Electricity: motive power systems – Synchronous motor systems
Reexamination Certificate
2000-10-06
2002-11-19
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
C318S705000, C318S806000, C318S811000, C318S609000, C363S176000
Reexamination Certificate
active
06483270
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a method for controlling a position sensorless motor and a control device thereof, and also a method and a device for frequency-to-voltage conversion mainly used for a position sensorless motor, and more particularly to a method and a device which can make accurate positioning control of a motor when controlling by using a counter electromotive force produced in stator coils.
BACKGROUND ART
The brushless DC motor is often used in view of its long life and low noise for motor-driven devices and motor-driven mechanisms used in each field of industrial world. For example, the servomotor, the stepping motor and the like are conventionally used as a motor for positioning in order to reciprocate a printing head of an ink-jet printer.
The servomotor has a position sensor and a speed sensor to make positioning by close loop control. On the other hand, the stepping motor makes positioning by open loop control.
The servomotor can provide very accurate positioning control but requires a very accurate sensor and also complex control. Therefore, it has a drawback that its controlling device and the like cost high in manufacturing.
The stepping motor, on the other hand, does not use a sensor of any sorts to make the open loop control. Therefore, the controlling device and the like therefor do not involve a high cost in manufacturing, but there is a drawback that vibrations and noises are liable to occur in use. In this connection, Japanese Patent Application Laid-Open Publication No. Hei 10-52094 proposes a technology that vibrations and noises can be reduced when the stepping motor is driven in position sensorless close loop driving.
The stepping motor of position sensorless drive detects a rotation position of the rotor by virtue of a counter electromotive force produced in stator coils. Therefore, the rotation position of the rotor cannot be detected when the motor is stopped. When the motor is started, synchronized operation is made to excite the stator coils in synchronization with a position command pulse in the same way as the drive of an ordinary stepping motor, and when the counter electromotive force reaches a level sufficient to detect the rotation position of the rotor, the position sensorless operation takes over from the synchronized operation.
In the position control system of the position sensorless motor as shown in
FIG. 8
, upon receiving a deviation of the present position detected by a rotation position detector from a commanded position, a PI compensator so controls to bring the present position to agree with the commanded position to control the input into the motor.
Where the speed cannot be increased quickly because of a load on the motor, it is necessary to gradually increase a frequency of a position command pulse. In this case, the motor is rotated in synchronization with the position command pulse during the synchronized operation.
But, when the synchronized operation is changed to the position sensorless operation, the motor position agrees with the position command, and a deviation between them becomes “0”. When the deviation is “0”, the PI compensator has a settled result “0”, and the motor has a drive voltage “0”. Therefore, the motor speed drops. Then, the output level of the counter electromotive force produced in the stator coils drops, disabling the detection of the rotation position of the rotor. Consequently, there is a possibility that the position sensorless drive cannot be made, and out of synchronism may be caused because the position sensorless drive can not be made.
Even if the out of synchronism could be avoided, the speed is largely changed as shown in FIG.
9
. Therefore, after shifting to the position sensorless operation, deviations are accumulated to cause vibrations in the system, and settling time becomes long.
Where the motor decelerates and stops, there may be a drawback that the counter electromotive force produced in the stator coils can not be detected because of the lowering of the motor speed, out of synchronism is caused, and it becomes impossible to stop the motor at the target position.
In view of the circumstances described above, a primary object of the present invention is to provide a method for controlling a position sensorless motor and a control device thereof which can securely drive even a position sensorless motor and stop it at a target position.
For example, an ink-jet printer injects ink to commanded positions while reciprocally moving a printing head to print characters and pictures on a sheet. And the motor for reciprocating the printing head is called a CR control motor, for which a hybrid type stepping motor is used, for example.
One example of a mechanical structure of such a conventional hybrid type stepping motor will be described with reference to
FIG. 12
to FIG.
14
.
This hybrid type stepping motor has an opening on top and bottom portions of a casing
201
, bearings
202
,
203
disposed in the openings and a rotation shaft
204
rotatably supported by the bearings
202
,
203
as shown in FIG.
12
.
A rotor
205
is mounted on the rotation shaft
204
. The rotor
205
comprises a magnet
206
which is inserted into and fixed to the be rotation shaft
204
, and rotor pole pieces
207
,
208
which are inserted into the rotation shaft
204
and fixed to the top and bottom of the magnet
206
. The magnet
206
is magnetized in a direction of thickness (axial direction), and the rotor pole pieces
207
,
208
are formed of layered steel plates.
A stator
209
is concentrically disposed around the rotor
205
, a plurality (six to nine) of stator pole pieces
210
are disposed at equal intervals on the side of the rotor
205
of the stator
209
, and each of the stator pole pieces
210
and the rotor
205
are mutually opposed with a space having a predetermined width therebetween. And, a stator coil
211
is wound around the respective stator pole pieces
210
.
A plurality (e.g., 6) of small tooth poles
210
a
are disposed on the leading end of each of the stator pole pieces
210
so to be arranged in right and left directions from the center as shown in FIG.
13
and FIG.
14
. And, a large number (e.g., 36) of small tooth poles
207
a
are formed on the outer periphery of the rotor pole piece
207
as shown in FIG.
13
and FIG.
14
. Similarly, a large number (e.g.,
36
) of small tooth poles
208
a
are formed on the outer periphery of the rotor pole piece
208
(see FIG.
14
). The small tooth poles
207
a
of the rotor pole piece
207
and the small tooth poles
208
a
of the rotor pole piece
208
are arranged in a state displaced by a half pitch, namely arranged with a phase displaced by 180° in electrical angle from one another.
The stepping motor configured as described above has the rotor
205
rotated by switching an electric current flowed in the stator coils
211
in synchronization with the pulse input from the outside, and its positioning can be made readily, so that its control circuit can be configured with ease. And, the hybrid type stepping motor is suitable as a CR control motor because it can realize an electrically delicate step angle.
Such a stepping motor can have its control circuit configured readily, but an electric current is flowed through the stator coils regardless of the position of the rotor, different from a brushless DC motor or the like, therefore, uneven rotations and noises are caused due to vibrations caused in the rotor. And, in order to prevent the occurrence of out of synchronism, an electric current with an allowance with respect to a load is flowed to the stator coils, resulting in a drawback that the motor generates a large volume of heat.
It has been demanded to provide a method for controlling a stepping motor which can realize smooth rotations and a low noise level through the elimination of vibrations of the rotor. To smoothly rotate the rotor, it is necessary to detect a rotation position of the rotor and to flow an electric current to the stator coils with appropriate timing. To do so, an encoder can be di
Ikegami Akihiko
Miyazaki Shinichi
Shinkawa Osamu
Tabata Kunio
Uetake Akihito
Kaneska & Takeuchi
Martin Edgardo San
Nappi Robert E.
Seiko Epson Corporation
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