Image forming apparatus

An image forming apparatus includes an image forming unit; a conveyance unit; a motor; a stop order unit that orders stoppage of the motor; a detection unit, which detects a rotated number of the motor or a conveyance distance during a post-order period, and the post-order period being a period from when the stop order unit ordered stoppage of the motor until when the motor stops; a storage unit; a sensor, which detects passage of the recording medium; a timing determination unit, which determines a pre-order period as a appropriate order timing, wherein the pre-order period is calculated from dividing a difference between a first distance and a second distance by a conveyance speed, wherein the first distance is the conveyance distance stored in the storage, and the second distance is a distance between a position where the sensor detects passage of the recording medium and the target stoppage position.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority from Japanese Patent Application No. 2009-116637 filed on May 13, 2009 the entire subject matter of which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to an image forming apparatus having a image forming unit that forms an image on a recording medium and, more specifically, to an image forming apparatus that conveys the recording medium through the image forming unit by a conveyance unit activated by a motor.

BACKGROUND

Because properties of the motor or the load of the motor has varied or a load torque of the motor is fluctuated by aging change temperature change, and the like, a known image forming apparatus can not controls accurately a stoppage position of the recording medium,

SUMMARY

Even if properties of the motor or the load of the motor has varied or a load torque of the motor is fluctuated by aging change, temperature change, and the like, the image forming apparatus controls accurately a stoppage position of the recording medium.

DETAILED DESCRIPTION

According to one exemplary embodiment of the invention, an image forming apparatus comprising: an image forming unit that forms an image on a recording medium; a conveyance unit that conveys the recording medium passing through the image forming unit; a motor that activates the conveyance unit; a stop order unit that orders stoppage of the motor; a detection unit, which detects a rotated number of the motor or a conveyance distance during a post-order period, and the post-order period being a period from when the stop order unit ordered stoppage of the motor, which rotate at a constant speed rotation, until when the motor stops; a storage unit, which stores the rotated number or the conveyance distance detected by the detection unit; a sensor, which is disposed in a conveyance path of the recording medium, and which detects passage of the recording medium; and a timing determination unit, which determines a pre-order period as a appropriate order timing for stoppage of the recording medium at a target stop position, wherein the pre-order period is calculated from dividing a difference between a first distance and a second distance by a conveyance speed during the constant speed rotation of the motor, wherein the first distance is one of either the conveyance distance stored in the storage unit or a second conveyance distance corresponding to the rotated number stored in the storage unit, and wherein the second distance is a distance between a position where the sensor detects passage of the recording medium and the target stoppage position for the recording medium.

Accordingly, the image forming unit forms an image on the recording medium when the conveyance unit activated by the motor conveys the recording medium through the image forming unit.

An embodiment of the present invention is described by reference to the drawings as follows. As shown inFIG. 1, a laser printer1(as an example of a image forming apparatus) is an electro-photographic laser printer forms an image on a sheet3(as an example of a recording medium) by developer of nonmagnetic mono-component. The laser printer1includes a feeder unit4that feeds sheets3and a printing unit5(as an example of an image forming unit) for forming an image on the sheet3, and both units is accommodated in a main casing (not shown).

The feeder unit4includes a sheet feeding tray6, which is removably attached to a bottom in the main casing, and a sheet feeding roller7, which is provided at upper end on one side of the sheet feeding tray6. The sheet feeding tray6has a box shape whose top face is opened so as to stack the sheet3. The sheet3housed in the sheet feeding tray6is hold up by a sheet pressure plate (not shown) and is conveyed sheet by sheet by the sheet feeding roller7. The sheet3conveyed by the sheet feeding roller7is turned along a conveyance path100by unit of a guide (not shown) and is conveyed to a registration roller9disposed at positions above the sheet feeding tray6.

