JP4898971B1 - Inductance change detection circuit, displacement detection device, and metal detection device - Google Patents

Abstract

An inductance change detection circuit for effectively detecting a change in inductance with a single coil and realizing a small and highly accurate displacement sensor, and a displacement detection device and a metal detection device using the same are provided. To do.
A coil is connected to a rectangular wave AC voltage source, an AC current flowing through the coil is converted into a voltage signal, and then passed through a comparator having a hysteresis characteristic, whereby the inductance change of the coil is changed as a phase change at the rising edge of the pulse. To detect. Since there is no need to provide two coils as in the prior art and the number of parts is reduced, a highly accurate displacement sensor can be realized at low cost.
[Selection] Figure 3

Description

The present invention relates to an inductance change detection circuit, a displacement detection device, and a metal detection device.
More specifically, the present invention relates to an inductance change detection circuit constituting an active displacement sensor (displacement detection device) that realizes highly sensitive and stable detection performance, and a displacement detection device and a metal detection device using the inductance change detection circuit.

  Unlike the resistance slider, the applicant manufactures and sells an active displacement sensor that can output a linear displacement amount with a linear analog voltage without an electromechanical contact part. Hereinafter, the operation principle of the active displacement sensor will be described.

FIGS. 10A, 10B, and 10C are graphs for explaining various characteristics of the coil for explaining the operation principle of the displacement sensor manufactured and sold by the applicant.
10A is a circuit diagram for explaining the principle of the displacement sensor, FIG. 10B is a diagram showing the state of the switch SW in the circuit of FIG. 10A, and FIG. 10C is FIG. It is a graph which shows the transient response characteristic of the both-ends voltage of the capacitor | condenser C of the circuit of (a).

As shown in FIG. 10A, when a switch SW, a resistor R, a coil L and a capacitor C are connected in series to the DC power source E, and the switch is turned on as shown in FIG. As shown in FIG. 10C, the both-end voltage gradually increases due to the action of the coil L and then converges to a certain voltage while vibrating. This is a typical step response waveform.
It is well known that the rising slope of the voltage waveform immediately after the switch SW is turned on becomes gentler as the inductance of the coil L becomes larger and becomes steeper as the inductance of the coil L becomes smaller.
The displacement sensor of the present applicant is realized by obtaining the change in inductance of the coil from a transient response phenomenon.

  When a non-magnetic metal is brought close to a coil carrying alternating current, an induced current is generated in the metal. That is, part of the magnetic flux generated from the coil is converted into heat, so that the inductance of the coil is reduced. Therefore, when a non-magnetic metal tube such as brass is put on a coil formed in a rod shape, the inductance changes linearly according to the degree of covering of the tube with respect to the coil. When this change in inductance is detected, a displacement sensor can be realized.

FIG. 10D is a graph showing the transient response characteristics of the voltage across the capacitor C when a metal is brought close to the coil.
When the switch SW of the circuit shown in FIG. 10A is turned on without bringing a metal object close to the coil L, the voltage across the capacitor C gradually increases due to the action of the coil L, as in FIG. Then, it converges to a certain voltage while vibrating (S1001).
When the switch SW of the circuit in FIG. 10A is turned on with a metal object close to a part of the coil L, the rising slope of the voltage across the capacitor C becomes steep as the inductance of the coil L decreases. (S1002).
When the switch SW of the circuit of FIG. 10 (a) is turned on with the entire coil L covered with a metal object, the rising slope of the voltage across the capacitor C becomes steeper as much as the inductance of the coil L is further reduced. (S1003).
Thus, the coil can obtain a linear signal according to the length of interference with the metal object. Therefore, the relative arrangement relationship between the coil and the metal object can be applied as a sensor capable of detecting an absolute distance.
Patent Document 1 and Patent Document 2 are prior art documents of displacement sensors by the applicant.

Japanese Patent Publication No. 6-23659 JP-A-2-201114

The technical contents disclosed in Patent Literature 1 and Patent Literature 2 are common to the technology for detecting the change in the inductance of the coil using the step response characteristics.
FIG. 11 is a circuit diagram of a detection circuit disclosed in Patent Document 1 and Patent Document 2.
The rectangular wave of the rectangular wave generator is applied to the coil A and the coil B through the transformer 1102.
The current flowing through the coil A flows to the capacitor C1104 through the diode D1103, and electric charge is accumulated in the capacitor C1104. A diode D1103 between the coil A and the capacitor C1104 limits the current flowing from the coil to the capacitor in one direction. During a period in which no current flows through the diode D1103, the electric charge accumulated in the capacitor C1104 is discharged by the resistor R1105 connected in parallel to the capacitor C1104. Therefore, a sawtooth wave is obtained at both ends of the capacitor C1104 (between the terminals 1110B and 1110C).

The current flowing through the coil B flows to the capacitor C1107 through the diode D1106, and electric charge is accumulated in the capacitor C1107. A diode D1106 between the coil B and the capacitor C1107 limits the current flowing from the coil to the capacitor in one direction. In a period in which no current flows through the diode D1106, the charge accumulated in the capacitor C1107 is discharged by the resistor R1108 connected in parallel to the capacitor C1107. Accordingly, a sawtooth wave similar to that on the coil A side is obtained at both ends of the capacitor C1104 at both ends of the capacitor C1107 (between the terminals 1110A and 1110C).
When signals obtained from the terminals 1110A and 1110B are differentially amplified by a differential amplifier circuit (not shown), a DC voltage is obtained. This DC voltage varies according to the inductances of the coil A and the coil B.

One of the coils A and B is used as a detection coil covered with a cylinder made of metal, and the other is prepared as a dummy coil not covered with a cylinder.
The same detection circuit is connected to each of the detection coil and the dummy coil, and sawtooth waves having the same phase are obtained as detection outputs. When both detection output signals are differentially amplified by the differential amplifier, the output voltage changes based on the change in the inductance of the detection coil.

  These detection circuits used in the prior art must prepare two coils on the principle of operation. Preparing two coils leads to an increase in the number of parts and hinders downsizing of the apparatus.

