Patent Publication Number: US-8121507-B2

Title: Image forming apparatus preventing sheet transfer deviation or slippage through fine detection of loop amount

Description:
This application claims priority to Japanese Patent Application No. 2007-198867 filed Jul. 31, 2007. The contents of Japanese Patent Application No. 2007-198867 are hereby incorporated by reference into the present application in their entirety. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present invention relates to an image forming apparatus that forms an image on a sheet material according to an electrophotographic method, and, more particularly, to an image forming apparatus capable of conveying a sheet material while keeping the loop of the sheet material between a transferring portion and a fixing portion at an appropriate amount. 
     2. Background Art 
     In an image forming apparatus using an electrophotographic method, a toner image on a photoconductor is first transferred to a sheet material by means of a transferring portion. Thereafter, the sheet material is allowed to pass through a conveying guide, and is guided to a nip of a fixing device. Herein, there is a case in which the trailing edge of the sheet material has not yet passed through the transferring portion when the leading edge of the sheet material enters the nip of the fixing device. 
     On the other hand, there is a case in which a difference arises between the sheet material conveying speed in the fixing device and the sheet material conveying speed in the transferring portion because of thermal expansion of a pressure roller provided in the fixing device, an individual difference, or a time-dependent change. In this case, if the sheet material conveying speed in the fixing device exceeds the sheet material conveying speed in the transferring portion, the sheet material carrying a not-yet-fixed toner image will be pulled toward the fixing device between the fixing device and the transferring portion, and, as a result, there is a fear that transfer deviation or slippage will occur. 
     To prevent the occurrence of this transfer deviation phenomenon, it has been conventionally performed to form a loop (i.e., a slackened state) for a sheet material conveyed between the transferring portion and the fixing device. However, there is a fear that the sheet material will be rumpled the next time if an excessive loop is given to the sheet material. Therefore, it is desired that the sheet material be conveyed while keeping its loop amount between the transferring portion and the fixing device at an appropriate amount. 
     A technique is known as an approach to respond to this request. This technique is carried out in the following way. In an image forming apparatus including a fixing unit and a transferring unit, drive systems are provided independently of each other for the fixing and transferring units, respectively. The upper limit and the lower limit of a loop amount formed by a sheet of paper at the front of an entrance of the fixing unit are detected. When its upper limit is detected, the speed of a driving portion provided on the side of the fixing unit is increased. When its lower limit is detected, the speed of the driving portion provided on the side of the fixing unit is decreased. Based on the loop amount detected in this way, the conveying speed of the sheet material is controlled. (See Japanese Published Unexamined Patent Application No. H7-234604). 
     However, in the conventional technique, the conveying speed of the sheet material is controlled based on the upper limit and the lower limit of the loop amount detected as above, and hence excellent results have not yet been achieved from the viewpoint of forming an image without transfer deviation or without rumples. In other words, to form an image without transfer deviation or rumples, there is a need to finely detect a loop amount and to reflect this in a control system for controlling the conveying speed. However, in the conventional technique, it is impossible to finely detect the loop amount at multiple stages including intermediate stages between the upper limit and the lower limit. Therefore, satisfactory results have not yet been achieved from the viewpoint of forming an image without transfer deviation or rumples. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to obtain an image forming apparatus capable of forming an image without transfer deviation or rumples. 
     To achieve the object, the image forming apparatus of the present invention is most prominently characterized by including an image carrier by which a toner image is carried, a transferring portion configured to transfer the toner image of the image carrier to a sheet material, a fixing portion configured to fix a transferred toner image of the sheet material, a fixing and driving portion configured to send the sheet material onto which the toner image has been fixed in the fixing portion, a conveyance guide member configured to guide the sheet material sent from the transferring portion toward the fixing portion, a loop amount detecting portion configured to detect a loop amount of the sheet material passing by the conveyance guide member with a plurality of thresholds at multiple stages, and a drive controller carrying out drive control of the fixing and driving portion based on a detection result of the loop amount detecting portion. 
