 1b are arranged in front of and behind the exciting coil 2 arranged similarly to the embodiment of FIG. The pair of MI elements 1a and 1b are arranged at the same distance from the exciting coil 2 and are supplied with a high frequency current from the same MI driving oscillation circuit 6.

In the signal processing section 4 of this embodiment, each MI element 1a, 1b is provided with a load resistor 15a, 15b corresponding to a reference impedance and a detection circuit 8a, 8b. Further, a differential amplifier circuit 18 for obtaining a difference between outputs from the respective detection circuits 8a and 8b, an amplifier circuit 19 for further amplifying the differential amplified output, and the like are provided. The other circuits in the signal processing unit 4 are the same as those in FIG. 1 and are set to perform the same movements as in FIG.

In the above structure, the gate circuit 9 is supplied with a differential amplification output through the amplification circuit 19, that is, a signal corresponding to the difference between the outputs from the MI elements 1a and 1b.
Therefore, the positive / negative sample-hold circuit 10 integrates the voltage corresponding to the difference in the magnitude of the eddy currents acting on the MI elements 1a and 1b in each sampling period, and the differential amplifier circuit 11 accumulates the integration result. A voltage signal is output.

Therefore, depending on the metal body F to be detected, each M
When a difference occurs in the output level from the I elements 1a and 1b, the difference is detected by the circuits described above, and the comparator 12
The comparison output of is turned on, and the object presence / absence signal from the output circuit 13 is also turned on.

Since the MI elements 1a and 1b are formed as thin film elements smaller than the coils, even if they are arranged as shown in FIG. 10, a sufficient distance is provided between the exciting coil 2 and each MI element 1. be able to. Therefore, the capacitance between each MI element 1 and the exciting coil 2 can be reduced. Further, since the MI element 1 is manufactured based on a fixed specification, it is possible to reduce variations in output between individual elements. Therefore, the noise due to the individual difference of the MI element 1 and the variation of the electrostatic capacitance between the MI element 1 and the exciting coil 2 can be reduced, and the output difference due to the eddy current magnetic flux T from the metal body F can be accurately sampled. It is possible to perform detection processing with much higher accuracy than that of the differential coil type proximity sensor of.

[0058]

According to the present invention, the magneto-impedance element is used as a means for detecting the eddy current magnetic flux generated by applying the exciting magnetic flux to the metal body.
Since the output from the magneto-impedance element is sampled and signal processing for metal object detection is executed based on the time when the current flowing in the exciting coil is reversed, the influence of the exciting magnetic flux is small and the eddy current magnetic flux It is possible to perform detection processing using a signal that reflects the size with high accuracy.
Therefore, the sensitivity to the metal body is significantly improved, the detectable distance to the metal body can be increased, and a high-performance proximity sensor can be provided.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration of a detection unit and a signal processing unit of a proximity sensor according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating a phase relationship between an excitation magnetic flux and an eddy current magnetic flux.

FIG. 3 is a diagram for explaining a phase relationship between an excitation magnetic flux and an eddy current magnetic flux using a vector.

FIG. 4 is a timing chart showing a specific example of signal processing for detecting a metal body.

FIG. 5 is an explanatory diagram showing another arrangement example of the exciting coil and the MI element.

FIG. 6 is an explanatory diagram showing another arrangement example of the exciting coil and the MI element.

7 is an explanatory diagram showing an example in which a core is used in the exciting coil of FIG.

FIG. 8 is an explanatory diagram showing characteristics of impedance change in the MI element.

FIG. 9 is an explanatory diagram showing an arrangement example of a coil for a bias magnetic field.

FIG. 10 is an explanatory diagram showing a specific example in which a pair of MI elements is used in the detection unit.

11 is a block diagram showing a configuration of a proximity sensor using the detection unit of FIG.

FIG. 12 is an explanatory diagram showing a configuration of a conventional differential coil type proximity sensor.

[Explanation of symbols]

1 MI element 2 excitation coil 4 Signal processing unit 5 Oscillation circuit for excitation 6 MI drive oscillator circuit 7 Timing control circuit 9 gate circuit 10 Positive / negative sample and hold circuit 17 Bias field coil S exciting magnetic flux T Eddy current magnetic flux F metal body

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Claims (4)

[Claims] 1. An exciting coil for causing an exciting magnetic flux to act on a metal body to be detected, a magnetic impedance element arranged in the exciting coil in a predetermined positional relationship, and connected to the exciting coil and the magnetic impedance element, respectively. A high-frequency oscillating circuit, and a signal processing unit that performs signal processing for detecting a metal body by sampling the output from the magneto-impedance element on the basis of the time when the current flowing through the exciting coil is reversed. Sensor. 2. When the exciting magnetic flux is applied to a metal body, the difference between the peak of the eddy current magnetic flux generated in the metal body and the time when the current flowing through the exciting coil becomes zero is within a predetermined time. The proximity sensor according to claim 1, wherein the oscillation frequency of a high-frequency oscillation circuit connected to the exciting coil is adjusted. 3. A magnet or coil for setting a predetermined bias magnetic field for the impedance element is provided in the vicinity of the magnetic impedance element, and when the exciting magnetic flux is applied to the metal body, The oscillating frequency of the high-frequency oscillation circuit connected to the exciting coil is set so that an eddy current magnetic flux having a constant polarity in a predetermined period centered around the time when the current flowing through the exciting coil becomes zero is generated from the metal body. The proximity sensor according to claim 1 or 2, wherein the signal processing unit is adjusted, and the signal processing unit executes the detection processing of the metal body by using a signal obtained by sampling and integrating an output from the magneto-impedance element within the period. . 4. An exciting coil for causing an exciting magnetic flux to act on a metal body to be detected, a pair of magneto-impedance elements arranged at symmetrical positions with the exciting coil sandwiched therebetween, and a first coil connected to the exciting coil. 1. The high frequency oscillation circuit of No. 1, the second high frequency oscillation circuit connected to each magneto-impedance element, and the differential signal of the output from each magneto-impedance element is sampled on the basis of the time point when the current flowing in the exciting coil is reversed. A proximity sensor including a signal processing unit that performs signal processing for detecting a metal body.