Image forming apparatus

An image forming apparatus includes: a fixing unit configured to include at least a first and second radiation heaters capable of heating printing medium; a power supply unit configured to supply power to the first and second radiation heaters; and a controller configured to select either one of the first and second radiation heaters and control to light or turn off the selected radiation heater, wherein a heating width of the printing medium of the second radiation heater is wider than that of the first radiation heater, and the controller determines whether to preheat the second radiation heater based on the number of times of lighting of the first and second radiation heaters and controls to light and turn off the second radiation heater when the controller has determined that it is necessary to preheat the second radiation heater.

The entire disclosure of Japanese Patent Application No. 2014-257758 filed on Dec. 19, 2014 including description, claims, drawings, and abstract are incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an image forming apparatus which fixes toner on printing medium by a fixing unit including a first and second radiation heaters which have heating widths different from each other.

2. Description of the Related Art

Conventionally, there has been an image forming apparatus disclosed in JP 2008-203685 A as the above-mentioned image forming apparatus. In JP 2008-203685 A, a heating roller of a fixing unit includes a short heater and a long heater as a first and second radiation heaters. Here, a heating width of the printing medium of the long heater is wider than that of the short heater. The temperature of the fixing unit is controlled so that lighting times of both heaters or use times of them converge on the same value.

In recent years, on/off control of the radiation heater has been performed so that a temperature fluctuation range of the heating roller of the fixing unit is reduced in order to improve image quality. The on/off control to the radiation heater included in the fixing unit has been performed in a short period in many cases. It has been considered so far that an inrush current of the radiation heater has had a small influence on a life of the radiation heater. Under a use condition where on/off control in the short period is often performed, the influence cannot be ignored.

At the time of warm-up or standby, it is necessary to heat a full width of the heating roller, and the long heater having a wide heating width is used. Therefore, the long heater is used at high frequency. Accordingly, when the on/off control is simply performed to the long heater, the number of the damages to the long heater due to an inrush current increases, and the life of the long heater ends earlier.

In many cases, the fixing unit is exchanged by a unit of the fixing unit not the radiation heater. Therefore, even though the short heater has enough time before the end of its life, there is a case where a user needs to exchange the fixing unit due to the end of the life of the long heater. In this way, it is not desirable that times to end the life of both radiation heaters are largely different from each other.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an image forming apparatus which can reduce the time difference between time to end the short heater and that of the long heater which have different heating widths from each other.

To achieve the abovementioned object, according to an aspect, an image forming apparatus reflecting one aspect of the present invention comprises: a fixing unit configured to include at least a first and second radiation heaters capable of heating printing medium; a power supply unit configured to supply power to the first and second radiation heaters; and a controller configured to select either one of the first and second radiation heaters and control to light or turn off the selected radiation heater so that a temperature of the fixing unit becomes a target value, wherein a heating width of the printing medium of the second radiation heater is wider than that of the first radiation heater, and the controller determines whether to preheat the second radiation heater based on the number of times of lighting of the first and second radiation heaters before a predetermined operation mode and controls to light and turn off the second radiation heater after lighting the first radiation heater when the controller has determined that it is necessary to preheat the second heater.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the illustrated examples.

First Column: Whole Structure and Printing Operation of Image Forming Apparatus

InFIG. 1, for example, an image forming apparatus1is a copying machine, a printer, a facsimile, or a multifunction machine including these functions. The image forming apparatus1prints an image on a sheet-shaped printing medium M (for example, paper sheet). To achieve the above, the image forming apparatus1generally includes a paper feeding unit2, a pair of register rollers3, an image forming unit4, a fixing unit5, a controller6, and a power supply unit7.

The printing medium M is mounted on the paper feeding unit2. The paper feeding unit2feeds the printing medium M one by one to a conveyance path FP indicated by a broken line inFIG. 1. The pair of register rollers3is provided on the conveyance path FP and provided on the downstream side of the paper feeding unit2. After temporarily stopping the printing medium M fed from the paper feeding unit2, the pair of register rollers3feeds the printing medium M to a secondary transfer region at a predetermined timing.

The image forming unit4generates a toner image on an intermediate transfer belt, for example, by using a known electrophotographic method and tandem system. The toner image is held on the intermediate transfer belt and is conveyed to the secondary transfer region.