     In the image forming apparatus of the present invention, the loop amount detecting portion to detect the loop amount of a sheet passing by the conveyance guide member detects the loop amount of the sheet with a plurality of thresholds at multiple stages, such as at least three stages. Consequently, in the drive controller, the loop amount finely detected at the multiple stages is fed back as a controllability driving factor of the fixing and driving portion, and, as a result, the sheet material can be conveyed while keeping the loop of the sheet material passing by the conveyance guide member at an appropriate amount, and hence a contribution can be made to the formation of an image without transfer deviation or rumples. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view illustrating a main part of an image forming apparatus according to an embodiment of the present invention. 
         FIG. 2  is an enlarged external view of a loop amount sensor according to the embodiment of the present invention. 
         FIG. 3  is an explanatory view for explaining the operation of the loop amount sensor according to the embodiment of the present invention. 
         FIG. 4  is an explanatory view for explaining one example of a loop amount conversion table according to the embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An image forming apparatus according to an embodiment of the present invention will be hereinafter described in detail with reference to the attached drawings. 
     As shown in  FIG. 1 , an image forming portion  11  of the image forming apparatus according to the embodiment of the present invention serves to convey a sheet material P upwardly from below in the vertical direction. An electrophotoconductor  13  serving as an image carrier is disposed on the upstream side in a sheet material conveying direction. The photoconductor  13  has an organic photosensitive layer formed on an outer circumferential surface of an electrically-conductive photoconductor base, and is rotationally driven at a predetermined peripheral speed (process speed) in the clockwise direction as shown by the arrow in  FIG. 1 . The circumferential surface of the photoconductor  13  is charged by a charging device (not shown) during its rotation so as to have a predetermined polarity and a predetermined electric potential. In this embodiment, its circumferential surface is primarily charged so as to have a predetermined electric potential of a negative polarity. The charged surface of the photoconductor is subjected to laser scanning exposure of image information by use of a laser scanner (not shown). As a result, the electric potential of an exposure bright section of the charged surface of the photoconductor is attenuated, and an electrostatic latent image corresponding to a scanning exposure pattern is formed. Thereafter, this electrostatic latent image is developed as a developer image (hereinafter, referred to as “toner image”) by use of a developing device (not shown). In this embodiment, the electrostatic latent image is subjected to reversal development (i.e., adhesion of a toner to the exposure bright section) by means of a negative toner charged to have a negative polarity. 
     In a transfer nip portion T with which the photoconductor  13  and a transfer roller (transferring portion)  15  are brought into contact, the toner image is sequentially transferred onto a sheet material P fed from a paper feed mechanism (not shown) In a process in which the sheet material P is conveyed by running a transfer roller drive motor (transfer driving portion)  15 M connected to the transfer roller  15  in a state in which the sheet material P is nipped in the transfer nip portion T, a predetermined transfer bias is applied to the transfer roller  15 , and the toner image on the surface of the photoconductor  13  is electrostatically transferred to the surface of the sheet material P in sequence. 
     The sheet sent from the transfer nip portion T is separated from the surface of the photoconductor  13 , and is conveyed to a fixing device (fixing portion)  17 . The surface of the photoconductor  13  from which the sheet has been separated is cleaned by a cleaner (not shown) so as to remove any adhered matter, such as a transfer residual toner, and is used to repeatedly form an image. 
     The fixing device  17  is a heating device of, for example, a pressure rotational body driving type, and is made up of a fixing roller  21  and a pressure roller  23  connected to the fixing roller  21  via a gear mechanism (not shown). A fixing nip portion N is formed between the fixing roller  21  and the pressure roller  23 . In the fixing nip portion N, a not-yet-fixed toner image conveyed from the transfer nip portion T is melted and fixed by being heated and pressed. The fixing roller  21  and the pressure roller  23  are members whose longitudinal direction is a direction perpendicular to the plane of the drawing paper in  FIG. 1 . While the sheet material P is passing through the fixing nip portion N and is being nipped in the fixing nip portion N, a fixing roller drive motor (fixing and driving portion)  21 M connected to the fixing roller  21  is driven, so that the fixing roller  21  and the pressure roller  23  are synchronously rotated by means of the gear mechanism (refer to the arrow direction in  FIG. 1 ), and, as a result, the sheet material P is discharged and conveyed toward mechanisms disposed on the downstream side. Instead of the thus formed structure, the following structure may be adopted. In detail, a pressure roller drive motor (not shown) is also connected to the pressure roller  23  without connecting the fixing roller  21  and the pressure roller  23  together via the gear mechanism, so that a fixing and driving portion is composed of the fixing roller drive motor  21 M and the pressure roller drive motor. Through the cooperative operation of the fixing and driving portions, the sheet material P is discharged and conveyed toward mechanisms disposed on the downstream side while being nipped in the fixing nip portion N. 
