US20060202408A1

US20060202408A1 - Image forming apparatus, sheet feeding method

Info

Publication number: US20060202408A1
Application number: US11/260,459
Authority: US
Inventors: Kazumasa Yasui; Masaya Arakawa
Original assignee: Toshiba Corp; Toshiba Tec Corp
Current assignee: Toshiba Corp; Toshiba Tec Corp
Priority date: 2005-03-10
Filing date: 2005-10-28
Publication date: 2006-09-14
Also published as: JP2006248678A; JP4468844B2

Abstract

The present invention provides an image forming apparatus, a sheet feeding method, and a sheet feeding program that can prevent adverse effect brought about when the temporary stop position of a sheet deviates from a predetermined stop position. The present invention provides an image forming apparatus which includes a drive control unit 101 that controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed, and a retention state information take-in unit 102 that takes in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily, wherein the drive control unit 101 resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit 102.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to an image forming apparatus and a sheet feeding method.
2. Description of the Related Art
Conventionally, in an image forming apparatus, skew correction and timing adjustment (or so-called aligning) for sheets being fed are carried out by a regist roller, etc. (refer to Jpn. Pat. Appln. Laid-Open Publication No. 11-184191). FIG. 6 shows a cross-sectional view indicative of the entire configuration of a conventional image forming apparatus.
In the image forming apparatus shown in FIG. 6, the following processing is performed. Firstly, sheets stocked in cassettes 31, 32, 33, 34 are picked up by pickup rollers 31 b, 32 b, 33 b, 34 b. Then, thus picked up sheets are separated to respective papers one by one by sheet supply rollers 31 a, 32 a, 33 a, 34 a and separation rollers 31 c, 32 c, 33 c, 34 c to be supplied to a sheet feeding path 35. Sheets fed to the sheet feeding path 35 are carried forward right before an intermediate feeding roller 38 by feeding rollers 36 a to 36 d. The intermediate feeding roller 38 is provided with a function of guiding sheets fed by an ADU (Automatic Duplexing Unit) 5, to be described later, or sheets fed along the sheet feeding path 35 to a regist roller 37 for performing skew correction and timing adjustment for sheets.
After undergoing skew correction and timing adjustment, toner images formed on the photosensitive surface of a photoconductor drum 20 are transcribed to sheets guided to the regist roller 37 through the intermediate feeding roller 38. Toner images transcribed to the sheets are fixed on the sheets by a fixing section 24. Sheets having images fixed thereon are directly discharged to the outside of the image forming apparatus in case of one-side printing, and are guided to a reverse feeding path 28 in case of duplex printing or in case sheets that have undergone one-side printing are discharged after being reversed.
In case of duplex printing, sheets directed to the reverse feeding path 28 are made to switchback to be fed by reverse rollers 29 and 30 to be sent to an ADU 5. Sheets sent to the ADU 5 are fed by feeding rollers 5 a to 5 d, and are directed to the intermediate feeding roller 38 again. Then, image forming processing by the photoconductor drum 20 and fixing section 24 are carried out for the other sides of sheets fed through the ADU 5, realizing duplex printing for sheets.
FIG. 7 shows a view for explaining the sheet feeding right before the regist roller 37, which is an enlarged view of part enclosed by a dotted line in FIG. 6.
As shown in FIG. 7, in the image forming apparatus of above-described configuration, in case of performing image forming processing for a plurality of sheets consecutively, that is, in case of feeding a plurality of sheets to the regist roller 37 sequentially, when a preceding sheet P1 is aligned by the regist roller 37, a succeeding sheet P2 is made to stand by temporarily at a predetermined position S right before the intermediate feeding roller 38 until a start command is issued. Similarly, a sheet fed to the regist roller 37 again through the ADU 5 is made to stand by temporarily at a predetermined position right before the intermediate feeding roller 38.
In case of performing image forming processing for sheets supplied from the plural cassettes 31 to 34 arranged on different positions on the sheet feeding route, the route on which sheets are fed to above-described predetermined position varies depending on the position of the cassettes 31 to 34. This means that, in case the cassette for supplying sheets is different, the feeding roller (including the sheet supply roller) through which sheets are fed varies.
Generally, under the environment of high temperature and high humidity, oil infiltrated to a bush of a bearing of a feeding roller (especially, a feeding roller using a clutch for transmitting driving force) is prone to steep, which reduces the rotation load of the feeding roller. This reduction of the rotation load leads to deterioration of the braking performance for sheets by a feeding roller, that is, when making a sheet stand by temporarily at above-described predetermined position, sometimes the sheet undesirably overruns the predetermined position (refer to a stop position S′ of the sheet P2 shown by a dotted line in FIG. 7). Furthermore, the braking performance to make a sheet stop at the temporary standby position depends on the number of rollers which apply braking force to the sheet.
In case there is raised a deviance (overrun) at the temporary standby position (temporary stop position) of a sheet, when the operation of feeding a sheet that is made to stand by temporarily is resumed, the sheet arrives at the regist roller 37 at the timing earlier than a regular timing. The misalignment in the arrival timing of a sheet to the regist roller 37 may undesirably lead to the malfunction of the regist roller 37, which may become the cause of sheet jam due to fold of a sheet.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to overcome the above-mentioned drawbacks by providing an image forming apparatus, a sheet feeding method, and a sheet feeding program that can prevent adverse effect brought about when the temporary stop position of a sheet deviates from a predetermined stop position.