Furthermore, the technical contents disclosed in Patent Document 1 and Patent Document 2, which are conventional techniques, have problems with respect to the mechanical strength of the displacement sensor. Hereinafter, this problem will be described.
For example, when a displacement sensor is mounted on a suspension of an automobile or a motorcycle and the displacement of the suspension is measured, since a strong force is applied to the suspension, the synthetic resin probe constituting the displacement sensor cannot withstand mechanical shock, It was sometimes damaged.
Therefore, the applicant and the inventor have started to study to replace the outer shell of the probe with metal in order to increase the mechanical strength of the probe.
However, since the displacement sensor is a mechanism for detecting a change in the inductance of the coil, if the outer shell of the probe is made of metal, a loss due to eddy current occurs in the outer shell, and the position detection sensitivity is significantly reduced.
Furthermore, in the circuit configuration of the conventional displacement sensor disclosed in FIG. 10, even if the current passed through the coil is increased in order to increase the detection sensitivity, only the eddy current loss increases, and this hardly contributes to the improvement in sensitivity. I understood.

  The displacement sensor disclosed in FIG. 10 can also be used as a proximity sensor that detects the presence of a metal object when binarized by a comparator. However, if this is housed in a metal housing in order to ensure mechanical strength, the same problem as that of the aforementioned displacement sensor will occur.

  The present invention solves such a problem, effectively detects a change in inductance with a single coil, and implements an inductance change detection circuit for realizing a small but highly accurate displacement sensor, and a displacement detection device using the same And it aims at providing a metal detection apparatus.

In order to solve the above-described problems, an inductance change detection circuit according to the present invention includes a rectangular wave AC voltage source, a coil connected to the rectangular wave AC voltage source to detect an inductance change, and a current flowing through the coil as a voltage signal. A current-voltage converter that converts the voltage signal into a binary signal with an upper limit threshold voltage and a lower limit threshold voltage lower than the upper limit threshold voltage, and outputs a first rectangular wave signal; and a first output from the comparator An exclusive OR gate that outputs an exclusive OR signal of one rectangular wave signal and a second rectangular wave signal synchronized with the rectangular wave AC voltage source, and a direct current is passed through the coil to change the DC resistance of the coil And a voltage control circuit for controlling a voltage for driving the rectangular wave AC voltage source .

In order to solve the above problems, a displacement detection device of the present invention includes a rectangular wave AC voltage source, a coil connected to the rectangular wave AC voltage source, and a current-voltage conversion that converts a current flowing through the coil into a voltage signal. A comparator having a hysteresis characteristic that outputs a first rectangular wave signal by binarizing the voltage signal with an upper threshold voltage and a lower threshold voltage lower than the upper threshold voltage, and a first rectangular wave signal and a rectangle output by the comparator An exclusive OR gate that outputs a signal of an exclusive OR with a second rectangular wave signal synchronized with a wave AC voltage source, and a rectangular wave AC based on a change in the DC resistance of the coil by passing a DC current through the coil A voltage control circuit that controls a voltage for driving the voltage source; and a nonmagnetic metal object that is provided so as to be continuously accessible in the winding direction of the coil and detects a relative displacement with the coil.

A coil is connected to a rectangular wave AC voltage source, an AC current flowing through the coil is converted into a voltage signal, and then passed through a comparator having hysteresis characteristics, thereby detecting an inductance change of the coil as a phase change at the rising edge of the pulse.
Since there is no need to provide two coils as in the prior art and the number of parts is reduced, a highly accurate displacement sensor can be realized at low cost.

  According to the present invention, an inductance change detection circuit for effectively detecting a change in inductance with a single coil and realizing a small and highly accurate displacement sensor, and a displacement detection device and a metal detection device using the same are provided. Can be provided.

It is the external appearance perspective view and partial cross section figure of the displacement sensor which concern on 1st embodiment of this invention. It is a block diagram of a displacement sensor. It is a circuit diagram of a detection part. It is a wave form diagram of each part of a displacement sensor in case the outer shell of a probe is a nonmetal. It is a wave form diagram of each part of a displacement sensor in case the outer shell of a probe is a metal. It is a circuit diagram of the detection part of the displacement sensor which concerns on 2nd embodiment of this invention. It is a circuit diagram of the detection part of the displacement sensor which concerns on 3rd embodiment of this invention. It is the schematic explaining the structure of a current transformer and a coil. It is the schematic of a coil and a core used for a metal detection apparatus. It is a graph explaining the various characteristics of a coil for explaining the principle of operation of the displacement sensor which an applicant manufactures and sells. It is a circuit diagram of the detection circuit of a prior art.

[First embodiment]
1A and 1B are an external perspective view and a partial sectional view of a displacement sensor according to a first embodiment of the present invention.
FIG. 1A is an external perspective view of the displacement sensor.
The displacement sensor 101 is a combination of a sensor main body 102 and a sleeve 103.
In the sensor main body 102, a cylindrical probe 104 is attached to one end of a housing 105 containing a detection circuit.
The housing 105 is provided with mounting holes 105a and 105b for fixing the sensor main body 102 to an arbitrary article.
The probe 104 incorporates a detection coil to be described later. A sleeve 103 that is a brass tube is inserted into the probe 104. The material of the sleeve 103 is a non-magnetic metal material such as brass, for example, in order to generate a loss due to eddy current in the coil L106.
When the sleeve 103 is inserted into and removed from the probe 104, the sensor main body 102 outputs an analog detection signal corresponding to the relative position of the sleeve 103 with respect to the probe 104.

FIG. 1B is a partial cross-sectional view showing the cross-sectional shapes of the sleeve 103 and the probe 104 in a state where the displacement sensor 101 is viewed from the side.
In order to improve rigidity, the outer shell 104a of the probe 104 is formed of austenitic stainless steel, which is a nonmagnetic material, instead of the resin mold of the applicant's conventional product, and incorporates a coil L106 which is a detection coil.
The coil L106 is formed in a spiral shape in the longitudinal direction of the probe 104 as shown in FIG.

