Electricity: motive power systems – Positional servo systems – With particular motor control system responsive to the...
Reexamination Certificate
2001-07-11
2003-04-29
Nappi, Robert E. (Department: 2837)
Electricity: motive power systems
Positional servo systems
With particular motor control system responsive to the...
C318S132000, C318S254100, C318S432000, C318S434000, C318S696000, C360S078130
Reexamination Certificate
active
06555985
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a stepping motor and the control of the same. More particularly, the present invention relates to a stepping motor having an encoder and a control device for controlling the stepping motor.
BACKGROUND ART
Stepping motors have features such as small size, high torque, and long life. Stepping motors are typically driven by open-loop control by utilizing the easy-to-control property. On the other hand, stepping motors have problems such as out-of-step, vibration, and a low rotational speed. To solve such problems, a method for driving a stepping motor by a closed-loop control has been proposed, where the stepping motor is provided with an encoder.
Japanese Patent Application No. 10-011069 describes the following arrangement. The number of output pulses in one cycle of an encoder is set to an integral multiple of the number of magnetic poles of a stepping motor. An exciting current to the stepping motor is switched every time a predetermined number of encoder pulses are detected with reference to an arbitrary rest position of the stepping motor. This allows the phase accuracy between the output signal of the encoder and the exalting current to the stepping motor to be smaller than or equal to a predetermined error.
In the case of this arrangement, it is necessary to cause a drive phase to be sufficiently advanced with respect to the actual angular position of a rotor in order to obtain a sufficiently large number of revolutions. However, a sufficiently advanced phase angle causes an actual phase to be excessively advanced when the rotor is operated at a low speed. In extreme cases, the rotor is adversely rotated in a reverse direction in the low-speed operation.
When the operation of the motor is initiated, the angular position of the rotor, which has been held due to microstep driving before the start of the operation, is determined by a ratio between the currents of each motor coil phase. The angular position of the rotor determined in such a manner includes an error. When an attempt is made to control the error, sufficient starting torque is sometimes not obtained, leading to a failure to start an operation.
Further, when a closed-loop driving is performed using such a stepping motor, the high positioning accuracy, which is a characteristic of the stepping motor, cannot be obtained only by closed-loop driving. Therefore, microstep driving is used in conjunction with closed-loop driving. Closed-loop driving is initially used to transfer a subject to be controlled at a high speed, thereafter, partway when the subject is decelerated, closed-loop driving is switched to microstep driving to perform accurate positioning. However, when closed-loop driving is switched to microstep driving, an unnecessary rotational amplitude often occurs, so that it is difficult to control a position and a speed accurately.
The above-described application describes no solution to these problems.
Further, conventionally, with the above-described arrangement in which a stepping motor may alternatively be used in place of a DC motor, speed control has been generally performed using an output signal of an encoder.
A problem with this arrangement is that when a head is moved at a high speed to an intended track (e.g., head movement control in a disk apparatus), a speed command value is considerably small at the point in time when the head reaches a position a few tracks away from the intended track, so that overshoot over the intended track, runaway of a motor, or the like are likely to occur due to an offset voltage or the like. To solve such a problem, Japanese Laid-open Publication No. 2-18766 discloses an arrangement in which a speed command value is increased when no signal is received from an encoder within a predetermined time.
However, it is difficult to provide an optimal value of the predetermined time with respect to any number of revolutions of a motor.
Specifically, when the predetermined time is provided so as to be suitable for a higher number of revolutions of a motor, then if the number of revolutions is small, it is often erroneously detected that temporal expansion of pulse intervals due to normal deceleration is abnormal. When the predetermined time is provided so as to be optimal for a lower number of revolutions, then if the number of revolutions is high, abnormalities cannot sometimes be detected.
Further, it is difficult to provide a corrected speed command value optimal for all cases.
Specifically, a driving system has variations in the frictional load of a motor or a transmission system, or the like. Therefore, it cannot be expected that the same increase in the speed command value leads to the same response. For example, even when the same increase in the speed command value is given to a motor, if the frictional load of a driving system is large, it may be impossible to inhibit the halt of the motor in spite of the increase. In this case, similar to the case where the speed command value is not increased, the device continues to wait for a next input pulse signal, resulting in no improved effect. Conversely, when the frictional load of the driving system is small, a high level of overshoot occurs due to an increase in the speed command value. In this manner, it is difficult to design such an increase in the speed command value that addresses variations in characteristics of a driving system. It is also difficult to perform reliable control.
The above-described application describes no solution to the above-described problems.
Hereinafter, a conventional technology will be described with reference to
FIGS. 20A through 26
.
FIG. 20A
is a schematic diagram showing an exemplary configuration of an optical disk drive using a conventional motor control device.
FIG. 20B
is a table showing a relationship between an angular position &thgr; of a rotor prior to starting and a command value for forced driving.
FIG. 21
is a timing chart showing a temporal relationship between driving voltages applied to exciting coils of conventional A-phase and B-phase stators, and an output of a position detecting means.
FIG. 22
is a diagram showing a conventional relationship of a phase between a rotor and driving with respect to the time of Ta in FIG.
21
.
FIG. 23
is a diagram showing a conventional relationship between the position of a rotor and electromagnetic force when the position of the rotor is shifted towards a rotational direction.
FIG. 24
is a flowchart used for explaining a conventional speed control operation.
FIGS. 25A and 25B
are diagrams used for explaining an excitation sequence, showing a time-varying current command value output from an instruction amplitude control means and a microstep driving means.
FIGS. 26A and 26B
are a conventional profile of an intended speed of a rotor and a conventional time chart of a current command value output from a command value selector.
In
FIG. 20A
,
301
indicates a head which optically records and reproduces information to and from an optical disk
302
. A nut piece
303
attached to the head
301
is engaged with the grooves of a lead screw
304
. The lead screw
304
has a screw pitch of 3 mm and is coupled with a stepping motor
305
. Therefore, the head
301
is straightly driven back and forth along a guide shaft
306
in accordance with the rotation of the stepping motor
305
. Reference numeral
307
indicates a bearing which is fixed to a chassis
308
and supports the screw
304
so that the screw
304
is freely rotated. A spindle motor
309
drives and rotates the optical disk
302
. When the head
301
is moved to an intended position, a direction and a distance in which the head
301
is moved are determined based on the addresses of a current position and an intended position. In accordance with the direction and distance, a control means
310
performs a control operation for the stepping motor
305
.
The driving means
311
includes an A-phase current driver
312
and a B-phase current driver
313
which are independent two-channel current drivers. The current drivers
312
Kajino Osamu
Kawabata Tohru
Mushika Yoshihiro
Nappi Robert E.
Renner Otto Boisselle & Sklar
Smith Tyrone
LandOfFree
Stepping motor control device does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Stepping motor control device, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Stepping motor control device will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3035718