The present invention relates in general to electrophotographic devices, and more particularly, to electrophotographic devices that support two or more image transfer rates and methods of operating electrophotographic devices at two or more image transfer rates.
In electrophotography, a latent image is created on the surface of an electrostatically charged photoconductive surface, e.g., a drum or belt, by exposing select portions of the photoconductive surface to laser light. Essentially, the density of the electrostatic charge on the photoconductive surface is altered in areas exposed to a laser beam relative to those areas unexposed to the laser beam. The latent electrostatic image thus created is developed into a visible image by exposing the photoconductive surface to toner, which typically contains pigment components and thermoplastic components. When so exposed, the toner is attracted to the photoconductive surface in a manner that corresponds to the electrostatic density altered by the laser beam. The toner is subsequently transferred from the photoconductive surface to a print medium such as paper, either directly or by using an intermediate transfer device. A fuser then applies heat and pressure to the print medium. The heat causes constituents including the thermoplastic components of the toner to flow into the interstices between the fibers of the medium and the fuser pressure promotes settling of the toner constituents in these voids. As the toner is cooled, it solidifies and adheres the image to the medium.
In a typical laser scanning system, a faceted rotating polygon mirror is used to sweep a laser beam across a photoconductive surface in a scan direction while the photoconductive surface advances in a process direction that is orthogonal to the scan direction. The polygon mirror speed is synchronized with the advancement of the photoconductive surface so as to achieve a desired image resolution, typically expressed in dots per inch (dpi) at a given image transfer rate, typically expressed in pages per minute (ppm). Thus, for example, to achieve a resolution of 600 dots per inch (236 dots per centimeter) in the process direction at an image transfer rate of 20 pages per minute, the photoconductive surface is operated at a speed sufficient to transfer toner images to twenty pages in one minute of time. Moreover, the polygon mirror velocity is configured to perform 600 scans across the photoconductive surface in the time it takes for the photoconductive surface to advance one inch (2.54 centimeters).
Slowing the operation of the photoconductive surface relative to a normal (full speed) operating image transfer rate can be desirable under certain circumstances. For example, slowing the photoconductive surface to one half of the full speed image transfer rate can provide double scan line addressability which, ideally, can improve the quality of the image printed on the medium. Additionally, by operating the photoconductive surface at half speed, greater time is available for fusing operations because the print medium is moving through the device at a slower speed. Relatively longer fusing times are desirable for example, when the print medium is relatively thick or where transparencies are used.
To operate satisfactorily at half speed, i.e., one half of the full speed image transfer rate, and to maintain double line addressability, the laser power needs to be reduced by one half of the full speed laser power so as to maintain output image consistency between full speed and half speed modes of printing. Unfortunately, the acceptable operating range of a typical laser diode may not allow such drastic changes in laser output power. As such, the prior art has attempted to reduce laser power output by using pulse width modulation of a full power laser beam such that the power output by the laser is reduced by one half. However, pulse width modulating a laser beam increases the complexity of the laser diode driver circuitry. Moreover, changing the duty cycle of a laser beam affects the “turn on” and “turn off” characteristics of the laser, which may affect overall consistency and print quality.
The present invention provides electrophotographic devices and methods of operating electrophotographic devices that are capable of operating at two or more image transfer rates such that the components of the laser system and paper feed path are operated within their normal ranges of operation. Further, a desired image characteristic, e.g., one or more of a predetermined process direction resolution and a total and/or average energy written to a photoconductive surface, is maintained regardless of the selected image transfer rate.
An electrophotographic device comprises a controller, a laser source, a photoconductive surface operable at two or more image transfer rates and a scanning device having a plurality of deflecting surfaces arranged to direct a beam from the laser source so as to sweep across the photoconductive surface in a scan direction. The laser source may alternatively include two or more laser devices, each capable of emitting an independently controllable laser beam. Where multiple beams are emitted from the laser source, the scanning device is further arranged to sweep each beam such that scan lines written by the beams are spaced from one another on the photoconductive surface by a predetermined beam scan spacing. The controller is arranged to maintain a desired image characteristic independent of a selected one of the image transfer rates by controlling the laser beam(s) so as to write image data only at select scan lines that have been identified from candidate scan lines. The candidate scan lines are defined by positions along the photoconductive surface that are determined at least by one or more of the selected image transfer rate, a predetermined rotational velocity of the scanning device, the number of independently controllable laser beams that may be swept across the photosensitive surface and their corresponding beam scan spacing.