As is well known, austenitic stainless steel is a non-magnetic metal having high rigidity and has a high resistivity. Therefore, when used for the probe 104, the eddy current can be suppressed to be smaller than that of a metal having a low resistivity. However, although stainless steel is a conductor even though the resistivity is high, generation of eddy current is inevitable when an alternating current is passed through the coil. Eddy currents are converted into thermal energy, resulting in a reduction in coil inductance.
The material of the outer shell 104a of the probe 104 is not necessarily limited to stainless steel, but certain conditions are required. As described above, it is a non-magnetic metal and has a high resistivity. The resistivity is preferably higher than the resistivity of the sleeve 103. In this embodiment, brass (brass), which is the material of the sleeve 103, is 5 to 7 × 10−8 Ωm, whereas stainless steel is 7.2 × 10−7 Ωm, which satisfies this condition.

As shown in FIG. 1 (b), the coil L 106 is wound around a cylindrical ferrite core 107 provided in the longitudinal direction inside the probe 104. That is, the probe 104 is formed by winding the coil L106 around the core 107 around the core 107 and covering the coil L106 with the outer shell 104a.
The length of the sleeve 103 is preferably the same as or slightly longer than the length of the coil L106, but is not necessarily limited to this depending on the application to which the displacement sensor 101 is applied.

FIG. 2 is a block diagram of the displacement sensor 101.
The displacement sensor 101 includes a detection unit 201 that detects a change in the inductance of the coil L106, a rectangular wave signal source 202 that outputs a rectangular wave signal having a constant period to the detection unit 201, and a rectangular wave generated from the rectangular wave signal source 202. It consists of a NOT gate 203 that inverts the logic of the signal. The rectangular wave signal source 202 outputs a signal having a waveform shown in FIG. The NOT gate 203 outputs a signal having a waveform shown in FIG.

FIG. 3 is a circuit diagram of the detection unit 201.
The first switch 301 is connected in series to the second switch 302 and applied with a power supply voltage + Vcc. The second switch 302 is connected between the first switch 301 and the ground.
The first switch 301 and the second switch 302 are transistor switches.

The first switch 301 is ON / OFF controlled by a control signal P204 output from the rectangular wave signal source 202. Similarly, the second switch 302 is ON / OFF controlled by a control signal P205 output from the NOT gate 203. That is, the first switch 301 and the second switch 302 are on / off controlled alternately.
The first switch 301 is connected in parallel with a known first freewheeling diode D303 for circuit protection. Similarly, a second freewheel diode D304 is connected in parallel to the second switch 302.
The first switch 301, the second switch 302, the first freewheel diode D303 and the second freewheel diode D304 are controlled by the rectangular wave signal source 202 and the NOT gate 203, and output a rectangular wave voltage of a pulsating current. Configure the source.

A capacitor C305, a coil L106, and a resistor R306 are connected in series at the midpoint between the first switch 301 and the second switch 302. One end of the resistor R306 is grounded.
Capacitor C305 is provided for cutting off the direct current, which applies only the alternating current component of the rectangular wave voltage source to coil L106.
The resistor R306 is provided to detect the current flowing through the coil L106 as a voltage, and has a low resistance of about 10Ω, for example.
It can be said that the rectangular wave voltage source and the capacitor C305 are rectangular wave AC voltage sources for applying an AC rectangular wave voltage to the coil L106 and the resistor R306.

A capacitor C307 is connected to a connection point between the coil L106 and the resistor R306. Furthermore, the other end of the capacitor C307 is connected to the inverting input terminal of the operational amplifier 309 through the resistor R308. A resistor R310 is connected between the inverting input terminal and the output terminal of the operational amplifier 309. Further, Vcc / 2, that is, a voltage half the power supply voltage is applied to the non-inverting input terminal of the operational amplifier 309.
The resistor R308, the resistor R310, and the operational amplifier 309 constitute a known inverting amplifier circuit. The voltage across the resistor R306 is inverted and amplified by the inverting amplifier circuit.
It can be said that the resistor R306 and the inverting amplifier circuit are a current-voltage converter that converts an alternating current flowing through the coil L106 into a voltage signal.

  In the circuit of the detection unit 201 of FIG. 3 according to the first embodiment, the capacitor C307 has a DC current flowing through the resistor R308 due to a DC potential difference between the ground potential and the non-inverting input terminal of the operational amplifier 309, and is extraneous to the output of the operational amplifier 309. This prevents the generation of an offset voltage. The embodiment of FIG. 3 has a circuit configuration that assumes operation with a single power supply, and the capacitor C307 is unnecessary when operating with a bipolar power supply. However, in the detection unit 601 of FIG. 6 according to the second embodiment described later, the capacitor C307 is essential. The role of the capacitor C307 will be described later in FIG. 6 of the second embodiment.

  The output terminal of the operational amplifier 309 is connected to the resistor R311. The resistor R311 is connected to the non-inverting input terminal of the operational amplifier 312. A resistor R313 is connected between the non-inverting input terminal and the output terminal of the operational amplifier 312. Further, Vcc / 2, that is, a voltage that is half the power supply voltage is applied to the inverting input terminal of the operational amplifier 312.

The resistor R311, the resistor R313, and the operational amplifier 312 constitute a known comparator. This comparator has a hysteresis characteristic by a resistor R311 and a resistor R313.
A comparator composed of a resistor R311 as an input resistor, a resistor R313 as a positive feedback resistor, and an operational amplifier 312 has an upper threshold voltage VH and a lower threshold when the voltage applied to the inverting input terminal of the operational amplifier 312 is Vcc / 2. The voltage VL can be calculated by the following equation.

VH = Vcc · (1 + R311 / R313) / 2
VL = Vcc. (1-R311 / R313) / 2

When the input voltage Vin applied to the resistor R311 which is an input resistor gradually increases from 0V and exceeds the upper threshold voltage VH, the output of the operational amplifier 312 is converted from 0V to + Vcc. This upper threshold voltage VH is a voltage higher than Vcc / 2.
Thereafter, even if the input voltage Vin applied to the resistor R311 as an input resistor gradually decreases from the voltage exceeding the upper threshold voltage VH and falls below the upper threshold voltage VH, it does not fall below the lower threshold voltage VL. The output of the operational amplifier 312 maintains + Vcc.
When the input voltage Vin applied to the resistor R311 that is an input resistor falls below the lower threshold voltage VL, the output of the operational amplifier 312 is converted from + Vcc to 0V. This lower limit threshold voltage VL is a voltage lower than Vcc / 2.

