Abstract:
A sheet transfer device is provided for transferring a sheet member along a transfer path having an upstream point and a downstream point. The sheet transfer includes a transfer mechanism for moving the sheet member along the transfer path from the upstream point to the downstream point, an upstream sensor arranged closer to the upstream point than to the downstream point for detecting presence and absence of the sheet member and a downstream sensor arranged closer to the downstream point than to the upstream point for detecting presence and absence of the sheet member. The transfer mechanism is arranged to discharge the sheet member only after the downstream sensor detects the presence of the sheet member and the upstream sensor detects the absence of the sheet member.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a sheet transfer device used for transferring sheet materials such as recording paper sheets and image-carrying paper sheets. In particular, the present invention relates to a sheet transfer device provided with sensors for monitoring the transfer condition of the sheet materials. 
     2. Description of the Related Art 
     Sheet transfer devices are widely used in various electrical appliances, such as printers, facsimile machines and the like. Referring to FIG. 5A, a conventional sheet transfer device incorporated in a printer for example, may include a paper feed tray  2 ′ for holding recording paper sheets  1 ′, a feed roller  3 ′ disposed adjacent to the paper feed tray  2 ′, a platen roller  4 ′ arranged in facing relation to a printhead  5 ′ of the printer, discharge rollers  6 ′ and a discharged paper tray  7 ′. 
     Between the paper feed tray  2 ′ and the discharged paper tray  7 ′ extends a paper transfer path P along which the recording paper sheet  1 ′ is transferred when the above-mentioned rollers  3 ′- 6 ′ are rotated. 
     The conventional sheet transfer device is also provided with sensors  8 ′- 9 ′ for monitoring the transfer procedure of the recording paper sheet  1 ′. These sensors are disposed at two locations along the sheet transfer path P: at an upstream position A between the feed roller  3 ′ and the platen roller  4 ′, and at a downstream position B between the platen roller  4 ′ and the discharge rollers  6 ′. Typically, these sensors may be mechanical devices which are arranged to generate presence signals or absence signals, depending on whether the sensors  8 ′- 9 ′ are held in contact with the recording paper sheet  1 ′ or not. 
     The sheet transfer device having the above arrangement operates as follows. Upon actuation of the feed roller  3 ′, a recording paper sheet  1 ′ is pulled out of the paper feed tray  2 ′ and moved along the transfer path P toward the printhead  5 ′ of the printer. On its way, when the front end portion of the recording paper sheet  1 ′ comes into contact with the upstream sensor  8 ′, the upstream sensor generate presence signals, as previously stated. These signals are sent to a CPU (central processing unit) of the printer. Receiving the signals, the CPU begins to control the rotation of the feed roller  3 ′ so that the front edge of the recording paper sheet  1 ′ is properly brought to a position C between the platen roller  4 ′ and the printhead  5 ′. (The operation described here is called “the paper feeding step” below.) 
     When the paper feeding step is finished, the printhead  5 ′ starts printing on the recording paper sheet  1 ′. While the printing is performed, the recording paper sheet  1 ′ is fed stepwise in a known manner. (The operation here is called “the printing step” below.) 
     When the printing step proceeds smoothly (without having the recording paper sheet  1 ′ crumpled, for example), the front edge of the recording paper sheet  1 ′ is brought into contact with the downstream sensor  9 ′. Then, the downstream sensor  9 ′ sends presence signals to the CPU. Upon receiving the signals, the CPU determines that the sheet transfer operation is being properly performed. 
     Finally, when the printing step comes to an end and the rear end portion of the recording paper sheet  1 ′ leaves the upstream sensor  8 ′, the upstream sensor sends absence signals to the CPU. At this instant, the CPU changes the transfer mode for the recording paper sheet  1 ′ from the currently-performed stepwise motion to a continuous discharging motion, so that the recording paper sheet  1 ′ is swiftly sent to the discharged paper tray  7 ′. 
     The conventional sheet transfer device, however, has been found to have the following disadvantage. 
