JP2005338252A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a low-cost image forming apparatus reducing variance in points where slack detection in recording medium between members is carried out while down sizing the apparatus. <P>SOLUTION: The image forming apparatus is provided with a medium transporting part 15 transporting a recording medium P on which a toner image is transferred and a fixing part 30 fixing and transporting the toner image on the recording medium P transported by this medium transporting part 15. Furthermore, the image forming apparatus is provided with a slack detection roller 52 installed downstream of at least the medium transporting part 15 and rotated in response to having the recording medium P transported by the medium transporting part 15 coming into contact. Medium transporting speed by the medium transporting part 15 and/or the fixing part 30 is controlled based on the detected result by the rotation of the slack detection roller 52 in the image forming apparatus. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an image forming apparatus that forms an image by fixing toner on a predetermined recording medium.

  Conventionally, there is known an image forming apparatus that forms an image by fixing toner on a predetermined recording medium. Such an image forming apparatus forms an electrostatic latent image on a photosensitive drum, develops the electrostatic latent image with toner to form a toner image, and then transfers the toner image on the photosensitive drum onto a recording medium. Further, the toner image on the recording medium is fixed by the heat and pressure of the fixing unit.

  In such an image forming apparatus, when the recording medium conveyed by the medium conveying unit is too loose, the recorded image is rubbed or wrinkled or jammed in the recording medium. When the tension is excessive, an image shift of the transfer toner, a step-out of the conveyance drive motor, and the like occur. Therefore, in the image forming apparatus, by detecting the slack of the recording medium conveyed by the medium conveying unit, the medium conveying speed in the medium conveying unit or the fixing unit is adjusted so as not to cause such a problem. As a medium slack detection method, a method using a combination of a lever arm and a photocoupler sensor has been widely used.

  Further, as a technique relating to the adjustment of the medium conveyance speed, for example, a technique described in Patent Document 1 is proposed.

JP 9-325544 A

  Specifically, this Patent Document 1 discloses a transport unit that forms a transport unit for transporting a fed recording medium and an image disposed downstream of the transport unit in the transport direction of the recording medium. Formed between a carrier and a transfer unit, and a transfer unit disposed opposite to the image carrier and forming a transfer unit for transferring a visible image of the image carrier to a recording medium. An image recording apparatus having a deflection relief portion for forming a deflection in a recording medium is disclosed. In such an image recording apparatus, since the recording medium is always bent at the bending relief portion, the recording medium is conveyed to the transfer portion without receiving a load when the recording medium is fed. Will not change, or horizontal stripes will not appear in the visible image.

  By the way, in the above-described medium slack detection method using a combination of a lever arm and a photocoupler sensor, it is common to rotate the lever and detect a part of the arm with a photocoupler sensor to obtain an on signal / off signal. It is.

  However, in such a method, due to variations in processing accuracy and assembly accuracy of the constituent members, it has been difficult to equally match the points where slack detection is performed between the members. Also, in this method, since the lever arm rotates, it is necessary to secure a large area as a mounting space for the lever arm, and it is not appropriate to apply to a corresponding part in an image forming apparatus that requires downsizing. There wasn't.

  The present invention has been made in view of such circumstances, and can reduce the variation in the point where the slack of the recording medium between the members is detected while reducing the size and cost, and the stable slack. An object of the present invention is to provide an image forming apparatus capable of performing detection.

  An image forming apparatus according to the present invention that achieves the above-described object includes an image forming unit that forms an image on a predetermined recording medium, and a medium conveying unit that conveys the recording medium onto which a toner image has been transferred by the image forming unit. A fixing unit that fixes and conveys the toner image on the recording medium conveyed by the medium conveying unit, and at least the recording medium that is disposed downstream of the medium conveying unit and conveyed by the medium conveying unit. A rotating body that rotates in response to contact, a detecting means that detects the rotation of the rotating body, and a medium conveying speed controlled by the medium conveying means and / or the fixing means based on a detection result by the detecting means. And a control means for performing the processing.

  Such an image forming apparatus according to the present invention detects looseness of a recording medium using a rotating body, and controls the medium conveying speed by the medium conveying unit and / or the fixing unit based on the detection result, It is possible to reduce the variation in the point where the slack of the recording medium between the members is detected.

  In addition, the image forming apparatus according to the present invention includes a retreating unit that retreats the rotating body out of the medium transport path while contacting the recording medium. In such an image forming apparatus according to the present invention, the rotating body is retracted out of the medium transport path while rotating in accordance with the contact with the recording medium, so that the rotating body has an obstacle in transporting the recording medium. It doesn't become a thing. Therefore, in the image forming apparatus according to the present invention, it is possible to eliminate an unnatural medium conveyance trajectory and to completely prevent rubbing on the printing surface side of the recording medium.

  As the detection means, an optical sensor that is turned on or off according to transmission or light shielding of sensor light according to the rotation of the rotating body can be used. Can be used.

  Further, the detection means is configured by using a magnet that circulates according to the rotation of the rotating body and a reed switch that is electrically turned on or off according to the approach or separation of the magnet. You can also. As a result, in the image forming apparatus according to the present invention, the detection unit that is not affected by heat can be configured, so that the rotating body and the detection unit can be disposed inside the fixing unit. Therefore, in the image forming apparatus according to the present invention, it is possible to detect the most loose portion among the slacks generated in the recording medium, and the slack detection performance can be improved. At this time, it is preferable that the reed switch is disposed so as to be in an on state or an off state at the same cycle when the rotating body is rotated at the same speed.

  In such an image forming apparatus according to the present invention, the rotating body and the detecting unit can be disposed on the back side of the printing surface of the recording medium. Thereby, in the image forming apparatus according to the present invention, it is possible to detect the slack due to the concave loop formed by the recording medium conveyed in the horizontal direction by the medium conveying unit and the fixing unit, and forcibly. By forming the loop in the concave direction, it is possible to avoid a situation that causes damage to the unfixed toner image due to rubbing of the recording medium.

  On the other hand, the rotating body and the detection means can be arranged on the printing surface side of the recording medium. At this time, the rotating body and the detecting unit are disposed at a point where the formation area of the loop in the convex direction formed by the recording medium conveyed in the horizontal direction by the medium conveying unit and the fixing unit becomes the highest. Is desirable. As a result, in the image forming apparatus according to the present invention, it becomes possible to detect slack due to the convex loop formed by the recording medium conveyed in the horizontal direction by the medium conveying means and the fixing means, and the loop is forcibly set. Since it is not necessary to form in the concave direction, the recording medium can be conveyed in the horizontal direction without distorting the medium conveyance path, and stable medium conveyance can be performed.

  Note that, in the image forming apparatus according to the present invention, by using a rotating body whose outer periphery is formed in a spur shape, even if the rotating body comes into contact with the printing surface of the recording medium, the printing surface side It is possible to prevent the unfixed toner image from being damaged.

  In the present invention, it is possible to reduce the variation in the point where the slack detection of the recording medium between the members is performed. Therefore, it is possible to perform the stable slack detection and to realize excellent print quality. In the present invention, since the rotating body and the detection unit can be arranged in a narrow mounting space, the fixing unit can be downsized and the entire image forming apparatus can be downsized. Furthermore, in the present invention, since no expensive member is used, it is possible to detect the slack of the recording medium at a very low cost.

  Hereinafter, specific embodiments to which the present invention is applied will be described in detail with reference to the drawings.

  This embodiment is an image forming apparatus that forms an image by fixing toner on a predetermined recording medium, and can be applied to a copying machine or a printer using a transfer type electrophotographic process. In particular, this image forming apparatus detects looseness of a recording medium using a rotating body, and controls the medium conveyance speed based on the detection result. In the following, “slack” refers to a state where a loop formed by the recording medium becomes too large and exceeds a predetermined allowable range.

  First, the image forming apparatus shown as the first embodiment will be described.