     A pair of conveyance guide members  25  and  27  that guide the conveyance of sheet materials are disposed in a path leading from the transfer roller  15  to the fixing device  17  so as to face each other across a sheet material P therebetween. The conveyance guide member  27 , which is one of the paired members, is provided with a loop amount sensor  29  detecting the loop amount of a sheet material. The loop amount sensor  29  is disposed approximately at the midpoint of a sheet material conveying path leading from the transfer nip portion T to the fixing nip portion N. The reason why the loop amount sensor  29  is disposed at that point is that, presumably, the loop amount of the sheet material leading from the transfer nip portion T to the fixing nip portion N is maximized near the midpoint. Preferably, the loop amount sensor  29  is disposed at a certain degree of distance from the fixing device  17 . The reason is that this arrangement makes it possible to previously avoid the influence of heat generated in the fixing device  17  on the loop amount sensor  29 . The loop amount sensor  29  serves to detect the loop amount of a sheet material at multiple stages. A drive controller  30  is connected to the drive systems, such as the transfer roller drive motor (transfer driving portion)  15 M and the fixing roller drive motor (fixing and driving portion)  21 M, in order to controllably rotate and drive these drive systems individually and independently. Based on the loop amount of the sheet material P detected by the loop amount sensor  29  at the multiple stages, the drive controller  30  controllably drives only the fixing and driving portion or controllably drives the transfer driving portion and the fixing and driving portion cooperatively with each other and independently of each other. As a result, the drive controller  30  can serve to convey sheet materials while keeping the loop of a sheet material P while passing between the conveyance guide members  25  and  27  at an appropriate amount. In other words, the drive controller  30  controllably drives the fixing and driving portion so as to decrease the conveying speed of the fixing and driving portion, for example, when the loop amount sensor  29  detects that the loop amount of the sheet material P is too small, whereas the drive controller  30  controllably drives the fixing and driving portion so as to increase the conveying speed of the fixing and driving portion when the loop amount sensor  29  detects that the loop amount of the sheet material P is excessive, thus adjusting the loop amount of the sheet material P to be kept at an appropriate amount. Instead of the thus formed structure, the following structure may be adopted. In detail, the drive controller  30  controllably drives the transfer driving portion and the fixing and driving portion cooperatively with each other and independently of each other so that the conveying speed of the fixing and driving portion becomes slower than that of the transfer driving portion, for example, when the loop amount sensor  29  detects that the loop amount of the sheet material P is too small, whereas the drive controller  30  controllably drives the transfer driving portion and the fixing and driving portion cooperatively with each other and independently of each other so that the conveying speed of the fixing and driving portion becomes faster than that of the transfer driving portion when the loop amount sensor  29  detects that the loop amount of the sheet material P is excessive, thus adjusting the loop amount of the sheet material P to be kept at an appropriate amount. 
     As shown in  FIG. 2 , the loop amount sensor  29  is disposed at the convex side of the loop in a sheet material P passing between the pair of conveyance guide members  25  and  27 , and is made up of a contact portion  31  coming into contact with a sheet material P passing between the conveyance guide members  25  and  27  and that changes its position in accordance with the loop amount of the sheet material P, a support shaft  33  configured to support rotatably the contact portion  31 , a spiral spring (urging member)  34  provided inside the support shaft  33  and configured to urge the contact portion  31  toward the back of the sheet material P, and a substantially fan-shaped detecting member  35  that pivots together with the contact portion  31  while being pivotally supported by the support shaft  33  and that detects the loop amount of the sheet material P. The contact portion  31  is bent at its intermediate part  32  so that its tip end forms an acute angle with the direction of movement of the sheet material P. This makes it possible to prevent the sheet material P from being damaged due to contact of the contact portion  31  with the sheet material P and to detect the loop amount of the paper P without missing a slight change in the amount thereof. In the spiral spring  34 , one end of a spring member is fixed to the stationary side, and the other end thereof is firmly fixed to the side of the support shaft  33 . Thus, the contact portion  31  is urged toward the sheet material P. 
