Patent Publication Number: US-6219508-B1

Title: Image forming apparatus with belt mark sensor

Description:
This application is a continuation of prior U.S. application Ser. No. 09/391,356 filed Sep. 8, 1999, now U.S. Pat. No. 6,038,417 which is a division of U.S. application Ser. No. 09/050,112 filed Mar. 30, 1998 now U.S. Pat. No. 5,978,626 issued Nov. 2, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, including a plurality of printing assemblies arranged in series, intended to make a multicolor record. 
     2. Description of the Related Art 
     The image forming apparatus of the electrostatic recording type comprises a photosensitive body (photosensitive drum), a charger,. an optical head for forming an electrostatic latent image on the photosensitive body, a developing device for developing the electrostatic latent image into a toner image, and a transfer element (transfer roller) for transferring the toner image onto paper. The charger, the optical head, the developing device and the transfer element are arranged to surround the photosensitive body to form a charged toner image while the photosensitive body rotates one revolution. The charged toner image is transferred from the photosensitive body onto paper by a transfer element, and the paper is then sent to a fixing device and is discharged. 
     The photosensitive body, the charger, the optical head, the developing device and the transfer element constitute a printing assembly. The image forming apparatus capable of making a multi-color record comprises a plurality of printing assemblies each including a photosensitive body, a charger, an optical head, a developing device and a transfer element. When, for example, developing agents of four colors, i.e., yellow, magenta, cyan and black are to be used, the image forming apparatus is equipped with four printing assemblies containing respective developing agents. The image forming apparatus of the tandem type in which a plurality of printing assemblies are arranged in series, is particularly effective in executing printing at high speed. 
     The paper is conveyed through the plurality of printing assemblies arranged in series, and a fixing device arranged after the assemblies, by a paper conveyer belt. The toner images formed by the printing assemblies of the respective colors are superposed on the paper to form a color image. In order to bring the printing positions into agreement, the printing assemblies print positioning marks on the paper conveyer belt, and an optical sensor reads the marks printed on the paper conveyer belt. Based on the marks thus read out, the printing positions and the printing timings of the plurality of printing assemblies can be adjusted. 
     The image forming apparatus is further equipped with a top cover which is rotatably mounted on the frame. Upon opening the top cover, the interior of the frame becomes accessible. That is, upon opening the top cover, the constituent members of the print assemblies arranged inside the frame can be repaired or replaced. In particular, the toner must be replenished as it decreases in the developing device. It is desired that the toner can be replenished with ease. Usually, the top cover is locked to the frame by a lock member, and is opened by hand after the locking of the lock member is released. 
     In the image forming apparatus of the electrostatic recording type, one print assembly includes a photosensitive body, a charger, an optical head, a developing device and a transfer element. The photosensitive body, the charger and the developing device are detachably mounted to the frame as a developing unit. The optical head is mounted to the top cover. When the top cover is opened, therefore, the optical head is lifted together with the top cover. Thus, the user is allowed to access the developing device and so on, and the manufacturer is allowed to maintain the optical heads. 
     In the image forming apparatus equipped with a plurality of printing assemblies, a plurality of optical heads are mounted to the top cover, and the developing units are detachably mounted to the frame. Moreover, the top cover includes a control board for controlling the optical heads, sheet metal for shielding the control board, and a base frame for mounting the optical heads and the control board. Therefore, the top cover is very heavy. Besides, the image forming apparatus has been designed to be capable of effecting the printing even on large papers and, hence, employs large constituent members, causing the top cover to become even heavier. If the top cover becomes heavier, it is difficult for the operator to lift the top cover. Besides, many constituent members must be efficiently arranged in the limited space in the interior of the apparatus. 
     A laser beam must not be allowed to leak to the exterior when the top cover is opened. A laser beam which leaks in unexpected directions when the top cover is opened may enter the eyes of the operator. This is not desirable. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an image forming apparatus which enables a top cover to be easily opened and closed, and enables the interior of the frame to be easily accessible upon opening the top cover, facilitating the replacement of expendables and the maintenance and testing of the apparatus. 
     Another object of the present invention is to provide an image forming apparatus in which the constituent members are efficiently arranged inside the frame. 
     A further object of the present invention is to provide an image forming apparatus capable of shutting off the optical passage of the optical reading unit when the apparatus is not in use. 
     The image forming apparatus, according to the present invention comprises a frame, a top cover movably mounted to the frame, a lock member for locking the top cover to the frame, a cover lifting mechanism for lifting the cover by a predetermined amount relative to the frame when the lock member is released, a plurality of print assemblies for forming images of different colors, a fixing device, and a paper conveyer belt for conveying paper through the plurality of printing assemblies and the fixing device. Each of the printing assemblies comprises a photosensitive body, a charger for charging the photosensitive body, an optical head for forming an electrostatic latent image on the photosensitive body, a developing device for developing the electrostatic latent image into a toner image, and a transfer element for transferring the toner image onto a paper, the optical head being mounted to the top cover. 
     Owing to this constitution, the top cover is lifted up by a predetermined amount relative to the frame by the cover lifting mechanism when the lock member is released. The predetermined amount will be, for example, a distance which permits fingers to be inserted between the frame and the top cover. Therefore, the operator is allowed to easily open the top cover using finger pressure. 
     The following constitution can be employed together with the above-mentioned constitution. 
     Provision is made of a switch which is linked to the cover lifting mechanism to detect the opening or closure of the top cover. 
     The apparatus has a front end and a rear end, the fixing device is disposed near the rear end of the apparatus, and the top cover is mounted to turn about a rotary support point disposed at a position on the rear side of the fixing device. 
     The cover lifting mechanism is disposed near the front end of the apparatus, and comprises a rod engageable with the top cover, a spring for urging the rod in a direction in which the top cover opens, and a stopper for stopping the rod at a predetermined position. 
     The photosensitive body, the charger and the developing device in each of the printer assemblies are formed as a developing unit, and the frame has a resin guide for positioning the optical head and the developing unit independently for each of the print assemblies. 
     The angle of the optical head when the top cover is closed is set to lie over a range of from 91 to 120 degrees with respect to the horizontal from the side of the rotary support point. 
