Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
1999-06-29
2001-01-30
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S06800R, C310S06700R, C338S03200R
Reexamination Certificate
active
06181036
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a rotational angle detector provided with a giant magnetoresistive element and a brushless or stepping motor using the detector.
2. Description of the Prior Art
The growth of the semiconductor technology and the digital technology in recent years has been encouraging all the industrial devices to succumb to digitalization and has been promoting the adoption of microcomputers therefor. Since motors which are capable of producing a digital positioning action are suitable as driving sources for these devices, stepping motors and servo motors have been heretofore used therefor. Likewise in the field of such applications as factory automation (FA) and robot, stepping motors which are smaller in size and larger in torque are used.
A conventional brushless or stepping motor (permanent magnet (PM) type stepping motor)
10
, as illustrated in FIG.
1
and
FIG. 2
, is composed of a magnetic rotor (hereinafter referred to as “rotor”)
12
formed of a permanent magnet and accommodated rotatably in a housing
11
, exciting oils
14
disposed around the rotor
12
as opposed thereto across a prescribed gap, and rotational angle detecting magnetic sensors interposed between the exciting coils
14
. By orderly exciting the exciting coils
14
externally, the rotor
12
is caused to change the position thereof depending on the state of coil electrization and produce a stepping motion, with the result that the angle of rotation and the speed of rotation will be controlled with the signal of excitation. It is not deniable, however, that the rotor
12
will possibly fail to rotate in spite of the excitation of the coils
14
on account of an unduly large load on the rotor
12
. For exact control of the rotational angle of the rotor
12
, therefore, a measure to use the magnetic sensors (Hall sensors)
15
for detecting changes in magnetic field of magnetized teeth
13
of the rotor
12
made of permanent magnet as the rotational angles or a measure to detect the rotational angles by the use of an encoder disposed outside the motor is resorted to.
In the miniaturization of the brushless or stepping motor, the permanent magnet rotor
12
provided with a plurality of magnetized teeth
13
and the housing
11
encompassing the exciting coils
14
can be miniaturized. The intervals between the individual teeth
13
of the rotor
12
of permanent magnet constructed as illustrated in
FIG. 2
can be decreased to the order of some tens of &mgr;m and, therefore, pose no problem to the miniaturization of the brushless motor. In the case of an application which requires the brushless motor to control the number of revolutions and the angle of rotation exactly, however, the miniaturization of this motor has its own limit because the Hall sensors
15
serving to detect the rotational angle cannot be reduced in size. The Hall sensors
15
each require at least four sensor leads (two voltage terminals
16
a
,
16
b
and two current terminals
17
a
,
17
b
) as illustrated in
FIG. 3
(the detection magnetic field perpendicular to the face of the paper in the bearings of
FIG. 3
) and, therefore, entail as an inherent problem the noise of induced electromotive force due to a change in the magnetic field generated by the lead wires. Further, since the Hall sensors are unserviceable at elevated temperatures exceeding 100° C. and since they are exposed, when operated in the stepping motor which usually gains in temperature while passing electric current, to such elevated temperatures, they are required to take into consideration the necessity for using a proper cooling means.
To overcome the problem, therefore, the idea of utilizing a magnetoresistive sensor
18
for which two sensor leads (current terminals
19
a
,
19
b
) suffice as illustrated in
FIG. 4
(the detection magnetic field in an arbitrary direction) may be conceived.
The term “magnetoresistance (MR) effect” as used herein means a phenomenon that the electric resistance offered by a given material is varied by applying a magnetic field to that material. Generally, a ferromagnetic material is used as an MR element. A CoFe alloy having a rate of change of about 5% and a permalloy having a rate of change of about 2%, in magnetoresistance, are typical examples of the MR element. The rate of change of the magnetoresistance effect (magnetoresistance ratio, MR ratio) is expressed by the following formula (1):
Magnetoresistance ratio(%)=[R(O)−R(H)]/R(O)×100 (1)
wherein R(O) represents the electric resistance in the absence of a magnetic field and R(H) represents the electric resistance in the presence of application of a magnetic field.
The utilization of the magnetoresistance effect is effective in realizing miniaturization of an angle detecting device as by reducing the number of necessary sensor leads to two and simplifying the layout of wires, for example. Examples of the motor which uses a rotational angle sensor utilizing the magnetoresistance effect are disclosed in published Japanese Patent Applications, KOKAI (Early Publication) No. 62-2353 and No. 8-289518. Since the brushless or stepping motor uses a magnet (having a surface magnetic field of not less than 100 [Oe]) as a rotor thereof and the exciting coil thereof for driving the rotor has a strong magnetic field (some hundreds of Oe), however, the sensor which utilizes a magnetoresistive element formed of a soft magnetic material represented by permalloy has the problem that it cannot detect the rotational angle because its detectable magnetic field (not more than some tens of Oe) is surpassed.
SUMMARY OF THE INVENTION
The rotational angle sensor for use in the small brushless or stepping motor described above is required to fulfill the following four requirements. The sensors of the conventional class, however, have the problem that none of them cannot satisfy all these requirements.
(1) The sensor should be capable of being easily miniaturized (miniaturization).
(2) The sensor should not suffer the detecting sensitivity of the magnetic field thereof to vary notably with temperature (temperature characteristic).
(3) The sensor should be capable of detecting a magnetic field of up to several kOe (magnetic field characteristic).
(4) The sensor should be capable of detecting an AC magnetic field of up to several kHz (frequency characteristic).
To satisfy these requirements, in accordance with one aspect of the present invention, there is provided a rotational angle detector for use in a brushless or stepping motor which comprises a detecting part of a wiring configuration having superposed through the medium of an insulating layer two sensor leads connected one each to the opposite terminals of a giant magnetoresistive element.
In accordance with another aspect of the present invention, there is provided a brushless or stepping motor which, by having the giant magnetoresistive element of the detector capable of reading a change in magnetic field of up to at least 10 kOe built in a motor housing as approximated closely to the magnetic teeth of a rotor of high magnetic field (some hundreds of Oe to several kOe), enables the giant magnetoresistive element to read directly the rotational angle of the rotor.
The giant magnetoresistive element mentioned above is formed of a magnetic particle dispersion type giant magnetoresistive material, an artificial lattice type giant magnetoresistive material, or a colossal magnetoresistive material and is capable of reading an AC magnetic field of at least not less than 5 kHz as a signal.
Preferably, the giant magnetoresistive element mentioned above is a magnetic particle dispersion type giant magnetoresistive element which is formed of a material having ferromagnetic particles, 5 to 500 nm in maximum major diameter, dispersed in a nonmagnetic (paramagnetic or diamagnetic) material. It may be otherwise an artificial lattice type giant magnetoresistive element which is formed of a material having a nonmagnetic (paramagnetic or diamagne
Katsumi Tetsuya
Kazama Noriaki
Miura Yoshinori
Takeda Hideki
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Lam Thanh
Ramirez Nestor
YKK Corporation
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