The registration roller9is configured a pair of rollers and feed the sheet3to a printing position at predetermined timing based on the detection of the sheet3by a pre-registration sensor55(to be described later). The printing position, where a toner image on a photosensitive drum13(to be described later) is transferred onto the sheet3, is a transfer position in the embodiment, where the photosensitive drum13(to be described later) contacts a transfer roller14.

The printing unit5is configured as a well-known electrophotographic printer engine including the photosensitive drum13, the transfer roller14, and a fixing unit15. First, a surface of the photosensitive drum13is rotated and is positively electro-statically charged uniformly by a scorotoron charger, or the like, (not shown). Next, the photosensitive drum13is exposed by a high-speed scan of a laser beam emitted from a scanner unit (not shown), and an electrostatic latent image is formed based on an image data. A positively electro-statically charged toner supplied from a developing cartridge (not shown) held to an electrostatic latent image formed on the surface of the photosensitive drum13. Namely, an exposed area on the uniformly and positively electro-statically charged surface of the photosensitive drum13, where an electric potential is reduced by exposing of the laser beam. Thus, the electrostatic latent image is visualized as a toner image, and a reversal development is formed.

The transfer roller14is disposed below and opposite the photosensitive drum13. During transfer of the toner image, a predetermined transfer current is applied to the transfer roller14by constant current control. The toner image held on the surface of the photosensitive drum13is transferred onto the sheet3by the transfer current during the sheet3conveyed from the registration rollers9passing between the photosensitive drum13and the transfer roller14.

The sheet3transferred toner image is conveyed toward the fixing unit15. The fixing unit15includes a heating roller16and a pressure roller17. The heating roller16includes a halogen lamp provided as a heater in a metal pipe. The pressure roller17is disposed below and opposite the heating roller16such that press the heating roller16from below. Therefore, the toner image on the sheet3conveyed to the fixing unit15is thermally fixed during passing between the heating roller16and the pressure roller17, and subsequently conveyed toward a sheet output roller19(as an example of conveyance unit).

The sheet output roller19is configured a pair of rollers, which sandwich and convey the sheet3, and is rotatable in both forward and backward directions depending on forward rotation and backward rotation of a motor70(to be described later). The sheet output rollers19forwardly rotate and output the sheet3fed from the fixing unit15to the sheet output tray (not shown). The sheet output rollers19backwardly rotate and convey a rear side of the sheet3to the printing unit5again. The rear side of the sheet3is a rear side in a conveyance direction by the sheet feeding roller7.

A re-conveyance roller21(as an example of re-conveyance unit) is provided between the printing unit5and sheet feeding tray6, and configures a plurality of pairs of rollers. A re-conveyance roller21conveys sheet3conveyed by the backward rotation of the sheet output rollers19to the rear side was conveyed to the position of the registration rollers9. A flapper23is disposed between the fixing unit15and the sheet output rollers19for switching the conveyance path100for the sheet3between a route extending from the fixing unit15toward the sheet output rollers19and a route extending from the sheet output rollers19to the re-conveyance rollers21. The flapper23is supported swayable in the main casing and can selectively switch the conveyance path100for the sheet3to any of the routes by excitation or non-excitation of a solenoid (not shown).

When the images are formed on both sides of the sheet3, the flapper23is first switched to a direction in which the sheet3is conveyed to the sheet output rollers19, and the sheet3is conveyed in an upward direction ofFIG. 1by the forwardly-rotating sheet output rollers19. Next, the sheet output rollers19are backwardly rotated at timing when the rear end of the sheet3has passed by the flapper23, and the flapper23is switched to a direction in which the sheet3is delivered to the re-conveyance roller21.

