JP3825205B2 - Stepping motor control method, apparatus and image reading apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a stepping motor control method, an apparatus thereof, and an image reading apparatus such as an image scanner, a digital copying machine, and a scanner of a facsimile machine.
[0002]
[Prior art]
In general, in an image reading apparatus such as an image scanner, a scanning optical system that electrically scans an original image in a main scanning direction by a CCD line sensor and exposes the original image to form an image on the CCD line sensor is sub-scanned by a motor. The original image is read two-dimensionally by scanning in the direction. In this case, as a motor that reciprocates the scanning optical system in the sub-scanning direction, a stepping motor is generally used because it can easily and accurately manage the reciprocal movement amount of the scanning optical system. Is used.
[0003]
FIG. 12 shows a configuration example of a conventional drive control circuit for such a stepping motor. Schematically, a drive clock CLK, a direction control signal DIR, and an initialization signal RST are supplied to a phase switching / phase current control circuit 2 from a speed / direction control circuit 1 constituted by a CPU or the like, and this phase switching / phase current is supplied. The control circuit 2 compares the voltage of the motor phase current detection resistor RS with the phase current control voltage VREF at the oscillation frequency (pulse width modulation PWM frequency) determined by the oscillation regulation capacitor Ct and the oscillation regulation resistor Rt, and the motor phase The drive signals UB, VB, and WB of the driver 4 for the three-phase UU, VV, and WW of the stepping motor 3 are controlled so that the peak voltage of the current detection resistor RS becomes equal to the phase current control voltage VREF.
[0004]
It should be noted that the control signals (U, UB, V, VB, W, WB) of the driver 4 are the speed and speed as shown in the time chart shown in FIG. 13 (FIG. 14 is an enlarged time chart). Controlled by signals RST, CLK, DIR from direction control circuit 1, initialized at rising edge (CLK ↑) of drive clock CLK when initialization signal RST is at H level, U, UB, WB are at H level, V, VB, W become L level (hence, U / UB / V / VB / W / WB: H / H / L / L / L / H), and the current IUU of the UU phase of the stepping motor 3 is negative (stepping The current IVV of the VV phase is positive (the direction of flowing into the stepping motor 3), but at this time, the drive signal VB is such that the current value is proportional to VREF at the aforementioned PWM frequency. Pulse width modulated. In this state, the current IWW of the WW phase is zero (IUU / IVV / IWW: − / + / 0).
[0005]
Next, the state changes at the rising edge (CLK ↑) of the drive clock CLK after the signal RST becomes 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, when the rising edge (CLK ↑) of the drive clock CLK at initialization is CLK0,
CLKn U / V / W / UB / VB / WB IUU / IVV / IWW Phase current direction
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
It becomes. When the direction control signal DIR becomes L level, the above order is advanced in the reverse direction.
[0006]
FIG. 15 shows an example of the configuration of the phase switching circuit 5 in the phase switching / phase current control circuit 2, which is a 6-bit bidirectional circuit composed of six flip-flops FF0 to FF5 and six multiplexers MPX0 to MPX5. The shift register 7 includes six OR gates 8a to 8f, 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, the AND gates 9a to 9c, and the inverters 10a to 10c also perform a level shift according to the operating potential of the driver 4. Also, an enable signal EN shown in FIG. 16 is connected to the AND gates 9a to 9c that generate the drive signals UB, VB, and WB.
[0007]
FIG. 16 shows a bidirectional circuit including a reference current source 13 including an amplifier 11, an internal reference voltage VR, an NMOS transistor 12, and an oscillation defining resistor Rt, two current mirrors 14 and 15, and two NMOS transistors 16 and 17. The current source 18, the oscillation regulation resistor Ct for charging / discharging the output current of the bidirectional current source 18, and the voltage of the oscillation regulation resistor Ct are monitored and discharged when the voltage rises to a constant voltage VTOP, and charged when discharged to a constant voltage VBOT. The oscillation circuit 22 including upper / lower limit comparators 19 and 20 for controlling the NMOS transistors 16 and 17 of the bidirectional current source 18 and the RS flip-flop 21 and the terminal voltage of the current detection resistor RS. Comparator 23 that compares current control voltage VREF, and AND gate 24 that ANDs the output of comparator 23 and the output OSCO of oscillation circuit 22 An enable signal EN is generated.
