JP4866974B1 - 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. Further, in order to improve the temperature characteristics, a period for fully charging and discharging the capacitor by the sequencer is provided.
[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. 9A, 9B and 9C are graphs for explaining various characteristics of the coil for explaining the operation principle of the magnetic sensor manufactured and sold by the applicant.
FIG. 9A is a circuit diagram for explaining the principle of the magnetic sensor, FIG. 9B is a diagram showing the state of the switch SW of the circuit of FIG. 9A, and FIG. 9C 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. 9A, 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. 9C, 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 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. 9D is a graph (BH curve) showing the relationship of the magnetic flux density with respect 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 is passed through the coil, the magnetic flux density of the yoke does not increase any more when the saturation magnetic flux density is reached (S901).
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 (S902).
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 (S903).
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 Japanese Patent No. 4684356

  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.

In order to solve the above-mentioned problems, the applicant and the inventor have developed a magnetic sensor disclosed in Patent Document 3. However, it has been found that the magnetic sensor of Patent Document 3 has a problem in temperature characteristics.
When the external magnetic field is increased, the coil current is relatively increased and the heat generation of the coil is increased. As a result, the DC resistance of the coil winding changes and the temperature characteristics deteriorate. It has also been found that when the magnetic field changes from a strong magnetic field to a non-magnetic field, hysteresis occurs in the output voltage due to the coil temperature equilibrium time.

  The present invention solves such a problem, and effectively suppresses a DC offset with a single coil, maintains a good temperature characteristic regardless of the strength of an external magnetic field, and provides a small but highly accurate magnetic sensor. For the purpose.

  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 for acquiring a change in the voltage across the capacitor, a charging period for measurement for charging the capacitor through the coil by turning on the first switch while turning off the second switch After the measurement charging period, the first switch is further turned on to charge the capacitor through the coil, and the first switch is turned on after the full charging period. A measurement discharge period in which the second switch is turned on while being controlled to discharge the capacitor through the coil, and a complete discharge period in which the second switch is further turned on after the measurement discharge period to discharge the capacitor through the coil. And a sequencer that outputs a control pulse signal to be transmitted.

  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 coil having a yoke to be connected, a voltage change acquisition unit for acquiring a change in voltage between both ends of the capacitor and the series connection of the coil, and turning on the first switch while turning off the second switch and passing the coil through the capacitor A charging period for charging, a full charging period in which the first switch is further turned on after the charging period for charging, and the capacitor is charged through the coil; A measurement discharge period in which the second switch is turned on and the capacitor is discharged through the coil while the first switch is turned off after the current period, and the second switch is further turned on after the measurement discharge period and the capacitor is coiled. And a sequencer for outputting a control pulse signal for forming a complete discharge period for discharging through the terminal.

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.
Furthermore, in order to make the temperature characteristic constant, a period for fully charging and completely discharging the capacitor is provided in the sequencer. In other words, in order to keep the heat generated from the coil constant regardless of the influence of an external magnetic field or the like, a sequencer pattern has been added so that the capacitor that is the basis of the current flowing through the coil is always fully charged and completely discharged.

  According to the present invention, it is possible to effectively suppress a direct current offset with a single coil, maintain good temperature characteristics regardless of the strength of an external magnetic field, and provide a small but highly accurate magnetic sensor.

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 an external appearance perspective view of the magnetic sensor which concerns on 2nd 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.

The present invention is an improvement of Patent Document 3.
The difference between the first embodiment of the present invention and Patent Document 3 is that, as shown in FIGS. 6A and 6B, the control pulse signal P1 and the control pulse signal P2 are fully charged with the capacitor C305. And a complete discharge period P604 (from time t18 to t20) for completely discharging the capacitor C305.

As shown in FIGS. 6A and 6B, the control pulse signal P1 and the control pulse signal P2 are alternately turned on and off.
The control pulse signal P1 includes a measurement charging period P601 (from time t1 to t3) for detecting a change in inductance of the coil L202 in the capacitor C305, and a full charging period P602 (from time t8) for fully charging the capacitor C305. up to t10). During the period between the measurement charging period P601 and the complete charging period P602, the voltage across the capacitor C305 is sampled by the sampling pulse signal P3.
The control pulse signal P2 includes a measurement discharge period P603 (from time t11 to t13) for detecting a change in inductance of the coil L202 in the capacitor C305, and a complete discharge period P604 (from time t18) for completely discharging the capacitor C305. up to t20). In the period between the measurement discharge period P603 and the complete discharge period P604, the voltage across the capacitor C305 is sampled by 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.

  FIG. 5B is a diagram illustrating a current flowing through the detection unit 203 during a period (from time t3 to t8) in which the control pulse signal P1 is turned off and the control pulse signal P2 is also turned off after the time t3. is there. 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.

  The control pulse signal P1 turns on the first switch 301 again from time t8 to time t10. That is, the state shown in FIG. The voltage across the capacitor C305 becomes equal to + Vcc by being fully charged, so that no current flows from time t9.