     The photosensitive body, the charger and the developing device in each of the printing assemblies are formed as a developing unit, and the frame has a guide which permits the developing unit to be attached or detached at an angle over a range of from 1 to 45 degrees with respect to the horizontal from the side of the front end. 
     The frame has a support portion having a recessed shape for receiving the tip of the optical head when the top cover is closed, the support portion being located at a position nearly in agreement with the surface of the photosensitive body. 
     The frame has a turn-stop portion for receiving a side portion of the optical head when the tip of the optical head is supported by the support portion, and further has an urging means for urging the optical head toward the turn-stop portion. 
     The top cover has side plates, the frame has side plates, the rotary support point is provided on the side plates of the top cover and the side plates of the frame, and the side plates of the top cover have protuberances that come into contact with the side plates of the frame. 
     The side plates of the frame have ridged portions that come into contact with the protuberances of the side plates of the top cover. 
     The rugged portions are formed at a position where the top cover is held in a fully opened condition. 
     The rugged portions are so formed that the protuberances on the side plates of the top cover will not ride over the rugged portions despite of the gravity of the top cover. 
     Provision is further made of an end cover located above the fixing device, the end cover being linked to the top cover. 
     The end cover remains at a closed position until the top cover is lifted from the closed position to a partially open position and opens, by being linked to the top cover, when the top cover is lifted from the partially open position to the fully opened position. 
     The present invention further provides an image forming apparatus comprising a closing cover mounted to the frame so as to be opened and closed, a plurality of printing assemblies each comprising a photosensitive body, a charger for charging the photosensitive body, an optical head for forming an electrostatic latent image on the photosensitive body, a developing device for developing the electrostatic latent image into a toner image, and a transfer element for transferring the toner image onto paper, a fixing device, a paper conveyer belt for conveying paper through the plurality of print assemblies, an optical sensor having a light-emitting element and a light-receiving element to read marks formed on the paper conveyer belt, and a movable cover which opens an optical passage of the optical sensor when the apparatus is in operation and shuts off the optical passage of the optical sensor when the apparatus is not in operation. 
     According to this constitution, since the movable cover is provided with a shut-off for shutting off the optical passage of the optical sensor, the laser beam does not leak to the exterior even though a portion of the apparatus is opened. 
     The following constitutions can be employed together with this constitution. 
     The optical sensor includes a laser diode and a photodiode. 
     The optical sensor includes a dust-preventing member which permits the transmission of detection light but prevents the infiltration of dust and dirt. 
     The movable cover includes a cleaning member capable of cleaning the dust-preventing member. 
     The movable cover is urged by an urging member toward a direction to shut off the optical passage. 
     The optical sensor has a plurality of detector units, and the movable cover is capable of simultaneously covering the plurality of detector units. 
     The optical sensor is retracted from the paper conveyer belt being linked to the movable cover. 
     The optical sensor is supported in a floating manner by the frame. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more apparent from the following description of the preferred embodiments, with reference to the accompanying drawings, in which: 
     FIG. 1 is a side view schematically illustrating an image forming apparatus according to the embodiment of the present invention, a top cover being at a slightly opened position; 
     FIG. 2 is a view illustrating the image forming apparatus in which the top cover is at the fully opened position; 
     FIG. 3 is a view illustrating one printing assembly; 
     FIG. 4 is a plan view illustrating the image forming apparatus in which the top cover is at the closed position relative to the frame; 
     FIG. 5 is a side view of the image forming apparatus of FIG. 4; 
     FIG. 6 is a plan view illustrating the top cover; 
     FIG. 7 is a front view the image forming apparatus, viewed in the direction of the arrow VII in FIG. 4; 
     FIG. 8 is a front view showing the interior of the image forming apparatus, with the front wall member removed from the frame in FIG. 7; 
     FIG. 9 is a front view, on an enlarged scale, of a portion of the image forming apparatus of FIG. 8; 
     FIG. 10 is a side view of the image forming apparatus of FIG. 9 when the top cover is opened by a predetermined amount; 
     FIGS. 11A and 11B are views. illustrating the cover lifting mechanism of FIGS. 1 and 2; 
     FIG. 12 is a view illustrating the operational relationship between the lock releasing button and the lock member of FIGS. 1 and 2; 
     FIG. 13 is a cross-sectional view illustrating a portion of the apparatus of FIG. 8 on an enlarged scale; 
     FIG. 14 is a side view of a portion of the apparatus of FIG. 13; 
     FIG. 15 is a view illustrating, in detail and on an enlarged scale, a portion of the side plate of the top cover of FIG. 6; 
     FIG. 16 is a view illustrating the relationship between the top cover and the end cover; 
     FIG. 17 is a view illustrating a modified example of a mechanism for moving the shut-off cover; 
     FIG. 18 is a view illustrating the paper conveyer belt and the optical sensor; 
     FIG. 19 is a view illustrating the operation of the optical sensor; 
     FIG. 20 is a perspective view illustrating the shutoff cover; 
     FIG. 21 is a side view of the shut-off cover of FIG. 21; 
     FIG. 22 is a perspective view illustrating another example of the shut-off cover; 
     FIG. 23 is a side view of the shut-off cover of FIG. 22; 
     FIG. 24 is a view illustrating a further example of the optical sensor and the shut-off cover; 
     FIG. 25 is a partly cut-away perspective view illustrating, in detail, the optical sensor including the shut-off cover of FIG. 24; 
     FIG. 26 is a view illustrating the shut-off cover of FIG. 25 at a position to open the optical passage; 
     FIG. 27 is a view illustrating the shut-off cover of FIG. 25 is at a position to block off the optical passage; 
     FIG. 28 is a view illustrating in detail the shutoff cover when it is not covering the dust-preventing member that defines the optical opening of the optical sensor; 
     FIG. 29 is a view illustrating in detail the shutoff cover when it is covering the dust-preventing member that defines the optical opening of the optical sensor; 
     FIG. 30 is an enlarged view of the optical sensor of FIG. 28; 
     FIG. 31 is an enlarged view of the optical sensor and the shut-off cover of FIG. 29; and 
     FIG. 32 is a cross-sectional view illustrating the shut-off cover in detail. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIGS. 1 and 2 illustrate an image forming apparatus  10  according to the embodiment of the present invention. The image forming apparatus  10  is constituted as a full color printer. The image forming apparatus  10  has a frame  12  which includes a top cover  14  and a rear cover  16 . The rear cover  16  is not shown in FIGS. 1 and 2 but is shown in FIGS. 4 and 5. FIG. 1 illustrates the top cover  14  is at a slightly opened position with respect to the frame  12 , and FIG. 2 illustrates the top cover  14  at the fully opened position with respect to the frame  12 . Upon opening the top cover  14  and/or the rear cover  16 , the members inside the image forming apparatus  10  become accessible. 