As a consequence, the rear end side of the sheet3is conveyed by the re-conveyance rollers21to the printing unit5by way of the registration rollers9while turned inside out. A reverse face of the sheet3conveyed to the printing position is opposing contact with the photosensitive drum13. After a toner image has been transferred to the reverse face, the fixing unit15fixes the toner image and the images are formed on both sides. As shown inFIG. 1, a sheet rear end sensor53that detects presence or absence of the sheet3is provided at a downstream position with respect to the sheet feeding roller7in the direction of conveyance of the sheet. The pre-registration sensor55and a post-registration sensor56that detect presence or absence of the sheet3are provided at backward and forward to the registration rollers9. A sheet output sensor57(as an example of sensor) for detecting presence or absence of the sheet3is provided between the fixing unit15and the flapper23.

FIG. 2is a block diagram showing the configuration of a control system of the motor70. The motor70is configured as a well-known DC motor having a rotor and a stator. The motor70includes a FG pattern, which induce an inductive voltage having 45 pulses per a rotation of the rotor (a so-called FG pattern signal), or a hole element, which outputs a hole signal having one pulse per rotation of the rotor. The inductive voltage or the hole signal is input to an encoder76provided in a motor driver75, and the encoder76encodes the voltage or the signal to waveform shaping and generates a digital signal.

An output from the encoder76is input to a speed control unit81provided in an ASIC80and a rotated number count unit82, which is provided in the ASIC80, and which includes a counter, or the like. The ASIC80includes a CPU83, which performs various arithmetic processing operations, and the CPU83is coupled to the speed control unit81and the rotated number count unit82via a bus84. The bus84is connected with SDRAM85(as an example of storage unit) and a timer86, and coupled various sensors, such as the sheet output sensor57, via a sensor interface (sensor IF)87. An interrupt request, or the like, issued by the timer86is input to the CPU38via the bus84. A detection signal from the sheet output sensor57, or the like, is also input to the CPU38via the bus84.FIG. 2shows flows of the signals by narrow arrows. The ASIC80is further configured to output a drive signal (a well-known CW/CCW and a speed control command) to the motor70.

Controls by the ASIC80are now described by reference to the flowchart shown inFIG. 3AandFIG. 3B. This processing is performed at power-on time of the laser printer1but may also be performed at the time of resetting, or the like, of the laser printer1. As shown inFIG. 3A, when processing is started, the motor70is first (forwardly) rotated at preset, given speed in step S1(as an example of a initialization processing unit, reference symbol S denotes s step; the same also applies to corresponding explanations). Thus initialization processing (so-called idling) for driving the photosensitive drum13, and the like, is executed. During initialization processing, the sheet output rollers19, and the like, coupled to the motor70are also rotated via a well-known gear mechanism. However, since the sheet feed roller7is coupled to the gear mechanism via an electromagnetic clutch, the sheet feed roller7is held at a stoppage. Therefore, the sheet3is not conveyed. During initialization processing, the motor70is ascertained to be rotating at the predetermined speed.

When initialization processing of S1ends, the processing proceeds to S2(as an example of stop order unit), and a motor stop command and a motor rotated number measurement command are issued. When the motor stop command is issued, a speed control command for the motor70is set to zero. However, the motor70stops after having rotated to some extent by inertia rather than stopping immediately. Accordingly, in this step, the motor rotated number measurement command is issued simultaneously with issuance of the motor stop command. When the rotated number measurement command is issued, the ASIC80starts measuring a rotated number of the motor70in another routine on the basis of the number of counts of the rotated number count unit82.

In S3, the processing waits until the motor70stops (No in S3). When the motor70stopped (Yes in S3), the processing proceeds to S4. In S4(as an example of detection unit), the rotated number measured from when the motor stop command is issued in S2until when the motor70stopped (Yes in S3) is converted into a conveyance distance X1(as an example of a first distance) over which the sheet output rollers19conveyed the sheet3, on the basis of the rotated number of the motor70measured at the time. The distance is saved (stored) in the SDRAM85(as an example of a storage unit).

