Patent Grant
8340543
The present patent application claims priority pursuant to 35 U.S.C.§119 from Japanese Patent Application No. 2008-132372 filed on May 20, 2008, the contents of which are hereby incorporated by reference herein in their entirety.
1. Field of the Invention
The present invention relates to a temperature control method for use in a fixing device that fixes a toner image on a recording sheet by heating the recording sheet and an image forming apparatus incorporating such a temperature control method.
2. Discussion of the Background
In an image forming apparatus such as a copying machine, a printer, a facsimile machine or a multifunction device incorporating several of these functions, a fixing device for melting a toner image by heat and fixing it on a recording medium such as a sheet for printing or the like is often employed.
As shown in
In order to ensure that the fixing device performs reliably, a temperature of the fixing belt must be maintained at a target temperature set in advance. Thus, a temperature detector 160 for detecting a temperature of the fixing belt 130 is disposed as shown in
As a temperature control method for the fixing belt, for example, an ON/OFF control method in which the heater 150 is turned on/off according to the temperature of the fixing belt 130 as measured by the temperature detector 160 is known. Specifically, if the temperature of the fixing belt is lower than a target temperature, a heater is turned on, while if the temperature of the fixing belt is higher than the target temperature, the heater is turned off.
However, employing only the ON/OFF temperature control method, the temperature of the fixing belt might deviate substantially from the target temperature. In order to decrease a temperature difference (also referred to as a temperature ripple) between the temperature of the fixing belt and the target temperature, an image forming apparatus shown in Japanese Unexamined Patent Application Publication No. 2006-323093, for example, executes PID control. PID control is a control method for optimizing a plurality of parameters according to a deviation between a detected temperature and a target temperature by combining proportional, integral, and differential with a control algorithm.
PID control is described referring to
As an initial matter, if the temperature difference between a temperature T1 of a fixing belt and a target temperature T0 is large (e.g., 100 degrees or more), a heater duty D of a heater is increased for heat generation (proportional control). Thereafter, when the temperature T1 of the fixing belt approaches the target temperature T0, the heater duty D of the heater is decreased (differential control) so that the temperature T1 of the fixing belt does not exceed (overshoot) the target temperature T0. Then, in order to eliminate the difference between temperature T1 of the fixing belt and the target temperature T0, the heater duty D is adjusted (integral control).
When a toner image is fixed onto a printing sheet at a fixing nip, since the printing sheet draws heat from the fixing belt the temperature of the fixing belt decreases. At this time, in order to raise the lowered temperature of the fixing belt to a target temperature, a heater is caused to generate heat. However, it takes time for the heat generated by the heater to raise the temperature of the fixing belt, and as a result, the temperature of the fixing belt may not be maintained at an appropriate temperature and proper fixing might not occur.
Therefore, Japanese Patent No. 3216386, for example, discloses a temperature control method that compensates for heat drawn off by a printing sheet by electrifying a heater in advance, that is, before the printing sheet enters a fixing nip. Accordingly, responsiveness of the temperature control of a fixing belt is improved, and image quality is stabled.
However, a problem with the PID temperature control method described above is that, if the measured temperature of the fixing belt and the target temperature are close to each other, it is not possible to greatly increase the heater duty for heating. Consequently, increase of the heater duty is gentle even if the heater is electrified in advance before the entry. As a result, when the printing sheet enters the fixing nip when the temperature of the fixing belt and the target temperature are close to each other, the temperature of the fixing belt is rapidly lowered.
This phenomenon is illustrated in the graph shown in
Exemplary aspects of the present invention are put forward in view of the above-described circumstances, and provide a novel temperature control method for use in a fixing device that prevents a reduction in temperature of a fixing belt caused by passage of a recording medium into a fixing nip.
Other exemplary aspects of the present invention provide a novel image forming apparatus that prevents a reduction in temperature of a fixing belt caused by passage of a recording medium into a fixing nip.
