Abstract:
An electrophotographic device that allows a user to produce optimum image density images on a variety of recording medium. The electrophotographic device can sense the path a sheet of recording medium takes when being conveyed to an image control unit. From this information, the electrophotographic device can sense from which one of a plurality of input trays the sheet of recording medium originated from, and then automatically adjust the transfer voltage based on which tray the sheet of recording medium originated from. If the recording medium is fed through the automatic feeder, a first transfer voltage is automatically applied during the printing process. If the sheet of recording medium is inserted through a manual feed unit, a second transfer voltage having a greater magnitude than the first transfer voltage is automatically applied during the printing process. Thus, it is possible to achieve optimum image density for a variety of recording medium having a variety of thicknesses.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 arising from an application for automatic Transfer Voltage Changing Method In Accordance With Thickness of Printing Paper earlier filed in the Korean Industrial Property Office on Oct. 20, 1995 and there duly assigned Ser. No. 36343/1995. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an image forming apparatus using an electrophotographic developing method, and more particularly, to a method of automatically changing the transfer voltage depending upon a path of the recording media. 
     BACKGROUND OF THE INVENTION 
     In order to achieve optimum image density on a piece of recording media in the electrophotography process, the thickness of the document must be considered. If a thick document, such as a postcard, transparency, envelope or, label is to be printed, the transfer voltage needs to be increased in order to achieve a quality result. Contemporary practice in the art, represented by U.S. Pat. No. 5,486,903 for an Image Forming Apparatus With Paper Thickness Detector to Kanno et al., endeavors to construct an image forming apparatus by using a capacitive thickness detector for the recording medium. The recording medium is passed between the plates of a capacitor biased at a fixed voltage, and the capacitance is then measured. From the value of the measured capacitance, the thickness of the printing medium may be determined. After the thickness of the printing media is determined, memory is consulted to determine the appropriate transfer voltage that needs to be applied to the transfer roller to obtain optimum image density on the printing media. Thus, an appropriate bias voltage is applied in the electrophotographic process depending on the gauged thickness and electrical resistance of the printing media. Additional examples of recent efforts in the art are found in U.S. Pat. No. 5,530,522 for an Image Forming Apparatus With Controlled Transfer Voltage to Tsunerni, U.S. Pat. No. 5,455,664 for an Electrophotographic Printer For Transferring Images on Different Sized Print Medium And Transferring Method Of The Same to Ito et al, U.S. Pat. No. 5,374,981 for an Electrostatic Recording Apparatus Capable of Maintaining Constant Gap Between Flexible Recording Electrodes and Opposite Electrode By Flexible Recording Electrodes to Yamamoto et al., U.S. Pat. No. 5,504,565 for an Image Forming Apparatus Having Transfer Voltage Timing Control to Tomiki et al., and U.S. Pat. No. 5,250,999 for an Image Forming Apparatus Having Transfer Voltage And Process Speed Control to Kimura et al. In Tsunemi &#39;522 and Ito et al. &#39;644, the transfer voltage is adjusted depending on the width of the recording media, in Yamamoto et al. &#39;981 and in Tomiki et al. &#39;565, the transfer voltage is adjusted depending on the thickness and the size of the recording media, while in Kimuranet, et al. &#39;999 the transfer voltage is adjusted according to the type of recording medium. 
     Other efforts in the art sought to provide electrophotographic copiers that allow for and contain numerous input ports for inserting the recording media. For example, U.S. Pat. No. 5,237,378 for a Copier/Printer Employing a Roll Media Feed Apparatus and Dual Functions Sensors to McEwen discloses a copier/printer that has three inputs to accommodate for three different recording media. Each input has a sensor to detect whether or not that particular recording medium is being conveyed to the printer in order to detect jamming in the photocopying machine. Thus, McEwen &#39;378 uses sensors on each recording medium input feed path to aid in the process of the printing operation. 