  As shown schematically in FIG. 1, the image forming apparatus 1 includes a paper feed tray 10 and a manual feed tray 11 that store a recording medium P on which no image is recorded. The recording medium P stored in the sheet feeding tray 10 is fed from the sheet feeding tray 10 by the rotation of the first sheet feeding roller 12 disposed so as to be in contact with the surface thereof, and the medium conveyance path In response to the rotation of the first medium conveyance roller 13 and the second medium conveyance roller 14 disposed in the medium, the medium is conveyed to a medium conveyance unit 15 described later. On the other hand, the recording medium P stored in the manual feed tray 11 is fed from the manual feed tray 11 by the rotation of the second paper feed roller 16 disposed so as to be in contact with the surface thereof, and the second feed roller 16 is fed to the second feed tray 16. In accordance with the rotation of the medium conveying roller 14, the medium is conveyed to the medium conveying unit 15. Note that the image forming apparatus 1 includes a medium detection sensor 20 that detects that the front end of the recording medium P in the conveyance direction has reached the downstream of the second medium conveyance roller 14, so that the medium by the medium detection sensor 20 is detected. With respect to the medium conveyance from the time when the detection signal is turned on, the position of the recording medium P in the medium conveyance path can be recognized.

  Further, the image forming apparatus 1 transfers a recording medium P, which is fed out by the second medium conveyance roller 14 and transferred with a toner image, which will be described later, to a predetermined medium downstream of the second medium conveyance roller 14 in the medium conveyance path. A medium transport unit 15 which is a medium transport unit for transporting in the direction of arrow a at the transport speed is provided. In addition, the image forming apparatus 1 is opposed to the medium conveying unit 15 and has four image drum units 17 corresponding to four colors of yellow (Y), magenta (M), cyan (C), and black (K). Is provided. Each of the image drum units 17 stores an LED (Light Emitting Diode) head 18 that performs exposure, and a photosensitive drum 19 that is driven to rotate in the direction of arrow b at the medium conveyance speed by the medium conveyance unit 15. Composed. Further, the image forming apparatus 1 includes a medium slack detecting unit 50 that detects slack of the recording medium P downstream of the medium transport unit 15 in the medium transport path, an upper fixing roller 21 that is rotationally driven in the direction of arrow c in FIG. And a fixing unit 30 which is a fixing means mounted with a lower fixing roller 22 which is substantially in contact with the upper fixing roller 21 and is driven to rotate in the direction of the arrow d in FIG.

  In such an image forming apparatus 1, the recording medium P is conveyed by the medium conveying unit 15 in the direction of the arrow a at a predetermined medium conveying speed, and accordingly, the four image drum units 17 perform the recording on the recording medium P. An image is formed on. Specifically, the image forming apparatus 1 is formed on the LED head 18 when the recording medium P is placed on the placement surface of the medium transport unit 15 and an image signal to be recorded is input to the image drum unit 17. An LED array (not shown) emits light to expose and charge the photosensitive drum 19, and an electrostatic latent image is formed on the surface of the photosensitive drum 19. Then, the image forming apparatus 1 charges a toner (not shown) stored in the image drum unit 17 to the electrostatic latent image and attaches the toner to the recording medium P by electrostatic force. Further, in the image forming apparatus 1, when the recording medium P to which the unfixed toner is transferred is conveyed to the fixing unit 30 by the medium conveying unit 15, the upper fixing roller 21 and the lower fixing roller 22 in the fixing unit 30 are The toner image on the recording medium P is fixed by heat and pressure applied at the nip portion formed between the two, forming an image, and the recording medium P is conveyed. When the image is formed on the recording medium P in this way, the image forming apparatus 1 is provided with the recording medium by the separator 23 that is disposed downstream of the fixing unit 30 and is driven to swing in the direction of the arrow e in FIG. The paper discharge direction of P is sorted, and the recording medium P is discharged from the first paper discharge tray 24 or led to the second paper discharge tray 26 according to the rotation of the third medium transport roller 25. Then, the paper is discharged from the second paper discharge tray 26.

  In such an image forming apparatus 1, the medium slack detection unit 50 is mounted on the fixing unit 30. Here, the medium slack detection unit 50 will be described.

  As shown in the perspective view of the fixing unit 30 in FIG. 2, the medium slack detecting unit 50 guides the recording medium P conveyed by the medium conveying unit 15 to the inside of the fixing unit 30 to form a medium conveying path. A medium transport guide 51 is provided, and the medium transport guide 51 has a hole 51a. 3 shows an enlarged perspective view of the main part of the fixing unit 30 as viewed from above, and FIG. 4 shows an enlarged perspective view of the main part of the fixing unit 30 as viewed from below. As shown in the figure, it has a slack detection roller 52 that is a rotating body that rotates in response to the contact of the recording medium P conveyed by the medium conveying unit 15. The slack detection roller 52 is provided such that a part of the outer periphery thereof protrudes from the hole 51 a formed in the medium transport guide 51 onto the medium transport path.

  Specifically, as shown in FIGS. 5 and 6, the slack detection roller 52 includes a roller base 52a as a main portion and an outer roller 52b formed using a rubber material having a high friction coefficient. The Further, the roller base 52a is formed with a post 52c protruding from both main surfaces as a rotation axis thereof, and further, a slit radially formed on a surface perpendicular to the axial direction of the post 52c. A hole 52d is provided. Since the post 52c is rotatably held by the frame 53 fixed to the medium conveyance guide 51, the slack detection roller 52 can rotate smoothly. The slack detection roller 52 is a position where the outer periphery of the slack detection roller 52 comes into contact with the back surface of the recording medium P when a later-described loop formed by the recording medium P during conveyance is larger than a predetermined allowable range. It is arranged. The medium slack detection unit 50 is provided with a photocoupler sensor 54 held by a frame 53 at a position where the sensor light irradiates the slit hole 52 d of the slackness detection roller 52.

  Now, in the image forming apparatus 1 including such a medium slack detection unit 50, when the recording medium P is transported from the medium transport unit 15 to the fixing unit 30, the medium transport speed and the fixing by the medium transport unit 15 are fixed. Due to the difference in the medium conveyance speed by the unit 30, the recording medium P is relaxed between the medium conveyance unit 15 and the fixing unit 30, and a loop is formed. This is because when the medium conveyance speed by the fixing unit 30 is set to the standard speed mode, the medium conveyance speed is set slightly slower than the medium conveyance speed by the medium conveyance part 15. The purpose of setting the medium conveyance speed by the fixing unit 30 to be slower than the medium conveyance speed by the medium conveyance unit 15 is due to the excessive tension of the recording medium P caused by the medium conveyance speed by the fixing unit 30 being too high. This is for the purpose of preventing the deterioration of the print quality due to the deviation of the unfixed toner image and the occurrence of wrinkling of the recording medium P.

  Further, in the image forming apparatus 1, the image drum unit 17 and other structures are arranged on the printing surface side formed on the surface of the recording medium P, so that the loop of the recording medium P becomes large. If too much, it is assumed that the unfixed toner image is damaged by rubbing. In general image forming apparatuses, in order to avoid such a situation, a loop is often formed in a concave direction.

  However, in the image forming apparatus 1, due to variations in processing accuracy and assembly accuracy of various members constituting the image forming apparatus, the influence of the fixing temperature, and the like, the medium conveying speed by the medium conveying unit 15 and the fixing unit 30. When the difference from the medium conveyance speed due to the above becomes large, it is assumed that the formed loop becomes too large and exceeds the predetermined allowable range, resulting in occurrence of “sagging”. In the image forming apparatus 1, even when a long medium that is out of the specification standard formed in the medium conveyance direction is used as the recording medium P, the loop formed at the rear end of the medium is large. It is assumed that it will become too much and "sagging" will occur.

  Here, FIG. 7 shows the medium transport trajectory until the formed loop gradually increases and becomes slack. In the same figure, a track A is a medium transport track having a margin with respect to a track on which the recording medium P is too tight, and a track B is a medium transport track having a margin with respect to a track on which the recording medium P is too slack. is there. The above-mentioned “predetermined allowable range” leading to the occurrence of slack indicates the range from the trajectory A to the trajectory B. In other words, the trajectories A and B represent the upper limit and the lower limit of the allowable range, respectively. Therefore, in the image forming apparatus 1, it can be said that the state in which the recording medium P is conveyed while forming a loop within the range from the track A to the track B is satisfactory.