     The detecting member  35  has a pair of to-be-detected parts  37  and  39  that are coaxial with the support shaft  33  and that draw continuous trajectories in the circumferential direction. Each of the to-be-detected parts  37  and  39  is provided at a predetermined distance from the support shaft  33 . The pair of to-be-detected parts  37  and  39  have predetermined widths and predetermined lengths, respectively, and are disposed to differ from each other in phase in the circumferential direction. 
     A pair of first and second rotational position sensors  41  and  43  each of which detects the rotational position of the detecting member  35  are disposed at positions on the fixing side that correspond to the trajectories of the pair of to-be-detected parts  37  and  39 , respectively. Each of the first and second rotational position sensors  41  and  43  can be formed of, for example, a photointerrupter. The first and second rotational position sensors  41  and  43  emit light onto the pair of to-be-detected parts  37  and  39 , respectively, on the other hand, detect the presence or absence of the reflection of the light emitted therefrom. Based on gray code information concerning the presence or absence of reflected light detected thereby and based on a previously-stored loop amount conversion table (described later), the first and second rotational position sensors  41  and  43  serve to detect the rotational position of the detecting member  35 . To achieve this function, the pair of to-be-detected parts  37  and  39  undergo treatment or processing so that the respective trajectory parts have optical reflectance different from that of a general surface on the detecting member  35 . For example, the trajectory parts are cut off or are subjected to coloring (with black, for example) for photoabsorption. 
     A supplementary description of  FIG. 3  will be given before giving a description of the operation of the loop amount sensor  29 . The thick dotted line of  FIG. 3  represents a virtual reference line V. In the positional relationship between the detecting member  35  and the first and second rotational position sensors  41  and  43  shown in  FIG. 3 , let it be supposed that these are placed in reference positions (rotational angle: 0°) when the loop amount of a sheet material P is zero (0). Additionally, let it be supposed that solid lines a, b, c, and d shown in  FIG. 3  are rotated together with the detecting member  35 . 
     In  FIG. 3 , when the loop amount of a sheet material P passing between the pair of conveyance guide members  25  and  27  increases for the loop amount sensor  29 , the sheet material P presses and urges the contact portion  31 . As a result, the contact portion  31  changes its position in accordance with the loop amount of the sheet material P, and is rotated on the support shaft  33  in the clockwise direction indicated by an arrow Q. In relation to this, the detecting member  35  is also rotated in the clockwise direction indicated by an arrow R in  FIG. 3  together with the contact portion  31  while being supported by the support shaft  33 . 
     At this time, in an area (this area is referred to as “area A”) in which the detecting member  35  is rotated and moved (rotational angle: 0° to 7°, loop amount: small) from the reference position to the position where the solid line a and the virtual reference line V coincide with each other, neither the first rotational position sensor  41  nor the second rotational position sensor  43  detects trajectories of the pair of to-be-detected parts  37  and  39 , and hence both output signals of the first and second rotational position sensors  41  and  43  are brought into “OFF” (see column A of  FIG. 4 ). Although a description showing a positive logic output signal will be given as follows, the present invention can be applied to a case adopting a negative logic output signal in the same way without being limited to this example. 
     In an area (this area is referred to as “area B”) in which the detecting member  35  is further rotated and moved (rotational angle: 7° to 15°, loop amount: moderately small) from the position where the solid line a and the virtual reference line V coincide with each other to the position where the solid line b and the virtual reference line V coincide with each other, the first rotational position sensor  41  detects the trajectory of the to-be-detected part  37 , whereas the second rotational position sensor  43  does not detect the trajectory of the to-be-detected part  39 , and hence output signals of the first and second rotational position sensors  41  and  43  are brought into “ON” and “OFF,” respectively (see column B of  FIG. 4 ). 
     In an area (this area is referred to as “area C”) in which the detecting member  35  is further rotated and moved (rotational angle: 15° to 30°, loop amount: moderately large) from the position where the solid line b and the virtual reference line V coincide with each other to the position where the solid line c and the virtual reference line V coincide with each other, both the first and second rotational position sensors  41  and  43  detect the trajectories of the pair of to-be-detected parts  37  and  39 , respectively, and hence both output signals of the first and second rotational position sensors  41  and  43  are brought into “ON” (see column C of  FIG. 4 ). 
