Abstract:
A photoconductive drum assembly and an optical writer used in an electrophotographic printer/copier share the same mechanical fiducials for positioning these subsystems within the printer/copier frame or housing. Preferably, a pair of mounting pins used to position and support the drum assembly in a printer frame is also used to position the optical writer. As a result of this mounting scheme, a tolerance stacking problem is reduced and each subsystem can be removed from the printer housing without disturbing the position of the other. Further, the need for a mechanism to retract the optical writer from the vicinity of the drum surface to facilitate drum removal is eliminated, thereby lowering cost and increasing reliability.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Reference is made to the commonly assigned U.S. patent applications, the disclosures of which are incorporated herein by reference. 
     (1) Ser. No. 09/474,352, filed on Dec. 29, 1999 and entitled “Apparatus for Positioning Work Stations in a Document Printer/Copier.” 
     (2) Ser. No. 09/574,057, filed concurrently herewith and entitled “Skew Adjustment For Optical Writer in a Document printer/Copier.” 
    
    
     FIELD OF THE INVENTION 
     This invention relates to the field of document printing/copying. More particularly, it relates to improvements in apparatus for precisely and repeatedly positioning an electro-optical printhead or “optical writer,” e.g. a linear array of light-emitting diodes, relative to a reusable image-recording drum in a document printer/copier, that is, an electrophotographic printer and/or copier, to enable, for example, removal, servicing and replacement of the optical writer and/or image-recording drum without altering a desired positional relationship between the optical writer and image-recording drum inside the document printer/copier. 
     BACKGROUND OF THE INVENTION 
     The above-referenced U.S. patent application Ser. No. 09/474,352 discloses an electrophotographic document printer/copier in which the photosensitive recording element comprises a photoconductive drum assembly having structure for precisely positioning the various image-processing stations relative to the drum&#39;s photoconductive surface. The drum assembly generally comprises a photoconductive drum having axles extending from opposite ends thereof along an intended axis of drum rotation, and a pair of drum-support members, sometimes referred to as “spiders,” that support the drum for rotation. Each of the drum-support members has a centrally located bearing for rotatably supporting a drum axle, and a plurality of mechanical fiducials (in the form of rounded buttons or “bullets”) extending in an outward direction relative to the drum&#39;s axis of rotation. Each of these fiducials is adapted to engage and mate with a V-notched block or the like associated with one of the image-processing stations (e.g., the primary charger station or toning station) as these stations are moved from a standby position substantially spaced from the drum surface towards an operative position closely spaced from or actually touching the drum surface. When the respective fiducials of the drum assembly have engaged and become seated in the V-blocks of the processing stations, the operative elements of the processing stations (e.g., the corona wire(s) of the primary charging station, or the development brush of the toning station) will have become precisely spaced parallel to, and/or exert substantially uniform pressure on, the drum surface over the entire width of the drum. 
     In use, the above-described drum assembly is mounted between a pair of parallel and vertically extending “mech” plates comprising the front and rear walls of the printer frame. An opening in the front plate (as viewed from the front of the printer) enables the drum assembly, as well as most of the image-processing stations, to enter the region between the plates. A pair of parallel guide channels, extending between the front and rear walls of the printer frame, engage a pair of ears on the drum-support members and serves to guide the drum assembly to its operative position from a position outside the printer frame. Four dowel pins, two on each frame plate, engage holes in the drum-support members and serve to precisely locate the drum assembly in the printer frame. 
     Of the different image-processing stations in an electrophotographic printer or copier, the image exposure station is one that requires exceptionally precise positioning relative to the surface of the recording drum. Unless this station is precisely located relative to the nominal position required to produce optimum focus on the photoconductive surface of the drum, the ultimate image on the image-receiver sheet will be noticeably degraded. Ideally, its position within the printer frame should be precisely set by the manufacturer and left undisturbed during the lifetime of the printer. In one respect, this goal is realizable in that the expected lifetime of conventional solid-state printheads is relatively long, far exceeding that of the photoconductive drum assembly and the other image processing stations. However, in printers of the type described above, the printhead must be movably mounted for movement toward and away from the imaging drum to enable the drum assembly to be serviced or replaced. Each time the printhead is moved away from its optimal focus position, there is a chance that it will not be returned to its optimal position. Further, in many systems for positioning the image-processing stations relative to the photoconductive drum surface in a document printer/copier, the exact position of each station will depend on the results of “stacked” tolerances. For example, the final position of the printhead may depend on the respective positions of a series of mechanical fiducials, each having nominal position, within a certain tolerance range, that depends on the placement of other fiducials. Since these tolerances are additive, i.e., stacked atop each other, it is desirable to minimize the number of related fiducials required to position an image-processing station. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing discussion, an object of this invention is to reduce the afore-noted tolerance stacking problem associated with prior art printers insofar as it relates to the positioning of an optical writer relative to the surface of a photosensitive recording element. 
