CURRENT SENSOR

Abstract

Each of three magnetic sensors is positioned on a first virtual line that passes the centers of two magnetic shields in the width direction and is perpendicular to a virtual plane, so a value detected by the magnetic sensor is less likely to be influenced by an external magnetic field. The first virtual line and the center of a conductor intersecting the first virtual line are separated from each other by a certain distance. Therefore, even if a distance from the center of the conductor in the width direction to the magnetic sensor is short, it is possible to improve, on a high-frequency side, frequency characteristics in the detection sensitivity of the magnetic sensor for the frequency of the current under measurement.

Claims

1. A current sensor comprising: a plurality of plate-like conductors, each of which has a plate surface positioned in the same virtual plane, the plate-like conductors being aligned and extending in parallel to one another; and a plurality of current detectors, each of which detects a current flowing in one of the plurality of conductors; wherein the current detector has two plate-like magnetic shields, each of which has a plate surface parallel to the virtual plane, the two magnetic shields facing each other with the conductor intervening between the two magnetic shields, and a magnetic sensor positioned between the two magnetic shields, the two magnetic shields have the same length in a width direction parallel to the virtual plane and perpendicular to a length direction of the conductor, the magnetic sensor is positioned on a first virtual line that passes centers of the two magnetic shields in the width direction and is perpendicular to the virtual plane, the conductor has a first conductor part that is positioned between the two magnetic shields and intersects the first virtual line, and the first virtual line and a center of the first conductor part in the width direction are separated from each other.

2. The current sensor according to claim 1, wherein when a length of the first conductor part in the width direction is W, a length L of the magnetic shield in the width direction is at least 2×W and at most 5×W, and a distance D in the width direction by which the first virtual line and the center of the first conductor part in the width direction are separated from each other is larger than (W/2)×(16/32) and smaller than (W/2)×(20/32).

3. The current sensor according to claim 1, wherein when a length of the first conductor part in the width direction is W, a length L of the magnetic shield in the width direction is at least 2×W and at most 5×W, and a distance D in the width direction by which the first virtual line and the center of the first conductor part in the width direction are separated from each other is larger than (W/2)×(17/32) and smaller than (W/2)×(19/32).

4. The current sensor according to claim 1, wherein: the plurality of magnetic sensors, each of which is included in one of the plurality of current detectors, are positioned on a second virtual line parallel to the virtual plane and also parallel to the width direction; and the current sensor has a circuit board on which the plurality of magnetic sensors are mounted in common.

5. The current sensor according to claim 4, wherein: the conductor has second conductor parts, each of which is formed so as to extend, in parallel to the length direction, from one of both ends of the first conductor part in the length direction; and the length of the second conductor part in the width direction is longer than the length of the first conductor part in the width direction.

6. The current sensor according to claim 5, wherein: the plurality of conductors are three or more equally spaced conductors; and a third virtual line and the first virtual line intersect, the third virtual line passing a center, in the width direction, of each second conductor part included in one conductor disposed between two conductors, the third virtual line being parallel to the virtual plane, the first virtual line being in the current detector provided in correspondence to the one conductor.

7. The current sensor according to claim 6, wherein one edge of the first conductor part in the width direction and one edge of the second conductor part in the width direction are continuous to each other.

8. The current sensor according to claim 5, wherein a whole of the first conductor part is positioned between the two magnetic shields.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] FIG. 1 illustrates an example of the outside shape of a current sensor according to an embodiment of the present invention;

[0028] FIG. 2 illustrates an example of the internal structure of the current sensor illustrated in FIG. 1 with the cases omitted;

[0029] FIG. 3 illustrates the internal structure of the current sensor illustrated in FIG. 1 with the circuit board and terminal omitted;

[0030] FIG. 4 is a cross-sectional view of the internal structure of the current sensor illustrated in FIG. 3, as taken along line IV-IV;

[0031] FIG. 5 illustrates the internal structure of the current sensor illustrated in FIG. 3, as viewed in the direction indicated by arrow V; and

[0032] FIG. 6 illustrates magnetic flux density around a conductor in which a current flows.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0033] A current sensor according to an embodiment of the present invention will be described with reference to the drawings.

