Method of driving stepping motor

A method of driving a stepping motor comprises dividing a drive signal for each drive step into a plurality of portions in the direction of a time axis, and controlling the drive signal in connection with the plurality of portions thereof in a way of PWM so as to obtain a pattern of a gradual change in a on-duty ratio of the drive signal.

BACKGROUND OF THE INVENTION 
The present invention relates to a method of driving a stepping motor and 
more particularly, to a technique of restraining vibration of the stepping 
motor during drive thereof. 
One of conventional methods of driving a stepping motor is disclosed, for 
example, in JP-A 4-58790. In this reference, upon switching of a waveform 
of a drive signal for driving stepwise a stepping motor, the on-duty ratio 
of the drive signal is gradually changed by pulse width modulation (refer 
hereafter to as PWM) control so as to retrain a settling due to sudden 
step rotation. By this, the stepping motor has reduced vibration noise 
during step drive. 
However, the above conventional method of driving a stepping motor produces 
the following inconveniences: 
PWM control as adopted in the conventional method is designed to gently 
carry out a rise of drive current output to the stepping motor by 
restricting a time duration of supply of the drive signal, so that an 
average of drive current is lowered at each drive step, resulting in 
lowered torque of the stepping motor during PWM control. If source voltage 
of a stepping motor drive circuit is decreased during PWM control, drive 
torque is lowered further, resulting in a possible erroneous operation of 
the stepping motor due to the relationship between load and drive torque. 
By way of example, when the stepping motor serves as an actuator for 
driving damping-force-characteristic varying means of a 
variable-damping-force-type shock absorber for a motor vehicle, the 
damping-force-characteristic varying means undergo a load based on a fluid 
force. Thus, if source voltage of the stepping motor drive circuit is 
lowered during PWM control due to a great variation in source voltage 
according to a service of the other electric devices in the motor vehicle, 
etc., and/or the stepping motor undergoes a great load suddenly, the 
stepping motor is out of good operation, resulting in erroneous control. 
It is noted that a variation in source voltage can be corrected by 
providing a constant current circuit based on feedback control of current, 
which produces, however, another inconvenience of a cost increase due to 
the complicated circuit structure and increased number of parts. 
Moreover, according to the conventional method, the stepping motor is 
driven smoothly only in an initial portion of each drive step, but driven 
stepwise in accordance with the drive signal in the same way as in the 
prior art, failing to fully eliminate vibration of the stepping motor 
during step drive thereof. 
When driving the stepping motor for driving damping-force-characteristic 
varying means of a variable-damping-force-type shock absorber, a waveform 
of damping force has disarrayed portions due to intermittent switching of 
a damping-force characteristic, which is therefore increased with an 
enlargement of the width of each drive step of the stepping motor 3. 
It is, therefore, an object of the present invention to provide a method of 
driving a stepping motor which enables a prevention of an erroneous 
operation of the stepping motor and a restraint of occurrence of vibration 
of the stepping motor during step drive thereof. 
SUMMARY OF THE INVENTION 
According to one aspect of the present invention, there is provided a 
method of driving a stepping motor having a plurality of drive steps, the 
stepping motor being driven by a drive signal, the method comprising the 
steps of: 
dividing the drive signal for each drive step into a plurality of portions 
in a direction of a time axis; 
controlling the drive signal in connection with said plurality of portions 
thereof in a way of PWM so as to obtain a pattern of a gradual change in a 
on-duty ratio of the drive signal; and 
supplying the drive signal as controlled to the stepping motor. 
Another aspect of the present invention lies in providing, in a motor 
vehicle: 
a shock absorber including means for varying a damping-force 
characteristic; and 
a stepping motor mounted to said shock absorber at one end thereof, said 
stepping motor serving to drive said damping-force characteristic varying 
means of said shock absorber, said stepping motor having a plurality of 
drive steps and being driven by a drive signal, said stepping motor being 
driven in accordance with a method comprising the steps of: 
dividing said drive signal for each drive step into a plurality of portions 
in a direction of a time axis; 
controlling said drive signal in connection with said plurality of portions 
thereof in a way of PWM so as to obtain a pattern of a gradual change in a 
on-duty ratio of said drive signal; and 
supplying said drive signal as controlled to said stepping motor.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the drawings wherein like reference numerals designate like 
parts throughout the views, a description will be made with regard to 
preferred embodiments of a method of driving a stepping motor according to 
the present invention. 
