JP4747224B1 - 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 predetermined voltage and ground are alternately connected to a resonance circuit in which a coil and a capacitor are connected in series. In addition, two free wheel diodes are provided to receive an electromotive force generated from the coil immediately after being disconnected from the voltage or ground to accumulate electric charges in the capacitor and stabilize the circuit. Thereafter, the voltage across the load consisting of a capacitor or a series connection of a capacitor and a coil is acquired, and a change in inductance that occurs when a non-magnetic metal is close to and not close to the coil is detected.
[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 operating principle of this active displacement sensor will be described.

FIGS. 11A, 11B and 11C are graphs for explaining various characteristics of the coil for explaining the operation principle of the displacement sensor manufactured and sold by the applicant.
11A is a circuit diagram for explaining the principle of the displacement sensor, FIG. 11B is a diagram showing a state of the switch SW of the circuit of FIG. 11A, and FIG. 11C 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. 11A, 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. 11C, the both-end voltage gradually increases by 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. 11D 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 in FIG. 11A 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. 11C. Then, it converges to a certain voltage while vibrating (S1201).
When the switch SW of the circuit shown in FIG. 11A is turned on while a metal object is brought close to a part of the coil L, the rising slope of the voltage across the capacitor C becomes steeper as the inductance of the coil L decreases. (S1202).
When the switch SW of the circuit shown in FIG. 11A is turned on with the entire coil L covered with a metal object, the rising slope of the voltage across the capacitor C is steep as much as the inductance of the coil L is further reduced. (S1203).
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. 12 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 1202.
The current flowing through the coil A flows to the capacitor C1204 through the diode D1203, and electric charge is accumulated in the capacitor C1204. A diode D1203 between the coil A and the capacitor C1204 limits the current flowing from the coil to the capacitor in one direction. In a period in which no current flows through the diode D1203, the charge accumulated in the capacitor C1204 is discharged by the resistor R1205 connected in parallel to the capacitor C1204. Accordingly, a sawtooth wave is obtained at both ends of the capacitor C1204 (between the terminals 1210B and 1210C).

The current flowing through the coil B flows to the capacitor C1207 through the diode D1206, and charges are accumulated in the capacitor C1207. A diode D1206 between the coil B and the capacitor C1207 limits the current flowing from the coil to the capacitor in one direction. In a period in which no current flows through the diode D1206, the electric charge accumulated in the capacitor C1207 is discharged by the resistor R1208 connected in parallel to the capacitor C1207. Therefore, a sawtooth wave similar to that on the coil A side is obtained at both ends of the capacitor C1204 at both ends of the capacitor C1207 (between the terminals 1210A and 1210C).
When signals obtained from the terminals 1210A and 1210B 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.

  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 problems, an inductance change detection circuit according to the present invention includes a first switch to which a predetermined voltage is applied, a first freewheeling diode connected in parallel to the first switch, a first switch, and a ground. A second switch connected between the second switch, a second freewheeling diode connected in parallel to the second switch, a capacitor connected between the first switch and the second switch, and between the capacitor and ground a coil connected Ru is detected a change in inductance, after a sequencer for outputting a first switch and a control pulse signal for on-off control to alternately and a second switch, the first switch by the sequencer is turned on controlled, a first sample-and-hold circuit, the second switch by a sequencer is turned on to obtain the potential of the connection point between the first switch and the second switch After being your, a second sample-and-hold circuit for obtaining a potential at the connection point between the first switch and the second switch, and a differential amplifier circuit which subtracts the output of the first sample-and-hold circuit and a second sample-and-hold circuit Prepare.

