JP2013037084A - Image formation device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a technique that sufficiently reduces flicker, harmonic current distortion, and uneven glossiness of images in controlling electric power supply to a heater in a fixation heating apparatus used for an image formation device.SOLUTION: In an image formation device, by switching on and off of current from an AC power supply to a heater in a fixation heating apparatus, a phase control method, a wave number control method, or a combined control method combining the phase control and the wave number control can be selected as a method for controlling an energization quantity to the heater. An image printing rate is derived that shows a degree of a toner concentration in a specific image area, which is a specific area in storage media. A method to control the energization quantity to the heater is selected based on the image printing rate in the specific image area to be fixed by the fixation heating apparatus.

Description

  The present invention relates to an image forming apparatus such as a copying machine, a printer apparatus, a facsimile apparatus, or a multifunction machine having a plurality of these functions, and in particular, has a fixing heating apparatus that fixes a toner image onto a recording medium by heat. The present invention also relates to an image forming apparatus that controls the power of the heater.

  Conventionally, there are various image forming apparatuses such as a copying machine, a printer, and a facsimile using an image forming process such as electrophotography, electrostatic recording, and magnetic recording. In these image forming apparatuses, in order to make an unfixed toner image formed on a recording medium such as recording paper or an OHP sheet a permanent image, a fixing heating apparatus for melting the toner by heat and fixing the toner on the recording medium is provided. ing. As this fixing and heating device, for example, a heat roller type heat fixing device using a halogen heater as a heat source or a film fixing type heat fixing device using a ceramic heater as a heat source is used. In addition, as the fixing heating apparatus, those having various systems and configurations such as an IH fixing system, a flash heating system, and an oven heating system have been put into practical use.

  Above all, the film fixing type fixing heating device that heats a cylindrical film with extremely small heat capacity from the inside of the cylinder has high energy efficiency and a high heating rate, so the time from power-on to printing becomes short. In addition, power consumption during print standby can be greatly reduced.

  Here, generally, the heater of the fixing heating device is connected to an AC power source via a switching element such as a triac, and power is supplied by the AC power source. For example, the on / off control of the ceramic heater may be phase control of an AC power supply (for example, see Patent Document 1), wave number control (for example, see Patent Document 2), or control that combines phase control and wave number control (for example, , See Patent Document 3).

  In general, the fixing heating apparatus is provided with a temperature detection element such as a thermistor temperature sensing element. The temperature detection element detects the temperature of the fixing heating device, and the engine controller controls on / off of the switching element based on the detected temperature information. Thereby, the power supply to the heater is turned on / off, and the temperature of the fixing heating device is adjusted to the target temperature.

  Here, the phase control described above is control for supplying electric power to the heater by turning on the heater at an arbitrary phase angle within each half wave in the input voltage from the AC power supply. The wave number control is a control in which a period of a plurality of half waves is set as one control cycle, and the heater is turned on / off in units of half waves of the AC power supply. Further, in the control combining the phase control and the wave number control, a period composed of a plurality of half waves is set as one control cycle, and a part of the half waves are subjected to wave number control and the rest are subjected to phase control.

  In the above, the purpose of selecting phase control is to suppress flickering of lighting equipment, so-called flicker, and temperature ripple of the fixing heating device. Here, flicker is a phenomenon in which peripheral devices connected to the same AC power source, for example, lighting devices, flicker due to a sudden change in the voltage of the AC power source. On the other hand, in the phase control, since a current flows every half wave, a change amount and a change period of the current are small, and flicker generation can be suppressed. Further, in the phase control, the electric power can be controlled every half wave, so that the responsiveness of the temperature control of the fixing heating device is high and the temperature ripple of the fixing heating device can be reduced.

On the other hand, in the phase control, since the heater is turned on at an arbitrary phase angle within each half wave of the AC power source, there is a possibility that a sudden current fluctuation occurs when the heater is turned on. When a sudden current fluctuation occurs, a current waveform containing a large amount of harmonic current distortion due to a multiplied frequency component of the fundamental frequency (for example, 50 Hz or 60 Hz) of the AC power supply is obtained. This harmonic current distortion may cause malfunction between electronic devices, or distortion in the voltage waveform of the AC power supply.

  The purpose of selecting the wave number control is to suppress harmonic current distortion. In wave number control, heater on / off control is always performed when the power supply voltage is close to 0V (zero cross point), so there is little sudden current fluctuation, compared with phase control in which the heater is turned on halfway through the voltage waveform. This is because it is difficult to generate harmonic current distortion. However, although the wave number control is effective in suppressing the harmonic current distortion, since the fluctuation of the load current for each half wave is large, there is a disadvantage that the voltage fluctuation of the AC power source becomes large and flicker is likely to occur.

  In addition, the reason for selecting control that combines phase control and wave number control is advantageous in suppressing harmonic current distortion compared to the case where only phase control is selected, and when only wave number control is selected. In comparison, it is advantageous for flicker suppression.

  Here, the magnitude of the harmonic current distortion tends to increase as the voltage of the AC AC power supply used increases. Therefore, conventionally, the heater control is generally fixed to either phase control or wave number control in accordance with the voltage of the AC AC power supply in the area where the image forming apparatus is used. For example, phase control advantageous for flicker may be adopted for an area of AC AC power supply voltage of 100 to 127 V, and wave number control advantageous for harmonic current distortion may be adopted for an area of AC AC power supply voltage of 220 V to 240 V. Conceivable. However, with the increase in heater power of the fixing heating device due to the recent increase in the speed of image forming apparatuses, it is difficult to sufficiently suppress harmonic current distortion only by phase control, and flicker can be sufficiently suppressed only by wave number control. It may be difficult to suppress. On the other hand, control combining phase control and wave number control is expected as control having both effects of suppressing harmonic current distortion and flicker.

JP 09-106215 A JP 2000-268939 A JP 2003-123941 A

  In recent image forming apparatuses, full-color images are formed, and printing speed is increased and image quality is increased. First printout time (from the print standby state until the first recording medium is discharged) Reduction of time (FPOT) is required. Therefore, the heater power of the fixing heating device tends to increase. In other words, the increase in heater power means that the resistance value of the heater tends to decrease. However, the decrease in the resistance value of the heater increases the current of the AC power supply when supplying power to the heater. As a result, harmonic current distortion and flicker tend to increase.

  Further, while the recording medium passes through the fixing heating device, the fixing heating device is controlled to a temperature at which the toner image can be fixed to the recording medium. As the printing speed increases, the temperature of the fixing heating device increases. Ripple tends to appear as uneven gloss in the image. For this reason, it is required to suppress the temperature ripple more than ever.

In the situation as described above, it is difficult to sufficiently suppress all of harmonic current distortion, flicker, and gloss unevenness (temperature ripple) only by using heater control combining phase control and wave number control. . An object of the present invention has been made in view of the above circumstances. For example, in control during power supply to a heater of a fixing heating device used in an image forming apparatus, flicker, harmonic current distortion, and image It is to provide a technology capable of sufficiently reducing all of the gloss unevenness.

In order to achieve the above object, the first invention according to the present application,
Image forming means for forming a toner image on a recording medium;
Fixing heating means for heating and fixing the toner image to the recording medium;
Switch means for switching energization and interruption of the heating source of the fixing heating means with a current from an AC power source;
Zero-cross detecting means for detecting zero-cross of the AC power supply;
A storage unit that stores a plurality of types of supply power control tables that store switching timings of energization and interruption of current from the AC power supply,
Based on the timing of zero-crossing by the zero-crossing detection unit and the switching timing referenced using the supply power control table stored in the storage unit, the switch unit is switched from the AC power source to the heating source of the fixing heating unit. A control means for controlling an energization amount to the heating source by switching between energization and interruption of the current;
Image printing rate deriving means for deriving an image printing rate indicating the degree of toner density of a toner image in a specific image region which is a specific region of the recording medium;
With
The control unit selects the supply power control table referring to the switching timing according to the image printing rate in the specific image area fixed by the fixing heating unit, which is derived by the image printing rate deriving unit. An image forming apparatus characterized by the above.

  As described above, in the present invention, in controlling the heater power of the fixing heating device, a plurality of power control tables are stored, and the optimum table is selected and used according to the image to be fixed. It becomes possible to sufficiently suppress current distortion and uneven glossiness of an image.

