Sheet supplying device, image reading apparatus having the same and method of detecting overlapping sheets

A sheet supplying device includes a stacker for placing sheets; a delivery device for separating and feeding the sheets on the stacker; a register device for temporarily holding a sheet; a sheet conveying guide for guiding the sheet from the register device to a processing position; a sheet sensor disposed between the delivery device and the register device for detecting the sheet; at least one driving device for driving the delivery device and the register device; a conveyance control device for controlling the driving device so that the delivery device and the register device form a loop of the sheet according to a signal from the sheet sensor and an overlap sensor disposed between the register device and the processing position for detecting overlapping of the sheet. An overlap determining device determines overlapping of the sheet according to signals from the sheet sensor and the overlap sensor.

BACKGROUND OF THE INVENTION AND RELATED ART STATEMENT

The present invention relates to a sheet supplying device for sequentially separating sheets on a stacker and feeding a sheet to a processing platen for reading or printing an image, and a method of detecting overlapping of a plurality of sheets while the sheets are being fed.

A sheet supplying device sequentially supplies sheets stacked on a stacker to a processing platen of a device such as a printer, a copier, or a scanner. An image reading apparatus such as a scanner feeds documents on a stacker to a platen one by one, so that a photoelectric converting device reads an image on the document.

When such a device separates sheets on a stacker one by one and supplies the sheet to the processing platen, if a plurality of sheets (documents) is overlapped and fed (double feed), an erroneous processing may be executed at the processing platen. Accordingly, it is necessary to accurately separate the sheets into a single sheet and detect the double feed of the sheets before the sheet reaches the processing platen, so that the processing is stopped or processing data such as reading information is discarded not to be sent to a processing device such as a printer.

A conventional method of detecting the double feed of the sheets includes an ultrasonic sensor or a photo-sensor for detecting attenuation in an ultrasonic wave or an intensity of light passing through the sheet, thereby determining whether there is a single sheet.

Japanese Patent Publication (Kokai) No. 10-257595 discloses an ultrasonic sensor for detecting a sheet. The ultrasonic sensor includes a piezoelectric oscillation plate such as piezoelectric ceramic at a wave transmission side. A pulse voltage with a predetermined frequency is applied to the piezoelectric oscillation plate to generate oscillation, thereby transmitting ultrasonic waves. A similar oscillation plate is provided at a wave reception side for receiving the ultrasonic waves and converting to an electrical signal. Electric energy is compared with a reference value, thereby determining a single sheet or several sheets.

Japanese Utility Model Publication (Kokai) No. 06-49567 proposes a structure in which a wave transmission element and a wave reception element are arranged opposite to each other between a downstream roller and an upstream roller arranged with a predetermined distance in between, thereby making it possible to detect the double feed while the sheet is in a stable condition. More specifically, with such a structure, the double feed is detected while the downstream and upstream rollers nip the sheet in a straight position during transportation. Accordingly, it is possible to accurately detect the double feed since a leading edge or a trailing edge of the sheet is not curved or does not flip vertically.

When the ultrasonic sensor or optical sensor is used to detect the overlapping of the sheets, if the sheets have different quality, a thickness, or a size, it is difficult to accurately determine whether one or more sheets are being fed at a time. That is, when several sheets contact tightly with one another due to humidity or other environmental factors, it is difficult to determine between a single sheet having a large thickness and overlapped several sheets. When sheets with various sizes are overlapped and shifted in a longitudinal direction, it is difficult to determine between a single sheet having a large size and several sheets overlapped in the longitudinal direction. Moreover, when the sheets are flapped in a vertical direction at a position of the sensor, a transmitted quantity of sound wave or light varies, thereby making it difficult to accurately determine the double feed.

Japanese Utility Model Publication (Kokai) No. 6-49567 has proposed that the overlapping of the sheets is detected while the pair of the rollers supports the sheets. However, it is still difficult to detect the double feed when the sheets tightly contact with one another. Further, when the double feed is detected over a predetermined length to determine that the sheets are shifted in the longitudinal direction, the trailing edge of the sheets flaps upon leaving from the roller, thereby causing a misdetection.

In view of the problems described above, an object of the present invention is to provide a sheet supplying device that can accurately detect the overlapping of sheets even when the sheets tightly contact with one another or are shifted in the longitudinal direction thereof.

Another object of the present invention is to provide an image reading apparatus and a method of accurately detecting the overlapping of original documents while the sheets are fed from a stacker to a reading platen.

