JP4731633B1 - 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 rectangular wave is supplied to a circuit in which a coil L104 with a yoke 201 and a capacitor C305 are connected in series. A saturation phenomenon is generated in the yoke 201 within the period of the rectangular wave, and the current change is detected through the current-voltage conversion circuit and then integrated. A current based on the integrated output signal is generated by a voltage-current converter circuit and fed back to the coil L104. Since the feedback current is proportional to the external magnetic field, a magnetic sensor with good linearity can be obtained. Unlike the prior art, since a bidirectional current is passed through the coil L104, 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.
[Selection] Figure 8

Description

The present invention relates to a magnetic sensor.
More specifically, a highly sensitive active magnetic sensor has a servo function that detects a magnetic field with a single coil and controls the yoke of the coil to a zero magnetic field, thereby realizing stable characteristics with few hardware resources. The present invention relates to a novel magnetic sensor.

  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, the range of good linearity is limited due to the restriction of the magnetic characteristics of the yoke, and the output voltage varies depending on the temperature because the circuit constants and the magnetic characteristics of the yoke change depending on the temperature. There was a problem.

  An object of the present invention is to solve such a problem by combining a servo function for detecting a magnetic field with a single coil and controlling a yoke of the coil to a zero magnetic field, and an object of the present invention is to provide a small but highly accurate magnetic sensor. .

In order to solve the above-described problems, a magnetic sensor of the present invention includes a drive circuit that outputs a rectangular wave, a coil connected to the drive circuit, a coil with a yoke, a capacitor connected in series to the coil, and a current of the coil. Current-voltage conversion circuit for converting to voltage, positive / negative peak detection circuit for detecting positive and negative peak voltage of output signal of current / voltage conversion circuit, and positive peak voltage output by positive / negative peak detection circuit Integrates the sum signal of the negative peak voltage and outputs a magnetic detection signal. An active integration circuit having a capacitor in the feedback loop, and a connection between the coil and the capacitor by converting the output voltage of the integration circuit into a current. A voltage-current conversion circuit to be supplied to the point.

  A predetermined voltage and ground are alternately connected to a 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 provide a magnetic sensor with a small size and high accuracy, which has both a servo function for detecting a magnetic field with a single coil and controlling a yoke of the coil to a zero magnetic field.

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 an external appearance perspective view of the magnetic sensor which concerns on 2nd embodiment of this invention. It is a circuit diagram of a magnetic sensor. It is a wave form diagram of each part of a magnetic sensor. 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 includes a resin-molded circuit block 102 and a coil L104 connected to the circuit block 102 through a cable 103. The coil L104 is wound around a permalloy yoke.
When the magnet 105 is brought close to the coil L104, 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 the coil L104 wound around the 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 capacitor C305 and a coil L104 are connected in series at the midpoint between the first switch 301 and the second switch 302. One end of the coil L104 is grounded.
Two sample and hold circuits are connected to a midpoint between the first switch 301 and the second switch 302 as a circuit for detecting the voltage across the capacitor C305 and the coil L104.

  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 and the coil L104 as it is.

A third switch 308 is interposed between the capacitor C306 and the capacitor C305. The third switch 308 is a transistor switch that 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 a transistor switch that 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 and coil L104. is there. 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 capacitor C317 is connected between the inverting input and the output of the operational amplifier 314. For this reason, the operational amplifier 314 constitutes an integrator.
A voltage that is half of the power supply voltage + Vcc is applied as a bias voltage to the non-inverting input of the operational amplifier 314 constituting the integrator.
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 L104 by the magnet 105 or the like. Details of this mechanism will be described later.

The output signal of the operational amplifier 314 is connected to the non-inverting input of the operational amplifier 319 through the resistor R318.
Similarly to the operational amplifier 314, a voltage half the power supply voltage + Vcc is applied to the inverting input of the operational amplifier 319 through the resistor R320. The operational amplifier 319 has a resistance value of the resistance R320 on the inverting input side and the resistance value R318 on the non-inverting input side equal, and constitutes a known differential amplifier circuit. The resistor R321 that is a feedback resistor connected between the inverting input and the output of the operational amplifier 319 has a resistance value equal to the resistance values of the inverting input side resistor R320 and the non-inverting input side resistor R318.