     As shown in FIG. 5B, the front end portion of the recording paper sheet  1 ′ is not held between the platen roller  4 ′ and the printhead  5 ′ during the paper feeding step described above. In this state, the recording paper sheet  1 ′ may unduly sag or prematurely leave the upstream sensor  8 ′. As a result, the absence signals are sent to the CPU of the printer, whereby the CPU, determining that the rear end portion of the recording paper sheet  1 ′ left the upstream sensor  8 ′, erroneously changes the transfer mode for the recording paper sheet l′ to the continuous discharging motion. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention is to provide a sheet transfer device capable of overcoming the disadvantage described above. 
     Another object of the present invention is to provide a printer and a facsimile machine which incorporate such a sheet transfer device. 
     According to a first aspect of the present invention, there is provided a sheet transfer device for transferring a sheet member along a transfer path having an upstream point and a downstream point, the sheet transfer device comprising: 
     a transfer mechanism for moving the sheet member along the transfer path from the upstream point to the downstream point; 
     at least one upstream sensor arranged closer to the upstream point than to the downstream point for detecting presence and absence of the sheet member; and 
     at least one downstream sensor arranged closer to the downstream point than to the upstream point for detecting presence and absence of the sheet member; 
     wherein the transfer mechanism is arranged to discharge the sheet member only after the downstream sensor detects the presence of the sheet member and the upstream sensor detects the absence of the sheet member. 
     With such an arrangement, it is possible to prevent the sheet member from being prematurely discharged from a transfer system of the transfer device. Thus, unlike in the conventional sheet transfer device, the sheet member and elements of the transfer system are advantageously protected from damage which would otherwise be caused by improper discharging of the recording paper. 
     According to a preferred embodiment, the transfer mechanism includes a feed roller arranged between the upstream point and the upstream sensor, an intermediate roller arranged between the upstream sensor and the downstream sensor, and a discharge roller arranged between the downstream sensor and the downstream point. 
     The upstream sensor and the downstream sensor may be mechanical sensing devices which produce presence signals when the mechanical sensing devices are held in contact with the sheet member, and absence signals when the mechanical sensing devices are held out of contact with the sheet member. 
     The mechanical sensing device may include a plate member pivotable about an axis upon contact of the sheet member. 
     The sheet transfer device may further comprise a controller for controlling the transfer mechanism based on the presence signals and absence signals from the upstream sensor and the downstream sensor. Such a controller may be provided by a CPU. 
     According to a second aspect of the present invention, there is provided a printer for printing on a sheet member transferred along a transfer path having an upstream point and a downstream point, the printer comprising: 
     a transfer mechanism for moving the sheet member along the transfer path from the upstream point to the downstream point; 
     at least one upstream sensor arranged closer to the upstream point than to the downstream point for detecting presence and absence of the sheet member; 
     at least one downstream sensor arranged closer to the downstream point than to the upstream point for detecting presence and absence of the sheet member; and 
     a printhead arranged between the upstream sensor and the downstream sensor; 
     wherein the transfer mechanism is arranged to discharge the sheet member only after the downstream sensor detects the presence of the sheet member and the upstream sensor detects the absence of the sheet member. 
     According to a third aspect of the present invention, there is provided a facsimile machine comprising: 
     a transfer mechanism for transferring a sheet member along a transfer path from an upstream point to a downstream point of the transfer path; 
     at least one upstream sensor arranged closer to the upstream point than to the downstream point for detecting presence and absence of the sheet member; 
     at least one downstream sensor arranged closer to the downstream point than to the upstream point for detecting presence and absence of the sheet member; and 
     an operation member arranged between the upstream sensor and the downstream sensor; 
     wherein the transfer mechanism is arranged to discharge the sheet member only after the downstream sensor detects the presence of the sheet member and the upstream sensor detects the absence of the sheet member. 
     The operation member may comprise a printhead or an image reader, depending on the applications. 
