Scan unit for an imaging device and methods of using same

An imaging device scan unit, including an oscillator oscillating in a predetermined oscillation pattern; a light source generating a light beam for deflection by the oscillator and including a forward sweep portion when the oscillator moves in a first direction of the oscillation pattern and a reverse sweep portion when the oscillator moves in a second direction different from the first direction thereof; and components defining at least two optical paths for the light beam, the forward sweep portion of the light beam passes through a first optical path and the reverse sweep portion of the light beam passes through a second optical path, the second optical path reverses a sweep direction of the reverse sweep portion of the light beam such that the forward sweep portion and the reverse sweep portion of the light beam are in the substantially same direction when exiting the scan unit.

REFERENCE TO SEQUENTIAL LISTING, ETC.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates generally to scan units for image forming device, and particularly to scan units that reduce or otherwise eliminate distortion due to use of both forward and reverse sweeps of a laser beam.

2. Description of the Related Art

The thermal, power, start-up-time and acoustic advantages of micro-mirror-based laser scan units of electrophotographic imaging devices are well demonstrated when compared with scan units utilizing a polygonal mirror. A drawback in using the micro-mirror is its relatively low scan duty cycle, i.e., the time the light beam illuminates the photoconductive member versus the time the light beam illuminates areas off the photoconductive member. This relatively low duty cycle is driven by the sinusoidal motion of the micro-mirror in the scan direction and the practical need to have the light beam velocity in the scan direction acceptably high as it moves onto the photoconductive member.

A further loss of potential duty cycle is encountered by the “zig-zag” trajectory of the laser beam on the photoconductive member if both the left-to-right (forward) and the right-to-left (reverse) sweeps of the reflected laser beam are used to produce the latent image on the photoconductive member. Scan units that use both the forward and reverse sweeps to create scan lines on the photoconductive member, and print defects due to zig-zag distortion can be demonstrated with specific print patterns. These print patterns are not likely to be used by many print jobs, so this potential print defect is less of a concern for monochrome devices. With the need to increase print speed and/or resolution, or the desire to otherwise apply the advantages of a micro-mirror based scan units for color applications, eliminating zig-zag distortion is advantageous. For multi-emitter architectures to increase speed and/or resolution, the advantage is primarily to reduce the extreme scan spacing overlap. For color applications, the primary advantage is to enable overlapping halftone screens needed for color applications.

The root cause of zig-zag distortion is the use of both the forward and the reverse motion of an oscillating mirror to create scan lines on the photoconductive member, coupled with the substantially constant rotational speed thereof. As shown inFIG. 1, the location A in the process direction at which the beam exits the photoconductive member in a forward sweep (beam location 110 mm) versus where it enters on the immediately following reverse sweep at location B is estimated to be about one half of the distance from location C where the beam enters on the forward sweep (−110 mm beam location) versus where it exits at location D on the following reverse sweep. This change in sequential scan spacing is further exacerbated by a 2-on-2-off line pattern where the darkness of the left edge, center and right edge of a page are different.

If one looks at the impact of zig-zag distortion in a multi-emitter case, the issues are multiplied in that, for a two emitter scenario, the separation between the forward sweep of a first emitter versus the reverse sweep of the second emitter can, depending on the scan efficiency, go to zero or even negative. This situation is illustrated inFIG. 2. As can be seen, along the 0 mm scan direction, beam spacing is substantially uniform but along the +110 mm and −110 mm locations, the beam spacing is not at all uniform.

Based upon the foregoing, there is a need for an improved scan unit having a bidirectional scanning oscillator.

SUMMARY

Example embodiments overcome shortcomings seen in existing scan units for image forming devices and satisfy a need for scan units that reduce or substantially eliminate zig-zag distortion of scan units employing bidirectional scanning oscillators. According to an example embodiment, there is shown an imaging device including a controller; a photoconductive member coupled to the controller for moving the photoconductive member; and a scan unit coupled to the controller, the scan unit generating a light beam and repeatedly scanning at least portions of the light beam across the photoconductive member to form scan lines thereon. In an example embodiment, the scan unit includes a light source for emitting the light beam and an oscillator having a reflective surface which oscillates in a predetermined oscillation pattern and reflects the light beam, the light beam reflected by the reflective surface including forward sweep portions when the reflective surface moves in a first direction of the oscillation pattern and reverse sweep portions when the reflective surface moves in a second direction of the oscillation pattern, the scan unit being configured such that the scan lines formed by the forward sweep portions and scan lines formed by the reverse sweep portions of the light beam are substantially parallel to each other on the photoconductive member.

In an example embodiment, the predetermined oscillation pattern is substantially sinusoidal in both a scan direction and a process direction. The oscillator include at least one torsion bar defining a first oscillation axis about which the oscillator oscillates, and an actuator coupled to the at least one torsion bar for causing the torsion bar and the oscillator to modulate along a second oscillation axis.