In one exemplary embodiment, the fixing device fixes a toner image on a recording sheet by passing the recording sheet through a fixing nip, and includes a fixing member and a heater. The fixing member is disposed pressed against a pressure roller to form the fixing nip therebetween. The heater heats the fixing member to a target temperature. The novel temperature control method includes temperature detection, heater control, and duty control execution. The temperature detection detects a temperature of the fixing member with a temperature detector. The heater control controls operation of the heater by changing a duty thereof according to the detected temperature. The duty control execution executes a heater duty control to change a heater duty for a current control cycle discontinuously from that for a previous control cycle when the current control cycle precedes entry of the recording sheet into the fixing nip by a given period of time.
In one exemplary embodiment, the image forming apparatus includes a fixing device, a temperature detector, and a heater controller. The fixing device fixes a toner image on a recording sheet by passing the recording sheet through a fixing nip, and includes a fixing member and a heater. The fixing member is disposed pressed against a pressure roller to form the fixing nip therebetween. The heater heats the fixing member to a target temperature. The temperature detector detects a temperature of the fixing member. The heater controller controls operation of the heater by changing a duty thereof according to the detected temperature. The heater controller executes a heater duty control to change a heater duty for a current control cycle discontinuously from that for a previous control cycle when the current control cycle precedes entry of the recording sheet into the fixing nip by a given period of time.
A more complete appreciation of the disclosure and many of the attendant advantages thereof is readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:
In describing exemplary embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result.
Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views thereof, exemplary embodiments of the present patent application are described.
Each of the image forming portions 1Y, 1C, 1M, 1Bk has the same configuration, except that each contains toner of a color different from the others. Thus, a configuration of the image forming portion 1Y is described as an example.
The image forming portion 1Y is provided with a photoreceptor 2 as an image supporting body for supporting an electrostatic latent image, a charging device 3 for charging the surface of the photoreceptor 2, a development device 4 for forming a toner image on the surface of the photoreceptor 2, a cleaning device 5 for cleaning the surface of the photoreceptor 2, and the like. As the cleaning device 5, a cleaning blade, a cleaning roller, or a cleaning brush and the like can be employed, either singly or in combination.
Above the image forming portions 1Y, 1C, 1M, and 1Bk is disposed an exposure device 6 for forming an electrostatic latent image on the surface of the photoreceptor 2. Below the image forming portions 1Y, 1C, 1M, 1Bk, an intermediate transfer unit 7 is disposed. The intermediate transfer unit 7 has an intermediate transfer belt 11 extended among a plurality of extension rollers 8, 9, 10. The intermediate transfer belt 11 has at least one layer of an elastic coating formed on the surface of an endless belt base material, for example. The endless belt base material is constituted by a resin, rubber, or metal thin plate or the like. The elastic coating layer is constituted by a resin, rubber, elastomer, or the like.
Four primary transfer rollers 12 are pressed into contact with the four photoreceptors 2 through the intermediate transfer belt 11. As a result, the four photoreceptors 2 are pressed into contact with the outer peripheral face of the intermediate transfer belt 11, and a primary transfer nip is formed at a fixing nip between each of the photoreceptors 2 and the intermediate transfer belt 11. Also, a secondary transfer roller 13 is pressed into contact with one roller 10 of the above plurality of extension rollers through the intermediate belt 11. A secondary transfer nip is formed at a fixing nip where the secondary transfer roller 13 is pressed into contact with the outer peripheral face of the intermediate transfer belt 11.
At a lower part of the image forming apparatus 200, a recording medium supply portion 14 is disposed. The recording medium supply portion 14 is provided with a cassette capable of containing the recording media, which may be a stack of printing sheets, OHP films, or the like, a supply roller for feeding out the recording medium, and the like (not shown).
Between the recording medium supply portion 14 and the intermediate transfer unit 7, a pair of resist rollers 15a, 15b, a recording-medium feeding unit 16 having a feeding belt, and a fixing device 17 are disposed. On an outer wall of a main body of the image forming apparatus 200, a discharge tray 18 for stacking the recording media discharged to the outside is attached.