     Another example is found in U.S. Pat. No. 5,530,522 for an Image Forming Apparatus With Controlled Transfer Voltage to Inage et al. which uses an electrophotographic machine containing a plurality of recording medium trays and recording medium conveyer paths that merge into a single conveyer path before entering the printing unit. Inage et al. &#39;489 has a controller which dictates when a sheet of recording medium is to be dispensed, and from which tray the sheet is to be dispensed. The controller decides the sequence and the quantity and the timing for dispensation of the recording medium from each of the plurality of feeding trays. Inage et al. &#39;489 seeks to overcome the situation of having two subsequent sheets of recording medium overlap during the conveying process. Therefore, in Inage et al &#39;489, it is essential to know from which feeding tray each recording sheet originates from so that the timing between each sheet of recording medium does not go awry. 
     U.S. Pat. No. 5,321,485 for a Printing Apparatus Having Manual Sheet Feeding And Document Reading Capabilities to Nukaya discloses a manual feed electrophotographic printing apparatus containing two paths by which the paper may be transported. In addition, a CPU is connected to the electrophotographic machine. A path selector can be set to either a first position or a second position, where the first position ejects the document and the second position guides the sheet to a printer. If the document is ejected, a sensor senses this ejection and sends a signal to the CPU accordingly. If the document is sent to the printer, another sensor senses the presence of the document in the printer and sends a signal to the CPU. Thus, the electrophotographic operation depends upon signals received from sensors along the path taken by the recording medium. I have found that designs such as those represented by McEwen &#39;378, Inage &#39;489 and Nukaya &#39;485 lack an ability to adjust the transfer voltage based on any of these sensed signals. 
     U.S. Pat. No. 5,444,524 for a Method and Apparatus For Controlling a Print Engine Of A Page Printing Device to Lee endeavors to control the electrophotographic printing process, but lacks ability to accommodate multiple trays of a recording medium. I have discovered that there is a need for an electrophotographic machine that varies the transfer voltage on the basis of the path taken by the sheet of recording medium. In other words, it is my belief that what is needed is an electrophotographic process that automatically adjusts process parameters on a basis of the tray from which the sheet of recording medium originates, so that optimum image density is achieved. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is an object to provide an improved electrophotographic photocopier. 
     It is another object to provide an electrophotographic photocopying machine that optimizes image density by automatically adjusting the transfer voltage in accordance with the tray or feeding unit from which the sheet of recording medium originates. 
     It is still another object to sense the path of conveyance of a sheet of recording medium and to adjust the transfer voltage accordingly to achieve optimal image quality. 
     It is yet another object to provide an engine controller for the electrophotographic process that receives signals from sensors disposed to detect the path of a sheet of recording medium and to generate a representative control signal to a transfer voltage generator based on the representative signals from sensor signals. 
     These and other objects may be attained according to the principles of the present invention by recognizing that if an electrophotographic, device has both an automatic feeding cassette and a manual feeding tray, it may be possible to print quality images on a variety of recording media by sensing the path of the recording media instead of measuring its thickness. In other words, by being able to detect from which input cassette the recording media originates, the transfer bias may be appropriately adjusted in order to optimize image density and image quality. This may be implemented with a transfer voltage generator that generates a first transfer voltage when a first control signal is received from the engine controller, and outputs a second transfer voltage of a different magnitude than the first transfer voltage when a second control signal is received from the engine controller so that one optimal transfer voltage is applied when the recording medium originates from the manual feeding tray and that a relatively different but nonetheless optimal transfer voltage is applied to the electrophotographic process when the recording medium originates from the automatic feeding cassette. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detail description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a schematic diagram illustrating an engine mechanism of an electrophotographic printer constructed according to the principles of the present invention; 
     FIG. 2 is a schematic block diagram illustrating a printer of the present invention using an electrophotographic developing method; and 
     FIG. 3 is a flow chart illustrating operations of sensing the path of manually fed recording media, then automatically changing the transfer voltage according to the sensed path of the recording media. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, FIG. 1 shows the engine mechanism of the printer using the electrophotographic developing method, which includes a photosensitive drum  100 , a charging unit  102 , an exposing unit  104 , a developing roller  106 , a transfer roller  108 , a fixing unit  124 , a pick-up roller  116 , a feed roller  114 , a register-roller  110 , a register-sensor  112 , an exit sensor  126 , a manual paper feeding tray  122 , an automatic paper feeding cassette  120 , and a manual feed sensor  118 . In FIG. 1, charging unit  102  forms a layer having a uniform charge density on the photosensitive drum  100 , and exposing unit  104  forms electrostatic latent image on the photosensitive drum  100 . The developing unit  106  provides toner that is developed on the electrostatic latent image formed on the photosensitive drum  100 , and transfer roller  108  transfers the toner image formed on photosensitive drum  100  to the recording medium. The fixing unit  124  fixes the toner image to the recording medium by applying heat and pressure to the recording medium containing the toner image. 
