Electricity: measuring and testing – Magnetic – Magnetometers
Reexamination Certificate
2002-10-21
2004-12-14
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Magnetic
Magnetometers
C324S253000, C324S247000, C033S361000
Reexamination Certificate
active
06831457
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to high-sensitivity two-dimensional magnetic sensors that detect terrestrial magnetic fields and the like.
PRIOR ART
Concerning magnetic field detection devices which detect terrestrial magnetic fields, and invention which uses the highly sensitive MI element is disclosed in Japanese Patent Application Laid-open (Kokai) 2001-296127. The invention described therein places MI elements in two axes, uses a negative feedback circuit, etc., and performs heat compensation with a differential circuit.
SUMMARY OF THE INVENTION
An electronic compass with a miniature size and efficient power use suitable for portability is needed for mobile devices, including mobile phones. Such an electronic compass would need a two-dimensional, highly sensitive magnetic sensor.
Such a magnetic sensor would require a range in which output voltage is linear in response to the strength of the magnetic field (hereafter called the magnetic field measurability range) of ±2.7 G or more. When used in a device like a mobile phone, this two-dimensional magnetic sensor will inevitably be used in a variety of locations, some of which will have magnetic field strengths much greater than ±2.7 G.
The magnetic field detection device of Japanese Patent Application Laid-open (Kokai) 2001-296127, when not equipped with a negative feedback circuit, had a linear magnetic filed measurability range of ±40 A/m (that is, 0.5 G), and thus was not suitable as such for use as an electronic compass. To resolve that problem, the invention was equipped with a coil which provided negative feedback and used a circuit that was constantly passing current.
The above technology used two MI elements for each axis as well as a differential circuit to perform a differential operation on these two signals, and because it used a negative feedback circuit, the scale of the electric circuit increased and became unsuitable for miniaturization. Power consumption also increased due to the flow of negative feedback current which was used to enlarge the magnetic field measurability range. Finally, the size of the prior MI element was large—3 mm wide, 2 mm tall, and 4 mm long—making its application in miniature electronic devices difficult.
Thus, to resolve the above problems, it was necessary to develop a two-dimensional magnetic sensor that was smaller, used less power, and had a wider magnetic field measurability range.
Thereupon, the present inventors, having researched the miniaturization of the MI element, came upon the following construction.
A two-dimensional magnetic sensor which detects external magnetic fields comprises a first magneto-impedance sensor element, called the first MI element, provided to detect the first axial component of said external magnetic field, and which comprises a first magneto-sensitive element less than 2 mm long and a first electromagnetic coil which is wound around the said first magneto-sensitive element; a second magneto-impedance sensor element, called the second MI element, provided to detect the second axial component of said external magnetic field and which comprises a second magneto-sensitive element less than 2 mm long and a second electromagnetic coil which is wound around the said second magneto-sensitive element; and an integrated circuit comprising an oscillating means which supplies a pulse or high-frequency current, a means for switching current alternatingly between said first MI element and said second MI element, a means for detecting the output voltage from the electromagnetic coils of said first and second MI elements, and an amplifier which amplifies the output voltage of said detection means.
A feature of one embodiment of the present invention is that, by shortening the length of the first and second magneto-sensitive elements to 2 mm or less, which is shorter than those of the past, the range in which output voltage is linear with respect to the strength of the magnetic field (that is, the magnetic field measurability range) now reaches ±10 G without the use of a negative feedback circuit. Hereby an expansion of the magnetic field measurability range not previously possible, the miniaturization of the MI element and the omission of a negative feedback circuit, and a substantial reduction of power consumption via the omission of a negative feedback circuit are all realized.
In addition, the use of a switching means to switch current back and forth between the elements allows for a further reduction in power consumption.
In addition, in each of the above MI elements, the inside diameter is less than 200 &mgr;m by virtue of the fact that there is no circuit board fixing the magneto-sensitive element between the magneto-sensitive element and the electromagnetic coil and only an insulator surrounds the magneto-sensitive element, and the first electromagnetic coil has a coil spacing per unit length of less than 100 &mgr;m/turn.
In the above construction, because the electro-magnetic coil is held using only an insulator around the magneto-sensitive element, the inside diameter can be reduced to less than 200 &mgr;m, and a further overall miniaturization of the MI element may be achieved while improving the output voltage.
Finally, the output voltage increases because of the turns per unit length increase due to the reduction of coil spacing per unit length in the electromagnetic coil. In practice, it is favorable to have 100 &mgr;m/turn or less. If the present output voltage is ample, the MI element may be shortened.
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N. Pollock, Wireless World, pp. 49-54, “Electronic Compass Using a Fluxgate Sensor”, Oct. 1982.
Honkura Yoshinobu
Koutani Yoshiaki
Mori Masaki
Yamamoto Michiharu
Aichi Micro Intelligent Corporation
Patidar Jay
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