SUMMARY OF THE INVENTION

To accomplish the objects described above, according to the present invention, a sheet supplying device comprises a stacker for placing documents (sheets) toward a processing position such as an image reading platen, and a sheet conveying guide for guiding the sheet from the stacker to the processing position. The stacker is provided with a delivery device for separating the sheet from others and feeding the sheet. The delivery device is formed of, for example, a sheet feeding roller contacting the uppermost sheet on the stacker to convey the sheet toward the processing position, and a friction pad contacting the roller with pressure. A register device such as a pair of pressure contact rollers is placed in the sheet conveying guide for temporarily holding the sheets fed by the delivery device. A sheet sensor is disposed at an upstream side of the register device for detecting the sheets. An overlap sensor is disposed at a downstream side of the register device for detecting overlapping of the sheets. The delivery device and the register device are controlled so that the delivery device feeds the sheet to the register roller and forms a loop at a leading edge of the sheet. A control circuit, for example, a CPU and a driver circuit, controls a driving device such as a motor connected to the delivery device and the register device in accordance with a signal from the sheet sensor for detecting the leading edge of the sheet, so that the delivery device feeds the sheet by a controlled amount.

An overlap determining device determines the overlapping of the sheets on the basis of a detection signal from the overlap sensor and a detection signal from the sheet sensor. When the CPU or the like receives an overlap signal and the sheet sensor detects the sheets, the overlap determining device determines that the overlap signal is valid. When the CPU or the like receives an overlap signal, and within a predetermined period of time after the sheet sensor detects the trailing edge of the sheet, the overlap determining device determines that the overlap signal is valid. A clocking device such as a timer sets a period of time from the sensor to immediately before the trailing edge of the sheet passes (leaves) the register device as the predetermined time.

The overlap sensor detects the sheets that are bent and loosened by the register device. When the overlapping sheets are bent, the sheets are released from a tight contact state, so that the overlapping is surely detected from an air layer between the sheets. The overlap determining device determines that the overlap signal from the sheet sensor is valid when the register device nips the trailing edge of the sheet. Accordingly, the overlap determining device does not determine the overlapping from the detection signal while the trailing edge of the sheet leaves the register device and is flapping.

According to the present invention, the conveyance control device may be provided with a first clocking device for forming the loop (hereinafter referred to as a register loop) in the sheet after the sheet sensor detects the leading edge of the sheet, and a second clocking device to be activated when the register device starts feeding the sheet to a platen. The second clocking device sets a time equal to or longer than that of the first clocking device. The overlap determining device is configured to determine the overlapping of the sheets on the basis of an output signal from the overlap sensor after the time set for the second clocking device. Accordingly, the register loop is removed, thereby making it possible to accurately determine the overlapping while the documents extend along a conveying path.

According to the present invention, the sheet conveying guide may include a bent guide member. The overlap sensor is disposed in a bent area of the guide member, so that the overlapping sheets are detected after being bent and loosened, thereby improving detection accuracy.

According to the present invention, a method of detecting overlapping sheets (documents) comprises a sheet delivering step of separating each sheet from others on a stacker and delivering the sheet; a loop forming step of abutting the document against a conveying roller to bend the document in a loop form; a document feeding step of extending the document bent by the conveying roller and feeding the extended document to the platen; a conveyance status detecting step of detecting the document on an upstream side of the conveying roller and detecting the overlapping on a downstream side of the conveying roller during the document feeding step; and an overlap determining step of determining the overlapping on the basis of an output signal indicating the overlapping and an output signal indicating the document obtained in the conveyance status detecting step.

In the present invention, the register device bends the sheets delivered from the stacker in a loop form, and the downstream overlap sensor detects the overlapping of the sheets, thereby accurately detecting the overlapping. In particular, when an ultrasonic wave sensor is used as an overlap sensor, tightly contacting sheets are bent so as to form air layers between the sheets. Accordingly, it is possible to easily determine whether a thick sheet or several overlapping sheets. Further, the sheet sensor disposed between the stacker and the register device detects the leading edge of the sheet to regulate the loop of the sheet. The sheet sensor also detects the trailing edge of the sheet to determine whether an output signal from the overlap sensor indicates that the register device is holding the sheet or that the sheet leaves the register device and is flapping, thereby making it possible to accurately detect the overlapping.

