High precision current sensor

Abstract

A current sensor for measuring an electric current flowing through a current bus bar includes: a magnetic core with an opening, through which the current bus bar passes with a predetermined interval; and a first magnetic sensor for detecting magnetic flux density in the magnetic core, the magnetic flux density generated by the electric current passing through the current bas bar. The magnetic core has a first gap and a second gap. The first magnetic sensor is disposed in the first gap. The second gap is capable of preventing magnetic saturation of the magnetic core.

Claims

1 . A current sensor for measuring an electric current flowing through a current bus bar comprising: a magnetic core with an opening, through which the current bus bar passes with a predetermined interval; and a first magnetic sensor for detecting magnetic flux density in the magnetic core, the magnetic flux density generated by the electric current passing through the current bas bar, wherein the magnetic core has a first gap and a second gap, the first magnetic sensor is disposed in the first gap, and the second gap is capable of preventing magnetic saturation of the magnetic core. 2 . The current sensor according to claim 1 , wherein the second gap is disposed on an opposite side of the first gap. 3 . The current sensor according to claim 1 , wherein the magnetic core further includes a plurality of gaps except the first and the second gaps, and the plurality of gaps are capable of preventing magnetic saturation of the magnetic core. 4 . The current sensor according to claim 3 , further comprising: a bonding member disposed in at least one of the gaps except the first gap. 5 . The current sensor according to claim 4 , wherein the bonding member provides a mounting portion for mounting the magnetic core to an external object. 6 . The current sensor according to claim 4 , wherein the first magnetic sensor has dimensions equal to dimensions of the first gap, and the magnetic core, the first magnetic sensor and the bonding member provides one-piece core. 7 . The current sensor according to claim 1 , wherein the magnetic core has rectangular shape with four sharp corners, and the magnetic core is disposed in a plane perpendicular to the current bus bar. 8 . The current sensor according to claim 1 , further comprising: a second magnetic sensor for detecting magnetic flux density in the magnetic core, the magnetic flux density generated by the electric current passing through the current bus; and a current detection circuit coupled to the first and the second magnetic sensors, wherein the second magnetic sensor is disposed in the second gap, and the current detection circuit compensates for an effect of residual flux density in the magnetic core on the basis of outputs of the first and the second magnetic sensors. 9 . The current sensor according to claim 8 , wherein the second gap is wider than the first gap. 10 . The current sensor according to claim 9 , wherein the second gap is disposed on an opposite side of the first gap. 11 . The current sensor according to claim 9 , wherein the magnetic core further includes a plurality of gaps except the first and the second gaps, and the plurality of gaps are capable of preventing magnetic saturation of the magnetic core. 12 . A current sensor for measuring an electric current flowing through a current bus bar comprising: a magnetic core with an opening, through which the current bus bar passes with a predetermined interval; and a first magnetic sensor for detecting magnetic flux density in the magnetic core, the magnetic flux density generated by the electric current passing through the current bus bar, wherein the magnetic core has a first gap and a slit, the first magnetic sensor is disposed in the first gap, and the slit is capable of preventing magnetic saturation of the magnetic core. 13 . The current sensor according to claim 12 , wherein the slit is disposed on an opposite side of the first gap. 14 . A current sensor for measuring an electric current flowing through a current bus bar comprising: a magnetic core with an opening, through which the current bus bar passes with a predetermined interval; a first magnetic sensor for detecting magnetic flux density in the magnetic core, the magnetic flux density generated by the electric current passing through the current bus bar; a second magnetic sensor for detecting magnetic flux density in the magnetic core, the magnetic flux density generated by the electric current passing through the current bus bar; and a current detection circuit coupled to the first and the second magnetic sensors, wherein the magnetic core has one gap, the first magnetic sensor and the second magnetic sensor are disposed in the gap, and the current detection circuit compensates for an effect of residual flux density in the magnetic core on the basis of outputs of the first and the second magnetic sensors. 15 . The current sensor according to claim 14 , wherein the gap includes two different width portions, the first magnetic sensor is disposed in one width portion of the gap, and the second magnetic sensor is disposed in the other width portion of the gap. 16 . The current sensor according to claim 14 , wherein the first magnetic sensor and the second magnetic sensor are different in magnetic sensing capability.
