Abstract:
A constant torque split is maintained between a pair of drive motors for the photoreceptor belt of an electrophotographic printing machine. By varying the voltage applied to the motors according to the speed of the photoreceptor belt, the torque applied by each motor can be continuously balanced at a predetermined relationship to apply a constant torque and the desired speed may be accurately maintained. To better refine the implementation, the relationship include a ratio and an offset which may be applied, to one of the motors. Furthermore, this offset is ramped up during motor acceleration to optimize motion quality and system performance.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a dual motor system for driving a photoreceptor belt with a balanced torque to improve image registration in an electrophographic electrophotographic imaging system. 
     2. Brief Description of Related Developments 
     Electrophotographic printing machines employ photoreceptor members, typically in the form of a belt that is electrostatically charged to a potential so as to sensitize the surface thereof. The charged portion of the belt is exposed to a light image of an original document being reproduced. Exposure of the charged member selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image corresponding to the informational areas contained within an original document. After the electrostatic latent image is recorded on the photoreceptor member, a developer material is brought into contact therewith to develop the latent image. The electrostatic latent image may be developed using a dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto or using a liquid developer material. Toner particles are attracted to the latent image, forming a visible powder image on the surface of the photoreceptor belt. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a substrate, such as a sheet of paper. Thereafter, the toner image is heated to permanently fuse the image to the substrate. 
     In order to reproduce a color image, the printing machine includes a plurality of imaging stations each of which deposits a toner of a given color. Each station has a charging device for charging the photoreceptor surface, an exposing device for selectively illuminating the charged portions of the photoreceptor surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the electrostatic latent image. The images are developed, at least partially, in superimposed registration with one another to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a substrate. The transferred multi-color image is then permanently fused to the sheet forming the color print. To obtain a high quality color image, registration of the images at each of the developer stations is essential. 
     Registration is achieved by accurately positioning the photoreceptor belt at the various imaging and developing stations along the belt path using a drive mechanism that typically comprises drive rollers that advance a substrate along the path and backer bars that support the belt. Many such drive rollers have a coating commercially known as an EPDM elastomer that is applied to the surface thereof to improve friction coupling between the drive mechanism and the belt. Due to backer bar and subsystem drag, the drive rollers often experience slippage at the photoreceptor belt and at other locations along the belt when the surface of the drive roller encounters particle contamination. Slippage has a deleterious impact on image registration, particularly when latent images are applied at multiple imaging stations. 
     An auxiliary belt drive may address slippage problems, but in order to be effective, the torque level and proper location of the auxiliary drive is essential to attain optimum drive benefit while at the same time satisfying motion quality and registration requirements of the imaging system. In addition, belt tensioning and drive capacity requirements must also be met. 
     One solution to the slippage problem is presented in U.S. Pat. No. 6,421,523 which issued to the same assignee as this application. This patent describes a belt drive module that achieves the above goal by providing a torque assist drive that applies a torque assist force to the belt at a location between the drive roller and the tension roller. In this instance the torque assist force is provided by a constant torque friction clutch or a current limited DC motor. This system operates in a torque limiting manner. 
     Image registration may be more difficult in designs where low friction between the drive roll and the belt occurs due to a large wrap angle. In these situations dual drive rolls are needed to apply the required torque to the photoreceptor belt. It a purpose of this invention to provide a dual roll drive mechanism for a photoreceptor belt. It is also a purpose of this invention to distribute the torque between the drive rolls in a predetermined manner to maintain a constant torque on the belt. 
     SUMMARY OF THE INVENTION 
     The drive system of this invention consists of a pair of brushless motors, a first motor provides a main drive torque and a second motor provides a supplemental drive torque. The second drive motor distributes the applied torque according to a predetermined function of the main drive. A constant torque split is maintained between the drive motors by holding the ratio of the torque applied by each motor constant. By varying the voltage applied to the motors according to the speed of the photoreceptor belt, the torque applied by each motor can be continuously balanced at a predetermined ratio to apply a constant cumulative torque and the desired speed may be accurately maintained. In order to further optimize motion quality and performance of the system, an additional predetermined amount of voltage is applied to the assist motor referred to as offset. The offset magnitude is ramped as the motor accelerates and reaches its full magnitude when the system achieves its desired steady state speed. Ramping the offset value allows the system to avoid oscillations and instability that could otherwise occur at start up. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The drive system of this invention is explained in more detail below with reference to the accompanying drawing, in which: 
         FIG. 1  shows a belt drive module of an electrophotographic imaging system to illustrate an environment in which the present invention may be deployed. 
