Abstract:
A method and apparatus that measures a thickness of a sheet of recording medium prior to image formation. Once measured, the transfer roller and the fusing roller may be set apart at an optimum distance to achieve optimum image quality for any thickness of a sheet of recording medium. Employed is a simple, non-contact thickness sensing apparatus and method. Inductive electromotive force or intensity of light reflected off a movable metallic roller are employed to determine the thickness of the sheet of recording medium. The result is excellent image quality transferred from a photoreceptor web and onto a sheet of recording medium, regardless of the thickness of the sheet of recording medium.

Description:
CLAIM OF PRIORITY 
     This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from the inventor&#39;s application PRINTER AND METHOD OF CONTROLLING THE GAP OF FUSING ROLLER filed with the Korean Industrial Property Office on Feb. 21, 2000 and there duly assigned Serial No. 8180/2000. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a printer in which the gap between the transfer roller and the fusing roller is adjusted according to the thickness of paper inserted therebetween, and a fusing roller gap adjusting method thereof. 
     2. Description of the Related Art 
     In printing apparatuses, a transfer roller transfers an image from a photoreceptor web onto a sheet of recording medium. A fusing roller forms a nip with the transfer roller and is often diametrically opposite the photoreceptor web as a sheet of recording medium passes between the transfer roller and the fusing roller. If the sheet of recording medium is thin or of standard thickness, the image successfully transfers onto the sheet of recording medium. However, if the sheet of recording medium is unusually thick, the image transfer is poor as the gap is too small between the transfer roller and the fusing roller. 
     What is needed is a printing apparatus that can adjust the size of the gap between the transfer roller and the fusing roller automatically based on the thickness of each sheet of recording medium so that unusually thick sheets, as well as thin or standard thickness sheets, can obtain a good image quality. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an image forming apparatus where the gap between the fusing roller and the transfer roller can automatically be adjusted based on the thickness of the recording medium. 
     It is also an object to provide an image forming apparatus that can print high quality images on thin or standard thickness recording medium as well as on unusually thick sheets of recording medium. 
     It is further an object to provide an image forming apparatus that can measure the thickness of the sheet of recording medium prior to when said sheet of recording medium reaches the transfer roller and the fusing roller. 
     It is yet another object to provide an apparatus that can measure the thickness of a sheet of recording medium prior to when said sheet of recording medium reaches the transfer roller/fusing roller combination. 
     It is still another object to provide a mechanism for adjusting the gap between the transfer roller and the fusing roller by moving the fusing roller towards or away from the transfer roller prior to printing on a sheet of recording medium. 
     It is further an object of the present invention to provide a method for measuring a thickness of a sheet of recording medium, and based on said measurement, adjust the gap between the transfer roller and the fusing roller to provide optimum image quality on said sheet of recording medium. 
     Accordingly, to achieve the above object, there is provided a printer comprising a printing portion for forming a toner image on a circulating photoreceptor web, a transfer roller rotating in contact with the photoreceptor web so that the toner image formed on the photoreceptor web is transferred, a fusing roller installed to face the transfer roller, conveying rollers installed along a paper conveying path from a paper feeding portion to the transfer roller and the fusing roller, to be capable of being released corresponding to the thickness of the paper passing between the transfer roller and the fusing roller, a fusing roller gap adjusting portion for adjusting a separation distance of the fusing roller with respect to the transfer roller, a displacement measuring sensor installed to face the conveying roller for measuring a vertical displacement of the conveying roller varying according to the thickness of the paper passing the conveying roller, and a paper thickness calculating portion for calculating the thickness of the paper passing the conveying roller from the vertical displacement information output from the displacement measuring sensor and controlling the fusing roller gap adjusting portion so that a separation distance set to correspond to the calculated paper thickness is maintained between the transfer roller and the fusing roller. 
     It is preferred in the present invention that a surface of the conveying roller is formed of metal, the displacement measuring sensor is an electromagnetic induction sensor for forming an electromagnetic field with respect to the conveying roller and outputting an electric signal induced corresponding to the displacement of the conveying roller, and the paper thickness calculating portion calculates the thickness of the paper passing the conveying roller from the electric signal output from the electromagnetic induction sensor corresponding to the vertical displacement of the conveying roller. 
     Also, to achieve the above object, there is provided a fusing roller gap adjusting method of a printer comprising the steps of (A) calculating a reference position value of the conveying roller from the signal output from the displacement measuring sensor before the paper enters the conveying roller, (B) calculating the thickness of the paper by subtracting the reference position value from the vertical displacement value calculated from the value output from the displacement measuring sensor corresponding to the vertical displacement of the conveying roller when the paper passes the conveying roller, and (C) adjusting the position of the fusing roller so that a gap is set to correspond to the calculated paper thickness value is maintained between the transfer roller and the fusing roller. 