The fixing device 17 has a fixing roller 19, a pressure roller (pressure member) 22, a heating roller 20, and a fixing belt (fixing member) 21. The fixing roller 19 is constituted by an elastic layer made of silicon rubber or the like formed around a core metal constituted by aluminum, iron and the like. The pressure roller 22 is constituted by an elastic layer made of silicon rubber or the like provided around a hollow core metal constituted by aluminum, iron and the like and a release layer made of a fluorine resin layer or the like formed around it in order to ensure releasing property of toner. The heating roller 20 is constituted by a tubular body made of a highly heat-conductive material such as aluminum. The fixing belt 21 has a release layer made of a fluorine resin layer or the like formed on the surface of a belt base material such as polyimide or the like in order to ensure releasing property of the toner. Alternatively, an elastic layer made of silicon rubber or the like may be interposed between the belt base material and the release layer.
The fixing belt 21 is extended between the fixing roller 19 and the heating roller 20. The heating roller 20 has a heater 23 inside. By having the heater 23 generate heat so as to heat the heating roller 20, the fixing belt 21 is also heated.
The pressure roller 22 is pressed into contact with the outer peripheral face of the fixing belt 21 at a position opposing the fixing roller 19. A fixing nip is formed at a fixing nip where the pressure roller 22 and the fixing roller 21 press against each other through the fixing belt 21. It is to be noted that the configuration of the fixing device is not limited to that described in
A temperature detector 24 is disposed on the outer periphery of the fixing belt 21. As shown in
Specifically, the PID controller 26 calculates the heater duty 23 on the basis of the PID algorithm shown in the following equation 1:
Dn=Dn−1+Kp*(Tn−1−Tn)+Ki*(T−Tn)+Kd*(2*Tn−1−Tn−Tn−2) [Equation 1]
In the PID algorithm shown in the above equation 1, Dn is the heater duty calculated in the current control cycle, Dn−1 is the heater duty calculated in the control cycle preceding the current control cycle, T is a target temperature of the fixing belt, Tn is a temperature of the fixing belt detected in the current control cycle, Tn−1 is a temperature of the fixing belt detected in the control cycle preceding the current control cycle, Tn−2 is a temperature of the fixing belt detected in the control cycle prior to the preceding one, Kp is a proportional gain, Ki is an integral gain, and Kd is a differential gain. Hereinafter the heater duty Dn−1 calculated in the previous control cycle is referred to as the preceding heater duty. Also, the temperature Tn−1 of the fixing belt detected in the preceding control cycle is referred to as the previous temperature, and the temperature Tn−2 of the fixing belt detected in the control cycle prior to the preceding one is referred to as the detected temperature prior to the previous one.
The image forming apparatus 200 of the present invention executes control so that mainly a temperature at the center part in the width direction of the fixing belt 21 becomes the target temperature for favorable fixing. Therefore, the temperature of the fixing belt in the above PID algorithm is the temperature at the center part in the width direction of the fixing belt 21, and the above target temperature is the target temperature at the center part in the width direction of the fixing belt 21. It is to be noted that although the PID algorithm is set as the above equation 1 herein, it is not limited to this calculation equation.
The heater controller 25 is provided with the heater duty booster 28. The heater duty booster 28 obtains a value of the heater duty larger than the heater duty calculated on the basis of the above PID algorithm. For example, the heater duty booster 28 is configured to substitute 100[%] instead of the previous heater duty Dn−1 in the above PID algorithm. Also, it is configured with a control switch 29 to enable selective switching between control by the heater duty booster 28 (heater duty control) and the usual heater duty control executing the PID control not by the heater duty booster 28.
Basic operation of the above image forming apparatus 200 is described below referring to
First, an image forming operation is described using one image forming portion 1Y as an example.
The surface of the photoreceptor 2 is charged with a uniform high potential by the charging device 3. A laser beam irradiates the surface of the photoreceptor 2 from the exposure device 6 on the basis of image data, and the potential on the irradiated portion is lowered so that an electrostatic latent image is formed. On the portion on the surface of the photoreceptor 2 where the electrostatic latent image is formed, a toner charged by the development device 4 is electrostatically transferred so that a visible yellow toner image is formed thereat.
