JP4266787B2 - Charging voltage control circuit and image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus that charges an image carrier and a charging voltage control circuit thereof.

  As is well known, an electrophotographic image forming apparatus that records an image by an electrophotographic method has a drum-type electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum), and the surface thereof is made uniform at a predetermined potential during image formation. Is charged. In this charging process, corona charging, which is non-contact charging in which a corona generated by applying a high voltage to a thin corona discharge wire is applied to the surface of the photosensitive drum and charged, is generally used.

  On the other hand, in recent years, a contact charging method which is advantageous from the viewpoint of a low pressure process, a low ozone generation amount, a low cost and the like is becoming mainstream. In this contact charging method, for example, a roller charging member (hereinafter referred to as a charging roller) is brought into contact with the surface of the photosensitive drum, and a voltage is applied to the charging roller to charge the photosensitive drum. In order to obtain a desired potential Vd on the surface of the photosensitive member by the contact charging method, the charging roller has a DC voltage Vd + Vth (a discharge starting voltage (charging starting voltage) to the charged body when a DC voltage is applied to the charging member). Is applied.

Further, as disclosed in Japanese Patent Application Laid-Open No. H10-228867, in order to achieve further uniform charge, an AC voltage component (AC voltage component) having a peak-to-peak voltage that is at least twice Vth as a DC voltage corresponding to a desired potential Vd. ) Is applied to the contact charging member (an AC charging method) in which a voltage (an alternating voltage, a pulsating voltage, an oscillating voltage; a voltage whose voltage value changes periodically with time) is used.
In the AC charging method, charging can be made uniform by alternately applying positive voltage and causing discharge to the positive side and the negative side. For example, an AC voltage having a peak-to-peak voltage more than twice the discharge start voltage (charging start voltage) of the object to be charged when a DC voltage is applied, and a DC voltage (DC offset bias) are superimposed. It is known that a charging roller as a member to be charged can be charged almost uniformly by applying the alternating voltage (vibration voltage). Here, when a sinusoidal AC voltage is applied to the charging roller, a resistive load current that flows through a resistive load between the charging roller and the photosensitive drum, and a capacitive load current that flows through a capacitive load between the charging roller and the photosensitive drum, The discharge current between the charging roller and the photosensitive drum flows, and the sum of these flows to the charging roller. At this time, in order to stably charge the charging roller, it has been empirically known that the discharge current amount should be a predetermined value or more.

  FIG. 10 is a graph showing the characteristics of the charging current (Ic) flowing through the charging roller when the charging AC voltage (Vc) is applied to the charging roller. The voltage Vc is expressed as a peak voltage value of the AC voltage, and the current Ic is expressed as an effective value of the AC current. The peak voltage value of the AC voltage means a voltage value that is ½ of the peak voltage value of the AC voltage.

  In the figure, when the amplitude (peak voltage value) of the charging AC voltage (Vc) is gradually increased, the charging current (Ic) flows accordingly. Here, when the charging AC voltage (Vc) is equal to or lower than the predetermined voltage (Vth), the amplitude of the charging AC voltage is substantially proportional to the charging current. This is because the resistance load current and the capacitive load current are proportional to the voltage amplitude and the voltage amplitude (peak voltage) is small, so that no discharge phenomenon occurs and no discharge current flows. If the amplitude of the charging AC voltage (Vc) is further increased, the discharge phenomenon starts at the predetermined voltage (Vth), and the relationship between the charging AC voltage and the charging current (Ic) is not proportional, and only the discharge current (Is). The charging current (Ic) increases. In order to obtain stable charging, the charging voltage (Vt) may be set so that the discharge current (Is) becomes a predetermined value or more.

  However, when the discharge current (Is) to the photosensitive drum increases, the photosensitive drum may be deteriorated such as scraping of the photosensitive drum, and an abnormal image such as image flow in a high-temperature and high-humidity environment may occur due to the discharge product. there were. For this reason, in order to obtain stable charging and to solve the above problems, it is necessary to minimize the discharge that occurs alternately between the positive side and the negative side by applying the minimum necessary charging AC voltage. is there.

  However, in practice, the relationship between the voltage applied to the photosensitive drum and the amount of discharge is not always constant, and changes depending on the film thickness of the photosensitive layer and dielectric layer of the photosensitive drum, the environmental variation of the charging member and air, and the like. In a low-temperature and low-humidity environment (hereinafter referred to as L / L environment), the material dries and the resistance value rises, making it difficult to discharge. Therefore, a peak-to-peak voltage greater than a certain value is required to obtain uniform charging. In addition, even in the case of a high temperature and high humidity environment (H / H), the material absorbs moisture and the resistance value decreases even when the lowest charging AC voltage that provides uniform charging in this L / L environment is applied. Therefore, more discharge than necessary is generated. As a result, the amount of discharge increases, causing problems such as image defects, toner fusion, photosensitive drum scraping due to deterioration of the surface of the photosensitive drum, and shortening of life.

  Such a change in the discharge amount is caused by the above-described environmental fluctuations, as well as fluctuations in the charging member manufacturing resistance and dirt, resistance value fluctuations due to the durability, photosensitive drum electrostatic capacity fluctuations due to durability, and the high voltage generation device of the image forming apparatus main body. It has been found that this can occur even when there are variations in characteristics.

  In order to suppress such a change in the discharge amount, the “discharge current control method” of Patent Document 1 has a configuration in which the AC voltage applied to the charging member is variable, and the peak voltage of the AC voltage is the voltage at which the discharge phenomenon starts. In the voltage range below (Vth) and in the voltage range above Vth, AC voltage-AC current characteristics are detected, and the AC voltage value that provides the optimum discharge amount is calculated from the two detected characteristic lines. The voltage level of the peak voltage of the alternating voltage applied to the member is determined.