  The comparator compares the output signal of the operational amplifier 309 with the control signal P204 (output signal of the rectangular wave signal source 202) and outputs a rectangular wave signal. The rectangular wave signal output from the comparator is shifted in the phase of rising and falling in accordance with the change in inductance of the coil L106.

  The output signal of the comparator is supplied to an input terminal of an exclusive OR gate (hereinafter “EXOR gate”) 314. The control signal P205 (output signal of the NOT gate 203) is supplied to the other input terminal of the EXOR gate 314. As a result, the EXOR gate 314 outputs a PWM signal. Further, when the PWM signal is integrated by an integrating circuit including a resistor R315 and a capacitor C316, a voltage signal can be extracted.

[Operation]
4A, 4B, 4C, 4D, 4E, and 4F are waveform diagrams of each part of the displacement sensor 101. FIG. 4D is a waveform when the outer shell 104a of the probe 104 is a non-metal such as a synthetic resin.
First, FIG. 4A shows the waveform of the control signal P204 output from the rectangular wave signal source 202, and FIG. 4B shows the waveform of the control signal P205 output from the NOT gate 203.
FIG. 4C is a waveform diagram of a voltage applied to the coil L106. That is, it is the rectangular wave AC voltage E321 in which the DC component is cut by the capacitor C316 from the pulsating rectangular wave voltage output from the rectangular wave voltage source.
FIG. 4D is a waveform diagram of the output signal (inverted current waveform signal E322) of the operational amplifier 309.
When a positive voltage is applied to the coil L106, a current that increases in the positive direction flows through the coil L106. Due to the characteristics of the coil L106, the current gradually increases in the positive direction.
When a negative voltage is applied to the coil L106, a current that decreases in the negative direction flows through the coil L106. Due to the characteristics of the coil L106, the current gradually decreases in the negative direction.
Since this current waveform is converted into a voltage by the resistor R306 and amplified by the inverting amplifier, the output voltage of the operational amplifier 309 decreases while the positive voltage is applied to the coil L106, and the coil The voltage increases while a negative voltage is applied to L106.
In this way, the inverted current waveform signal E322 output from the operational amplifier 309 becomes a sawtooth voltage waveform that rises and falls around Vcc / 2 as shown in FIG. 4D.

When the probe 104 incorporating the coil L106 is not covered with the sleeve 103, the operational amplifier 309 outputs an inverted current waveform signal E322a shown by a solid line in FIG.
On the other hand, when the sleeve 103 is put on the probe 104 including the coil L106, the operational amplifier 309 outputs an inverted current waveform signal E322b shown by a dotted line in FIG. That is, since the inductance of the coil L106 decreases, the current easily flows through the resistor R306, and the current increases in both the positive direction and the negative direction, so that the peak voltage of the sawtooth waveform increases.

The inverted current waveform signal E322 output from the inverting amplifier is passed through a comparator having hysteresis.
When the output voltage signal of the inverting amplifier gradually increases from 0V and exceeds the upper threshold voltage VH, the output signal of the operational amplifier 312 is converted from 0V to + Vcc.
The output signal of the operational amplifier 312 once converted to + Vcc gradually decreases from the voltage at which the output voltage signal of the inverting amplifier exceeds the upper threshold voltage VH, and does not fall below the lower threshold voltage VL even if the output signal falls below the upper threshold voltage VH. As long as the output signal of the operational amplifier 312 is maintained at + Vcc.
When the output voltage signal of the inverting amplifier falls below the lower threshold voltage VL, the output of the operational amplifier 312 is converted from + Vcc to 0V.
The output signal of the operational amplifier 312 once converted to 0 V gradually rises from the voltage at which the output voltage signal of the inverting amplifier is lower than the lower threshold voltage VL, and does not exceed the upper threshold voltage VH even if it exceeds the lower threshold voltage VL. As long as the output signal of the operational amplifier 312 is maintained at 0V.
In this way, the comparator outputs a rectangular wave phase difference signal E323 shown in FIG.

  The rising timing of the phase difference signal E323 output from the comparator varies depending on the slope of the inverted current waveform signal E322 output from the inverting amplifier with respect to the upper threshold voltage VH and the lower threshold voltage VL. As can be seen from FIG. 4D, when the peak value of the inverted current waveform signal E322 increases, the slope of the inverted current waveform signal E322 becomes steep, and the upper threshold voltage VH and the lower threshold voltage VL are reached earlier. To reach. Therefore, when the inductance of the coil decreases, the phase of the phase difference signal E323 output from the comparator is advanced.

  Therefore, when the sleeve 104 is not covered with the probe 104 incorporating the coil L106 (the inductance of the coil L106 is not reduced), the rectangular wave phase difference signal E323 output from the comparator is a solid line in FIG. In the state where the sleeve 103 is covered with the probe 104 incorporating the coil L106 (the inductance of the coil L106 is reduced), the phase difference signal E323 is indicated by a solid line in FIG. The waveform E323b shown by the dotted line has a rising phase earlier than the waveform E323a.

When the phase difference signal E323 output from the comparator and the control signal P205 output from the NOT gate 203 are input to the EXOR gate 314 in this way, the PWM signal E324 can be extracted as shown in FIG. it can. In the PWM signal E324, when the inductance of the coil decreases, the duty ratio of the pulse width (the period of high potential / one period of the signal in one period of the signal) decreases.
In a state where the sleeve 104 is not covered with the probe 104 incorporating the coil L106 (the inductance of the coil L106 is not reduced), the PWM signal E324 output from the EXOR gate 314 is a waveform E324a indicated by a solid line in FIG. However, when the sleeve 103 is covered with the probe 104 including the coil L106 (the inductance of the coil L106 is reduced), the PWM signal E324 is pulsed more than the waveform E324a shown by the solid line in FIG. A waveform E324b indicated by a dotted line in which the duty ratio of the width is increased is obtained.
Since the signal output from the EXOR gate 314 is a PWM signal E324, it can be input as it is to a measuring device equipped with a digital input, or by passing through an integrating circuit consisting of a resistor R315 and a capacitor C316, a change in the inductance of the coil, An analog voltage signal whose voltage changes according to the relative position of the sleeve with respect to the probe 104 can be obtained.
When the control signal P204 (output signal of the rectangular wave signal source 202) is given to the EXOR gate 314 instead of the NOT gate 203, the logic of the PWM signal E324 in FIG. 4 (f) is inverted, and FIG. The waveform is inverted upside down. That is, when the coil inductance decreases, a signal with an increased pulse width duty ratio can be obtained.