Accordingly, it is possible to accurately determine the overlapping of the sheets regardless of whether the sheets tightly contacts with each other or are overlapped and shifted in the longitudinal direction. In particular, for an image reading apparatus, it is possible to handle various types of sheets having different quality, thickness, size, or the like.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Hereunder, embodiments of the present invention will be described with reference to the accompanying drawings.FIG. 1is a view showing a sheet feeding mechanism section of a sheet supplying device according to an embodiment of the present invention.FIG. 2is a diagram showing a structure of an overlap sensor in the sheet feeding mechanism section shown inFIG. 1.FIG. 3is a timing chart showing control timings for the sheet supplying device shown inFIG. 1.FIG. 4is a flowchart showing a control of the sheet supplying device shown inFIG. 1

As shown inFIG. 1, a sheet feeding stacker1and a sheet discharging stacker15are arranged on a platen2of an image reading apparatus (described later) such as a scanner. A generally U-shaped conveying path20is formed of a sheet conveying guide3extending from the sheet feeding stacker1to the sheet discharging stacker15. The sheet feeding stacker1is formed of a tray on which document sheets are stacked. The sheet feeding stacker1is provided with a separating device4for contacting and sequentially separating the uppermost sheet from other sheets and then feeding the sheet. The separating device4is formed of a separating roller4aand a friction pad4bpressingly contacting the separating roller4a. The separating device4may be composed of a belt. Alternatively, the separating device4may comprise a pickup roller. A register device5is provided at a downstream side of the separating device4. The register device5is formed of a pair of register rollers5aand5b. The register device5temporarily holds the sheet from the separating device4, while bending a leading edge of the sheet in a loop form to correct a skew and loosen overlapping sheets.

A sheet sensor7is provided between the separating device4and the register device5. The sheet sensor7has a light emitting element7aand a light receiving element7bformed of light emitting diodes or the like and arranged opposite each other with the sheet in between. The sensor7is not limited to a photo sensor, and may be formed of a combination of a micro switch and a lever contacting the sheet. An overlap sensor6(described later) is placed at a downstream side of the register device5.

The separating device4and the register device5are connected to a driving device M and rotate in a sheet conveying direction. The driving device M is a stepping motor that can rotate forward and backward. The driving device M is connected to a motor driving circuit16. The driving device M is supplied with power from a power source18via a pulse generator17. A one-directional transmission clutch such as a one-way clutch is provided for transmitting an opposite rotating force to the separating device4and the register device5. When the separating device4rotates to deliver the sheet from the stacker1, the leading edge of the sheet abuts against the register device5, so that the sheet is bent in a loop form. When the register device5is actuated to feed the sheet to the platen2, the separating device4is stopped not to deliver a subsequent sheet.

An image reading mechanism is placed in the platen at a position where the sheet is to be processed. The image reading mechanism is formed of a light source27for irradiating the sheet on the platen2; a lens for focusing light reflected from the light source27; and a photoelectric converting device38such as a CCD (Charge Coupled Device) for electrically converting light from the lens29. Reference numeral28in the figure denotes a polarizing mirror. The sheet conveying guide3is formed of guide members3aand3barranged with a small space in between as a path for passing the sheets. The sheet conveying guide3forms a generally U-shaped conveying path20connected to the platen2.

A pressing member19formed of an elastic film is provided on one of the guide members3aat a downstream side immediately after the ultrasonic sensor6for deflecting the sheet toward the other of the guide members3b. Accordingly, the sheet is pressed against the guide member3band stabilized, thereby preventing vertical flapping of the sheet and a misdetection. A feeding roller24, an unloading roller25, and a sheet discharging roller26are arranged on the conveying path20. The feeding roller24is placed at an upstream side of the platen2and formed of a pair of rollers for supplying the sheet to the platen. Each of the unloading roller25and the sheet discharging roller26is formed of a roller pair for conveying the sheet from the platen to the sheet discharging tray15.

The overlap sensor will be described with reference toFIG. 2. The overlap sensor is formed of an ultrasonic sensor6. The ultrasonic sensor6is normally formed of a wave transmitting element6aand a wave receiving element6bhaving a same structure. Each element has a housing case10formed of metal or the like, and a piezoelectric vibrator11such as a piezoelectric ceramic plate is embedded in an elastic resin12in the housing case10. Electrodes are formed on front and rear surfaces of the piezoelectric vibrator11with deposition. High-frequency power is supplied to the piezoelectric vibrator11through a lead13. The piezoelectric vibrator11tightly contacts the housing case10. The piezoelectric vibrator11vibrates at a particular frequency on the basis of a natural frequency common to the piezoelectric vibrator11and the housing case10. A wave transmitting surface10aforming a part of the case transmits an ultrasonic wave to an external apparatus. One of the leads13is grounded on the housing case10.

When high-frequency power is supplied through the lead13in the wave transmitting element6a, the piezoelectric vibrator11and the housing case10contacting the piezoelectric vibrator11vibrate at a predetermined frequency. An ultrasonic wave is emitted from the wave transmitting surface10a. In the wave receiving element6b, a wave receiving surface10band the piezoelectric vibrator11integrated with the wave receiving surface10bare resonated with the ultrasonic wave. Accordingly, electricity is generated in the piezoelectric vibrator11and output to an external apparatus via the lead13.