     FIG. 4 is a plan view illustrating the image forming apparatus in which the top cover  14  is closed with respect to the frame  12 , and FIG. 5 is a side view of the image forming apparatus  10  of FIG.  4 . The image forming apparatus  10  includes an operation panel P at the right end of FIG.  4 . In the drawings, the right end where the operation panel P exists is the front side of the apparatus, and the left end is the rear side of the apparatus. A lock-releasing button  17  of the top frame  14  exists on the front side of the apparatus. The rear cover  16  exists at an end on the side opposite to the operation panel P. 
     FIG. 6 is a plan view of the top cover  14 . As shown in FIGS. 1 and 6, the top cover  14  has a so-called cover portion  14 A and side plate portions  14 B that extend rearward and downward from both sides of the cover portion  14 A. In FIG. 5, furthermore, the top cover  14  can pivotally move about a rotation support point  14 X located at a rear portion of the apparatus, and the rear cover  16  can pivotally move about a rotary support point  16 X located under the rotary support point  14 X. In FIG. 6, the rotary support point  14 X is shown as holes formed in the side plate portions  14 B. A rotary support shaft is inserted in the holes formed in the side plate portions  14 B. FIGS. 1 and 2 show the rotary support point  14 X but does not show the rotary support point  16 X. 
     In FIGS. 1 and 2, the image forming apparatus  10  includes four printing assemblies  20 B,  20 C,  20 M and  20 Y arranged in series. An endless paper conveyer belt  22  is provided for the four printing assemblies  20 B,  20 C,  20 M and  20 Y. The paper conveyer belt  22  is formed of a suitable transparent synthetic resin material, and runs around four rollers  24   a ,  24   b ,  24   c  and  24   d . The roller  24   a  is a drive roller which also works as an AC electric charge-removing roller for removing electric charge from the paper conveyer belt  22 . The roller  24   b  is a driven roller which also works as a charging roller for imparting an electric charge to the paper conveyer belt  22 . The rollers  24   c  and  24   d  are both guide rollers, the roller  24   d  being a tension roller for imparting a suitable tension to the paper conveyer belt  22 . 
     A hopper  26  is provided under the paper conveyer belt  22 . A stack of papers P is stored in the hopper  26 . The papers P are delivered one by one from the hopper  26  by a pick roller  28 , and are conveyed onto the paper conveyer belt  22  by a paper feed roller  30 . The paper P is conveyed by the paper conveyer belt  22  to the printing assemblies  20 B,  20 C,  20 M and  20 Y and printing or recording is effected thereat. The recorded paper P is conveyed to a fixing device  32 , and is discharged to a stacker formed on the upper surface of the top cover  14  through suitable guide rollers (not shown). 
     The paper conveyer belt  22  is electrically charged by the driven roller  24   b . Therefore, the paper P that is introduced onto the paper conveyer belt  22  from the side of the driven roller  24   b  is electrostatically attract and held by the paper conveyer belt  22 . Accordingly, the paper P is held maintaining a predetermined positional relationship with respect to the paper conveyer belt  22 . On the other hand, the drive roller  24   a  works as the electric charge-removing roller and, hence, the electric charge is removed at the time the paper P passes the position of the drive roller  24   a ; i.e., the paper P being discharged from the side of the drive roller  24   a  is not entrained in the lower running portion of the paper conveyer belt  22  and is easily separated away from the paper conveyer belt  22 . 
     The four printing assemblies  20 Y,  20 M,  20 C and  20 B have identical structures. The printing assembly  20 Y contains a developing agent having a yellow toner component, and the printing assembly  20 M contains a developing agent having a magenta toner component. The printing assembly  20 C contains a developing agent having a cyan toner component, and the print assembly  20 B contains a developing agent having a black toner component. Therefore, these printing assemblies  20 Y,  20 M,  20 C and  20 B print a yellow toner image, a magenta toner image, a cyan toner image and a black toner image onto the paper P, that moves while being held by the paper conveyer belt  22 , to thereby form a resultant full-color toner image. 
     FIG. 3 illustrates one printing assembly  20 Y. Other printing assemblies  20 M,  20 C and  20 B have the same constitution as that of the printing assembly  20 Y. Therefore, the printing assembly  20 Y only will be described in detail. The printing assembly  20 Y is equipped with a photosensitive body (photosensitive drum)  36  which is rotated in the direction of the arrow in FIG. 3. A precharger  38 , an optical head (LED beam scanner)  40 , a developing device  42 , a transfer element (transfer roller)  44 , and a toner-cleaning device  46  are arranged in this order to surround the photosensitive body  36 . 
     The precharger  38  disposed above the photosensitive body  36  is, for example, a corona charger or a scorotron charger. Due to the precharger  38 , the surface of the photosensitive body  36  is continuously charged with a uniform electric charge. The optical head  40  is disposed after the precharger  38 , and writes an electrostatic latent image onto the electrically charged region of the photosensitive body  36  using an LED beam. In other words, the LED beam is energized based on binary image data obtained from a computer or a word processor, so that the electrostatic latent image is written as dot images. 
     The electrostatic latent image written onto the photosensitive body  36  is electrostatically developed into an electrically charged toner image using a toner of a predetermined color in the developing device  42 . The electrically charged toner image is electrostatically transferred onto the paper P by the transfer element  44  located under the photosensitive body  36 . The transfer element  44  is formed as an electrically conducting transfer roller made of a porous material (sponge). The transfer element  44  is urged against the photosensitive body  36  via the paper conveyer belt  22 , and gives an electric charge of a polarity opposite to that of the electrically charged toner image to the paper P conveyed by the paper conveyer belt  22 , whereby the electrically charged toner image on the photosensitive body  36  is electrostatically transferred from the photosensitive body  36  onto the paper P. 
