Sheet transport apparatus and image forming apparatus

A sheet transport apparatus includes a fixed driving roller and a driven roller capable of coming into contact with or being separated from the driving roller, the driving roller and driven roller being capable of rotating and transporting a sheet interposed therebetween; a rotating-body acceleration sensor moving together with the driven roller and capable of detecting acceleration of movement of the driven roller; and a determining unit for determining, based on the acceleration detected by the rotating-body acceleration sensor, the arrival of a sheet at the driving roller and the driven roller, and the thickness of a sheet.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to co-pending application Ser. No. 11/225,274 filed on Sep. 13, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sheet transport apparatus for transporting sheets and an image forming apparatus having the sheet transport apparatus.

2. Description of the Related Art

Known image forming apparatuses for forming images on a sheet are provided with a sheet transport apparatus for transporting sheets. Examples of image forming apparatuses include copiers, printers, facsimiles, and multifunction machines combining the functions of copiers, printers, and facsimiles.

Some sheet transport apparatuses detect the thickness of a sheet. Such sheet transport apparatuses are provided with a sheet detector that detects, for example, a position where, in the image forming apparatus, a sheet is currently being transported and the thickness of the sheet.

For example, a sheet detector included in an electrophotographic-type copier detects the movement of a sheet fed from the sheet cassette, and allows the detected timing to be used as information for controlling the sheet transport apparatus and image forming section on the downstream side.

Known sheet detectors will be described below.

Examples of Known Sheet Detector Detecting Arrival of Sheet

A first example of a sheet detector detecting the arrival of a sheet is a photointerrupter sensor258shown inFIG. 10. The photointerrupter sensor258includes a rotatable flag251arranged in a position that blocks the sheet path, and a photointerrupter253detecting that detection light253ais intercepted. A spring252apresses the flag251into contact with a stopper252b.

When a sheet S is brought into contact with the flag251in the photointerrupter sensor258, the flag251is rotated about the rotation shaft251aand causes a light-shielding section251bto block the detection light253a. When the detection light253ais blocked, the photointerrupter sensor258emits an electronic signal based on the determination that the sheet S has arrived. The electronic signal is transmitted to a controller (not shown) that controls the entire image forming apparatus.

A second example of a sheet detector is a light transmission sensor260shown inFIG. 11. The light transmission sensor260detects a sheet when the sheet blocks the optical axis. The light transmission sensor260includes a light emitter260aemitting detection light260cand a light receptor260b. Unlike the photointerrupter sensor258inFIG. 10, the light transmission sensor260has no flag blocking the sheet path. This is advantageous in that the front edge of the sheet is not damaged even if the sheet is thin.

Examples of Known Sheet Detector Detecting Arrival and Thickness of Sheet

Electrophotographic-type image forming apparatuses often detect not only the movement of a sheet, but also detect the thickness of a sheet to control the operation of the image forming section. For example, in an electrophotographic-type image forming apparatus, which uses electric power to transfer toner to a sheet, it is desired that a voltage applied to the sheet be adjusted according to the thickness of the sheet.

The thickness information is also used to control the sheet transport mechanism. Before enabling the sheet transport mechanism to feed a sheet to the image forming section, the image forming apparatus brings the front edge of the sheet into contact with a resist roller at rest to correct the skew of the sheet, adjusts timing for starting the rotation of the resist roller, thereby adjusting timing for feeding the sheet to the image forming section. After bringing the sheet into contact with the resist roller, the image forming apparatus causes a transport roller, which allows a sheet to be fed into the resist roller, to rotate for a predetermined time (t) to create a loop in front of the resist roller. The force of the loop causes the front edge of the sheet to be reliably pressed against the resist roller, thereby allowing the skew of the sheet to be corrected. The time (t) is determined according to the thickness of the sheet. For example, for a thin sheet, the time (t) must be long enough to ensure the pressing force with which the sheet is pressed against the resist roller.

Since it is often required for image forming apparatuses to detect the thickness of the sheet, the following sheet detectors are proposed.