[0034] FIG. 1 illustrates an example of the outside shape of the current sensor according to this embodiment. FIG. 2 illustrates an example of the internal structure of the current sensor illustrated in FIG. 1 with the cases 2 and 3 omitted. FIG. 3 illustrates the internal structure of the current sensor illustrated in FIG. 1 with the circuit board 5 and terminal 4 omitted. FIG. 4 is a cross-sectional view of the internal structure of the current sensor illustrated in FIG. 3, as taken along line IV-IV. FIG. 5 illustrates the internal structure of the current sensor illustrated in FIG. 3, as viewed in the direction indicated by arrow V; the magnetic shields (MH1 to MH3) that would otherwise appear in front as viewed in that direction are omitted.

[0035] The current sensor in the examples in the drawings referenced above is a three-channel current sensor that can independently measure currents flowing in three current paths. The current sensor has three conductors T1 to T3, each of which forms a current path, and three current detectors U1 to U3, which detect currents flowing in the conductors T1 to T3, respectively.

[0036] The conductors T1 to T3 are each an elongated plate-like conductor. They have plate surfaces positioned in the same virtual plane PL1. That is, the conductors T1 to T3 are placed so that their plate surfaces are flush with the virtual plane PL1. The conductors T1 to T3 extend in parallel to one another and are aligned in the virtual plane PL1. In the examples in the drawings referenced above, the conductors T1 to T3 are equally spaced.

[0037] In the drawings referenced above, the conductors T1 to T3 have equivalent shapes. Specifically, the conductors T1 to T3 have the same length in a direction in which they extend (sometimes, the direction will be simply referred to below as the length direction), the same length in a direction perpendicular to the length direction and parallel to the virtual plane PL1 (sometimes, the direction will be simply referred to below as the width direction), and the same thickness perpendicular to the virtual plane PL1. The conductors T1 to T3 each have holes at both ends, one at each end; each hole is used to attach, for example, a terminal for connection.

[0038] A current detector Ui (i is an integer from 1 to 3) is a block that detects a current flowing in a conductor Ti. The current detector Ui has two magnetic shields MHi and MLi and a magnetic sensor Pi. Specifically, a current detector U1 has two magnetic shields MH1 and ML1 and a magnetic sensor P1, a current detector U2 has two magnetic shields MH2 and ML2 and a magnetic sensor P2, and a current detector U3 has two magnetic shields MH3 and ML3 and a magnetic sensor P3.

[0039] The two magnetic shields MHi and MLi included in the current detector Ui are each a plate-like member having a plate surface parallel to the virtual plane PL1. They are formed from a ferromagnetic substance. The magnetic shields MHi and MLi are positioned so as to face each other with the conductor Ti intervening therebetween. Their lengths in the width direction are equal. In the drawings referenced above, the magnetic shields MHi and MLi have the same rectangular shape having an edge extending in the width direction and another edge extending in the length direction; their edges in the width direction have the same length and their edge in the length direction also have the same length. The magnetic shields MHi and MLi are positioned so as to be symmetric with respect to the virtual plane PL1.

[0040] The magnetic sensor Pi is a sensor that detects a magnetic field corresponding to a current flowing in the conductor Ti. The magnetic sensor Pi is formed from, for example, a magnetoresistance effect element (such as a giant magneto resistive (GMR) device). The magnetic sensor Pi is positioned between the two magnetic shields MHi and MLi. That is, the magnetic sensor Pi is positioned on a first virtual line LAi that passes the centers of the magnetic shields MHi and MLi in the width direction and is perpendicular to the virtual plane PL1. In the drawings referenced above, this first virtual line LAi also passes the centers of the magnetic shields MHi and MLi in the length direction.