Specifically, as shown inFIG. 4, when a processing status of the ASIC80(the system status) shifts to starting of initialization processing pertaining to S1, after a while, the motor70reaches a constant-speed rotating state in which the motor rotates at the predetermined speed. The rotated number count unit82measures the rotated number of the motor70from when the motor stop command is issued after completion of initialization processing (S2) until when the motor70stops. In S4, the conveyance distance X1of the sheet3corresponding with the measured rotated number is saved in the SDRAM85. The load on the motor70is also checked (not shown) as being well known during the course of processing pertaining to S1to S3

As shown inFIG. 3A, in S5(as an example of an anomaly determination unit), which is subsequent to S4, whether or not the conveyance distance X1saved in S4is smaller than a conveyance distance X2(as an example of a second distance) of the sheet3measured from when the rear end of the sheet3passed by the sheet output sensor57to when the rear end of the sheet3comes to a predetermined position between the flapper23and the sheet output rollers19. The predetermined position is a stop position for the sheet3that is suitable for conveying the sheet3toward the re-conveyance rollers21by swaying the flapper23and the backwardly rotating the sheet output rollers19. When X2is equal to or smaller than X1(No in S5), the rear end of the sheet3cannot be stopped at the predetermined position even when the motor stop command is issued immediately after the rear end of the sheet3has passed by the sheet output sensor57. Accordingly, in such a case (No in S5), the processing proceeds to S6, and well-known error processing is performed and the motor stops.

Meanwhile, when X2is more than X1(Yes in S5), the processing proceeds to S7. In S7, processing waits until a print command is received (No in S7) from an external device, such as a personal computer. When the print command is received (Yes in S7), the processing proceeds to S8inFIG. 3B. In S8, the motor70is rotated at the predetermined speed, and the sheet3is conveyed to the printing unit5by the sheet feeding roller7, or the like. In subsequent S9, a drive signal is output to the scanner unit, or the like, and the printing unit5performs print processing (the image forming operation).

When the print processing is completed by S9, the motor70is rotated at the predetermined speed and activate the heating roller16and the sheet output rollers19of the fixing unit15in S10. The sheet3is conveyed at conveyance speed V2corresponding to the predetermined speed. In S11, it is determined whether or not the rear end of the sheet3passed by the sheet output sensor57. When the rear end of the sheet3has not yet passed by the sheet output sensor57(No in S11), the processing return to S10and the conveyance of the sheet3is continually carried out. When the rear end of the sheet3passed by the sheet output sensor57(Yes in S11), the processing proceeds to S12.

In S12, the motor70is rotated at constant speed for only a period of time calculated by an expression (X2−X1)/V2, and the processing proceeds to S13. A calculation processing on (X2−X1)/V2in S12correspond to an example of timing determination unit.

In S13subsequent to S12, it is determined whether or not double-sided printing is ordered and whether or not double-sided printing is not completed (namely, printing of only one side has finished). When double-sided printing is ordered and when double-sided printing has not completed (Yes in S13), the processing proceeds to S14(as an example of stop order unit) and the motor stop command is immediately issued. As mentioned previously, the rear end of the sheet3is stopped at the predetermined position where the rear end of the sheet3reaches as result of being additionally conveyed over the conveyance distance X2after passed through the sheet output sensor57(Yes in S11).

In S15, the processing waits until the motor70stops (No in S15). When the motor70stopped (Yes in S15), the processing proceeds to S8. The flapper23is then swayed, and the motor70is also backwardly rotated. As a result, the sheet3is conveyed via the re-conveyance rollers21. The motor70is switched to forward rotation at appropriate timing, whereby the sheet3is conveyed to the printing unit5while turned inside out. By the processing of S9to S12, the reverse side of the sheet3also undergoes printing. When the processing proceeds to S13, the sheet has finished undergoing double-sided printing (No in S13) and the processing proceeds to S18. When single-sided printing is ordered at first (No in S13), the processing does not proceeds to S14, or the like, and the processing proceeds to S18after a determination is made S13in first.