     It is preferred in the present invention that, in step (A), the reference position value is determined by averaging displacement values calculated from the values output from the displacement measuring sensor while rotating the conveying roller at least one time. 
     Also, it is preferred in the present invention that, in step (A), the maximum value of displacement values calculated from the values output from the displacement measuring sensor while rotating the conveying roller at least one time is determined as the reference position value. 
    
    
     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 detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components: 
     FIG. 1 is a view illustrating a printer; 
     FIG. 2 is a view illustrating a printer according to the present invention; 
     FIG:  3  is a view illustrating a displacement measuring sensor according to a preferred embodiment of the present invention; and 
     FIG. 4 is a flow chart for illustrating the process of adjusting the gap between the transfer roller and the fusing roller of the printer according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 is a view illustrating a printer. Referring to the drawing, the printer includes a printing portion  10 , a transferring portion  20  and a paper feeding portion  3   0 . The printing portion  10  includes a photoreceptor web  11 , a discharger  12 , a charger  13 , a plurality of optical scanning units  14 , a plurality of developing units  15 , and a drying unit  16 . The discharger  12  erases an electrostatic latent image formed on a photoreceptor web  11  by emitting light to the photoreceptor web  11  circulating around a plurality of rollers  17 . The charger  13  charges the photoreceptor web  11  to a predetermined electric potential so that a new electrostatic latent image is written on the photoreceptor web  11 . The optical scanning units  14  selectively emit light corresponding to an image to form an electrostatic latent image on the photoreceptor web  11 . The developing units  15  develop the electrostatic latent image by supplying developer to the photoreceptor web  11 . The drying unit  16  is applied when a mixture of toner and liquid carrier component is used as developer. The drying unit  16  dries and removes the liquid carrier which remains on the photoreceptor web  11 . 
     The transferring portion  20  includes a transfer roller  21  which rotates in contact with the photoreceptor web  11  and a fusing roller  22  installed to face the transfer roller  21 , so that the toner image formed on the photoreceptor web  11  is transferred to a sheet of paper  31  inserted between the transfer roller  21  and the fusing roller  22 . The paper feeding portion  30  includes a paper feeding cassette  32 , a pickup roller  33  for picking up the paper  31  contained in the paper feeding cassette  32  and a plurality of conveying rollers  35  installed along a paper conveying path  34  from the pickup roller  33  to the transferring portion  20  to convey the picked up paper. 
     In the operation of the printer having the above structure, each of the optical scanning units  14  emits light corresponding to image information to the photoreceptor web  11  circulating at a constant speed. The developing units  15  develop the electrostatic latent image formed by the optical scanning units  14 . After passing the drying unit  16 , the toner image formed on the photoreceptor web  11  is transferred by the transfer roller  21  to the paper  31  which is supplied from the paper feeding portion  30 . Since the above printing process is continuously performed, while the existing toner image on the photoreceptor web  11  is transferred on the paper  31 , a new subsequent image is formed on the photoreceptor web  11  by the optical scanning unit  14  and the developing unit  15 . 
     In earlier image forming apparatuses, the transfer roller  21  and the fusing roller  22  are installed such that the relative positions thereof is maintained to be constant while rotating. As a result, when a thick paper is supplied and not smoothly inserted between the transfer roller  21  and the fusing roller  22 , the rotation of the transfer roller  21  is hindered due to its thickness. That is, as the gap between the transfer roller  21  and the fusing roller  22  cannot be adjusted corresponding to the thickness of the paper  31 , the paper  31  proceeding by the conveying rollers  35  along the paper conveying path  34  momentarily stops at the entry in the gap between the transfer roller  21  and the fusing roller  22 . In this case, the paper  31  momentarily restrains the rotation of the transfer roller  21  and accordingly the speed of the photoreceptor web  11  circulating in contact with the transfer roller  21  is momentarily lowered. Such a change in the rotation speed of the transfer roller  21  and the circulation speed of photoreceptor web  11  causes an image forming error. That is, while the optical scanning units  14  scan information about the next image onto the photoreceptor web  11  in tune with a set speed of the photoreceptor web  11 , the circulation speed of the photoreceptor web  11  is momentarily lowered due to the above disorder of conveying the paper  31  inserted between the transfer roller  21  and the fusing roller  22 . Here, since the interval between image lines currently formed on the photoreceptor web  11  is narrowed, the image forming error is generated. The above phenomenon becomes more serious as the thickness of the paper  31  increases. 