Referring toFIG. 12, a sheet detector281combines a sheet transport mechanism282and a contact-type probe sensor264. The sheet transport mechanism282is configured such that a sheet S is introduced into the nip point between a roller262aattached to a roller shaft263athat is vertically displaceable, and a roller262battached to a roller shaft263bthat is secured so as not to be vertically displaced. The sheet detector281uses the contact-type probe sensor264to measure the displacement of the roller shaft263a, the displacement being associated with the passage of the sheet S. This not only allows the detection of the arrival of the sheet S, but also allows the detection of the thickness of the sheet S. This configuration is disclosed in Japanese Patent Laid-Open No. 07-215538.

Similar to the sheet detector281inFIG. 12, a sheet detector283inFIG. 13measures the displacement of a roller271to detect the arrival and thickness of a sheet. The sheet detector283differs from the sheet detector281in that it has a sheet thickness sensor270using reflecting light270ato measure the displacement. The sheet detector283controls a transfer charging device274according to the thickness and electric resistance value of the sheet. The transfer charging device274transfers toner images on a photoconductive drum (not shown) onto the sheet. This configuration is disclosed in Japanese Patent Laid-Open No. 05-313516.

There is another proposed method to detect the displacement of a roller. In this method, a pressure sensor supported by an elastic member is pressed against a roller shaft, and a change in pressure is interpreted as the displacement of the roller. However, this method has problems in that the pressure sensor cannot easily detect the arrival of a thin sheet unless the spring constant of the elastic member is high enough, and that the nip pressure of the roller becomes unstable if the spring constant is too high.

The above-described sheet detectors that are proposed or already in practical use have problems described in the following (1) to (5). For example, known sheet detectors with such problems cannot easily transport a thin sheet, cannot be installed in a desired location, have a low accuracy in detecting the position or thickness of a sheet, and malfunction in the detection of the position or thickness of a sheet. Moreover, known image forming apparatuses having a sheet detector with these problems have a low accuracy in forming images on a sheet.

(1) The photointerrupter sensor258inFIG. 10may obstruct the transport of a thin sheet.

(2) Problems in installation space: In the photointerrupter sensor258, the rotation shaft251aof the flag251and the photointerrupter253must be placed close to the sheet paths. Business machines, which are typically required to be small in size, have many sections where a plurality of connected and crossed sheet paths are densely arranged. Such a section may not be able to provide enough space to accommodate the photointerrupter253. Similarly, installation space for the light transmission sensor260inFIG. 11, the contact-type probe sensor264inFIG. 12, and the sheet thickness sensor270inFIG. 13may not be large enough.

(3) Problems in Installation: Since it is required for the photointerrupter sensor258that the positional relationship between the rotation shaft251aand the photointerrupter253be kept constant, their attaching parts must be stable. If the rotation shaft251aand the photointerrupter253need to be attached to different members, instability of the attaching parts affects detection accuracy. The same applies to the light transmission sensor260inFIG. 11.

For the contact-type probe sensor264inFIG. 12and the sheet thickness sensor270inFIG. 13, an unstable positional relationship with respect to the respective rollers may cause detection errors. That is, instability of a base to which the sensor is attached causes detection errors.

(4) Problems of Dirt on Sensor: The light transmission sensor260inFIG. 11and the sheet thickness sensor270inFIG. 13may malfunction if the emitter or receiver of the detection light is soiled with paper dust from the sheet, abrasion dust and oil from the drive mechanisms, and the like.

(5) Problems of External Vibrations: The contact-type probe sensor264inFIG. 12and the sheet thickness sensor270inFIG. 13may malfunction if the roller262aor the roller271is displaced due to vibrations transmitted from outside the sheet transport apparatus or generated inside the sheet transport apparatus. Detection errors can be prevented, to some extent, if the frame of the sheet transport apparatus is provided with an acceleration sensor such that the amount of displacement of the roller can be compared to the acceleration detected by the acceleration sensor. However, since acceleration applied to the frame and the amount of displacement of the roller are different types of physical quantities, it is difficult to completely prevent detection errors even if some predictions can be made about the relationship between the acceleration and the amount of displacement. The same applies to the case where a pressure sensor is used to detect the displacement of the roller.