In order to solve the above problems, a displacement detection device of the present invention includes a first switch to which a predetermined voltage is applied, a first freewheeling diode connected in parallel to the first switch, a first switch and a ground. A second switch connected between the second switch, a second freewheeling diode connected in parallel to the second switch, a capacitor connected between the first switch and the second switch, and between the capacitor and ground A sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch, and the first switch and the second switch after the first switch is on-controlled by the sequencer . a first sample-and-hold circuit for obtaining a potential at the connection point of the switch, after the second switch is turned on controlled by the sequencer, the first switch and the second switch Provided in a second sample-and-hold circuit for obtaining a potential at the connection point, a differential amplifier circuit which subtracts the output of the first sample-and-hold circuit and a second sample-and-hold circuit, to be continuously close to the winding direction of the coil And a non-magnetic metal object whose relative displacement with respect to the coil is detected .

  A predetermined voltage and ground are alternately connected to a resonance circuit in which a coil and a capacitor are connected in series. In addition, two free wheel diodes are provided to receive an electromotive force generated from the coil immediately after being disconnected from the voltage or ground to accumulate electric charges in the capacitor and stabilize the circuit. Thereafter, the voltage across the load consisting of a capacitor or a series connection of a capacitor and a coil is acquired, and a change in inductance that occurs when a non-magnetic metal is close to and not close to the coil is detected.

  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 block diagram of a sequencer. It is a figure explaining the principle of operation of a displacement sensor. It is a wave form diagram of each part of a displacement sensor. It is a circuit diagram of the detection part of the displacement sensor which concerns on 2nd embodiment of this invention. It is an external appearance perspective view of the displacement sensor which concerns on 2nd embodiment of this invention. It is a part of circuit diagram of the detection part of the displacement sensor which concerns on 3rd embodiment of this invention. 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 probe 104 is attached to one end of a housing 106 that incorporates a detection circuit.
The housing 106 is provided with mounting holes 106a and 106b 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.
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.
The probe 104 is formed of a resin mold and incorporates a coil L105 that is a detection coil.
The coil L105 is formed in a spiral shape in the longitudinal direction of the probe 104 as shown in FIG.

FIG. 2 is a block diagram of the displacement sensor 101.
The displacement sensor 101 includes a detection unit 203 that detects a change in inductance of the coil L105, and a sequencer 204 that outputs a plurality of pulses having a constant period to the detection unit 203. The sequencer 204 outputs pulses having waveforms shown in FIGS. 6A, 6B, 6C, and 6D described later.

FIG. 3 is a circuit diagram of the detection unit 203.
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 pulse signal P 1 output from the sequencer 204. Similarly, the second switch 302 is ON / OFF controlled by a control pulse signal P2 output from the sequencer 204. Since the control pulse signal P1 and the control pulse signal P2 are turned on / off, the first switch 301 and the second switch 302 are not turned on at the same time.

  A first freewheel diode D303 is connected in parallel to the first switch 301. Similarly, a second freewheel diode D304 is connected in parallel to the second switch 302.

A coil L105 and a capacitor C305 are connected in series at the midpoint between the first switch 301 and the second switch 302. One end of the capacitor C305 is grounded.
Two sample and hold circuits are connected to the capacitor C305 as a circuit for detecting the voltage across the capacitor C305. The circuit subsequent to the capacitor C305 can be said to be a voltage change acquisition unit.

The sample and hold circuit includes a capacitor C306 and an operational amplifier 307 that can be called a first sample and hold circuit, and a capacitor C309 and an operational amplifier 311 that can be called a second sample and hold circuit.
The operational amplifier 307 constitutes a non-inverting amplifier with zero feedback resistance, that is, a voltage follower, and outputs the voltage across the capacitor C306 as it is.
Similarly, the operational amplifier 311 outputs the voltage across the capacitor C309 as it is.

A third switch 308 is interposed between the capacitor C306 and the capacitor C305. The third switch 308 is ON / OFF controlled by a sampling pulse signal P3 output from the sequencer 204.
A fourth switch 310 is interposed between the capacitor C309 and the capacitor C305. The fourth switch 310 is ON / OFF controlled by a sampling pulse signal P4 output from the sequencer 204.

  Capacitors C306 and C309 need to have a sufficiently small capacitance compared to capacitor C305 in order not to greatly change the amount of charge accumulated in capacitor C305 when acquiring the voltage across capacitor C305. As an example, it is desirable that it is 1/100 or less of C305.