     In FIG. 2, the same reference numerals as those in FIG. 1 denote the same elements having the same functions. Referring to FIG. 2, the printer includes the printing portion  10 , the transfer portion  20 , the paper feeding portion  30 , a displacement measuring sensor  40 , a fusing roller gap adjusting unit  50  for adjusting a gap between a fusing roller and a transfer roller, and a paper thickness calculating unit  60 . Reference numerals  36  and  37  denote sensors indicating arrival of paper proceeding along the paper conveying path  34  from their respective positions. The printing portion  10  includes the photoreceptor web  11 , the discharger  12 , the charger  13 , the optical scanning units  14 , the developing units  15  and the drying unit  16 . The transfer portion  20  includes the transfer roller  21  rotating in contact with the photoreceptor web  11  and a fusing roller  22 ′ installed to face the transfer roller  11  and capable of adjusting a relative gap between the transfer roller  21  and the fusing roller  22 ′ under the control of the fusing roller gap adjusting unit  50 . For example, the fusing roller  22 ′ has a shaft which can move up and down perpendicularly with respect to the transfer roller  21  by being driven by the fusing roller gap adjusting unit  50 . The fusing roller gap adjusting unit  50  adjusts the position of the fusing roller  22 ′ so that a gap according to control information output from the paper thickness calculating unit  60  can be maintained between the transfer roller  21  and the fusing roller  22 ′. 
     The displacement measuring sensor  40  outputs a signal corresponding to the thickness of the paper  31  on the paper conveying path  34  from the paper feeding portion  30  to the transferring portion  20 . Preferably, the displacement measuring sensor  40  can detect a vertical displacement of the conveying roller  35   a  selected from the conveying rollers  35  installed along the paper conveying path  34 . For example, a variable resistor (not shown) for determining a value of resistance corresponding to a vertical displacement, by being directly engaged with the vertical displacement of the conveying roller  35   a , is coupled to the conveying rolls  35   a , and a circuit for outputting a value of voltage corresponding to the vertical displacement is provided. Alternatively, a circuit for determining a value of electrostatic capacity corresponding to the vertical displacement of the conveying roller  35   a  provided. Alternatively, light is emitted to the conveying roller  35   a  from a fixed position and the change in the amount of light reflected by the conveying roller  35   a  corresponding to the vertical displacement of the conveying roller  35   a  is measured by a photodetector (not shown). Preferably, the displacement measuring sensor  40  has a structure so that the conveying roller  35   a  can be easily disassembled when the conveying roller  35   a  is to be replaced, and that a paper thickness measuring error is less generated even when foreign material separated from the paper  31  contaminates the conveying roller  35   a.    
     According to a preferred embodiment of the present invention, an electromagnetic induction sensor satisfying the above conditions is used as the displacement measuring sensor  40 . The electromagnetic induction sensor is installed to be separated a predetermined distance from the conveying roller  35   a  selected to detect the thickness of paper and forms an electromagnetic field with respect to the conveying roller  35   a  and outputs an electric signal which is induced in response to the displacement of the conveying roller  35   a . To apply such an electromagnetic induction sensor, the surface of the conveying roller  35   a  selected for detection of the thickness of paper is formed of metal. 
     FIG. 3 schematically illustrates the configuration of the electromagnetic induction sensor. Referring to the drawing, in an electromagnetic induction sensor  41 , a current conduction path is formed through a coil  42  wound around a ferrite material via a fixed resistor R from a voltage source Vcc. In the operation thereof, a magnetic field is formed by current flowing though the coil  42  with respect to the conveying roller  35   a  which is installed to be released from a lower roller  35   b  rotatably installed at a fixed position corresponding to the thickness of the inserted paper. When the conveying roller  35   a  is vertically lifted in a process of passing the paper  31  between the lower roller  35   b  and the conveying roller  35   a , the value of current flowing through the coil  42  varies due to an inductive electromotive force. Thus, the voltage output between a resistance device R and the coil  42  varies in response to the vertical displacement of the conveying roller  35   a . The electromagnetic induction sensor  41  can detect information on the paper thickness corresponding to the displacement of the conveying roller  35   a  by a non-contact type. Thus, it is easy to install and the accuracy in detecting the vertical displacement of the conveying roller  35   a  is not much affected by the contamination of the conveying roller  35   a  by the foreign material separated from the paper  31 . An analog/digital (A/D) converting unit  61  converts an analog signal output from the displacement measuring sensor  40  to a digital signal and outputs the converted signal to the paper thickness calculating unit  60 . The paper thickness calculating unit  60  calculates the thickness of the paper passing the conveying roller  35   a  from the information on vertical displacement output from the displacement measuring sensor  40 . Also, the paper thickness calculating unit  60  controls the fusing roller gap adjusting unit  50  so that the separated distance set corresponding to the calculated thickness of the paper can be maintained between the transfer roller  21  and the fusing roller  22  while the paper passes therebetween. The vertical displacement value of the conveying roller  35   a  corresponding to the signal input through the A/D converting unit  61  from the displacement measuring sensor  40  is recorded on a lookup table (LUT)  62 . Thus, the paper thickness calculating unit  60  calculates the thickness of the paper  31  by searching for the vertical displacement value of the conveying roller  35   a  corresponding to the signal input through the A/D converting unit  61  from the LUT  62 . Also, the paper thickness calculating unit  60  controls the fusing roller gap adjusting unit  50  so that the gap set corresponding to the calculated thickness of the paper can be maintained between the transfer roller  21  and the fusing roller  22 . The gap between the transfer roller  21  and the fusing roller  22  is set such that an image can be transferred from the transfer roller  21  without slippage and the disturbance of rotation of the transfer roller  21  in a process of conveying the paper  31  having the calculated thickness. The gap corresponding to the thickness of the paper is appropriately determined according to the material of the surfaces and the rotation speeds of the transfer roller  21  and the fusing roller  22 . 
