Current sensor

A current sensor includes: a conductive member through which a current to be measured flows; first and second magnetic sensors which output signals having reversed phases to each other due to an induction magnetic field from the current to be measured; and a control unit which performs differential operation on the output signal of the first magnetic sensor and the output signal of the second magnetic sensor, wherein sensing axis directions of the first magnetic sensor and the second magnetic sensor are fixed in the same direction, form a predetermined angle with respect to an application direction of the induction magnetic field from the current to be measured applied to the first magnetic sensor and the second magnetic sensor, and are fixed so that the induction magnetic fields are applied to the first magnetic sensor and the second magnetic sensor in reverse directions to each other.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a current sensor that measures the magnitude of current, and more particularly, to a current sensor capable of reducing noise from disturbance magnetism.

2. Description of the Related Art

In recent years, in fields such as an electric vehicle and a solar battery, a current value that is involved has been increased due to increases in output and performance of the electric vehicle or the solar battery device, and a current sensor that contactlessly measures a high DC current has been widely used. As such a current sensor, a sensor including a magnetic sensor that detects a current to be measured, which flows through a conductor, using a change in magnetic field in the periphery of the conductor has been suggested. In addition, as the current sensor, a current sensor that reduces noise from disturbance magnetism has been developed.

As the current sensor that reduces noise from disturbance magnetism, a current sensor which acquires a differential between output signals of a pair of magnetic sensors disposed on the same substrate has been suggested (for example, refer to Japanese Unexamined Patent Application Publication No. 2002-243766). In the current sensor, the pair of magnetic sensors are disposed on the substrate at positions that oppose each other with a current line interposed therebetween, which is orthogonal to the substrate. The sensing axis directions of the pair of magnetic sensors are the same direction. In the current sensor, since the output signals having reversed phases to each other are output from the pair of magnetic sensors, the output signals are processed to be added to each other by differential operation and thus output sensitivity is enhanced. In addition, noise components other than the output signals of the pair of magnetic sensors have the same phase and thus are removed by the differential operation.

However, in a case where the magnitude of current for driving the motor of an electric vehicle is measured by the current sensor, the current to be measured becomes a high current during motor driving and becomes a minute current during motor stoppage. Therefore, a current sensor capable of measuring both the high current and the minute current with high accuracy is preferable. In addition, with an expansion of applications of the current sensor in recent years, further reductions in the size and the thickness of the current sensor are preferable. In terms of the reductions in the size and the thickness of the current sensor, it is preferable that the magnetic sensors be disposed in the vicinity of the current line.

However, in the current sensor described in Unexamined Patent Application Publication No. 2002-243766, in the case where the pair of magnetic sensors are disposed in the vicinity of the current line, there is a problem in that the pair of magnetic sensors become magnetically saturated by an induction magnetic field from the current to be measured, which flows through the current line. Particularly, in a case where the current to be measured which is an object of measurement is a high current, the induction magnetic field from the current to be measured is increased, and thus the magnetic sensors are likely to become magnetically saturated. As such, in the current sensor according to the related art, it is difficult to realize both the measurement of the current to be measured, which is a high current, and the reductions in the size and the thickness of the current sensor.

SUMMARY OF THE INVENTION

The present invention provides a current sensor capable of reducing noise from disturbance magnetism, measuring a current to be measured, which is a high current, and realizing reductions in size and thickness.

According to an aspect of the present invention, there is provided a current sensor including: a conductive member through which a current to be measured flows; a first magnetic sensor and a second magnetic sensor which output signals having reversed phases to each other due to an induction magnetic field from the current to be measured; and a differential unit which performs differential operation on the output signal of the first magnetic sensor and the output signal of the second magnetic sensor, wherein sensing axis directions of the first magnetic sensor and the second magnetic sensor are fixed in the same direction, form a predetermined angle with respect to an application direction of the induction magnetic field from the current to be measured applied to the first magnetic sensor and the second magnetic sensor, and are fixed so that the induction magnetic fields are applied to the first magnetic sensor and the second magnetic sensor in reverse directions to each other.

