JP2001028896A - Method and device for controlling stepping motor, and image reader - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for controlling a stepping motor capable of reducing fluctuations in the speed of the stepping motor due to beat components of PWM frequency and phase current switching frequency. SOLUTION: By superposing modulation signals through a modulation circuit 51 and changing the PWM frequency controlling the phase current of a stepping motor 3, the beat components of phase current switching frequency and PWM frequency do not change into fixed frequency components, but have fluctuations suited to a change in the PWM frequency, so that the energy of the beat frequency components can be reduced more than in the case where the PWM frequency is fixed. Thus, it is possible to restrain fluctuations in the speed of the stepping motor 3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a stepping motor control method, an apparatus therefor, and an image scanner, and an image reading apparatus such as a scanner of a digital copying machine or a facsimile machine.

[0002]

2. Description of the Related Art Generally, in an image reading apparatus such as an image scanner, an original image is electrically scanned in a main scanning direction by a CCD line sensor, and the original image is exposed to light.
The scanning optical system for forming an image on the D-line sensor is scanned in the sub-scanning direction by a motor so that a document image is read two-dimensionally. In this case, a stepping motor is generally used as a motor for driving the scanning optical system to reciprocate in the sub-scanning direction, since it is possible to easily and accurately manage the same reciprocal movement of the scanning optical system. Used in

FIG. 12 shows a configuration example of a conventional drive control circuit for such a stepping motor. Schematically, a speed / direction control circuit 1 constituted by a CPU or the like
To drive clock CLK, direction control signal DIR, initialization signal RS
T is supplied to the phase switching / phase current control circuit 2, where the oscillation stipulating capacitance Ct and the oscillation stipulating resistor
At the oscillation frequency (pulse width modulation PWM frequency) determined by Rt, the voltage of the motor phase current detection resistor RS is compared with the phase current control voltage VREF, and the peak voltage of the motor phase current detection resistor RS is set to the phase current control voltage. The drive signals UB, VB, WB of the driver 4 for the three phases UU, VV, WW of the stepping motor 3 are controlled to be equal to VREF.

The control signals (U, UB,
V, VB, W, and WB) are time charts shown in FIG. 13 (FIG. 14 is a time chart showing an enlarged part of them).
Signals RST, CLK, DI from the speed / direction control circuit 1
R, and is initialized at the rising edge (CLK #) of the drive clock CLK when the initialization signal RST is at the H level.
B and WB are at H level, V, VB and W are at L level (therefore, U
/ UB / V / VB / W / WB: H / H / L / L / L / H), the UU-phase current IUU of the stepping motor 3 is negative (direction flowing out of the stepping motor 3), and the VV-phase current IVV
Is positive (direction in which the current flows into the stepping motor 3). At this time, the drive signal VB has a current value of V at the aforementioned PWM frequency.
Pulse width modulation is performed so that the value becomes proportional to REF. In this state, the current IWW of the WW phase is zero (IUU / IVV
/ IWW:-/ + / 0).

Next, the state changes at the rising edge (CLK #) of the drive clock CLK after the signal RST goes to L level, and U / UB / V / VB / W / WB: H / H / L / H / L / L
And the current of each phase is IUU / IVV / IWW: − / 0 / +. That is, assuming that the rising edge (CLK #) of the drive clock CLK at the time of initialization is CLK0, the direction of the phase currents CLKn U / V / W / UB / VB / WB IUU / IVV / IWW CLK0 H / L / L / H / L / H − / + / 0 VV → UU CLK1 H / L / L / H / H / L− / 0 / + WW → UU CLK2 L / H / L / H / H / L 0 / − / + WW → VV CLK3 L / H / L / L / H / H + / − / 0 UU → VV CLK4 L / L / H / L / H / H + / 0 / −UU → WW CLK5 L / L / H / H / L / H 0 / + / − VV → WW. When the direction control signal DIR becomes L level, the above order is reversed.