The printing medium M is fed from the pair of register rollers3to the secondary transfer region, and also, the toner image is conveyed from the image forming unit4to the secondary transfer region. In the secondary transfer region, the toner image is transferred from the intermediate transfer belt on the printing medium M.

The printing medium M fed from the secondary transfer region is introduced into the fixing unit5. The fixing unit5fixes the toner on the printing medium M by heating and pressurizing the introduced printing medium M. The printing medium M fed from the fixing unit5is discharged on a tray of the image forming apparatus1as a printed matter.

In the controller6, a CPU executes a program stored in a ROM while using a RAM as a working area. The controller6performs various controls. However, energization control to the fixing unit5is important in the present embodiment. Specifically, the controller6performs the control so that the detection result by a temperature detection unit55(refer toFIG. 2) becomes the target temperature.

Second column: Detailed structure of main part of image forming apparatusNext, the main part of the present embodiment, that is, the fixing unit5, the controller6, and the power supply unit7will be described. As illustrated inFIG. 2, the fixing unit5includes a heating roller51and a pressure roller52which abut on each other and form a nip. The heating roller51and the pressure roller52extend in the front-back direction.

For example, the heating roller51includes a cylindrical core bar extending in the front-back direction of the image forming apparatus1. For example, the thickness of the core bar is thinned to about one mm, and the outer diameter of the core bar is reduced to about 25 mm. Accordingly, a heat capacity of the heating roller51is reduced.

Further, in the heating roller51, a first radiation heater53and a second radiation heater54are included in the core bar. Both heaters53and54are optical heating system heaters such as a halogen heater. When an output voltage from the power supply unit7is applied to each of the heaters53and54, a current flows into a filament, and the filament is heated and lighted. For example, the filament is formed of tungsten. Here, as is well known, the resistivity of tungsten increases as the temperature gets higher. In other words, both heaters53and54have resistance temperature characteristic such that a resistance value increases as the temperature gets higher.

Further, the first radiation heater53is shorter than the second radiation heater54in the front-back direction. Specifically, the first radiation heater53has a light emission length (for example, about 210 mm) which is relatively short in the front-back direction. Whereas, the second radiation heater54has a light emission length (for example, about 310 mm) which is longer than the first radiation heater53in the front-back direction. By providing both the heaters53and54in the heating roller51, two kinds of heating widths which is relatively different from each other in the front-back direction are realized.

Further, the second radiation heater54has a power consumption (for example, equal to or larger than 1000 W) larger than that of the first radiation heater53, for example, in order to shorten the warm-up time. Conversely, the first radiation heater53has a power consumption (for example, equal to or smaller than 800 W) smaller than that of the second radiation heater54. Drive units81and82respectively perform control and switch on/off of the heaters53and54. However, since the second radiation heater54has large power consumption, the second radiation heater54receives larger influence of the inrush current than the first radiation heater53.

The pressure roller52and the heating roller51rotate based on a control signal from the controller6. When the printing medium M is fed to the nip, the printing medium M is pressurized by both the rollers51and52and also heated by the heating roller51. As a result, the toner is fixed on the printing medium M.

The temperature detection unit55is included near the heating roller51. For example, the temperature detection unit55is a thermistor. The temperature detection unit55outputs a signal correlating with the temperature of the heating roller51(as a matter of convenience, simply refer to as temperature below) to the controller6.

The power supply unit7rectifies all alternating currents supplied from a commercial power supply and performs DC conversion to it and generates a plurality of DC voltages based on the converted current. Then, the power supply unit7supplies them to the controller6and a drive unit which is not shown. On the other hand, power supply lines L (live) and N (neutral) of the power supply unit7are used to light the first radiation heater53and the second radiation heater54. The power supply line L is connected to an end of each of the heaters53and54via an excessive rising preventing unit83, for example, configured of a thermostat. The excessive rising preventing unit83has a function to cut off power supply from the power supply unit7when the fixing unit5is abnormally overheated.

The first drive unit81and the second drive unit82are respectively provided on another sides L1and L2of the respective heaters53and54, and the lines L1and L2are connected to the power supply line N. Both the drive units81and82include switching units such as a solid state relay (SSR), and the drive units are turned on/off under the control of the controller6. According to this, application voltages to the heaters53and54are turned on/off.

