Magnetic sensor apparatus, current sensor apparatus and magnetic sensor element

A magnetic sensor apparatus comprises a magnetic detector (101) that outputs a signal responsive to a magnetic field and a magnetic substance (110) having a cavity (111) in which the magnetic detector (101) is placed. The magnetic detector (101) is placed in the cavity (111) of the magnetic substance (110). The ratio between a magnetic field (H) to be measured and a magnetic field applied to the magnetic detector (101) is set to a specific value, based on at least one of a first demagnetizing factor depending on the shape of the magnetic substance (110) and a second demagnetizing factor depending on the shape of the cavity (111). The magnetic sensor apparatus further comprises a feedback coil (112) that applies a negative feedback magnetic field to the magnetic detector (101) and a reference magnetic field coil (113) that applies a reference alternating magnetic filed to the magnetic detector (101) for controlling the property of the magnetic detector (101).

TECHNICAL FIELD
 The present invention relates to a magnetic sensor apparatus for measuring
 a magnetic field, a current sensor apparatus for measuring an electric
 current through measuring a magnetic field generated by the current, and a
 magnetic sensor element for measuring a magnetic field.
 BACKGROUND ART
 Many types of magnetic sensor apparatuses and non-contact-type electric
 current sensor apparatuses utilizing magnetic sensor apparatuses have been
 long developed since such apparatuses are useful in industry. However,
 their application fields have been limited and the market scale have been
 thus limited. Consequently, development of such apparatuses in terms of
 cost reduction have not been fully achieved yet.
 However, emission control originating from the need for solving
 environmental problems has accelerated development of electric automobiles
 and solar-electric power generation. Since a direct current of several
 kilowatts to tens of kilowatts is dealt with in an electric car or
 solar-electric power generation, a non-contact current sensor apparatus is
 required for measuring a direct current of tens to hundreds of amperes.
 The demand for such current sensor apparatuses is extremely high. It is
 therefore difficult to increase the popularity of electric automobiles and
 solar-electric power generation unless the current sensor apparatuses not
 only exhibit excellent properties but also are extremely low-priced. In
 addition, reliability is required for a period of time as long as 10 years
 or more for a current sensor apparatus used in a harsh environment as in
 an electric car. As thus described, it has been requested in society to
 provide current sensor apparatuses that are inexpensive and have excellent
 properties and long-term reliability.
 For non-contact measurement of an electric current, an alternating current
 component is easily measured through the use of the principle of a
 transformer. However, it is impossible to measure a direct current
 component through this method. Therefore, a method is taken to measure a
 magnetic field where a current is generated through a magnetic sensor for
 measuring a direct current component. In general, such a current sensor
 apparatus has a configuration including a magnetic yoke interlinking a
 current to be measured and having a gap in which a magnetic sensor element
 of a magnetic sensor apparatus is placed. A Hall element is widely used as
 such a magnetic sensor element incorporated in such a current sensor
 apparatus. A magnetoresistive (MR) element and a fluxgate element are used
 in some applications, too.
 In applications such as an electric car or solar-electric power generation
 mentioned above, a current to be measured is 10 to 500 amperes. Therefore,
 a Hall element or a giant magnetoresistive (GMR) element suitable for
 measuring a high magnetic field is mainly used as a magnetic sensor
 element.
 Not only for a current sensor apparatus incorporating a Hall element or a
 GMR element but also for a current sensor apparatus in general, a
 technique has been known for improving linearity and the dependence of
 output on temperature. That is, as disclosed in Published Unexamined
 Japanese Patent Application Sho 62-22088 (1987), for example, based on an
 output of a magnetic sensor apparatus, a magnetic field is generated in
 the direction opposite to a magnetic field to be measured that is produced
 by a current to be measured. Negative feedback of the output of the
 magnetic sensor apparatus is thereby achieved, such that the apparatus
 operates in the state where the magnetic field in the magnetic yoke is
 nearly zero, that is, in the state where the field applied to the
 apparatus is nearly zero. This technique is hereinafter called a negative
 feedback method.
 For a current sensor apparatus, as disclosed in Published Examined Japanese
 Patent Application Sho 63-57741 (1988), for example, a technique has been
 known for improving measurement accuracy. That is, a specific alternating
 magnetic field is superposed on a magnetic field to be measured that is
 produced by a current to be measured. Control is performed to constantly
 maintain an output of the magnetic sensor apparatus responsive to the
 alternating magnetic field. This technique is hereinafter called an
 alternating current superposing method.
 Various types of magnetic sensor elements have been known, such as a Hall
 element, an MR element, a GMR element and a fluxgate element. Each of
 theses elements has its own suitable measurement range of magnetic fields.
 Therefore, it has been required in prior art to choose a magnetic sensor
 element in accordance with the magnitude of a magnetic field to be
 measured. However, each element has its own properties such as output
 magnitude, linearity, and dependence on temperature. Consequently, desired
 accuracy is not always achieved even though a magnetic sensor element that
 provides a measurement range suitable for magnetic fields to be measured
 is chosen. Another problem is that, in some cases, no magnetic sensor
 element that provides a measurement range suitable for fields to be
 measured is available.
 As described above, the negative feedback method may be applied for
 improving linearity and the dependence of output on temperature. However,
 the negative feedback method requires the generation of a negative
 feedback magnetic field in the opposite direction that is equal in
 magnitude to the field produced by the current to be measured. To measure
 a current of 100 amperes, for example, a feedback current of 1 ampere is
 required even though the number of turns of the coil for generating the
 negative feedback field is 100. As a result, the negative feedback method
 causes secondary problems such as an increase in coil dimensions, power
 loss, and heating. It is difficult in the prior art to solve these
 problems.
 Furthermore, in the negative feedback method, the magnetic sensor element
 constantly operates in the state where the magnetic field is nearly zero.
 Therefore, if a Hall element whose output is small is used as the magnetic
 sensor element, the element is strongly affected by drifts of its own or
 the direct current amplification circuit and the accuracy is reduced.
 With regard to a GMR element, although its output is large, it is
 impossible to determine the direction of a magnetic field to be measured
 (or the direction of a current to be measured in the case of a current
 sensor apparatus) since the magnetoresistive effect thereof is independent
 of the direction of the magnetic field. Therefore, in order to measure a
 magnetic field through a GMR element in the prior art, a bias magnetic
 field is applied such that the output of the magnetic sensor element
 monotonously changes in response to change in the field to be measured. In
 this case, however, if the direction of the field of the field to be
 measured is opposite to that of the bias field and the absolute value of
 the field to be measured exceeds that of the bias field, it is impossible
 to maintain the monotonicity of the output of magnetic sensor apparatus in
 response to changes in the field to be measured. Consequently, the
 negative feedback system may run away if the negative feedback method is
 applied.
 The alternating current superposing method is a technique for improving
 accuracy, too. However, this method is applicable on condition that
 linearity of the magnetic sensor apparatus is ensured. Using this method
 only is thus not enough to improve linearity.
 As described so far, it is impossible to measure a magnetic field or an
 electric current having a specific magnitude, or a great magnitude in
 particular, with accuracy, through the use of a magnetic sensor apparatus
 or a current sensor apparatus of prior art.
 For example, the following problems have been found in the current sensor
 apparatus utilizing a Hall element that has been most highly developed in
 prior art.
 (1) low sensitivity
 (2) inconsistent sensitivity
 (3) poor thermal characteristic
 (4) offset voltage that requires troublesome handling
 In addition to the above problems, a magnetoresistive element has a problem
 of poor linearity.
 Some methods have been developed for solving the problems of a Hall
 element. One of the methods is the negative feedback method described
 above. In this method, a reversed magnetic field proportional to an output
 of the magnetic sensor element is applied to the element so as to apply
 negative feedback, such that the output of the element is kept constant.
 Consistency in sensitivity, the thermal characteristic, and linearity are
 thereby improved.
 When the negative feedback method is used, however, it is required to apply
 an inverse magnetic field as large as the field to be measured to the
 element. Consequently, when a current as high as hundreds of amperes is
 measured in applications such as an electric car or solar-electric power
 generation, a feedback current obtained is several amperes even if the
 number of turns of the coil for generating a negative feedback field is
 100. Therefore, a current sensor apparatus embodied through this method is
 very large-sized and expensive.
 If the magnetic sensor element has high sensitivity, it is possible that a
 feedback current is reduced by applying only part (such as one hundredth)
 of the field to be measured to the element. However, this is difficult for
 a Hall element with low sensitivity used as the magnetic sensor element.
 A fluxgate element has been developed mainly for measurement of a small
 magnetic field while not many developments have been made in techniques
 for measuring a large current. However, with some modification a fluxgate
 element may be used as a magnetic detector of a current sensor apparatus
 for a large current since the fluxgate element has a simple configuration
 and high sensitivity.
 Reference is now made to FIG. 13 to describe the operation principle of a
 fluxgate element having the simplest configuration. FIG. 13 is a plot for
 showing the relationship between an inductance of a coil wound around a
 magnetic core and a coil current. Since the core has a magnetic saturation
 property, the effective permeability of the core is reduced and the
 inductance of the coil is reduced if the coil current increases.
 Therefore, if bias magnetic field B is applied to the core by a magnet and
 the like, the magnitude of external magnetic field H.sub.o is measured as
 a change in inductance of the coil when external field H.sub.o is
 superposed on the bias field. This is the operation principle of the
 simplest fluxgate element. In FIG. 13 each of bias field B and external
 field H.sub.o is expressed in the magnitude converted to the coil current.
 In this method the position of bias point B changes with factors such as
 the intensity of the magnetic field generated by the magnet or the
 positions of the magnet and the core in relation to each other. It is
 therefore required to maintain the inductance at a specific value when the
 external magnetic field is zero. However, it is extremely difficult to
 compensate for the instability of the inductance value due to temperature
 changes and other external perturbations. This method is therefore not
 suitable for practical applications.
 If a rod-shaped magnetic core is used, an open magnetic circuit is
 provided, so that the effect of hysteresis is generally very small.
 Assuming that the hysteresis of the core is negligible, the characteristic
 of variations in inductance is equal when the coil current flows in the
 positive direction and in the negative direction since the saturation
 characteristic of the core is independent of the direction of coil
 current. For example, in FIG. 13, it is assumed that point P+ and point P.
 represent the coil current in the positive direction and the coil current
 in the negative direction, respectively, whose absolute values are equal.
 In the neighborhood of these points, the characteristic of variations in
 inductance with respect to variations in the absolute value of the coil
 current is equal. Therefore, an alternating current may be applied to the
 coil such that the core is driven into a saturation region at a peak, and
 the difference in the amounts of decrease in inductance may be measured
 when positive and negative peak values of the current are obtained. As a
 result, the difference thus measured is constantly zero when the external
 magnetic field is zero, which is always the case even when the
 characteristics of the core change due to temperature changes or external
 perturbations. In the present invention a saturation region of the
 magnetic core means a region where an absolute value of the magnetic field
 is greater than the absolute value of the magnetic field obtained when the
 permeability of the core is maximum.