FIG. 15 shows a phase switching / phase current control circuit 2.
5 shows a configuration example of a middle phase switching circuit 5, a 6-bit bidirectional shift register 7 composed of six flip-flops FF0 to FF5 and six multiplexers MPX0 to MPX5, and six OR gates 8a to 8f. And three AND gates 9a to 9c
And three inverters 10a to 10c provided on the output side of the AND gates 9a to 9c. The OR gates 8a to 8f and the AND gates 9a to 9
c, the inverters 10a to 10c also perform a level shift according to the operating potential of the driver 4. Also, AND gates 9a to 9c for generating drive signals UB, VB, WB
Is connected to the enable signal EN shown in FIG.

FIG. 16 shows an amplifier 11, internal reference voltages VR and NM.
A reference current source 13 composed of an OS type transistor 12 and an oscillation regulating resistor Rt, and two current mirrors 14, 15 and 2;
A bidirectional current source 18 composed of two NMOS transistors 16, 17; an oscillation regulating resistor Ct for charging / discharging the output current of the bidirectional current source 18; and a voltage of the oscillation regulating resistor Ct monitored to a constant voltage VTOP. Discharge when rising, constant voltage
Bidirectional current source 1 to start charging when discharged to VBOT
The oscillator circuit 22 composed of upper and lower comparators 19 and 20 for controlling the eight NMOS transistors 16 and 17 and the RS flip-flop 21, the terminal voltage of the current detection resistor RS and the phase current control voltage VREF are compared. Comparator 23
And the output of the comparator 23 and the output OSCO of the oscillation circuit 22
The enable signal EN is generated by the AND gate 24 which takes the logical product of the enable signal EN.

The driver 4 has the same configuration for the UU / VV / WW phases. The drive signal UB / VB / WB is applied to the gates of the PMOS transistors QUB, QVB and QWB, and the signal U / V / W is applied to the NMOS.
Type transistors QU, QV, QW.

[0009]

In the case of the stepping motor drive control circuit having the configuration shown in FIGS. 12 to 16, there is no problem at all if the ratio between the phase current switching frequency and the pulse width modulation frequency is sufficiently high. However, when the number approaches several times, the influence of the difference in the number of times that the phase current is controlled by the pulse width modulation in each phase becomes remarkable, and the phase current of each phase becomes uneven. This unevenness has a frequency component (beat frequency) of a difference between an integral multiple of the phase current switching frequency and the pulse width modulation frequency. If the frequency of the phase current unevenness matches mechanical resonance, a large speed fluctuation occurs. In this state, the image data read by the image reading device undergoes periodic expansion and contraction of the image at the beat frequency (mechanical resonance frequency). The periodic expansion / contraction is very conspicuous to human eyes, and causes a great deterioration in image quality.

SUMMARY OF THE INVENTION It is an object of the present invention to provide a stepping motor control method and apparatus which can reduce the speed fluctuation of the stepping motor due to the beat components of the pulse width modulation frequency and the phase current switching frequency.

It is another object of the present invention to provide an image reading apparatus having a stepping motor driving system capable of suppressing deterioration of the quality of a read image by utilizing such a stepping motor control apparatus.

[0012]

According to a first aspect of the present invention, there is provided a stepping motor control method for controlling a phase current of a stepping motor by pulse width modulation, wherein the modulation frequency of the pulse width modulation is changed. I did it.

Accordingly, by changing the modulation frequency of the pulse width modulation, the phase current switching frequency and the beat component of the pulse width modulation frequency do not become constant frequency components. As a result, the energy of the beat frequency component can be reduced as compared with the case where the modulation frequency of the pulse width modulation is constant, and therefore, the speed fluctuation of the stepping motor can be suppressed to be small.

According to a second aspect of the present invention, in the stepping motor control method of the first aspect, the modulation frequency of the pulse width modulation is randomly changed.

Therefore, in realizing the first aspect of the present invention, by randomly changing the modulation frequency of the pulse width modulation, the probability that the beat frequency coincident with the mechanical resonance appears becomes random, and the speed of the stepping motor is reduced. The periodicity of fluctuation is lost.

According to a third aspect of the present invention, in the stepping motor control method according to the first aspect, the modulation frequency of the pulse width modulation is changed into a rectangular wave.

