The present application claims priority to and incorporates by reference the entire contents of Japanese priority documents 2007-074551 filed in Japan on Mar. 22, 2007 and 2007-288547 filed in Japan on Nov. 6, 2007.
1. Field of the Invention
The present invention relates to a sheet conveying device and an image forming apparatus.
2. Description of the Related Art
Intermediate transfer type color image forming apparatuses are widespread in use. In such image forming apparatuses, a toner image each formed on a photosensitive element is primarily transferred onto an intermediate transfer member, and a color image on the intermediate transfer member is secondarily transferred onto a recording medium (sheet). The image forming apparatuses can be used for various types of sheet media such as thin sheet, thick sheet, postcard, and envelope, and therefore has an advantage of having high versatility. As the intermediate transfer member, an intermediate transfer drum or an intermediate transfer belt is generally used.
However, when a sheet with a certain level of thickness enters into a secondary transfer unit, the speed of the intermediate transfer member being driven at a certain speed drops for a short period of time. This disturbs image forming operation in the primary transfer unit.
Furthermore, along with downsizing of color image forming apparatuses, the secondary transfer unit and a fuser become adjacent to each other, and transfer and fixation of the image can be performed simultaneously on the sheet (when fixation is performed at a front end of one sheet, the image is transferred to a rear end of the sheet). At this time, when a sheet with a certain level of thickness enters into the fuser, the speed of a fuser roller or a fuser belt being driven at a certain speed drops for a short period of time. This disturbs image forming operation in the secondary transfer unit as in the intermediate transfer unit.
There is an image forming apparatus adopting a simultaneous transfer and fixing method in which transfer and fixation of the toner image onto the sheet is performed simultaneously (at a time). In this case also, when a sheet with a certain level of thickness enters into a transfer-fixing unit, the speed of the intermediate transfer member being driven at a certain speed drops for a short period of time, thereby causing a problem that the image is disturbed in the primary and secondary transfer units, as at the time of entering into the secondary transfer unit.
Japanese Patent Application Laid-open No. 2005-107118 discloses a conventional color image forming apparatus, in which the speed of a belt is made constant by changing a speed control amount with respect to a driving source of an endless belt at a preset predetermined timing, by a predetermined amount, and for predetermined duration.
In the conventional image forming apparatus, however, a unit that detects a mechanical property of a sheet such as a thickness sensor is required.
Moreover, because a control target value preset based on the type, thickness and width of a sheet is used, it is difficult to perform optimum control with respect to all usable sheets. Furthermore, even with the same sheet, the thickness and firmness change according to environmental conditions such as temperature and humidity, and fluctuation of speed caused thereby is different, and therefore, optimum control is difficult to perform.
Besides, it is required to store control target values corresponding to various types of sheets. With an increase in the type of sheets that can be handled, a storage unit is required to have a larger memory capacity.
It is an object of the present invention to at least partially solve the problems in the conventional technology.
According to an aspect of the present invention, there is provided a sheet conveying device that includes a plurality of conveying units each including a drive roller and a driven roller to convey a recording medium while holding the recording medium between the drive roller and the driven roller, the conveying units including a first conveying unit and a second conveying unit located downstream of the first conveying unit in a conveying direction in which the recording medium is conveyed, speed of the drive roller of the second conveying unit being controllable; a measuring unit that obtains speed information of the first conveying unit; a storage unit that stores therein the speed information; and a calculating unit that calculates a target value based on the speed information stored in the storage unit. The speed of the drive roller of the second conveying unit is controlled based on the target value.
According to another aspect of the present invention, there is provided an image forming apparatus including a sheet conveying device. The sheet conveying device includes a plurality of conveying units each including a drive roller and a driven roller to convey a recording medium while holding the recording medium between the drive roller and the driven roller, the conveying units including a first conveying unit and a second conveying unit located downstream of the first conveying unit in a conveying direction in which the recording medium is conveyed, speed of the drive roller of the second conveying unit being controllable; a measuring unit that obtains speed information of the first conveying unit; a storage unit that stores therein the speed information; and a calculating unit that calculates a target value based on the speed information stored in the storage unit. The speed of the drive roller of the second conveying unit is controlled based on the target value.
The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings.
Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings.