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is based on Japanese Patent Applications No. 2004-255253 filed on Sep. 2, 2004, and No. 2004-255254 filed on Sep. 2, 2004, the disclosures of which are incorporated herein by references. FIELD OF THE INVENTION [0002] The present invention relates to a current sensor, and more particularly to high precision current sensor with a magnetic sensor. BACKGROUND OF THE INVENTION [0003] Electrical components mounted on automotive vehicles such as car navigation systems have increased in recent years. A current drain on car batteries has become too large accordingly, and a peak current has reached to several hundred amperes. A variety of technologies improving fuel economy have therefore applied to cars. An engine control system, which is one of the technologies, stops to operate a battery-charging generator under acceleration and operates the generator under deceleration. The engine control system requires precision detection capability of a battery current in order to control the battery charging properly. [0004] FIG. 15 shows a conventional current sensor 10 for measuring a large current such as a battery current. The sensor 10 has a C-shaped magnetic core 20 , a current bus bar 12 and a magnetic sensor 14 . The core 20 has a center opening where the bar 12 passes through. A gap Ga 1 is formed between both end surfaces of the core 20 . The magnetic sensor 14 is disposed in the gap Ga 1 . The magnetic sensor 14 detects magnetic flux density in the gap Ga 1 generated by a current flowing through the bar 12 . Then, the magnetic sensor produces signals corresponding to the magnetic flux density. The sensor 10 receives the signals, and thereby can measures a current. [0005] Current sensors such as the sensor 10 are disclosed, for example, in Japanese Patent Application Publication No. H14-286764, Japanese Patent Application Publication No. H14-303642, Japanese Patent Application Publication No. H15-167009, and Japanese Patent Application Publication No. 2002-350470. [0006] A current sensor disclosed in Japanese Patent Application Publication No. H14-286764 uses magnetoimpedance devices (i.e., MI device) as magnetic sensing elements. In the sensor, sensitivity of a weak direct current is improved by applying alternating current to the MI devices. [0007] A current sensor disclosed in Japanese Patent Application Publication No. H15-350470 uses two Hall effect ICs as magnetic sensing elements. One IC is for a large current, and the other IC is for a small current. The sensor automatically switches on and off between the two ICs according to an amount of current. [0008] However, the sensor 10 shown in FIG. 15 measures a certain amount of current, even when no current flows. The measurement error of current is caused by magnetic hysteresis effect in the core 20 , which is ferromagnetic. Specifically, when a large current flows through the bar 12 , the core 20 is magnetized. Then, after the current stops and become zero, magnetic force generated by the current is removed. But some magnetic flux remains in the core 20 because of magnetic hysteresis effect. The remaining flux is defined as residual flux. The magnetic sensor 14 detects the residual flux, and consequently the sensor 30 measures an error current. [0009] The measurement error can be corrected by storing fixed data corresponding to the error in ROM (Read-Only Memory), which is mounted on a current detection circuit of a current sensor, if a current flows in one direction. However, the error correcting method with ROM cannot be applied to a current sensor for measuring a battery current. This is because a battery current flows in both directions for charging and discharging a battery, and the residual flux density in the core 20 varies depending on the direction and magnitude of current. Consequently, it is difficult to correct a measurement error with ROM storing fixed data corresponding to the error. [0010] The sensor 10 has another problem. When a large current of around several hundred amperes flows though the bar 12 , magnetic flux density in the core 20 increases significantly and hysteresis effect is enhanced accordingly. Consequently, magnetic saturation occurs in the core 20 and the sensor 10 cannot measure an actual current. SUMMARY OF THE INVENTION [0011] In view of the above-described problem, it is an object of the present invention to provide a current sensor with which a large current can be accurately measured. [0012] According to a first aspect of the present invention, the current sensor comprises a magnetic core, a first magnetic sensor, and a current bus bar. The magnetic core includes a center opening where the current bus bar is disposed so as to pass through. The magnetic core further includes a plurality of gaps, in one of which the first magnetic sensor is disposed. The other gaps are designed for preventing magnetic saturation in the core. [0013] In the current sensor, the gaps increase magnetic flux leakage from the core, and thereby magnetic saturation can be prevented. Consequently, the current sensor can accurately measure a large current. [0014] According to a second aspect of the present