Therefore, the invention according to claim 1 can be easily realized.

According to a fourth aspect of the present invention, in the stepping motor control method of the first aspect, the modulation frequency of the pulse width modulation is changed in a triangular waveform.

Therefore, the invention described in claim 1 can be easily realized.

According to a fifth aspect of the present invention, in the stepping motor control method for controlling the phase current of the stepping motor by pulse width modulation, the modulation frequency of the pulse width modulation is set to an integral multiple of the phase current switching frequency. The phase current is controlled as the frequency.

Therefore, for example, the externally supplied clock is used as the modulation frequency of pulse width modulation, and the phase current switching frequency is generated with reference to the externally supplied clock. No frequency difference between the modulation frequencies of the modulation occurs, and no speed fluctuation of the stepping motor due to this occurs.

According to a sixth aspect of the present invention, there is provided a stepping motor control device for controlling a phase current of a stepping motor by pulse width modulation by a phase switching / phase current control circuit. And a modulation circuit for changing.

Therefore, by changing the modulation frequency of the pulse width modulation by the modulation circuit, the beat components of the phase current switching frequency and the pulse width modulation frequency do not become constant frequency components, and the modulation frequency of the pulse width modulation by the modulation circuit is not changed. As a result, the energy of the beat frequency component can be reduced as compared with the case where the modulation frequency of the pulse width modulation is constant, so that the speed fluctuation of the stepping motor can be suppressed to be small.

According to a seventh aspect of the present invention, in the stepping motor control device according to the sixth aspect, the modulation circuit comprises:
A modulation circuit for outputting a modulation signal of a random frequency to the phase switching / phase current control circuit.

Therefore, in realizing the invention according to claim 6, by using a modulation circuit that randomly changes the modulation frequency of pulse width modulation, the probability that a beat frequency coincident with mechanical resonance appears becomes random, The periodicity of the speed fluctuation of the stepping motor is lost.

According to an eighth aspect of the present invention, in the stepping motor control device according to the sixth aspect, the modulation circuit comprises:
A modulation circuit for outputting a rectangular wave-shaped modulation signal to the phase switching / phase current control circuit.

Therefore, by using a modulation circuit that outputs a rectangular wave-shaped modulation signal, the invention according to claim 1 can be easily realized.

According to a ninth aspect of the present invention, in the stepping motor control device according to the sixth aspect, the modulation circuit comprises:
A modulation circuit for outputting a triangular modulation signal to the phase switching / phase current control circuit.

Therefore, by using a modulation circuit that outputs a triangular modulation signal, the invention according to claim 1 can be easily realized.

According to a tenth aspect of the present invention, there is provided a stepping motor control device for controlling a phase current of a stepping motor by pulse width modulation by a phase switching / phase current control circuit. A frequency dividing circuit for setting the modulation frequency of the pulse width modulation to a frequency that is an integral multiple of the phase current switching frequency is provided therein.

Therefore, for example, the externally supplied clock is used as the modulation frequency of the pulse width modulation, and the phase current switching frequency is generated as an integral multiple of the frequency dividing circuit based on the externally supplied clock.
There is no frequency difference between the integral multiple of the phase current switching frequency and the modulation frequency of the pulse width modulation, which is a problem in the related art, and the speed fluctuation of the stepping motor due to this is not generated.

[0032] The image reading apparatus according to the eleventh aspect of the present invention,
The original image is electrically scanned in a main scanning direction by a CCD line sensor, and a scanning optical system for exposing the original image and forming an image on the CCD line sensor is scanned in a sub-scanning direction by a stepping motor, and the original image is scanned. 11. An image reading apparatus for two-dimensionally reading, comprising a stepping motor control device according to claim 6 for controlling a phase current of said stepping motor.

Therefore, by using the stepping motor control device according to the sixth to tenth aspects to perform sub-scanning with the stepping motor whose speed fluctuation is suppressed, it is possible to suppress deterioration in the image quality of the read image.

