Abstract:
A method of determining a relative speed between two separately driven members in an image forming apparatus, includes the steps of: transporting a print medium using a print media transport assembly including an exit nip, the print media transport assembly operable at a first transport speed; driving a rotatable member associated with an entrance nip using an electric motor at a second transport speed which is independent from the first transport speed; transferring the print medium from the exit nip to the entrance nip; detecting an electrical characteristic of the motor when the print medium is present in each of the exit nip and the entrance nip; and determining a relative speed between the first transport speed and the second transport speed.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to an image forming apparatus, such as an electrophotographic (EP) printer, and, more particularly, to a method of determining a relative speed between two separately driven members in such a printer.  
         [0003]     2. Description of the Related Art  
         [0004]     Cost and market pressures promote the design of the smallest possible printer with the shortest possible length of paper path. Short paper paths mean that media (especially legal-length media) are involved in more than one operation at once, and may span adjacent components. For example, a piece of paper in a printer which images directly onto paper may be at more than one imaging station while it is also in the fuser at the same time.  
         [0005]     Tandem color laser printers which image directly onto paper typically use a paper transport belt to move media past successive imaging stations before fusing the final image onto the media. Velocity variation is a problem created when fuser or machine component tolerances or thermal growth affect the speed ratio between the fuser and the paper transport system upstream from it. Rather than having a constant ratio between the fuser and the paper transport system, this speed ratio varies from machine to machine and from time to time or mode to mode within the same machine. This can cause registration errors, and can cause scrubbing or other print defects as well.  
         [0006]     For optimal registration of the imaging planes in tandem color laser printers, the surface speeds of the photoconductors and the media (in a direct-to-paper machine) must be precisely controlled. To achieve this, it is important that no external loads disturb the motor system moving the media. In a hot-roll fuser, the fusing nip is typically a high-force nip, with pressures on the order of 20 psi or more. This high-force nip has a sufficient grip on the media that the fuser will attempt to control the speed of the media regardless of what other systems are regulating its speed. The ability of a fuser to overwhelm other media feeding devices, and the problems this causes, may also be shared by other fuser technologies, such as belt fusers or fusers with belt backup members. For certain types of belt fusers, the backup roll is the driven member, so its effective drive diameter controls the speed of the media.  
         [0007]     In direct-to-paper machines, if media is pulled taut between an imaging nip and a fusing nip operating at a higher speed, the disturbance force transmitted via the media from the fuser to the paper transport belt causes image registration errors. To prevent these, the fuser is often under driven so that a media bubble accumulates between the transport belt and the fuser. Since the fuser runs more slowly, the media never becomes taut, so less disturbance force can be transmitted from the fuser to the transport belt. However, the pursuit of small machines means that media bubbles must be constrained to stay as small as possible. If a machine is designed for a certain maximum bubble size, large velocity variations can make the media try to form a bigger bubble. If this happens, the media will probably make contact with machine features which scrape across the image area, causing print defects. The media might also “snap through”, from the desired bubble configuration into a new one which is undesirable. This snapping action may also disturb the image and create print defects.  
         [0008]     Ideally, the fuser is just slightly under driven so that a small paper bubble develops, but does not occupy much space in the machine. However, many factors affect the relative speeds of the transport belt and the fuser, potentially creating a large range of relative velocity variation. The nominal under drive of the fuser must be set such that the worst-case velocity variation condition still results in fuser under drive or exact speed matching, but never fuser overdrive (which would create taut media).  
         [0009]     The speed of the media on a paper transport belt is set by the motion of the transport belt and photoconductive drums which form respective nips with the belt. The speed of the media in the fuser is controlled by the motion of the driven fuser member, roll compliance, drag on the backup roll, and friction coefficients between media and the two fuser rollers. In a hot-roll fuser, the hot roll is usually gear-driven while the backup roll idles on low-friction bearings. Therefore, the surface speed of the hot roll determines the speed of the media in the fuser. In some fuser systems where the backup roll is driven, the speed of that member controls the speed of the media.  