The ultrasonic sensor6described above is placed on the conveying path20. The ultrasonic sensor6is connected to an oscillation circuit and an oscillation receiving circuit23as shown inFIG. 2. The oscillation circuit22is formed of a high-frequency oscillation circuit22aand an amplification circuit31b. The oscillation circuit22supplies a piezoelectric member11with a high-frequency voltage of a particular frequency from a power source22c. The oscillation receiving circuit23is formed of an amplification circuit23aand a smoothing circuit23bformed of a transistor or the like. The high-frequency oscillation circuit22agenerates a high-frequency voltage of, for example, 30 to 400 KHz, and amplifies and applies the signal to the electrodes formed on the front and back surfaces of the piezoelectric vibrator11via the lead13. The high-frequency oscillation circuit22athus excites the piezoelectric vibrator11. The ultrasonic wave passes through the sheet, and excites the piezoelectric vibrator11of the wave receiving element. The ultrasonic wave is then output as an electric signal. The amplification circuit23aamplifies the output signal from the wave receiving element6b. The signal is rectified by the smoothing circuit23band smoothed by an integration circuit.

When power is supplied to the high-frequency oscillation circuit22a, the ultrasonic wave of a particular frequency is excited in the piezoelectric vibrator11of the wave transmitting element6a. The vibrator11emits the ultrasonic wave with a high frequency and specific amplitude (output level LV1) as shown inFIG. 6(a). The wave receiving element6b, located opposite the wave transmitting element6a, receives the ultrasonic wave through the sheet. The piezoelectric member11of the wave receiving element6bis resonated and outputs power generated as a result of the vibration. The ultrasonic wave passing through the sheet is attenuated differently between a case of one sheet shown inFIG. 6(B)(output level LV2) and a case of two sheets shown inFIG. 6(c) (output level LV3).

The amplification circuit23aand the smoothing circuit23bprocess electric energy output with waveforms shown inFIGS. 6(b) and6(c). Specifically, electric energy with a vibration waveform output by the wave receiving element6bis amplified and rectified. The smoothing circuit23bconverts the electric energy into a signal with an output level as shown inFIGS. 6(d) and6(e).

FIG. 6(d) shows the level LV2obtained when one sheet is conveyed. A part A indicates that the leading edge of the sheet from the register rollers5aand5breaches the sensor6and a detected value is disturbed. This is because the sheet is bent in a loop form when delivered by the register roller5, and the leading edge of the sheet flaps. A part B indicates that the sheet is nipped by the register roller5so as to extend along the sheet conveying guide3and a detected value is stable. A part C indicates that the trailing edge of the sheet leaves the register rollers5aand5b(passed through the rollers) and a detected value is disturbed.FIG. 6(e) indicates the output level LV3obtained when two sheets are conveyed while overlapping. Parts A, B, and C indicate the above states.

When a reference value is set at a level LVO shown by a hidden line, in the case of one sheet shown inFIG. 6(d) and the case of two sheets shown inFIG. 6(e), a relationship LV1>LV2>LV0>LV3is established at the stable part B. Accordingly, when a comparison circuit (means)23csuch as a comparator compares an output signal at the part B from the smoothing circuit23bwith the reference value (LV0), it is possible to determine the overlapping of the sheets.

When the reference value is determined, first, conditions such as a thickness, quality of the sheets, and a sheet conveying speed are determined according to an environment in which the device is used. Then, under these conditions, boundary values of the output levels of the wave receiving sensor in the cases of one sheet and two sheets are experimentally determined to be set as the reference value.

As described above, the reference values are determined in the cases of one sheet and two sheets. A plurality of reference values may be set for cases of one sheet, two sheets, and more sheets. Accordingly, when the output signals are compared with the reference values, it is possible to detect the number of the overlapping sheets. The high-frequency oscillation circuit22ainstantaneously applies a high-frequency voltage to the wave transmitting element6ato generate a burst wave, or consecutively applies a high-frequency voltage to the wave transmitting element6ato generate a standing wave. In this case, the output signal from the wave receiving element6bmay become unstable (vary depending on environmental conditions) due to the overlapping of the sheets. Accordingly, it is preferable that the burst wave is detected consecutively and repeatedly a number of times.

The wave transmitting element6aand the wave receiving element6bare arranged as described below.