SUMMARY OF THE INVENTION

The present invention is directed to a sheet transport apparatus capable of reliably detecting the position and thickness of a sheet without using a sensor flag, but using an acceleration sensor included in the sheet transport apparatus.

The present invention is directed to a sheet transport apparatus capable of reliably determining the state of a sheet, such as the position and thickness of a sheet, and an image forming apparatus with improved accuracy in the formation of images.

In one aspect of the present invention, a sheet transport apparatus includes a pair of rotating bodies configured to come into contact with or to separate from each other, to rotate, and to transport a sheet interposed therebetween; an acceleration sensor configured to detect acceleration of movement of the pair of rotating bodies coming into contact with or separating from each other; and a determining unit determining, based on the acceleration detected by the acceleration sensor, a state of the sheet being transported.

DESCRIPTION OF THE EMBODIMENTS

A sheet transport apparatus according to embodiments of the present invention, and a copier serving as an image forming apparatus having the sheet transport apparatus will now be described with reference to the drawings.

The image forming apparatus of the present invention is not only applicable to copiers, but also to printers, facsimiles, and multifunction machines combining the functions of a copier, printer, and facsimile.

The sheet transport apparatus is not only included in the image forming apparatus, such as a copier, but also in other apparatuses dealing with sheets, such as a perforating apparatus for perforating sheets and a bending apparatus for bending sheets.

Copiers

FIG. 1is a front cross-sectional view of a copier serving as an image forming apparatus. A copier30includes a reader32, a feeder31, an image forming section24, and a fixer25. A sheet from the feeder31is introduced into the nip point between a driven roller5, provided with a rotating-body acceleration sensor1(seeFIG. 2), and serving as a movable rotating body and a driving roller6serving as a fixed rotating body. The sheet is then brought into contact with a pair of resist rollers9and10. A skew of the sheet is thus corrected. Then, the sheet is sent, at a predetermined timing, to the image forming section24having a photoconductive drum27. A document image read by the reader32is formed as a toner image on the photoconductive drum27. The toner image is transferred by a transfer charging device28onto the sheet, which is further sent to the fixer25, by which the toner image is fixed to the sheet. Finally, the sheet is ejected from the main body30A of the copier.

Sheet Transport Apparatus of First Embodiment

A sheet transport apparatus according to the first embodiment of the present invention will now be described with reference toFIGS. 2 to 7.

A sheet transport apparatus26includes a driving roller6driven by a drive unit40a, a driven roller5pressed by a pressure spring2into contact with the driving roller6, a fixed bearing8supporting the driving roller6, a movable bearing3rotatably supporting the driven roller5, the rotating-body acceleration sensor1serving as a sheet detector integrally attached to the bearing3, and a controller40for detecting the arrival and exit timing of the sheet S and determining the thickness of the sheet S, based on acceleration “a” detected by the rotating-body acceleration sensor1.

The controller40controls the drive unit40acausing the driving roller6to rotate, and the drive unit40bcausing the pair of resist rollers9and10to rotate. The drive unit40aand the drive unit40bhave respective motors (not shown). The resist roller10is pressed by a spring13against the resist roller9in such a manner that variations in thickness of the sheet can be accommodated.

If the rotating-body acceleration sensor1affects the movement of the bearing3, the controller40cannot accurately detect the arrival and exit timing and the thickness of the sheet. As such, the rotating-body acceleration sensor1is small and light weight to easily move with the bearing3.

The rotating-body acceleration sensor1in the present embodiment is an extremely small and lightweight micro-electro-mechanical system (hereinafter abbreviated as “MEMS”) sensor, as small as several square millimeters. A MEMS acceleration sensor is a sensor produced using MEMS technology.