FIGS. 5A, 5B, 5C, and 5D are waveform diagrams of each part of the displacement sensor 101 when the outer shell 104a of the probe 104 is a metal.
FIG. 5A is a waveform diagram of a voltage (rectangular wave AC voltage E321) applied to the coil L106, as in FIG. 4C.
FIG. 5B is a waveform diagram of the output signal (inverted current waveform signal E322) of the operational amplifier 309, as in FIG. 4D.
FIG. 5C is a waveform diagram of the current flowing through the outer shell 104a when the outer shell 104a of the probe 104 is a metal, obtained by simulation. Eddy current is generated in the outer shell 104a by the coil L106. This eddy current has a substantially trapezoidal waveform with a predetermined vertical peak value. This waveform substantially matches the strength of the magnetic field generated by the coil.
FIG. 5D is a waveform diagram of a current flowing through the coil L106 when the outer shell 104a of the probe 104 is a metal. This waveform is the combined waveform of FIGS. 5B and 5C. The current flowing through the coil is the sum of the amount of eddy current generated in the outer shell 104a and the amount of current flowing through the coil itself. Therefore, an offset effect of the current that the eddy current component flows in the coil itself is produced.

Once again comparing the waveform of FIG. 4D with the waveform of FIG. 5D, the offset component of the current waveform caused by the eddy current has a good effect on the hysteresis characteristics of the comparator. I understand. If there is no offset component, the change in the phase of the PWM signal E324 output from the EXOR gate 314 is small with respect to the slope of the current waveform. However, by adding an offset component of the current waveform due to the eddy current to the current waveform and appropriately setting the upper limit threshold voltage VH and the lower limit threshold voltage VL of the comparator, the change in inductance of the current flowing through the coil itself can be reduced. Changes that occur in response can be captured in a wide range.
In FIG. 5B, the change t1 in the phase difference due to the change in the peak current cannot theoretically exceed ¼ of the period of the rectangular wave AC voltage E321. On the other hand, it can be understood from the comparison with FIG. 5B that the detected phase difference t2 can be detected in a wider range in the waveform of FIG.
That is, it is possible to obtain a very interesting effect that the resolution as a displacement sensor can be improved by introducing the metal outer shell 104a into the probe 104, which would otherwise reduce the detection sensitivity.

[Operation and Effect of First Embodiment / Displacement Sensor 101]
As described above, the displacement of the sleeve 103 with respect to the probe 104 changes the inductance of the coil L106. The change in inductance appears as a change in the peak current value of the current flowing through the coil L106. The displacement sensor 101 detects the displacement of the sleeve 103 from the change in inclination caused by the change in peak current value.
There are two ways to increase the difference in peak current values. One is to increase the pulse voltage, and the other is to increase the pulse period of the pulse voltage. In the latter case, the current value increases as a linear function of time.

In the case of the displacement sensor 101 of the present embodiment, the material of the outer shell 104a, which can be called a protective tube for protecting the coil L106, is austenitic stainless steel in order to ensure mechanical strength.
If the pulse voltage is increased, the eddy current generated in the stainless steel outer shell 104a increases. Then, the loss based on the eddy current increases, the current consumption increases, and heat is generated.
This phenomenon can also be explained by replacing it with a transformer. For the coil L106, the outer shell 104a made of stainless steel corresponds to the relationship between the temporary coil of the transformer and the secondary coil of one turn. For this reason, when the voltage of the rectangular wave voltage source is increased to increase the current of the coil L106, the voltage induced in the outer shell 104a corresponding to the secondary coil also increases, and the short circuit current increases.
That is, by making the outer shell 104a made of stainless steel, increasing the pulse voltage only unnecessarily increases the power consumption, and hardly contributes to the increase in sensitivity of the displacement sensor 101.
From the above, it is effective to increase the sensitivity of the displacement sensor 101 by holding the pulse voltage as low as possible and increasing the peak current instead by increasing the pulse period.

It should be noted that the peak current value of the coil L106 does not directly become the current consumption value of the circuit.
In the case of a ferrite core, the power consumption of an AC circuit having a coil is dominated by Joule heat due to winding resistance. For example, even if the peak current value is 50 mA, if the winding resistance is several Ω, the current consumption is 1 mA or less. Therefore, even if the peak current is increased in order to increase the detection sensitivity, the influence on the current consumption is small.

The most important operation principle of the displacement sensor 101 according to the first embodiment is to detect a change in the current flowing through the coil as a phase difference using a comparator having hysteresis.
If the peak value of the coil current is detected as it is, a dynamic range corresponding to the peak current value is required for the circuit. In other words, the circuit configuration is such that a slight change (current difference) is obtained from a signal having a large offset value. In the case of such a circuit configuration, the S / N ratio of the output signal decreases.
In the case of the displacement sensor 101 according to the first embodiment, since the detection is based on the phase difference, the S / N ratio is good and the sensitivity is high.

  That is, the displacement sensor 101 according to the first embodiment applies a rectangular AC voltage E321 to the detection coil, converts the current into a voltage signal, and detects it as a phase difference using a comparator having hysteresis; Even if the sensor coil is housed in the metal outer shell 104a whose sensitivity is lowered by setting the rectangular wave AC voltage E321 low, and instead setting the period of the rectangular wave AC large, the necessary and sufficient position detection is possible. Sensitivity can be obtained.

[Second Embodiment]
The displacement sensor according to the first embodiment has the greatest factor that deteriorates the displacement detection accuracy, that is, temperature change.
The coil is a winding obtained by winding a conductor long. That is, the coil itself inevitably contains a direct current resistance inherent in the conductor. The resistance value of the DC resistance component increases as the temperature increases. An increase in the internal resistance value of the coil L106 appears as a change in impedance, rebounds to the potential difference of the resistor R306 constituting the displacement detection signal, and deteriorates the detection accuracy.

