Electricity: motive power systems – Open-loop stepping motor control systems
Reexamination Certificate
2001-08-07
2003-06-03
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Open-loop stepping motor control systems
C318S685000, C318S434000
Reexamination Certificate
active
06573680
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a driving method and a driving circuit for a stepping motor which drives a carriage and the like of a scanner, for example, in an image forming apparatus and the like.
In general, for example, in the driving method for a stepping motor which drives a carriage and the like of a scanner, for example, a micro step driving is performed. That is, by adopting the micro step driving, the number of partitions is increased or decreased in relation to the vibration generated in a low velocity area of the stepping motor, thereby allowing a peak current value to be variable and decreasing the vibration.
Now, in
FIG. 1
, an ideal value of a phase current waveform (assumed to be of ⅛ partition herein) in the micro step driving is shown, and it will be described below.
In general, regarding a current control, a chopping system is adopted, in which an ON state for causing current to flow for a target current value and an OFF state for not causing current to flow are controlled by a higher frequency (20 to 50 KHz) than a motor driving frequency.
In the current control by this chopping system, a rise through rate and a fall through rate of a motor driver are constant.
Hence, according to characteristics of the rise through rate and the fall through rate of stepwise waveforms as shown in
FIG. 1
, the rise through rate and the fall through rate are controlled by current waveforms as shown in
FIGS. 2
to
9
. That is,
FIG. 2
shows a waveform when a current waveform at a point (X) of
FIG. 1
is controlled by the rise through rate “large” and the fall through rate “large”.
FIG. 3
shows a waveform when a current waveform at a point (Y) of
FIG. 1
is controlled by the rise through rate “large” and the fall through rate “large”.
Also,
FIG. 4
shows a waveform when a current waveform at a point (W) of
FIG. 1
is controlled by the rise through rate “large” and the fall through rate “large”.
FIG. 5
shows a waveform, in which a current waveform at a point (Z) of
FIG. 1
is controlled by the rise through rate “large” and the fall through rate “large”
In the current waveforms shown in these
FIGS. 2
to
5
, since the rise through rate is “large”, the rise of current is quick. Hence, an overshoot and an undershoot are generated largely so as to make a ripple of the current large.
Particularly, as evident from
FIGS. 3 and 4
, when a step difference of the current is small, the ripple of the current becomes larger than the step difference of the current and is not controlled correctly to the stepwise waveforms (Y) and (W). Also, as shown in
FIGS. 2 and 5
, when the step difference of the current is large, since the ripple of the current is smaller than the step difference of the current, it relatively comes closer to the stepwise waveform. However, the current ripple does not differ from
FIGS. 3 and 4
. The current being unable to be controlled to the correct stepwise waveforms due to the influence of the ripple of the current is a cause of generating vibration without being able to correctly control the rotation of the motor.
On the other hand,
FIG. 6
shows a waveform when the current waveform at the point (X) of
FIG. 1
is controlled by the rise through rate “small” and the fall through rate “small”.
FIG. 7
shows a waveform when the current waveform at the point (Y) is controlled by the rise through rate “small” and the fall through rate “small”.
Also,
FIG. 8
shows a waveform when the current waveform at the point (W) is controlled by the rise through rate “small” and the fall through rate “small”.
FIG. 9
shows a waveform when the current waveform at the point (Z) is controlled by the rise through rate “small” and the fall through rate “small”.
In
FIG. 6
, since the step difference of the current is large and the rise through rate is “small”, the rise of the current is delayed and takes a delay time of td
1
and does not follow the current waveform (X) of FIG.
1
. In
FIG. 9
, since the step difference of the current is large and the fall through rate is “small”, the fall of the current is delayed and takes a delay time of td
2
and does not follow the current waveform (Z) of FIG.
1
. The generating of the delayed time in such a manner means that the control of the motor is delayed. The current being unable to be controlled to the correct stepwise waveforms due to the influence of the delay time of the current is a cause of generating vibration without being able to correctly control the rotation of the motor.
In
FIGS. 7 and 8
, since the step difference of the current is small, even if the through rate is small, the waveform follows stepwise thereby it is possible to control the current. Also, since the rise through rate or the fall through rate is small, the ripple is also made small.
In this way, in the method for driving a stepping motor according to the prior art, since it is a control method by allowing the rise through rate and the fall through rate of the current to be constant, even if a waveform can be controlled in a part of the stepwise waveforms, in other parts thereof, a large ripple or a large delay time in a follow-up property of the current control has been generated. Thus, it is not possible to control the waveform to a correct stepwise waveform and the generation of the vibration has been caused.
Now, in
FIGS. 10A and 10B
, a relationship between a phase A and a phase B in the case where a relationship between a micro step driving current waveform and a drive through rate is used in the conventional rise through rate “large” and the fall through rate “large” is shown, and it will be described below.
The driving current waveform of the phase A at a point (P) of
FIG. 10A
denotes FIG.
4
. The driving current waveform of the phase B at a point (P) of
FIG. 10B
denotes FIG.
2
.
At a point (X) of the phase B, the current ripple is “large” and yet it follows the stepwise waveform. However, at a point (W) of the phase A, since the ripple is large, it is unable to follow the stepwise waveform. This shows that, at a point (W) of the phase B, a rotational angle can maintain a position, but at a point (X) of the phase A, the rotational angle cannot maintain the position. In the micro step driving of the stepping motor, the rotational angle is controlled by a value of current caused to flow to the phase A and the phase B. However, at the point (P), the angle position of the phase A fluctuates and therefore it shows that the angle position of the stepping motor fluctuates. The generating of the fluctuation of the rotational angle shows that the vibration is generated in the stepping motor.
The driving current waveform of the phase A at a point (Q) of
FIG. 10A
denotes FIG.
5
. The driving current waveform of the phase B at a point (Q) of
FIG. 10B
denotes FIG.
3
.
At a point (Z) of the phase A, the current ripple is “large” and yet it follows the stepwise waveform. However, at a point (Y) of the phase B, since the ripple is large, it is unable to follow the stepwise waveform. This shows that, at a point (Z) of the phase A, the rotational angel can maintain the position, but at a point (Y) of the phase B, the rotational angle cannot maintain the position. Due to the same reason as the fluctuation of the rotational angle at the point (P), at the point (Q) also, the rotational angle fluctuates and the vibration is generated in the stepping motor.
On the other hand, in
FIGS. 11A and 11B
, the relationship between the phase A and the phase B in the case where the relationship between the micro step driving current waveform and the drive through rate is used in the conventional rise through rate “small” and the fall through rate “small” is shown, and it will be described below.
The driving current waveform of the phase A at a point (P) of
FIG. 11A
denotes FIG.
8
. The driving current waveform of the phase B at a point (P) of
FIG. 11B
denotes FIG.
6
.
At a point (W) of the phase A, the ripple follows the stepwise waveform, but at a point (X) of the phase B, since the rise of the current is delaye
Foley & Lardner
Toshiba Tec Kabushiki Kaisha
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