Abstract:
An electronic development compensation method which is broadly applicable to SCMB development includes controlling image banding by actively correcting for mechanical development errors by modulating DC bias to a magnetic brush.

Description:
BACKGROUND 
       [0001]    1. Field of the Disclosure 
         [0002]    This application generally relates to printing, and in particular, eliminating banding in semi-conductive magnetic brush developed images. 
         [0003]    2. Description of Related Art 
         [0004]    Banding in printing systems has been and will continue to be an engineering challenge in xerographic marking engines based on semi-conductive magnetic brush (SCMB) development as shown, for example, in U.S. Pat. Nos. 5,539,505 and 6,285,837 B1. Image banding is an image quality defect that consists of halftone density variation in the process direction and manifests itself as light and dark bands in the cross-process direction. Banding is largely due to fluctuations in the photoreceptor (PR) drum to magnetic roll spacing resulting from photoreceptor and magnetic roll run-out. Mechanical variations in the development nip from photoreceptor and/or magnetic roll run-out can modulate the developer nip density (mass on roll) and hence developability resulting in banding. Banding is not always apparent at time-zero, but may manifest itself as the developer ages. Hence, other material state factors, such as: toner concentration/triboelectricity; toner age; and possibly material processing and flow properties. Material state factors may magnify the effect of even small initially acceptable variations in photoreceptor drum to magnetic roll spacing although they are not well understood. 
         [0005]    Consequently, banding has been a very difficult problem to overcome and a method is needed to compensate for this effect other than costly mechanical countermeasures involving tightening of parts tolerances. 
       BRIEF SUMMARY 
       [0006]    Accordingly, disclosed is an electronic development compensation method which is broadly applicable to SCMB development and comprises actively correcting for mechanical development errors by modulating the magnetic roll DC bias. Initially, the magnetic roll AC current is measured and filtered. Then, the low pass filtered current signal is amplified and AC coupled into a magnetic DC power supply error amplifier. A feedback circuit generates a time varying correction voltage that is applied to the DC bias on the developer power supply in phase with the AC current variation. All of these steps are accomplished in real-time with simple analog electronics. 
         [0007]    The disclosed system may be operated by and controlled by appropriate operation of conventional control systems. It is well known and preferable to program and execute imaging, printing, paper handling, and other control functions and logic with software instructions for conventional or general purpose microprocessors, as taught by numerous prior patents and commercial products. Such programming or software may, of course, vary depending on the particular functions, software type, and microprocessor or other computer system utilized, but will be available to, or readily programmable without undue experimentation from, functional descriptions, such as, those provided herein, and/or prior knowledge of functions which are conventional, together with general knowledge in the software of computer arts. Alternatively, any disclosed control system or method may be implemented partially or fully in hardware, using standard logic circuits or single chip VLSI designs. 
         [0008]    The term ‘printer’ or ‘reproduction apparatus’ as used herein broadly encompasses various printers, copiers or multifunction machines or systems, xerographic or otherwise, unless otherwise defined in a claim. The term ‘sheet’ herein refers to any flimsy physical sheet or paper, plastic, media, or other useable physical substrate for printing images thereon, whether precut or initially web fed. 
         [0009]    As to specific components of the subject apparatus or methods, it will be appreciated that, as normally the case, some such components are known per se&#39; in other apparatus or applications, which may be additionally or alternatively used herein, including those from art cited herein. For example, it will be appreciated by respective engineers and others that many of the particular components mountings, component actuations, or component drive systems illustrated herein are merely exemplary, and that the same novel motions and functions can be provided by many other known or readily available alternatives. All cited references, and their references, are incorporated by reference herein where appropriate for teachings of additional or alternative details, features, and/or technical background. What is well known to those skilled in the art need not be described herein. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Various of the above-mentioned and further features and advantages will be apparent to those skilled in the art from the specific apparatus and its operation or methods described in the example(s) below, and the claims. Thus, they will be better understood from this description of these specific embodiment(s), including the drawing figures (which are approximately to scale) wherein: 
           [0011]      FIG. 1  shows a printer in accordance with an embodiment; 
           [0012]      FIG. 2  is a chart showing magnetic roll AC current after full wave rectification and low pass filtering at 500 Hz; 
           [0013]      FIG. 3  is a chart showing the FFT of the AC current in  FIG. 2 ; 
           [0014]      FIG. 4  shows scanned images of black halftones before and after electronic correction applied to DC developer voltage; 
           [0015]      FIG. 5  shows banding FFT print scans; and 
           [0016]      FIG. 6  shows an exemplary electronic development compensation method in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0017]    While the disclosure will be described hereinafter in connection with a preferred embodiment thereof, it will be understood that limiting the disclosure to that embodiment is not intended. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the disclosure as defined by the appended claims. 
         [0018]    For a general understanding of the features of the disclosure, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to identify identical elements. 
         [0019]      FIG. 1  shows a schematic illustration of a printer  100 , in accordance with an embodiment. The printer  100  generally includes one or more sources of printable substrate media that are operatively connected to a printing engine  104 , and output path  106  and finisher  108 . As illustrated, the print engine  104  may be a multi-color engine having a plurality of imaging/development (SCMB) systems  110  that are suitable for producing individual color images. A stacker device  112  may also be provided as known in the art. 