MEMS Acceleration Sensor

MEMS technology is a technology for forming a minute mechanical structure and an electric circuit on a substrate through an exposure process used in semiconductor manufacturing. The MEMS technology allows the production of minute sensors and actuators of several millimeters in size, which was impossible with known technology, at extremely low cost. Acceleration sensors produced using MEMS technology have already been put to wide practical use. The structures of acceleration sensors produced using MEMS technology are disclosed in Japanese Patent Laid-Open Nos. 05-5750, 05-34370, and 06-331648. A MEMS acceleration sensor described in Japanese Patent Laid-Open No. 06-331648 will now be explained.

(2) Structure of MEMS Acceleration Sensor

As shown inFIG. 4, a glass substrate81serving as an insulating substrate of a MEMS acceleration sensor80is provided with fixed parts82made of silicon and serving as electrodes, and a movable part83serving as a detection part. In addition, the glass substrate81has a rectangular concave portion81A on which a mass portion84having a movable comb-shaped electrode85is arranged in a displaceable manner in a direction K (direction to which acceleration is applied).

The fixed part82is separately arranged on the respective left and right sides of the glass substrate81with a plurality of (for example, five) thin electrode plates86A disposed therebetween. The plurality of electrode plates86A constitute a fixed comb-shaped electrode86serving as a fixed electrode.

The movable part83includes two supporting parts87secured to the respective front and rear portions of the glass substrate81, the mass portion84supported by thin beams88, and a plurality of (for example, five) thin electrode plates85A protruding in the respective left and right directions from the mass portion84. The plurality of electrode plates85A constitutes the movable comb-shaped electrode85.

There are narrow spaces between the electrode plates85A of the movable comb-shaped electrode85and the electrode plates86A of the fixed comb-shaped electrode86. The application of acceleration in the direction K to the entire MEMS acceleration sensor80causes the mass portion84to move in the direction K, thereby changing the size of the spaces. The fixed parts82and the movable part83are connected to an amplifier89.

(3) Production Process of MEMS Acceleration Sensor

The production process of the MEMS acceleration sensor80will now be described with reference toFIGS. 4 to 6.

A silicon wafer with a diameter ranging from about 7.5 to 15.5 cm, and a thickness of about 300 μm is masked and etched to form a plurality of mass portions84, electrode plates85A, electrode plates86A, and fixed parts82.

A disk-shaped glass substrate having the same size as that of the silicon wafer is etched to form the plurality of concave portions81A.

The glass substrate and the silicon wafer are joined by anodic bonding. As shown inFIG. 6, the plurality of MEMS acceleration sensors80is thus formed on the glass substrate81.

The plurality of MEMS acceleration sensors80on the glass substrate81are cut into several-millimeter square chips.

With this production process, the MEMS acceleration sensors80are produced in quantities of several dozen at a time and are made compact and lightweight. The amplifier89inFIG. 4may also be produced on the glass substrate81at the same time using known semiconductor manufacturing technology. Structural bodies formed using MEMS technology, such as the MEMS acceleration sensor80, have a significant advantage in that peripheral circuits can be formed on the substrate simultaneously with the formation of the structural body.

(4) Operation of MEMS Acceleration Sensor

When acceleration is applied in the direction K as inFIG. 4, the MEMS acceleration sensor80changes the size of the narrow spaces between the electrode plates85A and the electrode plates86A, and causes the amplifier89to amplify and output this change as a change in capacitance. Based on the amount of this output, the MEMS acceleration sensor80transmits the amount of acceleration to the outside. Since the electrode plates85A and the electrode plates86A of the MEMS acceleration sensor80in this example are electrically connected in parallel, the amount of acceleration can be determined based on the total capacitance obtained by summing the capacitance between the electrode plates85A and the electrode plates86A. This improves sensitivity and accuracy of detection.