The output of the operational amplifier 307 and the output of the operational amplifier 311 constituting the sample hold circuit are input to the differential amplifier circuit.
The output signal of the operational amplifier 307 is input to the inverting input terminal of the operational amplifier 313 through the resistor R312. + Vcc / 2 is applied to the non-inverting input terminal of the operational amplifier 313 from a reference voltage forming circuit (not shown).
As is well known, the amplification factor of the operational amplifier 313 is determined by the ratio (R314 / R312) of the feedback resistor R314 and the resistor R312.

On the other hand, the output signal of the operational amplifier 311 is input to the inverting input terminal of the operational amplifier 316 through the resistor R315. + Vcc / 2 is also applied to the non-inverting input terminal of the operational amplifier 316 from a reference voltage forming circuit (not shown).
The amplification factor of the operational amplifier 315 is also determined by the ratio (R317 / R315) of the feedback resistor R317 and the resistor R315.

The output of the operational amplifier 313 is input to the inverting input terminal of the operational amplifier 316 through the resistor R318.
Here, the resistance values of the resistor R318 and the resistor R317 are equal. That is, the operational amplifier 316 inverts and amplifies the output signal of the operational amplifier 313 by a factor of 1.
Further, the resistance values of the resistor R312 and the resistor R315 are equal. Furthermore, the resistance values of the resistor R314 and the resistor R317 are also equal. That is, the amplification factor of the operational amplifier 313 for the output signal of the operational amplifier 307 and the amplification factor of the operational amplifier 316 for the output signal of the operational amplifier 311 are equal.
As a result, the output signal of the operational amplifier 307 is non-inverted and amplified by the amplification factor of the operational amplifier 313, the output signal of the operational amplifier 311 is inverted and amplified by the amplification factor of the operational amplifier 316 having the same amplification factor as that of the operational amplifier 313, and each signal is amplified. By adding in 316, differential amplification is realized.

Thus, the operational amplifier 316 outputs the change in the voltage between the terminals of the capacitor C305 as a continuous analog voltage.
The change in the voltage between the terminals of the capacitor C305 changes according to the positional relationship of the sleeve 103 approaching the coil L105. Details of this mechanism will be described later.

FIG. 4 is a block diagram of the sequencer 204.
The sequencer 204 outputs pulses having waveforms shown in FIGS. 6A, 6B, 6C, and 6D described later. In order to output this periodic pulse, the sequencer 204 includes a clock generator 401, a loop counter 402, a ROM 403, and a decoder 404.

The clock generator 401 outputs a rectangular wave pulse having a predetermined frequency.
The loop counter 402 receives the pulse generated by the clock generator 401, counts from 0 to a predetermined number N, and outputs the count value data. When the loop counter 402 counts up to N, it returns the count value to 0 again and repeats counting again.

  Count value data of the loop counter 402 is input as an address of the ROM 403. Since the pulse of the clock generator 401 is also input to the ROM 403, the data written in the ROM 403 is read in order from addresses 0 to N. The data is a combination of the types of the control pulse signal P1, the control pulse signal P2, the sampling pulse signal P3, and the sampling pulse signal P4 and the output logical value.

Data output from the ROM 403 is input to the decoder 404. The decoder 404 outputs a control pulse signal P1, a control pulse signal P2, a sampling pulse signal P3, and a sampling pulse signal P4 according to the input data.
The sequencer 204 is not limited to this configuration, and may be configured by combining a plurality of counters and logic circuits such as flip-flops, or may be configured by a microcomputer.

[Operation]
FIGS. 5A, 5 </ b> B, 5 </ b> C, and 5 </ b> D are diagrams for explaining the operation principle of the displacement sensor 101.
6A, 6B, 6C, 6D, 6E, and 6F are waveform diagrams of respective parts of the displacement sensor 101. FIG.
First, FIG. 6A shows the waveform of the control pulse signal P1, FIG. 6B shows the waveform of the control pulse signal P2, FIG. 6C shows the waveform of the sampling pulse signal P3, and FIG. d) is a waveform of the sampling pulse signal P4.

