Phase Control of a Seamed Photoreceptor Belt

This invention relates generally to an electrophotographic printing machine having a seamed, web-type photoreceptor suitable for the exposure of one or more document images on the surface thereof, and more particularly to a method and apparatus for controlling the location of the photoreceptor seam in relation to the document images.

The features of the present invention may be used in the printing arts and, more particularly in electrophotographic printing. In the process of electrophotographic printing, a photoconductive surface is charged to a substantially uniform potential. The photoconductive surface is then image-wise exposed to record an electrostatic latent image corresponding to the informational areas of an original document being reproduced. Thereafter, a developer material is transported into contact with the electrostatic latent image. Toner particles are attracted from the carrier granules of the developer material onto the latent image. The resultant toner powder image is then transferred from the photoconductive surface to a copy sheet and permanently affixed thereto. The foregoing description generally describes a typical single color electrophotographic copying machine.

A typical machine of this type would be the Xerox.RTM. 1090.RTM. copier. Such a machine employs a mechanical rephaser to control the position of the photoreceptor seam with respect to the exposed or latent image areas of the photoreceptor. Generally, the rephaser is a gear box having two speeds for control of the speed at which the copy sheet is advanced as it is brought into registration with the latent image on the photoreceptor. Generally, the copy sheet transport system is used to trigger the exposure mechanism which creates the latent image on the photoreceptor. Periodically, once per photoreceptor revolution, the location of the photoreceptor seam is sensed and the rephaser is energized or deenergized for a period of time necessary to correct for the positioning of the advancing copy sheet, and in turn, the position of the latent image on the photoreceptor web. More specifically, the photoreceptor belt is moved at a predefined velocity, and the rate of travel of the advancing copy sheet is controlled so as to regulate the exposure and transfer operations in accordance with the position of the advancing sheet. Minor variations in the speed of the main drive motor, due to variations in the power line voltage, result in a variation of the position of latent images on the photoreceptor. Unfortunately, these variations are cumulative in nature and must be corrected to assure that the latent images are exposed at generally the same position on the photoreceptor each time. If not corrected, the cumulative variation would eventually cause one of the exposed latent image areas to occur over the photoreceptor seam, subsequently resulting in an unacceptable copy.

Such a system works well for typical single color systems, such as the Xerox.RTM. 1090.RTM. copier, but lacks the reliability for accurate velocity and position control of the photoreceptor required in multicolor development systems. Also, after significant variations have occurred in the photoreceptor velocity, resulting in the mis-positioning of the photoreceptor seam, the system may require a "dead" or nonoperative cycle, during which the copier once again repositions the seam to the interdocument region. Furthermore, the rephaser mechanism is a relatively expensive apparatus which provides the mechanical drive linkage between the photoreceptor drive and the copy sheet transport system. Hence, a more flexible and less costly drive system would be desirable.

Another technique used to control two moving members in a reprographic system is illustrated by U.S. Pat. No. 3,917,400 to Rodek et al. (Issued Nov. 4, 1975) which discloses a method and apparatus for maintaining a predetermined phase relationship between signals representing the velocity of a first variable velocity movable member and the velocity of a second constant velocity movable member. A first sensor emits a pulse signal whenever one of a plurality of registration marks on the variable velocity movable member passes the sensor, and similarly, a second sensor emits a pulse whenever one of a plurality of registration marks on the constant velocity movable member passes the second sensor. A phase relationship between the two movable members is determined by measuring the phase relationship between the occurrence of the pulse signal of the first sensor and the pulse signal of the second sensor. A control signal, related to the phase relationship, is generated and is utilized to vary the velocity of the variable velocity movable member so that a predetermined phase relationship (i.e. zero phase difference) is established for the two signals. Furthermore, a portion of the control signal generated to reduce the signal to zero is used to reduce a subsequent phase difference calculation to zero, thereby compensating for the fact that the velocity of the variable velocity movable member is still being adjusted as the subsequent difference calculation is being made.

