Abstract:
A method of activating an electrophotographic machine includes determining at least one condition of the electrophotographic machine. Power is applied to a polygon mirror. A time period required for the polygon mirror to accelerate from a stationary condition to a target rotational velocity is measured. The measured time period and data associated with the at least one condition are stored in a memory device. The polygon mirror is decelerated back to the stationary condition. Power is reapplied to the polygon mirror at a point in time. The measured time period and the data are used to determine when to begin at least one process in the electrophotographic machine relative to the point in time.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an electrophotographic machine, and, more particularly, to a method of minimizing a delay in printing a first page in an electrophotographic machine. 
     2. Description of the Related Art 
     On laser printers and copiers with a rotating mirror and laser, time to first page can be of critical importance. A laser printer typically has a polygon mirror that, during printing, rotates at a constant angular velocity. Time to first page is a function of paper path length, image/media velocity and warm-up time of the polygon mirror. Typically, warm-up time involves waiting until the mirror is rotating at a specified velocity. Prior to printing, the polygon mirror accelerates from 0 rpm to some terminal rate of angular velocity. 
     In order to minimize time to first page, some processes that cannot be completed before the polygon mirror has achieved constant speed must nevertheless be started before the polygon mirror has achieved constant speed. Such concurrent processes may include bias voltage stabilization, performing an erase cycle on a photoconductive drum, and performing a paper pick operation. Traditionally, before the processes are commenced, the angular velocity of the mirror increases rapidly and then a long delay time is allowed in order to guarantee that the velocity is exactly equal to the desired rate before the concurrent processes are complete. Imaging on the photoconductor cannot begin until the angular velocity exactly matches the terminal rate. This typically affects the “time to first page”, i.e., the time the user must wait to get the first printed page output from the printer. 
     What is needed in the art is a method of minimizing the delay in time to first page associated with mirror motor warm-up time. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of predicting an amount of time needed for polygon mirror warm-up. 
     The invention comprises, in one form thereof, a method of activating an electrophotographic machine. At least one condition of the electrophotographic machine is determined. Power is applied to a polygon mirror. A time period required for the polygon mirror to accelerate from a stationary condition to a target rotational velocity is measured. The measured time period and data associated with the at least one condition are stored in a memory device. The polygon mirror is decelerated back to the stationary condition. Power is reapplied to the polygon mirror at a point in time. The measured time period and the data are used to determine when to begin at least one process in the electrophotographic machine relative to the point in time. 
     An advantage of the present invention is that the delay in time to first page associated with mirror motor warm-up is minimized. 
     Another advantage is that, with the reduced delay in time to first page, there is less wear on the printhead since the total time that the polygon mirror is rotating is reduced. 
     Yet another advantage is that adequate controls are provided to guarantee that printing does not begin too early, which could cause print defects. 
     A further advantage is that there is very little cost associated with implementing the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
     FIG. 1 is a side sectional view of a multicolor laser printer which can be used in conjunction with the method of the present invention; 
     FIG. 2 is a cross-sectional view of one of the polygon mirrors of FIG. 1 reflecting a laser beam; and 
     FIG. 3 is a schematic view of the controller and one of the printheads of FIG.  1 . 
    
    
     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 
     Referring now to the drawings and, more particularly, to FIG. 1, there is shown one embodiment of a multicolor laser printer  10  including laser print heads  12 ,  14 ,  16 ,  18 , a black toner cartridge  20 , a magenta toner cartridge  22 , a cyan toner cartridge  24 , a yellow toner cartridge  26 , photoconductive drums  28 ,  30 ,  32 ,  34 , an intermediate transfer member belt  36  and a controller  37 . The controller is a combination of Application Specific Integrated Circuits (ASIC&#39;s), microprocessors, and firmware suited to the tasks described. 
