JP3446334B2 - Image forming device - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrophotographic image forming apparatus for forming an image by using a laser scanning apparatus, and more particularly, to an image forming apparatus which changes an image forming speed even when the image forming speed is changed. The present invention relates to an image forming apparatus that suppresses an increase in speed fluctuation of a carrier and prevents the occurrence of density unevenness on a formed image. 2. Description of the Related Art In an image forming apparatus, for example, an electrophotographic laser printer or an electronic copying machine, a laser beam is applied to an image carrier rotating in a sub-scanning direction by a laser scanning apparatus in a main scanning direction. To form an image. For this reason, it is an important issue to suppress the speed fluctuation of the image carrier to prevent the density unevenness occurring in the image forming direction on the formed image. This speed fluctuation occurs with a certain period,
The reciprocal of the period is called an operating frequency of the speed fluctuation. As a conventional image forming apparatus which suppresses such speed fluctuation of an image carrier, for example, Japanese Patent Application Laid-Open No.
177190 and JP-A-63-75759. In the former, as shown in FIG. 9, a flywheel 4 having a large moment of inertia is attached to a photosensitive drum 1 driven by a drive motor 2 via a speed reduction mechanism 3 to reduce the speed fluctuation of the photosensitive member 1. It is suppressed by the inertia of the flywheel 4. On the other hand, as shown in FIG. 10, the photosensitive drum 1 is driven by a drive motor 2 via a speed reduction mechanism 3.
In addition, a speed detecting means 5 such as an encoder is attached to the apparatus, and the driving of the drive motor 2 is controlled by a feedback control circuit 6 based on the detection result of the speed detecting means 5, thereby suppressing the speed fluctuation of the photosensitive drum 1. However, according to the conventional image forming apparatus, the speed fluctuation of a drive transmission mechanism such as a gear or a belt for transmitting a drive torque from a drive motor to a photosensitive drum causes density unevenness. Even if the level is low enough not to cause a problem, you can select the quality of the recorded image, or use plain paper or OH
When the image forming speed is changed in accordance with the type of recording medium such as a P sheet, the operating frequency of the speed change changes, so that the frequency (resonance frequency) at which the maximum speed change occurs in the transfer characteristic of the drive transmission mechanism is equal to or close to the frequency. Then, there is an inconvenience that the speed fluctuation increases, density unevenness occurs and the image quality deteriorates. Further, those employing a mechanical structure such as a flywheel become large in size and limit the cost reduction. Accordingly, an object of the present invention is to provide an image forming apparatus capable of suppressing an increase in speed fluctuation of an image carrier even when an image forming speed is switched, and preventing the occurrence of density unevenness on a formed image. is there. Another object of the present invention is to provide an image forming apparatus which can prevent the size from becoming large and can reduce the cost easily. SUMMARY OF THE INVENTION In order to achieve the above object, the present invention provides an image carrier having a laser beam modulated in accordance with an image signal by a laser scanning device and rotated in a sub-scanning direction by a DC motor. In an image forming apparatus that scans in the main scanning direction to form an image on the image carrier, an image forming speed signal is input, and speed control for controlling the speed of the DC motor to a speed corresponding to the image forming speed signal. Means for inputting the image forming speed signal and adjusting the exciting current of the DC motor to transmit a drive torque from the DC motor to the image carrier. der to provide an image forming apparatus characterized by comprising a resonance frequency shift means for shifting a frequency corresponding to the image forming speed signal from the operating frequency of speed fluctuation of the drive transmission mechanism . According to the image forming apparatus of the present invention based on the above configuration, for example, when a high-speed image forming speed is selected, a high-speed image forming speed signal is sent to the speed control means and the resonance frequency shift means. Is output.
The speed control means increases the speed of the drive motor, and the resonance frequency shift means controls the transmission characteristics of the drive transmission mechanism including the drive motor. The transmission characteristic of the drive transmission mechanism has a resonance frequency at which the speed fluctuation is maximum, and the resonance frequency shift means shifts the resonance frequency to a higher frequency side by, for example, increasing the excitation current of the drive motor. When the drive motor is driven at a high speed, the operating frequency of the speed fluctuation of the drive transmission mechanism shifts to the high frequency side, but the resonance frequency of the above-described transmission characteristic also shifts to the high frequency side, so that both frequencies match.