  A resistor R322 is connected between the output of the operational amplifier 319 and the coil L104. The resistor R322 is a resistor that detects a current flowing through the coil L104. The non-inverting input of the operational amplifier 323 is connected to the terminal on the coil L104 side of the resistor R322. The inverting input of the operational amplifier 323 is directly connected to the output, and constitutes a voltage follower similar to the operational amplifiers 307 and 311. The output of the operational amplifier 323 is connected to the non-inverting input of the operational amplifier 319 via the resistor R324. The operational amplifier 319 and the operational amplifier 323 are supplied with positive and negative power supply voltages so that positive and negative currents can be supplied to the coil L104.

The operational amplifier 319, the operational amplifier 323, the resistor R318, the resistor R320, the resistor R321, the resistor R322, and the resistor R324 constitute a voltage-current conversion circuit and function as a feedback circuit for the detection unit. That is, a current proportional to the change in the output voltage of the operational amplifier 314 is passed through the coil L104.
In this voltage-current conversion circuit, it is known that when R318 / R324 = R321 / R320, the output current of the operational amplifier 319 does not depend on the DC resistance value of the coil L104 or the voltage across the coil L104.

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, 6B, 6C, 6D, 6E, 6F, and 6G are waveform diagrams of each part of the magnetic 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, while the control pulse signal P2 is turned on after the control pulse signal P1 is turned off (from time t3 to t7), and while the control pulse signal P2 is turned off (when the control pulse signal P1 is turned on) ( Between the time points t9 and t13), a period in which both are turned off is provided. During the period when both the control pulse signal P1 and the control pulse signal P2 are off, sampling of the voltage across the capacitor C305 and the coil L104 is performed 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 capacitor C305 and the coil L104, 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 t3 to t7). 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 L104. 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 t7 to t9) 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 L104, 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 is also turned off and flows to the detection unit 203 in the period (from time t9 to t13). 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 flowed in the coil L104 is generated. In order to release this electromotive force, a first freewheel diode D303 is provided.

6 (e), (f), and (g) will be described based on the operation of the circuit described in FIGS. 5 (a), (b), (c), and (d).
FIG. 6E is a waveform diagram of the current flowing through the capacitor C305. In FIG. 3, this is indicated by a current I5.
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 L104, the current gradually increases in the positive direction.
When a current flows through the coil L104, a magnetic field proportional to the current is generated in the coil L104. When the current reaches a certain value at time t2, the magnetic permeability of the yoke 201 of the coil L104 is saturated, and the inductance of the coil L104 decreases. When the inductance of the coil L104 decreases, the current increase slope 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 L104 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 L104 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 L104 increases. As the inductance of the coil L104 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 L104, is finished, the capacitor C305 is charged with electric current by the current flowing through the coil L104. When the sequencer 204 generates the sampling pulse signal P3 from time t6 after t5 when the transient of the coil L104 ends, the sample-and-hold circuit samples the voltage across the capacitor C305 and the coil L104 until t7.

When the control pulse signal P2 is turned on at time t7, a current indicated by an arrow in FIG. Due to the characteristics of the coil L104, the current gradually increases in the negative direction.
When a current flows through the coil L104, a magnetic field proportional to the current is generated in the coil L104. When the current reaches a certain value at time t8, the magnetic permeability of the yoke 201 of the coil L104 is saturated, and the inductance of the coil L104 decreases. When the inductance of the coil L104 decreases, the current increase slope becomes steep.

  When the control pulse signal P2 is turned off at time t9, a current indicated by an arrow in FIG. Since the magnetic permeability of the yoke 201 is saturated after the time point t8, the current flowing through the coil L104 rapidly decreases. However, at the time point t10, the magnetic field applied to the yoke 201 is weakened, and the saturation state is eliminated. The inductance of L104 increases. As the inductance of the coil L104 increases, the slope of the current phenomenon becomes gentler, and the current becomes zero at time t11.