     Another object of this invention is to obviate any need for retracting the optical writer from its operative position in a printer of the type described in order to remove the drum assembly. 
     According to the present invention, the photosensitive drum assembly and the optical writer used in an electrophotographic printer/copier of the type described share the same mechanical fiducials for positioning these elements within the printer/copier frame or housing. More specifically, two of the mounting pins used to position and support the drum-support members (spiders) of the drum assembly in the printer frame are also used to position the optical writer. Preferably, the point at which each drum-support member contacts the mounting pins is closely spaced, axially speaking, from the point at which the optical writer contacts the mounting pins. 
     As a result of this mounting scheme, the afore-mentioned tolerance stacking is reduced since the position of the optical writer is not determined by a fiducial carried by the drum-support members, but rather by the very same fiducial that serves to locate the drum-support member in the printer frame. Also, since the optical writer does not contact the drum-support members, there is no need to retract the writer from the drum-support members in order to free-up the drum assembly for removal from the printer. This results in a saving of parts, since here is no need to provide a retractable mount for the writer, and minimizes the need to flex electronic cables and cooling is lines connected to the optical writer, thereby improving reliability Another advantage of this mounting scheme is that, since the optical writer is mounted within the printer frame in a manner that is totally independent of the drum assembly, the optical writer can be removed from the printer without any mechanism for initially retracting the writer from contact with any portion of the drum assembly. 
     Thus, according to the present invention, an electrophotographic printer/copier comprises: (a) a frame; (b) a drum assembly comprising (i) a drum having a photoconductive surface upon which a plurality of work stations operate to produce an image; and (ii) a pair of drum-support members for rotatably supporting said drum for rotation about a drum axis; (c) an optical writer for projecting image information onto said photosensitive surface; and (d) a mechanical fiducial mounted on said frame, said drum assembly and said optical writer sharing said mechanical fiducial for positioning said drum assembly and said optical writer in said frame to achieve a desired positional relationship between the optical writer and the photoconductive surface of said drum. Preferably, the frame comprises a pair of spaced frame plates, and the mechanical fiducial comprises a pair of dowel pins, one of such pins extending outwardly from each of the frame plates at locations to be engaged by opposite ends of the drum assembly and optical writer. 
     The invention and its technical advantageous effects will be better appreciated from the ensuing detailed description of a preferred embodiment, reference being made to the accompanying drawings in which like reference characters denote like or functionally similar parts. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is end elevation of an electrophotographic printer embodying the present invention; 
     FIG. 2 is a perspective view of a portion of the FIG. 1 apparatus; 
     FIGS. 3A and 3B are enlarged perspective views of different portions of the electrophotographic printer shown in FIG. 1; 
     FIGS. 4 and 5 are perspective views of drum-support members; 
     FIG. 6 is an enlarged front perspective view of a frame piece supporting a dowel pin; 
     FIG. 7 is a front perspective view of a portion of an optical writer supported by the dowel pin shown in FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to the drawings, FIG. 1 shows an electrophotographic printer  10  embodying the present invention. As shown, printer  10  comprises a drum assembly DA 1  that includes an image-recording drum  12  having a photoconductive outer surface  14  on which toner images are formed in a conventional manner. The printer also comprises a second drum assembly DA 2  having an intermediate image-transfer drum  22  with an adhesive (non-stick) surface  25  to which toner images formed on the photoconductive outer surface of image-recording drum  12  are transferred prior to being re-transferred to a receiver sheet (not shown). Briefly, toner images are formed on the photoconductive surface of drum  12  by rotating the drum in a counter-clockwise direction (as viewed in FIG. 1) past a series of image processing stations that sequentially operate on a desired portion of the drum&#39;s photoconductive outer surface to produce a visible image. These image processing stations include a corona charging station  16  for uniformly charging the photoconductive surface  14  with electrostatic charges, a solid-state print head or optical writer  18  for imagewise exposing the charged photoconductive surface, line-by-line, to actinic radiation, thereby selectively dissipating the uniform charge and leaving behind a latent electrostatic charge image, and a toning station  20  for developing the charge image with pigmented electroscopic toner particles. The toner image thus formed is then transferred to the outer surface  25  of the image-transfer drum  22 , and residual toner on drum  12  is removed by a cleaning station  24 . Upon re-transferring the toner image on the intermediate transfer drum  22  to an image-receiver sheet, the surface of drum  22  is cleaned by a second cleaning station  26 . Preferably, each processing station, with the exception of the optical writer, as explained below, is mounted for slight movement (e.g. about 5 to 7 mm.) towards and away from its respective operative position adjacent the drum surface (shown in FIG. 1) to provide minimal clearance for installation and replacement of the drum assemblies and/or the processing stations. During such installation, the drum assemblies/processing stations are moved substantially parallel to drum&#39;s respective axis of rotation, through openings in the front mechanism plate FP of the machine frame F (shown best in FIG.  2 ). 