A related method of positioning an electrostatic latent image on a photoconductive belt is described in U.S. Pat. No. 4,980,723 to Buddendeck et al. (Issued Dec. 25, 1990), and is hereby incorporated by reference for the teachings therein. The reference discloses a system capable of adjusting the number of latent image regions which are exposed on the photoconductive belt. More specifically, a portion of the inter-image zone is utilized to accomodate the shifting of the latent image positions on the belt. Furthermore, a control system for automatically altering the pitch, or number of latent images on a photoconductive belt, during operation is taught by U.S. Pat. No. 4,588,284 to Federico et al. (Issued May 13, 1986), where a memory flag is monitored to control the selection of a different number of pitches. The flag is also used to control the clock signals used for the timed actuation of events with respect to the selected pitch. The relevant portions of U.S. Pat. No. 4,588,284 to Federico et al. are hereby incorporated by reference.

The present invention seeks to overcome the limitations of the mechanical rephaser type control system, by mechanically decoupling the photoreceptor drive from the copy sheet registration and transport drives. Moreover, the present system has the added advantage of being able to control the phase relationship between two independently variable elements, the photoreceptor speed and the advancing copy sheets, in a reliable manner.

In accordance with one aspect of the present invention, there is provided a method for controlling the velocity of the photoreceptor within a reprographic machine of the type having a seamed, web type photoreceptor, for producing a plurality of developed images thereon, said developed images being separated by unexposed interdocument regions or zones on the photoreceptor, and means for registering copy substrates with the developed images. The method of assuring that the seamed region of the photoreceptor lies within an interdocument region begins by first sensing an actual phase relationship between the photoreceptor seam and the activity of the sheet registration apparatus. The method then calculates a phase error value by comparing the actual phase relationship between the photoreceptor seam and the registration apparatus to a desired phase relationship. As the next step, the system determines a new photoreceptor speed as a function of the phase error. Finally, the photoreceptor is accelerated or decelerated to a new constant velocity during interdocument gaps. The new constant velocity remains in effect during the subsequent exposure of the latent images. During operation of the reprographic machine, the above steps are executed once per revolution of the photoreceptor.

Pursuant to another aspect of the present invention, there is provided an electrophotographic printing machine of the type having a seamed, web type photoreceptor, for producing a plurality of developed images thereon, where the developed images are separated by unexposed interdocument zones on the photoreceptor. The machine also has an independently driven copy substrate registering apparatus for registering copy sheets in synchronization with the developed images on the photoreceptor. Included in the machine are phase measurement means for quantizing the phase relationship between the photoreceptor seam and an edge of the advancing copy sheets, and phase error calculating means for determining the variation in the phase relationship with respect to a desired phase relationship. Also included is a controller for adjusting the photoreceptor speed as a function of the phase error, during the interdocument zones, and then driving the photoreceptor at a constant velocity during image exposure.

FIG. 1 is a schematic elevational view of an illustrative electrophotographic printing machine having the photoreceptor drive unit incorporating the present invention;

FIG. 2 is a perspective view of the photoreceptor and sheet registration apparatus of the present invention illustrating the locations and relationships of the active sensor elements of the present invention;

FIG. 3 is a schematic illustration of the photoreceptor drive unit incorporating the elements of the present invention;

FIG. 4 is a functional block diagram illustrating the control elements and interconnections associated with the photoreceptor drive unit;

FIG. 5 is a block diagram illustrating the control operations directly associated with the photoreceptor;

FIG. 6A is an illustration of the timing signals used to determine the phase error of the photoreceptor;

FIG. 6B is an illustration of the velocity profile of the photoreceptor to correct for phase error; and

FIGS. 7A and 7B are flowcharts of the control processes used to initially position the photoreceptor seam, and to maintain the position of the seam with respect to the interdocument zone, respectively.

The present invention will be described in connection with a preferred embodiment, however, it will be understood that there is no intent to limit the invention to that embodiment. On the contrary, the intent is 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.