  After the phenomenon from the time t7 to the time t11, which is a transient phenomenon of the coil L104, is finished, the electric charge is discharged to the capacitor C305 by the current flowing through the coil L104. When the sequencer 204 generates the sampling pulse signal P4 from time t12 after t11 when the transient phenomenon of the coil L104 ends, the sample hold circuit samples the voltage across the capacitor C305 and the coil L104 until t13.

FIG. 6F is a waveform diagram of the voltage across the coil L104. In FIG. 3, this is indicated by voltage E6.
The voltage between both ends of the coil L104 appears in the direction of hindering the flowing current in accordance with on / off of the voltage applied to the coil L104. Therefore, the voltage between both ends increases from the time point t1 to the time point t3, and when the time point reaches the time point t3, a voltage in the opposite direction (negative voltage) is generated. At time t5, the voltage returns to zero (ground). Similarly, the voltage between both ends increases in the minus direction from time t7 to time t9, and when the time reaches time t9, a voltage in the direction opposite to the voltage applied so far (positive voltage) is generated. At time t11, the voltage returns to zero (ground). Repeat this cycle.

FIG. 6G is a waveform diagram of the voltage across the coil L104 and the capacitor C305. In FIG. 3, this is indicated by voltage E7.
Thus, + Vcc / 2 is output to the output terminal (output of the operational amplifier 314) in a state where no external magnetic field is applied to the coil L104.

In the case where the feedback circuit by the voltage-current converter circuit described later is cut, when an external magnetic field acts on the coil L104, the following operation is performed.
When the magnetic field in the coil L104 by the current when the control pulse signal P1 is turned on and the external magnetic field are in the same direction, the time t2 when the yoke 201 is saturated is advanced, and the peak current at the time t3 increases. As a result, the charging voltage of the capacitor C305 increases.
On the other hand, when the magnetic field in the coil L104 by the current when the control pulse signal P2 is turned on and the external magnetic field are in the same direction, the time t2 when the yoke 201 is saturated is delayed, and the peak current at the time t3 decreases. As a result, the charging voltage of the capacitor C305 decreases.
Eventually, charging is better than discharging, and the average voltage of the capacitor C305 increases. The same applies to the case where the external magnetic field has a reverse polarity. Thus, the average voltage of the capacitor C305 is offset around + Vcc / 2 according to the polarity and strength of the external magnetic field.

[Action of feedback]
The detection unit 203 of this embodiment is provided with a voltage-current conversion circuit as feedback. The feedback operation will be described.
The output current of the operational amplifier 319 constituting the voltage-current conversion circuit is proportional to the difference between the voltage applied to the resistor R318 and the voltage applied to the resistor R320.
Now, in a state where no external magnetic field is applied to the coil L104, the voltage between the terminals of the coil L104 transitions around 0 V as described with reference to FIG. Then, the voltage between terminals of the coil L104 and the capacitor C305 (voltage E7) is acquired by the sample hold circuit (the capacitor C306 and the operational amplifier 307, and the capacitor C309 and the operational amplifier 311), and then integrated by the integration circuit (the operational amplifier 314 and the capacitor C317). The operational amplifier 314 outputs + Vcc / 2. Therefore, the current output from the operational amplifier 319 is 0 mA.

  From the above state, an external magnetic field is applied to the coil L104. Assume that the output voltage of the operational amplifier 314 is offset in the positive direction from the + Vcc / 2 state. Then, the operational amplifier 319 passes a current proportional to the offset potential difference to the coil L104. This current causes the coil L104 to generate a magnetic field in a direction that cancels the external magnetic field. For this reason, the coil L104 always maintains a zero magnetic field state by this feedback.