  Further, the medium transport trajectory exceeds the range from the trajectory A to the trajectory B, and the formed loop gradually increases according to the difference between the medium transport speed by the medium transport unit 15 and the medium transport speed by the fixing unit 30; The track C may be formed. In this case, the recording medium P has a predetermined bending energy due to the nip portion in the medium transport unit 15 and the nip portion in the fixing unit 30 and the stiffness of the recording medium P. When the deflection energy reaches a peak, the medium transport track forms an S-shaped track D, and further, the slack jumps upward to form a track E. In the layout of each member shown in the figure, as shown in the F part of the figure, the contact between the recording medium P and the image drum unit 17 generated when the track D is formed, or as shown in the G part of the figure. The contact between the recording medium P generated when the track E is formed and the constituent members of the fixing unit 30 is a main cause of the rubbing of the printed image caused by the slack.

  Therefore, in the image forming apparatus 1, as described above, the slack detection roller 52 is provided so that a part of the outer periphery protrudes from the hole 51a formed in the medium transport guide 51 onto the medium transport path. The slack of the recording medium P is detected by the photocoupler sensor 54 according to the rotation of the detection roller 52. In the image forming apparatus 1, the fixing conveyance driving motor 61 that rotates the contact roller 60 that contacts the lower fixing roller 22 in the fixing unit 30 in response to the detection of the slack of the recording medium P is fixed. The medium is controlled by the conveyance motor driving unit 76, and the medium conveyance speed by the fixing unit 30 is controlled.

  Hereinafter, a mechanism for detecting the slack of the recording medium P and its control will be described.

  FIG. 8 shows a state where the medium transport track is formed in the vicinity of the approximate center between the track A and the track B shown in FIG. 7, and the recording medium P is in a good transport state. In this case, in the image forming apparatus 1, the outer periphery of the slack detection roller 52 protruding from the hole 51 a formed in the medium conveyance guide 51 does not come into contact with the back surface of the printing surface of the recording medium P. The detection roller 52 does not rotate.

  On the other hand, FIG. 9 shows a state in which the medium transport track is formed large up to the lower limit of the above-described allowable range represented by the track B shown in FIG. In this case, the outer periphery of the slack detection roller 52 protruding from the hole 51 a formed in the medium transport guide 51 is in contact with the back surface of the printing surface of the recording medium P. Therefore, in the image forming apparatus 1, as the recording medium P is conveyed, the slack detection roller 52 rotates in the direction of the arrow f in FIG. Here, in the medium slack detection unit 50, the sensor light of the photocoupler sensor 54 is irradiated to the slit hole 52 d of the slackness detection roller 52, so that the sensor light is transmitted or transmitted according to the rotation of the slackness detection roller 52. It will be shielded from light. Accordingly, the photocoupler sensor 54 is turned on or off in response to transmission or shielding of sensor light according to the rotation of the slackness detection roller 52, and can output an on / off slackness detection signal.

  In the image forming apparatus 1, the slack detection signal detected by the photocoupler sensor 54 is supplied to a control unit (not shown) via a predetermined connection cable or the like, thereby controlling the medium conveyance speed by the fixing unit 30. To do.

  Specifically, the image forming apparatus 1 controls the slack of the recording medium P by controlling the medium conveyance speed by the fixing unit 30 by a control system as shown in FIG. In other words, this control system detects the rotation of the slack detection roller 52 and detects the slack of the recording medium P. The slack detection sensor 71 as a photocoupler sensor 54 is detected by the slack detection sensor 71. A control unit 72 that is a control unit that controls each unit based on the looseness detection signal, and an image forming unit 73 that is an image forming unit that forms an image on the recording medium P by the image drum unit 17 under the control of the control unit 72. A fixing processing unit 74 that performs fixing processing by the fixing unit 30 under the control of the control unit 72, and a medium transport motor driving unit 75 that controls the driving of the medium transport driving motor 63 under the control of the control unit 72. And a fixing / conveying motor driving unit 76 that controls the driving of the fixing / conveying driving motor 61 under the control of the control unit 72. The control unit 72 includes a timer 72a for measuring time.

  In the image forming apparatus 1, using such a control system, the slack of the recording medium P is controlled by transmitting and receiving signals at a timing as shown in FIG. In the figure, the medium detection signal by the medium detection sensor 20 described above, the slack detection signal by the slack detection sensor 71, and the timing of the motor drive signal generated by the fixing conveyance motor drive unit 76 based on this slack detection signal. The chart shows a time-series change of the motor speed of the fixing conveyance driving motor 61 by the slack detection signal.

  First, in the image forming apparatus 1, as the recording medium P is conveyed, when the leading end of the recording medium P in the traveling direction is detected by the medium detection sensor 20, the medium detection signal is turned on by the medium detection sensor 20. It becomes a state. In the image forming apparatus 1, when the recording medium P is further conveyed and slack is detected by the medium slack detection unit 50, a slack detection signal is supplied from the slack detection sensor 71 to the control unit 72. The slack detection signal is counted by.

Here, the control unit 72, when receiving a predetermined amount or more slack detection signal has been within a predetermined time T _A measured by the timer 72a, it is determined that slack occurs, the fixing conveyance motor driver 76 Is instructed to increase the speed of the fixing conveyance driving motor 61. At this time, the control unit 72, the slack detection signals received within a predetermined time T _A, not the length of time on or off state is continued, that the number of pulses slack occurs in the case where more than a predetermined amount Is determined. In response to this, the fixing conveyance motor driving unit 76 refers to a predetermined speed table, increases the speed of the fixing conveyance driving motor 61 by a predetermined amount and shifts to the high speed mode, and the slack of the recording medium P is eliminated. Until this high speed mode continues.

  In the image forming apparatus 1, the fixing conveyance driving motor 61 is driven at a higher speed than the standard to convey the recording medium P by setting the medium conveyance speed by the fixing unit 30 to the high speed mode. Is done. In the image forming apparatus 1, when the slack of the recording medium P is eliminated and the loop is within the predetermined allowable range described above, the outer periphery of the slack detection roller 52 is in contact with the back surface of the printing surface of the recording medium P as described above. Since the slackness detection roller 52 does not rotate without contact, the slackness detection signal by the slackness detection sensor 71 is not output.

Control unit 72, when the, predetermined time T in _B measured by the timer 72a has not been received more than a predetermined amount of slack detection signal, it is determined that slack is eliminated, the fixing conveyance motor driver 76 An instruction is given to decrease the speed of the fixing conveyance driving motor 61. In response to this, the fixing conveyance motor driving unit 76 refers to a predetermined speed table and shifts the fixing conveyance driving motor 61 to the standard speed mode.

In the image forming apparatus 1, the looseness of the recording medium P can be controlled by transmitting and receiving signals at such timing. The predetermined times T _A and T _B that are conditions for determining the occurrence of slack by the control unit 72, the number of pulses of the slack detection signal, and the medium conveyance speed in the standard speed mode or the high speed mode are obtained by experiments. It is determined from the value.

  Here, in the image forming apparatus 1, the medium conveyance speed is not always controlled by such a slack detection signal, but the leading end of the recording medium P passes through the rearmost end of the medium conveyance unit 15 and the fixing unit. The time until reaching the nip portion at 30 is not performed. This is due to the following reason.

  First, in an image forming apparatus, in order to avoid rubbing on the printing surface side when a recording medium forms a loop, as described above, in general, the loop is often formed in a concave direction. Specifically, as a method of forming the loop in the concave direction, for example, as shown in FIG. 12, there is a method in which the fixing unit 30 ′ is inclined with respect to the medium conveying unit 15 ′. Further, as a method of forming the loop in the concave direction, with respect to the upper fixing roller 21 ′ and the lower fixing roller 22 ′, the diameter, material, hardness, friction coefficient, arrangement position, etc. of the upper fixing roller 21 ′ and the lower fixing roller 22 ′ There is also a method of combining differently with the lower fixing roller 22 ′.