1 is a schematic view of a fixing heating device in an embodiment of the present invention. 1 is a schematic diagram of a heater drive circuit of an image forming apparatus in an embodiment of the present invention. It is explanatory drawing of heater electric power control by the phase control in the Example of this invention. It is explanatory drawing of the heater electric power control by the wave number control in the Example of this invention. It is explanatory drawing of heater electric power control by combined control of phase control and wave number control. It is explanatory drawing of the minimum image structural unit in the Example of this invention. It is explanatory drawing which shows the example of the fixed image in the Example of this invention. It is a heater electric power control table switching flow in Example 1 of this invention. It is time series explanatory drawing of the heater electric power control table switching control of Example 1. FIG. It is a figure which shows the relationship between the electric power supplied in Example 2, and a harmonic current distortion relative value. 10 is a flowchart of power control table switching control according to the second embodiment. It is a flowchart of the table 3 setting process in Example 2. 10 is a flowchart of speed change determination processing according to the second embodiment. It is a flowchart of the table 1 selection process in Example 2. 10 is a flowchart of a speed reduction process in the second embodiment. 10 is a flowchart of speed return processing in the second embodiment. It is a flowchart of the table 2 setting process in Example 2. 1 is a schematic view of an image forming apparatus in an embodiment of the present invention.

  The best mode for carrying out the present invention will be exemplarily described in detail below with reference to the drawings.

<Example 1>
FIG. 18 shows an example of an image forming apparatus having a fixing heating device using a ceramic heater. However, the present invention can be applied to all apparatuses using a fixing heating apparatus, and is not particularly limited to the image forming apparatus shown in FIG.

  First, the overall operation of the image forming apparatus shown in FIG. 18 will be described. In the image forming apparatus 100 of FIG. 18, an image processing controller 120 receives image information and a print instruction transmitted from an external device such as a personal computer and performs processing. A control unit 113 serving as a control unit is electrically connected to the image processing controller 120 and controls each part of the image forming apparatus 100 in accordance with an instruction from the image processing controller 120. When the image processing controller 120 receives a print instruction from an external device, an image forming process is executed by the following operation.

  The recording medium 101 is fed by the feeding roller 102 and conveyed toward the intermediate transfer member 103. The photosensitive drum 104 is rotated counterclockwise at a predetermined speed by a drive motor (not shown), and is uniformly charged by the primary charger 105 during the rotation process. Further, laser light modulated in accordance with the image signal is output from the laser beam scanner 106, and an electrostatic latent image is formed on the photosensitive drum 104 by selectively scanning and exposing the photosensitive drum 104. The electrostatic latent image is visualized as a toner image (developer image) by attaching powder toner as a developer in the developing unit 107 to the electrostatic latent image.

  The toner image formed on the photosensitive drum 104 is primarily transferred onto the intermediate transfer member 103 that rotates in contact with the photosensitive drum 104. Thereafter, the recording medium 101 conveyed at an appropriate timing synchronized with the rotation of the intermediate transfer member 103 is pressed against the intermediate transfer member 103 by a transfer roller 108 to which a transfer bias is applied, so that the intermediate transfer member 103 is pressed. The toner image is secondarily transferred onto the recording medium 101. The photosensitive drum 104, the primary charger 105, the laser beam scanner 106, and the developer 107 are arranged for four colors of black, yellow, magenta, and cyan, respectively, and toner images for the four colors are recorded on the recording medium 101. Secondary transfer. The configuration for forming a toner image on the recording medium 101 as described above corresponds to an image forming unit in this embodiment.

  A fixing heating device 109 serving as a fixing heating unit includes a film unit 111 including a ceramic heater 110 serving as a heating source, and a pressure roller 112 pressed against the film unit 111. The recording medium 101 is heated and pressed by the film unit 111 and the pressure roller 112 to fix the toner image on the recording medium 101. Then, the recording medium 101 on which the toner image is fixed is discharged out of the apparatus as an image formed product. Here, the control unit 113 manages the conveyance status of the recording medium 101 by the conveyance sensor 114, the registration sensor 115, the pre-fixing sensor 116, and the fixing paper discharge sensor 117 on the conveyance path of the recording medium 101. The control unit 113 has a storage unit that stores a temperature control program and a temperature control table of the heat fixing device 109.

  The power supply device 119 connected to the AC power supply 118 has a power supply circuit to the heat fixing device 109 and a rectification circuit from AC to DC, and the power consumed in the above-described process is transferred from the power supply device 119 to the image. It is supplied to each part of the forming apparatus 100.

Next, the fixing heating apparatus according to the present embodiment will be described with reference to FIG. FIG. 1 is a cross-sectional view of the fixing heating device 109. The heater holder 200 holds the ceramic heater 110 and guides the inner surface of the film 201. The heater holder 200 is formed of a material having high heat resistance, heat insulation, and rigidity, and is a horizontally long member having a longitudinal direction in a direction (perpendicular to the drawing) crossing the conveyance path of the recording medium 101.

  The ceramic heater 110 is inserted into a groove formed along the longitudinal direction on the lower surface of the heater holder 200, and is fixedly supported by a heat-resistant adhesive. The ceramic heater 110 is horizontally long with the direction transverse to the transfer material conveyance path as the longitudinal direction. It has the shape of A cylindrical heat-resistant film material 201 (hereinafter referred to as a fixing film 201) is loosely fitted on a heater holder 200 to which a ceramic heater 110 is attached. The stay 202 is a highly rigid member having a longitudinal direction in the direction perpendicular to the paper surface of FIG. 1 and is disposed inside the heater holder 200.

  On the other hand, the pressure roller 112 is arranged so as to be in pressure contact with the ceramic heater 110 with the fixing film 201 interposed therebetween. A range indicated by an arrow N is a fixing nip portion formed by the pressure roller 112 and the ceramic heater 110 sandwiching the fixing film 201. The pressure roller 112 is rotationally driven at a predetermined peripheral speed in the direction of arrow B by a fixing motor (not shown). In the fixing nip portion N, the rotational force of the pressure roller 112 directly acts on the fixing film 201 due to the frictional force between the pressure roller 112 and the outer periphery of the fixing film 201. Then, the fixing film 201 is driven to rotate in the direction of arrow C by sliding while being pressed against the lower surface of the ceramic heater 110.

  As described above, the heater holder 200 functions as a guide member on the inner surface of the fixing film 201 to facilitate the rotation of the fixing film 201. Further, in order to reduce the sliding resistance between the inner surface of the fixing film 201 and the lower surface of the ceramic heater 110, a small amount of a lubricant such as heat-resistant grease can be interposed therebetween.

  Fixing of the unfixed image on the recording medium 101 is executed by the following operation. The fixing film 201 is rotated following the rotation of the pressure roller 112, and at the same time, the temperature of the ceramic heater 110 is raised to a temperature at which an unfixed image can be fixed by heat and pressure as a permanently fixed image. Then, the recording medium 101 to be fixed is introduced into a fixing nip portion N formed by the fixing film 201 and the pressure roller 112 and is nipped and conveyed. At the fixing nip N, heat from the ceramic heater 110 is applied to the unfixed image on the recording medium 101 via the fixing film 201, and the unfixed image on the recording medium 101 is heated and fixed on the recording medium 101.

  The recording medium 101 that has passed through the fixing nip N is separated from the fixing film 201 and the pressure roller 112 and conveyed. Note that an arrow A in FIG. 1 indicates the conveyance direction of the recording medium 101. Further, the fixing heating device 109 has a thermistor 203 that is a temperature sensitive element for detecting the temperature of the ceramic heater 110. The thermistor 203 is pressed against the ceramic heater 110 with a force that can always be sensed by a spring (not shown) or the like, and detects the temperature of the ceramic heater 110.

  FIG. 2 shows a heater drive circuit of the fixing heating apparatus according to this embodiment. The AC power source 118 is a commercial power source connected to the fixing heating device 109, and supplies power to the heater 110 in accordance with a heater drive signal from the control unit 113, causing the heater 110 to generate heat. Control of the electric power supplied to the heater 110 is performed by switching energization / interruption of the triac 301. The resistors 302 and 303 are bias resistors, and the phototriac coupler 304 is provided to secure a creepage distance between the primary side and the secondary side. When the light-emitting diode 305 of the phototriac coupler 304 is energized, the triac 301 is turned on. Note that the switch means in the present embodiment is configured to include the triac 301.