[0034]

FIG. 1 shows a first embodiment of the present invention.
A description will be given based on FIG. The same parts as those shown in FIGS. 12 to 16 are denoted by the same reference numerals, and description thereof is omitted (the same applies to each of the following embodiments). This embodiment is applied to, for example, a scanner 31 in a digital copying machine as an image reading apparatus as shown in FIG. In the scanner 31, a contact glass 33 is provided on the upper surface of the digital copying machine main body 32, and a read document (not shown) is placed on the upper surface of the contact glass 33. And this contact glass 3
The first scanning unit 34 is movably supported at a position facing the first scanning unit 3, and the second scanning unit 35 is movably supported at a position facing the first scanning unit 34. The first scanning unit 34 has a halogen lamp 36 and a first mirror 37, and the second scanning unit 3
5 has second and third mirrors 38 and 39. A CCD line sensor 41 is provided at a position facing the third mirror 39 of the second scanning unit 35 via an imaging lens 40.
Are fixedly arranged.

Here, since the scanning speed of the first and second scanning units 34 and 35 constituting the scanning optical system is set to 2: 1, the first and second scanning units 34 and 35 are separated from the contact glass 33. The optical path length of the image forming optical path communicating with the CCD line sensor 41 through the first and second scanning units 3
Even if 4, 35 move, it is always constant. The image formed by the CCD line sensor 41 electrically scanning the reflected light of the read original placed on the contact glass 33 and illuminated by the halogen lamp 36 in the main scanning direction through the imaging optical path having the predetermined length. It is photoelectrically converted into a signal.

The first and second scanning units 34 and 35
Is a motor pulley 4 using a stepping motor 3 as a drive source.
3, mechanically reciprocatingly driven in the sub-scanning direction via a belt 44, a pulley 45, a wire (not shown), and the like.

The stepping motor 3 as a driving source for such mechanical scanning in the sub-scanning direction is shown in FIG.
The driving is controlled by a stepping motor control device D as shown in FIG. This stepping motor control device D is basically the same as that of the conventional example shown in FIG. 12, but in this embodiment, a modulation circuit 51 for changing the modulation frequency of pulse width modulation is added. Have been. Specifically, the oscillation regulating resistor Rt of the phase switching / phase current control circuit 2
Are connected by a modulation circuit 51 so as to superimpose a modulation current.

According to such a configuration, the modulation circuit 51 is connected to the connection of the oscillation regulating resistor Rt of the phase switching / phase current control circuit 2.
By superimposing the modulation current, the oscillation frequency (that is, the PWM frequency) of the oscillation circuit 22 of the phase switching / phase current control circuit 2 can be changed. According to this, the beat components of the phase current switching frequency and the PWM frequency do not become constant frequency components, but have a change in the beat frequency according to the superimposed current from the modulation circuit 51. That is, the energy of the beat frequency component corresponding to the mechanical resonance frequency of the mechanical scanning unit can be reduced as compared with the case where the PWM frequency is constant, and the speed fluctuation of the stepping motor 3 can be reduced.

FIGS. 3 and 4 show a second embodiment of the present invention.
It will be described based on. This embodiment shows a configuration example of a modulation circuit 51A which is an example of the modulation circuit 51 in FIG.

The modulation circuit 51A causes a current to flow through the Zener diode ZD1 by the resistor R1, generates a Zener voltage at the middle point, and converts the Zener voltage to the amplifier 52, the resistor R2,
The circuit is configured to amplify by an AC amplifier 53 by a capacitor C1, cut a DC component by a capacitor C2, convert to a current by a resistor R3, and output a modulation signal.

In this modulation circuit 51A, the noise of the Zener diode ZD1 is amplified and used as a modulation signal, so that the PWM frequency changes randomly. FIG. 4 shows an example of a random temporal change of the PWM frequency. For this reason,
The beat frequency is also random, and the stepping motor 3
The speed fluctuations are also random. In this case, not only is the energy of the beat frequency component corresponding to the mechanical resonance smaller than in the case where the PWM frequency is constant, but also the probability that the beat frequency corresponding to the mechanical resonance appears is random, and the cycle of the speed fluctuation of the stepping motor 3 is changed. Therefore, the deterioration of the image quality of the read image data as seen by a human can be suppressed to a small degree.

