Abstract:
A mechanism and process for detecting mottle or banding in a developed electrophotographic image. Within an electrophotographic reproduction apparatus  10 , a photoconductor is used for receiving and developing a latent image. The photoconductor traverses a path that passes a charging station  28 , an exposure station  34 , a toning station  38 , and a transfer station  46 . Either a densitometer  76  for measuring the density of the developed image on the photoconductor, or an electrometer  50   a  or  50   b  for detecting the voltage of the image on the photoconductor, detects mottle or banding on the developed image. The densitometer  76  or electrometer  50   a  or  50   b  has an aperture small enough to detect mottle or banding with wavelengths perceptible by human eyes. A logic and control unit  24  averages the image density or voltage measurements, calculates the variations of the measurements about the average and the periodicities of the measurements, and if the variations or periodicities indicate mottle or banding is present, changes the operation of one or more stations to reduce mottle or banding.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims the benefit of the priority date of Provisional patent application Ser. No. 60/302,457 filed Jul. 2, 2001. 
    
    
     FIELD OF INVENTION 
     This invention relates to electrophotographic recording apparatus such as that used in document copiers and printers, and more specifically to output quality control in an electrophotographic recording apparatus. 
     Definitions 
     The following terms well-known in the art are defined here: 
     I exp —Writer current used during exposure. 
     V exp —Writer voltage used during exposure. 
     E 0 —Light produced by the print head. 
     E—Actual exposure of photoconductor. 
     V 0 —Primary voltage (relative to ground) on the photoconductor just after the charger. This is sometimes referred to as the “initial” voltage. 
     V B —Development station electrode bias. 
     V grid —A grid control signal that controls the transfer of initial charge to the photoconductor. 
     Discussion of Prior Art 
     Process control for electrophotographic systems is based on measurement and control of image density. However, images that have acceptable density on average can have undesirable levels of banding or mottle. “Banding” refers to the appearance on an output image of darker or lighter bands, running in a direction perpendicular to the direction of motion of the image through the development process, in areas where no change in input image brightness exists. Banding is generally due to speed variations in image movement, often caused by gear noise, stepper motor frequencies, or scanner frequency variations. “Mottle” refers to the appearance on an output image of darker or lighter patches in areas where no change in input image brightness exists. In general mottle does not exhibit a regular pattern. 
     U.S. Pat. No. 6,121,986 (Regelsberger et al.), incorporated herein by reference, teaches the use of the densitometer to monitor development of test patches to provide real-time control of the electrophotographic process and to provide “constant” image quality output, and the use of the electrometer to measure a calibration patche in an interframe area on the photoconductor. U.S. Pat. No. 5,937,229 (Walgrove et al.), also incorporated herein by reference, reveals use of the densitometer and the electrometer in the same way. Both patents spell out the mechanism and process in detail. Neither of the above patents addresses the problem of banding. Both patents address mottle by increasing toner density based on overall test patch density measurement. 
     The parameters defined above are important for understanding the operation and control of typical electrophotographic systems. Light intensity E 0  produced by the print head illuminates the photoconductor and causes a particular level of exposure E of the photoconductor. In general contrast and toner density control are achieved by varying levels of V 0 , E 0 , and V B  as is well-known and described in the published literature. 
     For the structure and operation of a typical toning station core, see U.S. Pat. No. 4,602,863 (Fritz, et al.), incorporated herein by reference. 
     SUMMARY 
     The invention detects mottle or banding in a developed electrophotographic image. It operates within an electrophotographic reproduction apparatus with a photoconductor used for receiving and developing a latent image. The photoconductor traverses a path that passes a charging station, an exposure station, a toning station, and a transfer station. The charging station charges the photoconductor to a desired level of electric charge. The exposure station exposes the photoconductor to an input document or document image to selectively discharge the photoconductor and form a latent image of the input document or document image. The toning station applies toner to the photoconductor to develop the latent image. The transfer station transfers the developed latent image to a receiver sheet. The invention detects mottle or banding using either a densitometer for measuring the density of the developed image on the photoconductor, or an electrometer for detecting the voltage of the image on the photoconductor. The densitometer or electrometer has an aperture small enough to detect mottle or banding with wavelengths perceptible by human eyes. The invention&#39;s processor averages the image density or voltage measurements, calculates the variations of the measurements about the average and the periodicities of the measurements, and if the variations or periodicities indicate mottle or banding is present, changes the operation of one or more stations to reduce mottle or banding. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  shows the invention as installed in a typical electrophotographic printing system. 