Third column: Fixing temperature control at the time of warm-up or standby The controller6performs processing of fixing temperature control inFIG. 3when an operation mode of the image forming apparatus1is one of the time of warm-up or the time of standby. InFIG. 3, first, the controller6receives a temperature of the heating roller51from the temperature detection unit55and determines whether the received temperature is equal to or lower than a predetermined first reference temperature (S01and S02). The first reference temperature is a lower limit value in a target temperature range of the heating roller during warm-up or standby and is appropriately and adequately determined.

When it has been determined as Yes in S02, the controller6determines whether a flag F which has been set in a non-volatile memory and the like in the controller6is set to be one (S03). The flag F will be described in detail below.

When it has been determined as No in S03, the controller6considers that a preheating control execution condition is not satisfied and lights the second radiation heater54(S04). At this time, the controller6turns on the second drive unit82. As a result, the output voltage of the power supply unit7is applied to the second radiation heater54, and the current flows. This lights the second radiation heater54.

Next, the controller6increments a second counter provided in the non-volatile memory and the like by one in S05. The second counter counts the number of times of lighting Nl of the second radiation heater54. In addition, the controller6records lighting start time of the second radiation heater54in a storage area provided in the non-volatile memory and the like (S05). After that, the controller6performs the processing in S01again.

Further, when it has been determined as Yes in S03, the controller6considers that the preheating control execution condition is satisfied and lights the first radiation heater53(S06). The first radiation heater53is lighted in S06in order to preheat the second radiation heater54. In other words, the temperature of the heating roller51is not controlled. Therefore, the controller6may continuously light the first radiation heater53in a period of the preheating control.

Next, the controller6increments a first counter provided in the non-volatile memory and the like by one in S07. The first counter counts the number of times of lighting Ns of the first radiation heater53. In addition, the controller6records the lighting start time of the first radiation heater53in the storage area provided in the non-volatile memory and the like (S07).

Next, the controller6turns off the first radiation heater53after a predetermined time elapses (S08). After that, the controller6adds the lighting time from the lighting start time to turn-off time recorded in S07to a current value of a third counter provided in the non-volatile memory and the like in order to count cumulative lighting time Ts of the first radiation heater53(S09).

Next, the controller6lights the second radiation heater54by using a method similar to that in S04(S010). Next, similarly to S05, the controller6increments the second counter by one and records the lighting start time of the second radiation heater54(S011). After that, the controller6clears the flag F (in other words, set the flag F to zero) (S012) and performs the processing in S01again.

Further, when it has been determined as No in S02, the controller6determines whether the temperature received in S01is equal to or higher than a second reference temperature (S013). The second reference temperature is an upper limit value of the target temperature range for warm-up and the like and is set to a value larger than the first reference temperature.

When it has been determined as Yes in S013, the controller6turns off the second radiation heater54(S014). After that, the controller6adds the lighting time from the lighting start time to the turn-off time recorded in S05to a current value of a fourth counter provided in the non-volatile memory and the like in order to count the cumulative lighting time Tl of the second radiation heater54(S015). After that, the controller6terminates the processing inFIG. 3. Further, when it has been determined as No in S013, the controller6performs the processing in S01again.

Fourth Column: Effect of Fixing Temperature Control at the Time of Warm-Up or Standby

According to the fixing temperature control described in the third column, when the flag F to be described below is one, the second radiation heater54is preheated by lighting the first radiation heater53before being lighted. Due to the preheating, the resistance value of tungsten in the second radiation heater54increases, and after that, the output voltage of the power supply unit7is applied. Therefore, the inrush current which flows in the second radiation heater54is smaller than that of a case of no preheating. As a result, the damage to the second radiation heater54can be reduced, and the life of the second radiation heater54can be prolonged.

Fifth Column: Setting Value of Flag F

The controller6performs the processing inFIG. 4at every timing defined by a main flow (not shown) of the image forming apparatus1. InFIG. 4, the controller6reads the number of times of lighting Ns of the first radiation heater53and the cumulative lighting time Tl and the number of times of lighting N1of the second radiation heater54from the non-volatile memory in the controller (S11).

Next, the controller6sets a threshold n and a coefficient α for preheating determination based on the cumulative lighting time Tl read in S11(S12). Here, the threshold n and the coefficient α are variable values and defined by a program and the like so as to be smaller as the cumulative lighting time Tl gets longer. By defining the threshold n and the coefficient α in this way, when the second radiation heater54has been used for a long time, it is easy to perform the preheating control. Therefore, this is preferable to solve the problem set herein. It is preferable that the specific values of the threshold n and the coefficient α be appropriately determined.