Alternatively, there is no approach, and as a result, the speed fluctuation of the drive transmission mechanism does not increase. An image forming apparatus according to the present invention will be described below in detail with reference to the accompanying drawings. FIG. 1 shows the configuration of a laser printer to which the present invention is applied. The laser printer receives a driving force from a photoconductor driving unit 10 and rotates in a sub-scanning direction with a photoconductor drum 1, a charger 7 for charging the photoconductor drum 1, and an electrostatic latent image formed on the photoconductor drum 1. A developing unit 8 for developing the toner, a cleaning unit 9 for removing the toner remaining on the photoconductor drum 1, a photoconductor unit 11 detachably provided and a laser beam corresponding to an image signal are charged. A laser scanner 12 for exposing the photosensitive drum 1 to form an electrostatic latent image on the photosensitive drum 1; a paper feeding device 13 for supplying recording paper 32; A transfer device 14 for transferring the recording image to the recording paper 32 supplied from the printer, a fixing device 15 for fixing the transferred image transferred to the recording paper 32, and a discharge tray 42 for discharging the recording paper 32 having received the image recording. It has been made. FIG. 2 shows the configuration of the image forming apparatus of the present invention applied to the above laser printer. This image forming apparatus includes a stepping motor 2A for rotating the photosensitive drum 1 via a speed reduction mechanism 3, a driving circuit 17 for driving the stepping motor 2A, selection of image quality of a recorded image, plain paper, an OHP sheet, or the like. A speed control signal and a transfer characteristic signal corresponding to an image forming speed signal which is switched according to the type of the recording medium are output to the drive circuit 17 to adjust the drive speed of the stepping motor 2A and the transfer characteristic of the photoconductor driving means 10. The speed controller 16 is provided. FIG. 3 shows a circuit configuration (reference example) of the speed control means 16 and the drive circuit 17 described above.
The speed control means 16 includes a transfer characteristic variable signal generation circuit 18 that outputs a transfer characteristic variable signal, and a speed control signal generation circuit 19 that outputs a speed control signal. The transfer characteristic variable signal generation circuit 18 receives the image forming speed signal _Dref, and performs D / A conversion of the image formation speed signal _Dref.
It has a differential amplifier 21 that amplifies the output of the A converter 20 and outputs a reference voltage (transfer characteristic signal) _Vref . On the other hand, the speed control signal generation circuit 19, a frequency dividing circuit and the clock signal for driving the motor (speed control signal) a reference clock oscillogram aerator 23 at a division ratio corresponding to the image formation speed signal D _ref by dividing 22. When a print start signal is input, the drive circuit 17 controls the speed control signal generation circuit 1 of the speed control means 16.
A gate circuit 24 for passing the clock signal for driving the motor from No. 9; a distribution circuit 25 for distributing the clock signal passed through the gate circuit 24 to an A-phase to D-phase clock signal as shown in FIG. Based on the pulse timings of the A-phase to D-phase clock signals, the comparison circuits 27A and 27A
Excitation coils 28A-28 with currents corresponding to the 7B comparison output.
An excitation circuit 26 for exciting D, a reference voltage _Vref output from the transfer characteristic variable signal generation circuit 18 and an excitation coil 28
Resistance of voltage generated based on exciting currents of A to 28D 2
9A and 29B and comparison circuits 27A and 27B for comparing the divided voltage values of the resistors 30A and 30B. As shown in FIG. 5, the excitation circuit 26 has switching transistors 31A to 31D which are turned on based on the input of the pulse of the A-phase to D-phase clock signal, and the comparison outputs of the comparison circuits 27A and 27B. Power supply to _Vcc
33A, 33B for supplying power to the exciting coils 28A to 28D from the ground, flywheel diodes 34A to 34D inserted between the ground terminals of the exciting coils 28A to 28D and the power supply V _CC , and transistors 33A, 33
Zener diode 35 for maintaining emitter potential of 3B (voltage applied to exciting coils 28A to 28D) at a predetermined value.
A, 35B. Hereinafter, the operation of the present invention (reference example) will be described. In the following description, a case where the image forming speed is switched from the OHP sheet recording mode (low speed) to the plain paper recording mode (high speed) will be described. First, when an OHP sheet recording mode is selected on a control panel or the like and a print start command is given,
The drive circuit 1 outputs a print start signal (START) of “1”.
7 gate circuit 24, also, the image formation speed signal D _ref corresponding to the image formation speed of the OHP sheet printing mode is outputted to the speed control unit 16. When the speed control means 16 receives the image forming speed signal D _ref , the D / A converter 20 outputs the image forming speed signal D _ref.
After D / A conversion of the _ref , the output is amplified by the differential amplifier 21, and the reference voltage (transfer characteristic signal) V _ref is output from the differential amplifier 21 to the comparison circuits 27 A and 27 B of the drive circuit 17. At the same time, the frequency dividing circuit 22 divides the frequency of the reference clock of the oscillator 23 by the frequency dividing ratio according to the image forming speed signal _Dref, and outputs a motor driving clock signal (speed control signal) to the gate circuit of the driving circuit 17. 24. The comparison circuits 27A and 27B output the reference voltage V
_ref and excitation coils 28A-2 of stepping motor 2A
The voltage generated in the resistors 29A and 29B when the 8D is excited is compared, and as a result, a comparison signal corresponding to the difference from the reference voltage _Vref is output to the excitation circuit 26. On the other hand, since the print start signal is input to the gate circuit 24, the motor driving clock signal from the speed control signal generation circuit 19 is directly supplied to the distribution circuit 25.