The drive roller la in the upstream rollers 1 is driven by a motor (driving source) 5 via a small-diameter gear 6 and a large-diameter gear 7. The driven roller 1b is pressed by the drive roller 1a to rotate together therewith. A speed measuring unit 8 is attached to a shaft of the drive roller 1a. An output from the speed measuring unit 8 is sent to a controller 9 that controls the motor 5.
The drive roller 2a in the downstream rollers 2 is driven by a motor (driving source) 15 via a small-diameter gear 16 and a large-diameter gear 17. The driven roller 2b is pressed by the drive roller 2a to rotate together therewith. A speed measuring unit 18 is attached to a shaft of the drive roller 2a. An output from the speed measuring unit 18 is sent to a controller 19 that controls the motor 15.
The output of the speed measuring unit 8 in the upstream rollers is sent to a storage unit 13, and an output of the sheet detector 11 is sent to a calculating unit 14. An output of the sheet detector 12 in the downstream rollers is sent to the controller 19.
The drive rollers 1a and 2a and the driven rollers 1b and 2b in the sheet conveying device according to the first embodiment are made of metal; however, a roller surface can be coated with an organic material.
For the motors 5 and 15 as the driving source, a direct current (DC) motor, a pulse motor, an ultrasonic motor, or a direct drive motor can be used.
In the sheet conveying device according to the first embodiment, a drive transmission system from each driving source to each drive roller is formed of a gear. However, the drive transmission system can be formed of a gear and a synchronous belt, a V-belt and a pulley, or a planetary gear. When the ultrasonic motor or the direct drive motor is used for the driving source, the roller can be directly driven without using the drive transmission system in terms of the characteristics of these motors.
The controller 9 includes a feedback controller and a phase compensator. The feedback controller calculates drive voltage, drive current, and drive frequency of the driving source 5 based on the speed information of the drive roller 1a measured by the speed measuring unit 8, to control the driving source 5. The fed back speed information can be rotation speed information of the driven roller 1b or the driving source 5.
When the driving source 5 is the DC motor or the direct drive motor, a drive-current control method or a drive-voltage pulse-width-modulation (PWM) control method is used. When the driving source 5 is the pulse motor or the ultrasonic motor, a drive-frequency control method is used. The same applies to the driving source 15.
The speed measuring unit 8 can use a method of using a rotary encoder installed coaxially with the roller shaft, a method of directly measuring the surface speed of the roller by laser Doppler, or a method of using a magnetic encoder that measures magnetic information of a rotor of the motor by a magnetic sensor. When the motor as the driving source is the DC motor, a frequency generator (FG) signal output from the motor can be used. Alternatively, the drive current of the DC motor can be measured. The same thing applies to the speed measuring unit 18.
When the pulse motor or the ultrasonic motor is used for the driving source 5, the motor can be driven only by open-loop control without performing the feedback control. The phase compensator adjusts a control band and gain.
The feedforward controller 22 converts the control target value obtained by the calculating unit 14 to a control command value, for example, expressed by voltage, current, or frequency of the driving source (described in detail later).
The timing controller 23 gives time delay to the command value output from the feedforward controller 22 and outputs the delayed command value. The delay time is from detection of the sheet P by the sheet detector 12 until the sheet P enters into a pressed part between the drive roller 2a and the driven roller 2b. The sheet can be detected without the sheet detector 12 by using a drive signal or speed fluctuation information of the upstream rollers 1.
A specific control method is explained below.
To calculate the control command value easily, it is preferable that the upstream rollers 1 and the downstream rollers 2 have the same configuration. However, if the configuration is different from each other, an appropriate control command value can be calculated by a method described below.
When a sheet is held by both the upstream rollers 1 and downstream rollers 2, the entering condition of the sheet can be changed, respectively, in the upstream rollers 1 and the downstream rollers 2. Therefore, it is desired to arrange the upstream rollers 1 and the downstream rollers 2 away from each other by more than the sheet length to be used. In this case, a sheet conveying unit is separately required for conveying the sheet between the respective pairs. It is preferable that the sheet conveying unit have the same configuration.
In the example of
The calculating unit 14 converts the speed information stored in the storage unit 13 to a control target value.
First, Vs is subtracted from the stored data V1 to V5 to remove an offset in the steady state as shown in
A conversion method from the control target value to the control command value in the feedforward controller 22 is explained below in detail.