A substrate having the first magnetic sensor disposed on one surface and the second magnetic sensor disposed on the other surface is included, the conductive member forms a shape having a longitudinal direction and a lateral direction in a cross-sectional view, and the substrate is disposed to be parallel to the conductive member in the longitudinal direction so that an in-plane direction of the substrate and the longitudinal direction are substantially parallel to each other.

In this configuration, since the first magnetic sensor and the second magnetic sensor are disposed with the substrate interposed therebetween, which is disposed to be parallel to the conductive member, the thickness of the current sensor may be reduced, and the reductions in the size and the thickness of the current sensor are possible. Moreover, since the first magnetic sensor and the second magnetic sensor are disposed with the substrate interposed therebetween, the difference between disturbance magnetism applied to the first magnetic sensor and disturbance magnetism applied to the second magnetic sensor may be reduced, thereby effectively reducing an influence of noise due to the disturbance magnetism.

In the current sensor according to the aspect of the present invention, a position at a center in a thickness direction of the substrate may be disposed at the same position with respect to a position at a center in a height direction of the conductive member.

In this configuration, the thickness of the current sensor may be reduced, and the reductions in the size and the thickness of the current sensor are possible. Moreover, the difference between magnetic vectors in the sensing axis directions of the induction magnetic field applied to the first magnetic sensor and the second magnetic sensor may be reduced, and thus the detection accuracy of the current sensor may be particularly enhanced.

In the current sensor according to the aspect of the present invention, the conductive member may have a rectangular shape in the cross-sectional view. In this configuration, a change in the application direction of the induction magnetic field in the vicinity of the end portions in the lateral direction of the conductive member increases. Therefore, the magnetic vectors in the sensing axis directions of the first magnetic sensor and the second magnetic sensor may be reduced, and thus it is possible to measure the current to be measured, which is a high current.

In the current sensor according to the aspect of the present invention, the first magnetic sensor and the second magnetic sensor may be magnetoresistance elements.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1is a cross-sectional schematic view of a current sensor according to this embodiment. As illustrated inFIG. 1, the current sensor1according to this embodiment includes a case11having a space therein, and a conductive member12of which a part is disposed in the case11and which extends in one direction. In the conductive member12in the case11, a pair of first and second magnetic sensors14aand14barranged via a substrate13are disposed, and the magnitude of a current to be measured, which flows through the conductive member12, is measured by the first and second magnetic sensors14aand14b. In addition, inFIG. 1, a cross-sectional schematic view in a vertical direction with respect to the extension direction of the conductive member12is illustrated.

The case11is formed of, for example, an insulating material, but a part thereof may also be configured to include a material having high magnetic permeability such as silicon steel or permalloy and be configured to shield disturbance magnetism in the case11. The conductive member12has a rectangular shape in a cross-sectional view, and has a pair of main surfaces12aand12bat both ends in a lateral direction D2and a pair of end surfaces12cand12dat both ends in a longitudinal direction D3.

The substrate13is supported by a support member (not shown) between one end surface12cand the case11in the longitudinal direction D3of the conductive member12and is disposed to be parallel to the conductive member12so that the in-plane direction of the substrate13and the longitudinal direction D3are parallel to each other. In addition, the in-plane direction of the substrate13may not be completely parallel to the longitudinal direction D3and may be substantially parallel thereto in a range in which the effect of the present invention is exhibited. In addition, the substrate13is disposed so that a position at the center (point P1) in the thickness direction is aligned with a position at the center (point P2) in the height direction (the lateral direction D2) of the conductive member12(see F1). On one surface (hereinafter, referred to as an “upper surface”) of the substrate13, the first magnetic sensor14ais disposed, and under the other surface (hereinafter, referred to as a “lower surface”) of the substrate13, the second magnetic sensor14bis disposed. In addition, on the substrate13, a control unit20(seeFIG. 4) which operates and processes the output signals of the first and second magnetic sensors14aand14bis provided.