[0008]
The driver 4 has the same configuration for the UU / VV / WW phase, the drive signals UB / VB / WB are the gates of the PMOS transistors QUB, QVB, QWB, and the signals U / V / W are the NMOS transistors. It is connected to the gates of QU, QV, and QW.
[0009]
[Problems to be solved by the invention]
In the case of the stepping motor drive control circuit configured as shown in FIGS. 12 to 16, there is no problem if the ratio between the phase current switching frequency and the pulse width modulation frequency is sufficiently high. However, when it becomes close to several times, the influence of the difference in the number of times that phase current control by pulse width modulation is applied in each phase becomes significant, and unevenness occurs in the phase current of each phase. This unevenness has a frequency component (beat frequency) of the difference between the integral multiple of the phase current switching frequency and the pulse width modulation frequency. When the frequency of this phase current unevenness coincides with mechanical resonance, a large speed fluctuation occurs. In this state, the image data read by the image reading apparatus is periodically expanded and contracted at the beat frequency (mechanical resonance frequency). Periodic expansion / contraction is very conspicuous in the human eye, which causes a significant deterioration in image quality.
[0010]
Therefore, an object of the present invention is to provide a stepping motor control method and apparatus capable of reducing the speed fluctuation of the stepping motor due to the beat components of the pulse width modulation frequency and the phase current switching frequency.
[0011]
It is another object of the present invention to provide an image reading apparatus having a stepping motor driving system that can suppress deterioration in image quality of a read image by using such a stepping motor control apparatus.
[0012]
[Means for Solving the Problems]
The stepping motor control method according to claim 1 is a stepping motor control method for controlling a phase current of a stepping motor to have a predetermined current value by pulse width modulation , wherein the modulation frequency of the pulse width modulation and the phase current are controlled. The modulation frequency of the pulse width modulation is changed so that a difference of an integer multiple of the switching frequency of the pulse width modulation does not become a constant frequency component .
[0013]
Therefore, by changing the modulation frequency of the pulse width modulation, the beat component of the phase current switching frequency and the pulse width modulation frequency does not become a constant frequency component, and has a variation corresponding to the change of the modulation frequency of the pulse width modulation. Thus, 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.
[0016]
According to a second 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 to a rectangular wave shape.
[0017]
Therefore, the invention of claim 1 can be easily realized.
[0018]
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 in a triangular wave shape.
[0019]
Therefore, the invention of claim 1 can be easily realized.
[0020]
A stepping motor control method according to a fourth aspect of the present invention is the stepping motor control method for controlling the phase current of the stepping motor to have a predetermined current value by pulse width modulation, wherein the modulation frequency of the pulse width modulation is the phase current. The phase current is controlled so that the frequency difference between the two becomes zero as a frequency that is an integral multiple of the switching frequency.
[0021]
Therefore, for example, by using the externally supplied clock as the modulation frequency of pulse width modulation and generating the phase current switching frequency based on the externally supplied clock, the integer multiple of the phase current switching frequency and the modulation of pulse width modulation, which is a problem in the past, are generated. A frequency difference between the frequencies does not occur, and the speed fluctuation of the stepping motor due to this does not occur.
[0022]
A stepping motor control device according to a fifth aspect of the present invention is the stepping motor control device for controlling the phase current of the stepping motor to have a predetermined current value by the pulse width modulation by the phase switching / phase current control circuit. A modulation circuit is provided that changes the modulation frequency of the pulse width modulation so that a difference between an integer multiple of the modulation frequency of the modulation and the switching frequency of the phase current does not become a constant frequency component.