         [0010]     The transport speed variances of the fuser can be divided into two primary categories: 1) the effect of temperature variations on the fuser roll, and 2) manufacturing variances such as dimensional tolerances, varying physical properties of materials used in components, different preload nip pressures, etc. Effects of temperature variations of the fuser roll at different operating temperatures are addressed in a manner described in a separate patent application entitled “METHOD OF DRIVING A FUSER ROLL IN AN ELECTROPHOTOGRAPHIC PRINTER”, U.S. patent application Ser. No. 10/757,301, filed Jan. 14, 2004, which is assigned to the assignee of the present invention.  
         [0011]     Manufacturing variances have been addressed heretofore, but in much more complicated and expensive ways. Merely measuring the outside diameter of a fuser roll and its rotational speed and calculating its circumference or surface speed is not good enough because the roll deforms during rotation. This deformation means that the actual distance media travels during one roll revolution through the fuser is not the same as the circumference of the roll. One method is to place a piece of tape on a fuser roll, and then to fuse solid-coverage images using the fuser roll. The tape causes a print defect at the period of the effective roll circumference, allowing distance traveled during one roll revolution to be accurately measured. The reduction in size of the media as it loses moisture during the fusing process complicates this process, since this change must be accounted for in calculating the period of the print defect. The use of tape is also undesirable since it risks roll damage which could cause later print defects.  
         [0012]     U.S. Pat. No. 5,819,149 describes sensing methods for directly monitoring the size of a backup roll in a belt fuser. As the backup roll changes size, its peripheral velocity will change, so the media velocity going through the fuser will also change. Monitoring roll size allows the printer to maintain a desired media speed through the fuser. However, as discussed above, roll circumference will not strictly match the media advance distance during one roll revolution, so this method introduces errors.  
         [0013]     U.S. Pat. No. 5,170,215 describes the use of a separate media speed sensor to determine whether a fuser is pulling on continuous-form media. The additional required sensors undesirably increase the cost of the printer.  
         [0014]     U.S. Pat. No. 5,508,789 describes a speed measurement method for determining the photoconductor drum speed needed to match speeds between an intermediate transfer belt and the photoconductor drum. The speed of the drum is varied while monitoring current to the drum drive motor, while the belt is driven and servo-actuated independently. Over a long-period speed oscillation (200 seconds), large variations in current demand caused by dry friction between the drum and belt materials when their speeds nearly match are monitored. This dry friction phenomenon provides a large physical response at the point of matching speeds.  
         [0015]     Each of these known patented methods uses additional sensors for sensing continuously available parameters or measuring parameters while components are in direct continuous contact. This increases the complexity and cost of related printers.  
         [0016]     What is needed in the art is a method of determining and setting a transport speed of a downstream driven member relative to a transport speed of an independent upstream driven member, without requiring additional sensors, etc.  
       SUMMARY OF THE INVENTION  
       [0017]     The present invention provides a method of setting a transport speed of a downstream driven member relative to a transport speed of an upstream driven member by monitoring electrical characteristics of a drive motor for the downstream driven member, rather than utilizing additional sensors, etc.  
         [0018]     The invention comprises, in one form thereof, a method of determining a relative speed between two separately driven members in an image forming apparatus, including the steps of: transporting a print medium using a print media transport assembly including an exit nip, the print media transport assembly operable at a first transport speed; driving a rotatable member associated with an entrance nip using an electric motor at a second transport speed which is independent from the first transport speed; transferring the print medium from the exit nip to the entrance nip; detecting an electrical characteristic of the motor when the print medium is present in each of the exit nip and the entrance nip; and determining a relative speed between the first transport speed and the second transport speed.  
         [0019]     An advantage of the present invention is that the relative speed between the independently driven members can be determined without additional sensors.  
         [0020]     Another advantage is that the transport speed of the downstream member can be set at a predetermined amount less than the upstream member so as to avoid certain print defects.  
         [0021]     Yet another advantage is that the point at which the transport speed of the downstream driven member matches the transport speed of the upstream driven member can be established using a threshold value or a linear regression data fit.  