invention, the current sensor further comprises a second magnetic sensor disposed in one of the gaps. [0015] In the current sensor with two magnetic sensors, a current is calculated on the basis of each output of the two magnetic sensors. The current calculation process corrects a measurement error caused by residual flux, and thereby the current sensor can accurately measure a large current. BRIEF DESCRIPTION OF THE DRAWINGS [0016] The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: [0017] FIG. 1A is a perspective view showing a current sensor according to a first embodiment of the present invention, and FIG. 1B is a simplified graph showing a relation between a total number of gaps and maximum flux density in a magnetic core; [0018] FIGS. 2A to 2 D are perspective views explaining a manufacturing method of a magnetic core of the first embodiment; [0019] FIG. 3A is a perspective view showing a current sensor according to a first modification of the first embodiment, and FIG. 3B is a perspective view showing a current sensor according to a second modification of the first embodiment; [0020] FIG. 4A is a perspective view showing a current sensor according to a third modification of the first embodiment, and FIG. 4B is a perspective view showing a current sensor according to a forth modification of the first embodiment; [0021] FIG. 5A is a perspective view showing a current sensor according to a fifth modification of the first embodiment, and FIG. 5B is a perspective view showing a current sensor according to a sixth modification of the first embodiment; [0022] FIG. 6A is a side view showing a current sensor according to a seventh modification of the first embodiment, FIG. 6B is a top view showing a current sensor according to a seventh modification of the first embodiment, and FIG. 6C is a side view showing a current sensor according to a seventh modification of the first embodiment attached to a mounting part; [0023] FIG. 7A is a perspective view showing a current sensor according to an eighth modification of the first embodiment, and FIG. 7B is a perspective view showing a current sensor according to a ninth modification of the first embodiment; [0024] FIG. 8A is a perspective view showing a current sensor according to a second embodiment of the present invention, and FIG. 8B is a simplified graph showing a relation between magnetic flux density and an amount of current in both a wide gap and a narrow gap; [0025] FIG. 9 is a flow chart showing an error correcting method for a current sensor according to the second embodiment; [0026] FIGS. 10A to 10 C are perspective views explaining a manufacturing method of a magnetic core of the second embodiment; [0027] FIG. 11 is a flow chart showing an error correcting method for a current sensor according to a first modification of the second embodiment; [0028] FIG. 12A is a perspective view showing a current sensor according to a second modification of the second embodiment, and FIG. 12B is a perspective view showing a current sensor according to a third modification of the second embodiment; [0029] FIG. 13 is a perspective view showing a current sensor according to a fourth modification of the second embodiment; [0030] FIG. 14 is a perspective view showing a current sensor according to a fifth modification of the second embodiment; [0031] FIG. 15 is a perspective view showing a conventional current sensor according to a prior art; [0032] FIG. 16 is a simplified graph showing a relation between magnetic flux density and the density detected position in a gap; and [0033] FIG. 17 is a perspective view showing a current sensor according to a third embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First Embodiment [0034] The inventors have preliminary studies about a current sensor for improving magnetic saturation. There is a potential method for overcoming magnetic saturation. Expanding the width of the gap Ga 1 of the sensor 10 increases leakage flux in the core 20 , and thereby magnetic saturation can be prevented. However, this method requires that the magnetic sensor 14 is exactly disposed in a center of the gap Ga 1 . The reason is as follows: [0035] Graphs illustrated in FIG. 16 show a relation between magnetic flux density and a position in the gap Ga 1 , when a given amount of current flows. A vertical axis represents magnetic flux density, and a horizontal axis represents a deviation from the center C of the gap Ga 1 to ±Y direction. A solid line graph represents the sensor 10 with one gap Ga 1 . A chain double-dashed line graph represents a wider gap version of the sensor 10 . The flat area in the middle of each graph indicates effective sensing area for the magnetic sensor 14 . When the magnetic sensor 14 is disposed out of the area, wrong flux density can be detected. [0036] In the sensor 10 , although effective sensing area is wide, magnetic flux density is high. Consequently, magnetic saturation tends to occur in the core 20 . In the wider gap version of the sensor 10 , although magnetic flux density decreases, effective sensing area also decreases. Consequently, magnetic flux density cannot be accurately detected unless the magnetic sensor 14 is exactly