  As shown in FIGS. 6A and 6B, the control pulse signal P1 and the control pulse signal P2 are alternately turned on and off. Further, during the time when the control pulse signal P2 is turned on after the control pulse signal P1 is turned off (from time t2 to t6) and during the time when the control pulse signal P1 is turned on after the control pulse signal P2 is turned off ( From time t7 to t11), a period in which both are turned off is provided. During the period in which both the control pulse signal P1 and the control pulse signal P2 are off, the voltage across the capacitor C305 is sampled by the sampling pulse signal P3 and the sampling pulse signal P4.

  FIG. 5A is a diagram illustrating a current flowing through the detection unit 203 during a period (from time t1 to t2) when the control pulse signal P1 is on. Since the control pulse signal P1 turns on the first switch 301, the power supply voltage + Vcc is applied to the coil L105 and the capacitor C305, and the current indicated by the arrow flows as a transient response.

  In FIG. 5B, after the period of FIG. 5A, the control pulse signal P1 is turned off and the control pulse signal P2 is also turned off and flows to the detection unit 203 in the period (from time t2 to t6). It is a figure which shows an electric current. When the first pulse 301 is controlled to be off by the control pulse signal P1, an electromotive force in the direction opposite to the current that has flowed until then is generated in the coil L105. In order to release this electromotive force, a second freewheel diode D304 is provided.

  FIG. 5C is a diagram illustrating a current flowing through the detection unit 203 in a period (from time t6 to t7) in which the control pulse signal P2 is on after the period in FIG. 5B. Since the control pulse signal P2 turns on the second switch 302, the electric charge accumulated in the capacitor C305 flows to the ground through the coil L105, and the current indicated by the arrow flows as a transient response.

  In FIG. 5D, after the period of FIG. 5C, the control pulse signal P2 is turned off, and the control pulse signal P1 also flows to the detection unit 203 in the period (from time t7 to t11). It is a figure which shows an electric current. When the control pulse signal P2 controls the second switch 302 to be turned off, an electromotive force in the direction opposite to the current that has been flowing in the coil L105 is generated. In order to release this electromotive force, a second freewheel diode D304 is provided.

6 (e) and 6 (f) will be described based on the operation of the circuit described in FIGS. 5 (a), 5 (b), 5 (c) and 5 (d).
FIG. 6E is a waveform diagram of a current flowing through the coil L105.
When the control pulse signal P1 is turned on at time t1, the current indicated by the arrow in FIG. Due to the characteristics of the coil L105, the current gradually increases in the positive direction.
When a current flows through the coil L105, a magnetic field proportional to the current is generated in the coil L105.

  When the control pulse signal P1 is turned off at time t2, a current indicated by an arrow in FIG. At time t3, the current becomes zero.

  After the phenomenon from the time point t1 to the time point t3, which is a transient phenomenon of the coil L105, is finished, the electric charge is accumulated in the capacitor C305 by the current flowing through the coil L105. When the sequencer 204 generates a sampling pulse signal P3 from time t4 after t3 when the transient of the coil L105 ends, the sample-and-hold circuit samples the voltage across the capacitor C305 until t5.

When the control pulse signal P2 is turned on at time t6, a current indicated by an arrow in FIG. Due to the characteristics of the coil L105, the current gradually increases in the negative direction.
When a current flows through the coil L105, a magnetic field proportional to the current is generated in the coil L105.

  When the control pulse signal P2 is turned off at time t7, a current indicated by an arrow in FIG. At time t8, the current becomes zero.

  After the phenomenon from the time t6 to the time t8, which is a transient phenomenon of the coil L105, is finished, electric charge is accumulated in the capacitor C305 by the current flowing through the coil L105. When the sequencer 204 generates the sampling pulse signal P4 from time t9 after t8 when the transient of the coil L105 ends, the sample-and-hold circuit samples the voltage across the capacitor C305 until t10.