First, a control target value obtained from the upstream rollers 1 is converted to a control command value of the upstream drive roller la by a function f1. The control command value of the upstream drive roller la is then converted to a control command value of the downstream drive roller 2a by a function f2. Alternatively, functions f3 and f4 can be used to perform conversion. Functions f1 to f4 are calculated beforehand by experiments or numerical analysis.
When the upstream rollers 1 and the downstream rollers 2 have the same configuration, the control target value of the downstream rollers 2 can be obtained only by using the function f1.
As described above, the downstream drive roller 2a is controlled by using a control command value obtained by the feedforward controller 22.
A control command value (lower right in
The Y axis in the graph indicating the control command value in
The function f1 is an inverse function of a transfer function from an input of the driving source to an output of the pair of rollers in the upstream rollers 1. The function f4 is an inverse function of a transfer function from an input of the driving source to an output of the pair of rollers in the downstream rollers 2. These functions f1 and f4 can be calculated based on a physical constant of a component of the apparatus. Further, when the system configuration is complicated, these functions can be calculated by using a system identification method.
The function f2 converts the control command value in the upstream rollers 1 to the control command value in the downstream rollers 2. The function f3 converts the control target value in the upstream rollers 1 to the control target value in the downstream rollers 2. These functions can be identified based on the control command value or the control target value in the upstream rollers 1 and the downstream rollers 2.
The function f5 converts the control target value in the upstream rollers 1 to the control command value in the downstream rollers 2. The function f5 can be identified based on the control target value in the upstream rollers 1 and the control command value for the downstream rollers 2.
When the upstream rollers 1 and the downstream rollers 2 have the identical configuration, fluctuation of speed generated in the respective pairs of rollers is the same, and therefore, f1=f4=f5, f2=f3=1. If the upstream rollers 1 and the downstream rollers 2 have a similar configuration, the functions f1 and f4, and f5 become simpler conversion functions, and as their configuration becomes different (profile of fluctuation of speed is different), f1, f4, and f5 become more complicated conversion functions.
While it has been described that there are three conversion routes of f1→f2, f3→f4, and f5, and neither of these is particularly superior, there are merits and demerits of each conversion route, and they are described below.
In the case of converting the function f5, only one operation is required, and response is the best. However, because the operation itself becomes slightly complicated, a central processing unit (CPU) requires performance to some extent. In the case of conversions f1→f2 and f3→f4, the number of operations is two, and though the response is slightly inferior, one operation becomes simple. Therefore, an inexpensive CPU can be used. The conversions f1→f2 and f3→f4 are substantially equal in view of the performance. However, at the time of calculating the functions f1 and f4, calculation becomes easier and errors are reduced, as the configuration of the upstream and downstream rollers becomes simpler (e.g., not including a belt). Also in the functions f2 and f3, easiness of identification and errors vary according to the speed as the control target value and an intensity waveform of the control command value. Accordingly, the conversions f1→f2 and f3→f4 can be selected according to the easiness of calculation and the magnitude of errors.
The speed of the pair of rollers fluctuates not only when a thick sheet enters into the pair but also when the thick sheet is separated from the pair.
When the control target value is calculated, a plurality of pieces of speed information is stored in the storage unit 13, to calculate the control target value by averaging these pieces of speed information, thereby enabling to calculate the control target value with higher accuracy. Even if the same type of sheet is used, the same fluctuation of speed does not occur all the time, and fluctuation of speed is slightly different. Therefore, by calculating the control target value based on the pieces of speed information, more accurate control target value can be calculated, thereby enabling to obtain more stable effect. For example, a changeover switch or a mode selection unit can be provided so that a normal mode in which the control target value is calculated based on one piece of speed information, and a highly accurate mode in which the highly accurate control target value is calculated based on pieces of speed information can be selected. In the highly accurate mode, a more accurate control target value can be calculated to obtain more stable effect. On the other hand, in the normal mode, calculation of the control target value becomes simple to reduce a load on the CPU, and less memory capacity is required.
When it is known beforehand that the same sheets are continuously used, it is preferable to add a function for setting to use the control target value calculated for the first sheet repetitively. When this function is selected, repetition of the same process can be omitted, thereby enabling to reduce wasteful power consumption. For example, by providing the changeover switch or the mode selection unit, the target value can be calculated every time or only for the first time, according to the selected mode.