     Referring additionally to FIGS. 3A,  3 B,  4  and  5 , the photoconductive drum assembly DA- 1  comprises, in addition to photoconductive drum  12 , a pair of a pair of drum-support members  32 , 32 ′. These drum-support members are better described in the aforementioned U.S. patent application Ser. No. 09/474,352, the disclosure of which is hereby incorporated herein by reference. Briefly, members  32 , 32 ′ serve to precisely and repeatedly positioning the various image processing and other work stations of the printer relative to the outer surfaces of the image-recording drum  12  to enable, for example, removal, servicing and replacement of the individual work stations and/or drum without altering a desired positional relationship between the work stations and the drum. Each drum-support member  32 , 32 ′ has a centrally located bearing B or the like for rotatably supporting a drum axle (e.g., drum axle  12 A shown in FIG. 1 extending outwardly from an end of drum  12 ). The drum-support members, in turn, are supported in a predetermined position within the printer frame F, on a plurality of dowel pins P 1 -P 4  mounted on the front and rear mechanism plates FP,RP, respectively, of the printer frame F. Drum-support members  32 , 32 ′ are provided with a plurality of reference surface features  40 ,  41 ,  43 ,  44 ,  48 ; and  40 ′,  41 ′,  43 ′,  44 ′  48 ′, respectively, which cooperate with complimentary reference surface features (e.g. v-notched blocks) carried by the respective processing stations to precisely position the work stations relative to the outer surface of the drum. Thus, as the work stations are moved from their respective stand-by or loading positions toward their operative positions, shown in FIG. 1, the reference surface features of the work stations move into contact with and engage the reference surface features on the drum-support members to locate each work station in a desired position relative to the drum surface 
     FIGS. 4 and 5 are enlarged front views of the front and rear drum-support members  32  and  32 ′, respectively. Front member  32  supports a shaft encoder SE through which the rotational speed of drum  12  is monitored for the purpose of controlling the operation of the optical writer. A plurality of clamps D 1 -D 3  on each drum-support member serves to retain the bearing B used to rotatably support one of the drum axles. Ears  60 ,  62  carried by the front member  32 , and ears  60 ′,  62 ′ carried by rear member  32 ′ cooperate with a pair of guide channels (not shown) positioned between the frame plates for the purpose of slidably guiding the drum assembly to its operating position where it is supported by a plurality of dowel pins P 1 -P 4  on the front and rear walls of the printer frame. Pins P 1 -P 4  are adapted to engage holes H 1 -H 4 , respectively, formed in the front and rear drum-support members to precisely position the drum assembly within the printer frame. 
     In accordance with the present invention, dowel pins P 1  and P 3 , in addition to functioning to position the drum assembly within the printer frame, also function to position the optical writer relative to the photoconductive surface of drum  12 . Referring to FIG. 6, dowel pin P 1  is shown to be supported by a frame piece  66  that is adapted to be bolted to the rear surface of the front frame plate FP so as to become part of the front frame plate. Preferable, pin P 1  is made of hardened steel and has a diameter of about 20 mm. Frame piece  66  also made of hardened steel and comprises a flat bar portion  67  extending between a pair of tabs  68  and  69 . The indented space between tabs  68  and  69  is shaped to receive that portion P of drum-support member  32  in which hole H 1  is formed. See FIG.  4 . Thus, as the drum assembly slides inwardly into the printer frame and hole H 1  eventually engages and slides upon pin P 1 , its inward movement will be arrested as the rear surface of portion P engages the front surface  67 A of bar portion  67  of the frame piece  66 . Bar portion  67  has a hole  70  formed therein, and dowel pin P 1  is press fit therein. When fully seated on pin P 1 , that portion of pin P 1  in the vicinity of the surface  67 A acts as a fiducial or reference point for locating the drum assembly  12 . Preferably, hole  70  does not pass entirely through the bar portion  67 , and the top portion of the bar portion is milled away so that an outer surface area PA of dowel pin PI received in hole  70  will be exposed. As shown in FIG. 7, it is this portion PA of pin P 1  that is used to support and act as a fiducial for the optical writer. Preferably, a V-notch block  80  associated with the optical writer frame  82  is urged into contact with surface area PA at a location very closely spaced from the plane in which the pin supports the drum-support member  32  of drum assembly DA- 1 , i.e., in the vicinity of surface  67 A. Ideally, the planes in which pin P 1  supports both the optical writer and the drum assembly should coincide. In such case, the same reference point on the fiducial is used to locate the optical writer and drum assembly, and any tipping of the pin in bar  67  will have no effect on the relative positions of these subsystems. However, if the two reference points are within, say, 5 mm. of each other, the effect of any anticipated tipping of pin P 1  will be negligible. 