The first and second magnetic sensors14aand14boutput the output signals depending on an induction magnetic field M1(seeFIG. 2) from the current to be measured, which flows through the conductive member12. In addition, the first and second magnetic sensors14aand14bare disposed so that the sensing axis directions D1thereof are fixed in the same direction as the longitudinal direction D3of the conductive member12and the induction magnetic field M1(seeFIG. 2) from the current to be measured is applied at a predetermined angle θ with respect to the sensing axis direction D1. In addition, the sensing axis direction D1may not be completely the same direction as the longitudinal direction D3of the conductive member12and may be substantially the same direction in a range in which the effect of the present invention is exhibited.

Next, the induction magnetic field M1during measurement of the current to be measured of the current sensor1according to this embodiment will be described with reference toFIGS. 2 and 3.FIG. 2is an explanatory view of the induction magnetic field in the current sensor1according to this embodiment, andFIG. 3is a partial enlarged view of the first and second magnetic sensors14aand14billustrated inFIG. 2. In addition, in FIG.2, a cross-sectional schematic view of the current sensor1is illustrated, and for the convenience of description, the case11is omitted.

As illustrated inFIG. 2, when the current to be measured flows through the conductive member12, the induction magnetic field M1is generated in a predetermined range from the outer peripheral edge of the conductive member12around the conductive member12as the center. The induction magnetic field M1is in a rightward direction with respect to the flowing direction of the current to be measured. Therefore, in the vicinity of the center portion of the main surface12aon the upper surface side of the conductive member12, the direction of the induction magnetic field M1becomes a rightward direction (leftward direction), and in the vicinity of the center portion of the main surface12bon the lower surface side of the conductive member12, the direction of the induction magnetic field M1becomes a leftward direction (rightward direction).

In addition, in the vicinities of the end surfaces12cand12dof the conductive member12, the direction of the induction magnetic field M1changes to a curve shape from one end side toward the other end side of the end surfaces12cand12din the cross-sectional view. Therefore, the induction magnetic field M1is applied to the first and second magnetic sensors14aand14bdisposed in the vicinity of the end surface12cof the conductive member12in a direction tilted with respect to the sensing axis direction D1.

Next, the direction of the induction magnetic field M1applied to the first and second magnetic sensors14aand14bwill be described in detail with reference toFIG. 3. As illustrated inFIG. 3, to the first magnetic sensor14adisposed on the upper surface side of the conductive member12, the induction magnetic field M1is applied in a direction at a predetermined angle θ with respect to the sensing axis direction D1(rightward direction). In addition, to the second magnetic sensor14bdisposed on the lower surface side of the conductive member12, the induction magnetic field M1is applied in a direction at the predetermined angle θ with respect to the reverse direction (leftward direction) to the sensing axis direction D1.

Here, the induction magnetic field M1applied to a center point P3of the first magnetic sensor14ais decomposed into a magnetic vector M2in the sensing axis direction D1and a magnetic vector M3in a perpendicular direction to the surface of the substrate13. Therefore, the output signal output from the first magnetic sensor14ais output as an output signal depending on the magnitude of the magnetic vector M2.

In addition, the induction magnetic field M1applied to a center point P4of the second magnetic sensor14bin the application direction is decomposed into a magnetic vector M4in the reverse direction to the sensing axis direction D1and a magnetic vector M3in the perpendicular direction to the surface of the substrate13. Therefore, the output signal output from the second magnetic sensor14bis output as an output signal depending on the magnitude of the magnetic vector M4.

That is, in the current sensor1according to this embodiment, the induction magnetic field M1from the current to be measured is applied to the first and second magnetic sensors14aand14bby being decomposed into the magnetic vectors M2and M4in the reverse directions to each other. Therefore, even in the case where the current to be measured is a high current, magnetic saturation of the first and second magnetic sensors14aand14bmay be suppressed.