  When the sleeve 103 is not covered with the probe 104 incorporating the coil L105, a current shown by a solid line in FIG. 6E flows through the coil L105. On the other hand, when the sleeve 103 is put on the probe 104 containing the coil L105, the current shown by the dotted line in FIG. That is, since the inductance of the coil L105 decreases, the current easily flows through the capacitor C305, and the current increases in both the positive direction and the negative direction.

FIG. 6F is a waveform diagram of the voltage across the capacitor C305.
The electric charge accumulated in the capacitor C305 is increased or decreased by the current generated in the coil L105. Accordingly, the voltage between both ends increases from time t1 to time t3, stabilizes from time t3 to time t6, decreases between time t6 and time t8, and stabilizes from time t8 to time t11. Repeat this cycle.

When the sleeve 104 is not covered with the probe 104 incorporating the coil L105, the voltage across the capacitor C305 changes as indicated by the solid line in FIG. Accordingly, the period from time point t4 t5, by sampling the voltage across the capacitor C305 between time t9 of t10, when the differential amplifier, from the output of the operational amplifier 314, voltage Va - (voltage -Va) = 2Va A corresponding signal is obtained.

On the other hand, when the sleeve 103 is put on the probe 104 incorporating the coil L105, the voltage across the capacitor C305 changes as indicated by the dotted line in FIG. That is, since the inductance of the coil L105 decreases, the current easily flows through the capacitor C305, and as a result of the current increasing in both the positive direction and the negative direction, the voltage across the capacitor C305 is the same as when the sleeve 103 is not covered. A voltage Vb having a larger potential difference than the voltage Va appears in both the positive direction and the negative direction.
Accordingly, the period from time point t4 t5, when sampling the voltage across the capacitor C305 between the time point t9 t10 differential amplification, the output of the operational amplifier 314, the voltage Vb - corresponds to (voltage -Vb) = 2Vb Signal is obtained.

  As described above, when the sleeve 103 is covered with the probe 104 including the coil L105, the change in the voltage across the capacitor C305 increases in accordance with the length of the sleeve 103. The sample and hold circuit collects the change in the voltage across the capacitor C305. A stable detection output is obtained by doubling the change with the differential amplifier.

[Second Embodiment]
FIG. 7 is a circuit diagram of a detection unit of the displacement sensor according to the second embodiment of the present invention. The displacement sensor of the second embodiment differs from the displacement sensor 101 of the first embodiment only in the detection unit, and the other parts are common, so the description of the common part is omitted.

  7 differs from the detection unit 203 in FIG. 3 in that a capacitor C305 is connected to the midpoint between the first switch 301 and the second switch 302, and a coil L105 is connected to the tip of the capacitor C305. This is the point. Therefore, the sample and hold circuit samples the voltage across the series circuit of the capacitor C305 and the coil L105.

  The advantage of the detection unit 701 in FIG. 7 compared to the detection unit 203 in FIG. 3 is that the coil L105 is grounded. Since the coil L105 as a sensor is grounded, it is difficult to be affected by external noise on the coil L105. Therefore, as compared with the detection unit 203 in FIG. 3, it is relatively easy to arrange only the coil L105 from the detection unit 701 with a cable.

FIG. 8 is an external perspective view of the displacement sensor according to the second embodiment of the present invention.
As described above, with the detection unit 701 in FIG. 7, only the coil L105 can be taken out from the detection unit 701 and routed with a cable. Since the coil L105, which is a sensor, can be arranged away from the detection unit 701, the degree of freedom of arrangement as a displacement sensor is high compared to the displacement sensor 101 of the first embodiment.

[Third embodiment]
The displacement sensor described so far 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 L105 appears as a change in impedance, which also rebounds from the potential difference of the capacitor C305 constituting the displacement detection signal, thereby degrading the detection accuracy.

  In order to perform temperature compensation of the detection unit 203 and the detection unit 701 disclosed in the first and second embodiments, a temperature sensor may be provided in the vicinity of the coil L105. However, the coil L105 is incorporated in the probe 104 and is elongated in the longitudinal direction. Sometimes, depending on the installation state of the displacement sensor 101, only the distal end 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.

Considering the possibility of using the coil L105 itself as a temperature sensor.
If you look closely at the circuit diagram of FIG. 7 which is the second embodiment, the coil L105 is connected between the ground and the capacitor C305. That is, the coil L105 is DC-isolated by the capacitor C305. The current for detecting the change in inductance is only an AC component. Even if a separate DC is passed through the coil L105, the detection of the change in inductance is completely affected as long as the current is not large enough to cause a temperature change in the coil L105. do not do.
By applying a direct current to the coil L105, detecting the internal resistance, and reflecting the obtained change in the voltage of the pulse applied to the series circuit of the coil L105 and the capacitor C305, temperature compensation can be realized.

FIG. 9 is a part of a circuit diagram of a detection unit of a displacement sensor according to the third embodiment of the present invention. In the displacement sensor of the third embodiment, the voltage applied to the first switch 301 of the detection unit 701 of the second embodiment is not + Vcc, and the DC resistance of the coil L105 is between the coil L105 and the capacitor C305. 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 901 is an improved version in which a voltage control circuit for temperature compensation is added to the detection unit 701.
In FIG. 9, a resistor R902, a resistor R903, an operational amplifier 904, a resistor R905, a capacitor C906, a resistor R907, an operational amplifier 908, a resistor R909, a resistor R910, a variable resistor VR911, and a capacitor added to the detection unit 701 in FIG. C912 constitutes a voltage control circuit.

A resistor R902 is connected between the coil L105 and the capacitor C305. The resistor R902 forms part of a series circuit of resistors that is grounded from the power supply voltage + Vcc through the resistors Rt, R903, R902, and the coil L105. The resistance values of the resistors R902 and R903 are the same. Rt is the same value as the DC resistance value of the coil L105 at room temperature. However, this Rt can be omitted. The reason will be described later.
The inverting input terminal of the operational amplifier 904 is connected to the connection point between the resistors R902 and R903. The non-inverting input terminal of the operational amplifier 904 is supplied with + Vcc / 2 as a reference voltage. That is, a two-input inverting amplifier circuit having a first input resistance having “Rt + R903” as an input resistance value and a second input resistance having “DC resistance value of R902 + L105” as an input resistance value is configured. Connected to + Vcc and ground potential.

In the normal temperature state, the DC resistance value of the coil L105 is the same as the resistance Rt. Accordingly, currents of the same value in opposite directions flow through the resistors R902 and R903 , so that the current flowing through the R905 becomes zero, and the operational amplifier 904 outputs Vcc / 2 if it is at room temperature.

When the temperature rises from the normal temperature state, the DC resistance value of the coil L105 increases. Then, the current flowing through R902 decreases, the difference from the current flowing through R903 flows into R905, and the voltage at the output terminal of the operational amplifier 904 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 L105 decreases. Then, the potential at the inverting input terminal of the operational amplifier 904 decreases, and the voltage at the output terminal of the operational amplifier 904 increases from Vcc / 2 in the positive direction.
Thus, the operational amplifier 904 outputs a voltage signal that changes according to the temperature around the coil L105.

Resistors R902 and R903 are several kΩ. On the other hand, the DC resistance value of the coil L105 is at most about several Ω. That is, the DC resistance value and the resistance Rt of the coil L105 are values that do not satisfy the error range as compared with the resistances R902 and R903.
Further, the operational amplifier 904 should output a signal indicating that the DC resistance value of the coil L105 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 R905 and a capacitor C906 are connected in parallel between the inverting input terminal and the output terminal of the operational amplifier 904. Due to the presence of the capacitor C906, the operational amplifier 904 constitutes an integration circuit. The capacitor C906 is necessary for removing the AC component output from the coil L105.