(1) The wave transmitting element6aand the wave receiving element6bare arranged opposite each other so as to incline at a predetermined angle relative to a sheet traveling along the conveying guide3. As shown inFIG. 2, the elements are inclined at an angle α relative to a line N-N that perpendicular to the conveying guide. In the figure, the angle α is set at 35to 45 degrees. Accordingly, when the ultrasonic wave oscillated by the wave transmitting element6ais reflected from a surface of the sheet and returns to a surface (wave transmitting surface) of the wave transmitting element6a, the ultrasonic wave does not interferes with the oscillation wave. Similar interference between the sheet surface and a wave receiving surface10aof the wave receiving element6bis avoided. The angle α may be set on the basis of a distance between the sheet and the wave transmitting (receiving) surface as well as an area of the transmitting (receiving) surface.

(2) In the direction of gravity, the wave transmitting element is placed below the conveying guide3, and the wave receiving element is placed above the conveying guide3. As previously described, the intensity (LV1) of vibration on the wave transmitting surface of the wave transmitting element6ais greater that that of the wave receiving element6b. Further, to determine a difference in the level of resonance (intensity of vibration) on the wave receiving surface between the case of one sheet and the case of two sheets, it is necessary to reduce an external effect on the wave receiving surface. The wave transmitting element6ais disposed at a lower position and the wave receiving element6bis disposed at an upper position in the direction of gravity, so that an adverse effect of paper dusts falling from the sheet conveying guide on the detection accuracy is reduced.

(3) The wave transmitting surface10aof the wave transmitting element6alocated at a lower position is inclined at a predetermined angle (β) relative to the horizontal direction. The angle β is selected such that dusts fall from the surface naturally or in corporation with the ultrasonic vibration. In the figure, the angle β is set at 30 degrees, and is preferably closer to 90 degrees.

FIGS. 5(a) to5(e) are views showing a process of delivering the sheet in the sheet supplying device shown inFIG. 1.FIG. 5(a) shows a state immediately after the sheet is fed;FIG. 5(b) shows that the sheet is temporarily standing by;FIG. 5(c) shows a state immediately after the sheet is fed toward a platen;FIG. 5(d) shows that an overlap determination is started on a basis of an overlap detection signal; andFIG. 5(e) shows that the overlap determination is completed. An operation of the sheet supplying device will be described in accordance with a flowchart.

The sheet feeding stacker1is provided with an empty sensor21that detects the sheets placed on the stacker. When the device is powered on, a control CPU31uses the empty sensor21to detect the sheets are on the stacker1. The driving motor M rotates in a forward direction (FIG. 4, ST01) upon a signal (FIG. 3, S01) indicating that the empty sensor21detects the sheets. The driving motor M rotates the separating roller4aclockwise inFIG. 1, while the register roller5aremains stopped. The separating roller4afeeds the sheets on the stacker1to the left side inFIG. 1. The sheet passes through the sheet sensor7to the register roller5a.

Upon detecting the leading edge of the sheet in the state shown inFIG. 5(a), the sheet sensor provides a detection signal S02to activate a timer T1, i.e., the first clocking device. The timer T1sends a stop signal S03to stop the driving motor M after a set time (seeFIG. 4, ST02). During the set time, the separating roller4arotates, so that the leading edge of the sheet reaches the register roller5aand is then bent to form a predetermined loop as shown inFIG. 5(b). The timer T1counts, for example, a reference clock of the CPU31to determine the set time. The set time is obtained from a time for forming the predetermined loop according to a specification of the device. The conveyance control circuit31bof the control CPU31determines whether the leading edge of the sheet arrives, and the sheet sensor7sends a stop signal S03.

When a main body processing apparatus such as an image reading apparatus sends a sheet feed instruction signal S04, the driving motor M is driven backward to rotate the register roller5ato feed the sheet to the platen2. At the same time, in response to the sheet feed instruction signal S04, the control circuit31bactivates a timer T2that is the second clocking device and turns on the oscillation circuit22of the ultrasonic sensor6(FIG. 4, ST03). A set time for the timer T2is equal to or longer than that for the first clocking device T1. After the set time for the timer T2, the control circuit31bprovides an overlap detection start signal (S05) (FIG. 4, ST04). At this time, the sheet is transferred to the platen along the conveying guide3in a linear posture as shown inFIG. 5(d).

The clocking device (T2) is formed of a delay circuit for counting, for example, a reference clock of the control CPU31. The control circuit31breceives a signal indicating that the empty sheet sensor21detects the sheets, and supplies power to the oscillation circuit. The wave transmitting element6aof the ultrasonic sensor6generates the ultrasonic wave with a predetermined frequency. The wave receiving element6breceives the ultrasonic wave passing through the sheet. The wave receiving element6bthen provides an output corresponding to a condition of the sheet. The comparison circuit13cthen compares the reference value with the output processed at the amplification circuit13aand the smoothing circuit23b. A result of the comparison is stored in a buffer memory31cand transferred to a determining circuit31a.