     An automatic paper feeding cassette  120  stores printing papers, and pick-up roller  116  picks up the printing papers stored in the automatic paper feeding cassette  120  and then delivers them, one by one, to image forming unit  10 . Feed roller  114  feeds the printing paper transferred by the pick-up roller  116  to image forming unit  10 , and register-roller  110  arranges the papers fed by feed roller  114 . The register-sensor  112  senses the delivery state of the printing paper to sense whether or not a paper jam has occurred, and the manual paper feeding tray  122  transfers recording medium having a larger than standard thickness placed in manual paper feeding tray  122  to image forming unit  10 . 
     Hereinafter, an embodiment of the image forming procedure of the printer using the electrophotographic developing method will be explained with reference to FIGS. 1 and 2. First, if a print command from an external host computer is transmitted to engine controller  144 , engine controller  144  drives pick-up roller  116  to feed a sheet of the printing paper stored in the automatic paper feeding cassette  120 , while and at the same time, raises the temperature of the fixing unit  124  up to a printing temperature. At this time, feed roller  114  continuously conveys the printing papers to the image forming unit  10 , and photosensitive drum  100 , charging unit  102 , developing roller  106 , transfer roller  108 , and exposing unit  104  respectively operate in a predetermined time sequence under the control of print control unit  140 . The surface of the photosensitive drum  100  is uniformly charged by charging unit  102 , and generally has a surface potential between −600 and −800V. If the printing paper fed by feed roller  114  is delivered to the register-roller  110  through the register-sensor  112 , a paper sensing signal by register-sensor  112  is input to print control unit  140  through sensor circuit  148 . In, the meantime, the register-roller  110  arranges for the delivery of printing papers to image forming unit  10 . The print control unit  140 , which inputs the paper sensing signal, transmits received image data to exposing unit  104 , and then exposing unit  104  exposes light in accordance with transmitted image data onto photosensitive drum  100 . 
     At this time, the exposed portion on the photosensitive drum  100  loses the voltage thereon due to the generation of the light carrier and accordingly, toner can be attached to the exposed portion. Further, the developing roller  106  for delivering toner to the photosensitive drum  100  rotates, while a toner blade regulates the amount and thickness of toner delivered to photosensitive drum  100 . At this point, toner is attached on the exposed portion on the photosensitive drum  100  while no toner is attached to the unexposed portions. Thus, a toner image is formed on photosensitive drum  100 . Toner attached to the photosensitive drum  100  is transferred to the printing paper by an electric field formed between transfer roller  108  and photosensitive drum  100 . Toner transferred onto printing paper is fixed to the printing paper by the heating roller and the pressure roller of fixing unit  124 , and then, the printed paper is discharged to the outside of the electrophotographic device. 