In addition, the induction magnetic field M1is applied to the first magnetic sensor14ain the direction at the predetermined angle θ from the sensing axis direction D1, and the induction magnetic field M1is applied to the second magnetic sensor14bin the direction at the predetermined angle θ from the reverse direction to the sensing axis direction D1. Therefore, the output signals having reversed phases to each other are output from the first and second magnetic sensors14aand14b. Moreover, since the induction magnetic field M1is applied at substantially the same angle θ with respect to the sensing axis directions D1of the first and second magnetic sensors14aand14b, the magnitudes of the magnetic vectors M2and M4of the sensing axis directions D1are substantially the same. Therefore, the output signals output from the first and second magnetic sensors14aand14bhave substantially the same magnitude. Moreover, since the sensing axis directions D1of the first and second magnetic sensors14aand14bare fixed in substantially the same direction, substantially the same output signals having the same phase are output under disturbance magnetism Hc. Therefore, by performing differential operation on the output signals of the first and second magnetic sensors14aand14b, the output signals from the induction magnetic field M1are processed to be added to each other, and thus noise components from the disturbance magnetism are removed as the same phase. Accordingly, it is possible to enhance the measurement accuracy of the current sensor1.

FIG. 4is a function block diagram illustrating the current sensor according to the embodiment of the present invention. The first and second magnetic sensors14aand14bare magnetic balance type sensors and are respectively constituted by feedback coils141aand141bdisposed to be able to generate a magnetic field in a direction in which the magnetic field generated by the current to be measured is canceled, and bridge circuits142aand142beach of which includes two magnetoresistance elements that are magnetic detection elements and two fixed resistance elements. The control unit20includes a differential and current amplifier211which amplifies differential outputs of the bridge circuit142aof the first magnetic sensor14aand controls a feedback current of the feedback coil141a, an I/V amplifier212which converts the feedback current of the first magnetic sensor14ato a voltage, a differential and current amplifier213which amplifies differential outputs of the bridge circuit142bof the second magnetic sensor14band controls a feedback current of the feedback coil141b, an I/V amplifier214which converts the feedback current of the second magnetic sensor14bto a voltage, and a differential amplifier222which amplifies the differential outputs of I/V amplifiers212and214.

The feedback coils141aand141bare respectively disposed near the magnetoresistance elements of the bridge circuits142aand142band generate cancellation magnetic fields that cancel the induction magnetic field generated by the current to be measured. GMR (Giant Magneto Resistance) elements, TMR (Tunnel Magneto Resistance) elements, and the like may be exemplified as the magnetoresistance elements of the bridge circuits142aand142b. The resistance of the magnetoresistance element varies due to application of the induction magnetic field from the current to be measured. Each of the bridge circuits142aand142bis constituted by using the two magnetoresistance elements and the two fixed resistance elements, thereby realizing a current sensor with high sensitivity. In addition, by using the magnetoresistance element, the sensing axis is easily disposed in a direction parallel to the substrate surface on which the current sensor is installed, enabling a planar coil to be used.

Each of the bridge circuits142aand142bhas two outputs that cause a voltage difference due to the induction magnetic field M1caused by the current to be measured. The two outputs of each of the bridge circuits142aand142bare respectively amplified by the differential and current amplifiers211and213, and the amplified outputs are provided as currents (feedback currents) to the feedback coils141aand141b. The feedback currents correspond to voltage differences due to the induction magnetic field M1. Here, cancellation magnetic fields that cancel the induction magnetic field M1are generated by the feedback coils141aand141b. In addition, currents that flow through the feedback coils141aand141bwhen a balanced state in which the induction magnetic field M1and the cancellation magnetic field cancel each other is achieved are converted into voltages by the I/V amplifiers212and214, and the voltages become the sensor outputs.

In the differential and current amplifier211, by setting a power supply voltage to a value close to a value of the reference voltage for I/V conversion+(the maximum value of the rating of the feedback coil resistance×the feedback coil current at full scale), the feedback current is automatically restricted, achieving an effect of protection of the magnetoresistance elements and the feedback coil. Here, the differential between the two outputs of each of the bridge circuits142aand142bis amplified to be used as the feedback current. Alternatively, only midpoint potentials may be output from the bridge circuits to be used as feedback currents based on potential differences from a predetermined reference potential.