         FIG. 2   a  shows the detection of mottle on a test patch of toner, using a single detector photodiode. 
         FIG. 2   b  shows the detection of banding on a test patch of toner, using a single detector photodiode. 
         FIG. 3   a  shows the detection of mottle on a test patch of toner, using multiple detector photodiodes. 
         FIG. 3   b  shows the detection of banding on a test patch of toner, using multiple detector photodiodes. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     The machine  10  diagrammed in  FIG. 1 , an electrophotographic printer, is typical of devices containing the invention. In machine  10 , a moving recording member such as photoconductive belt  18  is driven by a motor  20  past a series of work stations of the printer. A logic and control unit (LCU)  24  has a digital computer that operates a stored program for sequentially actuating the electrophotographic stations. The invention&#39;s mottle and banding detection unit  62  provides signal inputs to LCU  24  to direct changes to operating parameters for machine  10 . Detection unit  62  is shown here as a separate component, to highlight the invention&#39;s structure and operation. Detection unit  62  may exist as a separate component or as an integrated subsystem of LCU  24 . 
     In typical devices such as machine  10 , charging station  28  sensitizes belt  18  by applying a uniform electrostatic charge of predetermined primary voltage V 0  to the surface of the belt  18 . The output of the charger  28  is regulated by a programmable controller  30 , which is in turn controlled by LCU  24  to adjust primary voltage V 0  in accordance with a grid control signal, V grid  that controls movement of charges from charging wires to the surface of the recording member, as is well-known. 
     Exposure station  34 , projects light from a write head to dissipate the electrostatic charge on the photoconductive belt  18  to form a latent image of the document being copied or printed. The write head preferably has an array of light-emitting diodes (LEDs) or some other light source such as lasers for exposing the photoconductive belt picture element (pixel) by picture element. LCU  24  determines the exposure intensity E 0  and directs its regulation using a data source programmable controller  36 . Alternatively, the exposure may be by optical projection of an image of a document onto the photoconductor. Another alternative is creating electrostatic latent images using needle-like electrodes or other known means for forming such latent images. 
     Where an LED or other electro-optical exposure source is used, a data source  36  such as a computer, a document scanner, a memory, or a data network provides image data for recording. Signals from data source  36  and/or LCU  24  may also provide control signals to a writer network and other components. Signals from the data source  36  and/or LCU  24  may also provide control signals to a writer interface  32  for identifying and selecting exposure correction parameters for use in controlling image density. In order to form test patches of specific densities, the LCU  24  may be provided with ROM memory to store patch creation data for each desired level of toner density. LCU  24  transfers the patch creation data as needed into data source  36 . Travel of belt  18  brings the areas bearing the latent charge images, including patches, into a development station  38 . Development station  38  has magnetic brushes in juxtaposition to the travel path of belt  18 . Magnetic brush development stations are well-known. See U.S. Pat. No. 4,602,863 (Fritz, et al.), already incorporated herein by reference. 
     In relation to the passage of the image areas, LCU  24  selectively activates the development station  38  containing latent images. This activation selectively brings the magnetic brush of development station  38  into engagement with, or a small spacing from, belt  18 . The electric charge of the latent image pattern attracts the charged toner particles of the engaged magnetic brush imagewise to develop the pattern on belt  18 . 
     As is well understood in the art, conductive portions of the development station  38 , such as conductive applicator cylinders, act as electrodes. The electrodes are connected to a variable supply of D.C. or A.C.+D.C. potential V B . V B  is supplied by programmable controller  40  that is regulated by LCU  24 . Details regarding the development station  38  are not essential to the invention. 