 An external magnetic field is assumed to be applied to the core. If
 external field H.sub.o is applied in the positive direction of the
 current, as shown in FIG. 13, the inductance value decreases at the
 positive peak of the current (point Q+ in FIG. 13, for example) and the
 inductance value increases at the negative peak of the current (point Q-
 in FIG. 13, for example). Therefore, the difference between the values is
 other than zero. Since the difference in inductance depends on the
 external magnetic field, the external field is obtained by measuring the
 difference in inductance.
 The method thus described is called a large amplitude excitation method in
 the present invention, that is, to apply an alternating current to the
 coil such that the core is driven into a saturation region at a peak, and
 to measure the difference in the amounts of decrease in inductance when
 positive and negative peak values of the current are obtained.
 Magnetic sensor apparatuses that utilize such a large amplitude excitation
 method are disclosed in Published Examined Japanese Patent Application Sho
 62-55111 (1987), Published Examined Japanese Patent Application Sho
 63-52712 (1988), and Published Unexamined Japanese Patent Application Hei
 9-61506 (1997), for example. In Published Examined Japanese Utility Model
 Application Hei 7-23751 (1995), a technique is disclosed to achieve
 measurement similar to the large amplitude excitation method through the
 use of two bias magnets.
 The large amplitude excitation method is an excellent method since the
 effects of temperature changes and external perturbations are eliminated.
 However, it is not so easy to apply an alternating current enough to drive
 the core into saturation. Accordingly, in prior art the large amplitude
 excitation method is limited to a magnetic sensor apparatus for detecting
 a small magnetic field through the use of an amorphous magnetic core and
 the like having a small saturation field.
 For non-contact measurement of a direct current, a method is generally
 taken to detect a magnetic field generated by a current through the use of
 a magnetic sensor element. In this method, for example, a magnetic yoke
 having a gap is provided around a current path, and a magnetic sensor
 element is placed in the gap. The magnetic field in the gap is measured by
 the sensor element. Intensity H of the field in the gap is I/g where the
 current value is I and the gap length is g.
 It is assumed that the negative feedback method is applied to a current
 sensor apparatus in which a fluxgate element made up of a single coil
 wound around a single magnetic core is used as a magnetic sensor element.
 Examples in which the negative feedback method is applied to a magnetic
 sensor apparatus incorporating a fluxgate element are disclosed in
 Published Unexamined Japanese Patent Application Sho 60-185179 (1985) and
 Published Unexamined Japanese Patent Application Hei 9-257835 (1997).
 In the case where the negative feedback method is applied to the current
 sensor apparatus incorporating a fluxgate element, a magnetic field
 generated by the current to be measured is applied to the coil, and the
 magnetic field generated by the coil through a negative feedback current
 exactly cancels the applied magnetic field. Therefore, in order to
 increase the range of current to be measured, it is required to increase
 the negative feedback current or to reduce the applied field by increasing
 length g of the gap of the magnetic yoke.
 However, an increase in gap g requires a large magnetic yoke, which is
 uneconomical. An increase in negative feedback current causes an increase
 in power consumption and leads to unfavorable phenomena such as heating of
 the coil.
 In order to make the field applied to the magnetic sensor element smaller
 than the field to be measured, a method may be taken to shunt the magnetic
 flux so that only part of it passes through the magnetic sensor element.
 However, it is difficult in this method to precisely determine the shunt
 ratio.
 As described so far, in order to increase the measurement range and to
 measure a large magnetic field or current by the magnetic sensor apparatus
 or the current sensor apparatus of prior art, it is required to increase
 the gap of the magnetic yoke or to increase the negative feedback current.
 In any case the above-mentioned problems are found and it is difficult to
 achieve the object.
 SUMMARY OF THE INVENTION
 It is a first object of the invention to provide a magnetic sensor
 apparatus and a current sensor apparatus for measuring a magnetic field or
 an electric current having a specific magnitude with accuracy.
 It is a second object of the invention to provide a magnetic sensor
 apparatus, a current sensor apparatus and a magnetic sensor element that
 easily achieve an increase in a range of magnetic fields or currents to be
 measured.
 It is a third object of the invention to provide a magnetic sensor
 apparatus, a current sensor apparatus and a magnetic sensor element for
 easily measuring a large magnetic field or electric current.
 A first magnetic sensor apparatus or a first current sensor apparatus of
 the invention comprises: a magnetic detector for outputting a signal
 responsive to a magnetic field applied thereto in response to a magnetic
 field to be measured; a negative feedback means for generating a negative
 feedback magnetic field used for negative feedback of an output of the
 magnetic detector to the magnetic detector; and a magnetic substance
 placed around the magnetic detector or forming part of the magnetic
 detector and having a demagnetizing factor with respect to the magnetic
 field to be measured and a demagnetizing factor with respect to the
 negative feedback magnetic field different from each other.
 According to the first magnetic sensor apparatus or current sensor
 apparatus of the invention, the magnetic substance is placed around the
 magnetic detector or forms part of the magnetic detector. The magnetic
 substance has a demagnetizing factor with respect to the magnetic field to
 be measured and a demagnetizing factor with respect to the negative
 feedback magnetic field different from each other. As a result, the
 magnitude of the negative feedback magnetic field is made different from
 the magnitude of the magnetic field to be measured.
 In the first magnetic sensor apparatus or current sensor apparatus, the
 magnetic substance may be placed around the magnetic detector and have a
 cavity in which the magnetic detector is placed. The magnetic detector may
 be placed in the cavity of the magnetic substance.
 In the first magnetic sensor apparatus or current sensor apparatus, the
 magnetic detector may have a magnetic core and a coil wound around the
 core and provided for detecting the magnetic field to be measured. The
 magnetic substance may be the magnetic core forming part of the magnetic
 detector. In the present invention the magnetic core is a core made of a
 magnetic substance having a magnetic saturation property on which the coil
 is wound.
 A second magnetic sensor apparatus or a second current sensor apparatus of
 the invention comprises: a magnetic detector for outputting a signal
 responsive to a magnetic field applied thereto in response to a magnetic
 field to be measured; and a magnetic substance having a cavity in which
 the magnetic detector is placed. The magnetic detector is placed in the
 cavity of the magnetic substance. The ratio between the magnetic field to
 be measured and the magnetic field applied to the magnetic detector is set
 at a specific value, based on at least one of a first demagnetizing factor
 depending on the shape of the magnetic substance and a second
 demagnetizing factor depending on the shape of the cavity.
 According to the second magnetic sensor apparatus or current sensor
 apparatus of the invention, the ratio between the magnetic field to be
 measured and the magnetic field applied to the magnetic detector is set at
 a specific value, based on at least one of the first demagnetizing factor
 depending on the shape of the magnetic substance and the second
 demagnetizing factor depending on the shape of the cavity. As a result,
 the value of the magnetic field applied to the magnetic detector falls
 within the measurement range of the magnetic detector.
 In the second magnetic sensor apparatus or current sensor apparatus, the
 cavity may have an opening that opens toward a direction intersecting a
 direction of passage of a magnetic flux generated by the magnetic field to
 be measured.
 In the second magnetic sensor apparatus or current sensor apparatus, the
 magnetic detector may have a detection sensitivity with a high sensitivity
 direction and may be placed in the cavity such that the high sensitivity
 direction coincides with a direction of passage of a magnetic flux
 generated by the magnetic field to be measured.
 The second magnetic sensor apparatus or current sensor apparatus may
 further comprise a negative feedback field application means for applying
 a negative feedback magnetic field used for negative feedback of an output
 of the magnetic detector to the magnetic detector. In this case, the
 negative feedback field application means may be placed in the cavity such
 that a demagnetizing factor of the magnetic substance with respect to the
 magnetic field to be measured is different from a demagnetizing factor of
 the magnetic substance with respect to the negative feedback magnetic
 field.
 The second magnetic sensor apparatus or current sensor apparatus may
 further comprise a reference field application means for applying a
 reference alternating magnetic field to the magnetic detector. The
 reference field is used for controlling a property of the magnetic
 detector with respect to the magnetic field to be measured. In this case,
 the reference field application means may be placed outside the magnetic
 substance.
 A third magnetic sensor apparatus or a third current sensor apparatus of
 the invention comprises: a fluxgate magnetic sensor element having a
 magnetic core and a coil wound around the core, the coil being provided
 for detecting an applied magnetic field to be measured; and a detection
 means for detecting the magnetic field to be measured by detecting
 variations in inductance of the coil. The magnetic core has such a shape
 that a demagnetizing factor thereof with respect to the magnetic field to
 be measured is different from a demagnetizing factor thereof with respect
 to a magnetic field generated by the coil.
 According to the third magnetic sensor apparatus or current sensor
 apparatus of the invention, the demagnetizing factor with respect to the
 magnetic field to be measured is different from the demagnetizing factor
 with respect to the magnetic field generated by the coil. As a result,
 when a negative feedback current is supplied to the coil, it is possible
 to make the current different from the current that would be supplied when
 the two demagnetizing factors are equal.
 In the third magnetic sensor apparatus or current sensor apparatus, the
 magnetic core may have such a shape that the demagnetizing factor thereof
 with respect to the magnetic field to be measured is greater than the
 demagnetizing factor thereof with respect to the magnetic field generated
 by the coil.
 In the third magnetic sensor apparatus or current sensor apparatus, the
 magnetic core may form an open magnetic circuit with respect to both the
 magnetic field to be measured and the magnetic field generated by the
 coil.
 In the third magnetic sensor apparatus or current sensor apparatus, the
 magnetic core may form an open magnetic circuit with respect to the
 magnetic field to be measured and form a closed magnetic circuit with
 respect to the magnetic field generated by the coil.
 The third magnetic sensor apparatus or current sensor apparatus may further
 comprise a negative feedback means for having the coil generate a negative
 feedback magnetic field used for negative feedback of an output of the
 detection means, by supplying a negative feedback current to the coil. The
 negative feedback current is used for negative feedback of the output of
 the detection means.
 A fluxgate magnetic sensor element of the invention comprises a magnetic
 core and a coil wound around the core. The coil is provided for detecting
 an applied magnetic field to be measured. The magnetic core has such a
 shape that a demagnetizing factor thereof with respect to the magnetic
 field to be measured is different from a demagnetizing factor thereof with
 respect to a magnetic field generated by the coil.