[0041] The three magnetic sensors P1 to P3 may be positioned on a second virtual line LB parallel to the virtual plane PL1 and also parallel to the width direction (see FIGS. 3 and 5). The current sensor according to this embodiment may have a circuit board 5 on which the three magnetic sensors P1 to P3 are mounted in common. The circuit board 5 has an elongated rectangular shape extending along the second virtual line LB. In the drawings referenced above, a terminal 4 used to connect a cable connector (not illustrated) or the like is attached to the circuit board 5.

[0042] As illustrated in FIG. 5, the conductors T1 to T3 each have a first conductor part B1 with a narrow width and second conductor parts B2 with a wide width, each of which is formed at one of both ends of the first conductor part B1. Each second conductor part B2 may be formed so as to extend, in parallel to the length direction, from one of both ends of the first conductor part B1 in the length direction. The length V of the second conductor part B2 in the width direction may be longer than the length W of the first conductor part B1 in the width direction (see FIG. 4).

[0043] The first conductor part B1 is positioned between the two magnetic shields MHi and MLi, and intersects the first virtual line LAi (see FIG. 4). In the example in FIG. 5, the entire first conductor part B1 is positioned between the two magnetic shields MHi and MLi. The magnetic sensor Pi positioned on the first virtual line LAi detects a magnetic field generated by a current flowing in the first conductor part B1.

[0044] As illustrated in FIG. 4, the center of the first conductor part B1 in the width direction may be spaced by a distance D from the first virtual line LAi, which intersects the first conductor part B1. To suppress the detection sensitivity of the magnetic sensor Pi from being changed depending on the frequency, this spaced distance D may be set so that it is larger than (W/2)×(16/32) and is smaller than (W/2)×(20/32). Preferably, the spaced distance D may be set so that it is larger than (W/2)×(17/32) and is smaller than (W/2)×(19/32). When the spaced distance D is set within this range, to assure the effect of suppressing an external magnetic field (mainly, a magnetic field generated by a current flowing in an adjacent conductor), the effect being provided by the magnetic shield effect of the two magnetic shields MHi and MLi, the length L of the magnetic shields MHi and MLi in the width direction may be set to, for example, at least 2×W and at most 5×W.

[0045] As illustrated in FIG. 5, the center of each second conductor part B2 of the conductor Ti in the width direction is at the same position in the width direction (horizontal direction in FIG. 5) as the centers of the magnetic shields MHi and MLi in the width direction. That is, a third virtual line LCi and the first virtual line LAi intersect, the third virtual line LCi passing the center, in the width direction, of each second conductor part B2 of the conductor Ti, the third virtual line LCi being parallel to the virtual plane, the first virtual line LAi passing the centers of the magnetic shields MHi and MLi of the current detector Ui provided in correspondence to the conductor Ti (see FIG. 3). Since the center of each second conductor part B2 in the width direction matches the centers of the magnetic shields MH2 and ML2 in the width direction as described above, the conductor T2 positioned between the two conductors T1 and T3 provides a match between a spaced distance by which the magnetic sensor P1 is spaced from the second conductor part B2 and a spaced distance by which the magnetic sensor P3 is spaced from the second conductor part B2.

[0046] Furthermore, as illustrated in FIG. 5, one edge of the first conductor part B1 and one edge of each second conductor part B2 may be continuous to each other in the width direction. Specifically, one edge 11 of the first conductor part B1 and one edge 12 of each second conductor part B2 may be continuous to each other without a step. However, there is a step with a length of (V−W) between another edge 21 of the first conductor part B1 and another edge 22 of each second conductor part B2. If there is a margin in a space in which the current sensor is disposed, when the step is inclined, electromagnetic compatibility (EMC) can be enhanced.

[0047] The current sensor according to this embodiment has the cases 2 and 3 in which the current detectors U1 to U3 and circuit board 5 are accommodated, as illustrated in, for example, FIG. 1.