The following description makes reference to a collection of Appendices (A-D) which are included with this specification, the contents of which may be briefly characterized as follows:

Appendix A is a listing of the microcontroller assembly code for the main module of the servomotor control software, which serves as a background loop for many of the other modules and calls procedures listed in Appendices B, C, and D;

Appendix B is an assembly code listing of the the module associated with maintaining the positional relationship between the latent image and the belt seam, which utilizes positional information gathered during microcontroller interrupts to determine the position or phase error;

Appendix C is an assembly code listing for the motor control software; and

Appendix D is a listing of the assembly code for the interrupts which are processed by the microcontroller.

For a general understanding of the illustrative electrophotographic printing machine incorporating the features of the present invention, reference is made to the drawings. In the drawings, like reference numerals have been used throughout to designate identical elements. FIG. 1 schematically depicts the various components of an electrophotographic printing machine incorporating the photoreceptor drive controller of the present invention. Although the photoreceptor drive control of the present invention is particularly well adapted for use in the illustrative printing machine, it is equally well suited for use in a wide variety of printing machines.

Referring now to FIG. 1, the two color electrophotographic printing machine employs a belt 20, i.e. a charge retentive member, having a photoconductive surface deposited on a conductive substrate. In one embodiment, the photoconductive surface is made from a trigonal selenium alloy with the conductive substrate being made preferably from an electrically grounded aluminum alloy. Belt 20 moves in the direction of arrow 22 to sequentially advance successive portions through the various processing stations disposed about the path of movement. Belt 20 is entrained about tensioning roller 24, encoded drive roller 26, and stripping roller 30. Motor 32 rotates roller 26 to advance belt 20 in the direction of arrow 22. Roller 26, coupled to motor 32 by suitable means such as a belt drive, is further coupled to an encoder (not shown) so that the velocity of the roller may be monitored.

Initially, successive portions of belt 20 pass through charging station A, where a corona discharge device, such as a scorotron, corotron or dicorotron indicated generally by the reference numeral 34, charges the belt 20 to a selectively high uniform positive or negative potential. Preferably, the the photoreceptor is charged to a negative potential. Any suitable control, well known in the art, may be employed for controlling corona discharge device 34.

Next, the charged portions of the photoconductive surface are advanced through exposure station B. At exposure station B, the uniformly charged photoconductive surface or charge retentive surface is exposed to a laser based input and/or output scanning device 36 which causes the charge retentive surface to be discharged in accordance with the output from the scanning device. Preferably the scanning device is a three level laser Raster Output Scanner (ROS). An electronic sub system (ESS) 38 provides the control electronics which prepare the image data flow between the data source (not shown) and ROS 36. Alternatively, the ROS and ESS may be replaced by a conventional light/lens exposure device. The photoconductive surface, which is initially charged to a high charge potential, is discharged image wise in the background (white) image areas and to near zero or ground potential in the highlight color (i.e. color other than black) parts of the image.

At development station C, a magnetic brush development system, indicated generally by the reference numeral 42 advances developer materials into contact with the electrostatic latent images. The development system 42 comprises first and second developer units 44 and 46, respectively. Preferably, each magnetic brush developer unit includes a pair of magnetic brush developer rollers mounted in a housing. Thus, developer unit 44 contains a pair of rollers 48, 50, and developer unit 46 contains a pair of magnetic brush rollers 54, 56. Each pair of rollers advances its respective developer material into contact with the latent image. Appropriate developer biasing is accomplished via power supplies (not shown) electrically connected to the respective developer units 44 and 46.

Color discrimination in the development of the electrostatic latent image is achieved by moving the latent image recorded on the photoconductive surface past two developer units 44 and 46 in a single pass with the magnetic brush rolls 48, 50, 54 and 56 electrically biased to voltages which are offset from the background voltage, the direction of offset depending on the polarity of toner in the developer housing. First, developer unit 44 develops the discharged areas of the latent image with colored developer material having triboelectric properties such that the colored toner is driven to the discharged image areas of the latent image by the electrostatic field between the photoconductive surface and the electrically biased developer rolls. Conversely, second developer unit 46, develops the highly charged image areas of the latent image. This developer unit contains black developer material having a triboelectric charge such that the black toner is urged towards highly charged areas of the latent image by the electrostatic field existing between the photoconductive surface and the electrically biased developer rolls in the second developer unit.