 According to the magnetic sensor element of the invention, the
 demagnetizing factor with respect to the magnetic field to be measured is
 different from the demagnetizing factor with respect to the magnetic field
 generated by the coil. As a result, when a negative feedback current is
 supplied to the coil, it is possible to make the current different from
 the current that would be supplied when the two demagnetizing factors are
 equal.
 In the magnetic sensor element, the magnetic core may have such a shape
 that the demagnetizing factor thereof with respect to the magnetic field
 to be measured is greater than the demagnetizing factor thereof with
 respect to the magnetic field generated by the coil.
 In the magnetic sensor element, the magnetic core may form an open magnetic
 circuit with respect to both the magnetic field to be measured and the
 magnetic field generated by the coil.
 In the magnetic sensor element, the magnetic core may form an open magnetic
 circuit with respect to the magnetic field to be measured and form a
 closed magnetic circuit with respect to the magnetic field generated by
 the coil.
 Other and further objects, features and advantages of the invention will
 appear more fully from the following description.

BEST MODES FOR CARRYING OUT THE INVENTION
 Preferred embodiments of the invention will now be described in detail with
 reference to the accompanying drawings.
 FIRST EMBODIMENT
 FIG. 1 is an explanatory view for illustrating the configuration of a
 magnetic sensor apparatus of a first embodiment of the invention.
 The magnetic sensor apparatus comprises a magnetic detector 101 that
 outputs a signal corresponding to a magnetic field and a magnetic
 substance 110 having a cavity 111 in which the magnetic detector 101 is
 placed. The magnetic detector 101 is placed in the cavity 111 of the
 magnetic substance 110. The ratio between magnetic field H to be measured
 and a magnetic field applied to the magnetic detector 101 is set at a
 specific value, based on at least one of a first demagnetizing factor
 depending on the shape of the magnetic substance 110 and a second
 demagnetizing factor depending on the shape of the cavity 111.
 The cavity 111 may have an opening that opens toward a direction
 intersecting the direction of passage of a magnetic flux generated by
 magnetic field H, such as toward the direction orthogonal to the direction
 of passage of the flux. In this case, the opening may be blocked with a
 magnetic substance other than the magnetic substance 110, if necessary.
 The magnetic detector 101 may have a detection sensitivity having a
 dependence on an angle with respect to the applied magnetic field and
 having a high sensitivity direction. In this case, it is preferred that
 the magnetic detector 101 is placed in the cavity 111 such that the high
 sensitivity direction coincides with the direction of passage of a
 magnetic flux generated by magnetic field H.
 The magnetic sensor apparatus further comprises a feedback coil 112 as a
 negative feedback field application means (a negative feedback means) that
 applies a negative feedback magnetic field to the magnetic detector 101.
 The negative feedback magnetic field is used for negative feedback of an
 output of the magnetic detector 101. The feedback coil 112 is, for
 example, placed in the cavity 111 and wound around the detector 101. If
 the coil 112 is placed in the cavity 111, the demagnetizing factor of the
 magnetic substance 110 with respect to the magnetic field to be measured
 is different from the demagnetizing factor thereof with respect to the
 negative feedback magnetic field. It is not necessary that the coil 112 is
 applied to the detector 101. Furthermore, it is not necessary that the
 negative feedback field application means is coil-shaped as the coil 112
 as long as the negative feedback field is applied to the detector 101.
 The magnetic sensor apparatus further comprises a reference magnetic field
 coil 113 as a reference field application means for applying a reference
 alternating magnetic field to the magnetic detector 101. The alternating
 magnetic field is used for controlling the property of the magnetic
 detector 101 with regard to magnetic field H to be measured. The coil 113
 is, for example, placed outside the magnetic substance 110 and wound
 around the magnetic substance 110. It is not necessary that the coil 113
 is placed directly around the magnetic substance 110 but the coil 113 may
 be placed in the magnetic path of the field applied to the magnetic
 substance 110.
 The magnetic detector 101 may be chosen from various types of magnetic
 sensor elements including a Hall element, an MR element, a GMR element
 such as a spin valve element, and a fluxgate element. If a Hall element is
 used, some means, such as direct current amplification, are required since
 an output of the Hall element is small.
 Magnetic detector connection lines 121 are connected to both ends of the
 magnetic detector 101. Feedback coil connection lines 122 are connected to
 both ends of the feedback coil 112. Reference field coil connection lines
 123 are connected to both ends of the reference field coil 113. A
 processing circuit 124 is connected to the lines 121. The processing
 circuit 124 processes an output signal of the detector 101 and outputs a
 signal in response to the magnetic field to be measured to an output
 terminal 127. A feedback current source 125 for supplying a feedback
 current to the coil 112 is connected to the lines 122. The feedback
 current supplied from the source 125 is controlled by the processing
 circuit 124. An alternating current source 126 for supplying a specific
 alternating current to the coil 113 is connected to the lines 123.
 If the cavity 111 has an opening, the magnetic detector connection lines
 121 and the feedback coil connection lines 122 may be drawn from the
 opening to outside. If the opening is blocked, the lines may be drawn out
 through the use of known techniques such as forming a conductor pattern in
 the magnetic substance 110.
 The operation of the magnetic sensor apparatus shown in FIG. 1 including
 the processing circuit 124, the feedback current source 125 and the
 alternating current source 126 will now be described. A magnetic field
 having a specific ratio to magnetic field H to be measured as described
 later is applied to the magnetic detector 101 placed in the cavity 111 of
 the magnetic substance 110. The processing circuit 124 processes an output
 signal of the detector 101 and outputs a signal responsive to the magnetic
 field to be measured to the output terminal 127. The processing circuit
 124 controls the feedback current source 125 such that a feedback current
 responsive to the output signal of the detector 101 is supplied from the
 feedback current source 125 to the feedback coil 112. In this manner the
 coil 112 generates a magnetic field in the direction opposite to the
 direction of the magnetic field applied to the detector 101 in response to
 the field to be measured. The magnetic field thus generated by the coil
 112 has an absolute value equal to that of the field applied to the
 detector 101. Control is thus performed to keep the magnetic field applied
 to the detector 101 constantly nearly zero. Inconsistent sensitivity and
 output variations due to the dependence on temperature of the detector 101
 are thereby reduced.
 The alternating current source 126 supplies a specific alternating current
 to the reference field coil 113. An alternating magnetic field is then
 generated by the coil 113. A reference alternating magnetic field
 responsive to the alternating magnetic field is superposed on the magnetic
 field responsive to the field to be measured, and then applied to the
 detector 101. The processing circuit 124 outputs a signal produced through
 eliminating reference alternating magnetic field component from the output
 signal of the detector 101. The processing circuit 124 extracts the
 reference alternating magnetic field component from the output signal of
 the detector 101, and adjusts an output signal of the processing circuit
 124 such that the magnitude of the reference alternating field component
 is kept constant. The measurement accuracy of the magnetic sensor
 apparatus is thereby improved.
 The feedback coil 112 and the reference field coil 113 are provided for
 improving the measurement accuracy of the magnetic sensor apparatus, such
 as improving linearity or the dependence of output on temperature.
 Therefore, both or either of the coils 112 and 113 may be eliminated in
 some cases.
 FIG. 2 shows an example of a method of forming the cavity 111 in the
 magnetic substance 110. In this example a shallow concave recess to be the
 cavity 111 is formed in one surface of a first magnetic substance 110A
 having the shape of a rectangular solid, for example. The magnetic
 detector 101 is placed in the recess, and the recess is then blocked with
 a second magnetic substance 110B that is plate-shaped, for example. The
 blocked cavity 111 is thus formed. In this case, the magnetic substance
 110 is made up of the first magnetic substance 110A and the second
 magnetic substance 110B.
 FIG. 3 shows another example of the method of forming the cavity 111 in the
 magnetic substance 110. In this example the cavity 111 is formed through
 making an opening having a specific cross-sectional shape in the magnetic
 substance 110 having the shape of a rectangular solid, for example. The
 opening opens toward the direction orthogonal to the direction of passage
 of the magnetic flux generated by the magnetic field to be measured. The
 magnetic detector 101 is placed in the cavity 111, and the opening of the
 cavity 111 may be then blocked with another magnetic substance, if
 necessary.
 If ferrite is used for the magnetic substance 110, ferrite powder formed
 into a desired shape may be baked to make the magnetic substance 110
 having the cavity 111. No extra processing costs are required in this
 case.
 The following is a description of one of the features of the magnetic
 sensor apparatus of this embodiment that the ratio between magnetic field
 H to be measured and a magnetic field applied to the magnetic detector 101
 is set at a specific value, based on at least one of the first
 demagnetizing factor depending on the shape of the magnetic substance 110
 and the second demagnetizing factor depending on the shape of the cavity
 111.
 At both ends of a magnetic substance placed in a magnetic field, magnetic
 poles opposite to the direction of the magnetic field are induced. As a
 result, the field inside the magnetic substance has a value obtained by
 subtracting the field generated by the induced poles from the external
 field, which is smaller than the value of the external field. The ratio of
 the field inside the magnetic substance decreasing with respect to the
 external field is represented by the factor known as a demagnetizing
 factor or a self-demagnetizing factor. The demagnetizing factor of a
 magnetic substance is determined by the shape of the substance only. For
 example, the demagnetizing factor of a thin and long stick-shaped magnetic
 substance placed parallel to the external magnetic field is nearly zero.
 The demagnetizing factor of a thin plate-shaped magnetic substance placed
 orthogonal to the external field is nearly 1. Consequently, the internal
 magnetic field of the thin and long stick-shaped magnetic substance placed
 parallel to the external field is nearly equal to the external field. The
 internal field of the thin plate-shaped magnetic substance placed
 orthogonal to the external field is equal to the external field multiplied
 by one fraction of the relative permeability of the magnetic substance. In
 the present invention the demagnetizing factor of the magnetic substance
 depending on the shape of the substance is called the first demagnetizing
 factor. If the shape of the magnetic substance is suitably designed and
 the first demagnetizing factor is set at a desired value, the external
 field is converted to the field inside the magnetic substance through
 multiplication by a factor in the range between 1 and one fraction of the
 relative permeability of the magnetic substance.
 In contrast, in the cavity of the magnetized magnetic substance, the
 magnetic field generated by the poles induced by the wall of the cavity
 has a direction equal to that of the field inside the magnetic substance.
 Therefore, this field generated by the poles functions such that the field
 inside the cavity is greater than the field inside the magnetic substance.