  The transistor 307 operates in accordance with a heater drive signal supplied from the control unit 113 via the resistor 308. The phototriac coupler 304 is also turned on / off in response to the on / off of the transistor 307. The resistor 306 is provided to limit the current of the phototriac coupler 304. The input power supply voltage from the AC power supply 118 is also input to a zero cross detection circuit 309 that is a zero cross detection means.

  The AC voltage from the AC power supply 118 is half-wave rectified by the rectifiers 310 and 311. This half-wave rectified AC voltage is input to the base of the transistor 316 via the resistors 312 and 313, the capacitor 314, and the resistor 315. Accordingly, the transistor 316 is turned on when the neutral side potential is higher than the hot side potential, and the transistor 316 is turned off when the neutral side potential becomes lower than the hot side potential.

  The photocoupler 317 is an element for securing a creepage distance between the primary side and the secondary side, and resistors 318 and 319 are provided to limit a current flowing through the photocoupler 317. When the Neutral side potential is higher than the Hot side potential, the transistor 316 is turned on, so that the light emitting diode 320 of the photocoupler 317 is turned off, the phototransistor 321 is turned off, and the output voltage of the photocoupler 317 becomes High. On the other hand, when the neutral side potential becomes lower than the hot side potential, the transistor 316 is turned off, so that the light emitting diode 320 of the photocoupler 317 emits light, the phototransistor 321 is turned on, and the output voltage of the photocoupler 317 becomes low.

  The output of the photocoupler 317 is input to the control unit 113 as a zero cross signal via the resistor 322. The zero-cross signal is a pulse signal having a signal period equal to the frequency of the AC power supply, and the signal level changes according to the potential polarity of the AC power supply. The control unit 113 detects the rising and falling edges of the zero cross signal, turns on / off the triac 301 at an arbitrary timing, and thereby controls the power supply to the heater 110. The temperature detected by the thermistor 203 is detected as a partial pressure between the resistor 323 and the thermistor 203 and input to the A / D port as a TH signal to the control unit 113.

  In the above configuration, the temperature control of the fixing heating device 109 is performed as follows. That is, when the control unit 113 monitors the TH signal, if the temperature reaches a temperature at which an unfixed image can be fixed by heat and pressure as a permanently fixed image, energization of the heater 110 is stopped by the heater drive signal. . When the temperature is lower than the temperature at which heat and pressure fixing is possible, the heater 110 is energized. As a result, the temperature of the fixing heating device 109 can be kept constant.

  Next, phase control, wave number control, and control combining phase control and wave number control, which are power control methods when supplying power to the heater, will be described. FIG. 3 is a diagram for explaining an example of heater power control by phase control. In FIG. 3, the logic of the zero cross signal switches at the point where the AC power supply voltage waveform switches from positive to negative and from negative to positive. The heater drive signal is turned on after Ta time from the rising and falling edges of the zero cross signal. If it does so, a heater will energize and will supply electric power in the part shown with the oblique line in the graph which shows a heater applied voltage.

  In this example, after the heater is turned on, the energization to the heater is turned off at the next zero cross point. For this reason, by turning on the heater drive signal again after time Ta from the edge of the zero cross signal, the same power is supplied to the heater even in the next half wave. Further, when the heater drive signal is turned on after a time Tb different from the time Ta, the energization time to the heater changes, so that the power supplied to the heater can be changed. In this way, the power supplied to the heater can be controlled by changing the time from the edge of the zero cross signal to turning on the heater drive signal every half wave.

  In the phase control, as shown in FIG. 3, since the energization of the heater is turned on in the middle of the half wave of the AC power supply waveform, the current flowing through the heater suddenly rises and the harmonic current flows. This harmonic current increases as the rising amount of the current increases, and thus becomes maximum when the phase angle is 90 °, that is, when the supplied power is 50%. In addition, since a rising edge of this current occurs every half wave, many harmonic currents flow, and it is essential to comply with harmonic regulations. For this reason, circuit components such as filters are often required for phase control. On the other hand, in phase control, a current smaller than the maximum value in each half wave flows every half wave, so the amount of change in current per half wave is small, and the change period is also fast, so the effect on flicker is small. It is.

  FIG. 4 shows an example of heater power control by wave number control. In wave number control, on / off control is performed in half-wave units of an AC power supply. Therefore, the heater drive signal is turned on in synchronization with the edge of the zero cross signal. Then, for example, eight continuous half waves are set as one control cycle (one control cycle), and the power supplied to the heater is controlled by changing the number of half waves turned on in one control cycle. In FIG. 4, since four half waves of the eight half waves are turned on, the power supplied to the heater is 50%. As described above, in the wave number control, by changing the wave number to be continuously turned on, the heater supply power is determined in advance in four steps (25% increments) between 0% and 100%, The heater power control is performed based on the energization control pattern.

  The wave number control is characterized in that the heater is always turned on / off at zero crossing, so that there is no steep rising edge of current as in phase control and there is little harmonic current. On the other hand, since the current flows in half-wave units, the amount of change in current per half-wave is large and the change cycle is long compared to phase control. For this reason, the influence on flicker becomes large in wave number control. Further, the temperature ripple of the fixing film 201 tends to increase, and gloss unevenness of the image tends to occur.

FIG. 5 shows an example of heater power control that combines phase control and wave number control. In the example of FIG. 5, eight continuous half waves are set as one control period, of which two half waves are controlled by wave number control and two half waves are controlled by phase control, and the heater power supply ratio d is 33.3%. Show. Here, the control unit 113 performs phase control by transmitting an ON signal to the transistor 307 at the timing of Tc so that the power duty of the first wave and the second half wave is 33.3%. Then, by turning on the second half of the remaining six half-waves by wave number control and turning off all the other half-waves, about 33.3% of power is supplied in one control cycle.

  In this way, an energization control pattern in which 0% to 100% of the heater power supply is divided into arbitrary levels is determined in advance, and the control unit 113 performs heater energization control based on the energization control pattern. Since this control includes both phase control and wave number control, flicker is suppressed compared to the case where only wave number control is performed, and harmonic current distortion is suppressed compared to the case where only phase control is performed. There are features.

  Next, uneven glossiness of the image will be described. The gloss unevenness is more likely to occur as the temperature ripple of the fixing film 201 is larger, and is more likely to occur when the heater power control is performed by wave number control than by phase control. In heater power control in which phase control and wave number control are combined, temperature ripple may increase at portions where wave number control is performed, and gloss unevenness may occur. Also, the level of gloss unevenness varies depending on the image, and there is a tendency that it appears more noticeably in an image having a wide image area and a large amount of toner.

  On the other hand, in the case of an image with a small amount of toner such as a text image, the level of gloss unevenness is low, and there are many cases where there is almost no problem. In the present invention, by utilizing this tendency, the optimum heater power control corresponding to the image is selected, thereby suppressing uneven glossiness of the image and reducing harmonic current distortion and flicker.

  Below, the detail of the heater control sequence which controls the energization amount to the heater in a present Example is demonstrated. A storage unit (not shown) in the control unit 113 stores a plurality of types of heater power control tables (corresponding to a supply power control table in this embodiment). Specifically, three types of heater power control tables are stored: table 1: phase control table, table 2: wave number control table, and table 3: combination control table combining phase control and wave number control. Each table also stores data relating to heater on / off timing. In the present embodiment, when the image portion that is likely to cause uneven glossiness passes through the fixing nip portion N when passing through the fixing nip portion N of the recording medium 101, the table 1 is applied to the image portion that is less likely to cause uneven glossiness. Uses Table 3.

  By using only the phase control of the table 1 only for the image portion where gloss unevenness is likely to occur, the gloss unevenness can be more efficiently suppressed while suppressing the harmonic current distortion. On the other hand, both the harmonic current distortion and the flicker can be suppressed to a necessary and sufficient level by using the control table 3 that combines the phase control and the wave number control for the image portion where it is difficult to generate gloss unevenness. For the same reason, the table 3 is used at the time of starting up the fixing heating device, at the time of forward rotation, and at the time of rear rotation. Further, when printing a plurality of sheets, the table 2 is used for a conveyance interval (hereinafter referred to as a sheet interval) of the recording medium 101 in order to further reduce the harmonic current distortion.