A constant-voltage or constant-current controlled voltage of a polarity opposite to the charging polarity of the toner is applied to the primary transfer roller 12. As a result, a transfer electric field is formed at the primary transfer nip between the primary transfer roller 12 and the photoreceptor 2. At the primary transfer nip, the toner image on the rotating photoreceptor 2 is transferred to the intermediate transfer belt 11 travelling in the direction of the arrow Y in
Similarly, a toner image is formed on the photoreceptor 2 in each of the other image forming portions 1C, 1M, 1Bk and transferred onto the intermediate transfer belt 11, so that the toner images are superimposed with each other. As a result, a synthetic toner image in which the toner images in four colors are superimposed is formed on the intermediate transfer belt 11.
Each of the cleaning device 5 removes remaining toner adhering to the surface of the photoreceptor 2 having going through the primary transfer process. After that, any charge remaining on the photoreceptor 2 is removed by a destaticizing device such as a destaticizing lamp or the like, not shown.
On the other hand, the supply roller of the recording medium supply portion 14 is rotated so as to feed out a recording medium P. The recording medium P fed out of the recording medium supply portion 14 is stopped once by the resist rollers 15a, 15b.
After the synthetic toner image is formed on the intermediate transfer belt 11 as mentioned above, the driving of the resist rollers 15a, 15b is resumed, and the recording medium P is fed to the secondary transfer nip between the secondary transfer roller 13 and the roller 10 in synchrony with the synthetic toner image on the intermediate transfer belt 11. Then, the synthetic toner image on the intermediate transfer belt 11 is transferred onto the recording medium P fed to the secondary transfer nip.
The recording medium P onto which the synthetic toner image has been transferred is fed to the fixing device 17. More specifically, the recording medium P is fed to the fixing nip formed between the fixing roller 19 and the pressure roller 22. While the recording medium P passes through the fixing nip, the toner constituting the synthetic toner image is melted and fixed on the recording medium P. After that, the recording medium P on which the synthetic toner image has been fixed is discharged onto the discharge tray 18 and stacked.
A description is now given of a temperature control method for the fixing device according to the present invention.
As shown in
If a printing request is made from a user to the image forming apparatus 200, the recording medium on which the image has been transferred is fed to the fixing device. In a control cycle a prior to the entry of the recording medium fed to the fixing device into the fixing nip by a predetermined time Tx, the heater duty control is executed. The heater duty control is enabled when the heater duty booster 28 is connected to the PID controller 26 by switching the control switch 29 shown in
1Tp to 7Tp shown in
In each of the above embodiments according to the present invention, with the heater duty control, the heater can be activated with a larger heater duty value as compared with that of the usual heater duty control. This is described referring to
In
The reason for this difference in rate of change of the heater duty is that, in the related-art PID control, since the temperature T1 of the fixing belt is close to the target temperature T0 at the point in time A in
By contrast, in the heater duty control of the present invention, even if the temperature T1 of the fixing nip is close to the target temperature T0 at the point in time B in
In addition, it is to be noted that there is a time lag till the heat of the heater reaches the fixing belt. Thus, in the temperature control method of the present invention, the heater duty control of the heater is executed in the control cycle the predetermined time Tx prior to entry of the recording medium into the fixing nip. As a result, heat can be supplied to the fixing belt when the heat of the fixing belt is deprived of by the recording medium.
In the above embodiments, the heater duty control is executed by substituting 100[%] for the previous heater duty Dn−1 in the above PID algorithm. However, the value to be substituted is not limited to 100[%]. Thus, if a value larger than the heater duty calculated by the above usual heater duty control can be calculated, the value to be substituted may be 95[%] or 90[%], for example.