  In the image forming apparatus 1, regardless of which of these methods is employed, when the recording medium P is conveyed from the medium conveying unit 15 to the fixing unit 30 due to its side effects, Since the leading end portion is conveyed slightly downward, the leading end portion of the recording medium P may come into contact with the looseness detection roller 52 immediately before the recording medium P is nipped at the fixing unit 30. Further, in the image forming apparatus 1, even when the recording medium P whose front end is curled downward is transported or when the recording medium P that is weak such as thin paper is transported, the front end of the recording medium P is The slack detection roller 52 may be contacted.

  In the image forming apparatus 1, the slack detection signal is received by the control unit 72 until the leading end of the recording medium P reaches the nip portion of the fixing unit 30 after passing through the rearmost end of the medium transport unit 15. However, this is ignored and the medium conveyance speed is not controlled. FIG. 11 shows signal waveforms for the two systems of slack detection signals Exe1 and Exe2. The slack detection signal Exe1 starts slack control when the recording medium P reaches the nip portion of the fixing unit 30. The slack detection signal of several pulses is received by the control unit 72 until the time until the recording medium P is reached, but the slack detection signal is controlled when the recording medium P actually reaches the nip portion of the fixing unit 30 and the slack control is started. The example which is not received by the part 72 is shown. On the other hand, the slack detection signal Exe2 is an example in which the slack detection signal is continuously received by the control unit 72 before and after the recording medium P reaches the nip portion in the fixing unit 30 and the slack control is started. Is shown.

  As described above, in the image forming apparatus 1, the medium based on the slack detection signal is used until the leading end of the recording medium P passes through the rearmost end of the medium conveying unit 15 and reaches the nip portion of the fixing unit 30. By not controlling the conveyance speed, even if the leading end of the recording medium P contacts the slack detection roller 52 before the recording medium P is nipped in the fixing unit 30, slack occurs. It is possible to prevent a mistaken determination.

  As described above, in the image forming apparatus 1 shown as the first embodiment of the present invention, in order to detect the slack of the recording medium P, the slack detecting roller 52 that is a rotating body is used to detect the slack of the medium. By configuring 50, it is possible to reduce the variation in the point at which the slack detection of the recording medium P between the members is performed, so that it is possible to perform stable slack detection and to realize excellent print quality. Further, in this image forming apparatus 1, since the medium slack detection unit 50 can be disposed in a narrow mounting space, a small fixing unit 30 can be provided, and the image forming apparatus 1 as a whole is also small. Can be achieved. Further, in this image forming apparatus 1, since no expensive member such as a laser displacement meter or an ultrasonic sensor is used, the medium slackness detection unit 50 can be provided at an extremely low cost.

  Next, an image forming apparatus shown as the second embodiment will be described.

  The image forming apparatus shown as the second embodiment is improved with respect to the image forming apparatus 1 shown as the first embodiment, except that only the configuration of the medium slack detecting unit provided in the fixing unit is different. In other words, the slack detection roller is retracted out of the medium conveyance path after the slack of the recording medium is detected. Therefore, in the description of the second embodiment, the same components as those in the description of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. The image forming apparatus shown as the second embodiment is referred to as an image forming apparatus 200 in order to distinguish it from the image forming apparatus 1 shown as the first embodiment.

  First, prior to the description of the second embodiment, improvements to the image forming apparatus 1 shown as the first embodiment will be described.

  In the image forming apparatus 1, the medium conveyance speed is changed by the fixing unit 30 under the control of the control unit 72 that has received the slack detection signal, but the information that the recording medium P has determined that slack has occurred. Since the slackness continues to increase until it is reflected in each drive system, the slackness detection roller 52 becomes an obstacle, and an unnatural medium transport path is formed in the medium slackness detection unit 50 as shown in FIG. It will be. Further, in the image forming apparatus 1, when the bending energy of the recording medium P increases, an S-shaped medium conveyance track is formed, and the recording medium P is thereby lifted from the medium conveyance unit 15, and is shown in FIG. As described above, the printing surface comes into contact with the image drum unit 17, and the recording medium P jumps upward and comes into contact with each member including the fixing unit 30. Further, in the image forming apparatus 1, the slack detection delay due to the undulation of the recording medium P causes the same phenomenon. Here, the swell of the recording medium P indicates a state in which the sag occurs in the main scanning direction perpendicular to the medium transport direction in addition to the slack in the medium transport direction described above. It shows behavior.

  In order to prevent such a phenomenon, the image forming apparatus 200 shown as the second embodiment makes it possible to retract the slackness detection roller out of the medium conveyance path after detecting the slackness of the recording medium P. is there.

  FIG. 14 is an enlarged perspective view of the main part of the fixing unit 30 as viewed from above. The medium slack detection unit 250 includes a slack detection roller 252 that is a rotating body that rotates in accordance with the contact of the recording medium P transported by the medium transport unit 15. The slack detection roller 252 is provided such that a part of the outer periphery thereof protrudes from the hole 251a formed in the medium transport guide 251 onto the medium transport path.

  Specifically, as shown in FIGS. 15 and 16, the slack detection roller 252 includes a roller base 252a as a main part and an outer roller 252b formed using a rubber material having a high friction coefficient. The Further, the roller base 252a is formed with a post 252c projecting from both main surfaces as a rotation axis, and further, slits radially formed on a surface perpendicular to the axial direction of the post 252c. A hole 252d is provided. Such a slack detection roller 252 is held by a frame 253 fixed to the medium transport guide 251. The frame 253 is formed with a roller guide groove 253a that restricts movement of the slack detection roller 252 in the medium conveyance direction and allows movement in the vertical direction. In addition, two plate springs 255 are fixed to the frame 253. Each of the plate springs 255 is disposed so that one end thereof engages with a groove formed in the roller holder 256 that also functions as a bearing loosely fitted to both ends of the post 252c of the looseness detection roller 252. The The slack detection roller 252 is held in a state of being movable in the vertical direction via a roller guide groove 253a formed in the frame 253, and both ends of the post 252c are loosely fitted to the roller holder 256. The roller holder 256 can rotate smoothly. The roller guide groove 253a and the plate spring 255 are configured as a retracting unit that retracts the slack detection roller 252 out of the medium conveyance path while contacting the recording medium P. Further, the slack detection roller 252 is arranged at a position where the outer periphery of the slack detection roller 252 contacts the back surface of the recording medium P when a loop formed by the recording medium P during conveyance is larger than a predetermined allowable range. Established. Further, in the medium slack detection unit 250, a photocoupler sensor 254 held by the frame 253 is disposed at a position where the sensor light irradiates the slit hole 252d of the slackness detection roller 252.

  Now, in the image forming apparatus 200 including such a medium slack detection unit 250, the slack of the recording medium P is detected by the photocoupler sensor 254 according to the rotation of the slack detection roller 252, as in the image forming apparatus 1. Accordingly, the fixing conveyance motor driving unit 76 controls the driving of the fixing conveyance driving motor 61 and controls the medium conveyance speed by the fixing unit 30.

  Here, FIG. 17 shows a state where the medium transport track is formed in the vicinity of the approximate center between the track A and the track B shown in FIG. 7 and the recording medium P is in a good transport state. In this case, in the image forming apparatus 200, the outer periphery of the slack detection roller 252 protruding from the hole 251a formed in the medium conveyance guide 251 does not come into contact with the back surface of the printing surface of the recording medium P, and the slack The detection roller 252 does not rotate.

  On the other hand, FIG. 18 shows a state where the medium transport path is formed to the lower limit of the allowable range described above. In this case, the outer periphery of the slack detection roller 252 protruding from the hole 251 a formed in the medium transport guide 251 is in contact with the back surface of the printing surface of the recording medium P. Therefore, in the image forming apparatus 200, as the recording medium P is conveyed, the slack detection roller 252 rotates in the direction of the arrow g in FIG. Here, in the medium slack detection unit 250, the sensor light of the photocoupler sensor 254 is irradiated to the slit hole 252d of the slack detection roller 252, so that the sensor light is transmitted or transmitted according to the rotation of the slack detection roller 252. It will be shielded from light. Therefore, the photocoupler sensor 254 can output an on / off slack detection signal in accordance with transmission or shielding of sensor light according to the rotation of the slack detection roller 252. In the image forming apparatus 200, the slack detection signal detected by the photocoupler sensor 254 is supplied to the control unit 72 via a predetermined connection cable or the like, and the signal is exchanged at the timing shown in FIG. By controlling, the medium conveyance speed by the fixing unit 30 is controlled.

