JP4684356B1 - Magnetic sensor - Google Patents

Abstract

Disclosed is a magnetic sensor that effectively suppresses a DC offset with a single coil and is small but highly accurate.
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.
[Selection] Figure 3

Description

The present invention relates to a magnetic sensor.
More specifically, the present invention relates to a novel magnetic sensor that removes an offset that could not be removed so far from a high-sensitivity active magnetic sensor and realizes stable characteristics with less hardware resources.

  The applicant manufactures and sells an active type magnetic sensor that realizes a high-sensitivity characteristic of three orders of magnitude or more compared with a known Hall element. Hereinafter, the operating principle of this active magnetic 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 magnetic sensor manufactured and sold by the applicant.
10A is a circuit diagram for explaining the principle of the magnetic sensor, FIG. 10B is a diagram showing the state of the switch SW of 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 magnetic sensor of the present applicant is realized by obtaining the change in inductance of the coil from a transient response phenomenon.

  If a yoke with high magnetic permeability is interposed in the center of the coil, the inductance of the coil increases. As a magnetic material having a high magnetic permeability, there is a permalloy known for a magnetic head. In addition to high magnetic permeability, this permalloy has the property that when a strong magnetic field is applied, the magnetic flux density does not increase further after a certain point, and the magnetic permeability is substantially zero. There is a saturation magnetic flux density that magnetizes when a strong magnetic field is applied to a ferromagnet and the magnetic flux density does not increase any more, but in the case of permalloy, the residual magnetic flux density is small, so it is suitable for use as a yoke for transformers, coils, etc. Material. Therefore, a magnetic sensor can be realized by preparing a coil using permalloy for the yoke and detecting a change in inductance depending on whether or not a magnetic field is applied from the outside.

FIG. 10D is a graph (BH curve) showing the relationship of the magnetic flux density to the strength of the magnetic field applied from the outside of the coil using Permalloy as the yoke.
When an external magnetic field is applied in a state where no current flows through the coil, the magnetic flux density of the yoke does not increase any more when the saturation magnetic flux density is reached (S1001).
When a positive magnetic field is applied to the yoke while a positive current is applied to the coil, and the magnetic flux density of the yoke is observed in the same manner, the magnetic flux density is positive by the amount that a positive magnetic field is generated by the current. The saturation magnetic flux density is reached with an external magnetic field weaker than that in a state where no current flows (S1002).
When a current in the reverse direction is applied to the coil and a magnetic field in the positive direction is applied to the yoke, and the magnetic flux density of the yoke is observed in the same manner, the magnetic flux density is in the negative direction as much as the negative magnetic field is generated by the current. The saturation magnetic flux density is reached with an external magnetic field stronger than that in a state where no current flows (S1003).
Thus, a coil using permalloy as a yoke can be applied as a sensor that can distinguish and detect magnetic fields in both the positive and negative directions.
Patent Document 1 and Patent Document 2 are prior art documents of a current sensor using a magnetic sensor by the present inventor, to which the above-described technology is applied.

Japanese Patent No. 3907488 JP 2001-153895 A

  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. A pulsed DC voltage is continuously applied to the coil connected in series with the capacitor, and changes in the coil current are captured. In Patent Document 1 and Patent Document 2, a change in current is detected by envelope detection using a diode.

In the detection circuits used in these conventional techniques, a DC offset component is included in the detection voltage due to the operation principle. The offset voltage varies depending on circuit constants, and fluctuates due to environmental changes such as temperature. For this reason, the prior art prepares two coils, subtracts the respective outputs, and cancels the offset to avoid it.
However, preparing two coils leads to an increase in the number of parts and hinders downsizing of the apparatus.

  An object of the present invention is to solve such a problem and to provide a magnetic sensor with high accuracy while being small in size by effectively suppressing a DC offset with a single coil.

  In order to solve the above problems, a magnetic sensor 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, and between the first switch and ground. A second switch connected to the second switch, a second freewheeling diode connected in parallel to the second switch, a coil having a yoke connected between the first switch and the second switch, and a coil and ground A capacitor connected in between, a voltage change acquisition unit that acquires a change in voltage across the capacitor, and a sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch.