In an embodiment, the scan unit includes optical components defining at least two optical paths for the light beam, the forward sweep portions of the light beam passes through a first optical path and the reverse sweep portions of the light beam passes through a second optical path, and the optical components defining the second optical path reverse a sweep direction of the corresponding reverse sweep portions of the light beam such that the forward sweep portions and the reverse sweep portions of the light beam are in the substantially same direction when exiting the scan unit. In this example embodiment, the optical components in the second optical path reverse the direction of the reverse sweep portions of the light beam and may include one or more parabolic mirrors or a prism.

DETAILED DESCRIPTION

Spatially relative terms such as “top”, “bottom”, “front”, “back” and “side”, “above”, “under”, “below”, “lower”, “over”, “upper”, and the like, are used for ease of description to explain the positioning of one element relative to a second element. Terms such as “first”, “second”, and the like, are used to describe various elements, regions, sections, etc. and are not intended to be limiting. Further, the terms “a” and “an” herein do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item.

Furthermore, and as described in subsequent paragraphs, the specific configurations illustrated in the drawings are intended to exemplify embodiments of the disclosure and that other alternative configurations are possible.

Referring now toFIG. 3A, a schematic layout of scan unit100is shown according to an example embodiment of the present disclosure. Scan unit100may include a housing incorporating a light source102, a scanning device104, pre-scan optics106, and post-scan optics108.

Light source102may emit a light beam and may be implemented, for example, using a laser diode or any other suitable device for generating a beam of light. Scan unit102may also include driver circuitry (not shown) communicatively coupled to a controller for receiving video/image information and/or control data that may be utilized to set and/or vary the optical power used by light source102.

Pre-scan optics106may include a lens for collimating or converging the light beam emitted by light source102, and/or a pre-scan lens to direct and focus the light beam towards scanning device104.

Scanning device104may include at least one reflective surface for receiving and reflecting light incident thereon. In an example embodiment, scanning device104may be a bidirectional scanning oscillator, such as a torsion oscillator or resonant galvanometer, controlled to operate bidirectionally at a scanning frequency to scan the light beam emitted by light source104for creating scan lines on the surface of photoconductive member120. Scanning device104oscillates according to a predetermined oscillation pattern which generally includes a forward oscillation direction and a reverse oscillation direction such that the light beam includes forward sweep portions when reflected from scanning device104moving in the forward oscillation direction and also includes reverse sweep portions when reflected from scanning device104moving in the reverse oscillation direction.

Post-scan optics108may include a post-scan lens used to focus the light beam onto the surface of photoconductive member120. In an example embodiment, the housing of scan unit100may include an opening and post-scan lens110may be disposed to cover the opening in order to prevent outside contaminants from entering therein. Scan unit100may also include synchronization components including horizontal synchronization (hsync) sensors (not shown) to intercept or pick off the light beam for synchronizing scan line operations.

FIGS. 4A-4Band5A-5B illustrate a portion of scan unit100according to a first example embodiment. To address zig-zag distortion seen in imaging devices with scan units employing oscillators for creating scan lines that sweep the photoconductive member in forward and reverse scan directions, scan unit100ofFIGS. 4A-4Band5A-5B reverses one of the scan directions so that the scan lines generated by both of the forward and reverse oscillation directions of scanning device104are in the same direction. This is accomplished by having the forward sweep portions of the light beam traverse a first optical path in scan unit100and the reverse sweep portions of the light beam traverse a second optical path in scan unit100that is different from first optical path FP.

Causing the forward and reverse sweep portions of the light beam to take differing optical paths in scan unit100may be accomplished by controlling scanning device104so that it follows an oscillation pattern allowing for different optical path travel. In an example embodiment, scanning device104is controlled to have a substantially oval shaped oscillation pattern.FIG. 3Bdepicts such an oval shaped oscillation pattern in which the forward sweep portions are along first optical path FP and the reverse sweep portions are along second optical path SP. As shown, first optical path FP and second optical path SP are separated from each other. Such separation allows for optical components to be placed in forward sweep path FP and reverse sweep path RP having different optical functions from each other.

In the embodiment ofFIGS. 4 and 5, scan unit100includes two pairs of mirrors. Each mirror pair includes a parabolic mirror and a circular mirror disposed adjacent the parabolic mirror. A first mirror pair includes parabolic mirror402and circular mirror404, and a second mirror pair includes parabolic mirror406and circular mirror408. Mirror pair402,404may be disposed on one side of scanning device104and mirror pair406,408disposed on the other. Circular mirrors404,408are placed in first optical path FP of the light beam and parabolic mirrors402,406are placed in second optical path SP.