The differential amplifier222processes the differential value between the output signals of the I/V amplifiers212and214as a sensor output. By performing this processing, an influence of an external magnetic field such as geomagnetism on the output signals of the first and second magnetic sensors14aand14bis canceled, thereby measuring currents with higher accuracy. In this example, the magnetic balance type is exemplified. Alternatively, a magnetic proportion type in which a differential is acquired from the outputs of the first and second magnetic sensors as they are may also be employed.

Next, an example performed to clarify the effect of the present invention will be described.

Here, description will be provided by comparing a measurement range of the current to be measured of the current sensor1according to this embodiment to a measurement range of a current to be measured of a current sensor according to the related art as a comparative example.FIG. 5Ais a diagram illustrating the relationship between the magnetic field strength and the output signal of the current sensor1according to this embodiment, andFIG. 5Bis a diagram illustrating the relationship between the magnetic field strength and the output signal of the current sensor according to the comparative example. In addition, inFIGS. 5A and 5B, output signals in a case where induction magnetic fields having the same strength are applied to the current sensor1according to this embodiment and the current sensor according to the comparative example are illustrated.

As illustrated inFIGS. 5A and 5B, in the current sensor1according to this embodiment, the output signal linearly changes with respect to the magnetic field strength applied by the induction magnetic field from the current to be measured. On the other hand, in the current sensor according to the comparative example, magnetic saturation in which an increase in the output signal is reduced as the magnetic strength is increased occurs. As such, according to the current sensor1according to this embodiment, it is found that magnetic saturation may be suppressed even in the case where the current to be measured is a high current.

As described above, in the current sensor1according to this embodiment, the sensing axis directions D1of the first and second magnetic sensors14aand14bare disposed to form the predetermined angle θ with respect to the application direction of the induction magnetic field M1from the current to be measured applied to the first and second magnetic sensors14aand14b. Accordingly, the output signal depending on the magnetic vector M2in the sensing axis direction D1is output from the first magnetic sensor14a, and the output signal depending on the magnetic vector M4in the reverse direction to the sensing axis direction D1is output from the second magnetic sensor14b. As a result, even in the case where the current to be measured is a high current, it is possible to suppress magnetic saturation of the first and second magnetic sensors14aand14b. Therefore, it is possible to enlarge the measurement range (dynamic range) of the current sensor, and it is also possible to dispose the first and second magnetic sensors14aand14bto approach the conductive member, thereby realizing the reductions in the size and the thickness of the current sensor1.

In addition, the sensing axis directions D1of the first and second magnetic sensors14aand14bare fixed in the same direction, and the magnetic vectors M2and M4of the induction magnetic field M1are applied from the reverse directions to each other to the first and second magnetic sensors14aand14b. Therefore, the output signals having reversed phases to each other are output from the first and second magnetic sensors14aand14b. Therefore, by performing differential operation on the output signals of the first and second magnetic sensors14aand14b, the output signals of the first and second magnetic sensors14aand14bare added, thereby increasing detection sensitivity. Moreover, since disturbance magnetism is applied in the same direction to the first and second magnetic sensors14aand14b, it is possible to cancel noise due to the disturbance magnetism by performing differential operation on the output signals of the first and second magnetic sensors14aand14b.

Moreover, in the current sensor1according to this embodiment, the substrate13is disposed in the vicinity of the end surface12cof the conductive member12, and the pair of first and second magnetic sensors14aand14bare disposed with the substrate13interposed therebetween, thereby reducing the distance between the first and second magnetic sensors14aand14b. As such, by disposing the first and second magnetic sensors14aand14bto approach each other, the difference between the disturbance magnetism applied to the first magnetic sensor14aand the disturbance magnetism Hc applied to the second magnetic sensor14bmay be reduced, thereby effectively canceling an influence of the disturbance magnetism Hc.

Particularly, in the current sensor1according to this embodiment, by disposing the position at the center (the point P1) in the thickness direction of the substrate13and the position at the center (the point P2) in the lateral direction D2of the conductive member12in the same surface F1, the magnitude of the magnetic vector M2applied to the first magnetic sensor14aand the magnitude of the magnetic vector M4applied to the second magnetic sensor14bare maximized. As a result, the detection sensitivity of the current to be measured may be enhanced.

