Patent Publication Number: US-7596330-B2

Title: Determining a location of an uncharged region on a photoconductive drum

Description:
The present invention relates to a method and device for determining a location of an uncharged region on a photoconductive drum. 
   An electrophotographic or liquid electrophotographic, LEP, process is utilized in a plurality of machines such as copying machines, facsimile machines, digital presses, and laser printers. As illustrated in  FIG. 1 , the process involves charging a surface of a photoconductor drum  10  with a charge roller  12 , and exposing the charged surface of the drum to light produced by a modulated light source,  14 , for example a laser, LED array or reflected light from an original document (in the case of an analogue document copier), to form an electrostatic latent image thereon. The latent images are developed by a developing unit  16  to create visible images, which are transferred to an intermediate transfer device  18  or directly to a sheet of media. 
   Some types of photoconductor drum comprise a seam or uncharged region. When implementing an electrophotographic or liquid electrophotographic, LEP, process with a photoconductor drum comprising a seam region, it is often desired to synchronize particular actions with a given angular location on the drum, for example, when changing the charging levels or activating a writing process. 
   A known method of determining a given angular location on the drum is to install an encoder on the rotating drum. However, this involves costly hardware and a calibration process to ensure the photoconductor drum conforms to strict tolerances. Even where such an encoder were available, it may not provide the accuracy of measurement required for some applications. 
   U.S. Pat. No. 7,102,661 discloses an apparatus comprising two laser systems and a photoconductive drum having a surface. Light beams projected from the laser systems overlap on the surface of the drum, thereby providing a reference mark. A position detection sensor is provided to detect the reference mark and activate its output at every revolution of the photoconductive drum. This enables actions requiring synchronization with the reference mark on the drum to be carried out. However, again, this involves costly hardware. 
   U.S. Pat. No. 7,116,922 discloses an apparatus comprising a photosensitive drum having a peripheral surface which is charged by a charge roller, to which a voltage is applied. The apparatus is further provided with a charge current measurement circuit for measuring the charge current that flows to the charge roller through the drum and a control circuit having a current detecting circuit for detecting current of a specific type. U.S. Pat. No. 7,116,922 is concerned with controlling the voltage source of the charge roller in such a manner that either AC voltage or DC voltage or both are applied to the charge roller, based on the charge current data determined from the charge current measurement circuit, in order to minimize the discharge between the drum and the roller while preventing the drum from being unsatisfactorily charged. 

   
     An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: 
       FIG. 1  illustrates a prior art electrophotographic device for implementing an electrophotographic process; 
       FIG. 2  illustrates an electrophotographic device for implementing an electrophotographic process according to an embodiment of the present invention; 
       FIG. 3  is a flow chart depicting the processing performed in an embodiment of the present invention; and 
       FIG. 4  depicts graphically current versus time measurements used in the processing of  FIG. 3 . 
   