     Each of laser print heads  12 ,  14 ,  16  and  18  projects a respective laser beam  38 ,  40 ,  42 ,  44  off of a respective one of polygon mirrors  46 ,  48 ,  50  and  52 . As each of polygon mirrors  46 ,  48 ,  50  and  52  rotates, it scans a respective one of reflected laser beams  38 ,  40 ,  42  and  44  in a scan direction, perpendicular to the plane of FIG. 1, across a respective one of photoconductive drums  28 ,  30 ,  32  and  34 . Each of photoconductive drums  28 ,  30 ,  32  and  34  is negatively charged to approximately −1000 volts and is subsequently discharged to a level of approximately −300 volts in the areas of its peripheral surface that are impinged by a respective one of laser beams  38 ,  40 ,  42  and  44 . During each scan of a laser beam across a photoconductive drum, each of photoconductive drums  28 ,  30 ,  32  and  34  is continuously rotated, clockwise in the embodiment shown, in a process direction indicated by direction arrow  54 . The scanning of laser beams  38 ,  40 ,  42  and  44  across the peripheral surfaces of the photoconductive drums is cyclically repeated, thereby discharging the areas of the peripheral surfaces on which the laser beams impinge. 
     The toner in each of toner cartridges  20 ,  22 ,  24  and  26  is negatively charged and is transported upon the surface of a developer roll biased to approximately −600 volts. Thus, when the toner from cartridges  20 ,  22 ,  24  and  26  is brought into contact with a respective one of photoconductive drums  28 ,  30 ,  32  and  34 , the toner is attracted to and adheres to the portions of the peripheral surfaces of the drums that have been discharged to −300 volts by the laser beams. As belt  36  rotates in the direction indicated by arrow  56 , the toner from each of drums  28 ,  30 ,  32  and  34  is transferred to the outside surface of belt  36 . As a print medium, such as paper, travels along either path  58  or duplexing path  60 , the toner is transferred to the surface of the print medium in nip  62 . 
     One embodiment of a polygon mirror  46  is shown in FIG. 2 as viewed in the direction of arrow  64  in FIG.  1 . Polygon mirror  46  is shaped as an octagon with eight reflective sides or facets  66 . As polygon mirror  46  rotates in the direction indicated by arrow  68 , laser beam  38  reflects off of facets  66  between points  70  and  72  toward photoconductive drum  28 . Thus, as polygon mirror  46  rotates in direction  68 , the reflected laser beam  38  is caused to scan across the peripheral surface of photoconductive drum  28  in scan direction  74 . 
     Printhead temperature, age and voltage are among the factors that affect the time required for the mirror motor to reach constant speed. Printer  10  identifies some of these initial conditions prior to printing and stores a recent history of the mirror motor ramp-up time as a function of the initial conditions. It is then possible to reduce the “time to first page” by more accurately predicting when the polygon mirror will rotate at the terminal speed. The concurrent processes, such as, for example, bias voltage stabilization, performing an erase cycle on a photoconductive drum, and performing a paper pick operation, can be started sooner while still being assured that the processes will not be completed before the polygon mirror reaches terminal speed. Printheads  12 ,  14 ,  16 ,  18  are structurally substantially identical. Accordingly, to simplify the discussion and for ease of understanding the invention, only the structure of printhead  12  will be described in detail below in relation to FIG.  3 . However, it is to be understood that the discussion that follows with respect to printhead  12  also applies to each of printheads  14 ,  16  and  18 . 
     Inside printhead  12  is a thermistor  76  (FIG. 3) whose resistance is proportional to the temperature of printhead  12 . Also inside printhead  12  is an optical HSYNCn or start-of-scan sensor  78 . Sensor  78  provides a pulse that is active once per facet  66  of mirror  46 . For an eight-sided mirror  46 , HSYNCn sensor  78  pulses eight times per revolution. HSYNCn sensor  78  pulses when the beam of a laser diode (not shown), caused by activation of the signal on the VIDEOn line, reflects off the surface of polygon mirror  46  and impinges upon HSYNCn sensor  78 . 
     Control lines STARTn, LOCKn and REFCLKn interconnect controller  37  and printhead  12 . REFCLKn carries a clock signal sent from controller  37  to printhead  12 . The clock is typically active while power is applied to printer  10 . Polygon mirror  46  rotates at a speed proportional to the clock signal. STARTn carries a signal that tells polygon mirror  46  to rotate or to stop. LOCKn carries a signal that is “high” when mirror  46  is not rotating near, i.e., within a predetermined range of, its terminal speed, and is “low” when mirror  46  is rotating near its terminal speed. 
     Controller  37  uses the information from thermistor  76  to determine if printhead  12  is hot or cold. Typically, printhead  12  accelerates faster if printhead  12  is warm. Controller  37  measures the output of thermistor  76  prior to activating mirror  46  in order to indicate the likely acceleration time and stabilization time of mirror  46 . This information assists printer  10  in minimizing the time before imaging can begin. 