According to the present invention, there is provided an image forming apparatus, including a drive control unit that controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed, and a retention state information take-in unit that takes in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily, wherein the drive control unit resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit.
According to the present invention, there is also provided an image forming apparatus, including a drive control unit that controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed, a retention state information take-in unit that takes in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily, and an environment detection unit that detects at least one of the temperature and humidity, wherein, in case at least one of the temperature and humidity detected by the environment detection unit goes beyond a predetermined threshold value, the drive control unit resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit.
According to the present invention, there is also provided a sheet feeding method, including a drive control step that controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed, and a retention state information take-in step that takes in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily, wherein the drive control step resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a block diagram for explaining the image forming apparatus in the present embodiment;
FIG. 2 shows a flow chart for explaining the sheet feeding method in the present embodiment;
FIG. 3 shows a flow chart for explaining detailed processing of the sheet feeding method in the present embodiment;
FIG. 4 shows a flow chart for explaining detailed processing of the sheet feeding method in the present embodiment;
FIG. 5 shows a timing chart indicative of drive timing of a regist roller 37, an intermediate feeding roller 38, and feeding rollers excluding the intermediate feeding roller 38 of the image forming apparatus in the embodiment;
FIG. 6 shows a cross-sectional view indicative of the entire configuration of a conventional image forming apparatus; and
FIG. 7 shows a view for explaining the sheet feeding right before a regist roller.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will further be described below with reference to the accompanying drawings.
FIG. 1 shows a block diagram for explaining the image forming apparatus in the present embodiment. The basic configuration of the sheet feeding route etc., within the image forming apparatus in the present embodiment is similar to that within the conventional image forming apparatus shown in FIG. 6 and FIG. 7. Therefore, parts or components similar to those shown in FIG. 6 and FIG. 7 are indicated with the same reference numerals, and detailed explanation of which will be omitted.
The image forming apparatus in the present embodiment includes, in addition to the configuration shown in FIG. 6, a drive control unit 101, a retention state information take-in unit 102, an environment detection unit 103, a CPU 104, and a MEMORY 105.
The drive control unit 101 is provided with a function of controlling the operation of driving feeding rollers (including the pickup rollers 31 b to 34 b, sheet supply rollers 31 a to 34 a, separation rollers 31 c to 34 c, feeding rollers 36 a to 36 d, feeding rollers 5 a to 5 d shown in FIG. 6 in this embodiment) for feeding sheets to a regist roller that performs skew correction for sheets being fed.
The retention state information take-in unit 102 is provided with a function of taking in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily. The information concerning the retention state includes information concerning the supply start position of the sheet to the feeding route (cassette of which stage the sheet is supplied from, whether or not the sheet is sent through an ADU), and information concerning the size of the sheet (A4, A3, etc.).
The environment detection unit 103 includes a temperature sensor and a humidity sensor, and detects at least one of the temperature and humidity.
The CPU 104 is provided with a function of carrying out various processing in the image forming apparatus, and is also provided with a function of realizing various functions by executing programs stored in the MEMORY 105. The MEMORY 105 may be a ROM, a RAM, etc., and is provided with a function of storing various information and programs to be utilized in the image forming apparatus.
The drive control unit 101 resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit 102 in case at least one of the temperature and humidity detected by the environment detection unit 103 goes beyond a predetermined threshold value. Details of controlling the operation of driving the feeding rollers by the drive control unit 101 will be explained later.
FIG. 2 shows a flow chart for explaining the sheet feeding method in the present embodiment.
The drive control unit 101 controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed (drive control step) (S801).
The retention state information take-in unit 102 takes in information concerning the retention state (information concerning the supply start position of a sheet to the feeding route and information concerning the size of a sheet) of a sheet that is retained by the feeding rollers to be made to stand by temporarily (retention state information take-in step) (S802).
The environment detection unit 103 detects at least one of the temperature and humidity (environment detection step) (S803). Any of the retention state information take-in step (S802) and the environment detection step (S803) can be performed first so long as both the steps are performed prior to drive control step (S804) to be described later.
