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.

Description:
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.