disposed in the center C of the gap Ga 1 . [0037] FIG. 1A shows a current sensor 100 according to a first embodiment of the present invention. The sensor 100 is designed to measure a charging and discharging current of car batteries. The sensor 100 includes a magnetic core 20 , a current bus bar 12 and a magnetic sensor 14 . The core 20 has a center opening 21 and rounded corners. The core 20 is evenly divided into four pieces by four gaps. The four gaps are composed of a gap Ga 1 and three gaps Gb 1 . The gap Ga 1 and one of the gaps Gb 1 divide the core 20 into upper and lower pieces. Likewise, the others of the gaps Gb 1 divide the core 20 into right and left pieces. The magnetic sensor 14 is disposed in the gap Ga 1 , and bonding members 22 are disposed in the gaps Gb 1 , respectively. The magnetic sensor 14 can also serves as a bonding member, and thus the four pieces of the core 20 are integrated into the core 20 by the magnetic sensor and the bonding members 22 . The bar 12 is disposed in a plane perpendicular to the core 20 and inserted in the opening 21 with a given space until the core 14 is positioned in the middle of the bar 12 . The magnetic sensor 14 employs Hall effect devices as its magnetic sensing element and detects magnetic flux density in the gap Ga 1 generated by a current flowing through the bar 12 . [0038] The gaps Gb 1 are designed for preventing magnetic saturation in the core 20 . When a large current flows through the bar 12 , the magnetic flux density in the core 20 increases significantly. Then, the gaps Gb 1 allow some magnetic flux leakage, and therefore, the magnetic flux density decreases. Consequently, magnetic saturation can be prevented. [0039] Each of the gap Ga 1 and the gaps Gb 1 has a width of 1 mm. The bar 12 is made of high-conductive metal such as brass. The bar 12 has a width of 20 mm and a thickness of 2 mm. A current sensor housing (not shown) contains the core 20 including the bar 12 and the magnetic sensor 14 . [0040] A graph in FIG. 1B shows a relation between the number of the gaps (i.e., the number of divided pieces of the core) and maximum flux density in the core 20 . The graph indicates that the maximum flux density is almost inversely proportional to the number of the gaps. In fact, maximum flux density of the sensor 100 with four gaps is reduced to a quarter compared to that of the sensor 10 with one gap. [0041] As described above, the graphs illustrated in FIG. 14B shows a relation between magnetic flux density and the density detected position in the gap Ga 1 , when a given amount of current flows. The vertical axis represents magnetic flux density in the gap Ga 1 and the horizontal axis represents a deviation from the gap center C to ±Y direction. [0042] The solid line graph represents the sensor 10 with one gap Ga 1 . A chain line graph represents a two-gap type of the sensor 10 . A dot line graph represents a four-gap type (i.e., the sensor 100 ) of the sensor 10 . [0043] Compared to the sensor 10 with one gap, magnetic flux density of the two-gap type is reduced to a half. Likewise, magnetic flux density of the four-gap type is reduced to a quarter. In other words, the graph indicates that magnetic flux density in the gap Ga 1 is almost inversely proportional to the number of the gaps in the core 20 . In the sensor 100 with four gaps, consequently, magnetic saturation may be prevented when a large current flows. Further, the graphs indicate there is no relation between the effective sensing area for the magnetic sensor 14 and the number of gaps in the core 20 . The two-gap type and the sensor 100 have effective sensing area as large as the sensor 10 . Specifically, the sensor 100 has a sufficient wide region to accommodate the sensor 14 in the gap Ga 1 . [0044] As mentioned above, the sensor 100 can prevent magnetic saturation in the core 20 without decreasing the effective sensing area for the magnetic sensor 14 . In addition, in the sensor 10 , the magnetic flux in the core 20 reaches the maximum density in a position across the bar 12 from the gap Ga 1 . In the sensor 100 , one of the gaps Gb 1 is formed in the position, and thereby magnetic saturation is efficiently prevented. Consequently, the sensor 100 can correctly measure a large current. [0045] The magnetic core 20 of the sensor 100 is manufactured as follows. [0046] FIG. 2A shows a magnetic plate 24 with a predetermined shape. At first, an upper left piece 24 L of the core 20 is manufactured by using multiple plates 24 (e.g., three plates). The plates 20 are pressed together into the piece 24 L. Likewise, an upper right piece 24 R is manufactured. Second, as shown in FIG. 2C , an upper half 24 A of the core 20 is manufactured by using the piece 24 L, the piece 24 R and a bonding member 22 made of insulating resin. The bonding member 22 is sandwiched between one end surface of the piece 24 L and one end surface of the piece 24 R. The bonding member 22 has adhesive on both contact surfaces with the two pieces 24 L, 24 R. Thus, the two pieces 24 L, 24 R are bonded together into the upper half 24 A by the bonding member 22 . Likewise, a lower half 24 B of the core 20 is manufactured. Finally, a