         FIG. 2  is a schematic illustration of the drives system of this invention; 
         FIG. 3  is a block diagram of a control circuit for applying power to the drive motors of this invention; 
         FIG. 4   a  is a graph of the input voltages to the drive motors of this invention; and 
         FIG. 4   b  is a graph of the offset increment supplied to the assist drive motor of this invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     As an illustration of the context of the system of this invention, a single pass multi-color printing machine is shown in FIG.  1 . This printing machine employs a photoreceptor belt  10 , supported by a plurality of rollers or backer bars  12 . Belt  10  advances in the direction of arrow  14  to move successive portions of the external surface of photoreceptor belt  10  sequentially along a path including various image processing stations. 
     The illustrative printing machine includes five image recording stations indicated generally by the reference numerals  16 ,  18 ,  20 ,  22 , and  24 , respectively. Initially, belt  10  passes through image recording station  16 . Image recording station  16  includes a charging device and an exposure device. The charging device includes a corona generator  26  that charges the exterior surface of belt  10  to a relatively high, substantially uniform potential. After charging of the exterior surface of photoreceptor belt  10 , the charged portion thereof advances to an exposure device. The exposure device includes a raster output scanner (ROS)  28 , which illuminates the charged portion of the exterior surface of photoreceptor belt  10  to record a first electrostatic latent image thereon. 
     Developer unit  30  develops this first electrostatic latent image. Developer unit  30  deposits toner particles of a selected color on the first electrostatic latent image. After the highlight toner image has been developed on the exterior surface of belt  10 , belt  10  continues to advance in the direction of arrow  14  to a second image recording station  18  where the imaging process is repeated at recording stations  18 ,  20 ,  22 , and  24 , as described in incorporated U.S. Pat. No. 5,946,533, assigned to the same assignee hereof. Recording stations  18 ,  20 ,  22 ,  24  include components similar to recording station  16 , but are arranged to deposit a different color toner. 
     At each recording station, a latent image is recorded in registration with the previous latent image. Photoreceptor belt  10  ultimately advances the multi-color toner powder image to a transfer station, indicated generally by the reference numeral  56 . At transfer station  56 , a receiving medium, i.e., paper, is advanced from stack  58  by a sheet feeder and guided to transfer station  56 . At transfer station  56 , a corona generating device  60  sprays ions onto the backside of the paper. This attracts the developed multi-color toner image from the exterior surface of photoconductive belt  10  to the sheet of paper. Stripping assist roller  66  contacts the interior surface of photoconductive belt  10  and provides a sufficiently sharp bend thereat so that the beam strength of the advancing paper strips from photoreceptor belt  10 . A vacuum transport moves the sheet of paper in the direction of arrow  62  to fusing station  64 . 
     Fusing station  64  includes a heated fuser roller  70  and a backup roller  68 . The back-up roller  68  is resiliently urged into engagement with the fuser roller  70  to form a nip through which the sheet of paper passes. In the fusing operation, the toner particles coalesce with one another and bond to the sheet in image configuration, forming a multi-color image thereon. After fusing, the finished sheet is discharged to a finishing station where the sheets are compiled and formed into sets, which may be bound to one another. These sets are then advanced to a catch tray for subsequent removal therefrom by the printing machine operator. 
     Invariably, after the multi-color toner powder image has been transferred to the sheet of paper, residual toner particles remain adhering to the exterior surface of photoreceptor belt  10 . The photoreceptor belt  10  moves over isolation roller  78 , which isolates the cleaning operation at cleaning station  72 . At cleaning station  72 , the residual toner particles are removed from belt  10 . The belt  10  then moves under spots blade  80  to also remove toner particles therefrom. 
     A drive system  101  for a photoreceptor belt  102 , according to this invention, is shown schematically in FIG.  2  and is constructed having a main drive motor  103 , an assist drive motor  104 , and a steering motor  105 . The drive motors  103  and  104 , are operatively connected to rollers  107  and  108  respectively to rotate the rollers. Photoreceptor belt  102  is wrapped around the rollers  107 - 109  under tension for rotation, driven by the motors  103  and  104  in the direction of arrow  110 . An encoder  106  is positioned in contact with the belt  102  to generate a signal indicative of the actual belt speed ω. The steering motor  105  is a stepping motor which is connected independently to adjust the tilt angle of roller  109  in response to control processor  111 . The tilt angle of roller  109  causes a force to be applied to the belt that has a component transverse to the primary direction  110  of belt movement. Steering motor  105  is controlled to prevent sideways walking of the belt and to maintain alignment of belt  102  on the rollers  107 - 109 . Edge position sensors (not shown) may be used to provide a feedback signal to the control processor  111  for the required tilt compensation. 