         [0022]     A still further advantage is that the method of determining and setting the relative transport speed of the downstream driven member can occur during manufacture or upon replacement of the downstream driven member. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]     The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein:  
         [0024]      FIG. 1  is a simplified side, sectional view of an EP printer which may be used to carry out an embodiment of the method of the present invention;  
         [0025]      FIG. 2  is a schematic, side view of a portion of the paper transport assembly, fuser and electrical circuit of the EP printer shown in  FIG. 1 ;  
         [0026]      FIG. 3  is a graphical illustration of pulse width modulation settings corresponding to load on a fuser motor, at a fuser speed of approximately 104.991 mm/sec.;  
         [0027]      FIG. 4  is a graphical illustration of pulse width modulation settings corresponding to load on a fuser motor, at a fuser speed of approximately 106.647 mm/sec.;  
         [0028]      FIG. 5  is a graphical illustration of pulse width modulation settings corresponding to load on a fuser motor, at a fuser speed of approximately 107.030 mm/sec.;  
         [0029]      FIG. 6  is a graphical illustration of pulse width modulation settings corresponding to load on a fuser motor, at a fuser speed of approximately 107.284 mm/sec.;  
         [0030]      FIG. 7  is a graphical illustration of pulse width modulation settings corresponding to load on a fuser motor, at a fuser speed of approximately 107.540 mm/sec.;  
         [0031]      FIG. 8  is a graphical illustration of a linear regression data fit to determine an approximate matching speed between the fuser and transport belt. 
     
    
       [0032]     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates one preferred embodiment of the invention, in one form, and such exemplification is not to be construed as limiting the scope of the invention in any manner.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     Referring now to the drawings and particularly to  FIG. 1 , there is shown an embodiment of an EP printer  10  of the present invention. Paper supply tray  12  contains a plurality of print media  14 , such as paper, transparencies or the like. A print medium transport assembly (not numbered) includes a plurality of rolls and/or transport belts for transporting individual print media  14  through EP printer  10 . For example, in the embodiment shown, the print medium transport assembly includes a pick roll  16  and a paper transport belt  18 . Pick roll  16  picks an individual print medium  14  from within paper supply tray  12  and transports print medium  14  to a bump-align nip defined in part by roll  20  to paper transport belt  18 . Paper transport belt  18  transports the individual print medium past a plurality of color imaging stations  22 ,  24 ,  26  and  28  which apply toner particles of a given color to print medium  14  at selected pixel locations. In the embodiment shown, color imaging station  22  is a black (K) color imaging station; color imaging station  24  is a yellow (Y) color imaging station; color imaging station  26  is a magenta (M) color imaging station; and color imaging station  28  is a cyan (C) color imaging station.  
         [0034]     Paper transport belt  18  transports an individual print medium  14  ( FIG. 2 ) to fuser  32  where the toner particles are fused to print medium  14  through the application of heat. Fuser  32  includes a hot fuser roll  34  and a back up roll  36 . In the embodiment shown, fuser roll  34  is a driven roll and back-up roll  36  is an idler roll; however, the drive scheme may be reversed depending upon the application.  
         [0035]     Techniques for the general concepts of heating fuser roll  34  and rotatably driving fuser roll  34  or back-up roll  36  using gears, belts, pulleys and the like (not shown) are conventional and not described in detail herein. Fuser roll  34  is schematically illustrated as being connected via phantom line  38  to drive motor  40 , which is in turn connected to and controllably operated by electrical processing circuit  42 , such as a microprocessor.  
         [0036]     In the embodiment shown, print medium  14  is in the form of a legal length print medium. As is apparent, print medium  14  is concurrently present at the nips defined by a photoconductive (PC) drum  44  of color imaging station  26 ; a nip defined by PC drum  46  of color imaging station  28 ; a nip defined between fuser roll  34  and back-up roll  36 ; a nip defined by fuser exit rolls  48  and a nip defined by machine output rolls  50 . The leading edge of print medium  14  is received within output tray  52  on the discharge side of machine output rolls  50 .  