     In an area (this area is referred to as “area D”) in which the detecting member  35  is further rotated and moved (rotational angle: 300 to 500, loop amount: large) from the position where the solid line c and the virtual reference line V coincide with each other to the position where the solid line d and the virtual reference line V coincide with each other, the first rotational position sensor  41  does not detect the trajectory of the to-be-detected part  37 , whereas the second rotational position sensor  43  detects the trajectory of the to-be-detected part  39 , and hence output signals of the first and second rotational position sensors  41  and  43  are brought into “OFF” and “ON,” respectively (see column D of  FIG. 4 ). 
     Therefore, in the loop amount sensor  29  according to the embodiment of the present invention, the rotational position of the detecting member  35  i.e., the loop amount of a sheet material P can be detected at multiple stages, i.e., at four stages (small, moderately small, moderately large, large) with three thresholds based on output signals of the first and second rotational position sensors  41  and  43  and based on the descriptive contents of the loop amount conversion table shown in  FIG. 4 . 
     As described above, in the image forming apparatus according to the embodiment of the present invention, the loop amount sensor  29  detects the rotational position of the detecting member  35 , i.e., the loop amount of a sheet material P at multiple stages, i.e., at four stages (small, moderately small, moderately large, large) with three thresholds based on output signals of the first and second rotational position sensors  41  and  43  and based on the descriptive contents of the loop amount conversion table shown in  FIG. 4 . Consequently, in the drive controller  30 , the loop amount of the sheet material P finely detected by the loop amount sensor  29  at the multiple stages is fed back as a controllability driving factor of the transfer roller drive motor (transfer driving portion)  15 M and/or the fixing roller drive motor (fixing and driving portion)  21 M, and, as a result, the sheet material P can be conveyed while keeping the loop of the sheet material P while passing between the conveyance guide members  25  and  27  at an appropriate amount, and hence a contribution can be made to the formation of an image without transfer deviation or rumples. 
     The present invention is not limited to the above-mentioned embodiment, and can be properly changed or modified within a range not departing from the spirit of the present invention or the technical idea thereof that can be read from the appended claims or the entire description. Such a modified image forming apparatus is also included in the technical scope of the present invention. 
     Specifically, although a description has been given of the image forming portion  11  formed to convey a sheet material P upwardly from below in the vertical direction as an embodiment of the present invention, the present invention is not limited to this. The present invention can be applied, without changes, to the image forming portion  11  formed to convey a sheet material P in any direction, such as a horizontal direction or a oblique direction. 
     Additionally, although a description has been given of the loop amount sensor  29  functioning as a loop amount detecting portion of the present invention that is disposed on the convex side of the loop in a sheet material P passing between the pair of conveyance guide members  25  and  27  as an embodiment of the present invention, the present invention is not limited to this. The loop amount sensor  29  may be disposed on the concave side of the loop in a sheet material P passing between the pair of conveyance guide members  25  and  27 . 
     Additionally, although a description has been given of the detecting member  35  having the pair of to-be-detected parts  37  and  39  that are coaxial with the support shaft  33  at a predetermined distance from the support shaft  33  and that draw continuous trajectories in the circumferential direction as an example of the present invention, the present invention is not limited to this. In the detecting member  35 , three or four or more to-be-detected parts that are coaxial with the support shaft  33  and that draw continuous trajectories in the circumferential direction may be disposed at a predetermined distance from the support shaft  33 . Thus, this structure makes it possible to finely detect the loop amount of a sheet material P at multiple stages. 
     Still further, although a description has been given of the electrophotoconductor serving as an image carrier in the embodiment of the present invention, the present invention is not limited to this. A transfer belt may be employed instead of the photoconductor. 
     Lastly, although a description has been given of, in regard to the loop amount sensor  29  according to the embodiment of the present invention, the loop amount of a sheet material P detected at multiple stages, i.e., at four stages (small, moderately small, moderately large, large) with three thresholds as an embodiment of the present invention, the present invention is not limited to this. As long as the loop amount of a sheet material P can be finely detected at multiple stages, any kind of embodiment is, of course, included within the technical scope of the present invention.