Appropriate control can be performed regardless of the sheet thickness, sheet type, and environmental conditions, by generating the control target value based on fluctuation of speed of the upstream rollers 1, thereby enabling to reduce fluctuation of speed of the downstream rollers 2. Thus, according to the first embodiment, fluctuation of speed of the pair of rollers occurring when a thick sheet enters into the pair or leaves the pair can be reduced, and the speed of the pair to be controlled can be controlled constant at all times.
Because a thickness sensor or the like that detects the thickness of the sheet is not required, cost increase can be suppressed. Further, fluctuation of speed of a member that conveys the sheet can be effectively prevented regardless of the sheet type and environmental conditions.
An upstream sheet conveying unit can include an endless belt, while a downstream sheet conveying unit can include no endless belt. Besides, both the upstream and downstream sheet conveying units can include an endless belt. In addition, there can be three or more pairs of sheet conveying units, and any pair or pairs of sheet conveying units as well as all of them can include an endless belt. An endless belt can be located on the driven side (driven-roller side) as a sheet conveying unit.
Also in the sheet conveying device having such a configuration, fluctuation of speed of the pair of rollers occurring when the thick sheet enters into the pair (including the one using the endless belt) or leaves the pair can be reduced by the same control as in the sheet conveying device according to the first embodiment, and the speed of the pair to be controlled can be controlled constant at all times.
Because the thickness sensor or the like that detects the thickness of the sheet is not required, cost increase can be suppressed. Further, fluctuation of speed of the member that conveys the sheet can be effectively prevented regardless of the sheet type and environmental conditions.
When the sheet conveying unit includes the endless belt, the speed of the endless belt can be measured as the speed measuring unit of the pair of rollers. As means for detecting the belt speed, there is a method of measuring the surface speed of the belt by a laser Doppler velocimeter, or a method of measuring the speed by detecting a scale applied on the belt by an optical sensor.
The sheet conveying device of the above embodiments is effectively applied to any types of apparatuses required to convey a sheet, such as an electrophotographic image forming apparatus in which the sheet conveying unit is used in an intermediate transfer unit and a fuser. Explained below is such a tandem image forming apparatus of an intermediate transfer system.
An endless intermediate transfer belt 301 is provided in the center of the apparatus body 310 as an intermediate transfer member. The intermediate transfer belt 301 is spanned over three support rollers 302, 303, and 304 so that it can rotate clockwise in
Further, on the intermediate transfer belt 301 stretched over between the first support roller 302 and the second support roller 303 among the three support rollers, four imaging units 311 for yellow (Y), magenta (M), cyan (C), and black (B) are arranged horizontally along the movement direction of the belt to form a tandem image forming unit 350. In this example, the third support roller 304 is set as the drive roller. An exposure device 309 is provided on the tandem image forming unit 350.
While the image forming apparatus using an intermediate transfer belt is described here, the image forming apparatus can use an intermediate transfer drum. In this case, the support rollers 302, 303, and 304 are not required, and the image forming unit is arranged not horizontally but around the intermediate transfer drum. That is, the intermediate transfer unit can be an intermediate transfer belt as well as an intermediate transfer drum.
On the other hand, a secondary transfer unit 315 is provided on the opposite side of the tandem image forming unit 350, put the intermediate transfer belt 301 therebetween. In the illustrated example, the secondary transfer unit 315 is formed by spanning a secondary transfer belt 316, which is an endless belt, between two belt support rollers 317 and 318. The secondary transfer unit 315 is pressed against the third support roller 304 via the intermediate transfer belt 301, to transfer an image on the intermediate transfer belt 301 onto the sheet. A fuser 319 that fixes an unfixed image transferred on the sheet is provided at the side of the secondary transfer unit 315. The secondary transfer unit 315 also has a sheet conveying function for conveying the sheet after image transfer to the fuser 319. A transfer roller or a non-contact type charger can be arranged as the secondary transfer unit, and in this case, a conveying unit that conveys the sheet from the secondary transfer unit to the fuser needs to be provided separately.
The fuser 319 is formed by pressing a pressure roller 307 against a fuser roller 306. The fuser roller 306 has a heat generating mechanism therein, and is heated up to a temperature required for fixing an image. An unfixed image on a sheet is applied with heat and pressure and fixed on the sheet. The fuser can be a fixing belt or a fixing roller.
In the above example, a sheet reversing unit 308 that reverses the sheet to record images on the opposite sides of the sheet is provided below the secondary transfer unit 315 and the fuser 319, in parallel with the tandem image forming unit 350.