[0048] The case 3 is formed by being molded from a resin or the like so as to internally include the conductors T1 to T3 and magnetic shields ML1 to ML3. In the example in FIG. 1, the case 3 has an outside shape like a rectangular parallelepiped. The case 3 has an internal space in which the circuit board 5 is accommodated. The case 2 is formed by being molded from a resin or the like so as to internally include the magnetic shields MH1 to MH3. The case 2 is used as a lid that covers the space, described above, in the case 3.

[0049] Next, the frequency characteristics of the current sensor, according to this embodiment, structured as described above will be described with reference to FIG. 6.

[0050] FIG. 6 illustrates a magnetic flux density around a conductor in which a current flows in the structure in the present invention. The horizontal axis indicates the position of the first conductor part B1 in the width direction. Values on the horizontal axis have been normalized to match descriptions in Claims. The vertical axis indicates magnetic flux density. The results in FIG. 6 were obtained by a simulation performed under conditions described below. The conductor T2 at the center was measured. The length W of the first conductor part B1 in the width direction was 10.5 mm. The widths of the two magnetic shields were 25 mm. The centers of the two magnetic shields were shifted about 3 mm to the right from the center of the first conductor part B1. The value 0 on the horizontal axis indicates the center of the first conductor part B1. The value −32 on the horizontal axis indicates an edge of the first conductor part B1, the edge being on a side away from the centers of the two magnetic shields than the other edge. The value 32 on the horizontal axis indicates an edge of the first conductor part B1, the edge being on a side closer to the centers of the two magnetic shields than the other edge. In this embodiment, a value obtained by multiplying a value on the horizontal axis by 0.164 indicates a distance (spaced distance D) from the center of the first conductor part B1 in millimeters (mm). The positions of the centers of the two magnetic shields correspond to numeral 18 on the horizontal axis in FIG. 6.

[0051] FIG. 6 indicates that at a position at which the spaced distance D is larger than (W/2)×(16/32) and is smaller than (W/2)×(20/32), even if the frequency of a current changes, changes in magnetic flux density are small.

[0052] FIG. 6 further indicates that at a position at which the spaced distance D is larger than (W/2)×(17/32) and is smaller than (W/2)×(19/32), even if the frequency of a current changes, there are only extremely small changes in magnetic flux density.

[0053] Therefore, if the current sensor is designed so that the magnetic sensor P2 is placed at a position at which the spaced distance D is (W/2)×(18/32), it is possible to suppress changes in magnetic flux density even if the frequency of a current changes.

[0054] In the related art, described in Japanese Patent No. 4515855, in which two magnetic shields are not provided, there were only small changes in magnetic flux density in a range from a position at which the spaced distance D is (W/2)×(20/32) to a position at which the spaced distance D is (W/2)×(28/32). A comparison between the present invention and the related art indicates that the spaced distance D at a position at which changes in magnetic flux density in the present invention are small is shorter than in the related art.

[0055] When the spaced distance D is long, the distance between the adjacent conductor T3 and the magnetic sensor P2 becomes short, so the magnetic sensor P2 becomes likely to be influenced by an induced magnetic field generated by a current flowing in the adjacent conductor T3. That is, measurement precision is lowered. The spaced distance D in the present invention is shorter than in the related art, so a distance from the adjacent conductor T3 is long. This can suppress measurement precision from being lowered.

[0056] At a position at which the spaced distance D is larger than (W/2)×(16/32) and smaller than (W/2)×(20/32), magnetic flux density at a high frequency (2 kHz) is maximized. Thus, it is possible to suppress measurement precision at a high frequency from being lowered.

[0057] If it is enough only suppress changes in magnetic flux density with changes in the frequency of the current, FIG. 6 indicates that even a position at which the spaced distance D is about (W/2)×(−22/32) is sufficient. The negative sign of the spaced distance D indicates the leftward direction, that is, a direction away from the center of the two magnetic shields.

