Abstract:
A method and apparatus for correcting transfer belt positioning error in printers. A transfer belt subassembly includes a transfer belt, a plurality of rollers, and a storage device. The transfer belt also includes a home position indicator. The subassembly is measured and characterized before being installed in a printer. The measurement and calibration data for the transfer belt is stored in the storage device. When the transfer belt assembly is inserted into a printer, a controller within the printer is placed in communication with the storage device. A sensor is used to determine the home position of the belt from the indicator, and a resulting signal indicating the belt is at the home position is provided to the controller. The controller utilizes the measurement and calibration data from the storage device to control the motor to correct for belt positioning errors. In such a manner, the calibration data is predetermined before the belt assembly is inserted into the printer, thereby eliminating the need for calibration cycles after the belt assembly has been installed within the printer, while providing a high degree of alignment of the color planes onto the transfer belt.

Description:
BACKGROUND OF THE INVENTION 
     A frequent problem associated with color printers is misregistration or misalignment of one or more color planes. Alignment of the color planes is crucial in achieving a high quality image. The color planes are sequentially deposited onto a transfer medium such as an intermediate transfer belt that is used to transfer the color planes to medium such as a piece of paper. Alternately the medium itself may be transported and have the color planes sequentially deposited directly thereon. 
     Due to the fact that each individual color plane is transferred onto the belt or print medium at different locations along the travel path of the belt or print medium, the belt position within the travel path must be known or predicted with a high degree of precision. The position of the belt must be known to insure that the resulting image is of good quality. 
     There are many instances where belt positioning errors develop and cause a concomitant degradation in the resulting image. Drive roller runout, variations in the thickness of the belt, drive roller cylindrical imperfections, and variations in the belt tension are, in general, examples of factors that lead to belt positioning errors. In particular, the surface velocity of the belt is caused to run slower or faster depending upon whether: i) the belt is thin or thick as the belt passes over the drive roll; ii) the radius of the drive roller is longer or shorter as the belt passes over; and iii) the belt is tightly or loosely tensioned. 
     Others have tried to compensate for belt position errors by performing a calibration cycle within the printer at periodic intervals. The calibration cycle generates a test pattern from each color head to the transfer belt (typically toned line segments or symbols), detects the image position on the belt by way of a complex sensor, and corrects for belt speed or position based on the detected image. This manner of correcting for belt positioning to implement in the printer, wastes toner, and consumes time each occasion the calibration cycle is run. It would be desirable to have a method and apparatus that corrects for belt positioning errors which is inexpensive to implement in a printer, does not require user calibration, and does not add complexity to the printer. 
     BRIEF SUMMARY OF THE INVENTION 
     A method and apparatus for correcting transfer belt positioning error in printers is disclosed. A transfer belt subassembly includes a transfer belt, a plurality of rollers, and a storage device. The transfer belt also includes a home position indicator. The transfer belt subassembly is measured and characterized relative to the home position indicator before being installed in a printer. The measurement and calibration data for the transfer belt is then stored in the storage device that is part of the transfer belt subassembly. When the transfer belt subassembly is inserted into a printer, a controller within the printer is placed in communication with the storage device. A sensor is used to determine the home position of the belt from the indicator, and a resulting signal indicating when the belt is at the home position is provided to the controller. The controller utilizes the measurement and calibration data from the storage device to control the belt drive motor and print heads to correct for belt positioning errors. In such a manner, the calibration data is predetermined before the belt assembly is inserted into the printer, thereby simplifying the printer composition and eliminating the need for calibration cycles after the belt assembly has been installed within the printer. By use of the calibration and measurement data, precise alignment of the color planes onto the transfer belt or print medium is achieved. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING 
     The invention will be more fully understood from the following detailed description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a schematic diagram illustrating a first embodiment of the apparatus of the present invention; 
     FIG. 2 is a schematic diagram illustrating a second embodiment of the apparatus of the present invention; and 
     FIG. 3 is a flow chart illustrating the method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Image forming apparatus, such as color printers, sometimes utilize a transfer belt assembly to accumulate an image from a plurality of color planes. The color planes are placed onto the belt in succession as the transfer belt passes by the photoconductive (PC) drum, or other similar electrophotostatic devices, associated with each color print head. Once the belt has traversed all of the PC drums a resulting image, which will later be transferred to a print medium, is provided on the transfer belt. Alternately the transfer belt is used to transport a piece of print medium, such as paper, card stock or transparencies, and the color planes are deposited directly on the print medium as the medium passes by the PC drums of each color station. 
     Referring to FIG. 1, an apparatus  10  for providing transfer belt correction is shown. The apparatus  10  includes transfer belt subassembly  15 , a drive motor  30  and a controller  94 . The transfer belt subassembly  15  contains a transfer belt  20 , a first home position sensor  70 , a second home position sensor  71 , a temperature sensor  85  such as a thermistor, a memory device  80  and a plurality of rollers. While the presently described embodiment includes a belt movable about transfer rollers, other embodiments may include a movable platen wherein the color planes are transferred onto the platen. The plurality of rollers include a drive roller  40 , an end roller  41 , a first transfer roller  50 , a second transfer roller  51 , a third transfer roller  52 , a fourth transfer roller  53 , and an accumulated image transfer roller  55 . 