The reverse rotation of the driving motor rotates the register roller5aclockwise to feed the sheet to the processing platen2. At this time, the separating roller4aremains stopped. The loop in the leading edge of the sheet is removed, and the sheet is supported by the separating roller4aand the register roller5a. The timer T2provides an overlap detection start signal (S05). Each of the timers T1and T2is formed of a delay circuit that uses a counter to count the reference clock in the control circuit31.

In the overlap detection carried out by the determining circuit of the control CPU, an output signal from the wave receiving element6bis divided into pieces corresponding to a predetermined time, for example, 1 millisecond. The divided signal is then compared with the reference value, and the buffer memory23sends a result of the comparison to the determining circuit (seeFIG. 2). When the timer T2is up, the control CPU31receives an overlap detection start signal S04to clear the data stored in the buffer memory31c. While the sheet is transferred, an output signal from the wave receiving element6bsequentially carries the comparison data from the comparison circuit23cto the memory31c. The determining circuit31aof the control CPU31retrieves the comparison data to monitor whether the overlapping of the sheets occurs (FIG. 4, ST05)

When the output level of the comparison data from the comparison circuit23cis smaller than that of the reference value, that is, when the output level of the wave receiving element6bis smaller than that of the reference value, the determining circuit31aof the control CPU determines the overlapping in accordance with the following step (1) as ST6shown inFIG. 4. On the other hand, when the output level of the comparison data is greater than that of the reference value, the determining circuit31adetermines no overlapping. The determining circuit31aexecutes processing in accordance with the following step (2) as ST07shown inFIG. 4.

(1) When the comparison data indicates the overlapping, the determining circuit31adetermines the comparison data to be valid and executes overlap processing when a status signal from the sheet sensor7indicates presence of the sheets (FIG. 4, ST08). The overlap processing provides a trouble signal to a main body apparatus such as an image reading apparatus or an image forming apparatus to stop the operation of the main body apparatus. At the same time, a control panel provides an indication of the overlapping to warn a user. Alternatively, the overlap processing may store a order of pages for the overlapping sheets and continue to perform the next sheet processing operation. Then, once the whole processing is finished, the stored information may be displayed so that the user can execute the processing again on the basis of the information displayed to make required corrections.

(2) When the comparison data indicates that the overlapping does not occur, the determining circuit31aexecutes the sheet processing or continues the sheet processing being executed when the status signal from the sheet sensor7indicates absence of the sheets. In the case of the presence of the sheets, while the sheet processing is executed or the sheet processing is continued, the determining circuit31aloads the next comparison data to monitor the data overlapping (FIG. 4, ST07). After the sheet processing, the determining circuit31adetermines whether the next sheet is present on the sheet feeding tray on the basis of a signal from the empty sensor21(FIG. 4, ST09). When the next sheet is present, the process shifts to step ST03to process the next sheet document in the same manner. In this case, the next sheet is fed to the register roller. When the next sheet is not present on the sheet feeding tray, the determining circuit determines that the job is finished and stops the device.

The status signal from the sheet sensor7may determine the presence of the sheets based on whether a predetermined time elapses since the trailing edge of the sheet passes the sensor7. In other words, the timer may be started in response to a change in the status signal from the sensor7from the presence to the absence of the sheets. Then, whether the process is to shift to step ST06or ST08may be determined on the basis of whether an expected time elapses for the trailing edge of the sheet to pass through (leave) the register rollers5aand5b. The comparison data indicating the overlapping is determined to be valid depending on whether the register roller nips and supports the trailing edge of the sheets, thereby determining whether to shift to the overlap processing or sheet processing.

A method of detecting the overlapping will be explained according to an embodiment of the present invention.

A step of separating the sheet from others on the stacker and delivering the sheet includes placing a series of sheets on the sheet feeding tray, separating each sheet from the others, and delivering the sheet. In the above device, the conveyance control circuit31buses the separating roller4aand the friction pad4bto separate each sheet from the others on the sheet feeding tray and feed it, and is formed of a program of the control CPU.

The register device constitutes a loop forming step of abutting the document delivered in the above step against the conveying roller to bend the document in a loop form. The above device controls the separating roller4aand the register rollers5aand5b, so that the separating roller4adelivers the sheet to the register rollers5aand5bto bend the leading edge of the sheet.

In a document feeding step of using the conveying roller formed of the register roller to extend the bent document sheet and then feed it to the platen, the driving motor rotates the register rollers5aand5b.