FIGS. 1-6 show a first embodiment of the present invention. Referring first 
to FIG. 1, a method of the present invention is applied to a stepping 
motor 3 for driving damping-force-characteristic varying means of a 
variable-damping-force-type shock absorber SA. 
Referring to FIG. 2, the shock absorber SA is provided with a cylinder 1, a 
piston 2 serving to define an upper chamber A and a lower chamber B of the 
cylinder 1, an outer tube 4 serving to form a reservoir chamber C on the 
outer periphery of the cylinder 1, a base 5 serving to define the lower 
chamber B and the reservoir chamber C, a guide member 7 serving to 
slidably guide a piston rod 6 connected to the piston 2, a suspension 
spring 8 interposed between the outer tube 4 and a vehicular body (not 
shown), and a bumper rubber 9. Moreover, a rotary valve 10 as 
damping-force-characteristic varying means is arranged in the piston rod 
6, and the stepping motor 3 is mounted to the shock absorber SA at the 
upper end thereof so as to drive stepwise the rotary valve 10 through a 
control rod 11. 
Referring to FIG. 3, the stepping motor 3 is of a type of unipolar 
four-phase drive and 2--2 phase excitation. 
Referring again to FIG. 1, a control unit 12 comprises an interface circuit 
12a, an arithmetic circuit 12b, a drive signal generating circuit 12c, a 
PWM control circuit 12d, and a stepping motor drive circuit 12e. 
Input to the interface circuit 12a are signals out of an acceleration 
sensor or G-sensor 13, a rudder-angle sensor 14, and a vehicular velocity 
sensor 15. The arithmetic circuit 12b is arranged to obtain control 
signals for optimally controlling a damping-force characteristic of the 
shock absorber SA of a motor vehicle in accordance with the signals out of 
the interface circuit 12a. 
The drive signal generating circuit 12c is arranged to output a drive 
signal for driving stepwise the stepping motor 3 of the shock absorber SA 
to a target step position in accordance with the control signals input 
from the arithmetic circuit 12b. 
The PWM control circuit 12d is arranged to divide, for each drive step, the 
drive signal input from the drive signal generating circuit 12c into four 
in the direction of a time axis, each being subjected to PWM control so 
that the on-duty ratio of the drive signal at each drive step is increased 
or decreased stepwise, which is supplied to the stepping motor drive 
circuit 12e. 
Referring to FIGS. 4A and 4B, time charts show the on-duty ratio of the 
drive signal and applied voltage at each divided drive step with respect 
to the phases A, A', B, and B', respectively. In this embodiment, the 
on-duty ratio of the drive signal at each divided drive step is set to 0%, 
38%, 71% 92% and 100%, and can be thus switched in four grades between 0% 
and 100%. Moreover, the on-duty ratio can be subdivided in four grades for 
each drive step by a stepwise increase from 0% at the phase A, and a 
stepwise decrease from 100% at the phase B. 
Referring to FIG. 5A, a fully-drawn line shows in a bar graph the on-duty 
ratio (0%, 38%, 71%, 92%, and 100%) of the drive signal at each divided 
drive step. As shown in FIG. 5A, each drive step is partitioned equally so 
that the on-duty ratio has a waveform similar to a sine wave, and the 
phases A and B are shifted by 90.degree. with each other. By this, 
referring to FIG. 5B, as indicated by a fully-drawn line, a rotor of the 
stepping motor 3 can be held by 1/4 a conventional step drive pitch as 
indicated by a one-dot chain line due to a balance with applied current or 
magnetic force. That is, the width of each drive step can be reduced to 
1/4 the conventional step drive pitch, resulting in a great restraint of 
vibration generated during drive of the stepping motor 3. 
Referring to FIG. 6, a time chart shows a waveform of damping force when 
driving the stepping motor 3 according to the method of the present 
invention. As seen from FIG. 6, when a position of the 
damping-force-characteristic varying means of the shock absorber SA is 
shifted from a soft position to a hard position, the waveform of damping 
force has no disarrayed portion. 
As described above, the above method of driving the stepping motor 3 
produces the following effects: 
1) Vibration generated during drive of the stepping motor 3 can be largely 
restrained, resulting in a reduced level of noise transmitted in a 
vehicular room; and 
2) Switching of a damping-force characteristic is carried out in subdivided 
steps, having no disarrayed portion of the waveform of damping force. 