[0023]
Therefore, by changing the modulation frequency of the pulse width modulation by the modulation circuit, the beat component of the phase current switching frequency and the pulse width modulation frequency does not become a constant frequency component, and according to the change of the modulation frequency of the pulse width modulation by the modulation circuit Therefore, 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 thus the speed fluctuation of the stepping motor can be suppressed to be small.
[0026]
According to a sixth aspect of the present invention, in the stepping motor control device according to the fifth aspect , the modulation circuit is a modulation circuit that outputs a rectangular wave-shaped modulation signal to the phase switching / phase current control circuit.
[0027]
Therefore, the invention according to claim 1 can be easily realized by using a modulation circuit that outputs a rectangular wave-shaped modulation signal.
[0028]
A seventh aspect of the present invention is the stepping motor control apparatus according to the fifth aspect , wherein the modulation circuit is a modulation circuit that outputs a triangular wave-shaped modulation signal to the phase switching / phase current control circuit.
[0029]
Therefore, the invention according to claim 1 can be easily realized by using a modulation circuit for outputting a triangular wave-like modulation signal.
[0030]
A stepping motor control device according to an eighth aspect of the present invention is the stepping motor control device for controlling the phase current of the stepping motor to have a predetermined current value by pulse width modulation by the phase switching / phase current control circuit. A phase current control circuit is provided with a frequency dividing circuit that controls the phase current so that the frequency of the pulse width modulation is an integral multiple of the switching frequency of the phase current and the frequency of both is zero.
[0031]
Therefore, for example, by using an externally supplied clock as a modulation frequency of pulse width modulation and generating a phase current switching frequency by an integer multiple with a frequency dividing circuit based on the externally supplied clock, an integer of the phase current switching frequency that causes a problem in the past is used. There is no frequency difference between the double and the pulse width modulation modulation frequency, and there is no stepping motor speed fluctuation caused by this difference.
[0032]
According to a ninth aspect of the present invention, there is provided an image reading apparatus comprising: a stepping motor that scans an original image electrically in a main scanning direction by a CCD line sensor and exposes the original image to form an image on the CCD line sensor. comprising an image reading device for reading the document image by scanning in the sub-scanning direction in two dimensions, the stepping motor control apparatus of claims 5 to 8, wherein controlling the phase current of the stepping motor by.
[0033]
Therefore, by using the stepping motor control device according to the fifth to eighth aspects, sub-scanning is performed by the stepping motor in which the speed fluctuation is suppressed, so that deterioration of the image quality of the read image can be suppressed.
[0034]
DETAILED DESCRIPTION OF THE INVENTION
A first embodiment of the present invention will be described with reference to FIGS. 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 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 reading document (not shown) is placed on the upper surface of the contact glass 33. The first scanning unit 34 is movably supported at a position facing the contact glass 33, and the second scanning unit 35 is movably supported at a position facing the first scanning unit 34. ing. The first scanning unit 34 has a halogen lamp 36 and a first mirror 37, and the second scanning unit 35 has second and third mirrors 38 and 39. In addition, a CCD line sensor 41 is fixedly disposed via an imaging lens 40 at a position facing the third mirror 39 of the second scanning unit 35.
[0035]
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 CCD is connected from the contact glass 33 via the first and second scanning units 34 and 35. The optical path length of the imaging optical path communicating with the line sensor 41 is always constant even when the first and second scanning units 34 and 35 move. An image obtained by 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 by the fixed-length imaging optical path. Photoelectric conversion to signal.
[0036]
The first and second scanning units 34 and 35 are mechanically driven back and forth in the sub-scanning direction via the motor pulley 43, belt 44, pulley 45, wire (not shown), etc., with the stepping motor 3 as a drive source. .