[0058] At a position at which the spaced distance D is about (W/2)×(−22/32), however, the absolute value of the spaced distance D is larger than the position described in the claims of the present invention, so measurement precision is easily influenced by an induced magnetic field generated by the adjacent conductor T1, lowering measurement precision. Furthermore, the magnetic flux density of the target under measurement is low. Therefore, in measurement at a position at which the spaced distance D is about (W/2)×(−22/32), precision is lowered.

[0059] From the results of many different simulations, it was found that at the position at which the spaced distance D is (W/2)×(18/32), the change in magnetic flux density is smallest. It was also found that when the magnetic sensor P2 is placed so as to match the centers of the two magnetic shields in the width direction, the influence of the external magnetic field is smallest. Thus, it was found that the best mode is to place the two magnetic shields so that their centers in the width direction match the position at which the spaced distance D is (W/2)×(18/32). In this description, therefore, results obtained by placing the two magnetic shields so that their centers in the width direction match the position at which the spaced distance D is (W/2)×(18/32) were described as simulation results in the best mode.

[0060] The current sensor according to this embodiment can provide the following effects.

[0061] The magnetic sensor Pi is positioned between the two magnetic shields MHi and MLi. Therefore, due to the magnetic shield effect of the two magnetic shields MHi and MLi, a value detected by the magnetic sensor Pi disposed for the conductor Ti is less likely to be influenced by an external magnetic field generated by a current flowing in another conductor adjacent to the conductor Ti.

[0062] The magnetic sensor Pi is positioned on the first virtual line LAi that passes the centers of the two magnetic shields MHi and MLi in the width direction and is perpendicular to the virtual plane PL1. Therefore, it is possible to further reduce the influence of the external magnetic field when compared with a case in which the magnetic sensor Pi is positioned along edges of the two magnetic shields MHi and MLi in the width direction, so error in current measurement can be effectively suppressed.

[0063] There is a space between the first virtual line LAi on which the magnetic sensor Pi is positioned and the center of the first conductor part B1 in the width direction, the first conductor part B1 intersecting the first virtual line LAi. Therefore, it is possible to improve, on a high-frequency side, frequency characteristics in the detection sensitivity of the magnetic sensor Pi for the frequency of the current under measurement when compared with a case in which the center of the first conductor part B1 in the width direction is placed close to the first virtual line LAi. Therefore, it is possible to widen the frequency band in which a current can be measured.

[0064] There is an offset between the first virtual line LAi passing the centers of the magnetic shields in the width direction and the center of the first conductor part B1 in the width direction. Therefore, even if an amount by which the magnetic sensor Pi is deviated from the center of the first conductor part B1 in the width direction is reduced, the frequency characteristics can be improved. By comparison, in the related art described in Japanese Patent No. 4515855, the spaced distance D has been needed to be set to at least (W/2)×(10/16). In the present invention, however, even if the setting of the spaced distance D is smaller than (W/2)×(10/16), the frequency characteristics can be improved. The shorter the spaced distance D is, the longer the distance from the adjacent conductor is. Therefore, even if downsizing is performed, the conductor Ti is less likely to be influenced by an external magnetic field generated by a current flowing in another conductor adjacent to the conductor Ti.

[0065] When the length of the first conductor part B1 in the width direction is W, the lengths L of the magnetic shields MHi and MLi in the width direction are set to at least 2×W and at most 5×W, and the spaced distance D in the width direction between the first virtual line LAi and the center of the first conductor part B1 in the width direction is set within a range larger than (W/2)×(16/32) and smaller than (W/2)×(20/32). Therefore, it is possible to obtain an effect of improving the frequency characteristics due to a space between the first virtual line LAi and the center of the first conductor part B1 in the width direction, without the effect of suppressing an external magnetic field being hindered, the effect being provided by the magnetic shield effect of the two magnetic shields MHi and MLi.