  As described above, the polarity and strength of the external magnetic field is proportional to the current flowing from the voltage-current conversion circuit. That is, the output voltage of the operational amplifier 314 constituting the integrating circuit is a voltage proportional to the polarity and strength of the external magnetic field. Since a magnetic field is generated in the coil L104 in a direction that cancels the external magnetic field, the sum of positive and negative peak currents generated by the drive pulses generated by the first switch 301 and the second switch 302 is always zero. That is, the current waveform in FIG. 6E does not change even when an external magnetic field is applied to the coil L104. Further, the waveforms shown in FIGS. 6F and 6G, which are voltage waveforms, do not change even when an external magnetic field is applied.

The output impedance of the operational amplifier 319 constituting the voltage-current conversion circuit is theoretically infinite. For this reason, even if the voltage-current conversion circuit is connected to the connection point between the coil L104 and the capacitor C305, there is no AC effect.
Further, when a current is passed through the coil L104 from the current-voltage conversion circuit, a voltage is generated in the coil L104. However, since this voltage is a DC component, it is separated by the capacitor C305. Therefore, the subsequent sample hold circuit and the like are not affected.
Even if the magnetic characteristics of the yoke 201 of the coil L104 change due to a temperature change, the zero magnetic field is maintained in the DC direction in the coil L104, so that the balance between the positive and negative symmetrical magnetic characteristics is maintained. Therefore, the temperature characteristics are good. In addition, the current flowing through the coil L104 is proportional to the external magnetic field, and a zero magnetic field is maintained in the coil L104 in a direct current, so that excellent linearity is obtained without being limited by the magnetic characteristics of the yoke 201. I can do it.

  The features of the detection unit 203 of the first embodiment are different from the technical contents disclosed in Patent Document 1 and Patent Document 2, as described in FIGS. 5 (a) to 5 (d) and FIG. 6 (g). In addition, a bidirectional current flows through the coil L104 and the capacitor C305, and a zero magnetic field is maintained in the direct current. 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 L104 as in the prior art, and a magnetic field can be detected correctly with only one coil L104.

[Second Embodiment]
FIG. 7 is an external perspective view of the magnetic sensor according to the second embodiment of the present invention.
The magnetic sensor 701 is formed as an integrated sensor incorporating a resin-molded circuit.
A coil L104 wound around a permalloy yoke is incorporated in the detection projection 701a of the magnetic sensor 701.
When the magnet 105 is brought close to the detection protrusion 701a, an analog detection signal is obtained.

FIG. 8 is a circuit diagram of the magnetic sensor 701.
The magnetic sensor 701 of the second embodiment has no sequencer 204 compared to the magnetic sensor 701 of the first embodiment.

  The oscillation circuit 801 outputs a rectangular wave having a predetermined frequency. The output signal of the oscillation circuit 801 is input to the inverting input of the operational amplifier 803 through the resistor R802. The operational amplifier 803 forms an inverting amplifier together with a resistor R804 that is a feedback resistor. The oscillation circuit 801, the resistor R802, the operational amplifier 803, and the resistor R804 can be said to be a drive circuit 815.

  The output signal of the operational amplifier 803 is supplied to the coil L104. Coil L104 is connected to capacitor C305. The capacitor C305 is connected to an inverting input of an operational amplifier 806 that constitutes a current-voltage conversion circuit 816 together with a resistor R805 that is a feedback resistor.

The output signal of the operational amplifier 806 is connected to the anode of the diode D807 and the cathode of the diode D808.
Since the diode D807 passes the voltage in the positive direction of the output signal, the charge in the positive direction is accumulated in the capacitor C809, and as a result, the peak voltage in the positive direction of the output signal appears in the capacitor C809.
Similarly, since the diode D808 passes the negative voltage of the output signal, negative charge is accumulated in the capacitor C810, and as a result, the negative peak voltage of the output signal appears in the capacitor C810. .
The diode D807, the diode D808, the capacitor C809, and the capacitor C810 can be said to be a positive / negative peak detection circuit 817.