[0037]
The stepping motor 3 as a drive source for such mechanical scanning in the sub-scanning direction is driven and 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. Has been. Specifically, the modulation circuit 51 connects the modulation current to the connection portion of the oscillation regulation resistor Rt of the phase switching / phase current control circuit 2.
[0038]
According to such a configuration, the modulation current is superimposed on the connection portion of the oscillation regulation resistor Rt of the phase switching / phase current control circuit 2 by the modulation circuit 51, so that the oscillation circuit 22 of the phase switching / phase current control circuit 2 The oscillation frequency (that is, the PWM frequency) can be changed. According to this, the beat component of the phase current switching frequency and the PWM frequency is not a constant frequency component, and has a beat frequency variation 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 suppressed small.
[0039]
A second embodiment of the present invention will be described with reference to FIGS. The present embodiment shows a configuration example of a modulation circuit 51A that is an example of the modulation circuit 51 in FIG.
[0040]
The modulation circuit 51A causes a current to flow through the Zener diode ZD1 through the resistor R1 to generate a Zener voltage at the midpoint. The circuit configuration is such that the portion is cut and converted into a current by a resistor R3 to output a modulation signal.
[0041]
In this modulation circuit 51A, the noise of the Zener diode ZD1 is amplified and used as a modulation signal, so the PWM frequency changes randomly. FIG. 4 shows an example of a random temporal change in the PWM frequency. For this reason, the beat frequency is also random, and the speed fluctuation of the stepping motor 3 is also random. In this case, not only is the energy of the beat frequency component corresponding to the mechanical resonance smaller than when the PWM frequency is constant, but also the probability that a beat frequency that matches the mechanical resonance will be random, and the speed fluctuation period of the stepping motor 3 will be random. Therefore, the degradation of the image quality in the human appearance in the read image data can be suppressed to a small level.
[0042]
A third embodiment of the present invention will be described with reference to FIGS. The present embodiment shows a configuration example of a modulation circuit 51B which is an example of the modulation circuit 51 in FIG.
[0043]
The modulation 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 the feedback path. The output of the inverting integration circuit 55 is fed back to the hysteresis generator of the comparator 54 via the feedback resistor R6. Yes. A DC cut capacitor C4 and a current conversion resistor R7 are connected to the output side of the amplifier 56.
[0044]
In such a configuration, first, when the output of the amplifier 56 is positive, the + input of the amplifier 56 is also positive by the resistors R3 and R4, and the output of the amplifier 57 in the inverting integration circuit 55 changes toward negative. Therefore, 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. become. 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 positive input of the amplifier 56 becomes positive and the output of the amplifier 56 also becomes positive. Thus, the output of the amplifier 56 is a rectangular wave. Therefore, the output of the amplifier 56 is converted into a current by removing the DC component by the capacitor C4 and the resistor R7, and is output to the phase switching / phase current control circuit 2. FIG. 6 shows the temporal change of the PWM frequency in this case.
[0045]
Also in this embodiment, as in the case of the above-described embodiment, the energy of the beat frequency component corresponding to the mechanical resonance frequency can be reduced compared to the case where the PWM frequency is constant, and the speed of the stepping motor 3 can be reduced. Variations can be kept small.
[0046]
A fourth embodiment of the present invention will be described with reference to FIGS. The present embodiment shows a configuration example of a modulation circuit 51C as an example of the modulation circuit 51 in FIG.
[0047]
The modulation circuit 51C of the present embodiment is composed of a comparator 54 and an inverting integration circuit 55 as in the modulation circuit 51B, but the modulation signal for the phase switching / phase current control circuit 2 is an amplifier of the inverting integration circuit 55. 57 is taken out. On the output side of the amplifier 57, a DC cut capacitor C5 and a current conversion resistor R8 are connected.
[0048]
According to such a configuration, the output of the amplifier 57 has a triangular wave shape. Therefore, the output of the amplifier 57 is converted into a current by removing the DC component by the capacitor C5 and the resistor R8, and is output to the phase switching / phase current control circuit 2. FIG. 8 shows the temporal change of the PWM frequency in this case.