A method is also provided of controlling an electrophotographic device that is capable of two or more image transfer rates such that a desired image characteristic is maintained independent of a selected one of the image transfer rates. The method comprises providing one or more laser beams and a scanning device having a plurality of deflecting surfaces arranged so as to sweep the laser beam(s) in a scan direction across the photoconductive surface. In a given sweep in which multiple laser beams are turned on or are otherwise modulated, the respective beams are spaced from one another on the photoconductive surface in a process direction that is nominally orthogonal to the scan direction by a predetermined beam scan spacing.
The scanning device is controlled to rotate at a predetermined velocity, and based upon a selected one of the image transfer rates, candidate scan lines are identified for laser beams and deflecting surfaces of the scanning device. Candidate scan lines thus essentially identify relative process direction positions from which the controller may opt to sweep a beam when writing image data to the photoconductive surface. The controller operates the laser source so as to write image data to the photoconductive surface at select ones of the candidate scan lines to achieve an output image corresponding to the desired image characteristic.
The following detailed description of the preferred embodiments of the present invention can be best understood when read in conjunction with the following drawings, where like structure is indicated with like reference numerals, and in which:
In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration, and not by way of limitation, specific preferred embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and that changes may be made without departing from the spirit and scope of the present invention.
Referring now to the drawings, and particularly to
To form the overlaid toner patterns, the imaging section 12 includes four printhead units 24, 26, 28, 30, four toner cartridges 32, 34, 36, 38, four photoconductive drums 40, 42, 44, 46 and an intermediate transfer belt 48. Printhead unit 24 generates two independently controllable laser beams 50a, 50b that are modulated in accordance with bitmap image data corresponding to the black color image plane to form a latent image on the photoconductive drum 40. Printhead unit 26 generates two independently controllable laser beams 52a, 52b that are modulated in accordance with bitmap image data corresponding to the magenta color image plane to form a latent image on the photoconductive drum 42. Printhead unit 28 generates two independently controllable laser beams 54a, 54b that are modulated in accordance with bitmap image data corresponding to the cyan color image plane to form a latent image on the photoconductive drum 44. Similarly, Printhead unit 30 generates two independently controllable laser beams 56a, 56b that are modulated in accordance with bitmap image data corresponding to the yellow color image plane to form a latent image on the photoconductive drum 46.
Each photoconductive drum 40, 42, 44, 46 continuously rotates clockwise (as shown) according to the directional arrow 58 past their associated toner cartridge 32, 34, 36, 38 such that toner is transferred to each photoconductive drum surface in a pattern corresponding to the latent image formed thereon. As the intermediate transfer belt 48 travels past each photoconductive drum 40, 42, 44, 46, as indicated by the directional arrow 60, the corresponding toner patterns are transferred to the outside surface of the intermediate transfer belt 48. The timing of the laser scanning operations on each of the photoconductive drums 40, 42, 44, 46, the speed of the intermediate transfer belt 48 and the timing of the travel of a print media 18 along the paper path 16 are coordinated such that a forward biased transfer roll 62 transfers the toner patterns from the belt 48 to the print media 18 at the nip 64 so as to form a composite color toner image on the print media 18.
The print media 18 is then passed through a fuser 66 at the fusing section 14. Generally, heat and pressure are applied to the print media 18 as it passes through a nip 68 of the fuser 66 so as to adhere the color toner image to the print media 18. The print media 18 is then discharged from the printer 10 along the media discharge path 22.
Referring now to
Referring to
Similarly, post-scan optics 84 direct the laser beams 52a, 52b emitted from the printhead unit 26 so as to form scan lines on the photoconductive drum 42, which are spaced from one another in the process direction by a beam scan spacing D2. Post-scan optics 86 direct the laser beams 54a, 54b emitted from printhead unit 28 so as to form scan lines on the photoconductive drum 44, which are spaced from one another in the process direction by a beam scan spacing D3. Similarly, post-scan optics 88 direct the laser beams 56a, 56b emitted from printhead unit 30 so as to form scan line lines are on the photoconductive drum 46, which are spaced from one another in the process direction by a beam scan spacing D4.