     The paper P onto which the electrically charged toner image is transferred is liberated from the paper conveyer belt  22  and is sent toward the fixing device  32 . The toner that was not transferred onto the paper P remains adhered to the surface of the photosensitive body  36  after the transfer onto the paper P is finished. The residual toner is removed by the toner-cleaning device  46 . The residual toner that is removed is returned back to the developing device  42  by a toner returning device constituted by a screw conveyer  48  and the like, and is used again as the toner for developing. 
     In FIG. 3, the developing device  42  is equipped with a developing agent container  50  for containing a one-component developing agent comprising a toner (fine powdery particles of a colored resin). In the port of the developing agent container  50  is arranged a magnet roller or a developing roller  52  in a manner that a portion of the surface thereof is exposed. The developing roller  52  comprises a shaft/core portion supported by the two side walls of the developing agent container  50  and a sleeve disposed to rotate about the periphery of the core portion and is formed of a nonmagnetic material such as aluminum. When the developing device  42  is in operation, the sleeve of the developing roller  52  rotates in the direction indicated by the arrow in the drawings. 
     A toner hopper  53  is provided on the developing agent container  50  so as to be detachably mounted thereon. A toner is contained in the toner hopper  53 . When the toner in the developing agent container  50  is depleted, the toner in the toner hopper  53  is supplied into the developing agent container  50 . When the toner is supplied to the developing gent container  50  and is depleted in the toner hopper  53 , this fact is informed as an error to the outside of the apparatus. The toner is replenished by replacing an old toner hopper  53  containing no toner by a new toner hopper  53  filled with the toner. The interior of the toner hopper  53  is divided into two sections by a wall  53 - 1 . The one portion is filled with the new toner before the toner hopper is mounted, and the other portion is in an empty state. When the toner hopper  53  is mounted to the apparatus, the waste toner recovered by the toner-cleaning device  46  is recovered through a recovery pipe  48  and is contained in the empty portion of the toner hopper  53 . Upon replacing the toner hopper  53 , therefore, the new toner is replenished while discarding the waste toner. The recovery pipe  48  is provided with a screw by which the toner is conveyed. 
     When the developing device  42  is installed in the apparatus, the exposed surface of the developing roller  52 , i.e., the sleeve, is opposed to the surface of a carrier which carries the electrostatic latent image such as the photosensitive body. The bottom wall of the developing agent container  50  serves as a developing agent reservoir in which a reset roller  54  is provided. The reset roller  54  is rotatably supported by both side walls of the developing agent container  50 , and is rotated in the direction indicated by the arrow in the drawing when the developing device  42  is in operation. The reset roller  54  recovers the developing agent that was not supplied to the photosensitive body  36  but remains on the developing roller  52 . 
     With the rotation of the developing roller  52 , the developing agent is conveyed to the region opposed to the photosensitive body  36 , i.e., conveyed to the developing region. A developing agent-limiting blade  56  is provided under the developing roller  52 , while being opposed thereto, in order to limit, to a predetermined value, the amount of the developing agent conveyed by the developing roller  52  to the developing region. 
     The developing device  42  executes the following developing process. When, for example, the toner component in the developing agent is electrically charged to a negative polarity, a uniformly charged region of the negative polarity is formed on the rotary surface of the photosensitive body  36  due to the precharger  38 . When the electrically charged region of the photosensitive body  36  is irradiated with an LED beam emitted from the optical head  40 , the electric charge of the negative charge escapes from the irradiated portion giving rise to the occurrence of a potential difference. That is, the electrostatic latent image is written as a potential difference onto the electrically charged region of the photosensitive body  36 . For example, when the electrically charged region of the photosensitive body  36  has a potential of −600 volts, the potential of the electrostatic latent image drops to about −15 volts. On the other hand, a negative developing bias voltage of, for example, −400 volts is applied to the developing roller  52 , and an electric field is formed between the developing roller  52  and the photosensitive body  36 . Due to the electric field formed between the developing roller  52  and the photosensitive body  36 , the toner component charged into the negative polarity flies toward the photosensitive body  36 , adheres onto the photosensitive body  36  and the image is developed. 
     As shown in FIG. 1, therefore, the paper P is introduced from a portion of the driven roller  24   b  of the belt conveyer means  10  into the printing region, and passes through the printing assemblies  20 Y,  20 M,  20 C and  20 B, successively, whereby toner images of four colors are formed in an overlapped manner on the paper P to thereby form a full-color image. The paper P is, then, sent toward the heat roller-type thermal fixing device  32  from the side of the drive roller  24   a  of the belt conveyer means  10 , and the full-color image is thermally fixed on the paper P there. 
     The photosensitive body  36 , the charger  38 , the developing device  42  and the toner-cleaning device  46  are formed as a developing unit U in each of the printing assemblies  20 Y,  20 M,  20 C and  20 B. Each developing unit U is detachably mounted to the frame  12 . The optical head  40  is mounted to the top cover  14 . The paper conveyer belt  22  and the rollers  24   a  to  24   d  are jointly formed as a belt unit, and the transfer element  44  is mounted to the belt unit. 
     By opening the top cover  14 , the constituent elements of the printing assemblies can be repaired or replaced. Moreover, the end cover  16  is disposed at a position to cover the fixing device  32 . Upon opening the end cover  16 , the fixing device  32  can be repaired or replaced. As shown in FIGS. 1 and 2, the rotary support point  14 X of the top cover  14  is located at a position behind the fixing device  32 . Therefore, the top cover  14  has an increased length and, as shown in FIG. 2, the radii of rotation of the optical heads  40  increase while the top cover  14  turns, making it possible to decrease the dead space in the frame  12  that is required for the optical heads  40  to turn. Concretely speaking, the gap between the two neighboring developing units U can be decreased. 
     The top cover  14  includes the four optical heads  40 , the control board for controlling the optical heads  40 , the sheet metals for shielding the control board, the base frames for mounting the optical heads  40 , etc. and is very heavy. It is therefore desired to provide means by which the top cover  14  can be easily opened. 