     Other objects, features and advantages of the present invention will be apparent from the detailed description of the embodiment given below with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
     FIG. 1 is a sectional side view showing a sheet transfer device incorporated in a facsimile machine embodying the present invention; 
     FIG. 2 is a circuit block diagram of a control system of the facsimile machine shown in FIG. 1; 
     FIG. 3 is a flow chart illustrating a printing procedure of the facsimile machine; 
     FIG. 4 schematically shows a CPU of the facsimile machine cooperating with sensors and rollers used in the sheet transfer device of FIG. 1; and 
     FIGS. 5A and 5B illustrate a conventional sheet transfer device and its operational disadvantage. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The preferred embodiment of the present invention will be described below with reference to the accompanying drawings. 
     Reference is first made to FIG. 1 which is a sectional side view showing a sheet transfer device (generally designated by reference A) embodying the present invention. In the illustrated embodiment, it is assumed that the sheet transfer device A is incorporated in a recording section  32  (see FIG. 2) of a facsimile machine. However, this arrangement is not limitative, and the sheet transfer device A of the present invention may be incorporated in a reading section  32  of the facsimile machine, or even in other electrical apparatus such as printers, photocopiers and the like. 
     As shown in FIG. 1, the sheet transfer device A includes a paper feed section  2  for holding recording paper sheets  1 , a feed roller  3  for pulling the recording paper sheets  1  from the paper feed section  2 , and a pair of discharge rollers  5  for discharging the recording paper sheet  1  into a discharge section  4 . 
     The recording paper sheet  1  comes into engagement with the feed roller  3  at a feed point  2   a,  while it comes out of engagement with the discharge rollers  5  at a discharge point  4   a . Though not shown, a predetermined sheet transfer path along which the recording paper sheet  1  is moved extends from the feed point  2   a  to the discharge point  4   a.    
     The feed roller  3  is rotated by a stepping motor (not shown) whose operation is controlled by a control system (described later) of the facsimile machine. A guide plate  3   a  is arranged adjacent to the feed roller  3 , so that the recording paper sheet  1  is transferred between the feed roller  3  and the guide plate  3   a.    
     The discharge rollers  5  are also driven by stepping motors (not shown) under the control of the above-mentioned control system. In operation, the recording paper sheet  1  is inserted between the discharge rollers  5  to be discharged into the discharge section  4 . 
     The illustrated facsimile machine has a printing unit  6  provided with a printhead  6   a  for performing printing on the recording paper sheet. As can be seen, the printhead  6   a  is arranged at an intermediate position along the sheet transfer path. Adjacent to the printhead  6   a , a platen roller  6   b  is provided in facing relation to the printhead. The platen roller  6   b  is also rotated by a stepping motor (not shown) under the control of the control system of the facsimile machine. 
     Ink ribbon  7  extends between the printhead  6   a  and the platen roller  6   b . As is shown, the ink ribbon  7  is supplied from a ribbon feed roller  8   a  and wound around another roller  8   b.    
     The sheet transfer device A also includes a plurality of sensors  9  (only one shown) and a plurality of sensors  10  (only one shown) for detecting the presence or absence of the recording paper sheet transferred along the sheet transfer path. The first or upstream sensors  9  are arranged between the feed roller  3  and the platen roller  6   b , while the second or downstream sensors  10  are arranged between the platen roller  6   b  and the discharge rollers  5 . 
     The upstream sensor  9  is a mechanical sensing device including a contact plate  9   a  pivotable about a horizontal axis. In FIG. 1, the initial position of the contact plate  9   a  is shown in solid lines. In the initial position, the first sensor  9  sends absence signals to a CPU (central processing unit) which is provided in the control system of the facsimile machine. 
     When the recording paper sheet  1  comes into contact with the contact plate  9   a  of the first sensor  9 , the contact plate  9   a  is rotated to take a position shown in double-dot chain lines. In this position, the first sensor  9  sends presence signals to the above-mentioned CPU. Thereafter, when the recording paper sheet  1  leaves the first sensor  9 , the contact plate  9   a  will return to the initial position. Thereafter, the first sensor  9  sends absence signals to the CPU. 