     Drive motors  103  and  104  can be brushless motors selected to provide the required torque to the rollers  107  and  108  respectively at available voltage levels. Control processor  111  adjusts the input voltage  114  (see  FIG. 4   a ) to main drive motor  103  in response to actual speed signals from encoder  106 . The belt  102  is driven by the combined torque of motors  103  and  104 , the applied torque is split between motors  103  and  104  at a predetermined function. The voltage  114  is therefore adjusted to obtain and maintain a torque contribution from motors  103  and  104  which will result in a predetermined operating speed for photoreceptor belt  102 . 
     Assist motor  104  is driven by a voltage  115 , which is provided at a percentage of voltage  114  by amplifier  112 . In this manner the applied torque is split between rollers  103  and  104  according to a predetermined function. 
     The control system for the motors  103  and  104  is shown schematically in the block diagram of FIG.  3 . Control processor (Compensator circuit)  111  generates a pulse width modulated signal to drive the main drive motor  103  and the assist drive motor  104 . The dual drive system  101  of this invention is particularly advantageous where the wrap angle of the belt  102  is large, thereby limiting the frictional engagement with the rollers  107 - 109 . Compensator circuit  111  includes firmware  116 , such as an ASIC, having an imbedded algorithm that calculates the required voltage that will provide the desired torque according to the characteristic torque profile of the motors used. 
     The motors  103  and  104  respond with a combined output torque in accordance with the duty cycle of the pulse width modulator signal  114 , which is adjusted, depending on the desired speed of the belt  102 . A feed back signal from encoder  106 , allows the actual belt speed to be monitored and the duty cycle of the drive signal  114  is adjusted if needed. As stated above, the main drive motor  103  receives the adjusted signal. 
     Assist motor  104  is driven by voltage  115  which is a function of the voltage applied to the main drive motor  103 . This function consists of a ratio or percentage of the main drive motor voltage plus an offset  113 . The ratio remains fixed to maintain a constant torque to the belt rollers  107  and  108 . The offset  113  is ramped in the same manor that the motor is ramped during acceleration. As shown in  FIG. 4   b , the offset  113  reaches its full magnitude when the belt encoder  106  indicates the operational belt speed. This optimizes motion quality and belt performance as the main drive motor  103  starts and reaches its destination operating speed. The assist drive signal to motor  104  therefore is governed by the relation V 15 =V 14 *K+b, where K is the assist ratio and b is the offset value. 
     As shown in the graph of  FIG. 4   a , an available supply voltage of, for example 36 volts, may be varied by adjusting the pulse width modulated drive signal  114  for different duty cycles, i.e. 100%=36 volts, 50%=18 volts, etc. As shown in  FIG. 3 , the assist motor drive signal is obtained from the output of the compensator  111  and adjusted by a fixed percentage, for example 70%, by amplifier  112 . The offset voltage  117  varies with belt speed according to a predetermined acceleration profile, for example as shown in  FIG. 4   b , as an addition to the drive voltage input for assist motor  104 . 
     Applied voltage  114  can be determined by the torque characteristics of the motors  103  and  104 . The overall applied torque is determined by the speed required for belt  102 . The applied torque is the combined torque (T 3 +T 4 =T applied ) contributed by motors  103  and  104 . In general the voltage needed to generate the applied torque can be calculated for a given speed of belt  102  by the relation: Torque=Kt*(V INPUT −(Kv*ω). Through this relationship a linear relation can be derived between the voltage input to drive motor  103 , for a given torque T applied , and the voltage input to the assist motor  104 . The imbedded algorithm of control processor  111  takes into consideration the difference between actual speed and desired speed according to a compensator routine to obtain voltage for motor  103 . The assist motor voltage  115  is calculated by applying the ratio plus the variable offset. In some circumstances, it may be desirable to apply a negative offset, for example, a mirror image of ramp  117  of  FIG. 4   b.    
     While the present invention is described in connection with preferred embodiments thereof, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims.