Electricity: motive power systems – Nonrunning – energized motor
Reexamination Certificate
2002-07-12
2004-07-06
Ro, Bentsu (Department: 2837)
Electricity: motive power systems
Nonrunning, energized motor
C318S500000
Reexamination Certificate
active
06759824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to a motor controller and more particularly relates to an apparatus for performing an accurate positioning control by using a DC motor of a small size and a method of driving a DC motor.
2. Description of the Related Art
A DC motor including a brush and a commutator has a simple structure and can be manufactured at a low cost. Also, a DC motor achieves high efficiency and high output although its size is small, and needs no special driver. For these reasons, a DC motor is now used in numerous appliances.
However, if it is necessary to control the stop angle of the rotor of a DC motor accurately enough or to rotate the motor at an extremely low number of revolutions per minute, it might be difficult for a DC motor to satisfy these requirements fully.
This is partly because a DC motor generates a cogging torque. Hereinafter, the cogging torque of a DC motor will be described.
As shown in
FIG. 8A
, a DC motor includes a rotor
94
and fields
95
. The rotor
94
includes magnetic poles
91
,
92
and
93
, each of which includes an iron core made of a magnetic material such as silicon steel and a coil that is wound around the iron core. Each of the fields
95
is a permanent magnet such as a ferrite magnet. The DC motor typically has three magnetic poles
91
,
92
and
93
and two fields
95
as shown in FIG.
8
A.
In a DC motor like this, the magnetic poles
91
,
92
and
93
, including magnetic bodies, are attracted to the fields
95
. Accordingly, even when no electrical power is applied to the DC motor, a torque is generated in such a direction as to rotate the rotor
94
. The torque rotates the rotor
94
so that the magnetic poles
91
,
92
and
93
are stabilized in the magnetic field that has been generated by the fields
95
. The torque that is going to rotate the rotor
94
is generated by the attraction between the magnetic poles
91
,
92
and
93
and the fields
95
. Thus, the angle of rotation of the rotor
94
at which the magnetic poles
91
,
92
and
93
are stabilized changes with the positional relationship between the fields
95
and the magnetic poles
91
,
92
and
93
.
The “stabilized state” normally refers to a state in which one of the magnetic poles
91
,
92
and
93
is closest to one of the fields
95
. For example,
FIG. 8A
illustrates one of those stabilized states in which the magnetic pole
91
is closest to the N pole field
95
. In such a state, there is no torque that is going to rotate the rotor
94
.
Suppose the rotor
94
is rotated clockwise from the position shown in FIG.
8
A. In that case, when the rotor
94
is rotated 60 degrees from the position shown in
FIG. 8A
, the magnetic pole
92
will be closest to the S pole field
95
as shown in FIG.
8
B. This is another stabilized state. Since the rotor
94
has the three magnetic poles
91
,
92
and
93
and the number of the fields
95
is two, there will be six stabilized states for one rotation of the rotor
94
. That is to say, every time the rotor
94
rotates 60 degrees, one of the six stabilized states appears.
FIG. 9
shows the magnitudes and directions of the torques that are generated by the magnetic attraction between the rotor
94
and the fields
95
. In
FIG. 9
, the state shown in
FIG. 8A
is regarded as an initial state. If a force is externally applied to the rotor
94
to rotate the rotor
94
clockwise from the initial state as indicated by the point A in
FIG. 9
with no electrical power applied to the DC motor, the attraction between the N pole field
95
and the magnetic pole
91
generates a torque in the direction opposite to the rotational direction. As the angle of rotation increases, this reverse torque increases its magnitude. And when the rotor
94
rotates approximately 15 degrees, the magnitude of the reverse torque is maximized as indicated by the point B in FIG.
9
. As the rotor
94
is further rotated, attraction is soon generated between the magnetic pole
92
and the S pole field
95
. Accordingly, the reverse torque applied to the rotor
94
decreases gradually. And when the rotor
94
rotates approximately 30 degrees, no reverse torque is applied to the rotor
94
anymore as indicated by the point C in FIG.
9
.
As the rotor
94
is further rotated, the attraction between the magnetic pole
92
and the S pole field
95
dominates, thereby generating a torque that rotates the rotor
94
clockwise. When the rotor
94
rotates approximately 45 degrees, the torque that rotates the rotor
94
clockwise is maximized as indicated by the point D in FIG.
9
. But that torque also decreases as the rotor
94
is further rotated. And when the rotor
94
rotates approximately 60 degrees, no torque that rotates the rotor
94
clockwise is applied to the rotor
94
anymore as indicated by the point E in FIG.
9
.
Actually, though, a friction torque is applied to the shaft of the rotor
94
as indicated by the one-dot chains in FIG.
9
. Accordingly, unless a torque that has a magnitude greater than that of the friction torque is generated and applied to the rotor
94
, the rotor
94
never rotates. Thus, the effective torque applied to the rotor
94
is indicated by the solid curve in FIG.
9
. As can be seen from
FIG. 9
, such a torque variation is repeatedly caused every time the rotor
94
rotates 60 degrees. As indicated by the solid curve in
FIG. 9
, the effective torque may be positive, negative or zero depending on the angle of rotation of the rotor
94
. This effective torque is the so-called “cogging torque”.
If the DC motor is stopped by discontinuing the supply of power to the DC motor, the magnitude and the direction of the cogging torque change with the stop angle of the rotor
94
. Accordingly, when the rotor
94
reaches such an angle as to generate zero cogging torque (e.g., approximately 30 degrees or approximately 60 degrees in the example shown in FIG.
9
), the rotor
94
can be stopped without being affected by the cogging torque.
However, if the rotor
94
should be stopped at such an angle as to generate a positive cogging torque, then that cogging torque is applied to the rotor
94
, thereby rotating the rotor
94
excessively (i.e., to an angle greater than the desired angle) until the rotor
94
is stabilized. In the example shown in
FIG. 9
, the rotor
94
is rotated unintentionally to around 60 degrees, around 120 degrees, etc. On the other hand, if the rotor
94
should be stopped at such an angle as to generate a negative cogging torque, then a cogging torque is generated and applied to the rotor
94
in such a direction as to rotate the rotor
94
in the backward direction. In that case, just before the rotor
94
stops rotating, the rotor
94
rotates in the backward direction until the rotor
94
is stabilized. In the example shown in
FIG. 9
, the rotor
94
retrogrades to around 0 degrees, around 60 degrees, etc. For these reasons, when a DC motor is used, it is difficult to control the stop angle of the rotor
94
accurately enough.
Furthermore, a non-uniform torque is generated around the shaft of a DC motor when power is supplied to the DC motor. A DC motor of a small size, in particular, has a small number of magnetic poles, and therefore, there is a significant variation in the torque generated during one rotation of the rotor
94
. The output of a DC motor is also affected by a variation in the power supplied to the DC motor to drive it or in the load connected to the motor. Consequently, there is a great variation in the output of the DC motor.
In addition, there is also a great variation in the load applied to a DC motor (e.g., friction caused at the bearing thereof). In particular, a load variation resulting from the difference between static friction and kinetic friction is a problem. More specifically, when power is supplied to a DC motor, the static friction caused at the bearing increases proportionally to the torque generated at the rotor
94
. However, once the rotor
94
has started to rotate, the static fri
Kawabata Toru
Mushika Yoshihiro
Akin Gump Strauss Hauer & Feld L.L.P.
Ro Bentsu
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