FIGS. 5 and 6 show a third embodiment of the present invention.
It will be described based on. This embodiment shows a configuration example of a modulation circuit 51B which is an example of the modulation circuit 51 in FIG.

The modulating circuit 51B includes a comparator 54
And an inverting integration circuit 55. The comparator 54 includes an amplifier 56 and feedback resistors R3 and R4, and has a hysteresis characteristic. The inverting integration circuit 55 includes an input resistor R5, an amplifier 57, and a capacitor C3 connected to its feedback path. The output of the inverting integration circuit 55 is fed back to the hysteresis generation section of the comparator 54 via the feedback resistor R6. I have. A DC cut capacitor C4 and a current conversion resistor R7 are connected to the output side of the amplifier 56.

In such a configuration, first, the amplifier 5
6 is positive, the resistors R3 and R4 allow the amplifier 56
Is also positive, and the output of the amplifier 57 in the inverting integration circuit 55 changes toward negative.
The + input of the amplifier 56 connected by the resistor R6 gradually decreases, and when the output of the amplifier 57 falls to a certain negative level, the + input of the amplifier 56 becomes negative and the output of the amplifier 56 also becomes negative. Then, the output of the amplifier 57 changes from decrease to increase, and changes toward positive. When the positive direction also rises to a certain voltage, the + input of the amplifier 56 becomes positive, and the output of the amplifier 56 also becomes positive. Thus, the output of the amplifier 56 becomes a rectangular wave. Therefore, the output of the amplifier 56 is connected to the capacitor C
4. The current is removed by the resistor R7 to remove the DC component and output to the phase switching / phase current control circuit 2. FIG. 6 shows a temporal change of the PWM frequency in this case.

Also in the case of this embodiment, the energy of the beat frequency component corresponding to the mechanical resonance frequency can be reduced as compared with the case where the PWM frequency is constant, as in the case of the above-described embodiment. 3 can be kept small.

FIGS. 7 and 8 show a fourth embodiment of the present invention.
It will be described based on. This embodiment shows a configuration example of a modulation circuit 51C which is an example of the modulation circuit 51 in FIG.

The modulation circuit 51C of this embodiment includes a comparator 54 and an inversion type integration circuit 5 like the modulation circuit 51B.
5, the modulation signal for the phase switching / phase current control circuit 2 is supplied to the amplifier 57 of the inverting integration circuit 55.
Has been taken from. A DC cut capacitor C5 and a current conversion resistor R8 are connected to the output side of the amplifier 57.

According to such a configuration, the output of the amplifier 57 has a triangular waveform. Therefore, the output of the amplifier 57 is converted into a current except for a DC component by the capacitor C5 and the resistor R8 and output to the phase switching / phase current control circuit 2. FIG. 8 shows a temporal change of the PWM frequency in this case.

Also in the case of this embodiment, the energy of the beat frequency component corresponding to the mechanical resonance frequency can be reduced as compared with the case where the PWM frequency is constant, as in the case of the above-described embodiment. 3 can be kept small.

A fifth embodiment of the present invention will be described with reference to FIGS. The phase current is controlled by setting the PWM frequency in the phase switching / phase current control circuit 2 to a frequency that is an integral multiple of the phase current switching frequency.

For this reason, in this embodiment, since the phase switching / phase current control circuit 2 does not have the oscillation circuit 22, the oscillation regulation capacitance Ct and the oscillation regulation resistor Rt are omitted as shown in FIG. Further, in the phase switching circuit 61 in the phase switching / phase current control circuit 2 of the present embodiment, as shown in FIG. 10, the driving clock CLK from the speed / direction control circuit 1 which is an externally supplied clock is frequency-divided. Thus, an n-ary counter 62 is provided as a frequency dividing circuit for setting the PWM frequency to a frequency that is an integral multiple (n times) of the phase current switching frequency. The clock divided by the n-ary counter 62 is supplied to the shift register FF
Input as clocks 0 to FF5. FIG. 11 shows a configuration example of the phase switching circuit of the present embodiment. As described above, in the present embodiment, since the oscillation circuit 22 is not provided, the logical product of the logical product with the output COMPS of the comparator 23 by the clock CLK by the AND gate 24 is used as the enable signal EN in the phase switching circuit 61. Supplying.