When a copy is made by using the electrophotographic device, an original document is set on an original table 341 in the ADF 340. Alternatively, the ADF 340 is opened to set the document on a exposure glass 331 of the scanner 330 and closed to hold the document. The term “document” as used herein refers to any medium including text, an image, a photograph, a chart, and a table.
When a start switch (not shown) is pressed, the document is conveyed onto the exposure glass 331, in a case that the document is set in the ADF 340. On the other hand, in a case that the document is set on the exposure glass 331, the scanner 330 is immediately driven. A first carrier 332 and a second carrier 333 are driven next. While beams are irradiated from a light source by the first carrier 332, reflected light from the document surface is further reflected toward the second carrier 333, and reflected by a mirror in the second carrier 333 toward a read sensor 335 through an imaging lens 334, thereby reading the document content.
In parallel with document read, the support roller 304 is rotated by a drive motor (not shown), to rotate other two support rollers, thereby rotating the intermediate transfer belt 301. Simultaneously, in the individual imaging unit 311, a photosensitive drum 312 is rotated to expose and develop an image respectively by using color information of yellow, magenta, cyan, and black, thereby forming a single color toner image. With the movement of the intermediate transfer belt 301, these single color toner images are sequentially transferred thereto to form a synthesized color image on the intermediate transfer belt 301.
On the other hand, concurrently with image formation, one of feed rollers 321 of the feed table 320 is selected and driven to feed a sheet from one of a plurality of feed cassettes 323 provided in a sheet bank 322. Sheet are separated by a pair of separation rollers 324 and conveyed one by one by a conveyor roller 326 to a feed path 325, on which the sheet abuts against a registration roller 328 and stops. Alternatively, a bypass feed roller 329 is rotated to feed sheets on a bypass tray 336. The sheets are separated by a pair of separation rollers 337 and conveyed one by one to a bypass feed path 338, on which each sheet abuts against the registration roller 328 and stops.
The registration roller 328 is rotated, with the timing matched with the synthesized color image on the intermediate transfer belt 301, to feed the sheet to between the intermediate transfer belt 301 and the secondary transfer unit 315, and the image is transferred by the secondary transfer unit 315 to record a color image on the sheet.
The sheet after image transfer is conveyed by the belt 316 and fed to the fuser 319, and applied with heat and pressure in the fuser 319 to fix the transferred image thereon. The sheet is then switched by a switching claw 339, ejected by an ejection roller 342, and stacked on a eject tray 343. Alternatively, the sheet is switched by the switching claw 339 to be put into the sheet reversing unit 308, where the sheet is reversed and guided again to the transfer position, so that an image is recorded also on the other side of the sheet. The sheet is then ejected onto the eject tray 343 by the ejection roller 342.
On the other hand, the intermediate transfer belt 301 after image transfer is cleaned by the intermediate-transfer-belt cleaning device 305 to remove the residual toner remaining on the intermediate transfer belt 301 after image transfer, to prepare for next image formation by the tandem image forming unit 350. The registration roller 328 is generally grounded and used; however, a bias can be applied thereto to remove dust on the sheet.
A black monochrome copy can be made by using the electrophotographic device. In this case, the intermediate transfer belt 301 is separated from the photosensitive drums 312Y, 312C, and 312M by a unit (not shown). Rotation of these photosensitive drums are temporarily suspended, and only the black photosensitive drum 312K is brought into contact with the intermediate transfer belt 301 to perform image formation and transfer.
The image forming apparatus shown in
The image forming apparatus of
A feed cassette 161 is arranged in the feeder 120 provided at the bottom of the apparatus body, and a feed unit 162 that feeds the sheet from the feed cassette 161 is provided. The sheet fed from the feed cassette 161 is conveyed by a pair of conveyor rollers 164 arranged on a sheet-conveying path 163, and is fed to the transfer-fixing unit 166 by a pair of registration rollers 165.
In the transfer-fixing unit 166, the surface of the sheet is heated to a temperature sufficient for melting the toner by the sheet heating unit 167. The heated sheet is inserted into a nip formed by the transfer and fixing roller 104, the pressure roller 168, and the intermediate transfer belt 101. At this time, the toner image on the intermediate transfer belt is melted by the heat of the sheet and is simultaneously pressurized by the nip, thereby transferring onto and fixed on the sheet.