  In such an image forming apparatus 200, one end of each of the two plate springs 255 fixed to the frame 253 is engaged with a groove formed in the roller holder 256. The slack detection roller 252 is urged upward by the bending force of the plate spring 255, but the movement of the post 252c is restricted by the roller guide groove 253a. Therefore, the slack detection roller 252 is shown in FIG. As described above, the state is stable while protruding from the medium transport guide 251. In the image forming apparatus 200, in this state, the slack detection roller 252 is movable in the vertical direction and is rotatably held, and the slack of the recording medium P does not occur as shown in FIGS. , The state is waiting for the detection of slack.

  Here, in the image forming apparatus 200, when the slackness becomes large due to the time lag within the time during which the slackness control is performed or the slackness detection delay due to the waviness of the recording medium P, the slackness becomes large. As shown in FIGS. 19 and 20, the detection roller 252 moves while rotating downward in response to the contact due to the slack of the recording medium P, retreats out of the medium conveyance path, and as shown in FIG. In addition, a part of the outer periphery does not protrude from the medium transport guide 251. Therefore, in the image forming apparatus 200, as shown in FIG. 21, the slack detection roller 52 does not become an obstacle, and a natural medium conveyance path is formed in the medium slack detection unit 50. Note that, as shown in FIG. 19, the photocoupler sensor 254 retracts the slackness detection roller 252 downward even when a part of the outer periphery of the slackness detection roller 252 protrudes from the medium transport guide 251. Even if a part of the outer periphery does not protrude from the medium transport guide 251, the sensor light S is disposed at a position where it irradiates the slit hole 252 d.

  As described above, in the image forming apparatus 200 shown as the second embodiment of the present invention, the same effects as those of the image forming apparatus 1 shown as the first embodiment can be obtained. That is, in the image forming apparatus 200, the variation in the point where the slack of the recording medium P between the members is detected can be reduced, so that stable slack can be detected and excellent print quality can be realized. it can. In the image forming apparatus 200, the medium slack detecting unit 250 can be disposed in a narrow mounting space. Therefore, the small fixing unit 30 can be provided, and the image forming apparatus 200 as a whole is small. Can be achieved. Furthermore, since this image forming apparatus 200 does not use expensive members such as a laser displacement meter and an ultrasonic sensor, the medium slackness detection unit 250 can be provided at an extremely low cost.

  In addition to such effects, the image forming apparatus 200 not only forms the medium slack detecting unit 250 using the slack detecting roller 252 that is a rotating body, but also detects the slack detecting roller after the slack of the recording medium P is detected. By rotating the 252 in accordance with the contact with the recording medium P and withdrawing it out of the medium transport path while outputting the slack detection signal, the slackness detection roller 252 does not become an obstacle, and slackness control is performed. It is possible to eliminate the unnatural medium conveyance trajectory caused by the time lag within the time and the delay of the slack detection due to the undulation of the recording medium P, and it is possible to completely prevent the rubbing on the printing surface side.

  Next, an image forming apparatus shown as a third embodiment will be described.

  The image forming apparatus shown as the third embodiment is different from the image forming apparatus 1 shown as the first embodiment in the configuration of the medium slack detection unit disposed in the fixing unit and the medium slack detection. The signal output from the printing unit is made different and improved, and the slack of the recording medium is detected using a reed switch, a magnet, and a slack detection roller. Therefore, in the description of the third embodiment, the same components as those in the description of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. The image forming apparatus shown as the third embodiment is distinguished from the image forming apparatus 1 shown as the first embodiment and the image forming apparatus 200 shown as the second embodiment. It is assumed to be called an image forming apparatus 300.

  First, prior to the description of the third embodiment, improvements to the image forming apparatus 1 shown as the first embodiment will be described.

  In the image forming apparatus 1, it is desirable to reduce the length of the space formed between the medium transport unit 15 and the fixing unit 30 as the size thereof is reduced. When the unit 50 is provided, the medium slack detection unit 50 is exposed to heat from the fixing unit 30. Further, in the image forming apparatus 1, the point where the loop formation area of the recording medium P is lowest is reduced by the length of the space formed between the medium transport unit 15 and the fixing unit 30. Often within 30.

  In the image forming apparatus 1, the medium slack detection unit 50 is disposed inside or outside the fixing unit 30. An inexpensive optical sensor generally used as the photocoupler sensor 54 in the medium slack detection unit 50 is used. In this case, it is difficult to dispose the medium slack detecting unit 50 inside the fixing unit 30 from the viewpoint of heat resistance, and it may be necessary to use an expensive heat resistant optical sensor.

  Therefore, the image forming apparatus 300 shown as the third embodiment can detect the slack of the recording medium P by using a reed switch, a magnet, and a slack detection roller in order to solve such a problem, and the fixing unit 30. A low-cost medium slack detection unit that can be disposed inside the printer is provided.

  22 and 23 show the configuration of the medium slack detection unit 350. FIG. The medium slack detection unit 350 includes a slack detection roller 352 that is a rotating body that rotates according to the contact of the recording medium P transported by the medium transport unit 15. The slack detection roller 352 includes a roller base 352a as a main part and an outer roller 352b formed using a rubber material having a high friction coefficient. Further, a post 352c is formed on the roller base 352a as a rotation axis so as to protrude from both main surfaces, and a magnet embedding hole formed in a circle on a surface perpendicular to the axial direction of the post 352c. 352d is provided. A magnet 351 is embedded in the magnet embedding hole 352d. Such a slack detection roller 352 can rotate smoothly because the post 352c is rotatably held by a frame 353 fixed to the medium transport guide. The slack detection roller 352 is arranged at a position where the outer periphery of the slack detection roller 352 contacts the back surface of the recording medium P when a loop formed by the recording medium P during conveyance is larger than a predetermined allowable range. Established.

  Further, the medium slack detection unit 350 includes a reed switch 354 fixed to the frame 353. The reed switch 354 contacts when the magnet 351 approaches within a predetermined range as the embedded magnet 351 rotates around the rotation axis of the slack detection roller 352 as the slack detection roller 352 rotates. Is configured to be mechanically driven, and a voltage is applied by a circuit (not shown) in response to contact of the contact. In the reed switch 354, when the magnet 351 approaches within a predetermined range and the contact comes into contact, the magnet 351 is moved away from the predetermined range while being conducted and a current flows to be turned on electrically. Then, when the contact of the contact is released, no current flows and it is turned off electrically. In such a medium slack detecting unit 350, as shown in FIG. 24, when the slack detecting roller 352 is rotated at the same speed, the reed switch 354 is turned on / off at the same cycle, and accordingly, Each member including the reed switch 354 is arranged so that the slack detection signal is output. The arrangement positions of such members include the magnetic sensitivity of the elements constituting the reed switch 354, the magnetic force of the magnet 351, the distance between the magnet 351 and the reed switch 354, the position of the reed switch 354 with respect to the magnetic flux of the magnet 351, and the like. Can be determined by.

  Now, in the image forming apparatus 300 including such a medium slack detecting unit 350, the slack of the recording medium P is detected by the reed switch 354 in accordance with the rotation of the slack detecting roller 352, and the fixing conveyance motor is correspondingly detected. The drive of the fixing conveyance driving motor 61 is controlled by the driving unit 76, and the medium conveyance speed by the fixing unit 30 is controlled.

  In other words, in the image forming apparatus 300, the outer periphery of the slackness detection roller 352 comes into contact with the back surface of the printing surface of the recording medium P when the medium transport path is formed to the lower limit of the allowable range described above. Therefore, in the image forming apparatus 300, as the recording medium P is conveyed, the slack detection roller 352 rotates in the direction of arrow h in FIG. In the image forming apparatus 300, in synchronization with the rotation of the slack detection roller 352, the magnet 351 repeatedly approaches or moves away from the reed switch 354, and the magnet 351 approaches within a predetermined range and contacts the reed switch 354. Is in the on state, and when the magnet 351 is moved away from the predetermined range and the contact of the contact of the reed switch 354 is released, the reed switch 354 is turned on. Electrically turned off. Therefore, the reed switch 354 can output an on / off slack detection signal based on contact or release of the contact according to the rotation of the slack detection roller 352. In the image forming apparatus 300, the slack detection signal detected by the reed switch 354 is supplied to the control unit 72 via a predetermined connection cable or the like, thereby controlling the medium conveyance speed by the fixing unit 30.