     Referring to FIGS. 1 and 2, the image forming apparatus  10  includes a lock member  60  for locking the top cover  14  to the frame  12 , a cover lifting mechanism  62  for lifting the top cover  14  relative to the frame  12  when the lock member  60  is released, and a switch  64  linked to the cover lifting mechanism  62  to detect the opening or closing of the top cover  14 . 
     FIG. 7 is a front view of the image forming apparatus  10  seen from the direction of the arrow VII in FIG. 4, FIG. 8 is a view illustrating the interior of the image forming apparatus  10  with the front wall removed from the frame  12  in FIG. 7, and FIGS. 9 and 10 are views illustrating portions of the image forming apparatus  10  of FIG. 7 on an enlarged scale. As shown in these views (and particularly in FIG.  8 ), two lock members  60  are arranged on both sides at the front end of the apparatus and mounted to a support shaft  66 . The frame  12  has a left-side frame member  12 L and a right-side frame member  12 R, and the support shaft  66  is supported by these side frame members  12 L and  12 R. The lock members  60  have hooked tips as shown in FIGS. 1,  2  and  10 . Lock holes  68  are formed in the front surface of the top cover  14 . The top cover  14  is locked to the frame  12  when the hooked tips of the rock members  60  have engaged in the lock holes  68  in the top cover  14 . 
     FIG. 12 is a view illustrating the operational relationship between the lock-release button  17  and the lock member  60 . The lock-release button  17  is disposed so that it can be downwardly depressed, and is urged by a spring  17   a  to return upwards. An intermediate lever  70  is disposed between the lock-release button  17  and the lock member  60 . When the lock-release button  17  is depressed, the intermediate layer  70  turns causing the lock member  60  to turn clockwise as indicated by the arrow in the drawing, whereby the hooked tip of the lock member  60  is disengaged from the lock hole  68  in the top cover  14 , and the top cover  14  is released from the frame  12 . The two lock members  60  move together with the support shaft  66 . A spring  60   a  urges the lock member  60  toward the locking position. 
     FIGS. 11A and 11B are views illustrating the cover lifting mechanism  62 . In FIGS. 11A and 11B and in the above-mentioned drawings, the cover lifting mechanism  62  is disposed near the front end of the apparatus and includes a rod  72  capable of engaging with the top cover  14 , a spring  74  for urging the rod  72  in the direction in which the top cover  14  opens, and a stopper  76  for stopping the rod at a predetermined position. In FIGS. 11A and 11B, the angle-like bracket  78  is secured to the frame  12 , the upper wall of the bracket  78  serves as the stopper  76 , and the lower wall of the bracket  78  serves as a spring seat  80 . The rod  72  has a retaining ring  82  fitted in a groove formed therein. The spring  74  is disposed around the rod  72  between the stopper  76  and the spring seat  80 . 
     The tip of the rod  72  comes into contact with a predetermined portion of the top cover  14  as shown in FIG.  10 . When the top cover  14  is closed by hand, the top cover  14  pushes the rod  72  down against the spring  74 , so that the lock members  60  lock the top cover  14 . The rod  72  moves from the position of FIG. 11A to the position of FIG.  11 B. When the top cover  14  is opened, the lock-release button  17  is depressed and the lock members  60  are moved via the intermediate lever  70  so that the lock members  60  are disengaged from the lock holes  68  and the top cover  14  is unlocked from the frame  12 . Then, the rod  72  moves upwards due to the action of the spring  74  to thereby force the top cover  14  to open. The upward motion of the rod  72  stops at a position where the retaining ring  82  comes into contact with the stopper  76 . 
     The stroke of the rod  72  may be such that a gap is formed between the top cover  14  and the frame  12  to permit fingers to be inserted therein, so that the top cover  14  can be easily opened by hand. Concretely speaking, the top cover  14  may be lifted by more than 10 mm. 
     FIG. 1 illustrates the image forming apparatus  10  under this state. In the state of FIG. 1, therefore, it is possible to insert a hand under the top cover  14  to a sufficient degree, so that the top cover  14  can be fully opened by hand. FIG. 2 illustrates the image forming apparatus  10  in which the top cover  14  is at the fully opened position. 
     A force for lifting the top cover  14 , other than the cover lifting mechanism  62 , may exist. For example, the top cover  14  may include urging springs for positioning the optical heads  40  and urging springs for positioning the developing units U, that will be described later, but these springs are not capable of lifting the top cover  14  by a predetermined amount when the top cover  14  is unlocked. The urging springs for the developing units U do not act when the developing units U are removed. The cover lifting mechanisms  62  may be provided in a plural number. The top cover  14  can be reliably lifted up with a small force when the cover lifting mechanism  62  is disposed at the front end of the top cover  14 . 
     The switch  64  is disposed in a passage where the rod  72  of the cover lifting mechanism  62  moves, and is turned on or off by the rod  72 . The switch  64  is connected to a control circuit that is not shown and turns off the power source for predetermined parts in the frame  12  when the top cover  14  is opened. When the top cover  14  is closed, the power source is turned on for the predetermined parts in the frame  12 . The switch of any other type (e.g., a microswitch, a transmission type switch or the like) can be used in addition to the switch  64  that is shown. 
     Arranging the rotation pivot  14 X of the top cover  14  at the rear part of the apparatus, as described above, makes it possible to reduce a dead space required for turning (pivoting) the optical heads  40  mounted to the top cover  14  and, hence, to effectively and compactly arrange the plurality of printing assemblies  20 B,  20 C,  20 M and  20 Y. If the rotation pivot  14 X of the top cover  14  is located between the fixing device  32  and the final printing assembly  20 B, the radii of turn of the optical heads  40  decrease, resulting in an increase in a dead space due to the turn of the optical heads  40 . 
     FIG. 13 is a view illustrating a portion of the apparatus of FIG. 8 on an enlarged scale. In FIGS. 8 and 13, the frame  12  has the left-side frame member  12 L and the right-side frame member  12 R as described above. The photosensitive body  36 , the charger  38  and the developing device  42  are formed as a developing unit U in each printing assembly, and each of the side frame members  12 L and  12 R has a resin guide member  84  for positioning the optical head  40  mounted to the top cover  14  and the developing unit U independently for each of the printing assemblies. Upon providing the resin guide member  84  for each of the printing assemblies, it is allowed to form the guide member  84  in a smaller size than that when the similar guide member is commonly provided for all printing assemblies, and the amount of thermal deformation due to a rise in temperature caused by the heat of fixing and printing can be suppressed to be smaller than that when the guide member  84  is formed in a large size. In each printing assembly, therefore, the constituent members are reliably positioned and maintain a predetermined positional relationship. Decreasing the thermal deformation is effective in preventing the deviation in positions of the light-emitting portions of the optical heads  40 . 