  In FIG. 10, points A, B, C, and D indicated by circles indicate points to be detected. By sampling two points A and B at a voltage equal to or lower than the voltage (Vth) at which the discharge phenomenon starts, the characteristic LINE-A of the charging AC voltage (Vc) and charging current (Ic) in the region where no discharge current is generated can be obtained. Measured. Similarly, by sampling two points C and D after discharge, the characteristic LINE-B of the charging AC voltage (Vc) and the charging current (Ic) in the region where the discharge current is generated is measured. The charging AC voltage value for setting the discharge current value to a predetermined value is calculated from the relationship between the two characteristic lines obtained by such a method, and the fluctuation of the discharge amount is controlled by controlling the charging AC voltage accordingly. Control to suppress is performed.

  FIG. 11 is a diagram illustrating the timing at which the above-described sampling is performed in a conventional discharge current control type image forming apparatus.

When the main power supply of the apparatus main body is turned on, a pre-multi-rotation process for performing a series of processes such as driving the fixing device and raising the fixing device to a predetermined temperature is performed, and then the standby state is set. Next, when a print start command is received from an external device such as an external personal computer, a pre-rotation process, which is a predetermined print preparation stage, is executed, and then a printing operation is performed on a recording paper by a series of electrophotographic processes to go into. Here, in the mode in which a plurality of printing operations are performed, a predetermined process is performed in the inter-sheet process until the printing operation for the next recording sheet is performed, and then the process proceeds to the second and subsequent printing processes. . When the printing process of the last recording paper is completed, it returns to the standby state after the post-rotation process. The above sampling of points A, B and C, D is executed in the pre-rotation process, and after setting the charging AC high voltage level based on the result, the process proceeds to the printing process. Thus, by performing sampling at a timing other than the printing process, it is possible to prevent problems such as the occurrence of abnormal images caused by a charging voltage equal to or lower than the discharge start voltage (Vth) output during the execution of sampling.
JP-A 63-149668 JP 2001-201921 A

  However, the conventional discharge current control method has a problem that the value of the discharge current fluctuates during the printing operation when the “continuous print mode” in which a plurality of recording sheets are continuously printed is performed. This problem occurs when the temperature rises around the photosensitive drum during the printing operation in the continuous print mode, and the relationship between the voltage applied to the charging roller and the discharge current changes. That is, the characteristic lines LINE-A and LINE-B shown in FIG. 10 fluctuate during the printing operation, and it is possible to control to the desired discharge current even when a charging AC voltage set according to the sample result performed at the start of printing is applied. It will disappear. To solve this problem, the print operation is stopped at a predetermined interval in the continuous print mode for a certain period, the peak voltage of the charging AC voltage is lowered to the discharge start voltage (Vth) or less, and the sampling is performed again. A method of resetting the charging voltage output can be considered. However, in this case, since the printing speed of the image forming apparatus decreases, it has not been an effective measure.

  The present invention has been made in view of the above problems, and is a charging voltage control circuit that generates a predetermined amount of discharge with high accuracy and performs uniform charging regardless of environmental fluctuations and variations in characteristics of the charging member during manufacturing. And a control method thereof and an image forming apparatus.

  An image forming apparatus according to the present invention has the following configuration.

An image forming apparatus for charging an image carrier and transferring an image formed on the image carrier to a recording medium to form an image.
AC voltage generating means for generating AC voltage;
A charging member to which an AC voltage from the AC voltage generating means is applied;
Control means for controlling the AC voltage generation means so that a constant current according to a control value flows through a path for supplying current from the AC voltage generation means to the charging member;
First output means connected to the path via a capacitive member and outputting information according to a peak voltage of an alternating voltage applied to the charging member;
A second output unit connected to the path via the capacitive member and outputting information according to a change in an AC voltage applied to the charging member;
Wherein, the output result of the first output means and said second output means when said AC voltage generating means generates the said AC voltage as a discharge start voltage higher than a peak voltage of said image bearing member Based on this, the control value at the time of image formation is determined.

  The charging voltage control circuit according to the present invention has the following configuration.

A charging voltage control circuit for controlling a charging voltage supplied to a charging member for charging the image carrier,
A voltage generation circuit for generating a primary AC voltage to be input to the primary side of the voltage transformer;
A control circuit for controlling the primary AC voltage generated by the voltage generation circuit so that the current generated on the secondary side of the voltage transformer according to the primary AC voltage becomes a constant current according to a control signal;
Which is connected to a current generated in the secondary side of the voltage transformer via a capacitive member in a path for supplying to the charging member, and outputs the information corresponding to the peak voltage of the secondary AC voltage applied to the charging member A first output circuit;
Are connected via the capacitive element to said path, and a second output circuit for outputting information corresponding to the differential value of the secondary AC voltage applied to the charging member,
The control circuit, the output of the first output circuit and the second output circuit when the previous SL voltage generating circuit is a secondary alternating voltage as a discharge start voltage higher than a peak voltage of said image bearing member Based on the result, the control value for image formation is determined.

  According to the present invention, the first output means is connected to the path for supplying current to the charging member via the capacitive member, and outputs information corresponding to the peak voltage of the AC voltage applied to the charging member; A second output unit connected to the path via a capacitive member and outputting information corresponding to a change in the AC voltage applied to the charging member. Since the control value at the time of image formation is determined based on the output results of the first output means and the second output means when an AC voltage having a peak voltage equal to or higher than the discharge start voltage is set, environmental conditions and manufacturing It is possible to provide an image forming apparatus in which a constant amount of discharge is generated with high accuracy regardless of variations in characteristics of the charging member due to the charging.

  Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings.

  FIG. 1 is a configuration diagram of a laser beam printer (electrophotographic image forming apparatus) 100 according to an embodiment of the present invention.