     The second sensor  10  is basically similar to the first sensor  9 . Namely, the second sensor  10  is a mechanical sensing device including a contact plate pivotable about a horizontal axis. When the second sensor  10  is in the initial position shown in solid lines, the sensor  10  sends absence signals to the CPU. When the second sensor  10  is held in a position shown in double-dot chain lines, it sends presence signals to the CPU. 
     Reference is now made to FIG. 2 which is a circuit block diagram showing the control system of the facsimile machine. In addition to the already-mentioned CPU (reference number  21 ), the control system includes an NCU (network control unit)  22 , a RAM (random access memory)  23 , a modem (modulator-demodulator)  24 , a ROM (read-only memory)  25 , an EEPROM (electrically erasable and programmable ROM)  26 , a gate array  27 , a codec (coder-decoder)  28 , and a DMAC (direct memory access controller)  29 . 
     Further, the control system also comprises a reading section  31 , a recording section  32 , an operation section  33 , and a display section  34 . 
     The CPU  21 , the NCU  22 , the RAM  23 , the modem  24 , the ROM  25 , the EEPROM  26 , the gate array  27 , the codec  28  and the DMAC  29  are connected to each other via bus lines. The bus lines include data bus lines, address bus lines and control signal bus lines. The gate array  27  is connected to the reading section  31 , the recording section  32 , the operation section  33  and the display section  34 , respectively. The NCU  22  is connected to a telephone line  41 . 
     The CPU  21  provides an overall control of the facsimile machine. As will be described in detail hereinafter, the CPU  21  includes a transfer controller  21   a  (see FIG. 3) for controlling the operation of the feed roller  3 , platen roller  6   b  and discharge rollers  5 . 
     The NCU  22  is connected to the telephone line  41  for providing network control. The NCU  22  is also connected to the modem  24 . The modem  24  modulates the transmitting codes and demodulates the received codes. 
     The RAM  23  stores various data such as image data for example. Of course, an additional RAM or RAMs may be provided to increase the capacity of data storage. 
     The ROM  25  stores various programs, initial value data or the like as required for controlling the facsimile machine, whereas the EEPROM  26  stores registered data (e.g. shortcut dials) and/or various flags. 
     The gate array  27  functions as an I/O interface (input/output interface) for the CPU  21  for data transmission to and/or from the reading section  31 , the recording section  32 , the operation section  33  and the display section  34 . The gate array  27  also performs various kinds of processing for e.g., image signals from the reading section  31 . 
     The codec  28  performs coding of the transmitting image data and decoding of the received image data, for example. 
     The DMAC  29  provides memory access control with respect to the RAM  23  for example. 
     The reading section  31  includes an image reading apparatus for reading out images on an image carrying paper sheet (not shown) and supplying image signals corresponding to the images. 
     The recording section  32  performs printing of images on a recording paper sheet (not shown) on the basis of the image data received through the gate array  27 . The sheet transfer device A shown in FIG. 1 is arranged in the recording section  32 . However, as can be easily seen, an additional sheet transfer device according. to the present invention may be provided in the reading section  31  as well. 
     The operation section  33  has key switches to be operated by the user for output of operation signals, whereas the display section  34  includes an LCD (liquid crystal display) or the like for providing various indications under the control of the CPU  21 . 
     Referring to FIGS. 3 and 4, description will now be made to the function of the sheet transfer device A. First, in order to start a printing procedure, the CPU  21  sends driving signals to the stepping motor associated with the feed roller  3 . As a result, the feed roller  3  begins to rotate (S 1  in FIG. 3) to shift a recording paper sheet  1  from the paper feed section  2  toward the upstream sensor  9 . 
     Once the printing operation is started, the transfer controller  21   a  of the CPU  21  determines whether the upstream sensor  9  is sending presence signals or absence signals (S 2 ). When presence signals are received (S 2 :YES), the transfer controller  21   a  of the CPU  21  controls the rotation of the feed roller  3  so that the roller  3  is rotated through an angle sufficient enough to bring the front edge of the recording paper sheet  1  to a position between the platen roller  6   b  and the printhead  6   a  (S 3 ). 