     The transfer belt  20  surrounds and traverses an ellipsoidal path defined by rollers  40 ,  41  and  54 . The transfer belt  20  also includes a home position indicator  75  that is useful for accurately identifying a specific position of the transfer belt  20  with respect to the transfer belt subassembly  15 . 
     Roller  40  is used as a drive roller and is in mechanical communication with a drive motor  30  as will be described below. Roller  40  thus provides for movement of the transfer belt  20  through the belt path. 
     Transfer rollers  50 ,  51 ,  52  and  53  are used to aid in the transfer of color planes from respective PC drums onto the transfer belt  20 . While four color planes are described in this embodiment, it should be understood that any number of color planes and associated PC drums and transfer rollers travels over first transfer roller  50 , a first color plane is deposited onto the transfer belt by being placed in contact with a first PC drum  90 . As the same area of the belt further traverses the belt path a second color plane is transferred onto the transfer belt by being placed in contact with a second PC drum  91  opposite second transfer roller  51 . The second color plane is deposited overlaying the first color plane. As the area of the belt  20  continues to traverse further along the travel path, a third color plane is deposited over the first and second color planes by being placed in contact with a third PC drum  92  and third transfer roller  52 . As the area of the belt  20  continues further along the travel path, a fourth color plane is deposited over the first, second and third color planes by being placed in contact with a fourth PC drum  93  and fourth transfer roller  53 . The accumulated image is then transferred to a print medium (not shown) by transfer roller  55 . The print medium may comprise paper, card stock, transparencies or the like. 
     Referring to FIG. 2, a second embodiment of an apparatus  11  for providing transfer belt correction is shown. The apparatus  11  is similar to the apparatus disclosed in FIG. 1, except that the color planes are deposited directly onto a print medium disposed on and transported by the transfer belt. 
     In this embodiment the transfer rollers  50 ,  51 ,  52  and  53  are used to aid in the transfer of color planes from respective PC drums directly onto the print medium. As an area of the transfer belt  20  travels over first transfer roller  50 , a first color plane is deposited onto the print medium by being placed in contact with a first PC drum  90 . As the same area of the belt further traverses the belt path a second color plane is transferred onto the print medium by being placed in contact with a second PC drum  91  opposite second transfer roller  51 . The second color plane is deposited overlaying the first color plane. As the area of the belt  20  continues to traverse further along the travel path, a third color plane is deposited over the first and second color planes by being placed in contact with a third PC drum  92  and third transfer roller  52 . As the area of the belt  20  continues further along the travel path, a fourth color plane is deposited over the first, second and third color planes by being placed in contact with a fourth PC drum  93  and fourth transfer roller  53 . 
     Alignment of the color planes on the transfer belt or medium is crucial for providing a high quality resulting image. There are a number of factors that affect the alignment of the color planes on the transfer belt or medium. For example, the rollers can have various amounts of runout, there may be variations in the width or thickness of the belt, and there may be variations in the tension of the belt along the belt path. In the second embodiment, the print medium may move with respect to the transfer belt. 
     In order to provide for proper alignment of the color planes, the transfer belt subassembly  15  is measured and characterized in a special test fixture with simulated loads at the time the subassembly  15  is manufactured. This applies to the transfer belt subassembly  15  of either of the embodiments shown in FIGS. 1 and 2. An AC belt surface velocity relative to the home position indicator  75  is recorded. This measurement may be obtained by use of a calibrated surface wheel/tachometer in non-slip contact with the transfer belt near the drive roller. An average or DC belt surface velocity is also measured by recording the transfer belt transition time between the first home sensor  70  and the second home sensor  71 . The distance between the first home sensor  70  and second home sensor  71  is preferably equal to the distance between adjacent PC drums. The DC belt surface velocity may also be measured by use of a calibrated surface wheel/tachometer in non-slip contact with the transfer belt near the drive roller. A temperaturesensing element such as a thermistor measures the temperature on or near the drive roller. The measured temperatures are used to compensate for thermal variations of the printer components. The AC belt lateral position is measured at each color station and optionally at the transfer station. The measurements are made relative to a known or learned belt edge profile and are obtained by, for example, a photo-electric sensor. 
     The data that reflects the measured and characterized transfer belt subassembly  15  is stored in a storage device  80 , which is part of the belt subassembly  15 . The stored data includes, but is not limited to, the belt length, defined in zones, which is used for velocity control of the belt, the belt length, defined in zones, for start-of-imaging control for the respective print heads, and the belt DC travel time between the first home sensor  70  and the second home sensor  71  with respect to temperature. Additionally, the stored data includes the time between sensors with AC feed-forward, the travel time between sensors without AC feed-forward, different function enables for the printer, the AC belt velocity correction table, and belt start of scan delay correction tables for three of the color stations with respect to the fourth color station. Alternatively, scan correction tables for all four color stations with respect to position at another reference such as a second transfer to the print media. 