The operational amplifier 904 is an integrating circuit and constitutes an inverting amplifier. When the temperature rises and the DC resistance value of the coil L105 increases, the current of R902 decreases and the voltage at the output terminal of the operational amplifier 904 decreases. Further, when the DC resistance value of the coil L105 is increased due to the temperature rise, the current flowing through the capacitor C305 is decreased. Therefore, in order to perform temperature compensation on the coil L105, 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 904 needs to be further inverted and amplified.

The operational amplifier 908 connected from the output terminal of the operational amplifier 904 to the inverting input terminal through the resistor R907 has a resistor R909 connected between the inverting input terminal and the output terminal, and constitutes an inverting amplifier circuit. Furthermore, a variable resistor VR911 is connected to the inverting input terminal of the operational amplifier 908 through a resistor R910. One end of the variable resistor VR911 is connected to the power supply voltage + Vcc, and the other end is grounded. Further, the operational amplifier 908 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 908 outputs a drive voltage including a temperature compensation component and applies it to the first switch 301 and the first freewheel diode D303.

In addition to the embodiment described above, the following application examples are conceivable.
(1) In the detection units 203, 701, and 801, the voltage applied to the first switch 301 is the power supply voltage, but this does not necessarily need to be 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.

(3) If the coil L105 is deformed and the coil is housed in a ferrite core or the like, it can be applied to a metal detection device using eddy current.
10A and 10B are schematic views of a coil and a core used in the metal detection device.
FIG. 10A is an exploded perspective view of the coil and the ferrite core. FIG. 10B is a cross-sectional view of a ferrite core that houses a coil.
The coil L1001 is housed in a bobbin-shaped ferrite core 1002. The coil L1001 is incorporated as it is in the circuit of the detection unit disclosed in the first, second, and third embodiments. In the detection coil body 1003 formed in this way, when a metal approaches, an eddy current is generated in the metal, and the inductance of the coil L1001 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 predetermined voltage and ground are alternately connected to a resonance circuit in which a coil and a capacitor are connected in series. In addition, two free wheel diodes are provided to receive an electromotive force generated from the coil immediately after being disconnected from the voltage or ground to accumulate electric charges in the capacitor and stabilize the circuit. Then, obtain the voltage across the capacitor or the load consisting of the capacitor and coil connected in series, compare the voltage between the magnetic field due to the coil current and the external magnetic field with the same polarity and the opposite polarity, And detect the direction.

  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.

  In particular, in the second and third embodiments, the coil can be pulled out from the circuit block as a single sensor. At that time, since only one coil is required as compared with the prior art, the sensor can be miniaturized, and further, the number of signal cables connecting the circuit block and the coil can be saved, thereby further contributing to cost reduction. .

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

  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, 106 ... Case, 106a ... Hole, 203 ... Detection part, 204 ... Sequencer, 301 ... First switch, 302 ... Second switch, 307 Operational amplifier, 308 ... third switch, 310 ... fourth switch, 311 ... operational amplifier, 313 ... operational amplifier, 314 ... operational amplifier, 315 ... operational amplifier, 316 ... operational amplifier, 401 ... clock generator, 402 ... loop counter, 403 ... ROM, 404 ... Decoder, 701 ... Detection unit, 901 ... Detection unit, 904 ... Operational amplifier, 908 ... Operational amplifier, 1002 ... Ferrite core, 1003 ... Detection coil body, 1202 ... Transformer, 1210A ... Terminal, 1210B ... Terminal, 1210C ... Terminal

Claims (9)