The operation of scan unit100will be described. As mentioned, scanning device104is controlled to have an oval shaped pattern so that the forward sweep portions of the reflected light beam assume first optical path FP and the reverse sweep portions of the reflected light beam assume second optical path SP. During the time scanning device104is controlled so as to rotate generally in the clockwise direction, as shown inFIGS. 4A and 4B, the light beam generated by light source102is reflected by scanning device104in the clockwise direction, between forward sweep start position FS and forward sweep end position FE. Scanning device104is controlled during this part of the oscillation pattern so that the reflected light beam is incident on and reflected by circular mirror404. The light beam reflected by circular mirror404is directed towards circular mirror408in a clockwise sweep direction between forward sweep start position FS2and forward sweep end position FE2. The light beam reflected by circular mirror408is reflected in a clockwise sweep direction between forward sweep start position FS3and forward sweep end position FE3. In this way, the light beam leaving scan unit100includes forward sweep portions that are not reversed, relative to the forward sweep portions of the light beam reflected by scanning device104.

During the time scanning device104is controlled so as to rotate generally in the counter-clockwise direction, as shown inFIGS. 5A and 5B, the light beam generated by light source102is reflected by scanning device104in the counter-clockwise direction, between reverse sweep start position RS and reverse sweep end position RE. Scanning device104is controlled during this part of the oscillation pattern so that the reflected light beam is incident on and reflected by parabolic mirror402. Due to the nature of light reflection from parabolic mirrors, the light beam reflected by parabolic mirror402is directed towards parabolic mirror406in a clockwise sweep direction between reverse sweep start position RS2and reverse sweep end position RE2. The light beam reflected by parabolic mirror406is in a clockwise sweep direction between reverse sweep start position RS3and reverse sweep end position RE3. In this way, the light beam leaving scan unit100includes reverse sweep portions that are reversed, relative to the reverse sweep portions of the light beam reflected by scanning device104. The result is that the light beam exiting scan unit100(to be incident on photoconductive member120) includes forward sweep portions and reverse sweep portions that are in the same sweep direction, thereby causing the sweep portions on photoconductive member120being parallel to each other so as to eliminate zig-zag distortion.

FIGS. 6A-6Band7A-7B illustrate another approach to reversing one of the forward sweep portions and reverse sweep portions of the light beam that is reflected by scanning device104, according to another example embodiment. In this embodiment, instead of using mirrors in the first optical path FP and the second optical path SP, a prism is employed in second optical path SP. Specifically, scan unit500includes scanning light source102and scanning device104as before. In addition, scan unit500includes positive lens502, negative lens504and prism506. Positive lens502, which may be implemented with a collimation lens, receives the light beam which fans outwardly from scanning device104and changes the beam into substantially parallel rays. Negative lens504receives substantially parallel rays of a light beam and changes them to fan outwardly from the lens. Prism506rotates a received image. In an example embodiment, prism506is a dove prism.

The operation of scan unit500will be described. Scanning device104is controlled to have an oval shaped oscillation pattern, for example, so that the forward sweep portions of the reflected light beam assume first optical path FP and the reverse sweep portions of the reflected light beam assume second optical path SP. During the time scanning device104is controlled so as to rotate generally in the clockwise direction, as shown inFIG. 6A, the light beam generated by light source102is reflected by scanning device104so as to sweep in the clockwise direction, between forward sweep start position FS and forward sweep end position FE in first optical path FP. Scanning device104is controlled during this part of the oscillation pattern so that the reflected light beam is incident on positive lens502, which converts the fanned nature of the incident beam into parallel rays, as shown inFIG. 6A. First optical path FP is positioned so that the parallel rays from positive lens502are not incident on prism506but instead pass adjacent thereto, such as above or below, and are incident on negative lens504.FIG. 6Billustrates the light beam rays passing adjacent to prism506. Negative lens504receives the parallel rays of the reflected light beam and fans them outwardly, as shown inFIG. 6A. The fanned rays of the reflected light beam exit scan unit500, towards the photoconductive member120.

During the time scanning device104is controlled so as to rotate generally in the counter-clockwise direction, as shown inFIG. 7A, the light beam generated by light source102is reflected by scanning device104so as to sweep in the counter-clockwise direction, between reverse sweep start position RS and reverse sweep end position RE in second optical path SP. Scanning device104is controlled during this part of the oscillation pattern so that the reflected light beam is incident on positive lens502, which converts the fanned nature of the incident beam into parallel rays, as shown inFIG. 7A. Second optical path SP includes prism506so that the parallel rays from positive lens502are incident on prism506.FIG. 7Aillustrates prism506rotating the rays so that the rays exiting prism506sweep in the opposite direction from the sweep of the light beam reflected by scanning device104. Negative lens504receives the parallel rays from prism507and fans them outwardly, as shown inFIG. 7A. The fanned rays of the reflected light beam exit scan unit500, towards the photoconductive member120. As noted, the light beam exiting scan unit500from second optical path SP have sweep portions that are in a reversed direction, relative to the direction of the sweep portions of the light beam reflected by scanning device104. The result is that the light beam exiting scan unit500(to be incident on the photoconductive member120) includes forward sweep portions and reverse sweep portions that are in the same sweep direction, thereby causing the sweep portions on the photoconductive member120being parallel to each other so as to eliminate zig-zag distortion.