The drive control unit 101 resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step in case at least one of the temperature and humidity detected by the environment detection step goes beyond a predetermined threshold value (drive control step) (S804). On the other hand, the processing is not restricted to this, and the operation of feeding a sheet can be resumed under above-described predetermined timing regardless of the temperature and humidity.
In the drive control step, when resuming the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step, it is desired that the predetermined timing be delayed in case the number of feeding rollers which retain a sheet in the temporary standby state is small.
Generally, in case of temporarily stopping a sheet being fed, it is considered that the larger the number of feeding rollers which retain a sheet in the temporary stop state is, the larger the braking force becomes. Accordingly, it is presumed that the smaller the number of feeding rollers which retain a sheet in the temporary stop state is (the smaller the braking force is), the larger the value overrunning a predetermined temporary stop position S becomes. Thus, it becomes possible to make the leading edge of a sheet arrive at the regist roller under the regular timing by delaying the timing of resuming the sheet feeding from the temporary standby state.
Furthermore, in case the posterior edge of a sheet is retained by sheet supply rollers and separation rollers whose retention torque is large in the temporary stop state (it is presumed from the sheet size and a cassette for supplying sheet), it is considered that the sheet rarely overruns the predetermined temporary stop position. Accordingly, in this case, it is not necessary to delay the timing of resuming the sheet feeding from the temporary standby state.
In addition, in the drive control step, when resuming the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step, the predetermined timing can be delayed by a period of time longer than 0 msec and shorter than 60 msec from a predetermined reference timing in case the number of feeding rollers which retain a sheet in the temporary standby state is smaller than a predetermined number of rollers.
Furthermore, in case the size of a sheet in the temporary standby state is large (for example, A3 size), it is presumed that the number of feeding rollers which retain the sheet becomes necessarily large. Accordingly, the timing of resuming the sheet feeding is not made to delay from the predetermined reference timing (predetermined timing that can keep a suitable spacing between the leading edge of a sheet to be fed again and the posterior edge of a preceding sheet). On the other hand, in case the size of a sheet in the temporary standby state is small (for example, letter size), it is presumed that the number of feeding rollers which retain the sheet becomes necessarily small. Accordingly, the timing of resuming the sheet feeding is made to delay from the predetermined reference timing.
Moreover, in case the size of a sheet in the temporary standby state is large to some extent (for example, B4 size), and in case a cassette for supplying sheet is a lowermost cassette 34 that makes the sheet feeding distance to an intermediate feeding roller 38 long, the possibility that the pickup roller 34 b and the sheet supply roller 34 a whose braking force is large are in contact with the posterior edge of the sheet is presumed to be low. Accordingly, the timing of resuming the sheet feeding may be made to delay from the predetermined reference timing.
The intermediate feeding roller 38 of the image forming apparatus in the embodiment can feed sheets with speeds of at least three kinds or more. In this embodiment, the intermediate feeding roller 38 can be driven with a sheet supply speed, a process speed, and a duplex printing speed (referred to as ADU speed hereinafter).
Specifically, the sheet supply speed is a speed (for example, 420 mm/sec) at the time of feeding sheets from the cassettes to the regist roller, and the process speed is a speed (for example, 340 mm/sec) at the time when toner images are transcribed to sheets from a photoconductor drum 20, and the ADU speed is a speed (for example, 385 mm/sec) when sheets are fed from an ADU 5 to the intermediate feeding roller 38. In recent years, since it is desired that the sheet feeding efficiency and printing efficiency be improved, the sheet feeding speed is becoming high. Thus, in case of temporarily stopping sheets at the predetermined temporary stop position S, when the sheet feeding speed is high, overruns are prone to be raised frequently.
Accordingly, in case the sheet feeding speed right before being temporarily stopped is high, the possibility that sheets overrun the predetermined temporary stop position S is presumed to be high, and the timing of resuming the sheet feeding is made to delay. On the other hand, in case the sheet feeding speed right before being temporarily stopped is low, the possibility that sheets overrun the predetermined temporary stop position S is presumed to be extremely low, and the operation of feeding a sheet may be resumed under the predetermined reference timing.
Specifically, feeding rollers which resume the sheet feeding in the temporary standby state mean rollers including at least one of the feeding rollers 36 a to 36 d, pickup rollers 31 b to 34 b, and sheet supply rollers 31 a to 34 a. The number of feeding rollers which resume the sheet feeding may be increased or decreased according to need. For example, optimum sheet feeding can be realized by also changing the timing of resuming the sheet feeding using the feeding roller 36 d in addition to only controlling the timing of resuming the sheet feeding using the feeding roller 36 a.
As for the timing of resuming the sheet feeding, depending on the difference of the sheet supply speed and the process speed, for example, in case the sheet supply speed is 420 mm/sec and the process speed is 340 mm/sec, feeding a sheet is restarted when 100 msec elapses after a regist roller 37 is driven (corresponding to the predetermined reference timing) in case a sheet is fed from cassettes 31 and 32 or the temperature and humidity are in the ordinary state, while the restart timing of feeding a sheet is desired to be staggered by 30 msec from above-described reference timing in case a sheet is fed from cassettes 33 and 34.