A first switch to which a predetermined voltage is applied;
A first freewheeling diode connected in parallel to the first switch;
A second switch connected between the first switch and ground;
A second freewheeling diode connected in parallel to the second switch;
A coil connected Ru is detected a change in the inductance between the second switch and the first switch,
A capacitor connected between the coil and ground;
A sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch;
After the first switch is turned on by the sequencer, a first sample and hold circuit that acquires a voltage across the capacitor;
A second sample-and-hold circuit for obtaining a voltage across the capacitor after the second switch is turned on by the sequencer;
An inductance change detection circuit comprising: the first sample hold circuit; and a differential amplifier circuit that subtracts the output of the second sample hold circuit. A first switch to which a predetermined voltage is applied;
A first freewheeling diode connected in parallel to the first switch;
A second switch connected between the first switch and ground;
A second freewheeling diode connected in parallel to the second switch;
A capacitor connected between the first switch and the second switch;
A coil connected Ru is detected a change in the inductance between the ground and the capacitor,
A sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch;
After the first switch is turned on by the sequencer, a first sample and hold circuit that acquires a potential at a connection point between the first switch and the second switch ;
After the second switch is turned on by the sequencer, a second sample and hold circuit that acquires a potential at a connection point between the first switch and the second switch ;
An inductance change detection circuit comprising: the first sample hold circuit; and a differential amplifier circuit that subtracts the output of the second sample hold circuit. Furthermore,
The inductance change detection circuit according to claim 2, further comprising: a voltage control circuit that controls a voltage applied to the first switch by detecting a DC resistance of the coil by causing a direct current to flow through the coil. A first switch to which a predetermined voltage is applied;
A first freewheeling diode connected in parallel to the first switch;
A second switch connected between the first switch and ground;
A second freewheeling diode connected in parallel to the second switch;
A coil connected between the first switch and the second switch;
A capacitor connected between the coil and ground;
A sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch;
After the first switch is turned on by the sequencer, a first sample and hold circuit that acquires a voltage across the capacitor;
A second sample-and-hold circuit for obtaining a voltage across the capacitor after the second switch is turned on by the sequencer;
A differential amplifier circuit for subtracting outputs of the first sample hold circuit and the second sample hold circuit;
A displacement detection device 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 first switch to which a predetermined voltage is applied;
A first freewheeling diode connected in parallel to the first switch;
A second switch connected between the first switch and ground;
A second freewheeling diode connected in parallel to the second switch;
A capacitor connected between the first switch and the second switch;
A coil connected between the capacitor and ground;
A sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch;
After the first switch is turned on by the sequencer, a first sample and hold circuit that acquires a potential at a connection point between the first switch and the second switch ;
After the second switch is turned on by the sequencer, a second sample and hold circuit that acquires a potential at a connection point between the first switch and the second switch ;
A differential amplifier circuit for subtracting outputs of the first sample hold circuit and the second sample hold circuit;
A displacement detection device 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 device according to claim 5, further comprising: a voltage control circuit that controls a voltage applied to the first switch by detecting a DC resistance of the coil by causing a direct current to flow through the coil. A first switch to which a predetermined voltage is applied;
A first freewheeling diode connected in parallel to the first switch;
A second switch connected between the first switch and ground;
A second freewheeling diode connected in parallel to the second switch;
A coil wherein the first switch has a core is connected between the second switch, that will change the inductance by metal to the core it is close,
A capacitor connected between the coil and ground;
A sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch;
After the first switch is turned on by the sequencer, a first sample and hold circuit that acquires a voltage across the capacitor;
A second sample-and-hold circuit for obtaining a voltage across the capacitor after the second switch is turned on by the sequencer;
A metal detector comprising: a first sample hold circuit; and a differential amplifier circuit that subtracts an output of the second sample hold circuit. A first switch to which a predetermined voltage is applied;
A first freewheeling diode connected in parallel to the first switch;
A second switch connected between the first switch and ground;
A second freewheeling diode connected in parallel to the second switch;
A capacitor connected between the first switch and the second switch;
And the coil inductance you change by having a core, a metal is close to the core and is connected between ground and the capacitor,
A sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch;
After the first switch is turned on by the sequencer, a first sample and hold circuit that acquires a potential at a connection point between the first switch and the second switch ;
After the second switch is turned on by the sequencer, a second sample and hold circuit that acquires a potential at a connection point between the first switch and the second switch ;
A metal detector comprising: a first sample hold circuit; and a differential amplifier circuit that subtracts an output of the second sample hold circuit. Furthermore,
The metal detection device according to claim 8, further comprising: a voltage control circuit that controls a voltage applied to the first switch by detecting a DC resistance of the coil by causing a direct current to flow through the coil.

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