Next, the controller6obtains a value Nl-Ns as a difference in the numbers of times of lighting ΔN based on the numbers of times of lighting Ns and Nl read in S11. After that, the controller6determines whether ΔN≧n is satisfied (S13).

When it has been determined as No in S13, the controller6clears the flag F and an execution frequency A to be described (in other words, set the execution frequency A to zero) (S14and S15).

Further, when it has been determined as Yes in S13, the controller6determines whether the execution frequency A to be described is equal to or less than zero (S16). The execution frequency A is a parameter which means that the second radiation heater54is once preheated by the first radiation heater53for every A times of lighting.

When it has been determined as Yes in S16, the controller6sets the flag F to be one, and after that, sets the execution frequency A to be a value obtained by using a following formula (1) (S17and S18). After that, the controller6terminates the processing inFIG. 4.
A=α/(Nl−Ns)  (1)

Whereas, when it has been determined as No in S16, the controller6decrements the execution frequency A by one and clears the flag F (S19and S110). After S15, S18, and S110, the controller6terminates the processing inFIG. 4.

Sixth Column: Fixing Temperature Control at the Time of Printing

When the operation mode of the image forming apparatus1is to print, the controller6performs processing inFIGS. 5A and 5B. Here, since the fixing temperature control at the time of printing may be performed by using a known method, the description will be omitted. Processing regarding the cumulative lighting times Ts and Tl and the numbers of times of lighting Ns and Nl will be mainly described.

InFIG. 5A, first, the controller6receives the temperature of the heating roller51from the temperature detection unit55(S21). After that, the controller6determines whether a front-back direction width of the printing medium M to be used is equal to or shorter than a predetermined size (for example, short side length of A4 size) (S22).

When it has been determine as No in S22, the controller6determines whether the temperature received in S21is equal to or lower than the lower limit value of the target temperature of the heating roller51(S23). When it has been determined as Yes in S23, the controller6lights the second radiation heater54(S24). After that, the controller6increments the second counter by one and records the lighting start time of the second radiation heater54(S25and S26). After that, the controller6performs the processing in S21again.

Whereas, when it has been determined as No in S23, the controller6determines whether the temperature received in S21is equal to or higher than the upper limit value of the target temperature of the heating roller51(S27). When it has been determined as Yes in S27, the controller6turns off the second radiation heater54(S28). After that, the controller6adds the lighting time from the lighting start time to the turn-off time recorded in S24to the current value of the fourth counter (S29). After S29or when it has been determined as No in S27, the controller6performs the processing in S21again.

When it has been determined as Yes in S22, the controller6determines whether the temperature received in S21is equal to or lower than the lower limit value of the target temperature of the heating roller51(S210inFIG. 5B). When it has been determined as Yes in S210, the controller6lights the first radiation heater53(S211). After that, the controller6increments the first counter by one and records the lighting start time of the first radiation heater53(S212and S213). After that, the controller6performs the processing in S21again.

Whereas, when it has been determined as No in S210, the controller6determines whether the temperature received in S21is equal to or higher than the upper limit value of the target temperature of the heating roller51(S214). When it has been determined as Yes in S214, the controller6turns off the first radiation heater53(S215). After that, the controller6adds the lighting time from the lighting start time to the turn-off time recorded in S213to the current value of the fourth counter (S216).

Seventh Column: Action and Effect of Image Forming Apparatus

According to the image forming apparatus1, time difference between times to end the lives of the heaters53and54having different heating widths with each other can be reduced. An effect will be described below. Before the description, an idea of the cumulative damage is introduced as an index to determine whether the life ends in the present embodiment. The cumulative damage is basically defined as cumulative lighting time×the number of times of lighting. However, since the cumulative damage is influenced by the size of the inrush current at the time of lighting the radiation heaters53and54, it is necessary to consider the influence. Specifically, the size of the inrush current in a case where the preheating is controlled as in the present embodiment is different from that in a case where the preheating is not controlled. Therefore, the cumulative damage is defined in detail as the following formula (2).
cumulative damage=cumulative lighting time×(the number of times of lighting with no preheating×inrush current coefficient+the number of times of lighting with preheating)  (2)

Here, an inrush current coefficient is simply a value indicating an influence of the inrush current when the preheating is not performed. Therefore, there are various methods to define the inrush current coefficient. The inrush current coefficient is a value obtained by operating an actual machine of the image forming apparatus1. For example, each of the radiation heaters53and54has a specific value (1.1 to 1.5). Further, for example, the inrush current coefficient can be a ratio between an inrush current value with no preheating (maximum amplitude value) and an inrush current value with preheating (maximum amplitude value) under a condition where the application voltages are the same.