  The wave number control by Table 2 is a control in which flicker is likely to occur as described above. However, since the heater power between the sheets may be smaller than when the recording medium 101 passes through the fixing nip N, even if the table 2 is used between the sheets, the flicker will not be extremely deteriorated. In addition, even if a temperature ripple due to wave number control occurs between papers, there is no effect on the image. As described above, in the present invention, it is possible to sufficiently suppress uneven gloss, harmonic current distortion, and flicker by switching the heater control according to the situation.

Next, an example of a method for discriminating an image that easily causes gloss unevenness in the present embodiment will be described. In the image forming apparatus in this embodiment, a 3 × 3 dot set is used as a minimum image configuration unit (one pixel) for image formation, and the image density for each pixel is controlled by a known method as a whole. An image is formed.

  Here, for example, consider an image formed in the unit area A on the recording medium 101. The length of the recording medium in the unit area A in the conveyance direction is AL, and the width in the direction orthogonal to the recording medium is AW. In this embodiment, it is determined whether the image formed in the unit area A is an image in which gloss unevenness is likely to occur, and the heater power control table is switched. Therefore, it is necessary to set the conveyance direction length AL and the width AW of the unit area A to values suitable for the determination. For example, when the conveyance speed of the recording medium is 200 mm / sec and the frequency of the AC power supply 118 is 50 Hz, in this embodiment, the conveyance direction length AL is set to 8 mm (corresponding to 63 pixels at a resolution of 600 dpi).

  This is a length corresponding to two waves of the AC power supply frequency, and corresponds to one period of gloss unevenness that occurs when one wave is turned on and one wave is turned off by wave number control. On the other hand, the width AW of the unit area A may be determined based on the conspicuousness when gloss unevenness simply occurs, and is set to 8 mm which is the same as the length AL in this embodiment. In this embodiment, for all the unit areas A that can be selected in the image forming area (the minimum constituent unit of A is one pixel), it is determined whether or not the image formed there is an image that easily causes gloss unevenness. Determine.

FIG. 6 shows a schematic diagram of the unit area A included in the image forming region. FIG. 6 is an enlarged view of the left front end portion of the recording medium 101. The front end of the image forming area, the left end of the image forming area, one pixel that is the minimum unit (minimum image constituent unit) of image formation, and a plurality of unit areas _AXY are shown. It is shown. Unit area A
11 (thick solid line square) is a unit area A formed at the left end of the front end of the image forming area, and A ₂₁ , A ₃₁ , A ₄₁ ,... And the unit area A can be selected.

Similarly, A ₁₂ , A ₁₃ , A ₁₄ ..., And unit area A can be selected at a position moved pixel by pixel in a direction orthogonal to this. The total number of pixels included in the unit area A of the respective and PT _A, these actual image is formed (actually there are toner concentration (or the toner concentration, the degree of toner density) is not zero) image pixels the number of the PI _a (in the above settings _{PT a = 63 × 63 = 3969} ). Among the image pixel number PI _A, the toner amount is specified toner amount D (0% <D ≦ 100 %) or more (specified corresponds to at concentrations above.) Is a high-density pixels and PI _A ( D). The prescribed toner amount D means the image density of the corresponding pixel in the image information obtained by processing the image transmitted from the external device by the image processing controller 120, and is set to D = 75% in this example.

In this embodiment, referring to the ratio (percentage) RI of high density pixel number PI A _(D) for the total number of pixels PT _A shown by the following formula (1) as the printing rate (or print ratio).
RI = PI _A (D) / PT _A (1)
If the printing ratio RI is equal to or greater than the threshold value RI _LIM (greater than or equal to the predetermined threshold value) in a certain unit area A, it is determined that the unit area A is an image in which gloss unevenness is likely to occur. In this example, the value of the threshold value RI _LIM is set to 0.7. This value may be selected in accordance with the characteristics of the image forming apparatus so that gloss unevenness starts to be noticeable.

The table 1 (phase control) is used only when the unit area A whose printing rate RI is equal to or greater than the threshold value RI _LIM passes through the fixing nip N. In the present embodiment, the above-described image determination method is used in order to more surely prevent uneven glossiness. However, in order to simplify the control of image determination, the unit area A may be increased. Further, it may simply be an average image density of the total pixel number PT _A (average image density in the unit area A) a printing ratio RI. Furthermore, the printing rate RI, the method may be used, such as simply the ratio of the number of image pixels PI _A to the total pixel number PT _A (image existing ratio).

  The image processing controller 120 calculates the printing rate RI of the expression (1) for all the unit areas A described with reference to FIG. 6 from the image information transmitted from the external device, and notifies the control unit 113 of it. Then, the control unit 113 selects and switches the heater power control table according to the printing rate RI. Therefore, the image processing controller 120 corresponds to an image printing rate deriving unit in this embodiment.

  FIG. 7 schematically shows an image formed on the recording medium 101 as an example of how the heater power control table is switched in this embodiment. Here, in order to simplify the description, a simple image is taken as an example.

This image is composed of four parts IG-1 to IG-4, where IG-1 and IG-4 are characters (sentences) and have a unit area A within the region of IG-1 and IG-4. None of them has a printing ratio RI equal to or greater than a threshold value RI _LIM . On the other hand, IG-2 and IG-3 are, for example, photographic images, and it is assumed that the printing rate RI is equal to or greater than the threshold value RI _{LIM in} all the unit areas A in the region of IG-2 and IG-3. When fixing such an image on the recording medium 101, the IG-1 to IG-1 described above are used.
As a result of referring to the printing rate RI of the unit area A included in IG-4, in this example, the recording medium 101 is divided into sections 1 to 5 shown in FIG. Note that IG-1 to IG-4 correspond to specific image regions in this embodiment.

The division of the sections 1 to 5 is determined by whether or not the section area A includes a unit area A having a printing rate RI equal to or greater than the threshold value RI _LIM . Then, the heater power control table is switched as follows. Since there is no image in which gloss unevenness easily occurs in the section 1 which is the leading end portion of the recording medium 101, the table 3 is used when this portion passes through the fixing nip portion N. In the next section 2, there is IG-2, which is an image in which uneven glossiness is likely to occur, so the heater power control table is switched from table 3 to table 1 at the timing when section 2 reaches the fixing nip N.

  Since there is no glossy uneven image in the next section 3, the heater power control table is returned from the table 1 to the table 3 at the timing when the section 3 reaches the fixing nip N. Similarly, table 1 is used for section 4 and table 3 is used for section 5. As described above, in this embodiment, while the recording medium 101 passes through the fixing nip portion N, the table for heater power control is switched according to the image.

  Next, the above operation will be described using a flowchart. FIG. 8 shows a heater power control table switching flow when a print job is executed in this embodiment. In the heater power control table switching flow of FIG. 1, it is determined every time one control cycle of the heater power control whether any one of the tables 1 to 3 is selected.

  When the print job is started in FIG. 8, first, in S100, the table 3 is selected as a heater power control table used when the control unit 113 starts up the fixing heating device and performs the pre-rotation. This is a table of energization control patterns in which phase control and wave number control are combined, and is advantageous for both harmonic current distortion suppression and flicker suppression although it is disadvantageous for temperature ripple. by. Specifically, the energization control pattern is a combination of phase control and wave number control with four waves (eight half waves) as one control cycle, and the temperature control of the fixing heating device 109 is controlled by the temperature of the heater 110 detected by the thermistor 203. Done. And the control part 113 selects the electricity supply control pattern according to the electric power level required for every control period (four waves) from the table 3. FIG.

  Next, in S101, it is determined whether or not the first recording medium 101 reaches the fixing nip portion N within the next one control cycle (four waves). If an affirmative determination is made that the first recording medium 101 reaches the fixing nip N within the next one control cycle (four waves), the process proceeds to S103, which is a step of determining the image printing rate RI. move on. On the other hand, if it is determined that the first recording medium 101 does not reach the fixing nip N within one control cycle (four waves), the process proceeds to S102. In S102, it is determined whether or not the fixing heating device has been started up and is in the pre-rotation. In S102, when it is affirmed that the fixing heating device is started up and is in the pre-rotation, the process returns to the process before S100. The processes from S100 to S102 are repeated until the first recording medium reaches the fixing nip N.