  The laser printer 100 includes a deck 101 that stores recording paper P, a deck paper presence sensor 102 that detects the presence or absence of the recording paper P in the deck 101, and a paper size detection that detects the size of the recording paper P in the deck 101. The sensor 103, the pickup roller 104 that feeds the recording paper P from the deck 101, the deck paper feeding roller 105 that transports the recording paper P fed by the pickup roller 104, and the deck paper feeding roller 105 are paired with each other. A retard roller 106 is provided to prevent feeding. Downstream of the deck paper feed roller 105, the deck 101, a paper feed sensor 107 that detects a paper feed conveyance state from a double-side reversing unit described later, and a paper feed conveyance for conveying the recording paper P further downstream. A roller 108, a registration roller pair 109 that synchronously conveys the recording paper P, and a pre-registration sensor 110 that detects the conveyance state of the recording paper P to the registration roller pair 109 are provided.

  Further, downstream of the registration roller pair 109, a process cartridge 112 that forms a toner image on the photosensitive drum 1 based on a laser beam from a laser scanner unit 111 described later, and a toner image formed on the photosensitive drum 1 are displayed. A roller member 113 (hereinafter referred to as a transfer roller) for transferring onto the recording paper P, and a discharge member 114 (hereinafter referred to as static eliminating needle) for removing charges on the recording paper P and promoting separation from the photosensitive drum 1 are arranged. It is installed. Further, downstream of the static elimination needle 114, there are a conveyance guide 115 and a pair of a fixing roller 117 and a pressure roller 118 provided with a halogen heater 116 for heating in order to thermally fix the toner image transferred onto the recording paper P. A fixing unit is provided. A fixing paper discharge sensor 119 for detecting the conveyance state from the fixing unit and a double-sided flapper 120 for switching the destination of the recording paper P conveyed from the fixing unit to the paper discharge unit or the double-side reversing unit are provided. A paper discharge sensor 121 that detects the paper conveyance state of the paper discharge unit and a paper discharge roller pair 122 that discharges the recording paper are disposed downstream of the paper discharge unit.

  On the other hand, the recording paper P after one-sided printing is reversed in order to print on both sides of the recording paper P, and the recording paper P is fed to the double-side reversing part side for feeding again to the image forming part by forward / reverse rotation. The recording paper P is transported from a reverse roller pair 123 to be switched back, a reversing sensor 124 for detecting a paper transport state to the reverse roller pair 123, and a lateral registration portion (not shown) for aligning the lateral position of the recording paper P. A D-cut roller 125 for detecting the recording paper P in the double-side reversing unit, and a double-sided conveying roller pair 127 for conveying the recording paper P from the double-side reversing unit to the paper feeding unit. ing.

  The scanner unit 111 scans the photosensitive drum 1 with a laser unit 129 that emits a laser beam modulated based on an image signal transmitted from an external device (host computer) 128 described later. A polygon mirror 130, a scanner motor 131, an imaging lens group 132, and a folding mirror 133. The process cartridge 112 includes a photosensitive drum 1 necessary for a known electrophotographic process, a charging roller 2 as a charging member, a developing roller 134, a toner container 135, and the like, and is detachable from the laser beam printer 100. It is configured. In the figure, reference numeral 3 denotes a high voltage power supply unit having a high voltage circuit for supplying a desired voltage to the developing roller 134, the transfer roller 113, and the charge eliminating needle 114 in addition to a charging high voltage circuit described later. A main motor 136 supplies power to each part.

  Reference numeral 4 denotes a printer control unit for controlling the laser printer 100. The RAM 5a, ROM 5b, timer 5c, digital input / output port (hereinafter referred to as I / O port) 5d, analog-digital conversion input port (hereinafter referred to as A / D port) 5e, an MPU (microcomputer) 5 having a digital-analog output port (hereinafter referred to as D / A port) 5f, and various input / output control circuits (not shown). The printer control unit 4 is connected to an external device 128 such as a personal computer via an interface 138.

  FIG. 2 is a block diagram for explaining a charging bias generation circuit of the laser beam printer 100 according to the embodiment of the present invention. This circuit is provided in the printer control unit 4, and the parts in common with FIG. The same symbol is used.

  The charging bias generation circuit generates a charging voltage in which an AC high voltage is superimposed on a DC high voltage and outputs the charging voltage from the output terminal 299. When a clock pulse (PRICLK) is output from the I / O port 5d of the MPU 5, the transistor 239 performs a switching operation via the pull-up resistor 260 and the base resistor 238, and is connected to the pull-up resistor 237 via the diode 240. It is amplified to a clock pulse having an amplitude corresponding to the output of the operational amplifier 265. When this amplitude is large, the drive voltage amplitude (peak voltage) of a sine wave input to the high voltage transformer 204 described later also increases, and as a result, the peak voltage of the AC voltage that is a high voltage also increases.

  The clock pulse passes through the capacitor 224, and is a filter circuit constituted by resistors 218, 219, 222, 223, 225, 226, 229, 230 to 234, capacitors 216, 220, 227, 228, 233, and operational amplifiers 217, 221. The filter circuit 235 outputs a sine wave centered on + 12V. The output is amplified by a push-pull high voltage transformer drive circuit 205 and input to the primary winding of the high voltage transformer 204 via the capacitor 210. As a result, a sinusoidal AC high voltage is generated on the secondary winding side of the high voltage transformer 204. One side of the secondary side of the high voltage transformer 204 is connected to a DC high voltage generation circuit 247, and a high voltage bias in which an AC high voltage is superimposed on a DC high voltage output from the high voltage transformer 204 is connected to the output protection resistor 203. Is output from the output terminal 299 via A high charging voltage bias is applied to the charging roller 2 from the output terminal 299.

  Next, detection of the AC output current (It) of the charging bias generation circuit will be described.

