Electricity: measuring and testing – Measuring – testing – or sensing electricity – per se – Magnetic saturation
Reexamination Certificate
2000-01-18
2001-11-27
Metjahic, Safet (Department: 2858)
Electricity: measuring and testing
Measuring, testing, or sensing electricity, per se
Magnetic saturation
Reexamination Certificate
active
06323634
ABSTRACT:
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 meth
Nakagawa Shiro
Okita Yoshihisa
Tanaka Katsuaki
Yabusaki Katsumi
LeRoux Etienne P
Metjahic Safet
Oliff & Berridg,e PLC
TDK Corporation
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