  FIG. 5C is a diagram illustrating a current flowing through the detection unit 203 in a period (from time t11 to t13) in which the control pulse signal P2 is on after time t10. 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.

  FIG. 5D is a diagram illustrating a current flowing through the detection unit 203 in a period (from time t13 to t18) in which the control pulse signal P2 is turned off and the control pulse signal P1 is also turned off after the time t13. is there. 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.

  The control pulse signal P2 turns on the second switch 302 again from time t18 to time t20. That is, the state shown in FIG. The voltage across the capacitor C305 becomes equal to 0V by being completely discharged, and therefore no current flows from time t19.

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, the frequency of the clock generator 401, and the sequence pattern stored in the ROM 403 must be set appropriately to satisfy this condition. The same can be said for the case where the sequencer 204 is constituted by a known microcomputer. When configured by a microcomputer, the sequence pattern of the control pulse signals P1 and P2 and the sampling pulse signals P3 and P4 is configured by a program.

  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.

  The voltage across the capacitor C305 obtained by the sample and hold circuit between time points t6 and t7 is a result of the current flowing in accordance with the inductance of the coil L202 and the magnetic field applied to the yoke 201. That is, since the voltage across the capacitor C305 at this time changes according to the strength of the external magnetic field applied to the yoke 201, the period from the time point t1 to the time point t3 is referred to as a measurement charging period P601. This means that the amount of charge accumulated in the capacitor C305 between the time points t6 and t7 changes according to the strength of the external magnetic field applied to the yoke 201.

Now, there is a problem that the temperature characteristics vary due to the amount of heat generated by the coil L202 changing according to the strength of the external magnetic field. As is well known, the coil L202 has a slight DC resistance component. This direct current resistance causes heat generation. When the amount of heat generated by the coil L202 changes, the DC resistance component of the coil L202 varies. This fluctuation of the DC resistance component leads to measurement error. Further, hysteresis occurs in the output voltage due to the temperature equilibration time of the coil L202.
If the problem is that the amount of heat generated by the coil L202 changes according to the strength of the external magnetic field, the amount of heat generated by the coil L202 may be prevented from changing regardless of the strength of the external magnetic field. If the heat generation of the coil L202 becomes constant, the direct current resistance of the coil L202 becomes constant and measurement errors are less likely to occur. Further, since the heat generation of the coil L202 is constant, the magnetic characteristics of the yoke 201 are also constant. This eliminates variations in the temperature characteristics of the yoke 201.
As described above, the direct current resistance of the coil L202 causes heat generation. The amount of heat generated by the coil L202 is determined by the integration of the current flowing through the coil L202 and time. That is, the amount of heat generated by the coil L202 is substantially equivalent to the charge accumulated in the capacitor C305.

Therefore, after the measurement charging period P601, after the voltage across the capacitor C305 is sampled by the sampling pulse signal P3, a full charging period P602 is provided in which a current is further supplied to the capacitor C305 to fully charge the capacitor C305. This is the period from time t8 to t10.
The control pulse signal P1 becomes logic true again from time t8, and the first switch 301 is turned on again. A current flows through the coil L202 to the capacitor C305. At time t9 when the voltage across the capacitor C305 becomes equal to + Vcc, the capacitor C305 is in a fully charged state, and no more current flows.

When the control pulse signal P2 is turned on at time t11, 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 t12, 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 t13, a current indicated by an arrow in FIG. Since the magnetic permeability of the yoke 201 is saturated after the time t12, the current flowing through the coil L202 is rapidly reduced. However, at the time t14, 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 phenomenon becomes gentler, and the current becomes zero at time t15.

  After the phenomenon from the time t11 to t15, 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 t16 after t15 when the transient of the coil L202 ends, the sample-and-hold circuit samples the voltage across the capacitor C305 until t17.

  The voltage across the capacitor C305 obtained by the sample-and-hold circuit between time t16 and time t17 is a result of the current flowing according to the inductance of the coil L202 and the magnetic field applied to the yoke 201. That is, since the voltage across the capacitor C305 at this time changes according to the strength of the external magnetic field applied to the yoke 201, the period from the time t11 to the time t13 is referred to as a measurement discharge period P603. This means that the amount of charge remaining in the capacitor C305 between time t6 and time t7 changes according to the strength of the external magnetic field applied to the yoke 201.

Now, there is a problem that the temperature characteristics vary due to the amount of heat generated by the coil L202 changing according to the strength of the external magnetic field. As described above, if the problem is that the amount of heat generated by the coil L202 changes according to the strength of the external magnetic field, the amount of heat generated by the coil L202 may be prevented from changing regardless of the strength of the external magnetic field.
As is well known, the coil L202 has a slight DC resistance component. This direct current resistance causes heat generation. The amount of heat generated by the coil L202 is determined by the integration of the current flowing through the coil L202 and time. That is, the amount of heat generated by the coil L202 is substantially equivalent to the charge accumulated in the capacitor C305.

