Electricity: motive power systems – Open-loop stepping motor control systems
Reexamination Certificate
2001-03-29
2004-03-30
Masih, Karen (Department: 2837)
Electricity: motive power systems
Open-loop stepping motor control systems
C318S685000, C318S599000, C388S804000
Reexamination Certificate
active
06713985
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus for driving a stepping motor, and more particularly, to a method and apparatus for driving a stepping motor using a micro step driving mode where an exciting current which is increased and decreased step by step is energized to each phase.
2. Related Background Art
Various instruments are driven using a motor driven by pulse signals, for example, a stepping motor. A shutter device, for example, where a shutter blade is driven to be opened and shut using the stepping motor is known as disclosed in Japanese Utility Model Application Laid-Open No. 60-140934, Japanese Patent Publication No. 6-64281 and the like. A lens barrel device where a lens is moved along an optical axis using the stepping motor is also known as proposed in Japanese Patent Application Laid-Open No. 6-250070, for example. In the above mentioned stutter device and lens barrel, driving frequency or number of driving pulses (steps) of the stepping motor is controlled so that accurate control is carried out with respect to a speed and position of opening and closing of the shutter blade or a position of the lens along the optical axis, thereby permitting more proper photographing with a camera compared with the case of using a direct current motor or the like.
As a technique of minute control of positions, a micro step driving mode is known where an energized electrical current to a coil varies step by step to stop a rotor of the motor in a position corresponding to the energized electrical current. Also known is an energizing method to a coil of a stepping motor of two-phase permanent magnet type disclosed in Japanese Patent Application Laid-Open No. 9-047088 or the like.
A typical energizing method of the micro step driving is shown in FIG.
39
. In this figure, a horizontal axis shows passage of time and a vertical axis shows values of electrical currents which flows in phases A and B of coils which are shifted 90° each other. Each electrical current value is divided into a large number of steps and periodically repeats increasing and decreasing, and energizing the electrical current of such a value in each phase step by step allows the motor to be rotatably driven in small steps.
However, in the case where such a micro step driving mode is used to drive the shutter blade, lens or other delicate instruments by the stepping motor and stop to hold the rotor of the stepping motor in the position corresponding to the energized electrical current to the coil, there is a problem that stopping positions vary due to backlash of a reduction gear of a gear or a screw or due to inertial friction of a sliding portion.
Full step driving has a higher speed of rotation and output torque of the motor than the micro step driving does, while the micro step driving obviously has a higher rotational resolution of the output shaft of the motor than the full step driving does. For this reason, in case of situating a driven object in a predetermined position, the object is driven at high speed by the full step driving when far from a target position and on the way driven by the micro step driving when close to the target position to be minutely positioned, which permits positioning at high speed and accuracy.
Alternatively, in order to start moving smoothly, the object is driven at low speed by the micro step driving at first, gradually speeded up, and from more than predetermined speed, driven at much higher speed by the full step driving, which permits smooth driving of the driven object at high speed.
Energizing when switching the driving mode of the stepping motor from the micro step driving to full step driving or vice versa during driving is disclosed in Japanese Patent Application Laid-Open No. 9-023695 and herein shown in FIG.
40
. The stepping motor of PM type using a two phase permanent magnet is, for example, disclosed in Japanese Patent Application Laid-Open No. 9-331666.
This motor is configured in such a manner that cylindrically formed is a rotor in the form of the permanent magnet which is circumferentially equally divided and alternately attracted to different poles, that a first coil, a rotor and a second coil are arranged in turn axially of the rotor, that a first outer magnetic pole and a first inner magnetic pole excited by the first coil are faced to an outer surface and an inner surface of the rotor, and that a second outer magnetic pole and a second inner magnetic pole excited by the second coil are faced to the outer surface and the inner surface of the rotor, and a rotor axis as a rotary axis is taken out of the cylindrical permanent magnet.
A sectional view thereof is shown in
FIG. 41. A
relationship between phase of attraction of the rotor having the permanent magnet and the first outer magnetic pole and first inner magnetic pole is shifted 360°/(2×number of attracted poles) with respect to a relationship between the phase of attraction of the rotor in the form of the permanent magnet and the second outer magnetic pole and second inner magnetic pole, namely shifted 45° in Japanese Patent Application Laid-Open No. 9-331666 which has four poles.
However, in the case where energizing of the coil is changed from the micro step driving to full step driving as shown in
FIG. 40
, taking the stepping motor proposed in Japanese Patent Application Laid-Open No. 9-331666 mentioned above for example, the position of the rotor at a time point T
1
in
FIG. 40
is as shown in
FIGS. 42A and 42B
, and at a time point T
2
as shown in
FIGS. 43A and 43B
.
FIGS. 42A and 43A
show sections taken along A of
FIG. 41
, and
FIGS. 42B and 43B
show sections taken along B of FIG.
41
. There is a difference of
74
degree between positions of rotation of the rotor shown in
FIGS. 42A and 42B
and
FIGS. 43A and 43B
, which means that the change is not smoothly made. Therefore, vibration or step out occurs to make it impossible to smoothly drive the driven object at high speed and to position the same at high speed and accuracy.
In the case where the micro step driving mode is used to drive the lens or others by the stepping motor and stop to hold the rotor of the stepping motor in the position corresponding to the energized electrical current to the coil, there is the problem that the stopping positions vary due to the backlash of the reduction gear of the gear or screw or due to the inertial friction of the sliding portion.
As another problem of the micro step control, if a driving force is low for positioning the rotor in a position where the rotor is stopped, the rotor is influenced by the friction or driving load and cannot stop at the predetermined position, leaving a difference in stopping position. Especially, when the position of the rotor comes closer to an original position to be positioned by the micro step control, the rotational driving force becomes lower which is produced to try to position in the original position so that only a slight friction force is produced which makes it difficult to position in the original position.
On the other hand, when merely rotated, the rotor is easy to be rotated with a certain delay relative to an electrical signal even if there is the friction. Namely, if the position is delayed from the original position relative to a certain energizing condition, the driving force increases to a certain extent so that the force for trying to rotate to the original position is relatively high. For this reason, the influence of the friction is less in mere rotation compared to stopping to position in the original position as described above.
Increasing the driving force can reduce the difference in stopping position, which requires high current flown in the coil, so that there is a disadvantage of consuming high power and of heating the motor, which results in a poor characteristic.
A conventional stepping motor of a small cylindrical shape which carries out the micro step control or the like is as shown in
FIG. 44. A
stator
102
is configured in such a manner that a stato
Fitzpatrick ,Cella, Harper & Scinto
Masih Karen
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