 In the invention the ratio of the field inside the cavity becoming greater
 than the field inside the magnetic substance is called the demagnetizing
 factor, too. The demagnetizing factor of the cavity depends on the shape
 of the cavity. For example, the demagnetizing factor of a thin and long
 tube-shaped cavity placed parallel to the magnetic field inside the
 magnetic substance is nearly zero. The demagnetizing factor of a thin
 slit-shaped cavity placed orthogonal to the field inside the magnetic
 substance is nearly 1. Consequently, the magnetic field inside the thin
 and long tube-shaped cavity placed parallel to the field inside the
 magnetic substance is nearly equal to the field inside the magnetic
 substance. The magnetic field inside the thin slit-shaped cavity placed
 orthogonal to the field inside the magnetic substance is equal to the
 field inside the magnetic substance multiplied by the relative
 permeability of the magnetic substance. In the invention the demagnetizing
 factor of the cavity depending on the shape of the cavity is called the
 second demagnetizing factor. If the shape of the cavity is suitably
 designed and the second demagnetizing factor is set at a desired value,
 the field inside the magnetic substance is converted to the field inside
 the cavity through multiplication by a factor in the range between 1 and
 the relative permeability of the magnetic substance.
 As described so far, the shapes of the magnetic substance 110 and the
 cavity 111 are appropriately designed, so that the first and second
 demagnetizing factors are set at desired values. As a result, the ratio
 between magnetic field H to be measured and the magnetic field applied to
 the detector 1 is set at a specific value. It is also possible to set one
 of the first and second demagnetizing factors at a desired value, so that
 the ratio between magnetic field H and the magnetic field applied to the
 detector 1 is set at a specific value. However, it is preferred to set
 both of the first and second demagnetizing factors at appropriate values
 since the degrees of freedom of design of the shapes of the magnetic
 substance 110 and the cavity 111 are thereby increased.
 The following is a further description specifically illustrating the
 feature that the ratio between magnetic field H to be measured and a
 magnetic field applied to the magnetic detector 1 is set at a specific
 value, based on at least one of the first and second demagnetizing
 factors.
 The relative permeability of the magnetic substance 110 having the cavity
 111 is .mu.s. The first demagnetizing factor is N.sub.m and the second
 demagnetizing factor is N.sub.k. If the magnetic substance 110 is placed
 in magnetic field H, magnetic field H.sub.m inside the magnetic substance
 110 is represented by equation (1) below.
EQU H.sub.m =H/{1+N.sub.m (.mu..sub.s -1)} (1)
 If the cavity 111 is sufficiently smaller than the magnetic substance 110,
 magnetic field H.sub.k inside the cavity 111 is represented by equation
 (2) below.
EQU H.sub.k =H.sub.m {1+N.sub.k (.mu..sub.s -1)} (1)
 Consequently, from equations (1) and (2), magnetic field H.sub.k inside the
 cavity 111 is represented by equation (3) below.
EQU H.sub.k =H{1+N.sub.k (.mu..sub.s -1)}/{1+N.sub.m (.mu..sub.s -1)} (3)
 It is noted from equation (3) that if the shapes of the magnetic substance
 110 and the cavity 111 are suitably designed so that first demagnetizing
 factor Nm and second demagnetizing factor N.sub.k are set at desired
 values, the ratio between magnetic field H to be measured and magnetic
 field H.sub.k applied to the detector 101 placed in the cavity 111 is set
 at a specific value. It is thereby possible to measure a high magnetic
 field exceeding the measurement range of the detector 101 itself, in
 particular. Measurement of a large electric current is achieved, too,
 through incorporating this magnetic sensor apparatus into a current sensor
 apparatus.
 In practice, the second demagnetizing factor is more complicated since the
 magnetic flux distribution inside the magnetic substance 110 is changed
 due to the presence of the cavity 111. However, since it is no different
 from representing the nature of the invention, the following description
 is given, assuming that the above-mentioned equations (1) to (3) hold.
 If it is only required to make the magnetic field applied to the magnetic
 detector 101 smaller than the magnetic field to be measured, a method may
 be taken to shunt the magnetic flux such that only part of the flux passes
 through the detector 101. However, this method has problems that leakage
 flux often results and that the detector 101 is sensitive to the effect of
 a noise magnetic field. In contrast, this embodiment has advantages much
 more than those of the method of shunting the flux. That is, the
 advantages of the embodiment are: it is not necessary to consider leakage
 flux; the conversion rate of the magnetic field may be set at any value;
 the detector 101 is magnetically shielded by the magnetic substance 110
 and stable against the noise field; and so on.
 The effect produced when the cavity 111 has an opening will now be
 described. Even if the cavity 111 is not completely closed, the second
 demagnetizing factor is determined by the ratio between the
 cross-sectional area of the cavity 111 orthogonal to the direction of the
 magnetic field to be measured and the length of the cavity 111 in the
 direction of the field to be measured. Therefore, it is possible to form
 at least one opening, used for placing the detector 101 in the cavity 111,
 in a direction intersecting the field to be measured or the direction
 orthogonal to the field, for example. Placing the detector 101 is thus
 easy performed by forming the opening in such a manner.
 The effect produced when the magnetic detector 101 having detection
 sensitivity with a high sensitivity direction is used will now be
 described. If the cavity 111 is completely closed, it is natural that the
 detector 101 be entirely surrounded by the magnetic substance 110 and cut
 off from the external noise field. The operation thereof is thereby
 stabilized. If the cavity 111 has an opening, the detector 101 is
 surrounded by the magnetic substance 110 except the opening, so that the
 detector 101 is cut off from the external noise field and the operation
 thereof is stabilized. If the cavity 111 has an opening, the resistance to
 the noise field is further improved if the detector 101 having detection
 sensitivity with a high sensitivity direction is used, and detector 101 is
 placed such that the high sensitivity direction coincides with the
 direction of passage of the magnetic flux generated by field H to be
 measured and a low sensitivity direction faces toward the opening of the
 cavity 111.
 The following is a description of the effect produced by the feature that
 the magnetic sensor apparatus of the embodiment comprises the feedback
 coil 112 as the negative feedback field application means that applies a
 negative feedback magnetic field to the magnetic detector 101. The
 negative feedback magnetic field is used for negative feedback of an
 output of the detector 101. In prior art it is difficult to apply the
 negative feedback method very effective for improving linearity or the
 dependence of output on temperature to a current sensor apparatus for
 measuring a large direct current. This is because magnetomotive force
 generated by a feedback current and having a magnitude equal to
 magnetomotive force generated by a magnetic field to be measured is
 required.
 However, according to the magnetic sensor apparatus of the embodiment, the
 magnetic field applied to the detector 101 is made smaller than the field
 to be measured. As a result, the negative feedback field is made smaller
 than the field to be measured. For example, if the current to be measured
 is 100 amperes (A) and a gap of 10 mm is formed in the magnetic yoke
 interlinking the current to be measured, the magnetic field inside the gap
 is 10000 A/m. If the relative permeability of the yoke is 1000, the
 magnetic field inside the yoke is 10 A/m, that is, 1/1000 of the field
 inside the gap, which results from the continuity of the magnetic flux
 density.
 Assuming that the cavity 111 having a demagnetizing factor of N.sub.k is
 formed in the yoke, the magnetic field inside the cavity 111 is
 10.times.(1+N.sub.k.times.999) from equation (2). If N.sub.k =0.02 (which
 corresponds to the case where the ratio between the diameter of cross
 section and the length of the cavity 111 is about 10), the field inside
 the cavity 111 is approximately 210 A/m. Accordingly, the negative
 feedback method is applicable if a means is provided for the magnetic
 detector 101 for generating a magnetic field that is enough to cancel the
 field inside the cavity 111.
 It is possible to generate a field of 210 A/m by passing a current of 21 mA
 through a solenoid coil (10000 turns/m) on which an insulating copper wire
 having a diameter of 0.1 mm is tightly wound. According to this
 embodiment, it is thus possible to apply the negative feedback method
 having a remarkable effect of improving characteristics without requiring
 a large feedback current. Measurement accuracy is thereby improved.
 Although the cavity 111 is directly formed in the magnetic yoke in the
 foregoing description, it is possible to form the magnetic substance 110
 having the cavity 111 besides the yoke, place the detector 101 in the
 cavity 111, and combine the magnetic substance 110 with the yoke to make a
 magnetic path.
 The following is a description of the effect resulting from the feature
 that the magnetic sensor apparatus of the embodiment comprises the
 reference field coil 113 as the reference field application means for
 applying a reference alternating magnetic field to the magnetic detector
 101. The alternating magnetic field is used for controlling the properties
 of the magnetic detector 101 with regard to magnetic field H to be
 measured. Although the properties of the detector 101 are substantially
 improved by applying the above-mentioned negative feedback method, various
 types of variations that affect the measurement accuracy are not
 compensated, such as variations in the demagnetizing factor due to
 variations in dimensions of the cavity 111, or variations in magnetic
 fields applied to the detector 101 due to variations in dimensions of the
 gap of the yoke.
 However, the alternating current superposing method is applicable since the
 linearity of the detector 101 is ensured by applying the negative feedback
 method. For using the alternating current superposing method, for example,
 the reference field coil 113 is provided on the periphery of the magnetic
 substance 110 as shown in FIG. 1, or the coil 113 is provided in the
 magnetic yoke, and a specific alternating current is supplied to the coil
 113 to generate an alternating current magnetic field from the coil 113. A
 reference alternating magnetic field corresponding to this alternating
 magnetic field is applied to the magnetic detector 101. The processing
 circuit 124 extracts a reference alternating field component from an
 output signal of the detector 101 and adjusts an output signal of the
 processing circuit 124 such that the magnitude of the reference
 alternating field component is kept constant. It is thereby possible to
 completely compensate for various types of variations mentioned above that
 affect the measurement accuracy, such as variations in the demagnetizing
 factor due to variations in the dimensions of the cavity 111, or
 variations in magnetic fields applied to the detector 101 due to
 variations in the dimensions of the gap of the yoke. The measurement
 accuracy of the magnetic sensor is therefore improved.
 The effects of the magnetic sensor apparatus of the embodiment including
 the foregoing description are summarized as follows.
 According to the magnetic sensor apparatus, the ratio between magnetic
 field H to be measured and the magnetic field applied to the detector 101
 is set at a specific value, based on at least one of the first and second
 demagnetizing factors. It is thereby possible to measure a magnetic field
 exceeding the measurement range of the detector 101. Measurement of a high
 magnetic field and a large electric current is achieved, in particular.
 According to the magnetic sensor apparatus, the negative feedback method or
 the alternating current superposing method is easily applied. It is
 thereby possible to improve the linearity or the dependence of output on
 temperature if necessary and to improve the measurement accuracy. In
 addition, no large feedback current is required if the feedback coil 112
 as the negative feedback field application means is provided in the cavity
 111.
 According to the magnetic sensor apparatus, the detector 101 is surrounded
 by the magnetic substance 110, so that it is cut off from an external
 noise field and the operation is stabilized.