The timing of resuming the sheet feeding can be changed by 10 msec unit using an adjustment code through an operation input unit, not shown, by a service personnel etc. In this case, information concerning thus changed restart timing can be stored in a storage area of the MEMORY 105 or an external equipment. Accordingly, the timing of resuming the sheet feeding can be set to an optimum value depending on the environment for use, configuration of device, etc.
FIG. 3 and FIG. 4 show flow charts for explaining detailed processing of the sheet feeding method in the present embodiment.
When image forming processing is started, the environment detection unit 103 takes in temperature data (S101).
In case thus taken in temperature is higher than predetermined temperature threshold value Tbase (° C.) (S102, No), the drive control unit 101 reduces feeding time Ttr1 in feeding a sheet to the temporary stop position S using the feeding rollers by α tr1 (msec), or delays timing Tw to resume the operation of feeding a sheet in the temporary stop state by α w (msec) (S103).
Next, the environment detection unit 103 takes in humidity data (S104).
In case thus taken in humidity is higher than predetermined humidity threshold value Tbase (%) (S105, No), the drive control unit 101 reduces feeding time Ttr1 in feeding a sheet to the temporary stop position S using the feeding rollers by β tr1 (msec), or delays timing Tw to resume the operation of feeding a sheet in the temporary stop state by β w (msec) (S106).
The drive control unit 101 makes the sheet feeding be performed with thus corrected sheet feeding time before temporary stop under the restart timing of feeding a sheet after temporary stop.
Next, the processing of resuming the sheet feeding will be explained using a flow chart shown in FIG. 4.
When aligning processing by the regist roller 37 is completed and sheet feeding by the regist roller 37 is started, the drive control unit 101 determines which cassette a sheet that is made to temporarily stop right before the intermediate feeding roller 38 is fed from (S201, S203, S205), and selects the timing of resuming the sheet feeding set up corresponding to the respective cassettes (S202, S204, S206, S207).
Then, the drive control unit 101 makes the sheet feeding by the feeding rollers stand by until the restart timing of feeding a sheet (timing delayed by Tw from the predetermined reference timing) selected in above-described step (S208), and resumes the operation of feeding a sheet in the temporary standby state (S209). In this embodiment, the restart timing is determined in S103, S106 shown in FIG. 3, and S202, S204, S206, S207 shown in FIG. 4.
In the present embodiment, the respective steps of the processing in the sheet feeding method are realized by making the CPU 104 execute a sheet feeding program stored in the MEMORY 105.
FIG. 5 shows a timing chart indicative of drive timing of the regist roller 37, intermediate feeding roller 38, and feeding rollers excluding the intermediate feeding roller 38 of the image forming apparatus in the embodiment.
A sheet that is fed from a cassette is carried forward to the regist roller 37 by the intermediate feeding roller 38 with the sheet supply speed of 420 mm/sec. Then, the sheet has its leading edge applied to a nip portion of the stopped regist roller 37 to be aligned. Afterward, the intermediate feeding roller 38 and the regist roller 37 feed the sheet with the process speed of 340 mm/sec.
Then, a succeeding sheet that is made to stand by to form an image on one side thereof is carried forward to the regist roller 37 by the intermediate feeding roller 38 with the sheet supply speed of 420 mm/sec. FIG. 5 shows the state in which the timing to resume the sheet feeding by the feeding rollers (which correspond to a feed clutch 1 and a feed clutch 2 for transmitting motive energy to the feeding rollers, in this embodiment) is delayed by Tw.
In case image forming processing for one sheet side is performed for three sheets (in FIG. 5, first sheet is omitted), a sheet having an image formed on one side thereof is reversed to be carried forward, and is fed with an increased speed after switchback, and, after the speed is lowered right before the ADU 5, is carried forward to the regist roller 37 with the ADU speed of 385 mm/sec, to form an image on the other side thereof having formed thereon no image.
In above-described embodiment, the function for implementing the present invention is stored inside an image forming apparatus in advance. On the other hand, the present invention is not restricted to this, that is, similar function may be downloaded to an image forming apparatus from a network, or a recording medium having stored therein similar function may be installed to an image forming apparatus. The recording medium may be of any figuration such as a CD-ROM so long as the recording medium can store programs and can be read out by an image forming apparatus. Furthermore, function obtained by previously performed installing and downloading may be realized cooperatively with the OS (operating system) in an image forming apparatus.
As in the above, according to the present embodiment, stable sheet feeding in view of factors bringing about variation of the temporary stop position of sheets such as cassette stage, oil of a bush, load of feeding rollers can be realized under the environment of high temperature and high humidity.
While the invention has been described in accordance with certain preferred embodiments in detail, it should be understood by those ordinarily skilled in the art that the invention is not limited to the embodiments, but various modifications, alternative constructions or equivalents can be implemented without departing from the scope and spirit of the present invention.
According to the present invention, it becomes possible to provide an image forming apparatus, a sheet feeding method, and a sheet feeding program that can prevent adverse effect brought about when the temporary stop position of a sheet deviates from a predetermined stop position.