     The foregoing procedure was explained with the standard photocopying paper supplied from the automatic paper feeding cassette. On the other hand, in case of using extra thick recording medium having a higher electrical resistance than standard photocopying papers, the electric field between photosensitive drum  100  and transfer roller  108  is reduced due to the high resistance caused by the extra thickness of the recording medium, resulting in less toner being transferred from photosensitive drum  100  to the thick recording medium, resulting in a reduced image density. To solve the above problem, a sensor  118  is installed on a recording medium conveyor path next to manual recording media feeding tray  122 . Sensor  118  is to be triggered whenever a sheet of recording medium is inserted into the manual feeding tray  122  and is conveyed towards image forming unit  10 . 
     FIG. 2 shows a schematic block diagram of a printer according to the present invention connected a print engine unit  140 , which is connected to a host computer made up of a video controller  130  and an operating panel OPE  138 . The video controller  130  is further made up of a computer interface  132 , a video control unit  134  and an engine interface  136 . The computer interface unit  132  is connected between a host computer and video control unit  134 , and serves to interface an input or an output signal therebetween. Video control unit  134  includes a read-only memory ROM storing a control program as well as a random access memory RAM for temporarily storing various kinds of data to be input from the OPE  138  and from the host computer, and also changes code data received from the computer interface  132  to image data to be processed by the print control unit  140  and to transmit the image to the print control unit  140 . The engine interface  136  interfaces the signal to be input or an output transmitted between video control unit  134  and print control engine  140 . In addition, the OPE  138  is controlled by the video control unit  134 , and includes a plurality of keys for inputting various commands, and a display unit for displaying information according to operations.,of the printer. The print control unit  140  is made up of a video interface  142 , an engine controller  144 , an input/output interface  146 , a sensor circuit  148  and a transfer voltage generator  150 , and is connected to the video controller  130 . The video interface  142  provides an interface for transmitting and receiving signals between video controller  130  and engine controller  144 . Engine controller  144  controls each unit of the print control unit  140  including the transfer voltage generator  150 , according to the control of the video controller  130 , and then prints image data received from the video controller  130  on the recording medium. Furthermore, the engine controller  144  generates either a first or a second control signal, depending upon whether the manual paper feed sensing signal is received from the sensor circuit  148 , and outputs, via transfer voltage generator  150 , the appropriate voltage to be applied to transfer roller  108 . A first control signal indicates that the recording medium is of standard thickness, and a second control signal indicates that the recording medium has a thickness greater than the standard thickness. The I/O interface  146  is connected between the engine controller  144 , the sensor circuit  148  and the transfer voltage generator  150 , and serves to interface input/output signal of the engine controller  144 . The sensor circuit  148  drives various sensors for sensing the operating state of each unit of the print control unit  140 , including the manual feeding sensor  118  and the register-sensor  112 , and for sensing the amount off toner used, and provides a recording medium sensing signal to the engine controller  144  according to one embodiment of the present invention. The transfer voltage generator  150  generates a transfer voltage that depends upon whether the first or the second control signal is received from engine controller  144 . Transfer voltage generator  150  then applies the appropriate transfer voltage to transfer roller  108 . That is, if the first control signal is received by voltage transfer generator  150 , transfer voltage generator  150  applies the first voltage to transfer roller  108 . This voltage is applied to transfer roller  108 , resulting in an electric field intensity across the recording medium resulting in an image of optimum image density on the photocopying paper having a standard thickness. If the second control signal is received by voltage transfer generator  150 , voltage transfer generator  150  generates a second transfer voltage which is applied to transfer roller  108 . This second voltage is greater than the first voltage and is intended for recording medium having a thickness greater than that of standard printing paper. This results in an electric field intensity across the recording medium so that optimum image density is realized provided the recording medium has a thickness greater than the standard thickness for photocopying paper. 
     FIG. 3 shows a flow chart illustrating the control operation for sensing the origin of the recording medium and adjusting the electrophotographic process accordingly. First, if the printer using the electrophotographic process is energized, engine controller  144  initializes each unit of print control unit  140  according to an initializing program in step  200 , and then proceeds to step  202  to maintain a print stand-by mode. After that, the engine controller  144  checks whether or not a print command is received from video control unit  134 , as in step  204 , and if the print command is then received, the control operation proceeds to step  206  to check whether or not the manual feed sensing signal generated from sensor  118  is received. 