  Here, by detecting the current of the secondary winding of the high voltage transformer 204, feedback that generates an input voltage to the high voltage transformer drive circuit 205 according to the AC output current (It) output from the output terminal 299. Control is in progress. The AC output current (It) generated by driving the high-voltage transformer drive circuit 205 passes through the capacitor 248, the half wave in the arrow A direction passes through the diode 250, and the half wave in the arrow B direction passes through the diode 249. Each flows. The half wave in the direction of arrow A that has passed through the diode 250 is converted into a DC voltage by an integrating circuit including an operational amplifier 265, resistors 256 and 258, and a capacitor 254. In this case, the voltage (Vn) at the negative input terminal of the operational amplifier 265 is expressed by Expression (1).

Vn = Rs × Imean (1)
Here, Imean is an average value of a half wave of the charging alternating current (It), and Rs is a resistance value of the resistor 257.

  On the other hand, the current control signal (PRICNT) output from the D / A port 5f of the MPU 5 is input to the positive input terminal of the operational amplifier 265. This current control signal (PRICNT) is a signal for setting the amplitude level of the sinusoidal drive voltage input to the high voltage transformer 204, that is, the amplitude level of the AC output current (It), and varies between 0V and 5V. An analog signal corresponding to the digital value PRICNT. Here, when the voltage (Vn) of the negative input terminal of the operational amplifier 265 is smaller than the current control signal (PRICNT), the output of the operational amplifier 265 becomes large, and conversely, the voltage (V1) of the negative input terminal becomes the current control signal ( If it is larger than (PRICNT), the output of the operational amplifier 265 becomes small.

  As described above, when the output of the operational amplifier 265 increases, the amplitude of the clock pulse input to the filter circuit 235 increases and the AC high voltage peak voltage increases. With this configuration, the amplitude level of the AC high voltage (charging voltage) is controlled, and as a result, the AC output current (It) becomes a predetermined constant current value corresponding to the current control signal (PRICNT). Controlled. That is, constant current control according to the current control signal (PRICNT) is performed. The control value of the charging alternating current has the following characteristic.

Imean = PRICNT / Rs (2)
A transistor 260 is also connected to the positive input terminal of the operational amplifier 265. The transistor 260 is driven by a signal (PRION) output from the I / O port 5d of the MPU 5. By turning on the transistor 260 by setting the PRION signal to the high level, the potential of the positive input terminal of the operational amplifier 265 can be lowered to the low level, and the output of the charging AC voltage can be turned off.

Next, the voltage detection unit of the charging bias generation circuit will be described. In this charging bias generation circuit, there are two voltage detection circuits, a voltage detection circuit A and a voltage detection circuit B.
(A) Voltage detection circuit A
The voltage detection circuit A is a circuit that detects the peak voltage value of the charging AC voltage.

  FIG. 3 is a diagram showing the relationship between the waveform of the charging AC voltage and the peak voltage value detected by the voltage detection circuit A, in which the horizontal axis represents time and the vertical axis represents the charging AC voltage Vp.

  3000 indicates a case where the waveform of the charging AC voltage is a sine wave. In this case, the voltage Vp1 is detected by the voltage detection circuit A. On the other hand, reference numeral 3001 denotes a waveform when distortion occurs at the peak portion of the AC voltage waveform. The broken line shows a waveform of a waveform 3000 which is a sine wave, and distortion occurs at the peak portion, and the peak voltage is Vp2 which is lower than Vp1 by Δh. The voltage detection circuit A detects the voltage value Vp2.

  Next, the operation of the voltage detection circuit A will be described.

  The peak voltage is detected by detecting the current flowing through the capacitor 271 connected to the line having the same potential as the charging voltage output terminal 299. An alternating current corresponding to the voltage level of the charging alternating voltage flows through the capacitor 271, and this current is divided by the diodes 276 and 289. Here, the half-wave current in the direction of arrow D flows through the diode 276, and the half-wave current in the direction of arrow C flows through the diode 289.

  The half-wave current in the direction of arrow D is input to an integrating circuit composed of an operational amplifier 281, resistors 282 and 283, and a capacitor 288, and converted into a DC voltage. The peak voltage detection signal (PRIVS) corresponding to the DC voltage thus converted into voltage is input to the A / D port 5e of the MPU 5. The level of the peak voltage detection signal (PRIVS) can be expressed by equation (3).

PRIVS = C271 × f × R283 × 2 × Vp (3)
Here, C271 is the capacitance value of the capacitor 271, f is the frequency of the charging AC output, R283 is the resistance value of the resistor 283, and Vp is the peak voltage value of the charging AC voltage (corresponding to Vp1 in FIG. 3).
(B) Voltage detection circuit B
The voltage detection circuit B is a circuit that detects a peak voltage value (corresponding to Vp2 in FIG. 3) of the differential waveform of the charging AC voltage waveform.

  The differential waveform is a waveform corresponding to a change in the charging AC voltage waveform, and is a waveform whose change is maximum or minimum at the time when the AC voltage waveform becomes the vibration center voltage. Therefore, when the AC voltage waveform becomes the peak voltage, the differential waveform is not changed.

  300 in FIG. 3 shows a differential waveform of the AC voltage waveform 3001. In FIG. 3, the differential waveform 3002 is a peak voltage (Vp1) and the AC voltage waveforms 3000 and 3001 are peak voltages (Vp1 or Vp2) at time T1 when the AC voltage waveforms 3000 and 3001 become voltages at the vibration center. At time T2, the differential waveform 3002 is a vibration center voltage that does not change. The broken line portion indicates the shape of the sine wave 3000. In the region near the peak where the waveform 3001 is distorted, the differential waveform 3002 is also distorted from the sine wave. On the other hand, in the phase region where distortion is not generated in 3001, the differential waveform 3002 matches the shape of the sine wave, and the peak value thereof is Vp1 which is the same as the peak value of the waveform 3001. That is, the voltage detection circuit B detects and outputs the voltage Vp1 obtained by adding the distortion amount Δh to the peak voltage value (Vp2) of the charging AC voltage waveform in which distortion has occurred.