  The voltage between the terminals of the capacitor C809 and the capacitor C810 is supplied to the inverting input of the operational amplifier 314 through an adding circuit composed of a resistor R811 and a resistor R812, respectively. The operational amplifier 314 constitutes an integration circuit 818 together with the capacitor C317, and outputs a sum voltage of terminals between the capacitors C809 and C810.

  The output signal of the operational amplifier 314 is connected to the non-inverting input of the operational amplifier 319 constituting the voltage-current conversion circuit 819 through the resistor R318. The configuration of the voltage-current conversion circuit 819 is the same as that of the first embodiment except that the ground potential is connected to the non-inverting input of the operational amplifier 319, and thus the details are omitted.

[Operation]
FIGS. 9A, 9 </ b> B, 9 </ b> C, and 9 </ b> D are waveform diagrams of each part of the magnetic sensor 701.
FIG. 9A is a waveform diagram of the output signal voltage of the operational amplifier 803. This is the measurement point E11 in FIG. It can be seen that a square wave is output around the ground.
FIG. 9B is a waveform diagram of the output signal voltage of the operational amplifier 806. This is the measurement point E12 in FIG. That is, it is also the waveform of the current flowing through the capacitor C305. Similar to FIG. 6 (e), the saturation of the yoke 201 makes the slope of the current waveform steep from the middle.

FIG. 9C is a waveform diagram of the output signal voltage of the diode D807. This is the measurement point E13 in FIG. When the voltage having the waveform shown in FIG. 9B exceeds the ON voltage of the diode D807, the diode D807 becomes conductive, and a current flows through the capacitor C809. The voltage across the capacitor C809 is such that the charge of the capacitor C809 is gradually discharged through the resistor R811 and the operational amplifier 314 until the voltage having the waveform shown in FIG. 9B reaches the ON voltage of the diode D807. Then it gradually decreases.
FIG. 9D is a waveform diagram of the output signal voltage of the diode D808. When the voltage having the waveform shown in FIG. 9B exceeds the ON voltage of the diode D808, the diode D808 becomes conductive, and a current flows through the capacitor C810. The voltage across the capacitor C810 is such that the charge of the capacitor C810 is gradually discharged through the resistor R812 and the operational amplifier 314 until the next period when the voltage having the waveform shown in FIG. 9B reaches the ON voltage of the diode D808. Then it gradually decreases.

In the case of the circuit of FIG. 8, 0V is output to the output terminal in a state where no external magnetic field is applied to the coil L104. Except for this point, the operation is the same as that of the first embodiment.
Compared with the magnetic sensor 701 of the first embodiment, the magnetic sensor 701 of the second embodiment has a circuit configuration because the sequencer 204 is not necessary and the operational amplifier is omitted. Since it is simple, it is easy to reduce the cost of the apparatus.

The drive circuit 815, the current / voltage conversion circuit 816, the positive / negative peak detection circuit 817, the integration circuit 818, and the voltage / current conversion circuit 819 shown in FIG. 8 also exist in the detection unit 203 of the first embodiment.
The drive circuit 815 corresponds to the first switch 301, the second switch 302, and the sequencer 204 in FIG.
In the current-voltage conversion circuit 816, the capacitor C305 in FIG.
The positive / negative peak detection circuit 817 corresponds to the third switch 308, the fourth switch 310, the capacitor C306, the operational amplifier 307, the capacitor C309, and the operational amplifier 311 in FIG.
The integration circuit 818 corresponds to the operational amplifier 314 and the capacitor C317 in FIG.
The voltage-current conversion circuit 819 corresponds to the resistor R318, the operational amplifier 319, the resistor R320, the resistor R321, the resistor R322, the operational amplifier 323, and the resistor R324 in FIG.