[0066] The magnetic sensors P1 to P3 are positioned on the second virtual line LB parallel to the virtual plane PL1 and also parallel to the width direction, and are mounted on the circuit board 5 in common. Therefore, it is possible to reduce the number of parts, including the circuit board, used to mount the magnetic sensors P1 to P3, enabling the structure of a device to be simplified.

[0067] The second conductor parts B2, which are longer than the first conductor part B1 in the width direction, are formed at both ends of the first conductor part B1, one at each end. Therefore, a resistance loss in the conductor Ti can be made smaller than when the length in the width direction across the conductor Ti is the same as the narrow width of the first conductor part B1.

[0068] The conductors T1 to T3 are equally spaced. A third virtual line LC2 and a first virtual line LA2 intersect, the third virtual line LC2 passing the center, in the width direction, of each second conductor part B2 included in the conductor T2 disposed between the two conductors T1 and T3, the third virtual line LC2 being parallel to the virtual plane PL1, the first virtual line LA2 being in the current detector U2 provided in correspondence to the conductor T2.

[0069] Since the conductors T1 to T3 are equally spaced, the first virtual lines LA1 to LA3 in the current detectors U1 to U3 provided in correspondence to the conductors T1 to T3 are also equally spaced. Since the third virtual line LC2 in the second conductor part B2 included in the conductor T2 and the first virtual line LA2 in the current detector U2 provided in correspondence to the conductor T2 intersect, the two first virtual lines LA1 and LA3, which respectively correspond to the conductors T1 and T3, are symmetrically positioned with respect to the third virtual line LC2 of the conductor T2. Therefore, the magnetic sensors P1 and P3, which are respectively included in the two current detectors U1 and U3 provided in correspondence to the conductors T1 and T3, are separated from the second conductor part B2 included in the conductor T2 by the same distance. Accordingly, an external magnetic field generated by a current flowing in the second conductor part B2 included in the conductor T2 influences values detected by the magnetic sensors P1 and P3 to the same extent. That is, the external magnetic field does not largely influence only one of the magnetic sensors P1 and P3. Therefore, measurement error caused by the magnetic sensors P1 and P3 due to the influence of the external magnetic field can be reduced with good valance.

[0070] (9) One edge 11 of the first conductor part B1 in the width direction and one edge 12 of the second conductor part B2 in the width direction are continuous to each other without a step. Therefore, if the adjacent distance between the conductors T1 and T2 and the adjacent distance between the conductors T2 and T3 are shortened to a limit up to which the influence of the external magnetic field on values detected by the magnetic sensors P1 to P3 can be ignored, the lengths of the first conductor part B1 and second conductor part B2 in the width direction are not unnecessarily shortened by an amount equal to the step. That is, the lengths of the first conductor part B1 and second conductor part B2 are prolonged in the width direction. This enables a device to be made compact by shortening the adjacent distance between the conductors T1 and T2 and the adjacent distance between the conductors T2 and T3, and also enables resistance losses in the conductors T1 to T3 to be reduced.

[0071] (10) The entire first conductor part B1 is positioned between the two magnetic shields MHi and MLi. Therefore, even if the length of the first conductor part B1 in the length direction of the conductor Ti is relatively shortened, an even magnetic field can be easily generated in a magnetic sensor Pi by a current flowing in the first conductor part B1. This enables positional precision required in the placement of the magnetic sensor Pi to be relaxed. As the length of the first conductor part B1 with a narrow width is shortened, the ratio of the second conductor part B2 with a wide width to the conductor Ti is increased. This can further reduce the resistance loss in the conductor Ti. That is, resistance loss can be reduced by shortening the length of the first conductor part B1 in the length direction of the conductor Ti, and positional precision required in the placement of the magnetic sensor Pi can be relaxed.

[0072] The present invention is not limited to the embodiment described above. The present invention includes various variations.

[0073] Although, for example, a three-channel current sensor has been described as an example in the above embodiment, any number of channels measuring a current can be used. Furthermore, three-phase alternating currents or other mutually related currents may flow in the conductors, or mutually independent currents may flow in them.