Because the composite image developed on the photoreceptor consists of both positive and negative toner, a negative pre-transfer corona discharge member 58 is provided to condition the toner for effective transfer to a sheet using a positive corona discharge

A sheet of support material, 60, is moved into contact with the toner image at transfer station D. The sheet of support material is advanced to transfer station D by sheet transfer apparatus 62. Preferably, the sheet transfer apparatus receives sheet 60 from a sheet feeding apparatus (not shown) which includes a feed roll contacting the uppermost sheet of a stack of copy sheets (not shown). The feed rolls rotate so as to advance the uppermost sheet from the stack into the sheet transfer apparatus which directs the advancing sheet of support material into contact with the photoconductive surface of belt 20 in a timed sequence so that the composite toner powder image contacts the advancing sheet of support material at transfer station D.

Transfer station D includes a corona generating device 64 which sprays ions of a suitable polarity onto the backside of sheet 60. This simultaneously attracts the black and non-black portions of the toner powder image from belt 20 to sheet 60. After transfer, the sheet continues to move, in the direction of arrow 66, onto a conveyor (not shown) which advances the sheet to fusing station E.

Fusing station E includes a fuser assembly, indicated generally by the reference numeral 68, which permanently affixes the transferred powder image to sheet 60. Preferably, fuser assembly 68 comprises a heated fuser roller 70 and a pressure roller 72. Sheet 60 passes between fuser roller 70 and pressure roller 72 with the toner powder image contacting fuser roller 70. In this manner, the toner powder image is permanently affixed to sheet 60. After fusing, a chute (not shown) guides advancing sheet 60 to a catch tray (not shown) for subsequent removal from the printing machine.

After the sheet of support material is separated from the photoconductive surface of belt 20, the residual toner particles carried by the non-image areas of the photoconductive surface are charged to a suitable polarity and level by preclean charging device 74 to enable their removal. These particles are removed at cleaning station F where a vacuum assisted, electrostatic brush cleaner unit 78 is disposed. In the cleaner are two fur brush rolls that rotate at relatively high speeds creating mechanical forces that sweep the residual toner particles into an air stream provided by a vacuum source (not shown), then into a cyclone separator, and finally into a waste bottle. In addition, the brushes are triboelectrically charged to a very high negative potential which enhances the attraction of the residual toner particles to the brushes and increases the cleaning performance. Subsequent to cleaning, a discharge lamp (not shown) floods the photoconductive surface with light to dissipate any residual electrostatic charge remaining prior to the charging thereof for a successive imaging cycle.

Referring now to FIG. 2, which further details the active mechanical and electrical components of the photoreceptor and sheet transfer apparatus, sheet 60 is shown entering the input side of sheet transfer apparatus 62. As sheet 60 enters transfer apparatus 62 it is initially maintained between upper and lower guides 102 and 104 respectively. Advancing into the chute formed between guides 102 and 104, the sheet is engaged by a feed nip which is formed by idler rolls 106a, b in contact with transport belts 108a, b, respectively. Once engaged by the feed nip, the sheet is advanced further into the chute where it contacts fingers 110a, b of the registration switch (not shown), thereby producing an electrical signal ("registration fingers") indicating the position of the lead edge of the copy sheet.

At this time, copy sheet 60 is also forced against side registration edge 114 to enable the accurate registration of the sheet in a direction normal to the process direction. The sheet is forced against the registration edge by rotational motion of frictional ball element (not shown), as is commonly used to register sheets in paper transport systems.

Having been side registered, the sheet is subsequently advanced towards photoreceptor belt 20, where it will meet in synchronization with developed latent image area 116 thereon. To avoid having seam 118 of belt 20 within one of the latent image areas 116, the position, or velocity, of the belt must be carefully controlled. To accomplish such a rigorous positioning requirement, timing or belt hole 120, having been cut into belt 20 at a predetermined displacement from seam 118, is carefully monitored to determine the position of the seam. Alternatively, the location of the photoreceptor seam may be indicated with a notch, a raised bump, or other readable mark applied to the surface of photoreceptor 20. Belt hole sensor 122, preferably an optoelectronic sensor, detects the presence of belt hole 120 once per revolution of the belt. As belt 20 rotates, the position of seam 118 is maintained within the gap or interdocument zone (IDZ) 126 that exists between the latent electrostatic images thereon, by carefully controlling the velocity, and position of the belt during each revolution.