  In order to solve the above problems, a magnetic sensor 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 connected coil having a yoke, a voltage change acquisition unit for acquiring a change in voltage across the capacitor and the series connection of the coil, and a control pulse signal for alternately turning on and off the first switch and the second switch. And a sequencer for output.

  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.

  According to the present invention, it is possible to effectively suppress a direct current offset with a single coil, and to provide a highly accurate magnetic sensor while being small.

1 is an external perspective view of a magnetic sensor according to a first embodiment of the present invention. It is a block diagram of a magnetic 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 magnetic sensor. It is a wave form diagram of each part of a magnetic sensor. It is a circuit diagram of the detection part of the magnetic sensor which concerns on 2nd embodiment of this invention. It is a circuit diagram of the detection part of the magnetic sensor which concerns on 3rd embodiment of this invention. It is an external appearance perspective view of the magnetic sensor which concerns on 3rd embodiment of this invention. It is a graph explaining various characteristics of a coil for explaining an operation principle of a magnetic sensor which an applicant manufactures and sells.

[First embodiment]
FIG. 1 is an external perspective view of the magnetic sensor according to the first embodiment of the present invention.
The magnetic sensor 101 is formed as an integrated sensor incorporating a resin-molded circuit.
A coil wound around a permalloy yoke is incorporated in the detection projection 101 a of the magnetic sensor 101.
When the magnet 102 is brought close to the detection protrusion 101a, an analog detection signal is obtained.

FIG. 2 is a block diagram of the magnetic sensor 101.
The magnetic sensor 101 includes a detection unit 203 that detects a change in inductance of a coil L202 wound around a yoke 201, 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 L202 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 amplifiers 307 and 311 constitute a non-inverting amplifier with zero feedback resistance, that is, a voltage follower, and outputs the voltage across the capacitor C306 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 adder circuit.
The resistor R312 connected to the output of the operational amplifier 307 and the resistor R313 connected to the output of the operational amplifier 311 constitute an input unit of the adder circuit. The middle point of the resistors R312 and R313, that is, the addition signal is applied to the inverting input of the operational amplifier 314.

A voltage that is half of the power supply voltage + Vcc divided by the resistors R315 and R316 is applied as a bias voltage to the non-inverting input of the operational amplifier 314 constituting the inverting amplifier.
Thus, the operational amplifier 314 outputs the average voltage of the capacitor C305.
The average voltage of the capacitor C305 changes according to the magnetic field applied to the coil L202. 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 illustrating the operation principle of the magnetic sensor 101.
6A, 6 </ b> B, 6 </ b> C, 6 </ b> D, 6 </ b> E, and 6 </ b> F are waveform diagrams of each part of the magnetic sensor 101.
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, while the control pulse signal P2 is turned on after the control pulse signal P1 is turned off (from time t3 to t8), and between when the control pulse signal P2 is turned off and when the control pulse signal P1 is turned on ( Between the time points t10 to t15), 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 time t3) 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 L202 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 during the period (from time t3 to t8). 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 L202. 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 t8 to t10) 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 L202, 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 flows to the detection unit 203 during the period (from time t10 to t15). 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 L202 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 L202.
When the control pulse signal P1 is turned on at time t1, a current indicated by an arrow in FIG. Due to the characteristics of the coil L202, the current gradually increases in the positive direction.
When a current flows through the coil L202, a magnetic field proportional to the current is generated in the coil L202. When the current reaches a certain value at time t2, the permeability of the yoke 201 of the coil L202 is saturated, and the inductance of the coil L202 decreases. When the inductance of the coil L202 decreases, the current increase becomes steep.
The pulse width of the control pulse signal P1 needs to be a value until the yoke is sufficiently saturated. In other words, if the pulse width is narrow, it does not function as a magnetic sensor. The inductance of the coil L202 and the frequency of the clock generator 401 must be set appropriately to satisfy this condition.

  When the control pulse signal P1 is turned off at time t3, a current indicated by an arrow in FIG. 5B flows. Since the magnetic permeability of the yoke 201 is saturated after the time point t2, the current flowing through the coil L202 rapidly decreases. However, at the time point t4, the magnetic field applied to the yoke 201 is weakened, and the saturation state is eliminated. The inductance of L202 increases. As the inductance of the coil L202 increases, the slope of the current decrease becomes gradual, and the current becomes zero at time t5.