  Specifically, in the image forming apparatus 300, the slack of the recording medium P is controlled by exchanging signals at the timing shown in FIG. Here, as described in the first embodiment, when the slack detection signal at a predetermined time is counted by the control unit 72, the base point is the time when the received slack signal is equal to or greater than a predetermined amount. A control system for determining the slack by counting the slack detection signal at regular intervals, instead of counting the slack detection signal within a predetermined time will be described.

  In the image forming apparatus 300, as in the control system in the first embodiment, the medium conveyance speed is not always controlled by the slack detection signal, but the leading end of the recording medium P is the last of the medium conveyance unit 15. No time is taken until the nip portion of the fixing portion 30 is reached after passing through the end. In the image forming apparatus 300, at the same time that the leading end of the recording medium P reaches the nip portion of the fixing unit 30, a slackness determination time for determining the occurrence of slackness is set by the control unit 72. Every time elapses, a slack judgment time is set periodically.

  In such an image forming apparatus 300, when the control unit 72 receives a slack detection signal of a predetermined amount or more within the slack determination time, the control unit 72 determines that slack has occurred and sends it to the fixing transport motor driving unit 76. It instructs the fixing conveyance drive motor 61 to increase the speed. At this time, the control unit 72 assumes that the slackness detection signal received within the slackness determination time is not slackened when the number of pulses is equal to or more than a predetermined amount, not the time length during which the on state or the off state continues. judge. In response to this, the fixing conveyance motor driving unit 76 refers to a predetermined speed table, increases the speed of the fixing conveyance driving motor 61 by a predetermined amount and shifts to the high speed mode, and the slack of the recording medium P is eliminated. Until this high speed mode continues.

  In the image forming apparatus 300, the fixing conveyance driving motor 61 is driven at a higher speed than the standard to convey the recording medium P with the medium conveyance speed by the fixing unit 30 as a high-speed mode, the slack is eliminated, and the loop is the above-described predetermined loop. , The outer periphery of the slack detection roller 352 does not come into contact with the back surface of the printing surface of the recording medium P and the slack detection roller 352 does not rotate. The slack detection signal by the reed switch 354 and the slack detection sensor 71 as the magnet 351, which are detection means for detecting slack of the recording medium P, is also not output.

  The control unit 72 determines that the slack has been eliminated when the slack detection signal of a predetermined amount or more has not been received within the slack determination time, and determines that the slack has been eliminated. Instruct to reduce the speed of the. In response to this, the fixing conveyance motor driving unit 76 refers to a predetermined speed table and shifts the speed of the fixing conveyance motor 61 to the standard speed mode.

  In the image forming apparatus 300, the looseness of the recording medium P can be controlled by transmitting and receiving signals at such timing.

  As described above, in the image forming apparatus 300 shown as the third embodiment of the present invention, the same effects as those of the image forming apparatus 1 shown as the first embodiment can be obtained. That is, in the image forming apparatus 300, the variation in the point where the slack of the recording medium P between the members is detected can be reduced, so that the stable slack can be detected and excellent print quality can be realized. it can. Further, in this image forming apparatus 300, since the medium slack detecting unit 350 can be disposed in a narrow mounting space, the small fixing unit 30 can be provided, and the image forming apparatus 300 as a whole is also small. Can be achieved. Furthermore, since this image forming apparatus 300 does not use expensive members such as a laser displacement meter and an ultrasonic sensor, the medium slackness detection unit 350 can be provided at a very low cost.

  In addition to the above effects, the image forming apparatus 300 is not affected by heat by configuring the medium slack detecting unit 350 using the slack detecting roller 352 that is a rotating body, the reed switch 354, and the magnet 351. The medium slack detecting unit 350 can be disposed inside the fixing unit 30. Therefore, in this image forming apparatus 300, it is possible to detect the most slack portion among the slacks of the recording medium P generated between the medium transport unit 15 and the fixing unit 30 and improve the slack detection performance. Can be made.

  Finally, an image forming apparatus shown as the fourth embodiment will be described.

  The image forming apparatus shown as the fourth embodiment is different from the image forming apparatus 1 shown as the first embodiment in the configuration of the medium slack detecting unit arranged in the fixing unit and the arrangement position thereof. Thus, the recording medium can be stably transported without forcibly forming a loop in the concave direction. Therefore, in the description of the fourth embodiment, the same components as those in the description of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted. The image forming apparatus shown as the fourth embodiment includes the image forming apparatus 1 shown as the first embodiment, the image forming apparatus 200 shown as the second embodiment, and the third embodiment. In order to distinguish it from the image forming apparatus 300 shown in FIG.

  First, prior to the description of the fourth embodiment, improvements to the image forming apparatus 1 shown as the first embodiment will be described.

  In the image forming apparatus 1, when the recording medium P is conveyed from the medium conveying unit 15 to the fixing unit 30, a loop is formed in the recording medium P between the medium conveying unit 15 and the fixing unit 30. In order to avoid rubbing on the printing surface side when the recording medium P forms a loop, as shown in FIG. 12, the fixing unit 30 itself is inclined with respect to the medium conveying unit 15, For the upper fixing roller 21 and the lower fixing roller 22, the roller diameter, material, hardness, friction coefficient, arrangement position, and the like are set differently for the upper fixing roller 21 and the lower fixing roller 22, thereby recording. The tip of the medium P may be conveyed slightly downward to form a loop in the concave direction. However, in the image forming apparatus 1, as a result of forming the loop of the recording medium P in the concave direction, the medium conveyance path that should be horizontal is distorted, and the above-described undulation of the recording medium P is further promoted. However, there is a risk that the medium conveyance becomes unstable.

  Therefore, the image forming apparatus 400 shown as the fourth embodiment does not forcibly form the loop of the recording medium P in the concave direction in order to eliminate such a problem, and instead uses the recording medium P. By transporting in the horizontal direction, stable transport of the recording medium is realized. However, in the image forming apparatus 400, by conveying the recording medium P in the horizontal direction, the loop is formed not only in the concave direction but also in the convex direction. Therefore, the image forming apparatus 400 detects and eliminates slack due to the loop in the convex direction.

  27 and 28 show the positions where the medium slack detection unit 450 is disposed with respect to the fixing unit 30. FIG. The medium slack detection unit 450 is preferably disposed at a point where the loop formation region of the convex direction in the recording medium P formed between the medium transport unit 15 and the fixing unit 30 is the highest. As shown in FIG. 4, the upper cover 451 constituting the casing of the fixing unit 30 is fixed.

  As shown in FIG. 29, such a medium slack detection unit 450 includes a slack detection roller 452 that is a rotating body that rotates in response to the recording medium P transported by the medium transport unit 15 contacting, a frame 453, and the like. And a photocoupler sensor 454 and a plate spring 455. The slack detection roller 452 includes a roller base 452a that is a main part. In the roller base 452a, a post 452c is formed as a rotation axis so as to protrude from both main surfaces, and slit holes 452d are formed radially in a circumferential direction on a surface perpendicular to the axial direction of the post 452c. Is provided. The roller base 452a is formed with spur-like teeth 452b on the outer periphery thereof. As a result, even if the slack detection roller 452 comes into contact with the printing surface of the recording medium P, the printing surface side is not rubbed and the unfixed toner image is not damaged. Such a slack detection roller 452 is held by a frame 453 fixed to the upper cover 451. Although not particularly illustrated, the frame 453 restricts the movement of the slack detection roller 452 in the medium conveyance direction and can move in the vertical direction, similarly to the frame 253 in the image forming apparatus 200 shown as the second embodiment. A roller guide groove is formed. Further, two plate springs 455 are fixed to the frame 453. Each of the plate springs 455 is disposed so that one end thereof engages with the post 452c in the slack detection roller 452. In the medium slack detection unit 450, since the biasing force of the plate spring 455 is set weak, a member corresponding to the roller holder 256 in the image forming apparatus 200 shown as the second embodiment is provided. However, such a roller holder may be provided. Further, the photocoupler sensor 454 held by the frame 453 is disposed at a position where the sensor light irradiates the slit hole 452d in any state in which the slack detection roller 452 moves. The slack detection roller 452 is configured such that when the convex loop formed by the recording medium P during conveyance reaches the upper limit of a predetermined allowable range, the teeth 452b that form the outer periphery of the slack detection roller 452 correspond to the recording medium P. It arrange | positions in the position which contacts with the printing surface.