         [0037]     PC drum  46  and the corresponding backup roll define an exit nip from the print medium transport assembly, and fuser rolls  34  and  36  define an entrance nip to fuser  32 . As described above, it is undesirable to overdrive fuser roll  34  such that the fuser-controlled media velocity at the nip of fuser roll  34  exceeds the linear transport speed of paper transport belt  18 . The force on the media from the nip between fuser roll  34  and back-up roll  36  typically is larger than the combination of the forces from the nips at PC drums  44  or  46  and the electrostatic force acting on the print medium, and thus the nip pressure and transport speed at fuser roll  34  tend to dominate the transport speed of the print medium conveyed on paper transport belt  18 . If fuser roll  34  is overdriven such that the fuser-controlled media velocity is greater than that of paper transport belt  18 , then print defects may occur on print medium  14 . For this reason, fuser roll  34  may be under driven to cause a slight bubble  54  in the gap between the discharge side of paper transport belt  18  and the input side of the nip between fuser roll  34  and back-up roll  36 . This bubble  54  may be more pronounced, as illustrated by phantom line  56  in  FIG. 2 . If the size of bubble  54  becomes too large because of the velocity differences between fuser roll  34  and paper transport belt  18 , then print medium  14  may contact physical features within printer  10  resulting in print defects. That is fuser roll  34  should be under driven, but not to such an extent that defects resulting from scraping, etc. of print medium  14  occur.  
         [0038]     In the embodiment shown, each of fuser roll  34  and back-up roll  36  have a PFA sleeve at the outside diameter over an elastomeric layer. The outside diameter of fuser roll  34  and back-up roll  36  is approximately  36 mm at the outside diameter of the PFA sleeve when measured cold. It will be appreciated that the outside diameter of fuser roll  34  increases as the operating temperature of fuser roll  34  increases.  
         [0039]     According to an aspect of the present invention, the relative speeds between fuser roll  34  and transport belt  18  are measured to determine a desired nominal fuser speed in printer  10 . This method is carried out at the end of the printer manufacturing line, and is necessary if a fuser is replaced in the field. The method of the present invention accounts for manufacturing tolerances on fuser rolls which affect the speed of the media (such as paper  14 ) as it passes through fuser  32 . This measurement operation allows the relative speed between fuser  32  and transport belt  18  to be set in the middle of an acceptable range, so that media  14  will build an optimal paper bubble  54  between the two systems. Otherwise, during some operating modes, fuser  32  pulls media  14  too tight and affects color registration, or it slows down too much during other modes and builds too large of a paper bubble  56 , possibly causing tailflip and image smear.  
         [0040]     More particularly, one method of determining a relative speed between fuser  32  and transport belt  18  is to monitor commanded voltage of motor  40  while sending pages through fuser  32  at different speeds. A speed control feedback system inside printer  10  tries to maintain motor  40  at a constant commanded velocity. In order to do that, it monitors a fuser motor encoder and changes the commanded voltage applied to motor  40  to assure that the encoder and motor  40  are rotating at a consistent speed. When the load on motor  40  rises and its speed drops slightly, the speed control system raises the commanded voltage in order to restore the speed to the commanded value. The commanded voltage is generated by the electrical processor  42  within printer  10  as a pulse-width-modulation (PWM) duty-cycle setting which reduces the 24V motor supply voltage to a time-averaged intermediate voltage to drive motor  40 . This duty-cycle PWM setting can be monitored by processor  42  to assess the load on motor  40 .  