As shown in
A stack of sheets in a feed tray 216 is fed by a feed unit 217. The sheet fed from the feed tray 216 is conveyed by a pair of conveyor rollers 218 arranged on a sheet-conveying path and fed to the transfer-fixing unit 220 by a pair of registration rollers 219.
The toner image conveyed on the intermediate transfer belt 201 is transferred from the intermediate transfer belt 201 to the second intermediate transfer member 213. The secondarily transferred toner image is melted on the second intermediate transfer member 213 heated by the heater 215, pressed at the nip formed by the second intermediate transfer member 213 and the pressure roller 214, and transferred onto and fixed on the sheet.
The second intermediate transfer member is not limited to a roller shape in the illustrated example, and can be a belt shape. Also for the heating unit, an arbitrary heating unit can be used, such as a halogen heater, a ceramic heater, or an induction heater can be used, and the format and method are not limited. Further, the format and method of the pressurizing unit are not limited to the illustrated example.
Explained below is a case that the above embodiments are applied to a secondary transfer unit of the copier shown in
As shown in
A drive roller 328a of the registration roller 328, an opposite registration roller 328b, which is the driven roller, or the bypass feed roller 329 can be the roller whose speed is to be measured. For these rollers, however, to convey the sheet from a stopped state, starting time of the rollers and the driving source or a drive current value at the time of startup needs to be measured at the time of calculating the control target value. Therefore, it is desired to use the separation roller 337, into which the sheet enters during rotation thereof, as the roller whose speed is to be measured. Further, when there is a roller in the same state as the separation roller between the feed roller and the registration roller, the rotation speed of the roller can be measured.
An entrance detector for predicting that the sheet P enters into the secondary transfer unit formed of the third support roller 304 (drive roller) and the belt support roller 317 (driven roller) predicts the entering of the sheet based on a detection signal from a sheet detection sensor 383 arranged between the registration roller 328 and the secondary transfer unit. When the sheet detection sensor 383 is not used, an operation signal of the sheet conveying unit, such as an operation start signal of the registration roller 328 or an ON signal of a registration clutch is used for detecting the sheet or speed fluctuation information of the separation roller 337 can be used to detect the sheet.
When control is performed not only when the sheet enters into but also leaves the secondary transfer unit, a length detection sensor 384 that detect the sheet length needs to be installed. The type of the sensor to be used is the same as that explained in the first embodiment. As explained in the first embodiment, when the process in the calculating unit is performed in a sufficiently early stage, the sheet detection sensor 383 can also serve as the length detection sensor 384. The control is performed basically in the same manner as in the first embodiment, and the same explanation is not repeated.
Explained below is a case that the above embodiments are applied to the transfer-fixing unit of the image forming apparatus shown in
Explained below is a case that the above embodiments are applied to a fuser of the copier shown in
The speed of the third support roller 304 (drive roller) of the secondary transfer unit is measured by a speed detector 386 to obtain a control target value. Other than the third support roller 304, the roller whose speed is to be measured can be any roller of the belt support roller 317 (driven roller), the first support roller 302, the second support roller 303, and the intermediate transfer belt 301. The measuring unit used here is the same as the unit of the sheet conveying device according to the first and second embodiments.
The entrance detector for predicting the entering of the sheet P into the fuser including the fuser roller 306 and the pressure roller 307 predicts the entering of the sheet based on a detection signal from a sheet detection sensor 385 installed between the fuser and the secondary transfer unit. When the sheet detection sensor 385 is not used, the sheet can be detected by using an operation signal of the secondary transfer unit or the speed information of the secondary transfer unit. Further, the operation signal of the sheet conveying unit, such as the operation start signal of the registration roller 328 or the ON signal of the registration clutch is used for detecting the sheet or speed fluctuation information of the separation roller 337 can be used to detect the sheet.
Further, when control is performed as well when the sheet is separated from the secondary transfer unit, a sheet length detector is required. The length detection sensor 384 can be used as the sheet length detector. As explained in the first embodiment, when the process in the calculating unit is performed in a sufficiently early stage, the sheet detection sensor 385 can also serve as the length detection sensor 384. However, in the fuser, the necessity for performing the control at the time of sheet separation is low. The control operation is performed basically in the same manner as in the first embodiment, and the same explanation is not repeated.