In S18, the motor stop command is issued to the motor70. The motor stop command is issued after a lapse of a little time since processing shifted to S18rather than being issued immediately. The sheet3passes by the sheet output rollers19by inertial rotation of the motor70, or the like, and exit to the sheet output tray (S19), and processing temporarily ends. The ASIC80does not substantially perform the processing of S19shown inFIG. 3B. The processing of S19represents sheet output operation as operation of the laser printer1.

As mentioned above, in the present embodiment, the rotated number measured from a point in time (S2) when the motor stop command is issued after completion of initialization processing (S1) until when the motor70actually stops (Yes in S3) is converted into the conveyance distance X1over which the sheet output rollers19conveyed the sheet3, and the distance is saved (S4). Motor stop command issuance timing in double-sided printing is determined by the conveyance distance X1(S12and S14). Accordingly, even if properties of the motor70or the load of the motor70has varied or a load torque of the motor70is fluctuated by aging change, temperature change, and the like, the sheet3can accurately be stopped at the predetermined position as follows.

As shown in item B and C ofFIG. 5, there are two systems; a system1and a system2, in which values saved in S4as the conveyance distance X1differ from each other for reasons of the variations in properties, and secular change, temperature change, and the like, as mentioned previously. In the case of a system1, the time calculated by the foregoing expression (X2−X1)/V2is denoted by T1shown in item A ofFIG. 5. In the case of a system2, the time calculated by the foregoing expression (X2−X1)/V2in connection with the system2is denoted by T2in item A ofFIG. 5.

Therefore, as shown in item A ofFIG. 5, print processing (S9) has started, the motor70comes into a constant rotating state in which the motor rotates at the predetermined speed (corresponding to the sheet conveyance speed V2). In the case of the system1, the motor stop command is issued after a lapse of time T1since the rear end of the sheet3passed by the sheet output sensor57(Yes in S11). And in the case of the system2, the motor stop command is issued after a lapse of time T2since the rear end of the sheet3passed by the sheet output sensor57(Yes in S11). The timing for issuing the motor stop command changes between the system1and the system2according to the conveyance distance X1that is changed by the characteristics, or the like. The position of the sheet3achieved at the time of stoppage of the motor70can be uniformed respectively for the system1and the system2. Accordingly, in the embodiment, the sheet3can reliably be sent to the re-conveyance rollers21during double-sided printing.

In the embodiment, when X2is equal to or smaller than X1(No in S5) in the motor70stopped after completion of initialization processing (Yes in S3), the processing proceeds to error processing (S6). Therefore, an anomaly in the drive system, such as the rollers19, can be determined before starting of print processing.

The present invention is not limited to the embodiment mentioned above, and the like, and can be modified in various forms without departing the substance of the present invention. For instance, in the embodiment, when the motor stop command is issued (S2) after completion of initialization processing (S1), the conveyance distance X1is detected. However, the conveyance distance X1detected when the motor stop command is issued after completion of previous print processing can also be utilized. The processing for detecting the conveyance distance X1may also be performed special processing. When the conveyance distance X1is detected after completion of initialization processing as in the above embodiment, the processing can be made efficient in both a time and power conservation.

In the embodiment, the SDRAM85stores the conveyance distance X1. However, the rotated number detected via the rotated number count unit82for the purpose of detecting the conveyance distance X1can also be saved. In this case, if the rotated number is converted into the conveyance distance X1in S12, the processing similar to the embodiment can be performed. Moreover, in the embodiment, the present invention is applied to controlling a stop position for the sheet3being reconveyed during double-sided printing, but the present invention can also be applied to various control operations, such as operation for controlling stoppage of the sheet3on a sheet-feeding side prior to the printing unit5. If the position where the sheet3is to be stopped when reconveyed cannot accurately be controlled, a problem will arise in double-sided printing, such as the sheet3being snagged on the flapper23. Accordingly, when the stop position for the sheet3is controlled during double-sided printing as mentioned in connection with the embodiment, the advantage of the present invention is exhibited much noticeably.