  After the phenomenon from the time point t1 to the time point t5, which is a transient phenomenon of the coil L202, is finished, electric charges are accumulated in the capacitor C305 by the current flowing through the coil L202. When the sequencer 204 generates the sampling pulse signal P3 from time t6 after t5 when the transient of the coil L202 ends, the sample-and-hold circuit samples the voltage across the capacitor C305 until t7.

When the control pulse signal P2 is turned on at time t8, a current indicated by an arrow in FIG. Due to the characteristics of the coil L202, the current gradually increases in the negative direction.
When a current flows through the coil L202, a magnetic field proportional to the current is generated in the coil L202. When the current reaches a certain value at time t9, the magnetic permeability of the yoke 201 of the coil L202 is saturated, and the inductance of the coil L202 decreases. When the inductance of the coil L202 decreases, the current increase becomes steep.

  When the control pulse signal P2 is turned off at time t10, a current indicated by an arrow in FIG. Since the magnetic permeability of the yoke 201 is saturated after the time t9, the current flowing through the coil L202 decreases rapidly. The inductance of L202 increases. As the inductance of the coil L202 increases, the slope of the current phenomenon becomes gentler, and the current becomes zero at time t12.

  After the phenomenon from the time point t8 to t12, which is a transient phenomenon of the coil L202, is finished, electric charge is accumulated in the capacitor C305 by the current flowing through the coil L202. When the sequencer 204 generates the sampling pulse signal P4 from time t13 after t12 when the transient of the coil L202 ends, the sample-and-hold circuit samples the voltage across the capacitor C305 until t14.

  When a magnetic field is not applied to the coil L202, a current indicated by a solid line in FIG. On the other hand, when a negative magnetic field is applied to the coil L202, a current indicated by a dotted line in FIG. That is, since the BH curve of the coil L202 is offset in the negative direction, the current increases in the positive direction (time t1 to t5) and decreases in the negative direction (time t8 to t12).

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 L202. Therefore, the voltage between both ends increases from time t1 to time t5, stabilizes from time t5 to time t8, decreases between time t8 and time t12, and stabilizes from time t12 to time t15. Repeat this cycle.

  When no magnetic field is applied to the coil L202, the voltage across the capacitor C305 changes as shown by the solid line in FIG. Therefore, when the voltage across the capacitor C305 between time t6 and time t7 and between time t13 and time t14 is sampled and added, a signal corresponding to the voltage of the power supply voltage + Vcc / 2 is obtained from the output of the operational amplifier 314. .

On the other hand, when a negative magnetic field is applied to the coil L202, the voltage across the capacitor C305 changes as indicated by the dotted line in FIG. That is, since the BH curve of the coil L202 is offset in the negative direction, the current increases in the positive direction, and as a result, the voltage across the capacitor C305 is offset in the positive direction. Therefore, when the voltage between both ends of the capacitor C305 between time t6 and time t7 and between time t13 and time t14 is sampled and added, the output of the operational amplifier 314 has the power supply voltage + Vcc / 2 obtained at the time of no magnetic field. A signal offset in the positive direction is obtained from the signal corresponding to the voltage.
On the contrary, although not shown in FIG. 6, when a magnetic field in the positive direction is applied to the coil L202, the output of the operational amplifier 314 corresponds to the power supply voltage + Vcc / 2 obtained at the time of no magnetic field. As a result, a signal offset in the negative direction is obtained.

  As described above, when a DC magnetic field is applied to the coil L202, the change in the voltage across the capacitor C305 is offset in the positive direction or the negative direction depending on the direction of the DC magnetic field. The sample and hold circuit obtains a stable detection output by taking the average of changes in the voltage across the capacitor C305. This detection output is the power supply voltage + Vcc / 2 when the external magnetic field is zero, the capacitance error of the capacitor C305, the pulse width error of the control pulse signals P1 and P2, the inductance error of the coil L202, and the magnetic characteristics of the yoke 201. Unaffected by variations in circuit constants such as errors. That is, a stable output with no zero point drift can be obtained.

  The feature of the detection unit 203 of the first embodiment is different from the technical contents disclosed in Patent Document 1 and Patent Document 2, as described in FIGS. 5A to 5D and FIG. 6E. In addition, a bidirectional current flows through the coil L202 and the capacitor C305. That is, even when the circuit constant changes, the change in the detection voltage becomes the same magnitude and cancels due to the difference in the current direction, so that there is almost no offset in the no magnetic field state. Accordingly, it is not necessary to perform offset cancellation by arranging two coils L202 as in the prior art, and a magnetic field can be detected correctly with only one coil L202.