     As is also well-known, a transfer station  46  moves a receiver sheet S into engagement with the photoconductor on belt  18 , in register with the image, for transferring the image from belt  18  to receiver S. Alternatively, the image may be transferred to an intermediate member, and then from the intermediate member to receiver S. A cleaning station  48  downstream from transfer station  46  removes residual toner from belt  18  to allow reuse of the surface for forming additional images. A belt  18 , a drum photoconductor, or other structure for maintaining a charged image in toner may be used for supporting an image for toner transfer. After transfer of the unfixed toner images to receiver sheet S, sheet S is transported to a fuser station  49  where the image is fixed. 
     LCU  24  provides overall control of the apparatus and its various subsystems as is well-known. Programming commercially available microprocessors is a conventional skill well understood in the art. LCU  24  maintains and stores parametric values necessary for the operation of both the invention and the overall electrophotograhic apparatus  10 . 
     In a first embodiment, the invention measures the density of a process control patch with a small aperture densitometer  76  to determine both the average density and fluctuations in density that indicate mottle or banding. A densitometer  76  with an aperture of approximately 1 mm 2  is preferred, since the peak sensitivity of the human eye to noise is at spatial wavelengths of approximately ⅛ inch. In an alternate embodiment, an electrometer  50   a  or  50   b  with a small aperture and rapid response time is used to measure nonuniformities in the image voltage. The densitometer  76  or electrometer  50   a  or  50   b  is situated as shown between development station  38  and transfer station  48  along the path of movement of the developed latent image on photoconductive belt  18 . The two electrometer locations showing at  50   a  and  50   b  are presented to show the range of acceptable locations along the image path intermediate between the toning station  38  and transfer station  46 . The electrometer spacing from the photoconductor is typically 0.100″+/−0.035″. 
     Photodiodes typically used in densitometer  76  for this application include PIN silicon photodiodes types OP913SL and OP913WSL having acceptance angles of 10 degrees and 30 degrees respectively from the optical axis. These units can detect very low light levels, a characteristic making them qualified for use in the invention. The use of a pinhole opening to mask the photodiode reduces the photodiode&#39;s working acceptance angle, thereby allowing the detection of smaller nonuniformities in toner density as required. 
     In electrometer  50   a  or  50   b  for this application, electrostatic non-contact voltmeters used include the Trek Model 370 or equivalent, which has a response speed of approximately 50 microseconds and an aperture approximately 2 mm in diameter. Alternately, a CCD array with linearity of frequency response comparable to that of acceptable photodiode detectors is usable for measurement of optical density fluctuations. Density determination using a CCD array is done with image analysis software for spot and band detection and measurement, as is well-known. 
     The aperture and response time of the photodiode, the electrometer, and the CCD array are appropriate for detecting nonuniformities with spatial wavelengths on the order of ⅛ inch or less. 
     Using the detection inputs, LCU  24  calculates average density, variation about the average, and periodic variation. Process control adjusts density so that the average density is in an acceptable range. If either mottle or banding or both are present, LCU  24  directs the increase of toner density by making appropriate increases in E 0 , V B , and V 0 . If toner density level is acceptable but banding is present, LCU  24  increases the magnetic core speed of development station  38 . 
     A detection unit  62  detects mottle and banding, and distinguishes between them. In a basic embodiment, the invention uses a single densitometer as detector  76 , and takes multiple density readings from each test patch as required. The invention operates in this embodiment as follows. 