core 24 shown in FIG. 1A is manufactured by using the upper half 24 A, the lower half 24 B, the bonding member 22 and a magnetic sensor 14 . The bonding member 22 is sandwiched between one end surface of the upper half 24 A and one end surface of the lower half 24 B. The magnetic sensor 14 is sandwiched between the other end surface of the upper half 24 A and the other end surface of the lower half 24 B. The magnetic sensor 14 has adhesive on both contact surfaces with the two half 24 A, 24 B. Thus, the two half 24 A, 24 B are integrated into the core 20 by the bonding member 22 and the magnetic sensor 14 . In the sensor 100 , the gaps Ga 1 , Gb 1 can be accurately formed in a predetermined width. That is because the magnetic sensors 14 A and the bonding member 22 serve not only to bond the upper half 24 A and the lower half 24 B but also to form the gaps Ga 1 , Gb 1 . Modifications of the First Embodiment [0047] Several alternative modifications of the first embodiment are illustrated in FIGS. 3 to 7 . [0048] A current sensor 110 shown in FIG. 3A has gap Ga 1 and four gaps Gb 1 in the core 20 . The total gaps are five, and consequently maximum flux density in the core 20 is reduced to one-fifth compared to the sensor 10 . Likewise, a current sensor 120 shown in FIG. 3B has gap Ga 1 and five gaps Gb 1 in the core 20 . The total gaps are six, and consequently maximum flux density in the core 20 is reduced to one-sixth compared to the sensor 10 . [0049] A current sensor 130 shown in FIG. 4A has gap Ga 1 and three gaps Gb 1 in the core 20 . One of the gaps Gb 1 is formed across the bar 12 from the gap Ga 1 just as in the case of the sensor 100 . But the others of the gaps Gb 2 are formed over the bar 12 . In other words, no gaps are formed under the bar 12 . The gap-layout of the sensor 130 produces the same effect as the sensor 100 for reducing maximum flux density in the core. A current sensor 140 shown in FIG. 4B has the same gap-layout as the sensor 130 . But the gaps Gb 1 formed over the bar 12 has expanded widths. In the sensor 140 , maximum flux density in the core 20 easily leaks from the expanded gaps Gb 1 , and consequently maximum flux density in the core 20 can be greatly reduced. [0050] In a current sensor 150 shown in FIG. 5A , the core 20 has gap Ga 1 and three gaps Gb 1 just as in the case of the sensor 100 . Further, the gap-layout of the sensor 150 is similar to that of the sensor 100 . But, in the sensor 150 , the gap Gb 1 formed over the bar 12 is not in the same line as the gap Gb 1 formed under the bar 12 . The gap-layout of the sensor 150 produces the same effect as the sensor 100 for reducing maximum flux density. [0051] In a current sensor 160 shown in FIG. 5B , the core 20 has sharp corners 26 , whereas the core 20 of the sensor 100 - 150 has rounded corners. Magnetic flux leaks from the sharp corners 26 more easily than from rounded corners. Consequently, in the sensor 160 , magnetic saturation in the core 20 can be prevented and a large current can be accurately measured. [0052] In a current sensor 170 shown in FIGS. 6A to 6 C, the core 20 has gap Ga 1 and two gaps Gb 1 . Both of the gaps Gb 1 are formed over the bar 12 . The bonding members 22 are disposed in each gap Gb 1 , and besides, mounting holes 22 A are formed inside each bonding members 22 as shown in FIG. 6B . A mounting part 30 shown in FIG. 6C is used for fixing the sensor 170 to an object such as a car. The mounting part 30 includes a common body and two junction portions 32 extending from the common body in one direction. Each junction portion 32 has a hook 34 on its tip. Each junction portion 32 is inserted into each mounting hole 22 A in order to fix the core 20 to the mounting part 30 . Then, each hook 34 catches the core 20 , and thereby the core 20 can be fixed to the mounting part 30 . Thus, the sensor 170 can be easily attached to an object. [0053] In a current sensor 180 shown in FIG. 7A , the core 20 has rectangular slits 28 instead of the gaps Gb 1 of the sensor 100 . Some magnetic flux leaks from the slits 28 , when a current flows. Each slit 28 is designed to overlap alternately in the direction of magnetic flux in the core 20 so that magnetic flux leakage increases. In other words, the slits 28 serve to prevent magnetic saturation as well as the gaps Gb 1 . In a current sensor 190 shown in FIG. 7B , the core 20 has triangular slits 29 instead of the rectangular slits 28 of the sensor 180 . The triangular slits 29 are equal to the rectangular slits 28 in terms of the function of increasing magnetic flux leakage and preventing magnetic saturation. Therefore, the current sensors 180 , 190 can accurately measure a large current. [0054] Further, in the sensor 180 , 190 , the slits 28 , 29 are formed in opposite side of the gap Ga 1 . As describe above, magnetic flux in the core 20 reaches the maximum density in opposite side of the gap Ga 1 . Consequently magnetic saturation is efficiently prevented and the sensor 180 , 190 can correctly measure a large current. Second Embodiment [0055] A current sensor 200 according to a second embodiment of the present invention is shown in FIG. 8A . The sensor 200 is designed to