Explained below is a case that the above embodiments are applied to both the secondary transfer unit and fuser of the copier shown in
In this case, both the secondary transfer unit and fuser are formed as shown in
When the distance between the secondary transfer unit and the fuser is close to each other and the processing speed of the calculating unit is sufficiently high, the function of the sheet detection sensor 383, the sheet detection sensor 385, and the length detection sensor 384 can be performed only by the sheet detection sensor 383. Other configurations in this example are the same as the configuration shown in
Generally, the bypass tray is used for a sheet with a certain level of thickness, as explained in
In the copier in this example, as explained in the first and second embodiments, the normal mode the control target value is calculated based on one piece of speed information, and the highly accurate mode in which the highly accurate control target value is calculated based on pieces of speed information can be selectively provided. Further, when it is known beforehand that the same sheets are continuously used, the changeover switch or the mode selection unit is provided so that the control target value calculated for the first sheet can be repetitively used, and it can be changed over whether to calculate the target value every time or only for the first time, according to the selected mode. The changeover switch or the mode selection unit can be provided in the calculating unit of the copier.
As described above, according to the embodiments, fluctuation of speed of a pair of rollers can be controlled when a thick sheet enters into or leaves the secondary transfer unit, the fuser, or the transfer-fixing unit. Because fluctuation of speed of the rollers in the secondary transfer unit is suppressed, fluctuation of speed of the intermediate transfer belt 301 can be prevented, and image distortion in the primary transfer unit, for example, an out of color registration of the respective color images can be efficiently prevented. As a result, a high-quality full-color image can be obtained. Further, because fluctuation of speed of the rollers in the fuser is suppressed, image distortion such as blur in an unfixed toner image in the secondary transfer unit on the upstream side can be prevented. Further, because fluctuation of speed of the rollers in the transfer-fixing unit is suppressed, fluctuation of speed of the intermediate transfer member can be prevented, and image distortion occurring in the primary transfer unit or the secondary transfer unit can be also prevented, thereby enabling to obtain the high-quality full-color image.
While, in the above embodiments, the number of sheet conveying units of the sheet conveying device is explained as two pairs, by way of example and without limitation, the sheet conveying device can include three or more pairs of sheet conveying units. The sheet conveying unit can include an endless belt, and the endless belt can be arranged either on the drive side or the driven side. The speed measuring unit that obtains the speed information of the sheet conveying unit can adopt an appropriate method or configuration. A drive system that drives the sheet conveying unit has arbitrary configuration. The calculation method of the control target value and the procedure of converting the obtained control target value to the control command value are described by way of example only.
In addition, the image carrier (photosensitive element) is not limited to a drum shape, and a belt-shaped image carrier can also be used. The configuration of the imaging unit need not necessarily be as described above, and arrangement sequence of the imaging units of respective colors in the tandem system can be changed. Further, the configuration is not limited to the tandem system, and a configuration in which a plurality of developing devices is arranged around one photosensitive element or a configuration of using a revolver-type developing device can be also used. The image forming apparatus of the embodiments can be a full-color machine using three color toners, a multi-color machine using two color toners, or a monochrome machine. When the intermediate transfer member is used, not only an indirect transfer method but also a direct transfer method can be used. The image forming apparatus is explained above as a copier; however, it can be, for example, a printer, a facsimile machine, a scanner or a multifunction product (MFP) that combines any or all of functions of these.
While, in the above embodiments, the sheet conveying device is applied to an image forming apparatus, it can also be applicable to any devices that convey a sheet-type medium, for example, a reading device such as a scanner, an ADF, or the like. Such a scanner or an ADF can be incorporated in an image forming apparatus.
As set forth hereinabove, according to an embodiment of the present invention, fluctuation in moving speed of the sheet conveying unit can be suppressed, and the moving speed can be maintained constant. Thus, high-quality image output can be achieved.
Moreover, a control target value can be obtained with high accuracy every time a sheet passes. This reduces memory capacity required for storing the control target value as well as enabling appropriate control for any type of recording medium regardless of the thickness and width thereof and use environment.
Furthermore, the same process is not repeated to simplify the control operation, which reduces wasteful power consumption.
Although the invention has been described with respect to specific embodiments for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.
Number | Date | Country | Kind |
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2007-074551 | Mar 2007 | JP | national |
2007-288547 | Nov 2007 | JP | national |