     Referring to FIG. 13, the optical head  40  has its own mounting frame, and the mounting frame has a pin-like tip  40   a . The guide member  84  has a recessed (conical) support portion  84   a  for receiving the pin-like tip  40   a  of the mounting frame of the optical head  40 . The optical head  40  is downwardly urged by a spring  86  relative to the top cover  14 . When the top cover  14  is closed, the pin-like tip  40   a  of the optical head  40  is brought into reliable contact with the recessed support portion  84   a . The spring  86  upwardly urges the top cover  14  through the optical head  40 . Here, however, the spring  86  is not strong enough to produce a force and a movement to substantially open the top cover  14  when the lock member  60  is released. 
     The guide member  84  has a bearing  84   b  for receiving the shaft of the photosensitive body  36 . Therefore, the optical head  40  and the photosensitive body  36  are positioned by the common guide member  84 . The charger  38  and the developing device  42  are formed as a developing unit U together with the photosensitive body  36 . Therefore, these members, too, are positioned by the guide member  84 . 
     FIG. 14 is a side view of the apparatus of FIG. 13, and shows the above-mentioned optical head  40 , the photosensitive body  36  and the guide member  84 . The guide member  84  further has a guide portion (guide groove)  84   c . The developing unit U has its own mounting frame (not shown) for supporting the above-mentioned constituent members, and the mounting frame has a unit-positioning pin  88 . When the developing unit U is to be mounted to the frame  12  of the image forming apparatus  10 , the shaft of the photosensitive body  36  is brought into contact with the bearing  84   b  while moving the unit-positioning pin  88  along the guide portion  84   c.    
     As shown in FIG. 14, the support portion  84   a  is provided at such a position that the tip  40   a  of the optical head  40  comes nearly into agreement with the surface of the photosensitive body  36  that is supported by the bearing  84   b . Accordingly, the tip  40   a  of the optical head  40  is positioned to be nearly in agreement with the surface of the photosensitive body  36  at all times, and a predetermined positional relationship is maintained between the light-emitting portion of the optical head  40  and the surface of the photosensitive body  36 . Even in the case where the optical head  40  is deviated in the direction of rotation due to vibration or the like, a predetermined relationship is maintained between the surface of the photosensitive body  36  and the focal distance of the LED portion of the optical head  40  at all times, making it possible to accomplish favorable printing. The optical head  40  is in a floating condition with reference to the positioning point. 
     As shown in FIG. 2, when the top cover  14  is closed, the optical head  40  moves toward the guide  84  describing an arcuate locus and is supported by the recessed support portion  84   a  in the guide  84 . At a position where the optical head  40  is supported by the recessed support portion  84   a  of the guide  84  as shown in FIG. 14, the optical head  40  is located being tilted at an angle of from 91 to 120 degrees with respect to the horizontal from the side of the rotary support point  14 X. This helps decrease the dead space for turning the optical head  40  mounted to the top cover  14 , and it is allowed to arrange the plurality of printing assemblies effectively and compactly. When the optical head is located at 90 degrees, dead space increases due to the locus of turn. Space can be effectively utilized particularly when a wider optical head  40  is used. 
     The guide portion  84   c  of the guide member  84  permits the developing unit U to be detachably attached at an angle over a range of 1 to 45 degrees with respect to the horizontal from the side of the front end. This enables the neighboring two printing assemblies to be arranged at a smaller pitch. Besides, the developing units U need not be attached or detached in the order from an end but, instead, the developing unit U at any position can be attached or detached while the other developing units U are being attached. Since the developing unit U is attached or detached in the tilted position toward the operator side, it provides a good attaching or detaching operability. The printing assemblies are arranged being overlapped by each other so that they can be effectively mounted. When it is attempted to attach or detach the developing unit U located at the center with the printing assemblies being arranged in an overlapped manner, the central printing assembly may interfere with the front and rear printing assemblies and often cannot be removed. According to the present invention, however, the positioning pins for the developing units U are forcibly moved by the guides toward the direction for attachment or detachment, allowing the developing units U to be easily attached or detached. 
     As shown in FIG. 14, furthermore, the guide member  84  of the frame  12  has a turn-stop portion  84   d  for receiving the side portion of the optical head  40  when the tip of the optical head  40  is supported by the support portion  84   a . The spring  86  is arranged at an angle relative to the axis of the mounting portion of the optical head  40  so that the spring  86  urges the optical head  40  toward the turn-stop portion  84   d . Therefore, a predetermined focal distance can be maintained at all times, and the printing is favorably effected. 
     FIG. 15 is a view illustrating a portion of the apparatus of FIG. 6 in detail on an enlarged scale. In FIGS. 6 and 15, the top cover  14  has side plate portions  14 B that extend rearwardly and downwardly from both sides of the cover portion  14 A. FIG. 15 further shows side plate portions  12 B of the frame  12 . The side plate portions  14 B of the top cover  14  vertically extend in parallel with the side plate portions  12 B of the frame  12 , and the rotary support point  16 X comprises a shaft provided at the front ends of the side plate portions  14 B and the side plate portions  12 B. The side plate portions  14 B of the top cover  14  have protuberances  14 C that come into contact with the side plate portions  12 B of the frame  12 . The protuberances  14 C are shown in FIG. 1 too. The side plate portions  14 B of the top cover  14  easily undergo a deflection due to a force acting in a horizontal direction of the apparatus. Therefore, rigidity is imparted thereto by the provision of the protuberances  14 C that come into contact with the side plate portions  12 B of the frame  12 . The protuberances  14 C are located at positions away from the rotary support point  16 X, making it possible to increase the length of the arms from the rotary support point  16 X to the protuberances  14 C to a suitable degree and to impart increased rigidity to the right and left of the top cover. 