measure a battery current. As shown in FIG. 8A , the sensor 200 includes a magnetic core 20 , a current bus bar 12 , a first magnetic sensor 14 A and a second magnetic sensor 14 B. The core 20 has a center opening 21 and two gaps Ga 1 , Ga 2 . The core 20 is evenly divided into an upper half 24 A and a lower half 24 B by the gaps Ga 1 , Ga 2 . The magnetic sensor 14 A, 14 B are disposed in the gaps Ga 1 , Ga 2 , respectively. The bar 12 is disposed in a plane perpendicular to the core 20 and inserted in the opening 21 with a given space until the core 20 is positioned in the middle of the bar 12 . The magnetic sensors 14 A, 14 B detect magnetic flux density in the gaps Ga 1 , Ga 2 , respectively, generated by a current flowing through the bar 12 . [0056] The same Hall effect devices are employed in each magnetic sensor 14 A, 14 B as its magnetic sensing element. Consequently, the two magnetic sensor 14 A, 14 B have the same magnetic sensing capability. The gap Ga 2 is wider than the gap Ga 1 , and the gaps Ga 1 , Ga 2 have a width of 1 mm, 2 mm, respectively. The bar 12 is made of high-conductive metal such as brass. The bar 12 has a width W 1 of 20 mm and a thickness H 1 of 2 mm. [0057] Graphs illustrated in FIG. 8B show a relation between magnetic flux density in the gaps Ga 1 , Ga 2 and an amount of current flowing through the bar 12 . A vertical axis represents magnetic flux density B and a horizontal axis represents an amount of current I. Graphs GA 1 , GA 2 represent magnetic flux density in the gap Ga 1 , Ga 2 , respectively. The graphs GA 1 , GA 2 indicate that magnetic flux density in the gap Ga 1 is higher than that in the gap Ga 2 , when a current flows. That is because the gap Ga 2 is wider than the gap Ga 1 and accordingly more magnetic flux leaks from the gap Ga 2 than from the gap Ga 1 . Consequently, the magnetic sensor 14 A detects more magnetic flux than the magnetic sensor 14 B. That means the sensitivity of the magnetic sensor 14 A to a current is higher than that of the magnetic sensor 14 B. [0058] The graph GA 1 , GA 2 illustrated in FIG. 8B further indicate that magnetic flux density in the gap Ga 1 is equal to that in the gap Ga 2 , when no current flows. In short, residual flux density BR in each gap Ga 1 , Ga 2 is equal. The density BR caused by magnetic hysteresis effects result in a measurement error of current. Here, a graph GA 2 N in FIG. 8B represents magnetic density in the gap Ga 2 on the assumption of no magnetic hysteresis effects in the core 20 . As shown in the graphs in FIG. 8B , for instance, even when no current flows, the magnetic sensor 14 B detects the density B R and the sensor 200 measures a current I R equivalent to the density B R . Therefore, an actual current I C is obtained by subtracting the current I R from a current equivalent to the magnetic flux density detected on the magnetic sensor 14 B. When the magnetic sensor 14 B detects magnetic flux density B EXP , the actual current I C is obtained by subtracting the current I R from a current I EXP equivalent to the magnetic flux density B EXP (i.e., I C =I EXP −I R In the FIG. 8B , the actual current I C is represented by ΔI. [0059] As described above, the magnetic sensor 14 B detects magnetic flux density including the residual flux density B R . Likewise, the magnetic sensor 14 A detects magnetic flux density including the residual flux density B R . Moreover, the magnetic flux density detected by each magnetic sensor 14 A, 14 B are different. That is, the magnetic sensors 14 A, 14 B produces two different outputs including the same residual flux density B R . Therefore, the residual flux B R is eliminated by subtracting one output from the other output, and thereby an measurement error resulting from the residual flux B R can be corrected. Consequently, the sensor 200 can measure the actual current I C , even if the residual flux density B R varies depending on the magnitude and direction of current flow. [0060] FIG. 9 is a block diagram showing an error correcting method in the sensor 200 . Each magnetic sensor 14 A, 14 B is connected to a current detection circuit 30 . At first, a current I flowing through the bar 12 is converted to magnetic flux density B 1 , B 2 in the gap Ga 1 , Ga 2 , respectively. Then, the magnetic sensor 14 A, 14 B detects the magnetic flux density B 1 , B 2 , respectively. Finally, the actual current I C is calculated from outputs of the magnetic sensor 14 A, 14 B through a current calculating process built in the current detection circuit 30 . Here, the outputs of the magnetic sensor 14 A, 14 B are the magnetic flux density B 1 , B 2 , respectively. The current calculating process is explained below. [0061] The magnetic flux density B 1 , B 2 are represented by the following equations: B 1 =X A I C +B R   (1) B 2 =X B I C +B R   (2) [0062] I C represents an actual current flowing through the bar 12 ; X A represents the sensitivity of the sensor 14 A to the current; X B represents the sensitivity of the sensor 14 B to the current; B R represents residual flux density in the gaps Ga 1 , Ga 2 . [0063] The actual current I C can be calculated by the following equation derived from the above