The clock signal is distributed by the distribution circuit 25 to the A-phase to D-phase clock signals as shown in FIG. In the excitation circuit 26, the comparison circuits 27A, 27
The comparison signal output from B is applied to the transistors 33A and 33A.
When B is input, a current corresponding to the level of the comparison signal is to be supplied from the power supply V _CC to the excitation coils 28A to 28D. At this time, the transistors 31A to 31D are turned on based on the input of the pulse of the A-phase to D-phase clock signal output from the distribution circuit 25, and the excitation coils 28A to 28D are turned on at the pulse timing of the A-phase to D-phase clock signal. And the stepping motor 2A, that is, the photosensitive drum 1 is driven in the sub-scanning direction at an image forming speed (for example, about 40 mm / s) at the time of OHP sheet recording. As shown by the solid line in FIG. 6, the transmission characteristic of the photoconductor driving means 10 during the recording of the OHP sheet is sufficiently higher than the operating frequency f ₀ of the speed fluctuation of the driving transmission mechanism. Since the resonance frequency _fr exists, there is no possibility that the speed fluctuation will increase. When the photosensitive drum 1 is driven as described above,
The electrostatic latent image is formed on the photosensitive drum 1 by being charged by the charger 7 and exposed to laser light corresponding to the image signal by the laser scanner 12, and further formed into a toner image by the developing device 8. The recording paper 32 by the transfer device 14
After the transfer, a series of steps of finally removing the residual toner by the cleaning device 9 are performed. [0025] Here, when switching to the image formation speed of the fast plain paper printing mode than the image formation speed of the OHP sheet printing mode, the image formation speed signal D _ref corresponding to the image formation speed is outputted to the speed control means 16, Transfer characteristic variable signal generation circuit 18
Is a higher reference voltage V than in the OHP sheet recording mode.
_ref is output to the comparison circuits 27A and 27B. At the same time, the speed control signal generation circuit 19 outputs a clock signal having a higher frequency than in the OHP sheet recording mode to the gate circuit 24 of the drive circuit 17. Since the gate circuit 24 receives the print start signal, it passes the motor driving clock signal from the speed control signal generation circuit 19 to the distribution circuit 25, and the distribution circuit 25 converts the clock signal into an A-phase signal. To the D-phase clock signal and output to the excitation circuit 26. On the other hand, the comparison circuits 27A and 27B determine the reference voltage _Vref and the excitation coils 28A to 28A of the stepping motor 2A.
Resistors 29A and 29 generated by current flowing through 28D
B and a voltage based on the voltage division ratio of the resistors 30A and 30B are compared. At this time, since the reference voltage _Vref is higher than in the OHP sheet recording mode, the comparison circuit 27A,
The level of the comparison signal, which is the output of 27B, becomes higher than in the OHP sheet recording mode, and as a result, a large amount of current tends to flow through the exciting coils 28A to 28D. Further, since the frequency of the clock signal of the A phase to the D phase becomes higher than that in the OHP sheet recording mode,
The switching operation of the transistors 31A to 31D of the excitation circuit 26 becomes faster, and as a result, the stepping motor 2
A, that is, the image forming speed of plain paper faster than when the photosensitive drum 1 is in the OHP sheet recording mode (for example, about 160
mm / s). When the image forming speed is switched from low speed (OHP sheet recording mode) to high speed (plain paper recording mode) as described above, the exciting coils 28A to 28A of the stepping motor 2A are switched.
