Electricity: motive power systems – Open-loop stepping motor control systems
Reexamination Certificate
1999-08-05
2001-07-10
Masih, Karen (Department: 2837)
Electricity: motive power systems
Open-loop stepping motor control systems
C318S685000, C318S132000, C318S254100, C318S434000
Reexamination Certificate
active
06259225
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a stepping motor control unit and method, a printer employing them, and an information recording medium. More particularly, this invention relates to a stepping motor control unit and method, a printer employing the same, and an information recording medium, wherein a stepping motor is controlled by feeding back the magnitude of rotation in order to ensure a high rotating speed of the stepping motor, or a high print head moving speed or high paper feeding speed of a printer driven by the stepping motor.
BACKGROUND ART
The stepping motor is one type of motor for supplying power by rotating a rotor. The stepping motor has a plurality of phases magnetically defined therefor. The state of an applied magnetic field is predetermined in association with each phase. The rotor of the stepping motor has a stable orientation thereof determined in association with each phase or magnetic state. The stable orientation is a orientation in which the rotor is stable in terms of magnetic energy. When a magnetic field associated with a certain phase is applied, the rotor rotates to assume a stable orientation at the phase. When the rotor reaches the phase to assume the stable orientation, if a magnetic field associated with another phase is applied, the rotor rotates to assume the next stable orientation. If this sequence is repeated, the rotor can be rotated continuously. An angular movement made to switch one phase into another shall be referred to as one step. The time during which a magnetic field associated with a certain phase is applied shall be referred to as a step time.
The stepping motor is adopted as, for example, a print head moving unit or paper feeding unit for a printer because the magnitude of rotation (rotation angle) of the rotor can be controlled accurately. When the stepping motor is adopted as the print head moving unit for a printer, the stepping motor is initially stopped or is not rotated, then is rotated and accelerated, is rotated on steadily, is rotated and decelerated, and is then stopped again. This cycle is repeated.
The step time required for each step is measured experimentally in advance. Measured step times are stored in a recording medium such as a read-only memory (ROM). The stepping motor is then driven based on the set step times according to each state of rotation (acceleration, steady rotation, or deceleration). This technique is often adopted.
However, the stepping motor is different from product to product due to manufacturing or use conditions. Moreover, the print head moving unit for a printer, or the paper feeding unit therefor, is different from product to product in terms of load to be imposed during driving because of the manufacturing or use conditions. The magnitude of rotation of the stepping motor therefore cannot be controlled accurately by merely designating step times. Consequently, the magnitude of movement of the print head in a printer or the magnitude of paper feed therein cannot be controlled accurately.
Feedback control is adopted as a means for overcoming the foregoing drawbacks. More particularly, a means shown in
FIG. 7
is employed.
(a) A disk
703
having slits
702
coincident with phases is attached to a rotor
701
of a stepping motor. The disk
703
rotates about an axis of rotation
700
together with the rotor.
(b) An optical sensor
704
and a projector
705
are located on a portion of the stepping motor that is not rotated, for example, on a stepping motor body. Light emanating from the projector
705
passes through the slits
702
and reaches the optical sensor
704
.
(c) The motion of the rotor is detected using the slits
702
, optical sensor
704
, and projector
705
. When the rotor is rotated, a signal (hereinafter, a signal t) is generated by the optical sensor
704
.
(d) A magnetic field associated with a certain phase is constantly applied over a predetermined step time. The step time is set to a value recorded in advance on, for example, a ROM.
(e) When the predetermined step time elapses, it is determined from the signal t whether the rotor has been rotated.
(f) The magnetic field associated with the phase is constantly applied until the rotor assumes a stable orientation. The rotor will not be shifted to the next phase until the signal t demonstrates that the rotor has assumed the stable orientation.
The control steps (e) and (f) can be achieved by means of a rotor control circuit (not shown) realized with, for example, a logic circuit.
(g) The control steps (d) to (f) are repeated in order to rotate the rotor
701
continuously.
The slits
702
are bored as thinly as possible and are positioned at the center of between stable positions so that the rotor of a stepping motor is rotated bi-directionally according to the above control sequence.
However, when the stepping motor is controlled as mentioned above, there is difficulty in driving the stepping motor fast. This is because when phases are switched after the signal t is output, a large electrical advance angle cannot be attained and sufficient torque cannot be exerted. It has been discovered that the signal t is often generated late for a predetermined step time, especially in either of the following states:
(a) a state established from the instant the stepping motor is stopped to the instant it is accelerated; or
(b) a state of steady rotation.
When the stepping motor is accelerated to be driven fast or rotated on a steady basis, a wait time required until the stepping motor receives the signal t becomes longer. For example, when the stepping motor is adopted as a print head moving unit for a printer, it takes substantial time to accelerate. Consequently, a distance by which a print head can move in a state of steady rotation becomes shorter. The time required for the print head to reciprocate once is prolonged. Otherwise, since the stepping motor cannot be shifted to the next phase until the signal t is detected, a desired moving speed at which the print head is moved cannot be attained in the state of steady rotation.
Moreover, for driving the stepping motor fast, it is necessary to decrease the number of times, by which phases are switched for acceleration or deceleration, as much as possible. However, to decrease the number of times, energy to be given to the motor must be increased. Consequently, the rotor may be rotated too far. However, this depends on a product or a driven state. Another signal t may then be generated to indicate the next magnetically stable position. This incident occurs frequently, especially during deceleration. Consequently, the foregoing feedback control cannot be extended accurately.
A means for overcoming the above drawback is, for example, described in “Theory and Application of Stepping Motors” written by Hajime Ohki (Jikkyo Publishing co., Ltd, 1979, P. 162), wherein a plurality of optical sensors is employed. However, this means has a drawback in that control is complex and the cost of manufacturing is increased.
The present invention attempts to overcome the above drawbacks. An object of the present invention is to provide a stepping motor control unit and method suitable for fast driving, a printer employing the same, and an information recording medium.
DISCLOSURE OF THE INVENTION
A stepping motor control unit in accordance with the present invention comprises the following means:
(a) a rotor for supplying power from a stepping motor;
(b) a magnetic field applying means having a plurality of phases, to which the rotor is rotated, defined therefor;
(c) an orientation detecting means that has a first stable position and second stable position, to which the rotor is rotated by predetermined angles from a magnetically stable orientation in a direction in which the rotor is rotated, associated with a plurality of phases, and that when the rotor is rotated from the first stable position to the second stable position, detects the rotor at a given position near the first stable position;
(d) a control means for, after magnetic fields applied by the magnetic field applying
Gabrik Michael T.
Masih Karen
Seiko Epson Corporation
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