     In a first embodiment of a method of the present invention of activating a printhead, controller  37  activates REFCLKn so that printhead  12  knows the target rotational speed. Controller  37  then drops STARTn low. Next, controller  37  measures the time from STARTn going active to LOCKn going active. Once the LOCKn signal is active, controller  37  knows that printhead  12  will be stable within a time period specified by the manufacturer. This time period is typically on the order of 0.5 second. If the time from STARTn to LOCKn exceeds some predetermined time limit, printhead  12  issues an error. 
     As an example, for a printhead at rest to reach 20,000 rpm, the time period between STARTn going active and LOCKn going active may be 1.6 seconds at 25° C. With the tolerance of 0.5 second for printhead  12  to reach “true lock”, the total time is 2.1 seconds for printhead  12  to reach a state when printing can begin. This time period of 2.1 seconds at 25° C. is stored in the nonvolatile random access memory (NVRAM)  80  and is used by controller  37  in subsequent activations of printhead  12 . 
     The next time controller  37  decides to print, it can use the knowledge of the previous result to determine that printing can begin in 2.1 seconds plus some small safety factor, such as 0.5 second. This assumes that the printhead temperature is not less than some predetermined value. The temperature signal output by thermistor  76  at print time can be used to make adjustments in the time period allotted between STARTn and true lock. Typically, this time period will be longer for a cold printhead (less than 25° C.) and shorter for a warm printhead. 
     Controller  37  drops STARTn (REFCLK is already running) and measures the time to LOCKn going low. This new value is stored in NVRAM  80  and is used to calculate the total time to reach true lock for the next activation of printhead  12 . 
     In a second embodiment of a method of the present invention of activating a printhead, controller  37  activates REFCLKn so that printhead  12  knows the target rotational speed. Controller  37  then drops STARTn low. Next, controller  37  continuously measures the time between HSYNCn falling edges. Once the LOCKn signal is active and the HSYNCn to HSYNCn time is within defined tolerances, controller  37  knows that printhead  12  is stable or truly locked. If the time from STARTn to truly locked exceeds some predetermined time limit, printhead  12  issues an error. 
     As an example, for a printhead at rest to reach 20,000 rpm, the time period between STARTn going active and LOCKn going active may be 1.6 seconds at 25° C. The time from HSYNCn to HSYNCn to fall within tolerances may be about 0.5 second. Thus, 2.1 seconds is the time from starting the motor until true lock occurs. This time period of 2.1 seconds at 25° C. is stored in the nonvolatile RAM (NVRAM)  80  and is used by controller  37  in subsequent activations of printhead  12 . 
     The next time controller  37  decides to print, it can use the knowledge of the previous result to determine that printing can begin in 2.1 seconds plus some small safety factor, such as 0.5 second. This assumes that the printhead temperature is not less than some predetermined value. The temperature signal output by thermistor  76  at print time can be used to make adjustments in the time period allotted between STARTn and true lock. Typically, this time period will be longer for a cold printhead (less than 25° C.) and shorter for a warm printhead. 
     Controller  37  drops STARTn (REFCLK is already running). Next, controller  37  continuously measures the time between HSYNCn falling edges. Once the LOCKn signal is active and the HSYNCn to HSYNCn time is within defined tolerances, controller  37  knows that printhead  12  is stable or truly locked. This new value is stored in NVRAM  80  and is used to calculate the total time to reach true lock for the next activation of printhead  12 . 
     As printhead  12  ages, the time to first print increases due to wear of the printhead bearings, etc. The method of the present invention allows controller  37  to adjust the allowed time between STARTn and true lock as printhead  12  ages. The benefit is that the user gets the shortest possible time to first page with assured reliability. 
     The method of the present invention has been described herein as using NVRAM to store data. However, it is to be understood that static RAM, dynamic RAM, or some other type of volatile RAM can also be used to store data. 
     Actual results will vary depending on motor type, manufacturer and age. Since less time is required for a motor to accelerate to a lower speed than to a higher speed, the benefit is greater for electrophotographic machines that use higher speed motors. 
     The present invention provides a shortest possible time to first print for a given printhead motor, rather than assuming worst-case operation for every motor. This can provide tangible benefits to customers by reducing time to first page and reducing wear on the printhead for users who typically print short jobs. 
     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.