Claims

1. An image forming apparatus, comprising:

a drive control unit that controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed; and

a retention state information take-in unit that takes in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily,

wherein the drive control unit resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit.

2. An image forming apparatus, comprising:

a drive control unit that controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed;

a retention state information take-in unit that takes in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily; and

an environment detection unit that detects at least one of the temperature and humidity,

wherein, in case at least one of the temperature and humidity detected by the environment detection unit goes beyond a predetermined threshold value, the drive control unit resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit.

3. The image forming apparatus according to claim 1, wherein the retention state information take-in unit takes in information concerning the supply start position of the sheet to the feeding route.

4. The image forming apparatus according to claim 1, wherein the retention state information take-in unit takes in information concerning the supply start position of the sheet to the feeding route, and information concerning the size of the sheet.

5. The image forming apparatus according to claim 1, wherein, when resuming the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit, the drive control unit delays the predetermined timing in case the number of feeding rollers which retain a sheet in the temporary standby state is small.

6. The image forming apparatus according to claim 1, wherein, when resuming the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in unit, the drive control unit delays the predetermined timing by a period of time longer than 0 msec and shorter than 60 msec from a predetermined reference timing in case the number of feeding rollers which retain a sheet in the temporary standby state is smaller than a predetermined number of rollers.

7. A sheet feeding method, comprising:

a drive control step that controls the operation of driving feeding rollers for feeding sheets to a regist roller that performs skew correction for sheets being fed; and

a retention state information take-in step that takes in information concerning the retention state of a sheet that is retained by the feeding rollers to be made to stand by temporarily,

wherein the drive control step resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step.

8. The sheet feeding method according to claim 7, further comprising:

an environment detection step that detects at least one of the temperature and humidity,

wherein, in case at least one of the temperature and humidity detected by the environment detection step goes beyond a predetermined threshold value, the drive control step resumes the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step.

9. The sheet feeding method according to claim 7, wherein the retention state information take-in step takes in information concerning the supply start position of the sheet to the feeding route.

10. The sheet feeding method according to claim 7, wherein the retention state information take-in step takes in information concerning the supply start position of the sheet to the feeding route, and information concerning the size of the sheet.

11. The sheet feeding method according to claim 7, wherein, when resuming the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step, the drive control step delays the predetermined timing in case the number of feeding rollers which retain a sheet in the temporary standby state is small.

12. The sheet feeding method according to claim 7, wherein, when resuming the sheet feeding by the feeding rollers under predetermined timing based on information concerning the retention state of a sheet that is taken in by the retention state information take-in step, the drive control step delays the predetermined timing by a period of time longer than 0 msec and shorter than 60 msec from a predetermined reference timing in case the number of feeding rollers which retain a sheet in the temporary standby state is smaller than a predetermined number of rollers.