The predetermined time Tx is set on the basis of the thermal responsiveness of the fixing device. The thermal responsiveness of the fixing device is determined by the material of the fixing device, the heating capacity of the heater, and the like. For example, the predetermined time Tx may be set to a time from start of activation of the heater until the temperature of the fixing belt is raised by the activation of the heater. In each of the embodiments described above, an optimal value of the above predetermined time Tx for minimizing a temperature difference (temperature ripple) between the temperature of the fixing belt and the target temperature is set at 3 seconds. By setting the predetermined time Tx on the basis of the thermal responsiveness of the fixing device, even if the fixing speed or the like is different, there is no need to change the predetermined time Tx. Thus, the temperature control of the fixing device can be executed more easily.
For example, if the recording medium is a printing sheet, the heat absorbed from the fixing belt by the printing sheet when the printing sheet passes through the fixing nip is different depending on the mass of the printing sheet. Specifically, the smaller the mass of the printing sheet, the smaller the absorbed heat quantity, while the larger the mass of the printing sheet, the larger the absorbed heat quantity. If the absorbed heat quantity is large when the printing sheet passes through the fixing nip, the number of times the heater duty control is executed must be increased accordingly. Thus, as in the second embodiment of the present invention shown in
On the other hand, if the absorbed heat quantity is small when the printing sheet passes through the fixing nip, the number of times the heater duty control need to be executed may be small. Therefore, preferably, the number of times the heater duty control is executed is increased the larger the mass of the printing sheet passing through the fixing nip. Conversely, preferably, the number of times the heater duty control is executed is decreased the smaller the mass of the printing sheet passing through the fixing nip. Moreover, the larger the area or the mass (weight) per unit area of the printing sheet, the larger the mass of the printing sheet becomes. Thus, the larger the area or the mass per unit area of the printing sheet, the greater the number of times the heater duty control is executed. Conversely, the smaller the area or the mass per unit area of the printing sheet, the fewer the number of times the heater duty control is executed.
An example of the number of times the heater duty control times is executed as determined by the sheet size (sheet area or sheet length) and the mass (weight) of the printing sheet is shown in the following table 1. By preparing such a table in advance, the number of times the heater duty control is executed may be changed according to the printing sheet in use.
In addition, since the heat quantity absorbed when the recording medium passes through the fixing nip is different depending on the type of material constituting the recording medium (paper, OHP film and the like), preferably the number of times the heater duty control is executed is changed according to the type of recording medium material.
As described using
Further, in order to prevent defective fixing such as hot offset or the like, a threshold value of the temperature at the end part in the width direction of the fixing belt may be set in advance. In that case, if the temperature at the end part in the width direction of the fixing belt is not more than the threshold value, the heater duty control is executed in the control cycle the predetermined time Tx prior to entry of the recording medium into the fixing nip, whereas if the temperature at the end part in the width direction of the fixing belt exceeds the threshold value, the heater duty control is not executed. As a result, at the end part in the width direction of the fixing belt, any excessive temperature rise that might cause defective fixing such as hot offset or the like can be suppressed.
Finally, by adjusting the target temperature at the center part in the width direction of the fixing belt, the temperature at the end part in the width direction of the fixing belt can be adjusted. Specifically, control is executed such that, if the temperature at the end part in the width direction of the fixing belt exceeds a predetermined upper limit value, the target temperature at the center part in the width direction of the fixing belt is lowered, whereas if the temperature at the end part in the width direction of the fixing belt falls below a predetermined lower limit value, the target temperature at the center part in the width direction of the fixing belt is raised.
In a continuous image forming operation, it is also possible to suppress the temperature rise at the end part in the width direction of the fixing belt by interrupting printing in the middle or by lengthening the interval between successive passages of recording media through the fixing nip.
Numerous additional modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that, within the scope of the appended claims, the disclosure of this patent specification may be practiced otherwise than as specifically described herein. For example, the heater duty booster may obtain the heater duty without using the PID algorithm shown in the equation 1, and the temperature control method according to the present invention can be applied to the fixing device for calculating the heater duty using an equation other than the PID algorithm described above.
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