         [0041]     Except when a sheet of media  14  is on both transport belt  18  and in the fuser nip between rolls  34  and  36 , media  14  applies very little load to motor  40 . Most of the fuser motor power is used to rotate fuser rolls  34  and  36  (which deform against one another as they rotate under load), fuser exit rolls  48  and machine output rolls  50 . Even when a sheet  14  is on both transport belt  18  and in the fuser nip, if the media speed in fuser  32  is slower than the transport belt speed, a paper bubble  54  will develop, and little additional load will be imposed on motor  40 . However, if a sheet is on both transport belt  18  and in the fuser nip, and the media speed in fuser  32  is faster than the independently driven transport belt speed, then fuser  32  will pull on media  14  and transport belt  18 , raising the load on motor  40 . During normal operation, this is not desirable since the load on transport belt  18  could lead to color registration errors. However, during a speed measurement sequence of the present invention, this additional load can be monitored using the PWM setting of motor  40 . The presence or absence of this additional load, depending upon the relative speeds of transport belt  18  and fuser  32 , can be used to determine when the speeds are matched. With a known fuser speed which matches the transport belt speed, processor  42  adds an offset to slow fuser  32  so that a desired paper bubble is created, and the resulting sum is stored as a nominal fuser speed.  
         [0042]     This can be more easily explained via a graph of the fuser motor PWM setting (representing fuser load) and the fuser motor speed as a medium  14  passes through fuser  32 .  FIGS. 3-7  illustrate various PWM settings at different fuser speeds.  FIG. 7  is a graphical illustration at the fastest fuser speed and thus provides the most pronounced response for the examples shown in  FIGS. 3-7 . Since  FIG. 7  is also the easiest to visualize, it is initially used for illustration purposes herein.  
         [0043]      FIG. 7  illustrates a relatively fast fuser speed setting (107.540 mm/sec), with fuser  32  pulling on transport belt  18  via print media  14 . The fuser motor PWM settings can range between 0 and 4095, where higher numbers indicate voltage is being applied to motor  40  a greater percentage of the PWM period, thus providing higher average voltages to motor  40 . The higher voltages indicate a higher load on the fuser drive motor as previously described. The graph in  FIG. 7  represents empirical data recorded during the first page of a multi-page job, with the spike at +3.0 seconds being the leading edge of a following media  14  entering fuser  32 . Various events in  FIG. 3  are labeled A through F in Table 1 below:  
                         TABLE 1                       Event timing labels in FIGS. 3-7 for fuser motor PWM and speed graphs                                A =   Start of measurement period for “No-Paper PWM average”;       B =   End of measurement period for “No-Paper PWM average”;       C =   Paper leading edge enters fuser nip;       D =   Start of measurement period for “With-Paper PWM average”;       E =   Paper trailing edge exits last transfer nip; end of measurement           period for “With-Paper PWM average”; and       F =   Paper trailing edge exits fuser nip.                    
         [0044]     The method of the present invention is initiated from either an electronic signal over an interface cable or by an operator input menu of printer  10 , either after printer manufacture and color registration, or after a field replacement of fuser  32 . Fuser  32  must be at the nominal operating temperature. The sequence consists of the printing of a number of media  14  (e.g., around six), at progressively faster fuser speeds. The first fuser speed is chosen to be significantly slower (e.g. 1% slower) than the transport belt speed, so that media  14  will not exert any additional load on fuser  32 . During the printing of each media  14 , the important measurement interval is the period of time when the page is both attached to transport belt  18  and also in the fuser nip. During this time, the fuser motor PWM setting is averaged over one revolution of fuser rolls  34  and  36 . This average PWM level is compared to an earlier average PWM level, measured during one revolution of fuser rolls  34  and  36  before the media entered fuser  32 . The difference between these two average PWM levels quantifies the effect of media  14  on the fuser motor load at this slow fuser speed. This value is stored.  
         [0045]     Next, the measurement is repeated at successively faster speeds, at a nominal interval of 0.25% fuser speed increase per page. The effect of media  14  on the fuser motor load is measured and computed the same way for each speed (see, e.g., Table  2 ). Preferably, the later pages are printed slower-to-faster because a media transport speed which is too fast might risk motor over-current, causing a machine error which would interrupt the process. By operating slower-to-faster, the sequence can be stopped if motor current demands exceed a threshold below that which would cause an error.  