In general, the image transfer rate of an electrophotographic device defines a speed in which a toner image is transferred from the photoconductive surface to an associated image transfer device. The image transfer device may comprise for example, the intermediate transfer belt 48 described with reference to
Moreover, it is desirable in certain electrophotographic devices to provide two or more image transfer rates to support different modes of operation. Relatively slower image transfer rates generally result in the print media moving more slowly through the device, which may promote better fusing operations, e.g., to achieve translucence of color toners fused onto transparent media, or improve adherence of toner when printing thick, gloss or specialty papers. To this end, one approach is to slow down the image transfer rate by slowing down the intermediate transfer belt 48 and correspondingly slowing down the photoconductive drums 40, 42, 44, 46 and the associated transport of the print media 18. When slowing down the image transfer rate, either the laser output power, the rotational velocity of the polygon mirror, or both may be adjusted down in corresponding amounts to compensate for the new image transfer rate. However, a typical laser diode is not always adjustable to accommodate large variations in laser output power. For example, laser power adjustments over a wide range may result in spurious mode-hopping as the laser current approaches the laser power threshold for lasing. Moreover, the laser power must not exceed a specified maximum laser drive current level. Also, relatively large changes in laser power can affect the overall print quality due to changes in laser turn-on and turn-off timing. Relatively large variations in polygon motor velocity can also affect print quality, such as by causing jitter and otherwise unstable rotational velocity of the polygon mirror.
However, the speed of a brushless DC motor that is used to drive a photoconductive drum may be adjusted over a range of approximately 3:1 and still maintain a robust phase lock to maintain a relatively constant rotational velocity. As such,
For the discussion with reference to
Further, it is assumed that the electrophotographic device has a “facet resolution” that is nominally 300 dpi (118 dots per centimeter) at the full speed image transfer rate. The term “facet resolution” is used herein to denote a maximum process direction resolution that may be realized by sweeping a single laser beam across the photoconductive surface based upon the current image transfer rate and rotational velocity of the polygon mirror. Thus, using only a single laser beam at the full speed image transfer rate, the facet resolution or maximum process direction resolution realizable is 300 dpi (118 dots per centimeter). Correspondingly, the term “facet spacing” denotes the process direction spacing of a select laser beam on the photoconductive surface as a result of adjacent facets of the polygon mirror intercepting and sweeping that laser beam. In this instance, the facet spacing is 1/300th of an inch (84.6 microns). Without altering the rotational velocity of the polygon mirror, the facet spacing and corresponding facet resolution will change each time the image transfer rate changes because each is dependent, in part, upon the process direction speed of the photoconductive surface.
Still further, it is assumed that a printhead unit of the electrophotographic device, e.g., printhead unit 24, comprises two laser beams 50a-50b that are arranged so as to have a fixed nominal beam scan spacing D1 of 3/600th inch (approximately 127 microns). In this instance, the beam scan spacing of 3/600th inch (approximately 127 microns) is one and one half times the facet spacing of 1/300th of an inch (84.6 microns). It is preferable to set the beam scan spacing to a distance that is not the same as the facet resolution, or an integer multiple thereof. Under such an arrangement, there will be a redundancy in beam scans between the laser beams. That is, because the beam scan spacing will align with the facet spacing, each laser beam 50a-50b will write a scan line along the same position on the photoconductive surface. However, the above-described redundancy may be avoided where the beam scan spacing is set to a distance less than the facet resolution, or the beam scan spacing may be set to a non-integer multiple of the facet resolution. As will be described in greater detail herein, one exemplary approach where there are two laser beams per photoconductive surface is to set the beam scan spacing to one half the facet spacing, or to any odd multiple of one half the facet spacing.
With reference to
Similarly, the second beam, designated beam 2, is also enabled for each facet of the polygon rotation. As such, beam 2 will also be modulated in accordance with image data every 1/300th of an inch (84.6 microns) in the process direction (corresponding to the facet resolution). However, because there is a 3/600th of an inch (127 micron) spacing between laser 1 and laser 2, the modulated output of laser 2 will interlace with the modulated output of laser 1 and thus the effective scanning resolution is increased to 600 dpi (236 dots per centimeters) in the process direction. As can be seen by the chart, both laser 1 and laser 2 are modulated for each facet of rotation of the polygon mirror. Also, because the beam scan spacing ( 3/600th of an inch or 127 microns) is greater than the facet spacing ( 1/300th of an inch or 84.6 microns), there will be no scan line at 1/600th of an inch (42.3 microns) from the first scan line. As such, the RIP processor will have to account for this, for example, by buffering the image data with two blank lines or by disabling the first laser beam for the first facet. Thus, on the first facet, the first laser beam may write no image data, and the second beam may write the second line of bitmap image data. Under this arrangement, at facet 2, the first laser beam writes the first line of bitmap image data, and the second laser writes the fourth line of bitmap image data. This process continues for each facet until the entire image is written.