  Now, in the image forming apparatus 400 including such a medium slack detection unit 450, the slack of the recording medium P is detected by the photocoupler sensor 454 according to the rotation of the slack detection roller 452, as in the image forming apparatus 1. Accordingly, the fixing conveyance motor driving unit 76 controls the driving of the fixing conveyance driving motor 61 and controls the medium conveyance speed by the fixing unit 30. Hereinafter, such control will be described. Here, a case where slack is detected when the loop of the recording medium P is formed in the convex direction will be described.

  FIG. 30 shows a medium transport track in which a loop is formed in the convex direction. In the drawing, a track X is a medium transport track having a margin with respect to a track on which the recording medium P is excessively stretched in the convex direction, and a track Y is a margin with respect to a track on which the recording medium P is too slack in the convex direction. It is a medium conveyance orbit which has. Here, the above-mentioned “predetermined allowable range” that leads to the occurrence of slack when a loop is formed in the convex direction indicates the range from the trajectory X to the trajectory Y. In other words, the trajectories X and Y represent the lower limit and the upper limit of the allowable range, respectively. Therefore, in the image forming apparatus 400, it can be said that the state in which the recording medium P is conveyed while forming a loop in the range from the trajectory X to the trajectory Y is satisfactory.

  In the image forming apparatus 400, when the recording medium P is transported from the medium transport unit 15 to the fixing unit 30 and a loop is formed in the convex direction, the medium transport track is formed within the range from the track X to the track Y. Thus, the recording medium P is brought into a good transport state. In this case, in the image forming apparatus 400, the teeth 452b forming the outer periphery of the slack detection roller 452 do not contact the printing surface of the recording medium P, and the slack detection roller 452 does not rotate.

  On the other hand, in the image forming apparatus 400, the loop formed in the convex direction gradually increases according to the difference between the medium conveyance speed by the medium conveyance unit 15 and the medium conveyance speed by the fixing unit 30, and the medium conveyance trajectory. Increases to the upper limit of the predetermined allowable range, that is, the trajectory Y, the teeth 452b forming the outer periphery of the slack detection roller 452 come into contact with the printing surface of the recording medium P. Therefore, in the image forming apparatus 400, as the recording medium P is conveyed, the slack detection roller 452 rotates in the direction of arrow i in FIG. Here, in the medium slack detecting unit 450, the post 452c in the slack detecting roller 452 is held in a state of being movable in the vertical direction via a roller guide groove formed in the frame 453, and fixed to the frame 453. Since one end of each of the two plate springs 455 is engaged with the post 453c with a low load, the slack detection roller 452 is urged downward by the bending force of the plate spring 455. Since the movement of 452c is regulated by the roller guide groove, the slackness detection roller 452 is movable in the vertical direction and is held rotatably. Therefore, in the image forming apparatus 400, when the slackness becomes large due to the time lag within the time during which the slackness control is performed or the slackness detection delay due to the waviness of the recording medium P, the slackness detection is performed. The roller 452 moves in the direction of the arrow j while rotating downward according to the slack of the recording medium P, and retracts out of the medium conveyance path.

  Note that, as described above, the photocoupler sensor 454 is disposed at a position where the sensor light irradiates the slit hole 452d in any state in which the slackness detection roller 452 moves. Sensor light is transmitted or shielded according to the rotation of 452, and an on / off slack detection signal corresponding to the transmission or shield of the sensor light can be output. In the image forming apparatus 400, the slack detection signal detected by the photocoupler sensor 454 is supplied to the control unit 72 via a predetermined connection cable or the like, and at the timing shown in FIG. 11 or FIG. By transmitting and receiving signals, the medium conveyance speed by the fixing unit 30 is controlled.

  As described above, in the image forming apparatus 400 shown as the fourth embodiment of the present invention, the same effect as the image forming apparatus 1 shown as the first embodiment can be obtained. That is, in the image forming apparatus 400, the variation in the point where the slack detection of the recording medium P between the members can be reduced, so that the stable slack detection can be performed and excellent print quality can be realized. it can. Further, in this image forming apparatus 400, since the medium slack detection unit 450 can be disposed in a narrow mounting space, the small fixing unit 30 can be provided, and the image forming apparatus 400 as a whole is also small. Can be achieved. Furthermore, since this image forming apparatus 400 does not use expensive members such as a laser displacement meter and an ultrasonic sensor, the medium slackness detection unit 450 can be provided at an extremely low cost. Furthermore, in the image forming apparatus 400, it is possible to detect the most slack portion of the slack of the recording medium P that occurs between the medium transport unit 15 and the fixing unit 30 and improve the slack detection performance. Can be made.

  In the image forming apparatus 400, in addition to such effects, a medium slack detection unit 450 configured using a slack detection roller 452 formed with teeth 452b that are rotating bodies is arranged on the printing surface side of the recording medium P. By providing, it becomes possible to detect the loop formed in the convex direction, and it becomes unnecessary to forcibly form the loop in the concave direction. Therefore, in the image forming apparatus 400, the recording medium P can be transported in the horizontal direction without distorting the medium transport path, and the medium can be stably transported without undulation or fluttering of the recording medium P. It becomes.

  The present invention is not limited to the embodiment described above. For example, in all of the above-described embodiments, the medium slack detection unit is configured using the slack detection roller as the rotating body. However, the present invention configures the medium slack detection unit using a roller, a cam, or the like. You can also

  In all the embodiments described above, the slack detection roller rotates when the recording medium is slack, while the slack detection roller stops rotating when the recording medium is in a good conveyance state. As described above, in the present invention, on the contrary, when the recording medium is slack, the slackness detection roller stops, while when the recording medium is in a good conveyance state, the slackness detection roller performs the recording. The slack detection roller may be arranged so as to rotate in contact with the medium.

  Further, in all of the above-described embodiments, the medium conveyance speed is described as being switched in two stages (second speed) of the standard speed mode and the high speed mode. You may make it switch in the above multistage.

  Furthermore, in all the embodiments described above, the description has been made assuming that the medium conveyance speed by the fixing unit is controlled. However, in the present invention, the medium conveyance speed by the medium conveyance unit, the medium conveyance speed, and the fixing unit are controlled. You may make it control the medium conveyance speed by both.

  In the first embodiment and the second embodiment described above, the medium slack detection unit has been described as being disposed inside the fixing unit. However, the present invention specifically describes the outside of the fixing unit. It is also possible to dispose a medium slack detection unit between the medium conveyance unit and the fixing unit, and it is sufficient to dispose at least downstream of the medium conveyance unit 15 in the medium conveyance path.

  Furthermore, in the first embodiment and the second embodiment described above, when the slack detection signal at a predetermined time is counted by the control unit 72, the base point is the time when the received slack signal becomes a predetermined amount or more. In the present invention, the slack detection signal is counted by counting the slack detection signal every predetermined period as described in the third embodiment. You may make it do.

  Furthermore, in the above-described fourth embodiment, it has been described that the rotation axis of the slack detection roller is movable. However, the present invention uses a slack detection roller in which the rotation shaft is fixed so as not to move. It can also be realized.

  In the above-described fourth embodiment, the medium slack detection unit is described as being disposed inside the fixing unit. However, the present invention includes a slack that occurs between the medium transport unit and the fixing unit. For the purpose of detection at the most slack point, it is also possible to dispose a medium slack detection unit downstream of the medium conveyance unit in the medium conveyance path and outside the fixing unit.