[0049]
Also in this embodiment, as in the case of the above-described embodiment, the energy of the beat frequency component corresponding to the mechanical resonance frequency can be reduced compared to the case where the PWM frequency is constant, and the speed of the stepping motor 3 can be reduced. Variations can be kept small.
[0050]
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 an integer multiple of the phase current switching frequency.
[0051]
For this reason, in the present embodiment, the phase switching / phase current control circuit 2 does not have the oscillation circuit 22, and therefore, the oscillation defining capacitor Ct and the oscillation defining 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, the drive clock CLK from the speed / direction control circuit 1, which is an externally supplied clock, is divided as shown in FIG. Thus, an n-ary counter 62 is provided as a frequency dividing circuit that makes the PWM frequency an integer multiple (n times) the phase current switching frequency. The clock divided by the n-ary counter 62 is input as the clock of the shift registers FF0 to FF5. Further, 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 AND of the output COMPS of the comparator 23 at the clock CLK and the AND gate 24 is used as the enable signal EN to the phase switching circuit 61. Supply.
[0052]
In this embodiment, the phase current switching frequency is 2 * n times the PWM frequency, and the difference in the number of PWM control for each phase UU, VV, WW of the stepping motor 3 as described above does not occur. The frequency difference (beat frequency) between the integer multiple of the phase current switching frequency and the PWM frequency is also zero. For this reason, non-uniformity of each phase current does not occur, and the non-uniformity of the speed of the stepping motor 3 due to this does not occur.
[0053]
【The invention's effect】
According to the first aspect of the present invention, since the modulation frequency of the pulse width modulation for controlling the phase current of the stepping motor is changed, the beat component of the phase current switching frequency and the pulse width modulation frequency is a constant frequency component. Rather, it has fluctuations according to changes in the modulation frequency of the pulse width modulation, and the energy of the beat frequency component can be made smaller than when the modulation frequency of the pulse width modulation is constant. Can be kept small.
[0055]
According to the second aspect of the invention, since the modulation frequency of the pulse width modulation is changed to a rectangular wave shape, the first aspect of the invention can be easily realized.
[0056]
According to the invention described in claim 3, since the modulation frequency of the pulse width modulation is changed in a triangular wave shape, the invention described in claim 1 can be easily realized.
[0057]
According to the fourth aspect of the 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 as an integer multiple of the phase current switching frequency. By using the modulation frequency of the pulse width modulation and generating the phase current switching frequency based on the externally supplied clock, a frequency difference between the integer multiple of the phase current switching frequency and the modulation frequency of the pulse width modulation, which is a problem in the past, is generated. In addition, it is possible to control so as not to cause the speed fluctuation of the stepping motor due to this.
[0058]
According to the fifth aspect of the present invention, the modulation circuit for changing the modulation frequency of the pulse width modulation for controlling the phase current of the stepping motor is provided, and the modulation frequency of the pulse width modulation is changed by the modulation circuit. The beat component of the current switching frequency and the pulse width modulation frequency does not become a constant frequency component, but changes according 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 pulse width modulated. Therefore, the fluctuation in the speed of the stepping motor can be kept small.
[0060]
According to the invention described in claim 6, the invention described in claim 1 can be easily realized by using the modulation circuit that outputs a rectangular wave-shaped modulation signal.
[0061]
According to the seventh aspect of the present invention, the invention according to the first aspect can be easily realized by using the modulation circuit that outputs a triangular wave-like modulation signal.
[0062]
According to the eighth aspect of the invention, the phase switching / phase current control circuit for controlling the phase current of the stepping motor is provided with a frequency dividing circuit for setting the modulation frequency of the pulse width modulation to an integer multiple of the phase current switching frequency. Thus, for example, the externally supplied clock is used as the modulation frequency of pulse width modulation, and the phase current switching frequency is generated by an integer multiple with a frequency divider circuit based on the externally supplied clock. Control can be performed so that a frequency difference between the integral multiple of the frequency and the modulation frequency of the pulse width modulation does not occur, and the speed fluctuation of the stepping motor due to this does not occur.