Referring to
The techniques herein may be implemented even where an image transfer rate is a reduced speed rate defined by a reduction of the full speed image transfer rate by a factor other than two, i.e., where the image transfer rate is set to a speed other than full speed or half speed. Referring to
Changing the image transfer rate from full speed to one third speed has no effect upon the beam scan spacing, which is fixed at a 3/600th of an inch (127 micron) process direction spacing between laser beam 1 and laser beam 2. In this example, both laser beam 1 and laser beam 2 are enabled and scan the corresponding photoconductive surface in a repeating pattern that comprises both laser beam 1 and laser beam 2 enabled for a first facet, and disabled for the subsequent two facets. Thus, an effective output resolution of 600 dpi (236 dots per centimeter), is achieved. Notably, this one third output speed adjustment requires no modification of the laser diode output power and no adjustment of the polygon motor velocity. Further, because the laser power was not adjusted and the output resolution did not change, the average and total photoconductor exposure energy is nominally the same at both full and one third speeds. As such the desired image characteristic is once again met for the one third image transfer rate.
Also, because the beam scan spacing ( 3/600th of an inch or 127 microns) is greater than the facet spacing ( 1/900th of an inch or 28.2 microns), there will be no scan line at 1/600th of an inch (42.3 microns). As such, the RIP processor will have to account for this, e.g., by buffering the image data with two blank scan lines by disabling the first laser beam for the first facet. Thus, on the first facet, the first laser beam will write no image data, and the second beam will write the second line of bitmap image data. At facets 2 and 3, both laser beams are off. At facet 4, the first laser beam writes the first line of bitmap image data, and the second laser writes the fourth line of bitmap image data. This process continues for each facet until the entire image is written.
The chart of
Referring to
Each position thus defines a “candidate scan line” that the controller may opt to use or ignore. After identifying candidate scan lines for each laser beam 50a-50b for each facet of the polygon mirror, the controller may perform scan line selection to achieve the desired image characteristic. For example, the controller may select scan lines from the available candidate scan line positions based upon a predetermined or desired output resolution. Candidate scan lines may thus identify for a given image transfer rate, the relative process direction position of each laser beam for each facet of scanning by the polygon mirror. From the possible candidate scan lines, select scan lines are identified based upon the desired image characteristic. This essentially tells the controller which laser beams to enable for each facet of the polygon mirror to achieve the desired image characteristic when printing an image.
In the current example, the desired image characteristic defines a total exposure energy when the laser scanning rate in the process direction is 600 dpi. Conveniently, there are candidate scan lines that fall on 1/600th of an inch (42.3 micron) increments as indicated by the bolded position indications at 1/600th of in inch (42.3 micron) increments. Also, the “X” appearing in the “600 DPI” column indicates that a candidate scan line has been selected and the remainder of the facet/laser beam positions can be ignored, such as by disabling or not writing to the corresponding laser beam to skip the associated facet. The present invention can be expanded for any reasonable number of laser beams and any reasonable number of facets of the polygon mirror. Thus, the controller determines which laser beam or beams to modulate with image data, and which facet or pattern of facets to utilize for laser scanning for each facet of rotation based upon the selected candidate scan lines.
As demonstrated above, the present technique works even when the process direction spacing between adjacent candidate scan lines for a first one of the image transfer rates, e.g., the full speed image transfer rate, is an amount other than double the process direction spacing between adjacent candidate scan lines for a second one of the image transfer rates, e.g., the one third speed image transfer rate. For example, for the full speed image transfer rate illustrated in
Depending upon how the candidate scan lines are selected, some modification may be required to the laser beam output power to achieve the desired image characteristic. Keeping with the one third speed image transfer rate example of
However, by modulating only one laser beam at 900 dpi (354 dots per centimeter), and by reducing the laser power output of that laser beam for each scan to two thirds of the laser output power utilized for full speed mode printing, e.g., by reducing the laser diode drive current to an appropriate level, the total photoconductor exposure energy is nominally the same at both the full speed image transfer rate and one third speed image transfer rate. Thus, the desired image characteristic is maintained. That is, the total exposure energy of the photoconductive surface when writing an image at 600 dpi (236 dots per centimeters) using both laser beams, where the laser power of each beam is set to the level typically used when operating at the full speed image transfer rate, is the same as the total exposure energy of that same photoconductive surface when scanning a single laser beam at 900 dpi (354 dots per centimeter) where the laser beam power is two thirds the laser power at the full image transfer rate.