  Furthermore, in the present invention, the slack detection roller in which spur-like teeth are formed on the outer periphery described in the fourth embodiment using the reed switch and the Hall element described in the third embodiment described above. You may make it detect rotation.

  Furthermore, in the present invention, the detection mechanism for the loop formed in the concave direction described in the first to third embodiments is formed in the convex direction described in the fourth embodiment. It is also possible to detect a loop formed in any one of the concave direction and the convex direction at the same time by using it together with the loop detection mechanism.

  Thus, it goes without saying that the present invention can be modified as appropriate without departing from the spirit of the present invention.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus shown as a first embodiment of the present invention. 2 is a perspective view illustrating a configuration of a fixing unit included in the image forming apparatus. FIG. FIG. 3 is an enlarged perspective view when a main part of the fixing unit is viewed from above. FIG. 3 is an enlarged perspective view when a main part of the fixing unit is viewed from below. FIG. 3 is a perspective view illustrating a configuration of a medium slack detection unit provided in the image forming apparatus. It is a perspective view explaining the structure of the same medium slack detection part, and is an exploded perspective view for demonstrating especially the structure of a slack detection roller. FIG. 2 is a cross-sectional view of a main part for explaining the configuration of the image forming apparatus, and for explaining a medium transport trajectory until a loop formed by a recording medium is loosened. FIG. 2 is a cross-sectional view of a main part for explaining the configuration of the image forming apparatus, for explaining a state of a transport state in which a good loop is formed. FIG. 2 is a cross-sectional view of a main part for explaining the configuration of the image forming apparatus, for explaining a state in which a medium transport path is formed to a lower limit of an allowable range and recognized as slack. 2 is a block diagram illustrating a configuration of a control system that controls slack of a recording medium in the image forming apparatus. FIG. 6 is a timing chart of signals generated when controlling the slack of the recording medium in the image forming apparatus. FIG. 4 is a cross-sectional view of a main part for explaining the configuration of the image forming apparatus when the fixing unit is inclined with respect to the medium transport unit in order to form the loop in the concave direction. FIG. 10 is a cross-sectional view of a main part for explaining the configuration of an image forming apparatus shown as a second embodiment of the present invention, and is a diagram for explaining a medium transport path when a slack detection roller does not retreat out of the medium transport path. is there. FIG. 2 is an enlarged perspective view of a main part of a fixing unit provided in the image forming apparatus as viewed from above. FIG. 3 is an exploded perspective view illustrating a configuration of a medium slack detection unit provided in the image forming apparatus. It is a perspective view explaining the structure of the same medium slack detection part, and is a figure for demonstrating a mode that the slackness detection roller is controlled in the roller guide groove | channel, and is stable in the state protruded from the medium conveyance guide. FIG. 2 is a cross-sectional view of a main part for explaining the configuration of the image forming apparatus, for explaining a state of a transport state in which a good loop is formed. FIG. 2 is a cross-sectional view of a main part for explaining the configuration of the image forming apparatus, for explaining a state in which a medium transport path is formed to a lower limit of an allowable range and recognized as slack. It is a principal part front view explaining the structure of the same medium slack detection part, and demonstrates a mode that a photocoupler sensor is arrange | positioned in the position which sensor light irradiates a slit hole in any state to which a slack detection roller moves. FIG. FIG. 4 is a perspective view illustrating the configuration of the medium slack detection unit, illustrating how the slack detection roller moves while rotating downward according to contact due to slack of the recording medium and retracts out of the medium conveyance path. FIG. FIG. 3 is a cross-sectional view of a main part illustrating the configuration of the image forming apparatus, and illustrates how the slackness detection roller moves while rotating downward according to the contact due to slackening of the recording medium and retracts out of the medium conveyance path. It is a figure for doing. FIG. 10 is an exploded perspective view illustrating a configuration of a medium slack detection unit included in an image forming apparatus shown as a third embodiment of the present invention. It is a perspective view explaining the structure of the same medium slack detection part. In the same medium slack detection unit, it is a diagram for explaining a waveform example of a slack detection signal when the slack detection roller is rotated. It is a figure for demonstrating a mode that a detection signal is output. It is a perspective view explaining the structure of the same medium slack detection part, and is a figure for demonstrating the rotation direction of the slack detection roller according to conveyance of a recording medium. 6 is a timing chart of signals generated when controlling the slack of the recording medium in the image forming apparatus. FIG. 10 is a perspective view illustrating a configuration of a fixing unit included in an image forming apparatus shown as a fourth embodiment of the present invention, and is a diagram for explaining an arrangement position of a medium slack detection unit with respect to the fixing unit. FIG. 5 is a perspective view of a main part for explaining the configuration of the fixing unit, and is a diagram for explaining a state in which the medium slackness detecting unit is fixed to an upper cover constituting the casing of the fixing unit. It is a disassembled perspective view explaining the structure of the same medium slack detection part. FIG. 4 is a cross-sectional view of a main part for explaining the configuration of the image forming apparatus, and for explaining a medium transport track in which a loop is formed in a convex direction.

Explanation of symbols

1,200,300,400 Image forming apparatus 10 Paper feed tray 11 Manual feed tray 12 First paper feed roller 13 First medium transport roller 14 Second medium transport roller 15, 15 ′ Medium transport section 16 Second feed Paper roller 17 Image drum unit 18 LED head 19 Photosensitive drum 20 Medium detection sensor 21, 21 ′ Upper fixing roller 22, 22 ′ Lower fixing roller 23 Separator 24 First paper discharge tray 25 Third medium conveyance roller 26 Second Paper discharge tray 30, 30 'fixing unit 50, 250, 350, 450 medium slack detection unit 51, 251 medium transport guide 51a, 251a hole 52, 252, 352, 452 slack detection roller 52a, 252a, 352a, 452a roller Base 52b, 252b, 352b Outer roller 52c, 252c, 52c, 452c Post 52d, 252d, 452d Slit hole 53, 253, 353, 453 Frame 54, 254, 454 Photocoupler sensor 60 Contact roller 61 Fixing conveyance drive motor 63 Medium conveyance drive motor 71 Slack detection sensor 72 Control unit 72a Timer 73 Image forming section 74 Fixing processing section 75 Medium transport motor driving section 76 Fixing transport motor driving section 253a Roller guide groove 255, 455 Plate spring 256 Roller holder 351 Magnet 352d Magnet embedding hole 354 Reed switch 451 Upper cover 452b Teeth P Recording Medium S Sensor light

Claims (9)

Image forming means for forming an image on a predetermined recording medium;
Medium conveying means for conveying the recording medium onto which the toner image has been transferred by the image forming means;
Fixing means for fixing and conveying the toner image on the recording medium conveyed by the medium conveying means;
A rotating body that is disposed at least downstream of the medium conveying means and rotates in response to contact of the recording medium conveyed by the medium conveying means;
Detecting means for detecting rotation of the rotating body;
An image forming apparatus comprising: a control unit that controls a medium conveyance speed by the medium conveyance unit and / or the fixing unit based on a detection result by the detection unit. The image forming apparatus according to claim 1, further comprising: a retracting unit configured to retract the rotating body out of the medium conveyance path while being in contact with the recording medium. The image forming apparatus according to claim 1, wherein the detection unit is an optical sensor that is turned on or off according to transmission or light shielding of sensor light according to rotation of the rotating body. The detection means includes a magnet that circulates according to the rotation of the rotating body, and a reed switch that is electrically turned on or off according to the approach or separation of the magnet. The image forming apparatus according to claim 1. The image forming apparatus according to claim 4, wherein the reed switch is disposed so as to be in an on state or an off state at the same cycle when the rotating body is rotated at the same speed. The image forming apparatus according to claim 1, wherein the rotating body and the detecting unit are disposed on a back side of a printing surface of the recording medium. The image forming apparatus according to claim 1, wherein the rotating body and the detection unit are disposed on a printing surface side of the recording medium. The rotating body and the detecting means are disposed at a point where the convex loop forming area formed by the recording medium conveyed in the horizontal direction by the medium conveying means and the fixing means becomes the highest. 8. The image forming apparatus according to claim 7, wherein: The image forming apparatus according to claim 7, wherein an outer periphery of the rotating body is formed in a spur shape.