Since the current flowing through 28D increases, as a result, as shown in FIG. 6, the transfer characteristic of the photoconductor driving unit 10 changes from a solid line to a dotted line. That is, as the stepping motor 2 is driven at a high speed, the resonance frequency f _r changes to f _r ′ and shifts to a higher frequency side. Therefore, also the operating frequency of the speed variation image formation speed is switched from a low speed to a high speed is shifted to the high frequency side becomes f ₀ 'from f _0, the resonance frequency f _r in the transfer characteristic of the photosensitive body drive means 10 'Is also shifted to the higher frequency side, so that the operating frequency f ₀ ' and the resonance frequency f
_r 'does not match or approach, suppressing the increase in speed fluctuation, preventing the occurrence of density unevenness on the formed image,
Image quality can be improved. Conversely, when the image forming speed is switched from high speed to low speed, the resonance frequency shifts to the low frequency side, but the operating frequency also shifts to the low frequency side. frequency)
And the resonance frequency of the transfer characteristic do not match or approach. On the other hand, the operating frequency is higher than the resonance frequency,
In other words, when the image is present at a higher frequency side than the resonance frequency, the image forming speed increases, and the operating frequency becomes farther from the resonance frequency and shows a lower response. Problem does not occur. In the above embodiment, the reference voltage (transfer characteristic signal) _{Vref corresponding} to the image forming speed is output from the D / A converter 20 of the transfer characteristic variable signal generating circuit 18, but the image forming speed is changed in two steps. When only the degree is required, the reference voltage _Vref may be switched by a selector or the like without providing the D / A converter. FIG. 7 shows a second reference example of the present invention. In this reference example , a photoconductor driving unit 10 having a driving circuit 17 as shown in FIG. 2 is provided for each of photoconductor units 11A to 11D of each color (yellow, magenta, cyan, and black) of a color laser printer, Speed control means 16 for adjusting the transfer characteristics of the driving means 10 is provided. In such a configuration, each photoconductor unit 11A
11D, it is possible to prevent the occurrence of density unevenness on the formed image based on the switching of the image forming speed, and to improve the image quality of the color image. [0033] FIG. 8 is the actual施例image forming apparatus of the present invention is shown. The image forming apparatus of this embodiment is a case in which a DC motor 2B is used as a drive motor, and is configured to adjust the current to the DC motor 2B to change the transfer characteristic of the photoconductor driving unit 10, A speed detecting means 5 for detecting a driving speed of the body drum 1, a speed control signal,
A feedback control circuit 36 is provided which receives the speed detection signal and the image forming speed signal and controls the rotation speed and the exciting current of the DC motor 2B. The feedback control circuit 36 functions as a phase locked loop (PLL),
An F / V converter 37 for converting the pulse of the speed detection signal from the frequency F to the voltage V, a speed control signal, a speed detection signal, and an image forming speed signal are inputted, and the clock of the speed control signal and the speed detection signal are inputted. Phase comparison counter 38 for counting the phase difference between pulses with a clock having a frequency corresponding to the image forming speed signal
And D / A converting the output of the phase comparison counter 38 to D / A
The A converter 39 and the positive input terminal are connected to the ground potential via a resistor 46, and the F / V converter 37 is connected via a resistor 43.
Differential amplifier having a feedback variable resistor 45 for inputting the sum of the speed signal output from the D / A converter 39 via the resistor 44 and the phase difference signal output from the D / A converter 39 to the negative input terminal and adjusting the gain 40 , according to the sum of the speed signal and the phase difference signal
Controlling the excitation current of the DC motor 2B Te current amplifier 4
1 is provided. Also in such an image forming apparatus, by controlling the exciting current using the feedback control circuit 36, it is possible to change the transfer characteristic of the photosensitive member driving means 10 to shift the resonance frequency, and An increase in the speed fluctuation of the driving means 10 can be suppressed. As described above, according to the image forming apparatus of the present invention, the speed fluctuation of the drive transmission mechanism for transmitting the drive torque from the drive motor to the image carrier by inputting the image formation speed signal is obtained. Since the resonance frequency is shifted from the operating frequency of the speed fluctuation of the drive transmission mechanism by a frequency corresponding to the image forming speed signal, even when the image forming speed is switched, the increase in the speed fluctuation of the image carrier is suppressed, and the image forming speed is reduced. Can be prevented, and as a result, high quality printing can be performed. Also, since there is no need to adopt a mechanical configuration,
The size and cost of the device can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an explanatory view showing a laser printer to which the present invention is applied. FIG. 2 is an explanatory view showing a reference example of the present invention. FIG. 3 is a circuit diagram showing a speed control unit and a drive circuit according to a reference example . FIG. 4 is a timing chart showing a clock signal according to a reference example . FIG. 5 is a circuit diagram showing an excitation circuit according to a reference example . FIG. 6 is a graph showing a transfer function of a photoconductor driving unit according to a reference example . FIG. 7 is an explanatory view showing a reference example of the present invention. Circuit diagram showing an actual施例of the present invention; FIG. FIG. 9 is an explanatory view showing a conventional image forming apparatus. FIG. 10 is an explanatory view showing a conventional image forming apparatus.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) G03G 21/14 G03G 15/00 550 G03G 15/01 G03G 21/00 370-512 G03G 21/00 350

Claims (1)

(57) Claims: 1. A laser beam modulated in accordance with an image signal by a laser scanning device is scanned by a DC motor in a main scanning direction of an image carrier rotating in a sub scanning direction. An image forming apparatus for forming an image on an image carrier, speed control means for inputting an image forming speed signal and controlling the speed of the DC motor to a speed corresponding to the image forming speed signal; To adjust the excitation current of the DC motor to transmit the drive torque from the DC motor to the image carrier. The resonance frequency of the speed fluctuation of the drive transmission mechanism is calculated from the operating frequency of the speed fluctuation of the drive transmission mechanism. An image forming apparatus comprising: a resonance frequency shift unit that shifts by a frequency corresponding to an image forming speed signal.