Similarly, the average exposure energies of the photoconductive surface, e.g., in each 300×300 dpi square, is the same at the full speed image transfer rate and one third image transfer rate where a single beam scans at 900 dpi (354 dots per centimeter) at ⅔ the laser power. This example assumes that the laser output power can be adjusted down to two thirds the output power utilized for the full speed image transfer rate. The one third image transfer rate is achieved without requiring pulse width modulation of the laser power to achieve the desired photoconductor exposure energy. Still further, if there are no candidate scan lines that enable the desired resolution, the polygon mirror velocity can be adjusted to modify the facet resolution. Under this arrangement, the above method is repeated for the new facet resolution.
The above examples discussed with reference to
With reference to
Similarly, the second beam, designated beam 2, is also enabled for each facet of the polygon rotation. As such, beam 2 will also be modulated in accordance with image data every 1/600th of an inch (42.3 microns) in the process direction (corresponding to the facet resolution). However, because there is a 1/1200th of an inch (21 micron) spacing between laser 1 and laser 2, the modulated output of laser 2 will interlace with the modulated output of laser 1 and thus the effective scanning resolution is increased to 1200 dpi (472 dots per centimeter) in the process direction. As can be seen by the chart, both laser 1 and laser 2 are modulated for each facet of rotation of the polygon mirror. Also, because the beam scan spacing ( 1/1200th of an inch or 21 microns) is less than the facet spacing ( 1/600th of an inch or 42.3 microns), imaging can begin on the first encountered facet using both laser beams and there will be no need to skip the first facet with laser 1 as in the example described with reference to
Referring to
Also, the image transfer rate was adjusted from a full speed to half speed without modification of the laser diode power output and without modification of the polygon motor velocity. Thus, the desired image characteristic is further met because the total and average photoconductor exposure energy is nominally the same at both full and one half image transfer rates. Also, as noted in
Referring to
Changing the image transfer rate from full speed to one third speed has no effect upon the beam scan spacing, which is fixed at a 1/1200th of an inch (21 micron) process direction spacing between laser beam 1 and laser beam 2. In this example, both laser beam 1 and laser beam 2 are enabled and scan the corresponding photoconductive surface in a repeating pattern that comprises both laser beam 1 and laser beam 2 enabled for a first facet, and disabled for the subsequent two facets. Thus, an effective output resolution of 1200 dpi (472 dots per centimeter) is achieved. Notably, this one-third output speed adjustment requires no modification of the laser diode output power and no adjustment of the polygon motor velocity. Further, because the laser power was not adjusted and the output resolution did not change, the average and total photoconductor exposure energy is nominally the same at both full and one-third speeds. As such the desired image characteristic is once again met for the one-third image transfer rate. Also, it can be seen from the chart of
With reference to
With reference to
As with the example of
Although the above examples demonstrate two laser beams per photoconductive surface for convenience of discussion, the present invention may be applied to systems comprising one or more laser beams in practice. As an example, assume that only a single laser beam is provided for writing a latent image to a photoconductive surface, and that the full speed image transfer rate is 25 pages per minute at 600 dpi (236 dots per centimeter), which is also the facet resolution. Further assume that an image transfer rate of 10 pages per minute is desired. Using the techniques set out more fully herein, it can be seen that the 10 pages per minute image transfer rate may be obtained by slowing down the photoconductive drum motor by an appropriate amount. However, leaving all other parameters unchanged, the effective process direction resolution is now 1500 dpi (591 dots per centimeter) because the photoconductive surface is now moving at 40 percent of the speed it was moving at the full speed image transfer rate of 25 pages per minute. Note however, that by increasing the velocity of the polygon mirror by 20%, i.e., by a factor of 30/25ths, the effective process direction resolution increases to 1800 dpi (709 dots per centimeter).
The above can be conceptualized as a set of candidate scan lines at a spacing of 1800 dpi (709 dots per centimeter). Thus, a desired image transfer characteristic, e.g., an output resolution is achieved by selecting scan lines from the available candidate scan line positions. In the present example, a desired 600 dpi (236 dots per centimeter) output is achieved by skipping two facets for every facet utilized in a manner analogous to that described with reference to
Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention defined in the appended claims.