Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
2000-01-18
2001-11-13
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
Reexamination Certificate
active
06316931
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a magnetic sensor apparatus for measuring a relatively large magnetic field and an electric current sensor apparatus used for non-contact measurement of a large current through the use of the magnetic sensor apparatus.
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 generated by a current through a magnetic sensor for measuring a direct current component. A Hall element is widely used for such a magnetic sensor. A magnetoresistive element and a fluxgate element are used in some applications, too.
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 a so-called negative feedback method, that is, to apply a reversed magnetic field proportional to an output of the element to the element so as to apply negative feedback such that the output of the element is maintained 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 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.
As thus described, it is difficult in prior art to apply the negative feedback method to a current sensor apparatus used for non-contact measurement of a large current containing a direct current component. It is therefore difficult to implement an inexpensive current sensor apparatus having excellent characteristics.
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 detection unit 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. 25
to describe the operation principle of a fluxgate element having the simplest configuration.
FIG. 25
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
o
is measured as a change in inductance of the coil when external field H
o
is superposed on the bias field. This is the operation principle of the simplest fluxgate element. In
FIG. 25
each of bias field B and external field H
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 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. 25
it is assumed that point P
+
and point P
−
represent the coil current in the positive direction and the coil current of the negative direction, respectively, whose absolute values are equal to each other. 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 amount 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 patent application 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 when the permeability of the core is maximum.
An external magnetic field is assumed to be applied to the core. If external field H
o
is applied in the positive direction of the current, as shown in
FIG. 25
, the inductance value decreases at the positive peak of the current (point Q
+
in
FIG. 25
, for example) and the inductance value increases at the negative peak of the current (point Q
−
in
FIG. 25
, for example). Therefore, the difference between the values is other than zero. Since the difference in inductance depends on the external magnetic field, the
Itoh Kazuyuki
Nakagawa Shiro
Okita Yoshihisa
Yabusaki Katsumi
Leroux E P
Metjahic Safet
Oliff & Berridg,e PLC
TDK Corporation
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