         [0020]    The print engine  104  may mark xerographically; however, it will be appreciated that other marking technologies may be used, for example by ink-jet marking, ionographically marking or the like. In one implementation, the printer  100  may be a Xerox Corporation DC8000™ Digital Press. For example, the print engine  104  may render toner images of input image data on a photoreceptor  114 , where the photoreceptor  114  then transfers the images to a substrate. 
         [0021]    A display device  120  may be provided to enable the user to control various aspects of the printing system  100 , in accordance with the embodiments disclosed therein. The display device  120  may include a cathode ray tube, liquid crustal display, plasma, or other display device. 
         [0022]    AC biases are employed in the SCMB development systems  110  in order to control developer conductivity and improve image quality (i.e., background). In accordance with the present disclosure, each of the developer systems include a developer nip positioned between a charge retentive substrate or photoreceptor  114  and a magnetic roll (not shown) and a real-time measurement of the AC current flowing through the development nip during a print cycle at the AC bias set-points (Vpp, frequency, duty cycle). In an ideal development nip, the AC current would be constant because the photoreceptor/magnetic roll spacing is constant. In real systems, the photoreceptor/magnetic roll spacing varies periodically because of photoreceptor and magnetic roll run-out and imperfect centering of the drives with respect to the center of the photoreceptor and magnetic roll. Envisioning the development nip, the AC (capacitive) current peaks when the photoreceptor/magnetic roll spacing is at a minimum and vice versa. Hence, the AC current follows the periodic variations in photoreceptor/magnetic roll spacing. Similarly, developability follows the variation in photoreceptor/magnetic roll spacing. Whether or not the AC current and developability are perfectly correlated is not known, however, experience has taught that the correlation is good enough that the AC current variations are useful for applying a correction to the DC magnetic bias to substantially mitigate banding. A magnetic bias applied to the developer stations at  110  can be used as a real-time “probe” of development nip density and/or mechanical errors. This mechanical error is actively corrected by modulating the magnetic roll DC bias. 
         [0023]    In practice, the magnetic roll AC current on the developer bias line was measured in real-time during a print cycle as follows. The magnetic roll AC current was rectified through a full wave bridge and passed though an analog opto-coupler in order to measure the magnitude of the magnetic roll AC current. The latter signal was then low pass filtered to 100 Hz. An example of the latter signal is shown in  FIG. 2 . The lower curve represents the AC current taken at 15k developer print life during a test of Fuji Xerox FC2 toner in a Xerox DC8000™ printer, while the upper curve shows the results taken at 40K into the test. Banding was not observed at 15K, but was observed at 40K. Thus, the current measurement is capable of discriminating the banding performance of the machine. 
         [0024]    The low pass filtered current signal exemplified in  FIG. 2  was then amplified and AC coupled into the magnetic DC power supply error amplifier. The AC couple was in the DC correction, so as to not add a DC offset to the DC bias. A feedback circuit generates a time varying correction voltage that is applied to the DC bias on the developer power supply in phase with the AC current variation. In one test, where the nominal DC development voltage was 544V the correction voltages needed to cancel the banding was about 5Vp-p. The magnetic DC supply was measured to have a frequency response up to 50 Hz which is more than adequate for this and most applications since most corrections occur at less than 10 Hz. 
         [0025]    The frequency components of the AC current waveforms shown in  FIG. 2  are presented in  FIG. 3 . The fundamental and double of both the photoreceptor and magnetic roll rotational frequencies are seen to be the main components of the AC current variation and no components above 13 Hz were found in the test. 
         [0026]    The method detailed hereinbefore was used to actively correct or null out the banding frequency components below 50 Hz.  FIG. 4  shows a digital scan of the corrected and uncorrected prints side by side indicating visually the magnitude of the correction achieved.  FIG. 5  shows the banding FFT of the prints of  FIG. 3 . The FFT shows that the photoreceptor double and magnetic roll banding frequencies are eliminated from the halftones. 
         [0027]    In recapitulation, an exemplary electronic development compensation method to actively correct or null out the banding frequency components in real-time below 50 Hz in xerographic marking engines based on SCMB development is shown in  FIG. 6  as  200  and includes measuring the magnitude of the magnetic roll AC current in step  210 . Next, in step  220 , the signal is low pass filtered. Continuing to step  230 , appropriate correction amplification is applied to the signal. In step  240 , the signal is used to modulate magnetic roll DC power supply in phase with the AC current variation in step  210 . These steps are performed in real-time during a print cycle. 
         [0028]    The claims, as originally presented and as they may be amended, encompass variations, alternatives, modifications, improvements, equivalents, and substantial equivalents of the embodiments and teachings disclosed herein, including those that are presently unforeseen or unappreciated, and that, for example, may arise from applicants/patentees and others. Unless specifically recited in a claim, steps or components of claims should not be implied or imported from the specification or any other claims as to any particular order, number, position, size, shape, angle, color, or material.