  Next, the operation of the voltage detection circuit B will be described.

  The charging output voltage is divided by the capacitor 271 and the resistor 273 and converted to a low voltage level. Since the diode 289 is connected between the capacitor 271 and the resistor 273, a half-wave AC waveform is input to the positive input of the operational amplifier 286. Here, the impedance of the capacitor 271 is set to be sufficiently larger than the impedance of the resistor 273. In other words, an AC waveform half-wave waveform is generated at the positive input portion of the operational amplifier 286 by dividing the differential waveform of the charging AC voltage. This differentiated waveform is further an AC waveform generated at the negative input terminal of the operational amplifier 281 by the peak voltage detection circuit composed of the operational amplifier 286, operational amplifier 280, diodes 272, 279, capacitor 284, resistors 285, 298, and 299. It is converted into a DC voltage corresponding to the peak value and input to the A / D port 5e of the MPU 5 as a differential voltage detection signal (PRIDVS). The level of the differential voltage detection signal (PRIDVS) can be expressed by the following equation.

PRIDVS = C271 × f × R273 × π × 2 × Vd (4)
Here, C271 is the capacitance value of the capacitor 271, f is the frequency of the charging AC voltage output, R273 is the resistance value of the resistor 273, π is the circumference, and Vd is the peak voltage of the differential value of the charging AC voltage.

  From these formulas (3) and (4), the peak voltage detection signal (PRIVS) and the differential voltage detection signal (PRIDVS) are both proportional to the capacitance value of the capacitor 271. That is, it can be seen that the relative value between the two signals is constant even when the capacitance value of the capacitor 271 fluctuates due to environmental conditions or the like.

  Next, a discharge current detection method according to the first embodiment will be described. In the image forming apparatus according to the first embodiment, the peak value of the charging AC voltage and the peak value of the differential value of the charging AC voltage are detected, and the discharge current value is calculated based on the detected peak value. .

  FIG. 5A is a diagram showing the relationship between the peak value of the charging AC voltage applied to the charging roller 2 and the differential peak value of the charging AC voltage and the charging AC current value (Ic).

  A charging current (Ic) flows by applying a charging AC voltage to the charging roller 2. In a region where the charging AC voltage is equal to or lower than the discharge start voltage (Vth) (non-discharge generation region), the charging AC current increases linearly (in proportion) in proportion to the increase in charging AC voltage. In this non-discharge generation region, only the nip current corresponding to the resistive load and the capacitive load between the charging roller 2 and the photosensitive drum 1 flows.

  Next, when the charging AC voltage further rises and reaches a region (discharge generation region) exceeding the discharge start voltage (Vth), a discharge occurs between the charging roller 2 and the photosensitive drum 1, and the discharge current is added to the nip current. The charging current (Ic) to which is added flows. In the non-discharge generation region described above, the charging AC voltage peak value and the charging AC voltage differential peak value were proportional, but in this discharge generation region, the charging AC voltage peak value is the line LINE- in FIG. A value along A and the peak value of the charging AC voltage derivative becomes a value along line LINE-B in FIG. 5A, and a difference occurs between the two characteristics.

  The cause of the difference in characteristics as indicated by these two characteristic lines LINE-A and LINE-B will be described with reference to FIG.

  FIG. 6 is a diagram showing a charging AC voltage waveform when the charging AC voltage is set to a voltage Va1 higher than the discharge starting voltage (Vth), and the waveform distortion occurs near the peak of the AC waveform.

  This waveform distortion occurs because the output of the high-voltage transformer 204 is distorted by the occurrence of discharge. When the charging AC voltage exceeds the discharge start voltage (Vh), a discharge occurs at a timing near the peak of the AC voltage and a discharge current flows. This discharge current flows instantaneously with a sudden rise. For this reason, when a discharge current flows through the high-voltage transformer 204 that generates the charging AC voltage, a voltage drop occurs between the output terminals of the high-voltage transformer 204 due to the leakage inductance of the transformer 204, and the waveform of the output voltage is distorted. . At this time, the peak value of the charging AC voltage is Va1. On the other hand, the peak value of the differential value of the charging AC voltage is a voltage Vb1 obtained by adding a distortion amount to the voltage Va1. As a result, the characteristic lines LINE-A and LINE-B have different characteristics.

  As shown in FIG. 5A, the line LINE-A shows discontinuous characteristics with the discharge start voltage (Vh) as a boundary. On the other hand, the line LINE-B has a characteristic that changes linearly with respect to the peak value of the differential value of the charging AC voltage. This is because, due to the operating characteristics of the high-voltage transformer 204, the output power of the high-voltage transformer 204 operates at a constant regardless of the presence or absence of discharge.

  From the relationship indicated by these lines LINE-A and LINE-B, the discharge current value can be calculated. In the case of the charging AC voltage peak value (Vp), the charging AC voltage differential peak value (Vd), and the charging current (Ic), the discharge current value (Is) is expressed by Equation (5).

Is = Ic × (Vd−Vp) / Vd (5)
Furthermore, the discharge current can be calculated by the equation (6) from the equations (5), (3), and (4).

Is = Ic × (1−π × R273 / R283 × PRIVS / PRIDVS) Equation (6)
In the image forming apparatus according to the first embodiment, the charging AC voltage peak value (Vp) is detected by the voltage detection circuit A, the charging AC voltage differential peak value (Vd) is detected by the voltage detection circuit B, and the charging current ( Ic) is set by PRICNT in a constant current control circuit composed of an operational amplifier 265, diodes 249 and 250, capacitors 248 and 256, and the discharge current value is calculated.