In addition to the embodiment described above, the following application examples are conceivable.
(1) In the detection unit 203 and the magnetic sensor 701, the voltage applied to the first switch 301 and the resistor R315 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 rectangular wave is supplied to a circuit in which a coil L104 with a yoke 201 and a capacitor C305 are connected in series. A saturation phenomenon is generated in the yoke 201 within the period of the rectangular wave, and the current change is detected through the current-voltage conversion circuit and then integrated. A current based on the integrated output signal is generated by a voltage-current converter circuit and fed back to the coil L104. Since the feedback current is proportional to the external magnetic field, a magnetic sensor with good linearity can be obtained.

  Even if the magnetic characteristic of the yoke 201 of the coil L104 changes due to temperature change by combining the detection of the magnetic field with a single coil and the servo function for controlling the coil yoke to zero magnetic field, the inside of the coil L104 is DC Since a zero magnetic field is maintained, a balance of magnetic properties of positive and negative symmetry is maintained. Therefore, the temperature characteristics are good. In addition, since the current flowing through the coil L104 is proportional to the external magnetic field, and a zero magnetic field is maintained in the coil L104 in terms of direct current, good linearity can be obtained without being limited by the magnetic characteristics of the yoke 201. I can do it. Thus, a low-cost and highly accurate magnetic sensor can be realized.

  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 ... Circuit block, 103 ... Cable, 105 ... Magnet, 201 ... Yoke, 203 ... Detection part, 204 ... Sequencer, 301 ... First switch, 302 ... Second switch, 307 ... Operational amplifier, 308 ... First Three switches, 310 ... fourth switch, 311 ... operational amplifier, 314 ... operational amplifier, 319 ... operational amplifier, 323 ... operational amplifier, 401 ... clock generator, 402 ... loop counter, 403 ... ROM, 404 ... decoder, 701 ... magnetic sensor, 801 Oscillating circuit, 803 operational amplifier, 806 operational amplifier, 815 driving circuit, 816 current current converting circuit, 817 positive / negative peak detecting circuit, 818 integrating circuit, 819 voltage current converting circuit, C305 capacitor, C306 capacitor , C309 ... capacitor, C317 ... Capacitor, C809 ... capacitor, C810 ... capacitor, D303 ... first freewheel diode, D304 ... second freewheel diode, D807 ... diode, D808 ... diode, L104 ... coil, R312 ... resistor, R313 ... resistor, R315 ... resistor, R316 ... resistor, R318 ... resistor, R320 ... resistor, R321 ... resistor, R322 ... resistor, R324 ... resistor, R802 ... resistor, R804 ... resistor, R805 ... resistor, R811 ... resistor, R812 ... resistor

Claims (3)

A drive circuit that outputs a rectangular wave;
A coil with a yoke connected to the drive circuit;
A capacitor connected in series to the coil;
A current-voltage conversion circuit for converting the current of the coil into a voltage;
A positive / negative peak detection circuit for detecting a positive peak voltage and a negative peak voltage of the output signal of the current-voltage conversion circuit;
An active integration circuit having a capacitor in a feedback loop, which integrates the addition signal of the positive direction peak voltage and the negative direction peak voltage output by the positive / negative peak detection circuit and outputs a magnetic detection signal ;
A magnetic sensor comprising: a voltage-current conversion circuit that converts an output voltage of the integration circuit into a current and supplies the current to a connection point between the coil and the capacitor.   The period and voltage of the rectangular wave output from the drive circuit are such that the current flowing in the coil due to the rectangular wave causes magnetic saturation in the yoke while the rectangular wave maintains a predetermined voltage. The magnetic sensor according to claim 1, wherein an increase in current is determined to be steeper than before magnetic saturation occurs. The drive circuit is
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 sequencer that outputs a control pulse signal for alternately turning on and off the first switch and the second switch;
The current-voltage conversion circuit is also used as the capacitor,
The positive / negative peak detection circuit includes:
A first sample-and-hold circuit that acquires a voltage across the capacitor under the control of the sequencer after the first switch is turned on;
The magnetic sensor according to claim 2, further comprising a second sample and hold circuit that acquires a voltage across the capacitor under the control of the sequencer after the second switch is turned on.

Images