In S103, it is determined whether or not an image having a printing rate RI equal to or greater than the threshold value RI _LIM passes through the fixing nip N during the next one control cycle. When an affirmative determination is made here, the process proceeds to S104, and the heater power control table to be used is switched to Table 1 which is a heater power control table in which gloss unevenness is unlikely to occur. Therefore, an energization control pattern corresponding to the power level is selected from the table 1 (phase control) capable of suppressing gloss unevenness for an image in which gloss unevenness is likely to occur. Thereafter, the process returns from S104 to the front of S101, and an interval in which an image having a printing rate RI equal to or greater than the threshold RI _LIM exists until the recording medium 101 currently passing through the fixing nip N passes through the fixing nip N is shown in Table 1 ( Control in which the energization control pattern is selected from (phase control) is continued.

On the other hand, if it is determined in S103 that there is no image with the printing rate RI equal to or greater than the threshold value RI _LIM in the fixing nip portion N in the next one control cycle, the process proceeds to S105. In S105, the table for heater power control is returned from Table 1 to Table 3. And it returns to before S101 and the flow from S101 is continued.

  Thereafter, when it is time for the recording medium 101 to exit the fixing nip N during the next control cycle, the process proceeds from S101 to S106 via S102. In S106, it is determined whether or not printing is completed (whether the recording medium 101 currently passing through the fixing nip portion N is the last page of the job).

  If an affirmative determination is made in S106 that printing has ended, the flow proceeds to S108, the heater power control table is switched to the table 3 used during post-rotation, and the flow ends. On the other hand, if a negative determination is made in step S106 that printing has not ended, it is determined that the sheet is in the fixing nip portion N in the next one control cycle. Therefore, in this case, the process proceeds to S107, and the heater power control table is switched to the table 2 for the sheet interval. Thereafter, the process returns to S101, and during the interval between sheets, an energization control pattern corresponding to the power level is selected from Table 2 (wave number control) until the next recording medium 101 reaches the fixing nip N. In this embodiment, the table 2 (wave number control) is used for the inter-sheet space, but the table 3 may be used instead of the table 2 when it is desired to further suppress flicker.

Next, a part of the above flow will be described in time series with reference to FIG. FIG. 9 shows changes in the heater application voltage when the table for heater power control is switched from table 3 to table 1 in accordance with the image printing rate. The image printing rate RI up to time t2 is lower than the threshold value RI _LIM , and RI <RI _LIM is established, and the image printing rate RI is equal to or greater than the threshold value RI _LIM and RI ≧ RI _LIM is established for the image from time t2. In response to such a change in the image printing rate, in the flow of this embodiment, table switching for heater power control is executed at time t1.

  The time t1 is a timing earlier by td than the time t2 when the high printing rate image portion starts, and at the time t2, the heater power control table has already been switched to the table 1 (phase control) advantageous for suppressing uneven glossiness. Since the fixing heating device 109 used in this embodiment has a small heat capacity and good thermal responsiveness, there is no gloss unevenness from the beginning of the high printing rate image portion if the table is switched to the table 1 when the high printing rate image portion starts. A good image is obtained.

  As described above, in this embodiment, as the speed of the apparatus is increased, even when temperature ripple reduction (gloss unevenness suppression) of a high-heat-responsive fixing / heating apparatus is required, the phase is applied only to an image portion having a high printing rate. Use control. Further, wave number control or combination control of phase control and fractional control is used for portions other than an image with a high printing rate. As a result, the generation of uneven gloss and flicker can be suppressed, and at the same time, the harmonic current distortion can be kept to a minimum.

  In the present embodiment, the case where there are three types of heater power control tables stored in the recording unit has been described, but this is an example, and a plurality of tables of two or more types are recorded in the recording unit. Application is possible if it is stored in For example, in the case where the AC power supply voltage is high (for example, 220V to 240V), there is often a relatively large margin with respect to the flicker standard. Therefore, in such a case, only the two tables of Table 1 and Table 2 may be used without using the control combining the phase control and the wave number control of Table 3. Furthermore, when the AC power supply voltage is a low voltage (for example, 100 V to 127 V), there is often a relatively large margin with respect to the harmonic current distortion standard. Therefore, in such a case, only the two tables of Table 1 and Table 3 may be used without using the wave number control of Table 2.

<Example 2>
Next, a second embodiment of the present invention will be described. Since the configuration of the image forming apparatus, the heater configuration, the phase control, the wave number control, the control combining the phase control and the wave number, the power control table, and the like in the second embodiment are the same as those described in the first embodiment, the description thereof is omitted. To do. In Example 1, the power control table was switched according to the image printing rate RI, and the harmonic current distortion was suppressed while preventing uneven gloss and flicker. However, when only an image having an image printing rate higher than the threshold value RI _LIM continues, if the power control table switching flow of the first embodiment is continued, the harmonic current distortion is less than the required level although it is very rare. There was a possibility that it could not be suppressed. In view of such circumstances, the second embodiment is an example in which it is possible to prevent gloss unevenness while more surely suppressing harmonic current distortion and flicker.

  The outline of the switching control of the heater power control table in the second embodiment is as follows. The switching control of the heater power control table in accordance with the image printing rate is basically the same as in the first embodiment, but in the second embodiment, there is a possibility that the harmonic current distortion cannot be suppressed to a specified level or less. The process for avoiding this is performed. For this process, in the second embodiment, the power source wave number count NW, the table selection wave number count NT, and the phase control execution index PCC (NT) described below are used to switch the heater power control table and change the recording medium conveyance speed. I do.

  The power wave number count NW is a count value of the wave number of the AC power source to which the image forming apparatus is connected, and is counted up at a period and timing equal to the frequency of the AC power source 118. Therefore, if the AC power supply is 50 Hz, the value after 1 second from the start of counting is 50, and the value after 2 seconds is 100. The power supply wave number count NW starts when the image forming apparatus is turned on, and continues counting until the power is turned off.

  Next, the table selection wave number count NT corresponds to the control flow cycle of the heater power control table in the heater power control table switching control of this embodiment. As the table selection wave number count NT is incremented, the heater power control table to be used is continuously selected from Tables 1 to 3. The count cycle of the table selection wave number count NT is also equal to the frequency of the AC power supply 118, but the value is set to a value slightly ahead based on the value of the power supply wave number count NW.

  In this embodiment, for example, if table 3 is selected when table selection wave number count NT = n, then heater power control using table 3 is performed according to this selection when power supply wave number count NW = n. Shall be executed. Specific processing is as follows. Together with the selection of the table 3 in the table selection wave number count NT = n, the heater supply power corresponding to the detected temperature of the thermistor 203 is determined by the temperature control program of the fixing heating device 109 described above.

  Then, when the power supply wave number count NW = n, the heater power control is executed using the waveform pattern in the table 3 corresponding to the heater supply power. Here, the time lag from when the temperature control program determines the heater power supply to the time when it is actually supplied to the heater 110 is Δt, and the wave number corresponding to Δt at the frequency of the AC power supply 118 is Δn. At this time, the table selection wave number count NT in this embodiment is set such that NT = NW + Δn using the power wave number count NW and the wave number Δn.

  The phase control execution index PCC (NT) is an index that becomes a value of 0 ≦ PCC (NT) ≦ 1 according to the heater supply power in one wave when phase control is selected in the table selection wave number count NT. When other control is selected, it is fixed at 0. The phase control execution index PCC (NT) is set in units of one control cycle in each heater power control method (in units of one wave in phase control, in units of four waves in control combined with wave number control and phase control and wave number). When no power is supplied to the heater, it is set in units of one wave.

  FIG. 10 shows the relationship of the harmonic current distortion relative value (HLV (PWR)) to the heater supply power (PWR). As described above, the harmonic current distortion increases as the amount of rising current increases, and thus becomes maximum when the phase angle is 90 °, that is, when the power supplied to the heater 101 is 50%. In FIG. 10, the harmonic current distortion at this time is set as 1, and the harmonic current distortion when the supplied power is changed is shown as a relative value.

  In this embodiment, the value of the phase control execution index PCC (NT) is determined based on the relationship shown in FIG. That is, the relative value of harmonic current distortion (HLV (PWR)) with respect to a certain heater supply power (PWR) is set as the value of the phase control execution index PCC (NT) in the table selection wave number count NT using the heater supply power. In the present embodiment, a recording table (not shown) in the control unit 113 is provided with a correspondence table of harmonic current distortion relative values (HLV (PWR)) with respect to heater power supply (PWR). Then, referring to this, the value of the phase control execution index PCC (NT) in the table selection wave number count NT is determined.