  5B and 5C are diagrams showing the detection characteristics of PRIVS and PRIVDS corresponding to FIG.

  When the charging AC current (Ic) is Ic1, the charging AC voltage peak value is Va1 in FIG. 5A, and the PRIVS signal is PRIVS (1) (FIG. 5B). In FIG. 5A, the differential peak value of the charging AC voltage is Vb1, and the PRIDVS signal at this time is PRIDVS (1) (FIG. 5C). From the PRIDS and PRIDVS levels detected by the MPU 5, the discharge current (Is) can be calculated using the equation (6). As is clear from the equation (6), the capacitance value of the capacitor 271 is not related to the calculated value of the discharge current (Is). That is, when calculating the discharge current according to the first embodiment, it is possible to accurately calculate the discharge current Is even when the characteristics of the capacitor 271 change due to a temperature change or the like.

  Next, a series of charging high voltage control processing procedures during the printing operation of the image forming apparatus according to the first embodiment will be described.

  FIG. 4 is a diagram showing a sequence during the printing operation of the image forming apparatus according to the present embodiment.

  When the main power source of the laser beam printer 100 is turned on, a pre-multi-rotation process is performed in which a series of processes such as driving the fixing unit and raising the heater 116 of the fixing roller 117 to a predetermined temperature is performed, and then standby It becomes a state. Next, when an instruction to start printing is input from an external device 128 such as a personal computer, a pre-rotation process, which is a predetermined printing preparation stage, is executed, and then a printing process for printing on recording paper by a series of electrophotographic processes. enter. Here, in the mode in which a plurality of printing operations are performed, a predetermined process is executed in the inter-sheet process until the printing operation for the next recording sheet is performed, and then the second and subsequent printing processes are performed. Move. When the printing process of the last recording paper is completed in this way, the standby state is restored again after the post-rotation process.

  In the image forming apparatus according to the present embodiment, the process of determining the charging AC voltage level is continuously executed in the pre-rotation process period, the printing process, and the paper-to-paper process, and the charging AC voltage is real-time based on the result. Control the level.

  FIG. 7 is a flowchart for explaining a series of charging high voltage control processing during a printing operation in the image forming apparatus according to the first embodiment. A program for executing this processing is stored in the ROM 5b and is used for controlling the MPU 5. Executed below.

  When the printing operation is started, first, in step S1, a discharge current control value (Is (cnt)) is set. This control value (Is (cnt)) is set using a value stored in advance in the ROM 5b of the main body. In step S2, PRICNT for setting the initial value of the constant current control level of the charging current is set, and the output voltage of the D / A port 5f is set. This set value is stored in the ROM 5b of the image forming apparatus. In step S3, the charging DC bias is driven at a predetermined timing during the pre-rotation. In step S4, the charging AC bias driving signal (PRION) is switched to the low level. As a result, the transistor 260 is turned off and a charging AC bias is output from the operational amplifier 265. Subsequently, in step S5 to step S7, the discharge current is measured.

  Here, first, the detection value (PRIVS) of the voltage detection circuit A is read in step S5, and then the detection value (PRIDVS) of the voltage detection circuit B is read in step S6. In step S7, based on the PRIVS and PRDVS values read in steps S5 and S6, the discharge current value (Is) is calculated by the above-described equation (6).

  In step S8, the discharge current value (Is) calculated in step S7 is compared with the set value (Is (cnt)) set in step S1, and the discharge current value (Is) is set to the set value (Is (cnt). If greater than)), the process proceeds to step S9, where PRICNT for setting the constant current control level of the charging current is reduced by a predetermined amount (here, “1”), and the corresponding voltage is output from the D / A port 5f. As a result, the level of the charging AC voltage is controlled to be small.

  On the other hand, if the discharge current value (Is) is smaller than the control value (Is (cnt)) in step S8, the process proceeds to step S10, and PRICNT for setting the constant current control level of the charging current is set to a predetermined amount (here, “1”). ) Increase and output the corresponding voltage from the D / A port 5f. As a result, the level of the charging AC output is controlled to increase. Subsequently, in step S11, it is determined whether or not printing is completed. If printing is continued, the process returns to step S5 and the same processing is repeated. By performing such repeated processing, the output of the charging AC voltage is controlled so that the discharge current value (Is) becomes a desired level (Is (cnt)).

  On the other hand, if printing is completed in step S11, the process proceeds to step S12, the charging AC bias drive signal (PRION) is switched to a high level to turn on the transistor 260 to stop the charging AC bias, and then in step S13, charging is performed. The DC bias is stopped to complete a series of processes. Since the series of processes is continuously executed in the pre-rotation process period, the printing process, and the inter-paper process, the charging AC high voltage level is controlled in real time so that the discharge current value always becomes a desired value.

  As described above, according to the charging voltage control according to the present embodiment, the charging AC voltage peak value is detected by providing one capacitor in the charging voltage output unit and measuring the current flowing through the capacitor. The AC voltage differential peak value is detected by measuring the voltage generated in a resistor connected in series with the AC. Then, a discharge current is calculated from the detected charging AC voltage peak value and charging AC voltage differential peak value, and charging AC level control is performed so that the discharging current becomes a desired value.

  By adopting such a configuration, it becomes possible to detect the discharge current in the discharge generation region, and it is possible to always optimally control the discharge current value in the pre-rotation process period, the printing process, and the inter-paper process. This makes it possible to achieve uniform charging without causing problems such as deterioration of the photosensitive drum and image defects, regardless of fluctuations in the environment or characteristics of the charging member during manufacturing.

  Furthermore, even if the capacitance of the capacitor fluctuates due to environmental changes, the relative relationship between the peak value of the charging AC voltage and the differential peak value of the charging AC voltage does not fluctuate. Control can be realized and stable charging control can be performed.