     The side plate portions  12 B of the frame  12  have rugged portions  12 C that come into contact with the protuberances  14 C of the side plate portions  14 B of the top cover  14 . In this embodiment, the rugged portions  12 C are formed as two protuberances. As shown in FIG. 1, the protuberances  14 C and rugged portions  12 C are on a circle with the rotary support point  16 X as a center. When the top cover  14  is opened, the protuberances  14 C slide on the surfaces of the side plate portions  12 B of the frame  12 , ride over the first rugged portions  12 C, slide on the surfaces of the side plate portions  12 B of the frame  12  and, then, ride over the second rugged portions  12 C. After the protuberances  14 C have ridden over the second rugged portions  12 C, the top cover  14  reaches the fully opened position. 
     Upon providing the protuberances  14 C and rugged portions  12 C as described above, load is produced at the time when the protuberances  14 C ride over the rugged portions  12 C accompanying the opening or closing motion of the top cover  14 , and a click feeling is obtained in the operation. In particular, the (second) rugged portions  12 C located at positions just before the top cover  14  reaches the fully opened position make it possible to learn that the top cover  14  has reached the fully opened position, and enable the top cover  14  to be maintained at the fully opened position. The top cover  14  (as well as parts mounted thereon) may tend to be vigorously closed as it falls due to its own weight when it is to be closed. However, with the (first) rugged portions  12 C being located at positions at which the top cover  14  is likely to abruptly move, the top cover  14  is prevented from being suddenly closed. By providing at least one rugged portion at a position satisfying the relationship, the force causing the top cover  14  to turn and fall due to its weight &lt;the force required for the protuberances  14 C to ride over the rugged portions  12 C, the top cover  12  is prevented from being suddenly closed, avoiding the danger that a hand is caught by the top cover  12  that is closing. 
     Referring to FIGS. 6 and 15, the top cover  12  has coupling portions  14 D at a boundary portion between the cover portion  14 A and the side plate portions  14 B to couple the top cover  12  and the end cover  15  together. 
     FIG. 16 is a view illustrating, on an enlarged scale, a portion of the apparatus of FIG. 1 in which the end cover  16  is added to the top cover  12 . The end cover  16  includes a cover portion for covering the fixing device  32 , and side plate portions  16 B. The coupling portions  14 D include coupling grooves  14 E in an L-shape, and pins  16 E extending from the side plate portions  16 B of the end cover  16  are inserted in the coupling grooves  14 E. When the top cover  14  and the end cover  16  are at the closed position, the pins  16 E are located at the upper ends of the coupling grooves  14 E. When the top cover  14  moves from the closed position to a slightly opened position, the pins  16 E move along the vertical portions of the coupling grooves  14 E, and the end cover  16  remains still. When the top cover  14  further moves, the pins  16 E move along the horizontal portions of the coupling grooves  14 E, and the end cover  16  moves together with the top cover  14 . Thus, the end cover  16  permits the fixing device  32  to be repaired or replaced. 
     The guide provided independently for each of the printing assemblies makes it possible to decrease a change in the size thereof due to a rise in the temperature caused by heat produced by fixing and printing. This is particularly effective in regard to a focal distance that requires high dimensional precision. 
     In FIG. 1, under the paper conveyer belt  22  are provided an optical sensor  90  for reading marks printed on the paper conveyer belt, and a shut-off cover  92  which opens the optical passage of the optical sensor  90  when the apparatus is in operation and shuts off the optical passage of the optical sensor when the apparatus is not in operation. The optical sensor  90  will be described later. 
     The shut-off cover  92  moves by being linked to the opening or closure of the top cover  14 . That is, the shut-off cover  92  is provided with a push lever  94  that comes into contact with an engaging portion  14 F of the top cover  14  and a shutter lever  96  that comes into contact with the push lever  94 . When the top cover  14  is closed, the push lever  94  pushes the shut-off cover  92  via the shutter lever  96 ; i.e., the shut-off cover  92  is moved toward a position to open the optical passage of the optical sensor  90 . When the top cover  14  is opened, the push lever  94  returns the shut-off cover  92  via the shutter lever  96 ; i.e., the shut-off cover  92  is moved toward a position to shut off the optical passage of the optical sensor  90 . 
     FIG. 17 is a view illustrating a modified example of the mechanism for moving the shut-off cover  92 . In this example, provision is made of a push lever  95  that comes into contact with the belt unit  100  which jointly supports the paper conveyer belt  22 , instead of providing the push lever  94  that comes into contact with the top cover  14 . The push lever  94  moves the shut-off cover  92  via a shutter lever  96 . The shut-off cover  92  shuts off or opens the optical passage of the optical sensor  90 . The shut-off cover  92  can be driven by a drive means such as a motor or the like. Described below in detail is an example of using the push lever  94  that comes into contact with the top cover  14 . 
     FIG. 18 is a view illustrating the paper conveyer belt  22  and two optical sensors  90 . For example, at the start of use, the plurality of printing assemblies  20 B,  20 C,  20 M and  20 Y print marks  102  on the paper conveyer belt  22 . The optical sensors  90  read the marks  102  printed on the paper conveyer belt  22 , and a control means that is not shown adjusts the printing positions and printing timings of the plurality of printing assemblies based upon the marks  102  that are read out. 
     FIG. 19 is a view illustrating the operation of the optical sensor  90  which comprises a laser diode (L)  104  contained in a case, and a photodiode (PD)  106 . A laser beam emitted from the LD  104  and reflected by the paper conveyer belt  22 , is read by the PD  106 . The laser beam of the optical sensor  90  may unexpectedly enter the eyes when the top cover  14  is opened or the belt unit  100  is lifted up while the power source circuit is closed, which is not desirable. It is therefore desired to shut off the optical passage of the optical sensor  90  being linked to the top cover  14  or to the belt unit  100 . In front of the LD  104  and PD  106 , there are arranged dust-preventing members  108  which permit the transmission of the detection laser beam but prevent the infiltration of dust and dirt. 
     FIGS. 20 and 21 are views illustrating an example of the shut-off cover  92  which is secured to a support shaft  92   a . The shutter lever  96 , too, is secured to the support shaft  92   a . The support shaft  92   a  is rotatably supported by the frame  12  through bearings that are not shown. A spring  110  upwardly urges the shutter lever  96  (or the shut-off cover  92 ). 