equations (1), (2). I C =( B 2 −B 1 )/( X A −X B )  (3) [0064] In the sensor 10 according to prior art shown in FIG. 15 , the magnetic flux in the core 20 reaches the maximum density in a position across the bar 12 from the gap Ga 1 . In the sensor 200 , the gap Gb 1 is formed in the position, and thereby magnetic saturation is efficiently prevented. Thus the current sensor 200 can overcome the problems resulting from not only residual flux density but also magnetic saturation. Consequently the current sensor 200 can correctly measure a large current. [0065] The magnetic core 20 of the sensor 200 is manufactured as follows: [0066] FIG. 10A shows a magnetic plate 24 with a predetermined shape. At first, an upper half 24 A of the core 20 is manufactured by using multiple plates 24 (e.g., three plates). As shown in FIG. 10B , the plates 24 are pressed together into the upper half 24 A. Likewise, a lower half 24 B is manufactured. Then, the upper half 24 A are fixed above the bar 12 as shown in FIG. 8A . Likewise, the lower half 24 B are fixed below the bar 12 . Thus, the gap Ga 1 is formed between one end surface of the upper half 24 A and one end surface of the lower half 24 B. Likewise, the gap Ga 2 is formed between the other end surface of the upper half 24 A and the other end surface of the lower half 24 B. Finally, the magnetic sensor 14 A, 14 B is disposed in the gap Ga 1 , Ga 2 . [0067] The sensor 200 can include multiple gaps Gb 1 shown in FIGS. 1 A and 3 A- 7 B. Modifications of the Second Embodiment [0068] Several alternative modifications of the second embodiment are illustrated in FIGS. 11 to 12 . [0069] In the sensor 200 , the current detection circuit 30 employs the current calculating process. The process calculates an actual current I C from the outputs of the magnetic sensors 14 A, 14 B. [0070] FIG. 11 is a flow chart showing another current calculating process for the current detection circuit 30 . The process starts at step S 110 . Then, at step S 112 , it is checked whether it is in a predetermined cycle. If it is in a predetermined cycle, the process proceeds to step S 114 , where the residual flux density BR is calculated from the outputs of the magnetic sensor 14 A, 14 B. Then, the process proceeds to step S 116 . If it is not in a predetermined cycle at step S 112 , the process skips step S 114 and proceeds directly to step S 116 . At step S 116 , based on the output of the magnetic sensor 14 B, it is checked whether the current is large (e.g., 10 amperes or more). If the current is large, the process proceeds to S 118 , where the actual current I C is calculated from the output of the sensor 14 B with the residual flux density B R . If the current is not large at step S 116 , the process proceeds to step S 120 , where the actual current I C is calculated from the output of the sensor 14 A with the residual flux B R . After step S 118 or step S 120 is finished, the process proceeds to step S 122 and then returns to step S 110 . [0071] As described above, in the sensor 200 , the sensitivity of the magnetic sensor 14 A to a current is higher than that of the magnetic sensor 14 B. The process uses the sensor 14 A for a small current and uses the sensor 14 B for a large current. Thus, the sensor 200 employing the process has a wide range of current measurement capability from a small current to a large current with high accuracy. [0072] FIG. 12A shows a current sensor 210 according to a modification of the sensor 200 . In the sensor 210 , the magnetic sensors 14 A, 14 B have a cubic shape with the same dimension as the gaps Ga 1 , Ga 2 , respectively. Consequently, the magnetic sensors 14 A, 14 B tightly fit into the gaps Ga 1 , Ga 2 , respectively. Further, each magnetic sensor 14 A, 14 B has adhesive on its both sides, where each magnetic sensor 14 A, 14 B contacts the upper half 24 A and the lower half 24 B. Thus, the upper half 24 A and the lower half 24 B are bonded together into the core 20 by each magnetic sensor 14 A, 14 B. In the sensor 210 , the gaps Ga 1 , Ga 2 can be accurately formed in a predetermined width. That is because the magnetic sensors 14 A, 14 B serve not only to joint the upper half 24 A and the lower half 24 B but also to form the gaps Ga 1 , Ga 2 . [0073] The sensor 210 can include multiple gaps Gb 1 shown in FIGS. 1 A and 3 A- 7 B. [0074] FIG. 12B shows a current sensor 220 according to a modification of the sensor 210 . The sensor 220 has another two gaps Gb 1 in addition to the gaps Ga 1 , Ga 2 . Bonding materials 22 made of insulating resin are disposed in each gap Gb 1 . The gaps Gb 1 are designed for preventing magnetic saturation in the core 20 . The gaps Gb 1 allow more magnetic flux leakage in the core 20 , and thereby the magnetic saturation can be prevented. Consequently, the sensor 220 can accurately measure a large current. [0075] The sensor 220 can include multiple gaps Gb 1 shown in FIGS. 1 A and 3 A- 7 B. [0076] Yet another embodiments of the sensor 200 are illustrated in FIGS. 13 to 14 . [0077] FIG. 13 shows a current sensor 230 according to another modification of the sensor 200 . The sensor 230 has only one gap G in the core 20 . The gap G is composed of different width gaps Ga 3 , Ga 4 . The gap Ga 3 is narrower than the gap Ga 4 , and accordingly magnetic flux density in the gap Ga 3 is higher than that in the gap Ga 4 . The magnetic sensors 14 A, 14 B are disposed in the gaps Ga 3 , Ga 4 , respectively. Each magnetic sensor 14 A, 14 B employs the same Hall effect devices as its magnetic sensing element, and consequently the two magnetic sensor 14 A, 14 B are identical in magnetic sensing capability. In the sensor 230 , consequently, two magnetic sensors with the same magnetic sensing capability detect two different magnetic flux densities. In short, the different outputs can be produced from the magnetic sensors 14 A, 14 B, just as in the case of the sensor 200 . Thus, the processes applied to the sensor 200 can be applied to the sensor 230 , and thereby the sensor 230 can accurately measure a current without a measurement error caused by residual flux density. The sensors 230 are suitable for use in small current measurement The sensor 230 can include multiple gaps Gb 1 shown in FIGS. 1 A and 3 A- 7 B. [0078] FIG. 14 shows a current sensor 240 according to a modification of the sensor 230 . There are two differences between the sensor 230 and the sensor 240 . One difference is in their gap widths. The gap G of the sensor 240 has a uniform width all over the gap area, whereas the gap G of the sensor 230 has two different widths. In the sensor 240 , consequently, magnetic flux density in the gap G is uniform all over the gap area. The other difference is in their magnetic sensors. In the sensor 230 , the two magnetic sensors 14 A, 14 B have the same magnetic sensing capability. On the other hand, in the sensor 240 , the two magnetic sensors 14 A, 14 B have the different magnetic sensing capability. In the sensor 240 , consequently, the two magnetic sensors with the different magnetic sensing capability detect the same magnetic flux density. In short, two different outputs can be produced from the magnetic sensors 14 A, 14 B, just as in the case of the sensor 200 . Thus, the processes applied to the sensor 200 can be applied to the sensor 240 , and thereby the sensor 240 can accurately measure a current without a measurement error caused by residual flux density. [0079] The sensor 240 can include multiple gaps Gb 1 shown in FIGS. 1 A and 3 A- 7 B. [0080] The sensors 230 , 240 are suitable for use in small current measurement and can be easily manufactured because of their one-piece core. [0081] In the all above-mentioned embodiments according to the present invention, magnetic sensors can adopt magnetoresistance devices as their magnetic sensing elements in place of Hall effect devices. Third Embodiment [0082] A current sensor 300 according to a third embodiment of the present invention is shown in FIG. 17 . The sensor 300 includes two gaps G, Gb 1 . One of the gaps G is composed of different width gaps Ga 3 , Ga 4 , which is similar to the sensor 230 shown in FIG. 13 . The other one of gaps Gb 1 is disposed opposite to the gap G, which is similar to the sensor 100 shown in FIG. 1A . Here, the sensor 300 can include multiple gaps Gb 1 shown in FIG. 1A . For example, the sensor 300 can include three gaps Gb 1 . [0083] Thus, the sensor 300 according to the third embodiment is formed to combine the sensor 100 according to the first embodiment and the sensor 230 according to the second embodiment. Here, in the sensor 100 according to the first embodiment, the magnetic core 20 is divided into multiple parts so that the magnetic saturation is reduced. Therefore, to flow a large current in the core 20 , it is required to increase the number of the parts. However, when the sensor 100 includes the large number of the parts, the manufacturing steps for the sensor 100 are increased. Further, it is difficult to determine the positioning of the parts accurately. Further, in case of the sensor 100 shown in FIGS. 7A and 7B , the area of the slits 28 , 29 is increased as the number of the parts is increased. Thus, the mechanical strength of the core 20 is reduced. Therefore, the number of the parts is limited to a certain number. [0084] The sensor 230 according to the second embodiment includes two sensors 14 A, 14 B, which output sensor signals, respectively. Then, two signals are processed appropriately so that the sensor 230 can accurately measure a current without a measurement error caused by residual flux density. Specifically, a zero fluctuation caused by magnetic saturation is cancelled. To increase the current flowing through the core 20 , it is required to increase the gap widely. However, the increase of the gap widely causes to narrow the flat portion in the magnetic field, as shown in FIG. 16 . In this case, it is difficult to determine the positioning of the magnetic sensor 14 B. [0085] The sensor 300 can perform to detect the large current and to detect the current accurately, since the sensor 300 provides the merits of the first embodiment and the second embodiment. [0086] Such changes and modifications are to be understood as being within the scope of the present invention as defined by the appended claims.

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