Therefore, after the measurement discharge period P603, the voltage across the capacitor C305 is sampled by the sampling pulse signal P4, and then the capacitor C305 is further discharged to completely discharge the capacitor C305. This is the period from time t18 to t20.
The control pulse signal P3 becomes logic true again from time t18, and the second switch 302 is turned on again. Then, a current flows through the coil L202 to the capacitor C305. At time t19 when the voltage across the capacitor C305 becomes equal to 0V, the capacitor C305 is completely discharged, and no more current flows.

  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 t11 to t15).

FIG. 6F is a waveform diagram of the voltage across the capacitor C305.
The charge accumulated in the capacitor C305 that has passed the measurement charging period P601 and the charge that remains in the capacitor C305 that has passed the measurement discharge period P603 increase and decrease depending on the current generated in the coil L202. However, after the measurement charging period P601, the capacitor C305 is fully charged through the full charge period P602, and after the measurement discharge period P603, the capacitor C305 is fully discharged through the complete discharge period P604.
Therefore, the voltage between both ends increases from time t1 to time t5, stabilizes from time t5 to time t8, the voltage between both ends increases again to the power supply voltage + Vcc from time t8 to time t9, and from time t9 to time t11. The voltage is stable from time t11 to time t15, is stable from time t15 to time t18, is voltage is reduced to 0V from time t18 to time t19, and is stable from time t19 to time t21. . 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 t16 and time t17 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.
Conversely, 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.

  7 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 L702 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 L702.

  The advantage of the detection unit 701 in FIG. 7 compared to the detection unit 203 in FIG. 3 is that the coil L702 is grounded. Since the coil L702 as a sensor is grounded, it is difficult to be affected by external noise on the coil L702. Therefore, as compared with the detection unit 203 in FIG. 3, it is relatively easy to extend and arrange only the coil L702 from the detection unit 701 with a cable.

FIG. 8 is an external perspective view of a magnetic sensor according to the second embodiment of the present invention.
As described above, with the detection unit 701 in FIG. 7, only the coil L702 can be taken out from the detection unit 701 and routed with a cable. Since the coil L702, which is a sensor, can be arranged away from the detection unit 701, 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 and 801, the voltage applied to the first switch 301 and the resistor R315 is the power supply voltage. However, this is not necessarily the power supply voltage. How to determine the voltage applied to these elements is a matter of design.

  (2) 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.

  Furthermore, in order to prevent variation in characteristics and measurement errors due to heat generated in the coil L202 due to the strength of the external magnetic field, the sequencer pattern is improved, and the full charge period P602 and the full discharge period for the capacitor C305 after the sampling pulse. P604 was provided. By providing the full charge period P602 and the complete discharge period P604, the amount of charge charged and discharged to the capacitor C305 is always constant regardless of the strength of the external magnetic field. Therefore, the amount of heat generated by the coil L202 is also strong or weak in the external magnetic field. Regardless, it is always constant. For this reason, the hysteresis of the output voltage caused by the temperature equilibrium time of the coil L202 is also eliminated, and the possibility of erroneous detection is reduced.

  In particular, in the second 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 701 detector C305 capacitor C306 capacitor C309 D303 ... first freewheel diode, D304 ... second freewheel diode, L202 ... coil, L702 ... coil, R312 ... resistor, R313 ... resistor, R315 ... resistor, R316 ... resistor

Claims (4)

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 measurement charging period in which the first switch is turned on while the second switch is turned off to charge the capacitor through the coil, and the first switch is further turned on after the measurement charging period to A full charge period for charging a capacitor through the coil, and a measurement discharge period for discharging the capacitor through the coil by turning on the second switch while the first switch is turned off after the full charge period; A magnetic sensor comprising: a sequencer that outputs a control pulse signal that further controls the second switch after the measurement discharge period to form a complete discharge period in which the capacitor is discharged through the coil. The voltage change acquisition unit
A first sample-and-hold circuit for acquiring a voltage across the capacitor after the measurement charging period and before the full charging period by the sequencer;
A second sample and hold circuit for acquiring a voltage across the capacitor after the measurement discharge period and before the complete discharge period 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. 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 measurement charging period in which the first switch is turned on while the second switch is turned off to charge the capacitor through the coil, and the first switch is further turned on after the measurement charging period to A full charge period for charging a capacitor through the coil, and a measurement discharge period for discharging the capacitor through the coil by turning on the second switch while the first switch is turned off after the full charge period; A magnetic sensor comprising: a sequencer that outputs a control pulse signal that further controls the second switch after the measurement discharge period to form a complete discharge period in which the capacitor is discharged through the coil. The voltage change acquisition unit
A first sample-and-hold circuit for acquiring a voltage across the capacitor after the measurement charging period and before the full charging period by the sequencer;
A second sample and hold circuit for acquiring a voltage across the capacitor after the measurement discharge period and before the complete discharge period by the sequencer;
4. The magnetic sensor according to claim 3, further comprising an adder circuit that adds outputs of the first sample hold circuit and the second sample hold circuit.

Images