     During the above procedure (hereinafter referred to as “the paper feeding step”), the transfer controller  21   a  of the CPU is monitoring whether the upstream sensor  9  is still sending presence signals, or, instead, absence signals (S 4 ). If is is presence signals (S 4 :YES) that the CPU  21  receives, the paper feeding step is continued. 
     If it is absence signals (S 4 :NO), which means that the recording paper sheet prematurely left the upstream sensor  9 , the procedure goes to an error-fixing step (S 12 ). Then, under the control of the CPU  21 , necessary measures are taken, such as stopping the rotation of the feed roller  3  while informing the user of the error by turning on an error indicator, for example. 
     With such an arrangement, it is possible to prevent the recording paper sheet  1  from being unduly discharged even when the paper sheet  1  is prematurely detached from the upstream sensor  9 . 
     When YES in S 4  (namely, the paper feeding step is properly finished), the printhead  6   a  starts printing on the recording paper sheet  1  (S 5 ). During this procedure (hereinafter referred to as “the printing step”), the feed roller  3  and the platen roller  6   b  are rotated stepwise under the control of the transfer controller  21   a  of the CPU  21 . 
     After the printing step begins, the transfer controller  21   a  of the CPU is monitoring whether the upstream sensor  9  is still sending presence signals, or absence signals instead (S 6 ). If is is presence signals (S 6 :YES) that the CPU  21  receives, the printing step is continued. 
     If it is absence signals (S 6 :NO), which means that the recording paper sheet prematurely left the upstream sensor  9 , the procedure goes to the error-fixing step (S 12 ). Then, under the control of the CPU  21 , necessary measures are taken, such as stopping the rotation of the feed roller  3  while informing the user of the error by turning on an error indicator, as stated previously. 
     During the printing step, the CPU  21  monitors if a predetermined time has passed or not (S 7 ). Here, the predetermined time is defined as a time which is taken for the front edge of the recording paper sheet to contact with the downstream sensor  10  after the front edge leaves the platen roller  6   b . When NO in S 7 , the procedure returns to S 6 . 
     When YES in S 7 , the transfer controller  21   a  of the CPU  21  determines whether the downstream sensor  10  is sending presence signals, or absence signals instead (S 8 ). If is is presence signals (S 8 :YES), which means that the front edge of the recording paper sheet  1  properly came into contact with the downstream sensor  10  in the predetermined time, the printing step is continued. 
     If it is absence signals (S 8 :NO), which means that the recording paper sheet  1  failed to reach the downstream sensor  10  in the predetermined time, the procedure is subjected to the error-fixing step (S 12 ). 
     When YES in S 8 , the transfer controller  21   a  of the CPU  21  monitors whether the upstream sensor  9  is still sending presence signals, or absence signals instead. If it is presence signals (S 9 :YES) that are received, the transfer controller  21   a  continues the monitoring. 
     If it is absence signals (S 9 :NO), a discharging step is started (S 10 ) for sending the recording paper sheet to the discharge section  4 . 
     Finally, the transfer controller  21   a  of the CPU  21  monitors whether the downstream sensor  10  is sending presence signals or absence signals (S 11 ). If it is presence signals (S 11 :YES), the transfer controller  21   a  continues the monitoring. If it is absence signals (S 11 :NO), the entire printing procedures end. 
     As described above, according to the present invention, it is possible to prevent the recording paper sheet from being prematurely discharged from the transfer system of the printer. Thus, unlike in the conventional sheet transfer device, the recording paper sheet and elements of the transfer system are advantageously protected from damage which would otherwise be caused by improper discharging of the recording paper. 
     As previously mentioned, the sheet transfer device of the present invention may be incorporated in the reading section  32  of the facsimile machine. In that case, the printhead  6   a  shown in FIG. 1 may be replaced with an image reader. 
     The present invention being thus described, it is obvious that the same may be varied in many ways. Such variations should not be regarded as a departure from the spirit and scope of the present invention, and all such modifications as would be obvious to those skilled in the art are intended to be included within the scope of the following claims.