     As a result of this check, if sensor  118  does not sense the conveyance of recording medium from manual feeding tray  122 , and if engine controller  144  has already received a print command from video control unit  134 , the control operation proceeds to step  210  where engine controller  144  outputs the first control signal to the transfer voltage generator  150 . At this point, printing paper of standard thickness is being conveyed from automatic paper feeding cassette  120  and is being delivered to image forming unit  10 . Upon receiving a first control signal by transfer voltage generator  150 , transfer voltage generator  150  applies a first voltage in step  211  to transfer roller  108 , creating a first potential difference in the gap between transfer roller  108  and photosensitive drum  100 . Then, in step  212 , a sheet of the printing paper of standard thickness is conveyed between transfer roller  108  and photosensitive drum  100 , causing the toner image to be transferred from the photosensitive drum  100  onto the printing paper. Lastly, the toner image is fixed to the printing paper in step  220  by fixing unit  124 . 
     On the other hand, if sensor  118  does sense the passage of a sheet of recording medium from manual feeding tray  122 , a sheet greater than standard thickness is assumed to be conveyed from manual feeding tray  122 , and the control process proceeds to step  208  where engine controller  144  outputs a second control signal to transfer voltage generator  150 . Upon receiving the second control signal by transfer voltage generator  150 , transfer voltage generator  150  in step  209  applies a second and larger voltage to transfer roller  108  than in the case where the sheet originated from automatic paper feeding cassette  120 , creating a larger potential difference in the gap between transfer roller  108  and photosensitive drum  100 . Then, in step  212 , the sheet of recording medium is conveyed between transfer roller  108  and photosensitive drum  100 , causing the toner image to be transferred from photosensitive drum  100  onto the thick recording medium. Lastly, the toner image is fixed onto the recording medium in step  220  by fixing unit  124 . 
     The engine controller  144  controls timing and operation of photosensitive drum  100 , charging unit  102 , developing roller  106  and exposing unit  104 , among other components involved in the process, in a predetermined time sequence in step  212  to perform a printing operation. If the recording medium is fed through the manual paper feeding tray  122  and is detected by sensor  118 , a higher voltage is applied to transfer roller  108 , and the amount of toner transferred on the recording medium is increased, thereby obtaining optimum image density provided a thicker than standard sheet of recording medium was inserted into manual paper feeding tray  122 . In view of an experimental value, in case of using printing paper having a standard thickness, we can obtain optimum image density when transfer voltage is between 0.9 kV and 1.0 kV at a normal temperature and humidity. On the other hand, in case of using the recording medium having a greater than standard thickness, optimum image density is achieved when the transfer voltage is between 1.3 kV and 1.4 kV. Namely, the transfer voltage generated by the second control signal is greater than that generated by the first control signal. 
     According to the present invention, the manual feed tray  122  is referred to as a manual feed tray for supplying sheets of recording medium one by one as well as a unit (or tray) and for transferring the sheets of recording medium through the manual feed opening. As is apparent from the foregoing, when the recording medium having a larger than standard thickness is fed from manual paper feeding tray  122  to image forming unit  10 , a higher transfer voltage is needed than that necessary for transferring toner to the recording medium than for papers having just a standard thickness. This higher transfer voltage is achieved by applying a larger than normal voltage to transfer roller  108 , thereby increasing the amount of toner transferred to the thick recording medium. As a result, optimum image density is achieved in all cases, even when the thickness or resistance of the recording medium is greater than normal. 
     Therefore, it should be understood that the present invention is not limited to the particular embodiment disclosed herein as the best mode contemplated for carrying out the present invention, but rather that the present invention is not limited to the specific embodiment described in this specification except as defined in the appended claims.