Referring also to FIGS. 3 and 4, which further illustrate the electrical components and connections of the present invention, belt hole sensor 122 provides a direct interrupt input to photoreceptor servo printed wiring board (PWB) 140, thereby interrupting microcontroller 142, preferably an Intel.RTM. 8098.RTM. microcontroller, via the external interrupt (EXTINT). In addition, registration fingers switch 144, which is coupled to registration fingers 110a, b, is connected to high speed interrupt No. 3 (HSI-3) on the microcontroller, thereby enabling the recording of the "time" at which each registration fingers signal is received by microcontroller 142. The output of photoreceptor encoder 146, coupled to drive roller 26, is also input as an interrupt to microcontroller 142, via high speed input No. 0 (HSI-0), and periodically indicates the change in position of photoreceptor belt 20.

Also contained on PWB 140 is motor driver 150, preferably a Sprague.RTM. UDN 2936-120 driver, which provides the interface between microcontroller 142 and the three phase brushless servomotor, via the microcontroller 8-bit pulse width modulator (PWM) 152. Although not show, brushless DC motor 32 is controlled by the selective energization of two of the existing three coils in the motor at any specific time. By selectively altering the coil pairs that are energized the motor is caused to rotate. In addition, signals from Hall effect sensors, contained within motor 32, are monitored by motor driver 150 to determine which coil pairs should be energized. The speed at which motor driver 150 causes motor 32 to operate is controlled by the output of PWM 152 in a conventional manner.

EPROM 154 and address latch 156 are also contained on PWB 140, and enable microcontroller 142 to operate on programmed instructions contained within the EPROM. EPROM 154 contains instructions enabling the microcontroller to carry out the velocity position control method illustrated by control blocks 158, 160, 162, and 164 of FIG. 3. More specifically, block 158 continuously measures the velocity of belt 20, using the encoder input on HSI-0. HSI-0 is used as a clock input to the High Speed Interrupt block of the 8098 microcontroller. Preferably, the input signal is provided from photoreceptor drive encoder 146 which is coupled to photoreceptor servo motor 32 which drives the photoreceptor via drive roll 26. Subsequently, the measured digital velocity, output from block 158, is summed at block 162 with the desired velocity output from velocity command block 160, the result being input to digital compensation block 164. Digital compensation block 164 operates on the difference signal input to it, and provides an output to the PWM which directly regulates the velocity of motor 32.

Referring now to FIG. 5, which illustrates the control blocks associated with controlling the location of photoreceptor seam 118 in relation to the interdocument zone, IDZ 126, execution of the control scheme depicted in the figure is carried out in microcontroller 142 of FIG. 4. Reference is also made to the timing diagram of FIG. 6A, which shows the relationship between the signals input to the microcontroller. Initially, the input signals from belt hole sensor 122, and registration fingers switch 142, are received by microcontroller 142. Using machine clock input 168, FIG. 4, as a time reference, phase detector block 180 determines the delay (FIG. 6A), in machine clock pulses, from the leading edge of belt hole sensor input 210 to the leading edge of the next registration fingers switch pulse, 212. In one embodiment, the delay is measured using a memory location which is zeroed upon the valid detection of the belt hole, and incremented by each succeeding machine clock interrupt pulse, thereby maintaining an accurate count of the elapsed machine clocks. Alternatively, the delay may be measured with any software or hardware type counters responsive to the machine clock signal. Phase detector block 180 subsequently outputs the delay value, which is added with the desired phase value at adder block 182 to produce a phase error value according to the following equation:

where 401 is the number of machine clock pulses that would normally occur when seam 118 lies in the center of IDZ 126.