   Referring to  FIG. 2 , there is illustrated an electrophotographic device suitable for carrying out electrophotographic process according to an embodiment of the present invention. The device comprises a photoconductor drum  10 , in contact with a charging roller  12 . The drum  10  comprises a surface  20  with a seam region  22  provided thereon. A power supply  24  is provided for powering the charge roller  12 . 
   By rotating the drum, preferably at a constant speed, and activating the charge roller  12  by applying a voltage to it, the surface  20  of the drum  10  becomes charged. 
   As explained above, it is often desirable to synchronize certain actions with an angular or temporal location of the drum  10 . According to an embodiment of the present invention, synchronization is achieved by monitoring electrical characteristics of the charge roller  12  when applied to the drum  10 . A particular alteration in the electrical characteristics can indicate the location of the seam, thereby enabling synchronization of further processes with the location of the seam to be achieved. 
   In an embodiment, the electrical characteristics of the charge roller  12  are monitored by means of a current measuring circuit  26  provided in the power supply. However it will be appreciated that the current measuring circuit  26  may be provided at any suitable location. 
   In a first embodiment, the charge roller  12  is activated by the application of voltages that fall within normal operating range suitable for charging the photoconductor drum  10 . The DC charging current is then measured. This may be at a high sampling rate, for example, 16,264 Hz. For a normal drum rotation speed, this corresponds to an angular separation of 0.05° between samples on the drum  10 . The results of the measurement are analysed to determine a change in the current value. This change in current reflects a change in the charging of the photoconductor drum  10 , thereby indicating the seam region  22  of the drum  10 . A wide range of sampling rates may be used in other embodiments, both of higher and lower frequency that 16,264 Hz. 
   It will be appreciated that generally the higher the sampling rate, the more accurate and precise a current profile produced. In one embodiment, the sampling rate is high enough to allow sufficient measurement of values during transition from an entry point of the seam region to the seam region and from an exit point of the seam region to the remainder of the drum. In embodiments with noisy signals, a higher sampling rate may be used. 
   In some embodiments, where the location of the seam is to be determined with a positional accuracy given by A (length) and the photoconductors tangential velocity is V (length/time), the sampling rate R (samples/time) may be selected so that R&gt;&gt;V/A. 
   In alternative embodiments, the charge roller  12  is activated by the application of an AC voltage either alone or together with the DC voltage, the period of which is much lower than the temporal resolution required. In such an embodiment, the results of the measurement are analysed to determine a change in the current value. This change in current reflects a change in the charge roller  12  and/or photoconductor capacitance due to either a change in the geometry, for example, the distance between the charge roller  12  and the photoconductor, or the dielectric properties of the photoconductor in that region  22 . When the current measuring circuit is implemented through a non-contact “current-clamp” technique that is more sensitive to AC than to DC, it is preferable to utilise an AC current. 
   In both the AC and DC embodiments, a drop in the magnitude of the current value from an average value indicates the point at which the charge roller  12  enters the seam region  22  of the drum  10 , and a subsequent increase in the magnitude of the current towards its average level, indicates the point at which the charge roller  12  exits the seam region of the drum  10 . 
   With reference to  FIG. 3 , a specific example of an application of the preferred embodiment of the present invention as implemented on a Hewlett-Packard Indigo Digital Press is provided. 
   In this example, the drum  10  is charged  300  by the charging roller  12  in DC mode, as described above. The current of the charge roller ( 12 ) around the seam region  22  is sampled and averaged  310 . This step may be repeated more than once, for example 2, 5, 10 or 20 times or more in order to improve the quality of the result with samples from each rotation being averaged. Correlation of the samples from one rotation to the next can be performed by any number of suitable techniques.  FIG. 4  shows the results of this process graphically for the region around the seam. 
   From these measurements, the average current of the charge roller ( 12 ), before the seam region is determined,  320  and in the present example, this average is approximately −0.6 mA. 
   In step  330 , the time t 1  when two consecutive current points have a magnitude less than 20% of the average current value from step  320  i.e. less then approximately −0.48 mA, is determined. 
   In step  340 , the time t 2  after t 1  when two consecutive current points have a magnitude less than −0.1 mA is determined. 
   Extrapolating the times and current values at t 1  and t 2  provides a projected time t 3  when current is predicted to be the previously calculated average value, step  350 . This time t a  is deemed to be the entry point of the seam region  22 . 
   A similar process is applied to determine the exit point of the seam region. Thus, the average current of the charge roller ( 12 ) after the seam is determined,  360 . 
   In step  370 , working backwards towards the entry point, the time t 3  when two consecutive current points have a magnitude less than 20% of the average current value from step  360  i.e. less then approximately −0.48 mA, is determined. 
   In step  380 , the time t 4  before t 3  when two consecutive current points have a magnitude less than −0.1 mA is determined. 
   Extrapolating the times and current values at t 3  and t 4  provides a projected time t 5  when current is predicted to be the previously calculated average value, step  390 . This time t b  is deemed to be the exit point of the seam region  22 . 
   In a variation of the above technique, the times and current values at t 1  and t 2 ; and t 3  and t 4  can be extrapolated to provide respective projected times when current is predicted to be 0.0 mA, and these times can be deemed to be more closely defined entry and exit points of the seam region  22 . 
   Other variations of the measures taken above can also be used for defining seam exit and entry points, for example steps  370  and  380  can be reversed with their tests being for when points have magnitudes greater than −0.1 mA or 20% less than the average current value. 
   In an alternative embodiment, rather than calculating both the entry and exit points, determination of one of the entry point or exit point and knowledge of the size of the seam region  22  is used to estimate the other of the entry point or exit point of the seam region. In this embodiment, the other of the entry or exit point may be measured and determined for verification purposes. 
   In the particular cases of noisy signals, a further verification measure can be taken in all embodiments by comparing the entry and exit points of the seam with previously determined values, and rejecting these points if they are determined to be largely different. Furthermore, it should be ensured that the points fall within known limits of the system. 
   In the case of noisy signals, the determination of t 1 , t 2 , t 3  and t 4  can be improved be requiring that the threshold is crossed by more than one point. This assures that glitches in the signal will not cause a false trigger. 
   The current levels used to determine t 1 , t 2 , t 3  and t 4  may be adjusted according to the noise characteristics of the signal. If the noise level, defined as the standard deviation of the signal, is denoted as S, then the level change required, in certain embodiments, should be bigger than 3S but still low enough to allow a few dozens of measurement points to reside between t 1  and t 2  and between t 3  and t 4 . 
   If a seam region has a complex structure and charging by the charge roller still occurs to some extent in the seam, the current characteristic might differ considerably from the one shown in  FIG. 4 . The seam location can be nonetheless determined using an pattern recognition technique from that of  FIG. 3  and suitable to the current profile. 
   The method of the present invention is preferably carried out during a pre-print phase, as the processing overhead in sampling can be quite high and in general there tends to be little drift in the values determined for the seam location. However, it is appreciated that the method of the present invention may be carried out during a normal print process. 
   The invention is not limited to the embodiments described herein but can be amended or modified without departing from the scope of the present invention.