     The optical sensor  90  has optical openings  104   a  and  106   a  for the LD  104  and PD  106 , and has another optical opening  105   a  at the center thereof. The optical opening  105   a  has no direct relationship to the present invention and is not described here. The optical sensor  90  is rotatably supported by the frame  12  through the support shaft  90   a  in parallel with the support shaft  92   a . A spring  112  upwardly urges the optical sensor  90 . 
     The shut-off cover  92  consists of a pair of arm portions  92   b  mounted to the support shaft  92   a  and a cover portion  92   c  extending between the pair of arm portions  92   b . The cover portion  92   c  is disposed above the optical sensor  90 , and is so located as to open the optical openings  104   a ,  106   a , when the top cover  14  is closed, as shown in FIGS. 20 and 21. When the top cover  14  is opened, the push lever  94  is allowed to move, whereby the spring  110  causes the shutter lever  96  to turn upwards so that the shut-off cover  92  is turned in the clockwise direction. Thus, the cover portion  92   c  of the shut-off cover  92  covers the optical openings  104   a  and  106   a  thereby to shut off the optical passages of the optical sensors  90 . 
     When the shut-off cover  92  turns clockwise in this embodiment, the cover portion  92   c  of the shut-off cover  92  covers the optical openings  104   a  and  106   a  while contacting (wiping) the surfaces of the optical sensors  90 . The shut-off cover  92  includes a cleaning member such as a felt. In other words, the shut-off cover  92  is formed to work as a shutter as well as a cleaning member. 
     The optical sensor  90  is always urged onto the paper conveyer belt  22  by a spring  112 . The optical sensor  90  is supported by the frame  12  in a floating manner. Therefore, the marks  102  are read out while a gap between the paper conveyer belt  22  and the LD  104  and PD  106  of the optical sensor  90  is maintained constant. When the shut-off cover  92  turns clockwise, the cover portion  92   c  of the shut-off cover  92  comes into contact with the surface of the optical sensor  90 , and the optical sensor  90  is retracted from the paper conveyer belt  22 . 
     The member for executing the retracting motion comes into contact with the housing of the optical sensor  90  earlier than the cleaning member, so that it moves back, and, then, the cleaning member makes contact. Accordingly, the material of the cleaning member can be selected from a wide range, and the reliability of operation is enhanced. 
     The shut-off cover  92  can be arranged to satisfy the North America Safety Standards (CDRH Standards of the FDA), by selecting a gap size and preventing the operator from erroneously operating the apparatus. Moreover, the shut-off cover  92  is urged at all times by an urging mechanism such as a spring in the direction to close the optical openings. In order to prevent irregular operation, the shut-off cover  92  is opened by using a highly rigid member such as a link mechanism linked to opening and closing the top cover  14 , and is closed by a spring  110  or the like. 
     FIGS. 22 and 23 are views illustrating another example of the shut-off cover  92 . In this example, the shut-off cover  92  is divided into a cover portion  92 X and arm portions  92 Y, and the cover portion  92 X slides along a flat surface of the optical sensor  90 . The arm portions  92 Y are mounted to the support shaft  92   a  and are moved by the top cover  14  or the belt unit  100  via the push lever  94  and the shutter lever  96 . 
     FIG. 24 is a view illustrating a further example of the optical sensor  90  and the shut-off cover  92 . The optical sensor  90  has two pairs of LDs  104  and PDs  106 , and is suited for reading two rows of marks  102  printed on the paper conveyer belt  22  as shown in FIG.  18 . Dust-preventing members  108  are arranged on the surfaces of LDs  104  and PDs  106  to permit the transmission of the detection laser beams but to prevent the infiltration of dust and dirt. The dust-preventing members  108  are arranged in an arrangement corresponding to the top portions of “V”. The shut-off cover  92  is arranged inside the pair of dust-preventing members  108  in the case of the optical sensor  90 . The shut-off cover  92  shuts off the optical passage of the optical sensor  90 , and has cleaning members  93  for wiping the surfaces of the dust-preventing members  108 . The dust-preventing members  108  for the LD  104  and PD  106  can be formed of a single dust-preventing member. 
     FIG. 25 is a partly cut-away perspective view illustrating, in detail, the optical sensor  90  including the shut-off cover  92  of FIG.  24 . 
     FIGS. 26 and 27 are views illustrating the operations of the optical sensor  90  and of the shut-off cover  92  of FIG.  25 . FIG. 26 shows the top cover  14  at the closed position, the shut-off cover  92  not covering the dust-preventing member  108  that defines the optical opening of the optical sensor  90 . This state is best shown in FIG.  28 . FIG. 27 shows the top cover  14  at the open position, the shut-off cover  92  being covering the dust-preventing member  108  that defines the optical opening of the optical sensor  90 . This state is best shown in FIG.  29 . 
     FIG. 30 illustrates the state of FIG. 28 in further detail, and FIG. 31 illustrates the state of FIG. 29 in further detail. 
     FIG. 32 is a cross-sectional view illustrating the shut-off cover  92  in detail. The shut-off cover  92  comprises a plate-like base member  92   a  having an end cavity  92   b , a cap  92   c  adhered to the end of the base member  92   a , a cleaning member  93  adhered to a support plate  92   d , and a spring  92   e  disposed in the cavity  92   b . The cleaning member  93  is brought to a protruding position by the spring  92   e  that urges the support plate  92   d . The cleaning member  93  is a soft fibrous member suited for effecting the cleaning, such as a felt or a flannel. 
     According to the present invention, as described above, in the image forming apparatus equipped with a plurality of printing assemblies arranged in tandem it is possible to easily open and close the top cover which is large in size and is heavy in weight, and to reliably ensure precision for mounting the optical heads and the developing units. Since the top cover can be easily opened and closed, expendables can be easily replaced, and the apparatus can be easily checked during maintenance. 
     Since the shut-off cover is provided to shut-off the optical passage of the optical sensor, the laser beam does not leak to the exterior despite part of the apparatus is opened. Therefore, a person who is replacing the expendables or is checking the apparatus during maintenance is not irradiated with the laser beam and is not exposed to danger.