The present invention is not limited to the above-described embodiment, and various modifications can be made. For example, the sizes of the elements in the above-described embodiment may be appropriately changed. In addition, in the above-described embodiment, the case where the magnetoresistance element is used in the magnetic balance type current sensor is described. Alternatively, the magnetic balance type current sensor may also be configured using a Hall element or other magnetic detection elements. Besides, the present invention may be appropriately modified without departing from the scope of the present invention.

For example, in the current sensor1according to the above-described embodiment, the configuration in which the conductive member12having a rectangular shape in the cross-sectional view is used is described. However, the shape of the conductive member12is not limited to this configuration and may be appropriately modified. The shape of the conductive member12may be appropriately modified to, for example, an elliptical shape or a flat shape in the cross-sectional view as long as the shape is in a range in which the effect of the present invention is obtained.

In addition, the first and second magnetic sensors14aand14bare not limited to this configuration, and may have any configuration as long as the induction magnetic field from the current to be measured, which flows through the conductive member12, is applied at the predetermined angle θ with respect to the sensing axis directions D1of the first and second magnetic sensors14aand14band thus the output signals having reversed phases to each other are output. The reversed phases mentioned here include a range in which a phase shift to a degree that sufficient output signals are obtained after differential operation occurs.

Moreover, in the current sensor1according to the above-described embodiment, the angles θ of the induction magnetic field M1applied to the first and second magnetic sensors14aand14bmay be different angles as long as the angles are in a range in which magnetic saturation of the first and second magnetic sensors14aand14bis suppressed.

In addition, the substrate13is not limited to this configuration and may be any configuration as long as the configuration is in a range in which the reductions in the size and the thickness of the current sensor1are achieved. For example, the substrate13may be disposed so that the position at the center in the thickness direction is the same position as the position at the center in the height direction (the lateral direction D2) of the conductive member12. In addition, the position at the center in the thickness direction may not be completely the same position as the position at the center in the height direction (the lateral direction D2) of the conductive member12and may be substantially the same in a range in which the effect of the present invention is exhibited. Moreover, the substrate13may be disposed to be tilted at a predetermined angle with respect to the longitudinal direction of the conductive member12.

Moreover, in the current sensor1according to the above-described embodiment, a first control unit which operates and processes the output signal of the first magnetic sensor14aand a second control unit which operates and processes the output signal of the second magnetic sensor14bmay be separately provided. In this case, it is preferable that the first and second magnetic sensors14aand14band the first and second control units be provided on the upper surface and the lower surface of the substrate13to be vertically symmetrical. In this configuration, disturbance magnetism is uniformly applied to the first and second magnetic sensors14aand14band the first and second control units, and thus noise due to the disturbance magnetism may be effectively reduced.

In addition, in the current sensor1according to the above-described embodiment, it is preferable that the control unit20be provided at a position separated from the conductive member12in the surface of the substrate13in terms of reducing noise from the conductive member12.

Moreover, in the current sensor1according to the above-described embodiment, a shielding member (an electromagnetic shield) may be provided between the conductive member12and the substrate13. By providing the shielding member as such, noise caused by a voltage of the conductive member12may be absorbed, and thus the measurement accuracy of the current to be measured may be enhanced.

Furthermore, the above-described embodiment has the configuration in which the magnetic balance type sensors are used as the first and second magnetic sensors but is not limited to this configuration. The magnetic sensors may output the output signals having reversed phases to each other due to the induction magnetic field from the current to be measured which passes through the current wire, and may use, for example, magnetic proportion type sensors. By using the magnetic proportion type sensors, it is possible to reduce power consumption compared to the configuration in which the magnetic balance type sensor is used.

The present invention has effects of reducing noise from disturbance magnetism, measuring the current to be measured which is a high current, and realizing reductions in size and thickness. Particularly, it is possible to appropriately use the present invention for a current sensor which detects the magnitude of a current for driving the motor of an electric vehicle or a hybrid vehicle.