  In order to perform temperature compensation of the detection unit 201 disclosed in the first embodiment, a temperature sensor may be provided in the vicinity of the coil L106. However, the coil L106 is built in the probe 104 and is elongated in the longitudinal direction. Sometimes, depending on the installation state of the displacement sensor 101, only the tip portion of the probe 104 is heated, and other portions remain cold. There may be a situation. Normally, the temperature observation point of the temperature sensor is a point, and in order to detect the temperature widely in the longitudinal direction, a large number of temperature sensors must be provided. Such a solution is impractical.

Therefore, consider the possibility of using the coil L106 itself as a temperature sensor.
If the circuit diagram of FIG. 3 which is 1st embodiment is closely seen, the coil L106 and resistance R306 will be connected between the ground, the capacitor | condenser C305, and the capacitor | condenser C307. That is, the coil L106 is DC-isolated by the capacitor C305 and the capacitor C307. The current for detecting the change in inductance is only an alternating current component. Even if a separate direct current is passed through this coil L106, it does not affect the detection of the change in inductance unless the current is large enough to cause a temperature change in the coil L106. do not do.
When a direct current is passed through the coil L106, the internal resistance is detected, and the obtained change in current is reflected in the output voltage of the rectangular wave voltage source applied to the series circuit of the capacitor C305, the coil L106 and the resistor R306, temperature compensation Can be realized.

FIG. 6 is a circuit diagram of the detection unit 601 of the displacement sensor according to the second embodiment of the present invention. In the displacement sensor of the second embodiment, the voltage applied to the first switch 301 of the detection unit 201 of the first embodiment is not + Vcc, and the DC resistance of the coil L106 is between the capacitor C305 and the coil L106. Since a line for detecting a value is drawn and a part other than a new circuit is added, the description of the common part is omitted. That is, the detection unit 601 is an improved version in which a voltage control circuit for temperature compensation is added to the detection unit 201.
In FIG. 6, a resistor R602, a resistor R603, an operational amplifier 604, a resistor R605, a capacitor C606, a resistor R607, an operational amplifier 608, a resistor R609, a resistor R610, a variable resistor VR611, and a capacitor are added to the detection unit 201 in FIG. C612 constitutes a voltage control circuit.

A resistor R602 is connected between the capacitor C305, the coil L106, and the resistor R306. The resistor R602 forms part of a series circuit of resistors that is grounded from the power supply voltage + Vcc through the resistors Rt, R603, R602, the coil L106, and the resistor R306. The resistance values of the resistors R602 and R603 are the same. Rt is the same value as the sum of the DC resistance value of the resistor R306, which is a current detection resistor, and the DC resistance value of the coil L106 at room temperature. However, this Rt can be omitted. The reason will be described later.
An inverting input terminal of the operational amplifier 604 is connected to a connection point between the resistors R602 and R603. The non-inverting input terminal of the operational amplifier 604 is supplied with Vcc / 2 as a reference voltage. That is, a two-input inverting amplifier circuit having a first input resistance having (Rt + R603) as an input resistance value and a second input resistance having (R602 + R306 + L106 DC resistance value) as an input resistance value is configured. Connected to + Vcc and ground potential.

  In the normal temperature state, the total value of the DC resistance value of the coil L106 and the resistance R306 is the same as the resistance Rt. Therefore, currents of the same value flow in the opposite directions in the resistors R602 and R603, so that the current flowing in the resistor R605 becomes zero, and the operational amplifier 604 outputs Vcc / 2 in the normal temperature state.

When the temperature rises from the normal temperature state, the DC resistance value of the coil L106 increases. Then, the current flowing through R602 decreases, the difference from the current flowing through R603 flows into R605, and the voltage at the output terminal of the operational amplifier 604 decreases from Vcc / 2 in the negative direction.
Conversely, when the temperature drops from the normal temperature state, the DC resistance value of the coil L106 decreases. Then, the potential at the inverting input terminal of the operational amplifier 604 decreases, and the voltage at the output terminal of the operational amplifier 604 increases from Vcc / 2 in the positive direction.
Thus, the operational amplifier 604 outputs a voltage signal that changes according to the temperature around the coil L106.

Resistors R602 and R603 are several kΩ. On the other hand, the DC resistance value of the coil L106 is at most about several Ω, and the current detection resistance value is also about 10Ω. That is, the total value of the DC resistance value and the current detection resistance value of the coil L106 and the resistance Rt are values that do not satisfy the error range as compared with the resistances R602 and R603.
Furthermore, what the operational amplifier 604 should output is a signal indicating that the DC resistance value of the coil L106 has increased or decreased in response to a temperature change. The description of “room temperature” is for convenience of explanation, and it is not necessary to strictly determine the definition of “room temperature”.
Therefore, the resistor Rt may not be provided.

  A resistor R605 and a capacitor C606 are connected in parallel between the inverting input terminal and the output terminal of the operational amplifier 604. Due to the presence of the capacitor C606, the operational amplifier 604 forms an integrating circuit. The capacitor C606 is necessary for removing the AC component output from the coil L106.

  The operational amplifier 604 is an integrating circuit and constitutes an inverting amplifier. When the temperature rises and the DC resistance value of the coil L106 increases, the current of the resistor R602 decreases and the voltage at the output terminal of the operational amplifier 604 decreases. Further, when the direct current resistance value of the coil L106 increases due to the temperature rise, the current flowing through the capacitor C305 decreases. Therefore, in order to perform temperature compensation on the coil L106, it is necessary to increase the voltage applied to the capacitor C305 as the temperature rises. That is, the output signal of the operational amplifier 604 needs to be further inverted and amplified.