It is noted that in place of the control unit 12 as shown in FIG. 1, the 
control unit 12 as shown in FIG. 7 may be used wherein the arithmetic 
circuit 12b provides signals to both drive signal generating circuit 12c 
and PWM control circuit 12d, and the PWM control circuit 12d provides a 
signal to the drive signal generating circuit 12c so as to correct a drive 
signal thereof. Moreover, the control unit 12 may be as shown in FIG. 8, 
comprising a software having an arithmetic part 121, a drive signal 
generating part 122, a PWM control part 123, and a data part 124 arranged 
in a micro processing unit (MPU), which enables a great reduction in 
system cost. 
FIGS. 9-12C show a second embodiment of the present invention, which is 
substantially the same as the first embodiment. Referring to FIG. 9, the 
control unit 12 comprises the interface circuit 12a, arithmetic circuit 
12b, drive signal generating circuit 12c, PWM control circuit 12d, and 
stepping motor drive circuit 12e. 
A vehicular power source of DC 12V is connected to the stepping motor 3, 
and voltage of DC 5V obtained from the vehicular power source through a 
transformer circuit 12f is supplied to the arithmetic circuit 12b. Voltage 
supplied to the stepping motor 3 or source voltage is detected by a 
voltage detector circuit 12g, and fed back to the arithmetic circuit 12d. 
Input to the interface circuit 12a are signals out of the acceleration 
sensor or G-sensor 13, rudder-angle sensor 14, and vehicular velocity 
sensor 15, and brake sensor 16. The arithmetic circuit 12b is arranged to 
obtain control signals for optimally controlling a damping-force 
characteristic of the shock absorber SA in accordance with the signals out 
of the interface circuit 12a. 
The drive signal generating circuit 12c is arranged to output a drive 
signal for driving stepwise the stepping motor 3 of the shock absorber SA 
to a target step position in accordance with the control signals input 
from the arithmetic circuit 12b. 
The PWM control circuit 12d is arranged to divide, for each drive step, the 
drive signal input from the drive signal generating circuit 12c into four 
in the direction of a time axis, each being subjected to PWM control so 
that the on-duty ratio of the drive signal at each drive step is increased 
or decreased stepwise, which is supplied to the stepping motor drive 
circuit 12e. 
Moreover, the arithmetic circuit 12b serves to carry out selection control 
of a drive pattern of the drive signal that the PWM control circuit 12d 
provides to the drive signal generating circuit 12c in accordance with a 
variation in source voltage fed back from the voltage detector circuit 
12g. Specifically, when voltage supplied to the stepping motor S or source 
voltage is greater than DC 12V, selective switching control is carried out 
to a basic drive pattern, whereas when the voltage is smaller than DC 12V 
and greater than DC 9V, selective switching control is carried out to a 
first corrected drive pattern. Moreover, when the voltage is smaller than 
DC 9V and greater than DC 8V, selective switching control is carried out 
to a second corrected drive pattern. 
First, the basic drive pattern upon normal source voltage will be 
described. Referring again to FIGS. 4A and 4B, the time charts show the 
on-duty ratio of the drive signal and applied voltage at each divided 
drive step with respect to the phases A, A', B, B', respectively. In the 
basic drive pattern, the on-duty ratio of the drive signal at each divided 
drive step is set to 0%, 38%, 71%, 92%, and 100%, and can be thus switched 
in four grades between 0% and 100%. Moreover, the on-duty ratio can be 
subdivided in four grades for each drive step by a stepwise increase from 
0% at the phase A, and a stepwise decrease from 100% at the phase B. As 
shown in FIGS. 4A and 4B, this embodiment has four divisions of the drive 
signal in each divided drive step, with a sampling period of 6.66 ms and a 
chopping frequency of 4 KHz, so that a time duration of one division of 
the drive signal and the number of switching or repetition of turn-on and 
turn-off are as follows: 
Time duration of one division: 6.66+4=1.665 ms 
Number of switching: 1.665+0.25=6.66 times (note that one cycle of 4 KHz is 
0.25 ms) 
Referring again to FIG. 5A, the fully-drawn line shows in a bar graph the 
on-duty ratio (0%, 38%, 71%, 92%, and 100%) of the drive signal at each 
divided drive step. As shown in FIG. 5A, each drive step is partitioned 
equally so that the on-duty ratio has a waveform similar to a sine wave, 
and the phases A and B are shifted by 90.degree. with each other. By this, 
referring to FIG. 5B, as indicated by a fully-drawn line, a rotor of the 
stepping motor 3 can be held by 1/4 a conventional step drive pitch as 
indicated by a one-dot chain line due to a balance with applied current or 
magnetic force. That is, the width of each drive step can be reduced to 
1/4 the conventional step drive pitch, resulting in a great restraint of 
vibration generated during drive of the stepping motor 3. 