  Next, a second embodiment of the present invention will be described. The basic configuration of the image forming apparatus according to the second embodiment is the same as that of the first embodiment described above, and only the configurations of the voltage detection circuit A and the voltage detection circuit B in the charging output circuit are different.

FIG. 8 is a diagram showing the configuration of the charging control circuit according to the second embodiment of the present invention. Portions common to the configuration of FIG. 2 according to the above-described first embodiment are denoted by the same symbols, and description thereof is omitted. .
(A) Voltage detection circuit A
The voltage detection circuit A detects the peak value of the charging AC voltage as in the first embodiment. This peak value is detected by measuring the current flowing through the capacitor 271 connected to the line having the same potential as the charging voltage output terminal 299. An AC current corresponding to the level of the charging AC voltage flows through the capacitor 271 and is shunted by the diodes 801 and 802. A half-wave current in the direction of arrow D flows through diode 802, and a half-wave current in the direction of arrow C flows through diode 801. The half-wave current in the direction of arrow C is input to an integrating circuit including an operational amplifier 281, resistors 282 and 283, and a capacitor 288, and is converted into a DC voltage. This voltage-converted signal is input to the A / D port 5e of the MPU 5 as a peak voltage detection signal (PRIVS).

  The level of the peak voltage detection signal can be expressed by the following equation.

PRIVS = Vt2−C271 × f × R283 × 2 × Vp (7)
Here, C271 is the capacitance value of the capacitor 271, f is the frequency of the charging AC output, R283 is the resistance value of the resistor 283, and Vp is the peak voltage value of the charging AC output. Vt2 is the voltage at the positive input terminal of the operational amplifier 281. This voltage Vt2 is set to a value obtained by dividing a 5V power supply by resistors 804 and 805.
(B) Voltage detection circuit B
The voltage detection circuit B detects the peak voltage value of the differential waveform of the charging AC voltage waveform as in the first embodiment.

  The charging output voltage is divided by the capacitor 271 and the resistor 803. One end of the resistor 803 is connected to the negative input terminal of the operational amplifier 281. Since the positive input terminal of the operational amplifier 281 is held at the voltage Vt2, the voltage obtained by adding the DC voltage (Vt2) and the divided half-wave AC waveform to the positive input terminal of the operational amplifier 286. Is applied. Further, since the impedance of the capacitor 271 is set to be sufficiently larger than the impedance of the resistor 803, the positive input portion of the operational amplifier 286 has a half-wave waveform and a DC voltage level corresponding to the differential value of the charged AC voltage. A voltage to which (Vt2) is added is generated. The differentiated waveform is further converted into a DC voltage corresponding to the peak value of the differentiated AC waveform by a peak voltage detection circuit constituted by operational amplifiers 286, 280, diodes 272, 279, capacitor 284, resistors 285, 298, 299. And is input to the A / D port 5e of the MPU 5 as a differential voltage detection signal (PRIDVS). The level of this differential voltage detection signal (PRIDVS) can be expressed by equation (8).

PRIDVS = C271 × f × R803 × π × 2 × Vd × Vt2 (8)
Here, C271 is the capacitance value of the capacitor 271, f is the frequency of the charging AC output, R283 is the resistance value of the resistor 283, π is the circumference, and Vd is the peak voltage of the differential value of the charging AC voltage.

  FIG. 9A is a diagram for explaining the relationship between the charging AC voltage peak value and charging AC voltage differential peak value applied to the charging roller 2 and the charging AC current value (Ic).

  FIGS. 9B and 9C show the detection characteristics of the PRIVS signal and PRIVDS signal corresponding to FIG. 9A. When the charging AC current (Ic) is Ic1, the peak value of the charging AC voltage is Va1, and the PRIVS signal is PRIVS (1). From the levels of the PRIDS signal and the PRIDVS signal detected by the MPU 5, the discharge current (I) is calculated using the equation (9).

  This equation (9) is calculated from the equation (5, equation (7) and equation (8).

I = I * {1-π * (Vt2-PRIVS) / (PRIDVS-Vt2) * R803 / R283} (9)
The series of charging high voltage control methods during the printing operation of the image forming apparatus according to the second embodiment is the same as in the first embodiment, and only the calculation of the discharge current is performed by the above method.

  By performing such control, the same effect as in the first embodiment can be obtained.

  As described above, according to the charging high voltage control according to the second embodiment, the charging voltage output unit is provided with one capacitor, and the peak value of the charging AC voltage is detected by measuring the current flowing through the capacitor. The charging AC voltage differential peak value is detected by measuring the voltage generated in the resistor connected in series with the capacitor. Then, a discharge current is calculated from the detected charging AC voltage peak value and charging AC voltage differential peak value, and charging AC level control is performed so that the discharging current becomes a desired value.

  With such a configuration, it becomes possible to detect the discharge current in the discharge generation region, and the discharge current value can always be optimally controlled in the pre-rotation process period, the printing process, and the paper-to-paper process. It is possible to achieve uniform charging without causing problems such as deterioration of the photosensitive drum and image defects regardless of variations in the characteristics of the charging member at the time of manufacture.

  Furthermore, even if the capacitance of the capacitor fluctuates due to environmental changes, the relative relationship between the charging AC voltage peak value and the charging AC voltage differential peak value does not change, so accurate discharge current control can be achieved even when environmental fluctuations occur. And uniform charging can be achieved.

[Other embodiments]
The present invention achieves its object by supplying a software program for realizing the functions of the above-described embodiments to a computer or CPU, and the computer or CPU reads and executes the supplied program. it can.

  In this case, the program is supplied directly from a storage medium storing the program, or downloaded from another computer or database (not shown) connected to the Internet, a commercial network, a local area network, or the like. Supplied by

  The form of the program may be in the form of object code, program code executed by an interpreter, script data supplied to an OS (operating system), and the like.