In this embodiment, the phase current switching frequency is PW
The frequency is 2 * n times the M frequency, and there is no difference in the number of times of PWM control for each phase UU, VV, WW of the stepping motor 3 as described above. Is also zero (beat frequency). For this reason, unevenness of each phase current does not occur, and accordingly, speed unevenness of the stepping motor 3 does not occur.

[0053]

According to the first aspect of the invention, since the modulation frequency of the pulse width modulation for controlling the phase current of the stepping motor is changed, the phase current switching frequency and the beat component of the pulse width modulation frequency are changed. It does not become a constant frequency component, but has a fluctuation according to the change of the modulation frequency of the pulse width modulation, so that the energy of the beat frequency component can be made smaller than that in the case where the modulation frequency of the pulse width modulation is constant. Speed fluctuations of the stepping motor can be kept small.

According to the second aspect of the present invention, in realizing the first aspect of the present invention, the modulation frequency of the pulse width modulation is randomly changed, so that a beat frequency coincident with the mechanical resonance appears. Is also random, and the periodicity of the speed fluctuation of the stepping motor can be eliminated.

According to the third aspect of the present invention, since the modulation frequency of the pulse width modulation is changed in a rectangular wave shape, the first aspect of the present invention can be easily realized.

According to the fourth aspect of the invention, the modulation frequency of the pulse width modulation is changed in a triangular waveform, so that the first aspect of the invention can be easily realized.

According to the fifth aspect of the present invention, the phase current is controlled by setting the modulation frequency of the pulse width modulation for controlling the phase current of the stepping motor to a frequency that is an integral multiple of the phase current switching frequency. By using the supply clock as the modulation frequency of pulse width modulation and generating the phase current switching frequency with reference to the externally supplied clock, the frequency difference between the integral multiple of the phase current switching frequency and the modulation frequency of pulse width modulation, which is a problem in the past, is considered. Can be controlled so as not to occur and the speed fluctuation of the stepping motor caused by this does not occur.

According to the sixth aspect of the present invention, there is provided a modulation circuit for changing the modulation frequency of the pulse width modulation for controlling the phase current of the stepping motor, and the modulation circuit changes the modulation frequency of the pulse width modulation. Therefore, the beat components of the phase current switching frequency and the pulse width modulation frequency do not become constant frequency components, but have a fluctuation corresponding to the change in the modulation frequency of the pulse width modulation by the modulation circuit, and the energy of the beat frequency component is reduced. The modulation frequency of the pulse width modulation can be reduced as compared with the case where the modulation frequency is constant, and thus the speed fluctuation of the stepping motor can be suppressed to be small.

According to the seventh aspect of the present invention, in realizing the sixth aspect of the present invention, by using a modulation circuit for randomly changing the modulation frequency of the pulse width modulation, the beat frequency matching the mechanical resonance is used. Is random, and the periodicity of the speed fluctuation of the motor can be eliminated.

According to the eighth aspect of the present invention, the use of the modulation circuit that outputs a rectangular wave-shaped modulation signal makes it possible to easily realize the first aspect of the invention.

According to the ninth aspect of the present invention, the first aspect of the present invention can be easily realized by using a modulation circuit that outputs a triangular wave-shaped modulation signal.

According to the tenth aspect of the present invention, in the phase switching / phase current control circuit for controlling the phase current of the stepping motor, the frequency division for setting the modulation frequency of the pulse width modulation to an integral multiple of the phase current switching frequency. By providing a circuit, for example, the externally supplied clock is used as the modulation frequency of pulse width modulation, and the phase current switching frequency is generated as an integer multiple by the frequency dividing circuit based on the externally supplied clock. Control can be performed so that a frequency difference between an integral multiple of the phase current switching frequency and the modulation frequency of the pulse width modulation does not occur, and the speed fluctuation of the stepping motor caused by the difference does not occur.