In a conveyance status detecting step, the sheets are detected on an upstream side of the conveying roller means, and the overlapping status is detected on a downstream side of the conveying roller during the document feeding step of feeding the sheet to the platen. In the above device, the photo sensor is provided at an upstream side of the register roller5ato detect the sheets. The ultrasonic sensor is placed at a downstream side of the register roller to detect the overlapping status.

In an overlap determining step, the overlapping is determined on the basis of the results of detection of the document overlapping status and the sheets carried out in the conveyance status detecting step. The above device determines the overlapping on the basis of the overlap sensing signal from the ultrasonic sensor and the sheet presence signal from the sheet sensor.

An image reading apparatus according to an embodiment of the present invention will be explained next.FIG. 7shows an image reading device A and an image forming apparatus B having the image reading device A as a unit.FIG. 8shows a sheet supplying section of the image reading apparatus A. The image forming apparatus B having the image reading device A (described below) has a print drum102; a sheet feeding cassette101for supplying a sheet to the print drum102; a developing device108for developing an image on the print drum102with toner ink; and a fixing device104. These components are contained in a casing100. Reference numeral103denotes a print head that uses a laser or the like to form a latent image on the print drum192. The conveying roller105conveys the sheet from the sheet feeding cassette101to the print drum102. An image formed by the print head103is transferred to the print drum102. The fixing device104then fixes the image.

The image forming apparatus B is widely known as a printer and formed of a sheet feeding section, a printing section, and a discharged sheet housing section. The functional parts are not limited to those described above, and may have various functions such as ink jet printing and silk screen printing. The print head103is electrically connected to a storage device109such as a hard disk for storing image data and a data management control circuit122for sequentially transferring the image data to the print head. The image reading device A is mounted on an upper part of the image forming apparatus B as a unit.

In the image reading apparatus A, a platen112is mounted in the casing110. An optical mechanism114and a photoelectric converting element113are arranged in the casing110to read a document sheet via the platen. A CCD or the like is widely known as the photoelectric converting element113.

A sheet supplying device C shown inFIG. 8is installed in the platen112. In the sheet supplying device C, a sheet feeding stacker115and a sheet discharging stacker116are provided above the platen112in parallel in the vertical direction. The sheet from the sheet feeding stacker115is guided along a U-shaped conveying path to the sheet discharging stacker116via the platen112. An empty sensor117and a size sensor (not shown) are arranged on the sheet feeding stacker115, and the empty sensor117detects the sheets placed on the stacker115. Reference numeral133denotes a side guide that regulates the side edges of the sheet.

A separating roller119and a fixed roller120are arranged at an upstream side of the sheet feeding stacker115, and the fixed roller120pressingly contacts the separating roller119. A kick roller118is attached to a bracket119bmounted to a rotating shaft119aof the separating roller119. When the rotating shaft119arotates clockwise, the kick roller118lowers onto the sheet feeding stacker115. When the rotating shaft119arotates counterclockwise, the kick roller118elevates to a state shown in the figure (described in detail with reference toFIG. 10). A sheet sensor is placed at a downstream side of the separating roller119for detecting the leading and trailing edges of the sheet. The conveying path134is provided with register rollers125aand125b, feeding rollers127aand127b, an unloading roller129, and a sheet discharging roller116in this order. The sheet is conveyed from the sheet feeding stacker115to the sheet discharging stacker116.

An overlap sensor124is placed at a downstream side of the register roller125and formed of a pair of ultrasonic sensors. In each ultrasonic sensor, a wave transmitting element and a wave receiving element are arranged and configured as described above (seeFIG. 1). Reference numeral128denotes a pair of guide members128aand128bfor guiding the sheet to the platen112while maintaining the sheet in a U shape. The guide member128ais provided with a pressing member128cfor deflecting the sheet to the opposite guide member128b. The pressing member is formed of an elastic resin film. A lead sensor126is provided at a downstream side of the pressing member for detecting the leading edge of the sheet. Reference numeral131denotes a circulating path through which a sheet from the platen112is fed to the register rollers125aand125bthrough a path switching gate131a.

A driving mechanism of the conveying rollers will be described next.FIG. 9(a) shows a driving mechanism for the separating roller119and register roller125. A sheet feed driving motor140capable of rotating forward and backward drives the kick roller118, the separating roller119, and the register roller125.FIG. 9(b) shows a conveyance driving motor141for the feeding roller127, an unloading roller129, and a sheet discharging roller130as well as a transmission mechanism for the conveyance driving motor141. InFIG. 9(a), the sheet feed driving motor140rotates forward to drive the kick roller118and the separating roller119. The sheet feed driving motor140rotates backward to drive the register roller125. The sheet feed driving motor140controllably elevates and lowers the kick roller118. The sheet feed driving motor140transmits rotations to the register roller125via belts B1and B2in only one direction through a one way clutch142. The sheet feed driving motor140is also connected to a rotating shaft of the separating roller119through a one way clutch143. The one way clutches142and143are set so that they transmit opposite driving forces.