  For example, the phase control execution index PCC (NT) = 1.00 when the heater supply power (PWR) = 50% in the table selection wave number count NT, and the phase control execution index PCC when the heater supply power (PWR) = 30% and 70%. (NT) = 0.96. Further, when the heater supply power (PWR) = 10% and 90%, the phase control execution index PCC (NT) = 0.71. By continuously monitoring the phase control execution index PCC (NT), it is possible to predict the harmonic current distortion level during printing.

In this embodiment, the average value of the phase control execution index PCC (NT) is obtained in the prescribed wave number counting section (wave number count number = X), and the harmonic current distortion level is predicted from this value, and recorded accordingly. The conveyance speed of the medium 101 is changed. In this example, the value of X is set to a value corresponding to, for example, 5 minutes (15000 for 50 Hz, 18000 for 60 Hz). The average value of the phase control execution index PCC (NT) at the table selection wave number count NT is defined as a harmonic distortion index RPCC (NT) expressed by the equation (2).

In this embodiment, when the harmonic distortion index RPCC (NT) becomes a value equal to or higher than a threshold value (LD) that is a specified usage level (more than a specified usage level), the conveyance speed of the recording medium 101 is set to 1 / of the normal speed. Reduce to 3. In contrast to the first embodiment, the conveyance speed that is 1/3 of the normal speed uses a combination of phase control and wave number control for an image whose image printing rate is higher than the threshold value RI _LIM . This is because, even when control combining phase control and wave number control is performed on an image with a high image printing rate, by reducing the conveyance speed, the pitch interval of the uneven gloss can be made inconspicuous. By doing in this way, it becomes possible to reduce the average level of the harmonic current distortion in the specified section while suppressing uneven glossiness.

  Here, the threshold value LD is a margin more than the image length of one recording medium 101 with respect to the limit value of the harmonic distortion index that can reduce the harmonic current distortion in the specified section to a required level or less. It is good to set it to a value with. Specifically, after the harmonic distortion index RPCC (NT) reaches the threshold value LD, even when the phase control is performed by the number of waves corresponding to the image length in the conveyance direction for one recording medium, the harmonics in the specified section It is preferable to set the threshold value LD to such a value that the current distortion is less than the required level. This is because, during printing, when the harmonic distortion index RPCC (NT) becomes a value equal to or greater than the threshold value LD while the recording medium 101 passes through the fixing nip N, the image is affected by changing the conveyance speed at that time. This is to avoid the occurrence of a difference in gloss before and after the speed change.

  Even if the conveyance speed is changed after the recording medium 101 passes through the fixing nip N, the threshold LD is set as described above so that the harmonic current distortion can be suppressed below the required level. Realistic. Based on the above, the threshold value LD in this embodiment is set to 0.55.

In the present embodiment, in the state where the conveyance speed is lowered as described above, since the control combining the phase control and the wave number control is used even for the high printing rate image, the harmonic distortion index RPCC (NT) is the time. Decreases over time. If it is determined that the harmonic distortion index RPCC (NT) has sufficiently decreased, the conveyance speed of the recording medium 101 is returned to the speed at the start of printing, and the image print rate is higher than the threshold value RI _LIM. Is allowed to execute phase control (Table 1).

  Here, the threshold for judging that the harmonic distortion index RPCC (NT) has sufficiently decreased is LU. The threshold value LU is preferably set to a value that is sufficiently smaller than the threshold value LD that is used for the determination when the conveyance speed is lowered. In this example, this value is set to 0.4. This is to prevent further reduction in productivity when printing for a long time by repeating the shift of the conveyance speed in a short cycle. If the transfer speed change described above is repeatedly executed, a reduction in productivity can be minimized.

  Next, an example of the switching control of the heater power control table in the present embodiment will be described in detail using a flowchart. FIG. 11 shows a heater power control table switching flow of this embodiment when a print job is executed. FIG. 11 shows a control flow for sequentially setting the heater power control table at the table selection wave number count NT as the table selection wave number count NT is counted up.

  As described above, the heater power control table set in this control flow is actually used when the power supply wave number count NW becomes equal to the table selection wave number count NT when the heater power control table is set. It is. In the second embodiment, the heater power control table set here is used for temperature control of the fixing heating device 109 at the timing described above.

  When the print job is started, it is first determined in step S700 whether or not the image forming apparatus has just been turned on. Here, if it is determined that the power is not immediately after turning on, the process proceeds directly from S700 to S702. On the other hand, if an affirmative determination is made immediately after the power is turned on, the control parameters are initialized in S701 (initialization of the table selection wave number count NT, SCR = 0, speed flag = 0 (the SCR and speed flag will be described later). ) Is performed. Here, initialization of the table selection wave number count NT will be described. Although not described in the flow of FIG. 11, when the power is turned on, the power supply wave number count NW is started before the power supply to the heater 110 is started. Then, regardless of the flow of FIG. 11, the count-up is continued until the power is turned off at a cycle equal to the frequency of the AC power supply 118.

  The initialization of the table selection wave number count NT in S701 is a process of referring to the power supply wave number count NW at the time of S701 and setting the table selection wave number count NT to a value preceding by the aforementioned Δn (NT = NW + Δn). . The speed flag is a flag indicating the conveyance speed of the recording medium 101. The value when printing is performed at a normal speed is 0, and the value when the speed is decreased as described above is 1. It becomes. The SCR is a flag indicating whether or not there is a request for changing the recording medium conveyance speed. When a condition for decreasing or returning the recording medium conveyance speed is satisfied, SCR = 1, and when the speed change is performed, SCR = Return to zero.

Thereafter, in S702, the control unit 113 selects the table 3 that is a combination of phase control and wave number control as a heater power control table used when the fixing heating device 109 is started and pre-rotated.

  FIG. 12 shows a flowchart of the “table 3 selection process” subroutine. In this process, the value of the phase control execution index PCC (NT) for one control period four waves in Table 3 is set. When the process of “table 3 selection process” starts, table 3 is first selected in S800c. Also in this embodiment, as shown in FIG. 9, among the four control cycles of Table 3, the first two waves are phase controlled, and the second two waves are wave number controlled. The PCC (NT) value for four waves is set for each wave as NT is incremented.

  The value indicating the number of times of setting the PCC (NT) value is m, and m is decremented every time the PCC (n) value is set from the initial value m = 3 to the final value m = 0. In FIG. 12, the above-described harmonic current distortion relative value (HLV (PWR)) corresponding to the heater supply power for each wave is adopted as the value of the phase control execution index PCC (NT) for the first two waves. . (S801c-> S802c-> S804c-> S805c-> S806c-> S807c-> S802c) For the latter two waves, the value of PCC (NT) is fixed at 0 (S802c-> S803c-> S805c-> S806c-> S807c-> S802c) . When the above is completed, the process returns from “table 3 selection processing” to S703 in FIG.

  In the speed change determination process of S703, the above-described harmonic distortion index RPCC (NT) is calculated, and it is determined whether or not the conveyance speed needs to be changed. FIG. 13 shows a flowchart of the “speed change determination process”. The “speed change determination process” is executed when it is determined whether the conveyance speed of the recording medium 101 needs to be changed, and the value of the conveyance speed change request flag SCR is set according to whether or not the speed change is necessary. Perform the process until setting. When the “speed change determination process” is started, it is determined in S1000 whether or not the table selection wave number count NT has reached a prescribed wave number count number X. If the table selection wave number count NT has not reached the prescribed wave number count number X and a negative determination is made, the process proceeds to S1006. On the other hand, if the table selection wave number count NT is equal to or greater than the prescribed wave number count number X and an affirmative determination is made, the process proceeds to S1001.

In S1001, the harmonic distortion index RPCC (NT) shown by the above equation (2) is calculated. After the harmonic distortion index RPCC (NT) is calculated in S1001, the value of the speed flag (current recording medium conveyance speed) is confirmed in S1002. If the speed flag value at that time is 0 (normal speed), the process proceeds to S1003, where the harmonic distortion index RPCC (NT) is equal to or higher than the threshold LD that reduces the conveyance speed of the recording medium 101 to 1/3 of the normal speed. A determination is made whether there is any. On the other hand, if the current speed flag value is 1 (the conveyance speed of the recording medium 101 is 1/3 of the normal speed), the process proceeds to S1004, where the harmonic distortion finger RPCC (NT) indicates the conveyance speed of the recording medium 101. A determination is made as to whether or not the threshold LU is below the normal speed.