Referring again to FIG. 6, the time chart shows a waveform of damping force 
when driving the stepping motor 3 according to the method of the present 
invention. As seen from FIG. 6, when a position of the 
damping-force-characteristic varying means of the shock absorber SA is 
shifted from a soft position to a hard position, the waveform of damping 
force has no disarrayed portion. 
Next, the corrected drive patterns upon source voltage lowered will be 
described. 
In the first corrected drive pattern, the on-duty ratio of the drive signal 
at each divided drive step is set to 0%, 52%, 80%, 95%, and 100%. FIG. 10 
shows in a bar graph the on-duty ratio (0%, 52%, 80%, 95%, and 100%) of 
the drive signal at each divided drive step. As seen from FIG. 10, the 
initial on-duty ratio is increased as compared with the basic drive 
pattern to have the shape similar to a rectangle as a whole, obtaining an 
increased average of drive current at each divided drive step. 
In the second corrected drive pattern, referring to FIG. 11, in order to 
obtain a further increased average of drive current at each divided drive 
step, the on-duty ratio of the drive signal is set to 100% simply. That 
is, PWM control of the drive signal in the PWM control circuit 12d is 
stopped. 
Referring to FIGS. 12A-12C, graphs show the on-duty ratios of the drive 
signal in the above drive patterns at each divided drive step thereof. 
Specifically, FIG. 12A illustrates the basic drive pattern, FIG. 12B 
illustrates the first corrected drive pattern, and FIG. 12C illustrates 
the second corrected drive pattern. 
Therefore, upon source voltage lowered, since selective switching control 
is carried out to the first corrected drive pattern or the second 
corrected drive pattern in accordance with a degree of lowering S of 
source voltage, a decrease in torque generated by source voltage lowered 
can be corrected by an increase in an average of drive current. 
As described above, the above method of driving the stepping motor 3 
produces the following effects: 
1) An erroneous operation of the stepping motor S due to source voltage 
lowered can be prevented during PWM control; 
2) Vibration generated during drive of the stepping motor S can be largely 
restrained, resulting in a reduced level of noise transmitted in a 
vehicular room; and 
3) Switching of a damping-force characteristic is carried out in subdivided 
steps, having no disarrayed portion of the waveform of damping force. 
Third, fourth, and fifth embodiments of the present invention will be 
described, which are substantially the same as the second embodiment. 
Referring to FIG. 13, in the third embodiment, in accordance with a map of 
switching of the four drive patterns P with a variation in source voltage 
V, the on-duty ratio (%) of the drive signal is switched in seven grades 
in place of three grades in the second embodiment. 
In the fourth embodiment, the on-duty ratios (4) of the drive signal in the 
four drive patterns are obtained out of the formulae (1)-(4): 
EQU 1) D.sub.1 =-K.sub.1 .multidot.V+.alpha. (1) 
EQU 2) D.sub.2 =-K.sub.2 .multidot.V+.beta. (2) 
EQU 3) D.sub.3 =-K.sub.3 .multidot.V+.gamma. (3) 
EQU 4) D.sub.4 =100 (4) 
wherein .alpha.,.beta., and .gamma. are fundamental coefficients, and 
K.sub.1, K.sub.2, and K.sub.3 are correction coefficients in accordance 
with source voltage V. Referring to FIG. 14, each coefficient is 
established to a value which ensures a variable characteristic of the 
on-duty ratio of the drive signal with respect to source voltage V as 
indicated by D.sub.1, D.sub.2 or D.sub.3. 
Referring to FIG. 15, the fifth embodiment has six divisions of the drive 
signal in each divided drive step, with a sampling period of 5 ms and a 
chopping frequency of 8 KHz in place of four divisions with a sampling 
period of 6.66 ms and a chopping frequency of 4 KHz in the above 
embodiments. The increased number of divisions in each divided drive step 
enables further smoothed drive of the stepping motor 3. 
Having described the present invention in connection with the preferred 
embodiments, it is noted that the present invention is not limited 
thereto, and various changes and modifications are possible without 
departing from the spirit of the present invention. 
By way of example, in the embodiments, the stepping motor S is of a type of 
unipolar four-phase drive and 2--2 phase excitation. Alternatively, the 
stepping motor of the other type may be used. 
Moreover, in the embodiments, the present invention is applied to PWM 
control carried out at each drive step in its entirety. Alternatively, the 
present invention is applicable to PWM control carried out simply in an 
initial portion of each drive step.