Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2000-08-28
2001-11-13
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S225000, C324S11700H
Reexamination Certificate
active
06316939
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
With concern about the environment, considerable developments have been recently made in electric automobiles and solar-electric power generation that produce less environmental pollution. A direct current of several kilowatts to tens of kilowatts is dealt with in an electric car or solar-electric power generation. Therefore, a non-contact current sensor apparatus is required for measuring a direct current of tens to hundreds of amperes. Since the demand for such current sensor apparatuses is extremely great, it is requested in society to provide current sensor apparatuses that are inexpensive and exhibit high accuracy.
A current sensor apparatus incorporating a Hall element as a magnetic sensor is widely used for non-contact measurement of an electric current through measuring a magnetic field generated by the current with the magnetic sensor.
However, the Hall element has a problem of offset voltage that requires troublesome handling, which prevents a reduction in the price of the current sensor apparatus. The offset voltage means a residual output voltage when the magnetic field to be measured is zero.
There is a magnetic sensor apparatus or a current sensor apparatus that incorporates a fluxgate element as a magnetic sensor that utilizes saturation of a magnetic core. Attention has been given to such an apparatus that is expected to produce no offset voltage, according to the principle.
Reference is now made to
FIG. 12
to describe the operation principle of a fluxgate element having the simplest configuration.
FIG. 12
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
0
is measured as a change in inductance of the coil when external field H
0
is superposed on the bias field. This is the operation principle of the simplest fluxgate element. In
FIG. 12
each of bias field B and external field H
0
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, it is assumed that point P
+
and point P
−
of
FIG. 12
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 each 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 decreases in the 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. That is, no offset voltage is generated in this case. 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
0
is applied in the positive direction of the current, as shown in
FIG. 12
, for example, the inductance value decreases at the positive peak of the current (point Q
+
in
FIG. 12
, for example) and the inductance value increases at the negative peak of the current (point Q
−
in
FIG. 12
, for example). Therefore, the difference between these values is other than zero. Since the difference in the inductance values depends on the external magnetic field, the external field is obtained by measuring the difference in the inductance values.
With regard to a magnetic sensor apparatus or a current sensor apparatus incorporating a fluxgate element, the difference of the inductance values described above may be obtained from a signal obtained through differentiating the voltage generated across another inductance element connected in series to the sensor coil, that is, a signal equivalent to the second-order differential coefficient of the current flowing through the sensor coil.
The method thus described is called a large amplitude excitation method in the present patent application, that is, to apply an alternating current to the sensor coil such that the core is driven into a saturation region at a peak, and to measure the difference in the amounts of decreases in inductance at positive and negative peak values of the current.
In Published Unexamined Japanese Patent Application Hei 4-24574 (1992), an oscillation circuit including a resonant circuit part of which is made up of a sensor coil is disclosed. The oscillation circuit is provided as a means for applying an alternating current to the sensor coil.
When an external magnetic field is zero, it is required that the excitation current of the sensor coil has a wave with symmetrical positive and negative portions in order that the difference between the inductance values of the sensor coil at the positive and negative peaks of the current is zero.
However, the positive and negative portions of the waveform of the excitation current are not symmetrical, strictly speaking, if a drive circuit for exciting the sensor coil is actually fabricated and its operation is studied in detail. If a self-excited oscillation circuit is used as the drive circuit, in particular, asymmetry between the positive and negative portions of the wave of the excitation current is considerably great. Therefore, an offset voltage that is not negligible is generated in practice by a sensor apparatus utilizing the large amplitude excitation method, too.
The problems resulting from the offset voltage are that: the offset voltage causes a constant error in the output of the sensor apparatus; and that the offset voltage varies due to external perturbations such as a temperature and supply voltage.
It is known through observation that it is energy loss in the control input of an active element making up the oscillation circuit that induces the asymmetry between the positive and negative portions of the wave of the excitation current. It is also known that the major one of the external perturbations that cause variations in the asymmetry mentioned above is variations in the operating temperature of the active element making up the oscillation circuit.
Reference is now made to
FIG. 13
to
FIG. 15
to describe in detail the asy
Nakagawa Shiro
Yabusaki Katsumi
Oliff & Berridg,e PLC
Patidar Jay
TDK Corporation
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