  In S1003 or S1004, if the relationship between the harmonic distortion index RPCC (NT) set in each and the threshold value is satisfied and an affirmative determination is made, the process proceeds to S1005 and the value of the conveyance speed change request flag SCR is set to 1. If a negative determination is made, the process proceeds to S1006. In step S1006, NT is incremented at the timing already described, and the "speed change determination process" is exited and the process returns to step S704 in FIG.

  In S704, it is determined whether or not the value of the conveyance speed change request flag SCR is set to 1. If SCR = 0 and a negative determination is made, the process proceeds to S708. If SCR = 1 and a positive determination is made, the process proceeds to S705.

In S705, an execution reservation for the return of the conveyance speed (change from 1/3 of the normal speed to the normal speed) is made. Specifically, in order to specify the power supply wave number count NW for executing the speed recovery, a process of storing the value of the table selection wave number count NT in S704 in the storage unit as nu is performed. Then, when the value of the power supply wave number count NW reaches the above nu, the image forming apparatus executes a speed change from the 1/3 speed to the normal speed.

If an affirmative determination is made in step S704 that SCR = 1, the above processing is performed assuming that the current transport speed is automatically 1/3 of the normal speed. This is because the harmonic distortion index RPCC (NT) does not exceed the threshold LD that reduces the conveyance speed of the recording medium 101 to 1/3 of the normal speed at the time of starting up and rotating the fixing heating device 109. It is processing based on.

  This is due to the following reason. Since the table 3 is used during startup and pre-rotation of the fixing heating device, the average value of the phase control execution index PCC (NT) in one control cycle is only 0.50 at the maximum. Therefore, the harmonic distortion index RPCC (NT) during the start-up of the fixing heating device and during the pre-rotation does not exceed this value 0.50, and does not exceed the threshold value LD = 0.55 in this embodiment. Therefore, during the start-up and pre-rotation of the fixing heating device 109 in this embodiment, the speed cannot be changed from the normal speed to 1/3 of the normal speed. However, this is the value of the threshold LD in the present embodiment described so far, the startup of the fixing heating device, the type of the heater power control table used during the pre-rotation, the phase control execution index PCC (NT), the harmonics This is established when the method of calculating the distortion index RPCC (NT) is used. When these conditions are set differently, a control flow corresponding to the setting may be used, and the present invention is not limited to the control flow of FIG.

  As described above, in this embodiment, in order to simplify the control, when SCR = 1 in S704 and an affirmative determination is made, confirmation of the current speed is omitted, and an execution reservation for the return of the conveyance speed is made in S705. Done. When the return reservation for the transport speed is made in S705, the value of the speed flag is changed to 0 indicating the normal speed in S706, and the value of the transport speed change request flag SCR is returned to 0 in S707, and the process proceeds to S708. move on.

  In S708, whether the time during which the recording medium passes through the fixing nip portion N is included in the next one control cycle starting from the current table selection wave number count NT (here, the first recording medium is the fixing nip). Whether the part N is reached) is determined. If an affirmative determination is made including the time during which the recording medium passes, the process proceeds to S710. On the other hand, if there is no recording medium in the fixing nip portion N (here, the first recording medium has not yet reached the fixing nip portion N) during the next one control cycle, the process proceeds to S709. In S709, it is determined whether or not the fixing heating device has been started and is in the pre-rotation state. If an affirmative determination is made here, the processing flow from S702 is the timing at which the first recording medium reaches the fixing nip N. Repeat until.

If it is determined in S708 that the first recording medium reaches the fixing nip N during the next one control cycle, the process proceeds to S710 and the value of the speed flag is confirmed. Here, if the value of the speed flag is 0 (normal speed), the process proceeds to S711, and it is further determined whether or not an image with an image printing rate higher than the threshold value RI _LIM arrives during the one control cycle. On the other hand, if the value of the speed flag is 1 (speed of 1/3 of the normal speed) in S710, the process proceeds to S713 without checking the image printing rate, and the above-described “table 3 selection process” is executed.

If it is determined in S711 that an image having an image printing rate higher than the threshold value RI _LIM is present in the next one control cycle, the process proceeds to S712. In S712, processing for selecting Table 1 which is phase control is performed as a heater power control table used when fixing a high printing rate image at a normal conveyance speed. If it is determined in S711 that there is no image having an image printing rate higher than the threshold value RI _LIM during the next one control cycle, the process proceeds to S713, and the above-described “table 3 selection process” is executed. .

  Next, FIG. 14 shows a flowchart of “table 1 selection processing”. When the “table 1 selection process” starts, table 1 is selected in S800a, and the value of the phase control execution index PCC (NT) is set in S801a. When the process of S801a is completed, the process returns to S714 of FIG. In S800a, table 1 is selected, and an energization control pattern corresponding to the required heater supply power is selected from this table. In S801a, the above-described harmonic current distortion relative value (HLV (PWR)) corresponding to the heater supply power is adopted as the value of the phase control execution index PCC (NT).

  After the processing of S712 or S713 is performed, the above-described “speed change determination processing” is executed in S714. In S715, it is determined whether or not the value of the conveyance speed change request flag SCR is 1. If the conveyance speed change request flag SCR = 0 and a negative determination is made, the process returns to S708. On the other hand, if the condition for changing the conveyance speed is satisfied in S715 and an affirmative determination is made that SCR = 1, it is first determined in S716 whether or not the flag for the current conveyance speed is 1. If the current transport speed is the normal speed (speed flag = 0) in S716 and a negative determination is made, processing for changing the transport speed to 1/3 between sheets is executed in S717.

  FIG. 15 shows a flowchart of the “speed down process” in S717 of FIG. In the “speed-down process”, after the recording medium currently in the fixing nip N has passed through the fixing nip N, a process for actually reducing the conveyance speed of the recording medium is performed. First, in S900a, it is determined whether the recording medium has passed through the fixing nip N during the next control cycle. As a result, if it is determined in the next one control cycle that the recording medium passes through the nip portion N and is in the gap between sheets, an execution reservation for reducing the conveyance speed to 1/3 in S905a. Is done. Specifically, in order to specify the power supply wave number count NW for executing the speed reduction, the value of the table selection wave number count NT in S905a is stored in the storage unit as nd. Then, when the value of the power wave count NW becomes nd, the image forming apparatus 100 according to the second embodiment executes a speed change that reduces the conveyance speed from the normal speed to the 1/3 speed thereof.

On the other hand, if it is determined in S900a that the recording medium still exists in the fixing nip N during the next one control cycle, the process proceeds to S901a, where the image printing ratio is confirmed. As before, when the image printing rate is equal to or greater than the threshold value RI _LIM and the determination is affirmative, Table 1 (S902a) is selected, and when the determination is negative, Table 3 (S903a) is selected. In S904a, NT is incremented at the timing already described, and the process returns to S900a. In step S905a, the above flow is repeated until an execution reservation for reducing the conveyance speed to 1/3 is made.

  When execution of speed reduction is reserved in S905a, speed flag = 1 is set in S906a, and SCR = 0 is set in S907a. After that, the "speed reduction processing" is exited and the process returns to S719 in FIG. On the other hand, if it is determined in S716 that the current conveyance speed is 1/3 of the normal speed (speed flag = 1), processing for returning the conveyance speed to the normal speed between sheets is performed in S718. Run.

Next, FIG. 16 shows a flowchart of “speed return processing” in S718. In the “speed return process”, after the recording medium currently in the fixing nip N has passed through the fixing nip N, a process of returning the recording medium conveyance speed to the normal speed is performed. First, in S900b, it is determined whether the recording medium has passed through the fixing nip N during the next control period. Here, when the recording medium passes through the fixing nip portion N and is in the gap between sheets, a negative determination is made in S903b to execute the return of the conveying speed to the normal speed. Specifically, in order to specify the power supply wave number count NW for executing the speed return, the value of the table selection wave number count NT in S903b is stored as nu in the storage unit. Then, when the value of the power supply wave number count NW reaches the above nu, the image forming apparatus 100 according to the second embodiment executes speed recovery for returning the conveyance speed from 1/3 of the normal speed to the normal speed.