The operational amplifier 608 connected from the output terminal of the operational amplifier 604 to the inverting input terminal through the resistor R607 has a resistor R609 connected between the inverting input terminal and the output terminal, and constitutes an inverting amplifier circuit. Further, a variable resistor VR611 is connected to the inverting input terminal of the operational amplifier 608 through a resistor R610. One end of the variable resistor VR611 is connected to the power supply voltage + Vcc, and the other end is grounded. Further, the operational amplifier 608 itself is driven by a driving voltage that is output from a voltage doubler circuit (not shown) or the like that is higher than the power supply voltage + Vcc, and can output a voltage higher than the power supply voltage + Vcc. Further, if a slight decrease in output voltage is allowed, a single power supply voltage may be used.
In this way, the operational amplifier 608 outputs a drive voltage including a temperature compensation component and applies it to the first switch 301 and the first freewheel diode D303.

[Third embodiment]
In each of the displacement sensors according to the first embodiment and the second embodiment, the change in the current flowing through the coil L106 is converted into a voltage signal by the resistor R306, and the AC component of the voltage signal is extracted by the capacitor C307. .
Various other methods are conceivable for converting the current change of the coil L106 into a voltage signal. For example, a current-voltage conversion circuit using an operational amplifier. However, in the case of an operational amplifier current-voltage conversion circuit, the non-inverting input terminal of the operational amplifier needs to be grounded. That is, the potential should not change due to the input signal or other factors. On the other hand, the displacement sensors according to the first embodiment and the second embodiment are both configured by a single power source. This is because a change in inductance can be detected by a PWM signal, that is, a change on the time axis of the signal, and thus it is not necessary to obtain strictness in the power supply voltage that determines the amplitude of the signal. This point is very advantageous in terms of cost when configuring a displacement sensor. However, if the current-voltage conversion circuit of the operational amplifier is to be adopted, both power sources are inevitably adopted due to the circuit configuration, and the cost advantage of the corner is discarded. This is not very preferable in constructing a displacement sensor.

  Therefore, the use of a current transformer is considered as another current-voltage conversion method. Although the current transformer itself is a somewhat expensive component compared to other passive elements such as resistors and capacitors, it is extremely advantageous in terms of circuit design because it can separate the DC voltage so that the potential of other circuit elements need not be taken into account. There are many points.

  FIG. 7 is a circuit diagram of the detection unit 701 of the displacement sensor according to the third embodiment of the present invention. In the displacement sensor of the third embodiment, the resistor R306, the capacitor C307, the resistor R308, the operational amplifier 309, and the resistor R310 of the detection unit 601 of the second embodiment are replaced with a current transformer 702, an operational amplifier 703, and a resistor R704. Circuit configuration. Since other circuit configurations are the same as those in the second embodiment, description of common portions is omitted.

In FIG. 7, one end of the first winding of the current transformer 702 is connected to a midpoint between the first switch 301 and the second switch 302 through a capacitor C305. A coil L106 is connected to the other end of the first winding of the current transformer 702 and one end of the second winding. The other end of the second winding of the current transformer 702 is grounded.
The terminal of the third winding of the current transformer 702 is connected to the inverting input terminal and the non-inverting input terminal of the operational amplifier 703, respectively. Further, Vcc / 2, that is, a voltage half of the power supply voltage is applied to the non-inverting input terminal of the operational amplifier 703. A resistor R704 is connected between the inverting input terminal and the output terminal of the operational amplifier 703.
It can be said that the current transformer 702, the operational amplifier 703, and the resistor R704 are a current-voltage converter that converts an alternating current flowing through the coil L106 into a voltage signal.
Here, the number of turns N1 of the first winding and the number of turns N2 of the second winding of the current transformer 702 are equal. This is for canceling well-known common mode noise described later.

FIG. 8 is a schematic diagram illustrating the configuration of the current transformer 702 and the coil L106.
As described above, the coil L106 is connected to the other end of the first winding of the current transformer 702 and one end of the second winding. When the coil L106 and the current transformer 702 are connected by the two-core shielded wire 801, the common mode noise in riding on the signal line connected to the terminal of the coil L106 is applied in the opposite phase when reaching the current transformer 702. Therefore, noise canceling can be realized by the current transformer 702. Therefore, the detection unit 701 of the third embodiment is suitable for configuring a displacement sensor that extends the coil L106.

In addition to the embodiment described above, the following application examples are conceivable.
(1) In the detection unit 201, the voltage applied to the first switch 301 is the power supply voltage. However, this is not necessarily the power supply voltage. How to determine the voltage applied to these elements is a matter of design.

  (2) Although the sleeve 103 is brass, it is not limited to this as long as it is a non-magnetic metal. For example, copper, aluminum, etc. are mentioned. However, it is desirable that the resistivity is lower than that of the outer shell 104a.

(3) If the coil L106 is deformed and the coil is housed in a ferrite core or the like, it can be applied to a metal detector utilizing eddy current.
FIGS. 9A and 9B are schematic views of a coil and a core used in the metal detection device.
FIG. 9A is an exploded perspective view of the coil and the ferrite core. FIG. 9B is a cross-sectional view of a ferrite core that houses a coil.
The coil L901 is housed in a bobbin-shaped ferrite core 902. The coil L901 is incorporated as it is in the circuit of the detection unit 201 disclosed in the first, second, and second embodiments. In the detection coil body 903 formed in this manner, when a metal approaches, an eddy current is generated in the metal, and the inductance of the coil L901 is reduced according to the loss caused by the eddy current. This decrease in inductance is extracted as a metal detection signal.

In the present embodiment, a displacement sensor has been disclosed.
A coil is connected to a rectangular wave AC voltage source, an AC current flowing through the coil is converted into a voltage signal, and then passed through a comparator having hysteresis characteristics, thereby detecting an inductance change of the coil as a phase change at the rising edge of the pulse.
Since there is no need to provide two coils as in the prior art and the number of parts is reduced, a highly accurate displacement sensor can be realized at low cost.

  Furthermore, in the second embodiment, by using the coil itself as a temperature sensor, temperature compensation of the displacement sensor can be realized without providing a separate temperature sensor.

  In the third embodiment, the coil can be extended with a shield wire by adopting a current transformer as the current-voltage conversion method.

  The embodiment of the present invention has been described above. However, the present invention is not limited to the above-described embodiment, and other modifications may be made without departing from the gist of the present invention described in the claims. Includes application examples.