The phase error value is then passed to controller logic block 184, where a control algorithm, preferably a proportional integrating algorithm, is used to determine the correction necessary in the speed of photoreceptor belt 20 to correct for the phase error. Subsequently, controller logic block 184 outputs a speed correction value which is added to the nominal speed value at adder block 186. The output from adder block 186, the photoreceptor speed reference value, is an input to velocity command block 160.

As illustrated in FIG. 6B, the photoreceptor speed is generally controlled to cause photoreceptor encoder 146 to produce an output of approximately 1309 cycles/sec, which represents the process speed of the photoreceptor. During normal operation, phase correction is accomplished by software called Electronic Phase Control (EPC). During this time, should the phase error be positive and increasing, the velocity of the photoreceptor will be incremented while the interdocument zone is passing through the imaging station B of FIG. 1. Similarly, if the phase error is negative and increasing, the velocity of the photoreceptor will be decremented during that time. Additionally a gross phase error correction algorithm call Initial Phase Alignment (IPA) is provided. It allows for large corrections in phase error that occur during startup/initialization. The IPA will set the reference velocity between 1000 and 1700 cycles per second until all the phase error has been corrected for, Velocity profile 240 in FIG. 6B shows a typical correction profile when the phase error is a large positive number. Similarly, velocity profile 242 in FIG. 6B shows a typical correction profile when the phase error is a large negative number. The reference speed is then reset to the nominal of 1309 and EPC is then activated.

Referring now to FIGS. 8A and 8B, in conjunction with Appendices A, B, C and D, where the flowcharts illustrate the operations associated with controlling the phase relationship. More specifically, Appendix A is the code listing for the MAIN1 module executed by microcontroller 142, which contains the control loop for the the photoreceptor drive motor 32. Appendix B contains the code listings for the INC.sub.-- POS.sub.-- ERROR module which is called by the MAIN1 module to maintain the phase, or positional relationship between the seam and the interdocument zones. Appendix C contains the listings for the MOTOR.sub.-- IO module which details the machine instructions associated with the interrupts and other I/O functions of microcontroller 142. Finally, Appendix D contains the code listings associated with the GENMOT module, including the various branches of the MOT.sub.-- SEQUENCER routine, which enable microntroller 142 to interface with motor driver 150 via the pulse width modulator outputs.

FIG. 8A details the steps associated with the initial establishment of the phase relationship whenever the rotation of photoreceptor belt 20 is begun. Beginning at start block 300, microcontroller 142 analyzes the status of the HSI.sub.-- STATUS to determine if the MOTOR.sub.-- ON bit is cleared. If not, the motor is expected to be running and the phase relationship to be maintained. Cycle-up block 302 is executed whenever the BELT.sub.-- STATUS, INIT.sub.-- FLAG bit is cleared, by calling the MOT.sub.-- SEQUENCE procedure. The MOT.sub.-- SEQUENCER procedure begins execution at the MOT.sub.-- INIT: label. Subsequently, rotation of the servomotor is begun via motor driver 150 of FIG. 4, as shown by block 304. An initialization delay is executed, block 306, by the STANDBY and MOTOR.sub.-- OL branches of the MOT.sub.-- SEQUENCER procedure. Generally, these branches enable the servomotor to reach the nominal operating speed. As indicated in the code listings of the Appendices, the motor speed and phase relationship are generally maintained via the software loop which begins at the SERVICE.sub.-- MOTOR: label (Appendix B).

Following the initialization of the servomotor, microcontroller 142 executes a series of operations designed to establish the desired phase relationship between seam 118, as indicated by belt hole 120, and the interdocument zones 126. Block 308 continuously checks for the signal from belt hole sensor 122, via the external interrupt (EXTINT), and once detected, reinitializes the value of SPEED.sub.-- CNTR to zero, which is represented by block 312. Subsequently, the SPEED.sub.-- CNTR variable is incremented for each machine clock input on microcontroller pin HSI-1. When the next registration fingers signal is received, from switch 142, as detected by test block 314, the value of SPEED.sub.-- CNTR is copied to the MC.sub.-- COUNT variable, thereby recording the actual number of machine clocks (mc) elapsed since the belt hole was sensed. As indicated by block 316, the measurement is made, thereby allowing the operations of block 318 to determine the phase error value (PHASE.sub.-- ERROR). As illustrated in Appendix B at label ERR.sub.-- CALC:, the MC.sub.-- COUNT value is subtracted from 401, and the result becomes the PHASE.sub.-- ERROR value.