(5) Other Characteristics of MEMS Acceleration Sensor

As described in (3), peripheral circuits can be easily formed on the substrate of a sensor using MEMS technology. Therefore, the sensor may be provided with a transmitting and receiving circuit, as shown inFIG. 7, to create a wireless configuration. Such wireless technology has been put to practical use as radio frequency identification (RFID) tags and the like, and is disclosed in Japanese Patent Laid-Open No. 2002-337426 (corresponding to U.S. Pat. No. 6,827,279) and the like.

Referring toFIG. 7, the MEMS acceleration sensor80and a wireless circuit are disposed on a common substrate to form an acceleration sensor unit100. The MEMS acceleration sensor80is provided with an amplification circuit100e, a rectifying-smoothing circuit100d, a modulation circuit100a, and an antenna coil100b. The acceleration sensor unit100can wirelessly receive power from and transmit signals to a power-transmission/signal-receiving unit101. Power radio signals emitted from a power transmitter101dand a power supply coil101aare received by the antenna coil100bthat constitutes a resonance circuit together with the resonant capacitor100c,converted by the rectifying-smoothing circuit100dto power for operation, and then supplied to the entire acceleration sensor unit100. On the other hand, signals outputted from the MEMS acceleration sensor80are amplified by the amplification circuit100e, modulated by the modulation circuit100a, transmitted through the antenna coil100bto a data receiving coil101b, and transmitted further through a signal receiver101eto a control circuit101f.

In the acceleration sensor unit100, the wireless configuration allows the removal of communication cables for communicating with the external devices, and thus greatly improves the freedom of installation of the sensor. While the rotating-body acceleration sensor1of the present embodiment is attached to the bearing3, the installation of peripheral drive mechanisms may cause interference with wiring. The wireless configuration of the rotating-body acceleration sensor1gives a solution to such a problem.

Next, the operation of the sheet transport apparatus26having the rotating-body acceleration sensor1produced using MEMS technology will be described.

When the sheet S from the feeder31of the copier30is introduced into the nip point between the pair of rollers5and6, the driven roller5is pressed downward (seeFIGS. 2A and 2B). At this point, the rotating-body acceleration sensor1outputs, to the controller40, a change in acceleration “a” represented by a waveform inFIG. 3as a change in capacitance. The controller40obtains arrival timing t1of the sheet S from the waveform inFIG. 3and determines the thickness of the sheet S by evaluating the double integral of a peak waveform A. Exit timing at which the rear edge of the sheet S exits the pair of rollers5and6can also be determined from the acceleration waveform.

The front edge of the sheet S is brought into contact with the pair of resist rollers9and10that do not rotate. The controller40stops the rotation of the driving roller6, at predetermined timing, to create a loop Sa (seeFIG. 2C) of the sheet S for correcting a skew thereof. Stop timing at which the controller40stops the rotation of the driving roller6is determined based not only on the arrival timing t1, but also on the thickness of the sheet S. For example, if it is determined that the sheet S is thin, the controller40delays the stop timing of the driving roller6to increase the size of the loop Sa (seeFIG. 2C). If it is determined that the sheet S is thick, the controller40expedites the stop timing.

If the sheet S is thick paper, the size of the loop Sa is reduced. Even if the size of the loop Sa is small, the sheet S strikes the pair of resist rollers9and10at a strength sufficient to correct skew of the sheet S. If the size of the loop Sa is large, the sheet S is forced into the nip point between the pair of resist rollers9and10and may be folded.

After correcting the skew of the sheet S, the controller40waits for the image forming section24to be prepared, and feeds the sheet S into the image forming section24by rotating the resist roller9on the drive side.

The above-described sheet transport apparatus26of the first embodiment has the following advantages.

Since a flag, which is conventionally used, is not provided, transport of a thin sheet is not obstructed.

Since the rotating-body acceleration sensor1of several square millimeters is directly attached to the bearing3, the space occupied by the rotating-body acceleration sensor1can be minimized. Moreover, even if a plurality of sheet paths is complex, there is no need to change the shape of a guide plate for the sheet paths.