A bracket119bis supported on a rotating shaft of the separating roller119via a spring clutch144. A belt B3is used to transmit a driving force to the kick roller118attached to the bracket119b. The sheet feed driving motor140rotates forward to drive the separating roller119and the kick roller118. A spring of the spring clutch144is loosened to release the bracket119b. The bracket119bthus lowers from a withdrawn position where the bracket119bis elevated. Consequently, the kick rocker118contacts the sheet on the stacker. The sheet feed driving motor140rotates backward to transmit a driving force to the register roller125. The spring clutch144is contracted to elevate and return the bracket119bto the withdrawn position inFIG. 8.

A conveying section driving motor141is connected to the feeding roller127, unloading roller129, and sheet discharging roller130as shown inFIG. 9(b). Even though the motor rotates forward and backward, the one-way clutch allows the feeding roller127and the unloading roller129to always rotate in only one direction. The sheet discharging roller130rotates forward and backward as the motor rotates forward and backward.

A sensor is placed on the conveying path134to detect the leading edge of the sheet. The sensor will be described below together with an operation thereof. A plurality of sensors (not shown) is arranged on the sheet feeding stacker115for detecting a specific size of the sheet. These sensors detect the size of the sheet to control the conveyance of the sheets. The empty sensor117is provided at a tip portion of the sheet feeding stacker115to detect the sheets on the stacker. The empty sensor117detects that the final sheet is fed to provide a signal to a processing apparatus such as the image reading apparatus A. An ultrasonic sensor123and a sheet end detecting sensor124are provided at a downstream side of the separating roller119.

A lead sensor126is provided before the feeding roller127for notifying the image reading apparatus that the leading edge of the sheet arrives. The lead sensor126further determines a line on the sheet where reading or printing is to be started. When no sheet is detected even after a predetermined time since the feeding instruction signal is sent to the register roller125, the lead sensor126determines that a jam occurs. The lead sensor126stops the driving motor and sends a warning signal. A sheet discharge sensor145is placed at a downstream side of the unloading roller129for detecting the leading and trailing edges of the sheet. The sheet discharge sensor145thus determines whether a jam occurs.

An operation of the above apparatus will be described.FIG. 10shows a flowchart of the operation. An apparatus power source is turned on and the sheets are set (placed) on the sheet feeding stacker115. The empty sensor117detects the sheets, and actuates the sheet feed driving motor140(ST100). The sheet feed driving motor140rotates the kick roller118and the separating roller119to separate one sheet from the others. The sheet is then fed to the conveying guide128between the separating roller119and the register roller125. The sheet sensor124(referred to as the sensor124below) detects the leading edge of the sheet (ST101). A detection signal of the leading edge of the sheet operates the timer T1(seeFIG. 3) to stop the motor140after a predetermined time (ST102).

As shown inFIG. 11(a), the sensor124detects the leading edge of the sheet to operate the timer T1. As shown inFIG. 11(b), the leading edge of the sheet abuts against the register roller125and is thus bent in a loop form. In this state, the set time for the timer T1is over and the motor140is stopped. When a control section of the image reading apparatus A provides the sheet feeding instruction signal, the motor is actuated again and rotates backward (ST103). The sheet feeing instruction signal operates the timer T2. The timer T2(seeFIG. 4)clears a register loop to allow the sheet to be conveyed linearly supported between the separating roller119and the register roller125(ST104) as shown inFIG. 11(c). The overlap detection start instruction signal is provided to determine the overlapping of the sheets as described above with reference toFIGS. 1 to 6.

As shown inFIG. 11(e), the ultrasonic sensor123detects the overlapping until the sensor124detects the trailing edge of the sheet (ST105). The sensor124detects the trailing edge of the sheet fed as described above (ST106). Before the trailing edge of the sheet is detected, the lead sensor126detects the leading edge of the sheet. The feeding roller127thus feeds the sheet to the platen112. After the leading edge is detected by the lead sensor126, when the sheet reaches the platen112, the optical mechanism114and the photoelectric converting element113read the sheet to obtain an electric signal (ST107). After the reading process, the unloading roller129and the sheet discharging roller130discharge the sheet to the sheet discharging stacker116. The sheet discharge sensor145detects that the sheet is discharged (ST108).

The disclosure to Japanese Patent Application No. 2003-428192, filed on Dec. 24, 2003, is incorporated in the application.