 According to the magnetic sensor apparatus, it is possible that the
 detector 101 is made up of a magnetic sensor element that is not
 applicable to prior-art apparatuses since the measurement range thereof is
 not suitable for magnetic fields to be measured. It is therefore possible
 to use a magnetic sensor element that is not applicable to prior-art
 apparatuses with regard to the measurement range although the element
 ensures the monotonicity of output with respect to variations in field to
 be measured. If such an element is used, there is no possibility that the
 negative feedback system would run away although the negative feedback
 method is applied.
 Similarly, according to the magnetic sensor apparatus, it is possible to
 use a magnetic sensor element that is not applicable to prior-art
 apparatuses with regard to the measurement range although the element
 produces a large output. If such an element is used, it is possible to
 implement a magnetic sensor apparatus that produces a large output and is
 less affected by drifts.
 The magnetic sensor apparatus basically has the configuration in which the
 detector 101 is placed in the cavity 111 of the magnetic substance 110. It
 is therefore possible to provide the magnetic sensor apparatus having a
 simple structure and high accuracy at low costs. In particular, if the
 cavity 111 having an opening is formed in the magnetic substance 110, it
 is easy to place the detector 101. It is therefore possible to provide the
 magnetic sensor apparatus at lower costs. Through application of the
 alternating current superposing method, it is possible, without mechanical
 adjustment, to compensate for various types of variations that would
 affect the measurement accuracy, such as variations in the demagnetizing
 factor due to variations in the dimensions of the cavity 111, or
 variations in magnetic fields applied to the detector 101 due to
 variations in the dimensions of the gap of the yoke. The magnetic sensor
 apparatus with high accuracy is thereby provided at low costs.
 SECOND EMBODIMENT
 A magnetic sensor apparatus of a second embodiment of the invention will
 now be described with reference to FIG. 4. The magnetic sensor apparatus
 of this embodiment is designed for high magnetic field measurement. The
 apparatus comprises the magnetic detector 101 that outputs a signal
 responsive to a magnetic field and the magnetic substance 110 having the
 cavity 111 in which the magnetic detector 101 is placed. The magnetic
 detector 101 is placed in the cavity 111 of the magnetic substance 110.
 The ratio between magnetic field H to be measured and a magnetic field
 applied to the magnetic detector 101 is set at a specific value, based on
 at least one of the first demagnetizing factor depending on the shape of
 the magnetic substance 110 and the second demagnetizing factor depending
 on the shape of the cavity 111.
 The magnetic sensor apparatus further comprises the feedback coil 112 as
 the negative feedback field application means that applies a negative
 feedback magnetic field to the magnetic detector 101. The negative
 feedback magnetic field is used for negative feedback of an output of the
 magnetic detector 101. The feedback coil 112 is placed in the cavity 111
 and wound around the detector 101.
 The magnetic sensor apparatus further comprises the reference field coil
 113 as the reference field application means for applying a reference
 alternating magnetic field to the magnetic detector 101. The alternating
 magnetic field is used for controlling the property of the magnetic
 detector 101 with regard to magnetic field H to be measured. The coil 113
 is placed outside the magnetic substance 110 and wound around the magnetic
 substance 110.
 For the magnetic sensor apparatus of the embodiment, magnetic field H.sub.k
 inside the cavity 111 is 0.042 H from equation (3) where relative
 permeability .mu..sub.s of the magnetic substance 110 is 1000, first
 demagnetizing factor N.sub.m is 0.5, and second demagnetizing factor
 N.sub.k is 0.02.
 That is, the magnetic field applied to the detector 101 is 4.2 percent of
 magnetic field H to be measured. In other words, the magnetic sensor
 apparatus measures a magnetic field approximately 24 times as high as the
 maximum measurable field of the detector 101.
 For the magnetic sensor apparatus, it is enough that the magnetomotive
 force of the feedback coil 112 is 4.2 percent of the magnetic field to be
 measured. The magnetomotive force of the reference field coil 113 of about
 1 percent of the field to be measured is acceptable. Accordingly, the
 magnetic sensor apparatus of the embodiment is practical since the current
 consumption of each of the coils 112 and 113 is very low.
 The remainder of configuration, operation and effects of the second
 embodiment are similar to those of the first embodiment.
 THIRD EMBODIMENT
 Reference is now made to FIG. 5 and FIG. 6 to describe a magnetic sensor
 apparatus of a third embodiment of the invention. The apparatus of this
 embodiment is an example in which the first demagnetizing factor depending
 on the shape of the magnetic substance is only set at a desired value, so
 that the ratio between the magnetic field to be measured and the magnetic
 field applied to the magnetic detector is set at a specific value.
 FIG. 5 is an explanatory view showing an example of the configuration of
 the magnetic sensor apparatus of the embodiment. In this apparatus the
 magnetic detector 101 is placed in the cavity 111 of the magnetic sensor
 apparatus of the second embodiment, and the gap of the cavity 111 is then
 filled with a magnetic substance 115 such as a magnetic coating material.
 In the magnetic sensor apparatus of the third embodiment, the magnetic
 field inside the magnetic substance 110 is applied to the detector 101.
 This magnetic field is determined by external magnetic field H and first
 demagnetizing factor N.sub.m depending on the shape of the magnetic
 substance 110.
 FIG. 6 is an explanatory view showing another example of the configuration
 of the magnetic sensor apparatus of the embodiment. In this apparatus a
 single magnetic substance 130 is provided in place of the magnetic
 substances 110 and 115 of FIG. 5. The detector 101 is embedded inside the
 magnetic substance 130. The magnetic sensor apparatus having such a
 configuration is obtained through, for example, forming the magnetic
 substance 130 having a specific shape and made of a compound material of a
 resin and a magnetic material while the detector 101 is embedded inside
 the magnetic substance 130.
 The remainder of configuration, operation and effects of the third
 embodiment are similar to those of the second embodiment.
 FOURTH EMBODIMENT
 Reference is now made to FIG. 7 to describe a current sensor apparatus of a
 fourth embodiment of the invention. This current sensor apparatus
 incorporates a magnetic sensor apparatus of this embodiment. Although the
 current sensor apparatus will be mainly described, the following
 description applies to the magnetic sensor apparatus, too.
 The current sensor apparatus of the embodiment comprises: a magnetic yoke
 142 that forms a ring a portion of which is cut off and surrounds a
 conductor 141 through which an electric current-to be measured passes; and
 the magnetic substance 110 placed in the portion of the yoke 142 cut off.
 The magnetic substance 110 has the cavity 111 in which the magnetic
 detector 101 is placed.
 According to the current sensor apparatus, the ratio between the magnetic
 field to be measured generated by the current to be measured and the
 magnetic field applied to the magnetic detector 101 is set at a specific
 value, based on at least one of the first demagnetizing factor depending
 on the shape of the magnetic substance 110 and the second demagnetizing
 factor depending on the shape of the cavity 111.
 The current sensor apparatus further comprises the feedback coil 112 as the
 negative feedback field application means that applies a negative feedback
 magnetic field to the magnetic detector 101. The negative feedback
 magnetic field is used for negative feedback of an output of the magnetic
 detector 101. The feedback coil 112 is placed in the cavity 111 and wound
 around the detector 101.
 The current sensor apparatus further comprises the reference magnetic field
 coil 113 as the reference field application means for applying a reference
 alternating magnetic field to the magnetic detector 101. The alternating
 magnetic field is used for controlling the property of the magnetic
 detector 101 with regard to the magnetic field to be measured. The coil
 113 is wound around part of the magnetic yoke 142.
 The length of each of the gaps between ends of the magnetic substance 110
 and the yoke 142 is G.sub.1 and G.sub.2, respectively. The sum of the
 lengths of the gaps is G=G.sub.1 +G.sub.2.
 The operation of the current sensor apparatus of the embodiment will now be
 described. In the apparatus, a magnetic field is generated by an electric
 current to be measured flowing through the conductor 141 in the direction
 orthogonal to the drawing sheet. This magnetic field is called a magnetic
 field to be measured in this embodiment. The magnetic field to be measured
 is applied to the magnetic substance 110. A magnetic field having a
 specific ratio to the magnetic field to be measured is applied to the
 magnetic detector 101. The magnitude of the magnetic field to be measured
 changes in response to the magnitude of the current to be measured. The
 direction of the magnetic field to be measured changes in response to the
 direction of the current to be measured, too. The current sensor apparatus
 indirectly measures the current through measuring the magnetic field
 generated by the current to be measured. If the apparatus shown in FIG. 7
 is used as the magnetic sensor apparatus, the apparatus directly measures
 the magnetic field.
 In the current sensor apparatus of this embodiment, it is unpractical to
 make the magnetic yoke 142 and the magnetic substance 110 too large-sized.
 In addition, there is a limit to reducing the size of the cavity 111 since
 the magnetic detector 101 requires to have certain dimensions. Therefore,
 it is not always possible to determine the shape of the magnetic substance
 110 as desired and to determine first demagnetizing factor N.sub.m as
 desired.
 However, even though first demagnetizing factor N.sub.m depending on the
 shape of the magnetic substance 110 is small, the yoke 142 and the
 magnetic substance 110 nearly forms a closed magnetic circuit with gaps
 G.sub.1 and G.sub.2. If leakage flux is ignored, the magnetic field inside
 the magnetic substance 110 is one fraction of the relative permeability of
 the magnetic substance 110 if the relative permeability of the magnetic
 substance 110 is sufficiently greater than 1. This is equal to the fact
 that first demagnetizing factor N.sub.m is 1.
 Where the relative permeability of the magnetic substance 110 is
 .mu..sub.sm and the relative permeability of the magnetic yoke 142 is
 .mu..sub.sy, the relation holds that .mu..sub.sm &gt;&gt;1 and .mu..sub.sy
 &gt;&gt;1. Where the magnetic field inside the magnetic substance 110 is
 H.sub.m and the magnetic field in the gap having a length of G is H.sub.g,
 the relation holds that H.sub.g =1/G and H.sub.m =H.sub.g /.mu..sub.sm.
 Therefore, field H.sub.k inside the cavity 111 is expressed by the
 following equation.
EQU H.sub.k =(1/G.mu..sub.sm){1+N.sub.k (.mu..sub.sm -1)}
 Field H.sub.k inside the cavity 111 is obtained as follows where the
 current to be measured is 100 A, the total gap length G, that is, G.sub.1
 +G.sub.2 is 10 mm, second demagnetizing factor N.sub.k is 0.02 (which
 corresponds to the case where the ratio between the diameter of the cross
 section and the length of the cavity 111 is about 10), and .mu..sub.sm is
 1000.
EQU H.sub.k =209.8.apprxeq.210 A/m.