     Referring to FIGS. 2 and 3B, the rear drum-support member  32 ′ is supported on dowel pins P 3  and P 4  carried by the rear mech plate RP of the printer frame. Pins P 3  and P 4  engage holes H 3  and H 4 , respectively, formed in member  32 ′. When moved to it rearmost position on pin P 3 , the rear surface S of drum-support member  32 ′ will be slightly spaced (by about 6 mm) from the front surface of the rear mech plate RP by a plurality of legs L. As a result, a space PA′ is provided on the upper surface of pin P 3  for receiving and supporting a V-block  80 ′ mounted on the opposite end of the optical writer frame that supports V-block  80 . Again, owing to the close spacing between the points at which pin P 3  supports both the drum-support member  32 ′ and V-block  80 ′, virtually the same portion of the pin operates as the fiducial for both the drum assembly and the optical writer, thereby eliminating the tolerance stacking problem discussed above. 
     From the foregoing, it will be appreciated that a technically advantageous method and apparatus have been provided for mounting a solid-state optical writer in a document printer/copier. By using the same mechanical fiducial to locate both the photoconductive drum assembly and the optical writer, a stacking of mechanical tolerances is avoided. Further, by using a different portion of the fiducial to register each subsystem, either subsystem can be removed from the printer without disturbing the other. For example, the optical writer can be removed through the back of the printer through opening  90  in the rear mech plate, and the drum assembly can be removed through an opening in the front mech plate. 
     The invention has been described with reference to an electrophotographic printer/copier apparatus. An example of an electrophotographic printer/copier apparatus is described in U.S. patent application Ser. No. 08/900,696, filed in the name Tombs et al, the contents of which are incorporated herein by reference. The apparatus and method of the invention may be used to locate the writer or other process or work station, such as a charging station, development station, cleaning station which typically operates upon a photoconductive surface during an electrophotographic process. The invention may also be used for positioning a process or work station about other toner image bearing or recording surfaces such as an intermediate transfer member. The invention has been described with reference to positioning a process or work station about a drum. However, the invention also contemplates that the apparatus and method thereof may also be used to accurately position a process station in any electrostatographic apparatus about a toner image bearing or recording surface including surfaces formed as a web wherein the web is supported by a roller. The roller would be journaled for rotation and accurately located relative to the frame by having a first surface connected thereto engaging one portion of the pin and a process station operating on a toner image bearing or recording surface and having a second surface connected to the process station engaging a second portion of the pin to accurately locate the process station relative to the toner image bearing or recording surface. The apparatus and method of the invention may also be used in electrographic recording apparatus wherein stylus writers or other types of writers or other process stations are used to record or transfer electrostatic images on a surface and are required to be accurately positioned relative to the toner image bearing or recording surface. 
     While the invention has been described with reference to a particularly preferred embodiment, it will be appreciated that variations can be made without departing from the spirit of the invention, and such variations are intended to fall within the scope of the appended claims. 
     PARTS LIST 
       10 —electrophotographic printer 
       12 —image-recording drum 
       14 —photoconductive surface 
       16 —corona charging station 
       18 —print head 
       20 —development station 
       22 —image-transfer drum 
       24 —cleaning station 
       26 —cleaning station 
       32 , 32 ′—drum support members for drum  12   
       34 —drum support members for drum  22   
       36 —frame piece 
       37 —bar portion of frame piece  40 ,  40 ′,  41 ,  41 ′,  43 ,  43 ′,  44 ,  44 ′,  48 ,  48 ′—fiducials on drum support members for positioning printer work stations 
       50 —V-grooved block 
       60 , 62 —ears for guiding drum assembly 
       66 —frame piece 
       67 ,  68 ,  69 —portions of frame piece 
       67 A—front surface of member  67   
       80 , 80 ′—V-block fiducials for positioning optical writer 
       82 —optical writer frame 
       90 —opening in rear mech plate 
     DA 1 —drum assembly  1   
     DA 2 —drum assembly  2   
     A—drum axles 
     B—bearings 
     F—printer frame 
     L—legs on rear drum-support member  32 ′ 
     P—portion of front drum-support member  32   
     FP—front plate of printer frame 
     RP—rear plate of printer frame 
     SE—shaft encoder 
     D 1 -D 3 —bearing-retainers 
     P 1 -P 4 —mounting pins 
     H 1 -H 4 —mounting holes 
     PA,PA′—fiducials for positioning optical writer