[0063]
According to the image reading apparatus of the ninth aspect of the present invention, the scanning optical system is sub-scanned by the stepping motor in which the speed fluctuation is suppressed by using the stepping motor control apparatus according to the fifth to eighth aspects of the present invention. Image quality degradation can be suppressed.
[Brief description of the drawings]
FIG. 1 is a schematic configuration diagram of a scanner showing a first embodiment of the 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 showing a configuration example of a modulation circuit according to a second embodiment of the present invention.
FIG. 4 is a waveform diagram showing how the PWM frequency that is the output changes.
FIG. 5 is a circuit diagram showing a configuration example of a modulation circuit according to a third embodiment of the present invention.
FIG. 6 is a waveform diagram showing how the output PWM frequency changes.
FIG. 7 is a circuit diagram showing a configuration example of a modulation circuit according to a fourth embodiment of the present invention.
FIG. 8 is a waveform diagram showing how the output PWM frequency changes.
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 the phase current control circuit and the 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 example of the operation.
FIG. 14 is a time chart showing a part thereof in an enlarged manner.
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 motors 34 and 35 Scanning optical system 41 CCD line sensor 51 Modulating circuit 62 Dividing circuit D Stepping motor control device

Claims (9)

In the stepping motor control method for controlling the phase current of the stepping motor to be a predetermined current value by pulse width modulation,
A stepping motor control method characterized in that the modulation frequency of the pulse width modulation is changed so that a difference between an integral multiple of the modulation frequency of the pulse width modulation and the switching frequency of the phase current does not become a constant frequency component. . 2. The stepping motor control method according to claim 1, wherein a modulation frequency of the pulse width modulation is changed to a rectangular wave shape. 2. The stepping motor control method according to claim 1, wherein a modulation frequency of the pulse width modulation is changed in a triangular wave shape. In the stepping motor control method for controlling the phase current of the stepping motor to a predetermined current value by pulse width modulation,
  A stepping motor control method characterized in that the phase current is controlled so that the frequency difference between the two becomes zero, with the modulation frequency of the pulse width modulation being a frequency that is an integral multiple of the switching frequency of the phase current. In the stepping motor control device that controls the phase current of the stepping motor to a predetermined current value by pulse width modulation by the phase switching / phase current control circuit,
  Stepping motor control comprising a modulation circuit that changes the modulation frequency of the pulse width modulation so that a difference between an integer multiple of the modulation frequency of the pulse width modulation and the switching frequency of the phase current does not become a constant frequency component apparatus. 6. The stepping motor control device according to claim 5, wherein the modulation circuit is a modulation circuit that outputs a rectangular wave-shaped modulation signal to the phase switching / phase current control circuit. 6. The stepping motor control device according to claim 5, wherein the modulation circuit is a modulation circuit that outputs a triangular wave-like modulation signal to the phase switching / phase current control circuit. In the stepping motor control device that controls the phase current of the stepping motor to a predetermined current value by pulse width modulation by the phase switching / phase current control circuit,
  The phase switching / phase current control circuit includes a frequency dividing circuit that controls the phase current so that the frequency of the pulse width modulation is an integral multiple of the switching frequency of the phase current and the frequency of both is zero. A stepping motor control device characterized by the above. A scanning optical system that electrically scans the document image in the main scanning direction by the CCD line sensor, exposes the document image and forms an image on the CCD line sensor, scans the document image in the sub-scanning direction by the stepping motor. 9. An image reading apparatus comprising: a stepping motor control device according to claim 5 for controlling a phase current of the stepping motor in an image reading device for two-dimensional reading.

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