     The storage device  80  may be a semiconductor memory such as a DS1985 non-volatile 16 Kbit memory available from Dallas Semiconductor Corp. of Dallas, Tex. The stored data is also referred to as calibration data. 
     The home position indicator  75  of the transfer belt  20  provides a reference point for the measurement and calibration data. As such, the calibration data is in some manner associated with the home position indicator. For example, since the belt length and surface velocity are known, by measurement, a precise distance on the belt away from the home position indicator may be determined by sensing the home position indicator and then by measuring elapsed time. Other such examples can be deduced from the foregoing description. 
     As for a physical embodiment of the home position indicator, the indicator  75  may be realized as a notch or a hole punched in the transfer belt  20  or as indicia printed, adhered, painted, etc., on the belt. The indicator  75  may also be realized as a magnetic or an electrostatic device. While the first and second home position sensors  70 ,  71  are shown as part of the transfer belt subassembly  15  in this embodiment, the home position sensors  70 ,  71  could also be located external to the subassembly  15 . The home position sensors  70 ,  71  must be able to detect the presence of the home position indicator  75 . Thus, when the home position indicator  75  comprises a hole punched in the transfer belt  20 , an optical sensor may be used to detect the presence of the hole. When painted, adhered, or printed indicia are used to indicate the home position a reader must be used to sense the presence of the indicia. Similarly, when a magnetic or electrostatic device is used as the home position indicator a sensor sensitive to the magnetic or electrostatic device is used to determine the presence of the home position indicator  75 . 
     The subassembly  15 , after having its measurement and calibration data determined and stored in memory, is installed in a printer. The data from the subassembly storage device  80  is utilized by the controller  94  of the printer to control the motor  30  to correct the belt speed or belt position based on the previously stored measurement and calibration data in accordance with a pre-programmed algorithm which interprets the parametric correction data from the storage device  80 . In response to the home position sensors  70 ,  71  detecting the home position indicator  75 , and the data in the memory  80 , the controller  90  produces a signal that modulates the speed of the drive motor  30 . The drive motor  30  may be a brushless D.C. motor with encoder feedback, a brush D.C. motor with encoder feedback, a stepper motor, or a stepper motor with encoder feedback. The drive motor  30  drives the drive pulley  40  to provide movement of the transfer belt  20  around the belt path in accordance with the measurement and calibration data. Additionally, the start-of-scan delay for each color print head  95 - 98  is determined in order to provide for lateral alignment of the deposited color planes. Accordingly, the registrations of the various color planes transferred to the transfer belt  20  are precise, resulting in the production of a high quality image. 
     In a preferred embodiment, the transfer belt subassembly  15  is a field replaceable unit. That is to say that the subassembly is a self-contained unit within an image forming apparatus that may be replaced independently of other subassemblies of the apparatus, such as the cartridges, for example. As such, a worn transfer belt subassembly  15  can be easily replaced with another subassembly that also has its own stored calibration data. The printer can use the new subassembly without the need to be recalibrated while still providing a high quality image. 
     Referring now to FIG. 3, a flowchart showing a method  100  of providing transfer belt correction is provided. A first step  110  of the method comprises providing a transfer belt subassembly. The subassembly is manufactured and assembled as a separate field-replaceable unit. 
     During the next step  120 , calibration data relating to the transfer belt subassembly is obtained and stored in a memory. The memory is a non-volatile memory which is included as part of the subassembly. The calibration data is preferably of the type previously described. 
     The next step  130  comprises installing the subassembly into a printer. The installation could be into a new printer or as a replacement for a worn subassembly. 
     At step  140  a variable speed motor drives the transfer belt of the subassembly. The motor engages a drive pulley of the subassembly that in turn causes the transfer belt to traverse along the belt path. 
     At the following step  150 , a sensor or sensors detect the home position indicator of the belt. This provides a reference point for the calibration data with respect to the transfer belt. 
     At step  160  the belt positioning is controlled by a controller which provides a signal in response to the detection of the home position indicator by the sensor or sensors and the calibration data from the memory. As such, the calibration data is used dynamically to correct for belt positioning errors of the subassembly according to the particular characteristics of the subassembly. 
     By way of the above described apparatus and method, errors associated with transfer belt positioning are removed or significantly reduced. The complexity, cost, measurement time and toner waste associated with measuring and characterizing the assembly within the printer are eliminated. By including the memory device as part of the transfer belt subassembly, the transfer belt subassembly can be removed and a replacement installed without having to recalibrate the printer, while maintaining highly precise color plane registration on the transfer belt. 
     Having described preferred embodiments of the present invention it should be apparent to those of ordinary skill in the art that other embodiments and variations of the presently disclosed embodiment incorporating these concepts may be implemented without departing from the inventive concepts herein disclosed. Accordingly, the invention should not be viewed as limited to the described embodiments but rather should be limited solely by the scope and spirit of the appended claims.