                                                           TABLE 2                           Speed measurement via PWM settings                Actual   No-Paper   With-Paper               Fuser Speed   Fuser PWM   Fuser PWM   PWM Increase           (mm/sec)   (avg counts)   (avg counts)   (%)                            104.991   2089   2126   1.8           106.647   2108   2266   7.5           107.030   2101   2769   31.8           107.285   2112   2813   33.2           107.540   2106   3012   43.0                      
 
         [0046]     Graphs of the fuser motor PWM settings and fuser motor speeds are shown in  FIGS. 3-7 . The same labels shown in Table 1 apply, with the motor PWM setting averages computed during the timing windows indicated in Table 1. As is apparent, as fuser speed is increased, there is a progressive increase in the amount of influence from transport belt  18 , requiring additional fuser motor power, as quantified by the increase in the average PWM.  
         [0047]     Two methods may be used to detect an approximate matched speed between fuser  32  and transport belt  18 . One method applies a threshold to the PWM increase. For example, if 15% is set as a threshold value, then the illustrated transport speed of 106.647 mm/s is the matched speed, because it is the last speed point below the threshold PWM increase. Alternately, it is possible to interpolate between  106 . 647 mm/s and 107.030 mm/s to find the speed for exactly a 15% PWM increase, obtaining 106.765 mm/s.  
         [0048]     Another method of detecting an approximate matched speed uses linear regression and more of the data to find an intercept value. For example, referring to Table 2, normalize the PWM increase percentages by subtracting the PWM increase at the lowest speed. That power increase is likely due to the presence of paper  14  in the fuser nip, rather than any drag of transport belt  18  on fuser  32 . Second, fit a line to the PWM increase data and estimate the lowest fuser speed which does not require any increase in PWM values. The data is shown in Table 3:  
                                           TABLE 3                           Speed measurement via PWM settings            Actual       Normalized       Fuser Speed   PWM Increase   PWM Increase       (mm/sec)   (%)   (%)                    104.991   1.8   0.0       106.647   7.5   5.7       107.030   31.8   30.0       107.285   33.2   31.4       107.540   43.0   41.2                  
 
         [0049]     This data and the resulting line are plotted in  FIG. 8 . The intercept of the line is 106.41 mm/s, the estimated fuser speed to match the transport belt speed. Using the fuser speed which matches the speed of the transport belt, the nominal fuser speed is set about 0.75% slower than this speed, to put the nominal paper bubble in the middle of the range of its possible sizes.  
         [0050]     The method of the present invention can also detect other electrical characteristics of motor  40 . For example, this method can also be used with the signals from the fuser motor encoder. When a media  14  leaves transport belt  18  so that it is only in the fuser nip, a dramatic reduction in the fuser motor load occurs, which results in a brief over-speed condition on motor  40 . The resulting speed spike can be detected by monitoring the fuser encoder output. Either the rate of encoder pulses or transitions or the period between the pulses or transitions can be monitored to find the size of this spike, which is greater when motor  40  is driving the print media at a velocity that is faster than the transport belt. While this event is one of the few that the motor encoder output could be used to monitor, the same spike could also be monitored via motor current or motor PWM setting.  
         [0051]     Further, the method of the present invention as described above for determining a relative speed between two separately and independently driven members in an image forming apparatus may be used with independently driven members other than a fuser and a paper transport assembly. For example, a print medium may be transported from an exit nip of an upstream and independently driven bump-align motor to the entry nip of a transport belt. The present invention allows the relative speed between the transport speed at the exit nip of the upstream bump-align motor and the entry nip of a transport belt to be determined, and an adjustment made to one or both transport speeds, if necessary.  
         [0052]     Both bump-align and fuser interfaces to a transport belt may be measured in concert as long as print media is not in both bump-align and fuser nips simultaneously. The effect on the bump-align motor voltage can be determined while a page is in the bump-align nip and on the transport belt, but before the page enters the fuser nip. After the same page leaves the bump-align nip, the effect on the fuser motor voltage can be determined while the page is on the transport belt and in the fuser nip. During the measurement process, the successive pages printed at different speeds must be separated by large enough interpage gaps to ensure that a previous page has left the transport belt before a following page reaches the transport belt.  
         [0053]     While this invention has been described as having a preferred design, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.