  DESCRIPTION OF SYMBOLS 101 ... Displacement sensor, 102 ... Sensor main-body part, 103 ... Sleeve, 104 ... Probe, 105 ... Housing, 107 ... Core, 201 ... Detection part, 202 ... Rectangular wave signal source, 203 ... NOT gate, 301 ... First switch , 302 ... second switch, 309 ... operational amplifier, 312 ... operational amplifier, 314 ... EXOR gate, 601 ... detection section, 604 ... operational amplifier, 608 ... operational amplifier, 701 ... detection section, 702 ... current transformer, 703 ... operational amplifier, 801 ... two Core shield wire, C305, C307, C316, C606, C612 ... capacitor, D303 ... first freewheel diode, D304 ... second freewheel diode, L106 ... coil, R306, R308, R310, R311, R313, R315, R602, R603, R605, 607, R609, R610, R704 ... resistance, VR611 ... variable resistance

Claims (9)

A rectangular wave AC voltage source;
A coil connected to the rectangular wave AC voltage source to detect a change in inductance;
A current-voltage converter for converting a current flowing through the coil into a voltage signal;
A comparator having hysteresis characteristics that binarizes the voltage signal with an upper threshold voltage and a lower threshold voltage lower than the upper threshold voltage and outputs a first rectangular wave signal;
An exclusive OR gate that outputs an exclusive OR signal of the first rectangular wave signal output from the comparator and the second rectangular wave signal synchronized with the rectangular wave AC voltage source ;
An inductance change detection circuit, comprising: a voltage control circuit that controls a voltage that causes a direct current to flow through the coil and drives the rectangular wave AC voltage source based on a change in DC resistance of the coil . A rectangular wave AC voltage source;
A coil connected to the rectangular wave AC voltage source;
A current-voltage converter for converting a current flowing through the coil into a voltage signal;
A comparator having hysteresis characteristics that binarizes the voltage signal with an upper threshold voltage and a lower threshold voltage lower than the upper threshold voltage and outputs a first rectangular wave signal;
An exclusive OR gate that outputs an exclusive OR signal of the first rectangular wave signal output from the comparator and the second rectangular wave signal synchronized with the rectangular wave AC voltage source ;
A voltage control circuit for controlling a voltage for driving the rectangular wave AC voltage source based on a change in DC resistance of the coil by passing a DC current through the coil;
A displacement detection apparatus comprising: a non-magnetic metal object that is provided so as to be continuously accessible in the winding direction of the coil and detects a relative displacement with the coil. A rectangular wave AC voltage source;
A coil connected to the rectangular wave AC voltage source and having a core, and a coil whose inductance changes due to the proximity of a metal to the core;
A current-voltage converter for converting a current flowing through the coil into a voltage signal;
A comparator having hysteresis characteristics that binarizes the voltage signal with an upper threshold voltage and a lower threshold voltage lower than the upper threshold voltage and outputs a first rectangular wave signal;
An exclusive OR gate that outputs an exclusive OR signal of the first rectangular wave signal output from the comparator and the second rectangular wave signal synchronized with the rectangular wave AC voltage source ;
A metal detection device, comprising: a voltage control circuit that controls a voltage that drives a rectangular wave AC voltage source based on a change in DC resistance of the coil by passing a DC current through the coil . A rectangular wave AC voltage source;
A current transformer in which one terminal of the first winding is connected to the rectangular wave AC voltage source, and one terminal of the second winding is grounded;
A coil connected to the other terminal of the first winding and the other terminal of the second winding to detect a change in inductance;
A current-voltage converter that is connected to the third winding of the current transformer and converts a current flowing through the coil into a voltage signal;
A comparator having hysteresis characteristics that binarizes the voltage signal with an upper threshold voltage and a lower threshold voltage lower than the upper threshold voltage and outputs a first rectangular wave signal;
An inductance change detection circuit comprising: an exclusive OR gate that outputs an exclusive OR signal of a first rectangular wave signal output from the comparator and a second rectangular wave signal synchronized with the rectangular wave AC voltage source. Furthermore,
5. An inductance change detection circuit according to claim 4 , further comprising: a voltage control circuit that controls a voltage for driving the rectangular wave AC voltage source based on a change in DC resistance of the coil by passing a DC current through the coil. A rectangular wave AC voltage source;
A current transformer in which one terminal of the first winding is connected to the rectangular wave AC voltage source, and one terminal of the second winding is grounded;
A coil connected to the other terminal of the first winding and the other terminal of the second winding to detect a change in inductance;
A current-voltage converter that is connected to the third winding of the current transformer and converts a current flowing through the coil into a voltage signal;
A comparator having hysteresis characteristics that binarizes the voltage signal with an upper threshold voltage and a lower threshold voltage lower than the upper threshold voltage and outputs a first rectangular wave signal;
An exclusive OR gate that outputs an exclusive OR signal of the first rectangular wave signal output from the comparator and the second rectangular wave signal synchronized with the rectangular wave AC voltage source;
A displacement detection apparatus comprising: a non-magnetic metal object that is provided so as to be continuously accessible in the winding direction of the coil and detects a relative displacement with the coil. Furthermore,
The displacement detection apparatus according to claim 6 , further comprising: a voltage control circuit configured to control a voltage for driving the rectangular wave AC voltage source based on a change in DC resistance of the coil by passing a DC current through the coil. A rectangular wave AC voltage source;
A current transformer in which one terminal of the first winding is connected to the rectangular wave AC voltage source, and one terminal of the second winding is grounded;
A coil connected to the other terminal of the first winding and the other terminal of the second winding to detect a change in inductance;
A current-voltage converter that is connected to the third winding of the current transformer and converts a current flowing through the coil into a voltage signal;
A comparator having hysteresis characteristics that binarizes the voltage signal with an upper threshold voltage and a lower threshold voltage lower than the upper threshold voltage and outputs a first rectangular wave signal;
A metal detection device comprising: an exclusive OR gate that outputs an exclusive OR signal of a first rectangular wave signal output from the comparator and a second rectangular wave signal synchronized with the rectangular wave AC voltage source. Furthermore,
The metal detection apparatus according to claim 8 , further comprising: a voltage control circuit configured to control a voltage for driving the rectangular wave AC voltage source based on a change in DC resistance of the coil by causing a direct current to flow through the coil.

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