Unlike the known contact-type probe sensor264, there is no need to prepare a stable mounting base, as the rotating-body acceleration sensor1is directly attached to an object to be measured (bearing3of the driven roller5). In other words, all that is needed is to allow a surface to accommodate the rotating-body acceleration sensor1of several square millimeters. It is hardly necessary to change the peripheral configuration.

For a known sensor, such as the sheet thickness sensor270, that emits the reflecting light270ato an object to be measured, the surface of the object must be given a smooth finish by blasting or the like. For the rotating-body acceleration sensor1, it is not necessary to give a smooth finish to the surface of an object to be measured, as there is no need to emit detection light to the object.

Since there is no need for the rotating-body acceleration sensor1to emit detection light to an object to be measured, detection can be performed with little or no degradation in accuracy even if the rotating-body acceleration sensor1becomes soiled, to some extent, by oil of the drive unit of the sheet transport apparatus26and copier30, and dust and dirt, such as sheet dust.

Sheet Transport Apparatus of Second Embodiment

A sheet transport apparatus of the second embodiment will now be described with reference toFIG. 8andFIG. 9.

A sheet transport apparatus126of the second embodiment differs from the sheet transport apparatus26of the first embodiment in that a frame7serving as a supporting body is provided with a supporting-body acceleration sensor12serving as a second acceleration sensor. In the sheet transport apparatus126of the present embodiment, the components that are the same as those of the first embodiment are given the same reference numerals and their description will be omitted. The operation of the sheet transport apparatus126is also the same as that of the sheet transport apparatus26of the first embodiment unless otherwise specified.

The sheet transport apparatus126of the present embodiment is designed not to be affected by vibration of the frame7that may cause detection errors in the rotating-body acceleration sensor1.

Specifically, the frame7of the sheet transport apparatus126is provided with the supporting-body acceleration sensor12, which detects vibration of the frame7to compensate for vibration of the frame7detected by the rotating-body acceleration sensor1.

A further description will be given with reference to a detection waveform inFIG. 9. In processing output signals from the rotating-body acceleration sensor1, the controller40subtracts the output of the supporting-body acceleration sensor12(seeFIG. 9B) from the output of the rotating-body acceleration sensor1(seeFIG. 9A). Then the controller40obtains arrival timing t1of the sheet S from a peak C of the resultant signal waveform inFIG. 9C, and determines the thickness of the sheet S by evaluating the double integral of the waveform inFIG. 9C.

While the rotating-body acceleration sensor1detects externally-applied vibrations (for example, peaks B and D inFIGS. 9A and 9B) and may erroneously determine that the sheet S has arrived (when the peaks B and D inFIG. 9Aexceed a threshold E), the controller40can eliminate the effect of external vibrations, as shown inFIG. 9C, by subtracting the output of the supporting-body acceleration sensor12from the output of the rotating-body acceleration sensor1.

In the sheet transport apparatus126of the present embodiment, external vibrations and the displacement of the driving roller6and bearing3can be measured in the same physical quantity units (acceleration). Therefore, by determining the difference between their corresponding signal waveforms, the effect of external vibrations can be reliably eliminated and a detection error can be easily prevented. On the other hand, even if the supporting-body acceleration sensor12for measuring external vibrations would be added to the known sheet transport apparatuses shown inFIG. 12or13, it is difficult to completely eliminate the effect of external vibrations since different types of physical quantities, such as the amount of displacement and acceleration, are compared.

In the sheet transport apparatus126of the present embodiment, the effect of externally-applied vibrations can be eliminated.

While the sheet transport apparatuses26and126of the first and second embodiments are disposed at a location from which a sheet is fed to the pair of resist rollers9and10, the present invention is not limited to this configuration. The sheet transport apparatus may be provided at any location where the detection of arrival timing, exit timing, or thickness of a sheet is required. For example, the sheet transport apparatus may be attached to the pair of resist rollers and arranged near the cassette or manual paper feed such that the thickness of a sheet to be fed can be detected to control the speed of the fixer or the like.

This application claims the benefit of Japanese Application No. 2004-269017 filed Sep. 15, 2004, which is hereby incorporated by reference herein in its entirety.