[Second Embodiment]
FIG. 7 is a circuit diagram of the detection unit of the magnetic sensor according to the second embodiment of the present invention. The magnetic sensor of the second embodiment differs from the magnetic 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.

  The difference between the detection unit 701 shown in FIG. 7 and the detection unit 203 in FIG. 3 is that an integration circuit using a resistor R702 and a capacitor C703 is used instead of the sample hold circuit. Even with such a simple circuit, the magnetic sensor 101 can be realized. However, in the case of the detection unit 701 in FIG. 7, the response characteristics are slightly inferior to those of the detection unit 203 in FIG.

[Third embodiment]
FIG. 8 is a circuit diagram of the detection unit of the magnetic sensor according to the third embodiment of the present invention. The magnetic sensor of the third embodiment is different from the magnetic 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.

  8 is different 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 L802 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 L802.

  The advantage of the detection unit 801 of FIG. 8 compared to the detection unit 203 of FIG. 3 is that the coil L802 is grounded. Since the coil L802 as a sensor is grounded, it is difficult to be affected by external noise on the coil L802. Therefore, as compared with the detection unit 203 of FIG. 3, it is relatively easy to extend and arrange only the coil L802 from the detection unit 801 with a cable.

FIG. 9 is an external perspective view of a magnetic sensor according to the third embodiment of the present invention.
As described above, with the detection unit 801 in FIG. 8, only the coil L802 can be taken out from the detection unit 801 and routed with a cable. Since the coil L802, which is a sensor, can be arranged away from the detection unit 801, the degree of freedom of arrangement as a magnetic sensor is high compared to the magnetic sensor 101 of the first embodiment.

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 and the resistor R 315 is the power supply voltage, but this is not necessarily the power supply voltage. How to determine the voltage applied to these elements is a matter of design.

  (2) The yoke 201 is permalloy, but the material is not limited to this as long as the material has a high magnetic permeability and a low saturation magnetic flux density. For example, an amorphous alloy etc. are mentioned.

In the present embodiment, a magnetic 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.
Unlike the prior art, since a bidirectional current flows through the coil, even if the circuit constant changes, the change in the detection voltage becomes the same magnitude due to the difference in the current direction and cancels out. For this reason, an offset at the time of no magnetic field hardly occurs. Therefore, it is not necessary to provide two coils as in the prior art, and the number of parts is reduced, so that a highly accurate magnetic sensor can be realized at low cost.
In particular, in the third embodiment, 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. .

  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 ... Magnetic sensor, 102 ... Magnet, 201 ... Yoke, 203 ... Detection part, 204 ... Sequencer, 301 ... First switch, 302 ... Second switch, 307 ... Operational amplifier, 308 ... Third switch, 310 ... Fourth switch, 311... Operational amplifier 314... Operational amplifier 401... Clock generator 402. Loop counter 403 ROM ROM 404 decoder 701 detector 801 detector C305 C306 capacitor C309 capacitor C703 ... capacitor, D303 ... first freewheel diode, D304 ... second freewheel diode, L202 ... coil, L802 ... coil, R312 ... resistor, R313 ... resistor, R315 ... resistor, R316 ... resistor, R702 ... resistor

Claims (5)

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 having a yoke connected between the first switch and the second switch;
A capacitor connected between the coil and ground;
A voltage change acquisition unit for acquiring a change in the voltage across the capacitor;
A magnetic sensor comprising a sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch. The voltage change acquisition unit
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;
The magnetic sensor according to claim 1, further comprising an adder circuit that adds outputs of the first sample hold circuit and the second sample hold circuit.   The magnetic sensor according to claim 1, wherein the voltage change acquisition unit includes a low-pass filter. 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 having a yoke connected between the capacitor and ground;
A voltage change acquisition unit for acquiring a change in voltage across the capacitor and the coil connected in series;
A magnetic sensor comprising a sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch. The voltage change acquisition unit
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;
The magnetic sensor according to claim 4, further comprising an adder circuit that adds outputs of the first sample hold circuit and the second sample hold circuit.

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