FIG. 6is a graph of an exemplary change of the cumulative damage relative to the total number of printed sheets of the image forming apparatus1. As illustrated inFIG. 6, in a section P where the total number of printed sheets is of zero to Sa, even when the controller6performs the processing inFIG. 3, the value ΔN is not determined to be equal to or more than n in S13. Therefore, the flag F is constantly set to be zero in S14. Accordingly, since the second radiation heater54is not preheated at the time of warm-up and standby, the damage to the second radiation heater54is accumulated at a speed faster than that of the first radiation heater53(refer to thin solid line and thick solid line).

Whereas, when the total number of printed sheets reaches Sa and it is determined that the value ΔN is equal to or more than n in S13inFIG. 3, the second radiation heater54is preheated at least once at every A times of lighting. Therefore, according to the formula (2), the damage to the second radiation heater54is accumulated at a speed slower than that in the section P. By controlling the preheating, since the number of times of lighting increases, the damage to the first radiation heater53is easily accumulated (refer to section Q where the total number of printed sheets is equal to or more than Sa and less than Sb).

After the section Q, it is assumed that the image forming apparatus1perform printing to a large number of printing medium M which are smaller than a predetermined size. In this state, it is often determined as Yes in S22inFIG. 5A, and a frequency for using the first radiation heater53increases. As a result, the damage to the first radiation heater53is easily accumulated (refer to section R where the total number of printed sheets is equal to or more than Sb and less then Sc).

After the section R, in the image forming apparatus1, the number of times of determinations such that the value ΔN is equal to or more than n in S13inFIG. 3is reduced. Therefore, similar to a case of the section P, the damage is easily accumulated in the second radiation heater54(refer to section S where the total number of printed sheets is equal to or more than Sc and less than Sd). After the section S, the second radiation heater54is preheated at least once at every A times of lighting similarly to the section Q. Therefore, since the lighting frequency increases, the damage to the first radiation heater53is more easily accumulated than that in the section S (refer to section T where the total number of printed sheets is equal to or more than Sd and less than Se).

Relative to the above, regarding the image forming apparatus having no preheating control as the present embodiment, in the sections P and Q and the sections S and T, the damages are accumulated to both the radiation heater with a wide heating width and the radiation heater with a narrow radiation width according to the respective numbers of times of lighting (refer to thin broken line and thick broken line).

As described above, according to the present embodiment, time difference between times to the ends the lives of the radiation heaters53and54having different heating widths from each other can be reduced. Therefore, both the heaters53and54can be sufficiently used to the ends of their lives before the fixing unit5is exchanged.

Next, other effects will be described. First, an inrush current to the second drive unit82on the power supply line L2can be reduced by preheating the second radiation heater54. As a result, since the increase in the temperature of the second drive unit82due to the inrush current can be prevented, an effect to prolong the life of the second drive unit82can be expected.

Further, when the operation mode is to print, the second radiation heater54is not preheated. Accordingly, the heating roller51can be appropriately feedback controlled to be the target temperature.

Eighth Column: Supplementary Notes

In the fifth column, the description has been made in which the coefficient α is set to be a value which becomes smaller as the cumulative lighting time Tl gets longer. However, the coefficient α is not limited to this and may be set to be a value which becomes smaller as the lighting period of the second radiation heater54gets shorter. Accordingly, for example, in a case where a frequency in which the inrush current flows to the second radiation heater54increases, such as a case where a difference between the upper limit value and the lower limit value of the target temperature of the heating roller51is small, the execution frequency of the preheating to the second radiation heater54can be increased.

An image forming apparatus according to the present invention can reduce time difference between times to the ends the lives of first and second radiation heaters which have different heating widths from each other, and the image forming apparatus is suitable for a printer and the like.