After determining the phase error, the system then determines whether the magnitude of the error is within acceptable limits, block 320. If so, the Electronic Phase Control (EPC) is enabled by returning to the nominal velocity block, 326, and continuing execution of the EPC process of FIG. 7B. Otherwise, the Initial Phase Alignment (IPA) process of FIG. 7A is continued. In IPA process block 324, the error is first converted from machine clocks to photoreceptor clocks, since the closed loop algorithm tracks photoreceptor clocks. Secondly, the new reference speed is chosen to make up all of the position error within a one second profile. If this new reference speed is outside the range of 1000 HZ to 1700 HZ then a second reference speed is chosen which will make up all of the position within 2 seconds. Similarly, if the second reference speed is outside the 1000 HZ to 1700 HZ range, then a velocity profile and third reference speed is chosen that will correct for the position error within 3 seconds. Control is then passed to the EPC process shown in FIG. 7B.

In the Electronic Phase Control mode of FIG. 7B, phase error is calculated once per photoreceptor belt revolution, as illustrated by blocks 308 through 318. Subsequently, if the phase error is less than .+-.3 machine clocks the ref speed, NEW.sub.-- REF.sub.-- VALUE, is not altered, as illustrated by negative responses at blocks 330 and 332. If the absolute value of the phase error is greater than 3 machine clocks, but less than 30 machine clocks, the differential phase error, the present phase minus the phase measured from the previous correction, is determined. If the differential phase error is greater than three, and the absolute phase error is increasing, as detected by block 332, then the velocity is incremented or decremented to minimize that phase error, block 334.

Otherwise, the error is beyond reasonably correctable range, and the system must go into a nonfunctional or calibration mode, IPA block 336, to enable the reestablishment of the phase relationship. More specifically, block 336 represents the execution of two routines, the first being the code beginning at the CHECK.sub.-- MC.sub.-- COUNT: label, where the variation in the belt speed is reviewed, and the second being the reestablishment of the phase relationship, beginning once again with block 300 of FIG. A.

Having determined a valid NEW.sub.-- REF.sub.-- VALUE for the acceleration of the motor, during the IDZ, processing continues at block 338, where the microcontroller determines if operation of the motor is still required by the larger system. As in block 302 of FIG. 8A, this is determined by analyzing the MOTOR.sub.-- ON bit of the HSI.sub.-- STATUS input register. Should the bit be cleared, processing would continue at the STOP.sub.-- THE.sub.-- MOTOR: label of Appendix A, as represented by block 340. Otherwise, the looping structure of the control software enables the continuous monitoring of the phase relationship, once per belt revolution, to enable control of the relative delay between the belt hole sensor pulse and the registration fingers switch pulse, thereby controlling the relationship between the belt seam and the interdocument zones, and keeping the seam out of the exposed image areas on the photoreceptor.

In recapitulation, the latent image recorded on the photoconductive surface has charged image or document areas and interdocument zones therebetween. The phase control method and apparatus of the present invention enable the adjustment of the photoreceptor speed, while the interdocument zones travel through the imaging station, to compensate for any irregularities in the speed of the photoreceptor belt or the advancing copy sheet. The periodic adjustment to the photoreceptor speed does not impact the image quality of the system, but does provide the system with a means for correcting for the variations which are inherent in such systems.

It is, therefore, apparent that there has been provided in accordance with the present invention, a control apparatus and method for use in an electrophotographic printing or reprographic machine that fully satisfies the aims and advantages hereinbefore set forth. While this invention has been described in conjunction with a preferred embodiment thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications and variations that fall within the spirit and broad scope of the appended claims. ##SPC1## ##SPC2## ##SPC3## ##SPC4##