 Therefore, as the magnetic detector 101, it is possible to use a highly
 sensitive magnetic sensor element such as a spin-valve-type GMR element or
 a fluxgate element. Such a highly sensitive magnetic sensor element
 exhibits a high signal-to-noise (S-N) ratio of output and stable operation
 when the magnetic sensor element operates in the neighborhood of the
 magnetic field of zero as in the case where the negative feedback method
 is applied. Since many of spin-valve-type GMR elements and fluxgate
 elements ensure the monotonicity of output of the elements, there is no
 possibility that the feedback system would run away if such an element is
 used.
 If magnetic field H.sub.k inside the cavity 111 is 210 A/m, a negative
 feedback magnetic field of 210 A/m is required. It is possible to generate
 a field of 210 A/m by passing a current as small as 21 mA through a
 solenoid coil (100 turns/cm) on which an insulating copper wire having a
 diameter of 0.1 mm is tightly wound. The current required for the
 reference field coil 113 is 10 mA which is 1 percent of magnetomotive
 force of 100 A/m generated by the current to be measured where the number
 of turns of the coil is 100. According to the embodiment as thus
 described, it is possible to implement the current sensor apparatus with
 accuracy, stability, and the ability of reducing variations and so on that
 are equal to those of prior-art apparatuses, through the use of a smaller
 feedback current and a smaller alternating current for alternating current
 superimposing, compared to the prior-art apparatuses that require a large
 feedback current when the negative feedback method is used.
 The remainder of configuration, operation and effects of the fourth
 embodiment are similar to those of the first embodiment.
 FIFTH EMBODIMENT
 Reference is now made to FIG. 8 to describe a current sensor apparatus of a
 fifth embodiment of the invention. The apparatus of this embodiment is an
 example in which the second demagnetizing factor depending on the shape of
 the cavity in the magnetic substance is only set at a desired value, so
 that the ratio between the magnetic field to be measured and the magnetic
 field applied to the magnetic detector is set at a specific value.
 In the current sensor apparatus of this embodiment, the magnetic substance
 110 of the current sensor apparatus shown in FIG. 7 is replaced with a
 magnetic substance 150 that does not have a cavity and a magnetic detector
 inside. In this apparatus the cavity 111 is formed inside the magnetic
 yoke 142 of the current sensor apparatus shown in FIG. 7. The magnetic
 detector 101 is placed in the cavity 111. The yoke 142 corresponds to a
 magnetic substance having a cavity of the invention.
 In the current sensor apparatus the magnetic field inside the cavity 111 is
 applied to the detector 101. This magnetic field is determined by the
 magnetic field to be measured corresponding to the current to be measured
 and second demagnetizing factor N.sub.k depending on the shape of the
 cavity 111.
 The remainder of configuration, operation and effects of the fifth
 embodiment are similar to those of the fourth embodiment.
 According to the magnetic sensor apparatus or the current sensor apparatus
 of the invention including the first to fifth embodiments, the ratio
 between the magnetic field to be measured and the magnetic field applied
 to the magnetic detector is set at a specific value, based on at least one
 of the first demagnetizing factor depending on the shape of the magnetic
 substance and the second demagnetizing factor depending on the shape of
 the cavity. It is thereby possible that the magnetic field applied to the
 magnetic detector falls within the measurement range of the magnetic
 detector. As a result, it is easy to use the magnetic detector having
 excellent properties and techniques for improving measurement accuracy. It
 is thus possible to measure a magnetic field or an electric current having
 a specific magnitude with accuracy. Furthermore, the magnetic detector is
 magnetically shielded by the magnetic substance, so that it is stable
 against a noise field.
 The cavity may have an opening that opens toward the direction intersecting
 the direction of passage of the magnetic flux generated by the magnetic
 field to be measured. In this case, it is easy to place the magnetic
 detector in the cavity.
 The magnetic detector may have detection sensitivity with a high
 sensitivity direction, and the detector may be placed such that the high
 sensitivity direction coincides with the direction of passage of the
 magnetic flux generated by the magnetic field to be measured. In this case
 the resistance to a noise field is further improved.
 The magnetic sensor apparatus or the current sensor apparatus may comprise
 the negative feedback field application means that applies a negative
 feedback magnetic field to the magnetic detector. The negative feedback
 magnetic field is used for negative feedback of an output of the magnetic
 detector. In this case, it is possible to improve the linearity and the
 dependence of output on temperature and to improve the measurement
 accuracy.
 If the negative feedback field application means is placed in the cavity,
 it is further possible to reduce the feedback current even when a high
 magnetic field or a large electric current is measured.
 The magnetic sensor apparatus or current sensor apparatus may comprise the
 reference field application means for applying a reference alternating
 magnetic field to the magnetic detector. The alternating magnetic field is
 used for controlling the property of the magnetic detector with regard to
 the magnetic field to be measured. In this case, it is possible to
 compensate for various types of variations that would affect the
 measurement accuracy.
 SUMMARY OF SIXTH TO EIGHTH EMBODIMENT
 A summary of sixth to eighth embodiments of the invention will now be
 described. In those embodiments a fluxgate magnetic sensor element
 comprises a magnetic core and a coil wound around the core and detecting
 an applied magnetic field to be measured. The magnetic core has a shape
 that makes the demagnetizing factor with respect to the magnetic field to
 be measured different from the demagnetizing factor with respect to the
 magnetic field generated by the coil. In those embodiments, in particular,
 the magnetic core has a shape that makes the demagnetizing factor with
 respect to the magnetic field to be measured greater than the
 demagnetizing factor with respect to the magnetic field generated by the
 coil.
 The demagnetizing factor of the magnetic core with respect to the applied
 magnetic field to be measured and the demagnetizing factor of the core
 with respect to the magnetic field generated by the coil will now be
 considered.
 Magnetic field H.sub.s inside a magnetic substance placed in parallel
 magnetic field H.sub.g is expressed by equation (4) below.
EQU H.sub.s =H.sub.g /{1+N.sub.s (.mu..sub.s -1)} (4)
 The demagnetizing factor is represented by N.sub.s and the relative
 permeability of the magnetic substance is represented by .mu..sub.s. The
 demagnetizing factor will now be briefly described. At both ends of a
 magnetic substance placed in a magnetic field, magnetic poles in the
 direction opposite to the direction of the magnetic field are induced.
 Consequently, the magnetic field inside the magnetic substance is equal to
 the value of the external magnetic field from which the magnetic field
 generated by the induced poles is subtracted, which is smaller than the
 external field. The ratio of the field inside the magnetic substance
 becoming smaller than the external field is expressed by a factor known as
 a demagnetizing factor or self-demagnetizing factor. The demagnetizing
 factor of a magnetic substance is determined only by the shape of the
 substance.
 Where a magnetic field to be measured or a magnetic field generated by a
 current to be measured is represented by H.sub.g, demagnetizing factor
 N.sub.s of a rod-like magnetic core with respect to H.sub.g is equal to
 demagnetizing factor N.sub.c with respect to the magnetic field generated
 by the coil wound around the core (which approximates to a parallel
 magnetic field for convenience). Therefore, if the negative feedback
 method is applied, a magnetic field generated by the coil through the use
 of a negative feedback current is required to be -H.sub.g. Assuming that
 `n` turns of coil is wound around width `b` (meters) of the rod-like core,
 a simple approximation holds that the magnetic field generated is ni/b
 where the coil current is `i`. Magnetic field H.sub.c inside the core is
 expressed by equation (5) below.
EQU H.sub.c =(ni/b)/{1+N.sub.c (.mu..sub.s -1)} (5)
 If the negative feedback method is applied, the relation holds that H.sub.s
 =-H.sub.c. Therefore, equation (6) below is obtained where N.sub.s
 =N.sub.c.
EQU H.sub.g =-ni/b (6)
 Therefore, where the relation holds that N.sub.s =N.sub.c, it is required
 to reduce the width of the coil turns (the coil length in the axial
 direction) `b` or to increase the number of turns `n`, in order to reduce
 negative feedback current `i` without changing gap length `g` of the
 magnetic yoke, that is, without changing H.sub.g, or on the contrary, in
 order to increase H.sub.g without increasing negative feedback current
 `i`. However, there is a limit in either case, such as a problem of wire
 required to be too thin.
 If demagnetizing factor N.sub.s of the magnetic core with respect to field
 H.sub.g to be measured is different from demagnetizing factor N.sub.c of
 the core with respect to the magnetic field generated by the coil wound
 around the core (which approximates to a parallel magnetic field for
 convenience) where N.sub.s &gt;N.sub.c, the relation holds that H.sub.c
 &gt;H.sub.g from equations (4) and (5). It is thereby noted that the
 magnetic field to be measured or the current to be measured is increased
 without changing the other conditions.
 The demagnetizing factor depends on the cross-sectional area of the
 magnetic substance orthogonal to the direction of passage of the magnetic
 flux and the length of the magnetic substance along the direction of
 passage of the flux. Therefore, in order to make N.sub.s greater than
 N.sub.c, the apparent length of the magnetic core with respect to the
 magnetic flux of the applied field may be made different from the apparent
 length of the core with respect to the magnetic flux of the field
 generated by the coil. A simple example of implementing this is to provide
 a U-shaped magnetic core having a straight rod-like bottom portion and
 portions extending from ends of the bottom portion in the direction
 orthogonal to the orientation of the bottom portion. A coil is wound
 around the bottom portion and a magnetic field to be measured is applied
 in the axial direction of the bottom portion.
 Although a well-known fluxgate element incorporating a toroidal core has
 the demagnetizing factor of zero with respect to an excitation magnetic
 field, the configuration, objective, and effects of this fluxgate element
 are different from those of the magnetic sensor element of the present
 invention. The operation principle of the former fluxgate element is as
 follows. An annular path is provided for the excitation magnetic flux in
 the toroidal core. As a result, the magnetic flux generated by the
 external magnetic field applied to the excitation flux in the toroidal
 core in parallel is added in part of the annular path and subtracted in
 another part of the annular path. Consequently, the magnetic flux is large
 in some portions and small in other portions inside the toroidal core. If
 the core is driven into saturation where the flux is increased through
 addition of the flux generated by the external field, the excitation flux
 leaks out of the core. If the excitation flux is constant, the magnitude
 of the leakage flux depends on the magnitude of the external field.
 Therefore, the entire toroidal core including the excitation winding is
 inserted to another coil, and this coil detects the leakage flux so as to
 detect the external field. In the magnetic sensor element of the
 invention, in contrast, the coil wound around the magnetic core functions
 as the coil for detecting the external field and detects the external
 field by detecting variations in inductance of the coil. A coil for
 detecting leakage flux is therefore not required. Furthermore, no
 consideration is given to the fluxgate element incorporating the toroidal
 core with regard to the shape affecting the demagnetizing factor with
 respect to the external field.