According to the image reading apparatus of the present invention, the scanning optical system is sub-scanned by the stepping motor whose speed fluctuation is suppressed by using the stepping motor control apparatus of the present invention. ,
Deterioration of the image quality of the read image can be suppressed.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of a scanner according to a first embodiment of the present invention.

FIG. 2 is a block diagram illustrating a configuration example of a stepping motor control device according to the present embodiment.

FIG. 3 is a circuit diagram illustrating a configuration example of a modulation circuit according to a second embodiment of the present invention.

FIG. 4 is a waveform diagram showing a state of a change in a PWM frequency as an output.

FIG. 5 is a circuit diagram illustrating a configuration example of a modulation circuit according to a third embodiment of the present invention.

FIG. 6 is a waveform chart showing how the output PWM frequency changes.

FIG. 7 is a circuit diagram illustrating a configuration example of a modulation circuit according to a fourth embodiment of the present invention.

FIG. 8 is a waveform diagram showing a state of a change in a PWM frequency as an output.

FIG. 9 is a block diagram illustrating a configuration example of a stepping motor control device according to a fifth embodiment of the present invention.

FIG. 10 is a block diagram showing a configuration example of the phase switching circuit.

FIG. 11 is a circuit diagram showing a configuration example of a phase current control circuit and a driver.

FIG. 12 is a block diagram illustrating a configuration example of a conventional stepping motor control device.

FIG. 13 is a time chart showing an operation example thereof.

FIG. 14 is a time chart showing an enlarged part thereof.

FIG. 15 is a block diagram showing a configuration example of the phase switching circuit.

FIG. 16 is a circuit diagram showing a configuration example of the phase current control circuit and the driver.

[Explanation of symbols]

 2 phase switching / phase current control circuit 3 stepping motor 34, 35 scanning optical system 41 CCD line sensor 51 modulation circuit 62 frequency dividing circuit D stepping motor control device

Claims (11)

[Claims] 1. A stepping motor control method for controlling a phase current of a stepping motor by pulse width modulation, wherein the modulation frequency of the pulse width modulation is changed. 2. The stepping motor control method according to claim 1, wherein a modulation frequency of said pulse width modulation is changed at random. 3. The stepping motor control method according to claim 1, wherein the modulation frequency of the pulse width modulation is changed in a rectangular wave shape. 4. The stepping motor control method according to claim 1, wherein a modulation frequency of said pulse width modulation is changed in a triangular wave shape. 5. A stepping motor control method for controlling a phase current of a stepping motor by pulse width modulation, wherein the phase current is controlled such that a modulation frequency of the pulse width modulation is an integral multiple of a phase current switching frequency. A stepping motor control method. 6. A stepping motor control device for controlling a phase current of a stepping motor by pulse width modulation by a phase switching / phase current control circuit, comprising a modulation circuit for changing a modulation frequency of the pulse width modulation. Stepping motor control device. 7. The stepping motor control device according to claim 6, wherein the modulation circuit is a modulation circuit that outputs a modulation signal having a random frequency to the phase switching / phase current control circuit. 8. The stepping motor control device according to claim 6, wherein the modulation circuit is a modulation circuit that outputs a rectangular modulation signal to the phase switching / phase current control circuit. 9. The stepping motor control device according to claim 6, wherein said modulation circuit is a modulation circuit that outputs a triangular modulation signal to said phase switching / phase current control circuit. 10. A stepping motor control device for controlling a phase current of a stepping motor by pulse width modulation by a phase switching / phase current control circuit, wherein the phase switching / phase current control circuit includes a modulation frequency of the pulse width modulation. A stepping motor control device comprising a frequency dividing circuit for setting the frequency to an integral multiple of the current switching frequency. 11. A scanning optical system for electrically scanning a document image in a main scanning direction by a CCD line sensor and exposing the document image to form an image on the CCD line sensor is scanned by a stepping motor in a sub-scanning direction. 6. An image reading apparatus for two-dimensionally reading said document image by controlling a phase current of said stepping motor.
0. An image reading apparatus comprising the stepping motor control device according to 0.