  Further, the present invention supplies a computer or CPU with a storage medium storing a software program for realizing the functions of the above-described embodiments, and the computer or CPU reads and executes the program stored in the storage medium. Can also be achieved.

  In this case, the program code itself read from the storage medium realizes the functions of the above-described embodiments, and the storage medium storing the program code constitutes the present invention.

  As a storage medium for storing the program code, for example, ROM, RAM, NV-RAM, floppy (registered trademark) disk, hard disk, optical disk (registered trademark), magneto-optical disk, CD-ROM, MO, CD-R, CD -RW, DVD-ROM, DVD-RAM, DVD-RW, DVD + RW, magnetic tape, nonvolatile memory card, etc.

  The function of the above-described embodiment is not only by executing the program code read from the computer, but the OS or the like running on the computer performs part or all of the actual processing based on the instruction of the program code. Can also be realized.

1 is a diagram illustrating a configuration of an image forming apparatus (laser beam printer) according to an embodiment of the present invention. It is a block diagram which shows the structure of the charging voltage control circuit which concerns on Embodiment 1 of this invention. It is a figure which shows the relationship between the charging alternating current waveform which concerns on this Embodiment, and the peak voltage value detected by the voltage detection circuit A. FIG. 5 is a diagram illustrating a sequence during a printing operation of the image forming apparatus according to the present embodiment. A graph showing the relationship between the peak value of the charging AC voltage applied to the charging roller and the differential peak value of the charging AC voltage and the charging AC current value (Ic), and the PRIVS signal and PRIVDS corresponding to FIG. It is a figure (B, C) which shows the detection characteristic of a signal. It is a figure which shows a charging alternating voltage waveform when charging AC voltage is Va1 higher than discharge start electricity (Vth). 4 is a flowchart showing a procedure of high charging voltage control in the laser beam printer according to the first embodiment. It is a block diagram which shows the structure of the charging voltage control circuit which concerns on Embodiment 2 of this invention. Charging AC voltage peak value to be applied to charging roller, charging AC voltage differential peak value, charging AC current value (Ic) and characteristics (A) and detection of PRIVS signal and PRIVDS signal corresponding to FIG. 9 (A) It is a figure (B, C) which shows a characteristic. It is a figure explaining the conventional discharge control. It is a figure which shows the timing which performs the sampling in the image forming apparatus of the conventional discharge current control system.

Claims (11)

An image forming apparatus for charging an image carrier and transferring an image formed on the image carrier to a recording medium to form an image.
AC voltage generating means for generating AC voltage;
A charging member to which an AC voltage from the AC voltage generating means is applied;
Control means for controlling the AC voltage generation means so that a constant current according to a control value flows through a path for supplying current from the AC voltage generation means to the charging member;
First output means connected to the path via a capacitive member and outputting information according to a peak voltage of an alternating voltage applied to the charging member;
A second output unit connected to the path via the capacitive member and outputting information according to a change in an AC voltage applied to the charging member;
Wherein, the output result of the first output means and said second output means when said AC voltage generating means generates the said AC voltage as a discharge start voltage higher than a peak voltage of said image bearing member An image forming apparatus characterized in that the control value for image formation is determined based on the control value.   The image forming apparatus according to claim 1, wherein the information according to the change in the AC voltage is information according to a difference between a maximum value and a minimum value of the change rate of the AC voltage. The value first output means and said second output means outputs are identical when said AC voltage generating means to generate said alternating voltage as a discharge start voltage or less of the peak-to-peak voltage of the image bearing member The image forming apparatus according to claim 2. It said first output means, to any one of claims 1 to 3, characterized in that outputs information corresponding to the peak voltage of the AC voltage by detecting the alternating current flowing through the capacitive member The image forming apparatus described.   The first output means outputs information corresponding to a peak voltage of the AC voltage by detecting an average value of a half-wave current of the AC current flowing through the capacitive member. The image forming apparatus described.   The second output means outputs information according to a difference between a maximum value and a minimum value of the change rate of the AC voltage based on a peak value of a half-wave current of the AC current flowing through the capacitive member. The image forming apparatus according to claim 2. A charging voltage control circuit for controlling a charging voltage supplied to a charging member for charging the image carrier,
A voltage generation circuit for generating a primary AC voltage to be input to the primary side of the voltage transformer;
A control circuit for controlling the primary AC voltage generated by the voltage generation circuit so that the current generated on the secondary side of the voltage transformer according to the primary AC voltage becomes a constant current according to a control signal;
Which is connected to a current generated in the secondary side of the voltage transformer via a capacitive member in a path for supplying to the charging member, and outputs the information corresponding to the peak voltage of the secondary AC voltage applied to the charging member A first output circuit;
Are connected via the capacitive element to said path, and a second output circuit for outputting information corresponding to the differential value of the secondary AC voltage applied to the charging member,
The control circuit, the output of the first output circuit and the second output circuit when the previous SL voltage generating circuit is a secondary alternating voltage as a discharge start voltage higher than a peak voltage of said image bearing member A charging voltage control circuit, wherein the control value for image formation is determined based on a result. The information corresponding to the differential value of the secondary alternating voltage, the charging voltage control according to claim 7, wherein the the information corresponding to the difference between the maximum value and the minimum value of the rate of change of the secondary alternating voltage circuit. The first output circuit and the signal second output circuit outputs upon the discharge start a voltage below the peak voltage the secondary AC voltage of the voltage generating circuit is the image bearing member are the same The charging voltage control circuit according to claim 7. Wherein the first output circuit is charged according to claim 9, characterized in that outputs information corresponding to the peak voltage of the secondary alternating voltage by detecting the secondary alternating current through the capacitive element Voltage control circuit. The second output circuit 7 through claim and outputs information corresponding to the differential value of the secondary alternating voltage by detecting the peak value of the secondary alternating current through the capacitive element The charging voltage control circuit according to claim 10.

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