 SIXTH EMBODIMENT
 A magnetic sensor element of a sixth embodiment of the invention will now
 be described. FIG. 9 is a cross-sectional view showing the configuration
 of the magnetic sensor element of the embodiment. The element is a
 fluxgate magnetic sensor element comprising a magnetic core 1 and a coil 2
 wound around the core 1 and detecting an applied magnetic field to be
 measured. The magnetic core 1 corresponds to a magnetic substance of the
 invention.
 The magnetic core 1 is a drum-shaped core having a cylindrical core portion
 1a and disk-shaped brim portions 1b formed at both ends of the core
 portion 1a. The magnetic core 1 forms an open magnetic circuit both for
 the magnetic field to be measured and for the magnetic field generated by
 the coil 2. For example, the core portion 1a may have a diameter of 0.8 mm
 and a length of 1.5 mm and each of the brim portions 1b may have a
 diameter of 2 mm and a thickness of 0.5 mm. The core 1 may be made of
 Ni--Cu--Zn--base ferrite and have relative permeability .mu..sub.s of 500,
 for example.
 The coil 2 is wound around core portion 1a of the magnetic core 1. For
 example, the coil 2 may be made of 180 turns of urethane-coated wire
 having a diameter of 0.03 mm.
 The inductance of the magnetic sensor element shown in FIG. 9 is 350 .mu.H.
 The coil current that reduces the inductance to the half is 60 mA.
 The operation of the magnetic sensor element of the embodiment will now be
 described. This element is incorporated into a magnetic sensor apparatus
 or a current sensor apparatus. To be specific, the element is placed such
 that the axial direction of the core portion 1a is parallel to the
 magnetic field to be measured indicated with H in FIG. 9 (including the
 magnetic field to be measured generated by the current to be measured).
 For example, if the large amplitude excitation method is used, an
 alternating current that drives the core 1 into a saturation region at a
 peak is applied to the coil 2. If the negative feedback method is used, a
 negative feedback current for generating an inverse magnetic field having
 a magnitude equal to the magnitude of the field to be measured is supplied
 to the coil 2.
 It is difficult to analytically obtain demagnetizing factor N.sub.s of the
 core 1 with respect to external field H when external field H to be
 measured is applied to the magnetic sensor element of this embodiment and
 demagnetizing factor N.sub.c with respect to the magnetic field generated
 by the coil 2 wound around the coil 2. However, it is possible to estimate
 these factors based on measured values as follows.
 In the magnetic sensor element shown in FIG. 9, the coil current that makes
 the magnetic field inside the coil 2 zero which is obtained through actual
 measurement is 5 mA for H=1000 AT/m. Assuming that demagnetizing factors
 N.sub.s and N.sub.c are equal, coil current `i` that makes the magnetic
 field inside the coil 2 zero is approximately obtained through equation
 (6) as follows.
EQU Since H=(180.times.i/1.5).times.1000,
EQU i=8.3mA where H=1000 AT/m
 The measured value of coil current that makes the magnetic field inside the
 coil 2 zero is about 1/1.6 of the coil current value where demagnetizing
 factors N.sub.s and N.sub.c are equal. Therefore, it will be noted that
 the equivalent demagnetizing factor obtained from the ratio of the current
 value is N.sub.s.apprxeq.1.6N.sub.c. That is, according to the magnetic
 sensor element of the embodiment, the negative feedback current for
 canceling the same external magnetic field (the magnetic field to be
 measured) is reduced to 1/1.6 of the current required where demagnetizing
 factors N.sub.s and N.sub.c are equal.
 The reason that equivalent demagnetizing factor N.sub.s is not much greater
 than demagnetizing factor N.sub.c is as follows. Attention being given to
 the length of the magnetic path on the center line of the magnetic core 1,
 demagnetizing factor N.sub.s of the core 1 with respect to the magnetic
 field to be measured is, broadly speaking, equivalent to the demagnetizing
 factor of a magnetic core having a diameter of 0.8 mm and a length of 2.5
 mm. Similarly, demagnetizing factor N.sub.c of the core 1 with respect to
 the magnetic field generated by the coil 2 is equivalent to the
 demagnetizing factor of a magnetic core having a diameter of 0.8 mm and a
 length of 4 mm. Therefore, the ratio between the magnetic path lengths is
 not so great.
 According to the magnetic sensor element of the embodiment thus described,
 demagnetizing factor N.sub.s of the core 1 with respect to the applied
 magnetic field to be measured is different from demagnetizing factor
 N.sub.c of the core 1 with respect to the magnetic field generated by the
 coil 2. As a result, when a negative feedback current is supplied to the
 coil 2, it is possible to make the current different from the current
 supplied when demagnetizing factors N.sub.s and N.sub.c are equal. It is
 therefore easy to increase the measurement range of magnetic fields or
 electric currents. According to this embodiment, in particular,
 demagnetizing factor N.sub.s of the core 1 with respect to the applied
 magnetic field to be measured is greater than demagnetizing factor N.sub.c
 of the core 1 with respect to the magnetic field generated by the coil 2.
 As a result, when a negative feedback current is supplied to the coil 2,
 it is possible to make the current smaller than the current supplied when
 demagnetizing factors N.sub.s and N.sub.c are equal. It is therefore easy
 to measure a large magnetic field or electric current.
 SEVENTH EMBODIMENT
 A magnetic sensor element of a seventh embodiment of the invention will now
 be described. FIG. 10 is a cross-sectional view showing the configuration
 of the magnetic sensor element of the embodiment. The element comprises
 the magnetic core 1 and the coil 2 similar to those of the magnetic sensor
 element of the sixth embodiment. In addition, the exterior of the coil 2
 is coated with a magnetic coating material made of a mixture of ferrite
 powder and a resin coating material to form a coating layer 3. The coating
 layer 3 connects the two brim portions 1b of the core 1 to each other.
 For example, the average thickness of the coating layer 3 may be 0.5 mm.
 The relative permeability of the coating layer 3 may be 12. The inductance
 of the coil 2 of the magnetic sensor element of this embodiment is 1 mH.
 The coil current that reduces the inductance to the half is 30 mA.
 In this embodiment, assuming that the core 1 and the coating layer 3 make
 up the magnetic core, this magnetic core forms an open magnetic circuit
 with respect to the magnetic field to be measured but forms a closed
 magnetic circuit with respect to the magnetic field generated by the coil
 2. Consequently, demagnetizing factor N.sub.c of the core with respect to
 the magnetic field generated by the coil 2 is greatly reduced. In this
 case, the coil current that makes the magnetic field inside the coil 2
 zero which is obtained through actual measurement is 2.4 mA for H=1000
 AT/m. Accordingly, the equivalent demagnetizing factor is
 N.sub.s.apprxeq.3.5N.sub.c. That is, according to the embodiment, the
 negative feedback current required is reduced to 1/3 or less of the
 current required where demagnetizing factors N.sub.s and N.sub.c are
 equal.
 The remainder of configuration, operation and effects of the embodiment are
 similar to those of the sixth embodiment.
 EIGHTH EMBODIMENT
 A current sensor apparatus of an eighth embodiment of the invention will
 now be described. FIG. 11 is a circuit diagram of the current sensor
 apparatus of the embodiment. The current sensor apparatus incorporates the
 magnetic sensor element of the sixth embodiment. The current sensor
 apparatus includes a magnetic sensor apparatus of the eighth embodiment.
 The current sensor apparatus comprises a magnetic yoke 62 that surrounds a
 conductor 61 through which a current to be measured flows. Part of the
 yoke 62 has a gap in which the magnetic sensor element of the sixth
 embodiment is placed. The magnetic sensor apparatus is the part of the
 current sensor apparatus shown in FIG. 11 except the magnetic yoke 62.
 The circuit configuration of the current sensor apparatus of the embodiment
 will now be described. Positive and negative power supply circuits for an
 operational amplifier are not shown, according to the practice.
 An end of a detection coil 20 is connected to an end of the coil 2. The
 other end of the detection coil 20 is grounded. An end of a coil 6 used
 for a feedback current path is connected to the other end of the coil 2.
 The other end of the coil 6 is grounded through a capacitor 7.
 The current sensor apparatus further comprises: a drive circuit having a
 series resonant circuit part of which is made up of the coil 2 and
 supplying a resonant current flowing through the series resonant circuit,
 as an alternating current for driving the core 1 into a saturation region,
 to the coil 2; and a detection and feedback circuit for detecting a
 magnetic field to be measured by detecting variations in resonant current
 flowing through the coil 2 that corresponds to inductance variations of
 the coil 2 and for supplying a feedback current used for the negative
 feedback method to the coil 2 so as to have the coil 2 generate a negative
 feedback magnetic field used for the negative feedback method. The
 detection and feedback circuit corresponds to a negative feedback means of
 the invention.
 The drive circuit has an oscillation circuit including the series resonant
 circuit. The configuration of the oscillation circuit is as follows. The
 oscillation circuit incorporates a transistor 11. The base of the
 transistor 11 is connected to the other end of the coil 2 through a
 capacitor 12 used for resonance. An end of a capacitor 13 used for
 feedback is connected to the base of the transistor 11. An end of the
 capacitor 14 used for feedback and the emitter of the transistor 11 are
 connected to the other end of the capacitor 13. The other end of the
 capacitor 14 is grounded. The emitter of the transistor 11 is grounded
 through a load coil 15. The collector of the transistor 11 is connected to
 a power input 16 and to the base through a bias resistor 17. The
 configuration of this oscillation circuit is that of a Clapp oscillation
 circuit wherein Cs&lt;&lt;Cb and Cs&lt;&lt;Ce, the capacitance of each of
 the capacitors 12, 13 and 14 being Cs, Cb and Ce, respectively.
 The configuration of the detection and feedback circuit is as follows. An
 end of a capacitor 21 is connected to the connection point between the
 coil 2 and the detection coil 20. The other end of the capacitor 21 is
 grounded through a resistor 22. The capacitor 21 and the resistor 22 make
 up a differentiation circuit for differentiating the voltage generated
 across the coil 20 and outputting a signal corresponding to the magnetic
 field to be measured.
 The anode of a diode 23 and the cathode of a diode 25 are connected to the
 connection point between the capacitor 21 and the resistor 22. The cathode
 of the diode 23 is grounded through a capacitor 24. The anode of the diode
 25 is grounded though a capacitor 26. The diode 23 and the capacitor 24
 make up the positive peak hold circuit. The diode 25 and the capacitor 26
 make up the negative peak hold circuit.
 An end of a resistor 27 is connected to the connection point between the
 diode 23 and the capacitor 24. An end of a resistor 28 is connected to the
 connection point between the diode 25 and the capacitor 26. The other end
 of each of the resistors 27 and 28 is connected to an end of a resistor
 31. The resistors 27 and 28 make up the resistor adding circuit for adding
 a positive output value held at the positive peak hold circuit to a
 negative output value held at the negative peak hold circuit. A detection
 signal corresponding to the external magnetic field is present at an end
 of the resistor 31.