  If it is determined in S900b that the recording medium is still in the fixing nip N during the next one control cycle, the process proceeds to S901b, and a process of selecting the table 3 is executed regardless of the image printing rate. Then, after NT is incremented at the timing already described in S902b, the process returns to S900b. Then, in S903b, the above flow is repeated until execution reservation for the conveyance speed return is made. When the execution reservation for the conveyance speed return is made in S903b, the speed flag = 0 is set in S904b, and SCR = 0 is set in S905b, and then the "speed return process" is exited and the process returns to S719 in FIG.

  If a negative determination is made in S719 that it is not the timing for ending printing, a process of selecting Table 2 (wave number control), which is a heater power control table for the sheet interval, is selected in S720. FIG. 17 shows a flowchart of “table 2 selection processing”. In this process, the value of PCC (NT) for one control cycle of four waves in Table 2 is set. When this process starts, first, table 2 is selected in S800b. Then, after S801b, the PCC (NT) value for four waves is set to 0 for each wave.

  The value indicating the number of times the PCC (NT) value is set is set as m. In S801b, the initial value m is set to 3, and m is decremented every time the PCC (NT) value is set. Then, the PCC (NT) value is repeatedly set to 0 until the final value m = 0 (S802b → S803b → S804b → S805b → S802b). Then, after NT is incremented at the timing already described in S806b, the “table 2 selection process” is exited, and the process returns to S708 in FIG. The above-described flow is repeated until the print end timing. If an affirmative determination is made in S719 that printing has ended, the process proceeds to S721. In S721, a process of selecting the table 3 as a heater power control table used when the fixing heating device performs the post-rotation is performed, and the heater power control table switching flow ends.

  In the second embodiment, the conveyance speed is changed by changing the peripheral speed of the laser beam scanner 106, the photosensitive drum 104, the intermediate transfer member 103, the transfer roller 108, the pressure roller 112, the fixing film 201, the conveyance roller of each part, and the like to the target speed. It can be realized by a known method for changing to Also, for example, when using a laser beam scanner equipped with a polygon mirror having nine mirror surfaces, the conveyance speed of the recording medium 101 can be changed without changing the rotational speed of the polygon mirror if the mirror surface is used by skipping two surfaces. A method is also disclosed. If such a method is used, when changing the rotation speed of the polygon mirror, time is not required for stabilizing the rotation of the polygon mirror at the changed specified rotation speed. For this reason, it is possible to change the conveyance speed more quickly.

  In the present embodiment, by executing the control described above, it is possible to suppress harmonic current distortion and flicker without degrading the image quality similarly to the first embodiment. This effect can be maintained even when printing is performed continuously. In the above description, the method of continuously maintaining the harmonic current distortion below the required level by reducing the conveyance speed of the recording medium when the high printing rate image continues for a long time has been described. However, the same effect can be obtained by widening the conveyance interval (inter-paper) of the recording medium without changing the conveyance speed.

If wave number control or control with a combination of phase control and wave number control is performed while the conveyance interval (between papers) is not widened, gloss unevenness can be suppressed simply by widening the conveyance interval (between papers) of the recording medium. However, the execution ratio of the phase control can be lowered. In this case, unlike the case of changing the conveyance speed described above, it is not always necessary to change the selection method of the heater power control table before and after the conveyance interval (between sheets) is increased. Whether to decrease the recording medium conveyance speed or widen the recording medium conveyance interval (paper interval) may be selected according to the characteristics of the apparatus, and the present invention is not limited to either method.

  In the description of the second embodiment, the value corresponding to the heater supply power is used as the value of the phase control execution index PCC (NT) when executing the phase control. The value of the phase control execution index PCC (NT) may be 1 (fixed value). In this case, it is not necessary to provide a correspondence table of harmonic current distortion relative values (HLV (PWR)) with respect to the heater supply power (PWR), and the capacity of the storage unit necessary for this can be reduced. On the other hand, the harmonic distortion index RPCC (NT) is a parameter indicating the execution ratio of simple phase control in the specified section (= the usage ratio of Table 1), and the accuracy in predicting the actual harmonic current distortion level is the above. It may be lower than the method used. For this reason, it is desirable to use for the apparatus which has a comparatively sufficient harmonic current distortion with respect to the requested | required level.

  DESCRIPTION OF SYMBOLS 100 ... Image forming apparatus, 104 ... Photosensitive drum, 105 ... Primary charger, 106 ... Laser beam scanner, 107 ... Developing device, 109 ... Fixing heating apparatus, 113 ... Control unit 118... AC power source 119... Power supply device 120... Image processing controller 309.

Claims (8)

Image forming means for forming a toner image on a recording medium;
Fixing heating means for heating and fixing the toner image to the recording medium;
Switch means for switching energization and interruption of the heating source of the fixing heating means with a current from an AC power source;
Zero-cross detecting means for detecting zero-cross of the AC power supply;
A storage unit that stores a plurality of types of supply power control tables that store switching timings of energization and interruption of current from the AC power supply,
Based on the timing of zero-crossing by the zero-crossing detection unit and the switching timing referenced using the supply power control table stored in the storage unit, the switch unit is switched from the AC power source to the heating source of the fixing heating unit. A control means for controlling an energization amount to the heating source by switching between energization and interruption of the current;
Image printing rate deriving means for deriving an image printing rate indicating the degree of toner density of a toner image in a specific image region which is a specific region of the recording medium;
With
The control unit selects the supply power control table referring to the switching timing according to the image printing rate in the specific image area fixed by the fixing heating unit, which is derived by the image printing rate deriving unit. An image forming apparatus.   The image printing rate is a minimum image constituent unit in which the toner density is equal to or higher than a predetermined density among minimum image constituent units that are the minimum unit image area that controls the image density of the toner image and that constitutes the specific image area. 2. The image forming apparatus according to claim 1, wherein the image forming apparatus is a ratio occupied by or an average image density in the specific image area. The plurality of types of power supply control tables stored in the storage unit are:
A phase control table for executing phase control for energizing current from the AC power supply at a timing of a predetermined phase angle from the zero cross;
A wave number control table for executing wave number control to perform energization and interruption of current from the AC power supply in half wave units of the AC power supply in synchronization with the zero cross,
3. The image forming apparatus according to claim 1, wherein at least two types of supply power control tables are used in a combination control table for executing both the phase control and the wave number control in combination. 4. The storage unit stores three types of tables: the phase control table, the wave number control table, and the combination control table.
The control means selects the phase control table when the area including the specific image area where the image printing rate is equal to or higher than a predetermined threshold is fixed by the fixing heating means,
4. The image forming apparatus according to claim 3, wherein when the other area is fixed by the fixing heating means, the wave number control table or the combination control table is selected. The control unit is responsive to the image printing rate in the specific image area fixed by the fixing heating unit, derived from the image printing rate deriving unit, and the usage degree of a specific table of the supply power control table. And
4. The image forming apparatus according to claim 1, wherein the supply power control table is selected, and the conveyance speed or the conveyance interval of the recording medium is changed. 5. When detecting the usage of the specific table,
The image forming apparatus according to claim 5, wherein at least one of a usage ratio of the specific table and power supplied to a heating source of the fixing heating unit is referred to. The specific table is a phase control table for executing phase control in which a current from the AC power supply is energized at a timing of a predetermined phase angle from the zero cross,
When the usage level is equal to or higher than a predetermined usage level, the conveyance speed of the recording medium is reduced,
When the fixing heating unit fixes the region including the specific image region in which the image printing rate is equal to or higher than the predetermined threshold in a state where the conveyance speed of the recording medium is reduced, the control unit includes the control unit, Selecting a combination control table for executing both the wave number control and the phase control in combination with the half-wave unit of the AC power supply in synchronism with energization and interruption of the current from the AC power supply in the zero crossing; The image forming apparatus according to claim 5 or 6. The specific table is a phase control table for executing phase control in which a current from the AC power supply is energized at a timing of a predetermined phase angle from the zero cross,
When the usage level is equal to or higher than a predetermined usage level, the conveyance interval of the recording medium is extended,
In a state where the conveyance interval of the recording medium is extended, while the portion between the recording media passes through the fixing heating unit, the control unit synchronizes the energization and interruption of the current from the AC power source with the zero cross. A wave number control table for executing wave number control performed in units of half waves of the AC power supply or a combination control table for executing both phase control and wave number control in combination is selected. Item 7. The image forming apparatus according to Item 5 or 6.

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