     The process of adjusting the gap between the transfer roller  21  and the fusing roller  22 , with improved accuracy in measuring the paper thickness, considering that a reference position of the conveying roller  35   a  prior to passage of the paper changes due to the foreign material separated from the paper  31  adhering to the surface of the conveying roller  35   a , is described with reference to FIG.  4 . First, in a print mode, as a preparation step prior to starting a print job, a value of the reference position of the conveying roller  35   a  is calculated from signals output from the displacement measuring sensor  40  in the step before the paper arrives at a conveying roller  35   a  which is selected to measure the thickness of the paper. Preferably, while the conveying roller  35   a  is rotated at least one time (step  100 ), a reference position value is determined by averaging position values calculated from the value output from the displacement measuring sensor  40  (step  10 ). Alternatively, the maximum value of the position values calculated from the values output from the displacement measuring sensor  40  while rotating the conveying roller  35   a  at least one time is determined as the reference position value. When the reference position value is determined from the position values sampled during the rotation of the conveying roller  35   a , an appropriate initial reference value can be determined for the case in which foreign material adheres to the outer circumferential surface of the conveying roller  35   a  or the outer diameter of the conveying roller  35   a  changes due to abrasion. Then, it is determined by the paper passage confirmation sensor  36  provided before the selected conveying roller  35   a  whether the paper  31  has arrived (step  120 ). When the paper  31  is determined to arrive in step  120 , the conveying roller  35   a  rotates to pass the paper  31  (step  130 ). While the paper  31  passes the conveying roller  35   a , a vertical position value of the conveying roller  35   a  is calculated from the value output from the displacement measuring sensor  40  corresponding to the vertical movement of the conveying roller  35   a  (step  140 ). In this step, the vertical displacement value of the conveying roller  35   a  is calculated by averaging the position values calculated from the values output from the displacement measuring sensor  40  while the conveying roller  35   a  rotates at least one time. Alternatively, the maximum value of the position values calculated from the values output from the displacement measuring sensor  40  while the conveying roller  35   a  rotates at least one time is calculated as the vertical displacement value of the conveying roller  35   a . Next, the thickness of the paper  31  is calculated by subtracting the reference position value calculated in step  110  from the calculated vertical position value (step  150 ). After obtaining the paper thickness, the gap between the transfer roller  21  and the fusing roller  22  is calculated according to the calculated paper thickness (step  160 ). When the gap of the fusing roller  22  is calculated in step  160 , the vertical position of the fusing roller  22  is adjusted accordingly (step  170 ). When a paper passage signal is received from the paper passage confirmation sensor  37  provided between the transfer roller  21  and a paper exhaust outlet, the fusing roller  22  is lowered to the initial position (step  180 ). Although the case in which the photoreceptor web  11  is used as a photoreceptor medium is described, the present invention can be applied to a case in which a photoreceptor drum is used as the photoreceptor medium. 
     As described above, according to the printer and the fusing roller gap adjusting method thereof according to the present invention, the gap between the transfer roller and the fusing roller is adjusted according to the thickness of the paper inserted therebetween. Thus, when the paper passes the transfer roller, an obstacle to rotation of the photoreceptor web can be reduced, thus preventing an image forming error. 
     As stated above, the preferred embodiment of the present invention is shown and described. Although the preferred embodiment of the present invention has been described, it is understood that the present invention should not be limited to this preferred embodiment but various changes and modifications can be made by one skilled in the art within the spirit and scope of the present invention as hereinafter claimed.