 The other end of the resistor 31 is connected to the inverting input of an
 operational amplifier 32. The noninverting input of the operational
 amplifier 32 is grounded through a resistor 33. The output of the
 operational amplifier 32 is connected to the noninverting input through a
 resistor 34. The amplifier 32 and the resistors 31, 33 and 34 make up an
 inverting amplifier.
 The output of the operational amplifier 32 is connected to an end of a
 resistor 35 for detecting an output. The other end of the resistor 35 is
 connected to the connection point between the coil 6 and a capacitor 7.
 The one end of the resistor 35 is connected to the noninverting input of
 an operational amplifier 38 through a resistor 36. The other end of the
 resistor 35 is connected to the inverting input of the amplifier 38. The
 noninverting input of the amplifier 38 is grounded through a resistor 39.
 The output of the amplifier 38 is connected to the inverting input through
 a resistor 40 and to an detection output 41. The amplifier 38 and the
 resistors 36, 37, 39 and 40 make up a differential amplifier.
 The detection coil 20, the coil 6 and the capacitor 7 are not only part of
 the oscillation circuit as the drive circuit but also part of the
 detection and feedback circuit.
 The operation of the current sensor apparatus of the embodiment will now be
 described. An alternating current is supplied to the coil 2 by the
 oscillation circuit such that the core 1 is driven into the saturation
 region. The alternating current is a resonant current that is equal to the
 current value limited by the supply voltage multiplied by value Q of the
 resonant circuit. A method taken in this embodiment is to detect
 variations in waveform of resonant current as a method of capturing
 variations in inductance of the coil 2 as an output signal of the magnetic
 current apparatus. To be specific, the voltage across the detection coil
 20 connected to the coil 2 in series and having a large saturation current
 is differentiated at the differentiation circuit made up of the capacitor
 21 and the resistor 22. A positive output value of an output of the
 differentiation circuit is held at the positive peak hold circuit made up
 of the diode 23 and the capacitor 24. A negative output value of the
 output of the differentiation circuit is held at the negative peak hold
 circuit made up of the diode 25 and the capacitor 26. The positive and
 negative output values are added to each other at the resistor adding
 circuit made up of the resistors 27 and 28. A detection signal
 corresponding to the external magnetic field is thus obtained.
 When no external magnetic field is present, the positive and negative
 portions of the differential waveform of the voltage across the detection
 coil 20 are symmetric, and the sum of positive and negative peak values
 (the difference between the absolute values) of the differential waveform
 is zero. In contrast, when the external field is applied to the coil 2,
 the positive and negative portions of the differential waveform are
 asymmetric. As a result, the sum of positive and negative peak values (the
 difference between the absolute values) of the differential waveform is
 other than zero, which depends on the external magnetic field. According
 to the embodiment, in such a manner, the external magnetic field is
 measured by obtaining the sum of positive and negative peak values (the
 difference between the absolute values) of the differential waveform.
 As thus described, the detection and feedback circuit detects the magnetic
 field to be measured, based on a portion of the resonant current flowing
 through the coil 2 that drives the core 1 into the saturation region. In
 other words, the detection and feedback circuit 4 detects the magnetic
 field to be measured, based on asymmetric positive and negative components
 of the resonant current flowing through the coil 2.
 The detection signal obtained at the adding circuit made up of the
 resistors 27 and 28 is inverted and amplified at the inverting amplifier
 made up of the operational amplifier 32 and the resistors 31, 33 and 34.
 The signal is then carried through the resistor 35 and applied to the
 connection point between the coil 6 and the capacitor 7. A negative
 feedback current is thereby supplied to the coil 2 through the coil 6 and
 magnetomotive force in the direction opposite to the external magnetic
 field is applied to the coil 2. In this embodiment, since the inverting
 amplifier has outputs of both positive and negative polarities, negative
 and positive feedback currents (wherein one of the directions of the
 external field is defined as positive) corresponding to the positive and
 negative polarities of the external magnetic field are supplied from the
 output of the inverting amplifier to the coil 2. Therefore, the end of the
 inverting amplifier on the side of the coil 2 is grounded.
 The external magnetic field is measured as follows. The feedback current,
 that is, the current corresponding to the external field, is converted to
 a voltage by the resistor 35. The voltage is amplified at the differential
 amplifier made up of the operational amplifier 38 and the resistors 35,
 36, 39 and 40, and then given to the detection output 41. A detection
 output signal corresponding to the external field is then outputted from
 the detection output 41.
 The balance between the external field and the magnetomotive force
 generated by the negative feedback current would not change unless the
 ampere turn of the coil 2 changes. Consequently, the current sensor
 apparatus of the embodiment achieves reduced sensitivity variations,
 excellent linearity, and excellent stability against changes in
 temperature, supply voltage and so on. In addition, since the large
 amplitude excitation method is taken, the offset is zero, according to the
 principle, and no drift due to external perturbations occurs.
 A specific example of the current sensor apparatus actually fabricated will
 now be described. In this example, the yoke 62 is a toroidal core made of
 Mn--Zn--base ferrite. The yoke 62 has an outer diameter of 20 mm, an inner
 diameter of 10 mm and a thickness of 5 mm and has a gap of 8 mm in width.
 In this example the dimensions of the entire apparatus are 20 mm by 35 mm
 by 6 mm and very small. The apparatus is operated at a power source of
 .+-.5 V. Current consumption is +27 mA and -2 mA when the current to be
 measured is zero. In the apparatus an increase in current consumption due
 to a negative feedback current is 5 mA per current to be measured of 10 A.
 The weight of the apparatus is 10 g.
 FIG. 12 shows an example of the relationship between an output voltage of
 the current sensor apparatus and a current to be measured passing through
 the conductor 61 placed inside the magnetic yoke 62. As shown, a linear
 output voltage characteristic is obtained in a good condition in an
 extremely wide current range, according to the current sensor apparatus of
 the embodiment. FIG. 12 shows the output characteristic wherein an offset
 bias not shown in FIG. 11 is applied.
 According to the current sensor apparatus of the embodiment, it is possible
 to suppress an increase in current consumption due to the negative
 feedback current to the minimum while the negative feedback method is
 used. As a result, problems of heating, for example, are prevented. The
 apparatus therefore contributes to industry and particularly to
 controlling direct current in an electric car or solar-electric power
 generation.
 According to the embodiment, a resonant current of the resonant circuit is
 supplied to the coil 2. As a result, an alternating current that drives
 the core 1 into the saturation region is easily supplied to the coil 2. In
 addition, the configuration of the apparatus is simple since it is not
 required to wind any coil for excitation around the core 1 besides the
 coil 2.
 According to the embodiment, the feedback current used for the negative
 feedback method is supplied to the coil 2 through the coil 6 connected to
 the coil 2 in parallel in terms of alternating current. As a result, the
 feedback current is easily supplied to the coil 2 without causing a loss
 of resonant current.
 According to the embodiment, a detection output of the order of volts is
 easily obtained by inserting the detection coil 20 to the resonant
 circuit, without reducing value Q of the resonant circuit, that is,
 without causing insufficiency in resonant current supplied to the coil 2.
 In addition, the peak hold circuit is implemented by the simple and
 inexpensive one utilizing the diode and the capacitor. The detection coil
 20 is able to obtain a sufficiently large output even when the inductance
 value thereof is a few percent of the inductance value of the coil 2.
 Therefore, since the number of turns of the detection coil 20 is small and
 the saturation current value is sufficiently large in general, the
 detection coil 20 will not be saturated by the drive current (resonant
 current) of the coil 2.
 Through those techniques, the large amplitude method and the negative
 feedback method are applicable while a magnetic core, such as a ferrite
 core, having a large saturation field and large nonlinearity is used. It
 is thereby possible to use a fluxgate element for detecting a large
 magnetic field or a large electric current.
 The features of the current sensor apparatus of the embodiment, in addition
 to the feature that an increase in negative feedback current is
 suppressed, are listed below.
 (1) Since the negative feedback method is taken, sensitivity variations are
 reduced and thermal characteristics are improved automatically.
 (2) Thus, no sensitivity adjustment or thermal characteristic compensation
 is required.
 (3) No offset adjustment is required.
 (4) The apparatus exhibits excellent properties since the large amplitude
 excitation method is applied.
 (5) No special method is required for fabricating the sensor section.
 (6) Since resonant current is utilized, the sensor coil is driven at a low
 supply voltage and a high frequency.
 (7) No special material or method is required for fabricating the apparatus
 and the circuit is very simple. As a result, the apparatus is manufactured
 at an extremely low cost and it is possible to meet the great demand.
 (8) The apparatus has an excellent frequency response.
 (9) Power consumption is low since resonant current is used.
 (10) The apparatus is small and light-weight since the configuration is
 simple.
 The magnetic sensor element of the seventh embodiment may be used as the
 magnetic sensor element of the current sensor apparatus shown in FIG. 11.
 The shape of the magnetic core is not limited to the one illustrated in
 the sixth or seventh embodiment but may be any other shape that allows the
 demagnetizing factor with respect to the applied magnetic field to be
 different from the demagnetizing factor with respect to the magnetic field
 generated by the coil.
 Although the Clapp oscillation circuit is used as an example of the
 oscillation circuit in the eighth embodiment, the invention is not limited
 to the circuit but may be applied to cases where any other oscillation
 circuit such as a Colpitts oscillation circuit or a Hartley oscillation
 circuit is utilized.
 According to the magnetic sensor element, the magnetic sensor apparatus, or
 the current sensor apparatus of the invention including the sixth to
 eighth embodiments, the demagnetizing factor of the magnetic core with
 respect to the applied magnetic field to be measured is different from the
 demagnetizing factor of the core with respect to the magnetic field
 generated by the coil. As a result, when a negative feedback current is
 supplied to the coil, it is possible to make the current different from
 the current supplied when the two demagnetizing factors are equal. It is
 therefore easy to increase the measurement range of magnetic fields or
 electric currents.
 The demagnetizing factor of the core with respect to the applied magnetic
 field to be measured may be greater than the demagnetizing factor of the
 core with respect to the magnetic field generated by the coil. As a
 result, when a negative feedback current is supplied to the coil, it is
 possible to make the current smaller than the current supplied when the
 two demagnetizing factors are equal. It is therefore easy to measure a
 large magnetic field or electric current.
 Obviously many modifications and variations of the present invention are
 possible in the light of the above teachings. It is therefore to be
 understood that within the scope of the appended claims the invention may
 be practiced otherwise than as specifically described.