     See  FIGS. 2   a  and  2   b , showing a test patch  96 , with mottle  98  and banding  99  respectively. A detection unit  62  has a single detector photodiode  76 . For convenience of illustration, detector  76  is shown as moving in direction  120  with respect to test patch  96 . The detector is actually in a fixed position in machine  10 , while belt  18  and the test patch on it are moving in the direction opposite to that shown. Mottle conditions will cause the detector  76  to change readings at irregular intervals, as shown by the density level trace  108  and its first derivative trace  108   d . Banding conditions will cause the detector  76  to change readings on a regularly periodic basis, as shown by the density level trace  109  and its first derivative trace  109   d . A test patch  96  on photoconductive belt  18  moves past detector  76  at a distance enabling detector  76  to detect toner nonuniformities of approximately 2 mm in size in test patch  96 . Detector  76  detects changes in density of test patch  96  along the direction of travel of the test patch. Detection unit  62  takes multiple densitometer readings for each test patch  96 . Detection unit  62  counts each significant change in density on a test patch  96 , producing a positive count pulse for each increase and a negative count pulse for each decrease. Detection unit  62  records the time intervals between successive pairs of positive count pulses. Detection unit  62  sums the positive count pulses in a first sum, and the negative count pulses in a second sum, from all detectors. If detection unit  62  detects counts above a specific threshold for both the first sum and the second sum, it signals a mottle or banding condition. Detection unit  62  compares the time intervals between successive pairs of positive count pulses. If time intervals between successive pairs of positive count pulses are approximately equal, detection unit  62  signals a banding condition. If detection unit  62  detects a mottle condition or a banding condition, it directs an increase in toner density via LCU  24 . If detection unit  62  detects a banding condition, it directs an increase in magnetic core speed via LCU  24 . 
     In summary, detection unit  62  compares the intervals between succeeding count pulses from a test patch. If the time interval between pulses A and B matches that between B and C, and that between C and D, the regularity of appearance of the pulses implies a banding condition. Pulses appearing irregularly imply a mottle condition. The condition detected drives adjustment of toner density and/or development station core speed as required. 
     Alternate Embodiments of the Invention 
     In another embodiment, the invention replaces the single detector by multiple detectors disposed across the test patch in a row perpendicular to the direction of travel. See  FIGS. 3   a  and  3   b , showing a test patch  96 , with mottle  98  and banding  99  in the respective figures. A detection unit  62  has two detectors  76   a  and  76   b . Additional detectors may be disposed along the same line as detectors  76   a  and  76   b , as desired. Again, for convenience of illustration, detector  76  is shown as moving in direction  120  with respect to test patch  96 . The detector is actually in a fixed position, while belt  18  and the test patch on it are moving in the direction opposite to that shown. Mottle conditions will cause detectors  76   a  and  76   b  to change readings at irregular intervals, as shown by the density level traces  108   a  and  108   b . Banding conditions  99  as shown in  FIG. 3   b  will cause most or all detectors to change readings synchronously, as shown by the density level traces  109   a  and  109   b . The small-aperture detectors  76   a  and  76   b  detect changes in density of the test patch along the direction of travel of the test patch. Using the detector inputs, detection unit  62  counts each significant change in density on the test patch, emitting a positive count pulse for each increase and a negative count pulse for each decrease. Detection unit  62  sums the count pulses from all detectors. If detection unit  62  detects counts above a specific threshold, it signals a mottle or banding condition. If pulses from most or all detectors arrive synchronously, detection unit  62  signals a banding condition. If detection unit  62  detects a mottle condition or a banding condition, it directs an increase in toner density. If detection unit  62  detects a banding condition, it directs an increase in magnetic core speed. 
     In still another embodiment, the invention replaces the densitometer or electrometer with a CCD array for detecting and reporting test patch density fluctuations. The CCD detects the amount of light transmitted through the film and density patch. 
     In still further embodiments, detector photodiodes may be replaced by photocells or other photodetectors with substantially the same detection performance characteristics. 
     Conclusion, Ramifications, and Scope of Invention 
     This invention allows production of images that have acceptable, low toner stack heights, minimal mottle, and minimal banding. The invention adjusts toner density to address mottle and banding conditions accurately. This accuracy reduces toner consumption by obviating the manual setting of toner density at a too-high level to avoid mottle or banding. From the above descriptions, figures and narratives, the invention&#39;s advantages in these respects should be clear. 
     Although the description, operation and illustrative material above contain many specificities, these specificities should not be construed as limiting the scope of the invention but as merely providing illustrations and examples of some of the preferred embodiments of this invention. 
     Thus the scope of the invention should be determined by the appended claims and their legal equivalents, rather than by the examples given above.