FIGS. 8A-8Band9A-9B illustrate another approach to reversing one of the forward sweep portions and reverse sweep portions of the light beam that is reflected by scanning device104, according to yet another example embodiment. Scan unit800includes scanning device104, prism802and mirror804. In this embodiment, mirror804is included in first optical path FP and prism802is included in second optical path SP. Similar to prism506, prism802rotates a received image.

The operation of scan unit800will be described. Scanning device104is controlled to have an oval shaped oscillation pattern, for example, so that the forward sweep portions of the reflected light beam assume first optical path FP and the reverse sweep portions of the reflected light beam assume a separate second optical path SP. During the time scanning device104is controlled so as to rotate generally in the clockwise direction, as shown inFIG. 8A, the light beam generated by light source102is reflected by scanning device104so as to sweep in the clockwise direction, between forward sweep start position FS and forward sweep end position FE in first optical path FP. Scanning device104is controlled during this part of the oscillation pattern so that the reflected light beam is incident on mirror804, which reflects the light beam so as to have the sweep direction shown inFIG. 8A. The light beam reflected by mirror804exits scan unit800towards photoconductive member120.

During the time scanning device104is controlled so as to rotate generally in the counter-clockwise direction, as shown inFIG. 9A, the light beam generated by light source102is reflected by scanning device104so as to sweep in the counter-clockwise direction, between reverse sweep start position RS and reverse sweep end position RE in second optical path SP. Scanning device104is controlled during this part of the oscillation pattern so that the reflected light beam is incident on prism802, which rotates the light beam so as to have the sweep direction shown inFIG. 9A, having the same sweep direction as the sweep direction of the light beam in first optical path FP. The result is that the light beam exiting scan unit800(to be incident on the photoconductive member120) includes forward sweep portions and reverse sweep portions that are in the same sweep direction, thereby causing the sweep portions on the photoconductive member being parallel to each other so as to eliminate zig-zag distortion.

Another technique involves controlling the movement of scanning device104to follow an oscillation pattern in which a sinusoidal pattern is applied along each of two oscillation axes. In an example embodiment, scanning device104is controlled to follow a Lissajous pattern, which may be described by the equations
x=Asin(at+δ) andy=Bsin(bt),
where, in this case, a=1 and b=2.FIG. 10shows the oscillation pattern where both the scan (x) and process (y) direction trajectories are sinusoidal. It is noted that the oscillation in the process direction is much smaller in magnitude and twice the frequency as the oscillation in the scan direction.FIG. 11illustrates the resulting light beam location on a photoconductive member120relative to a light beam that is not modulated in the process direction. In the drawing, the scan lines between the vertical dashed lines represent the portions of the light beam that are incident the photoconductive member120. As can be seen, the forward and reverse scan lines are substantially parallel to each other, thereby substantially eliminating the occurrence of zig-zag distortion.

In this embodiment in which scanning device104is controlled to follow a Lissajous oscillation pattern, post-scan optics are not used to reverse a sweep portion of the light beam. Further, modulation in the process direction is a function of process speed. For an image forming device requiring process speeds either tightly grouped or proper fractions of the fastest supported process speed, the advantages offered by even a fixed modulation amplitude could be significant. The nearly parallel nature of the forward and reverse sweeps makes multiple emitter and color applications possible. Given the extremely small magnitude of the modulation in the process direction, the scan unit may utilize post-scan optics that would attenuate the process direction modulation, allowing a larger mirror deflection while still maintaining the same modulation at the photoconductive member120.

Creating the two-axis sinusoidal modulation may be accomplished, for example, through careful adjustment of the mass balance of scanning device102or through active motion in the process direction. The mechanical method would fix the design of scanning device102to a particular path length from scanning device102to the photoconductive member and process speed. While theoretically possible to actively achieve the desired modulation using a familiar 2-axis scanning device, such as those used in projector applications, the extremely small modulation angle makes a more specific design more desirable. According to an example embodiment, the torsion bar that is used to suspend the micro-mirror of a 1-axis scanning device is mounted to a structure with a very small maximum travel path that can be modulated. For example, a piezo-electric actuator may be coupled to a torsion bar to implement the desired small, rapid but controlled oscillatory motion.FIG. 12illustrates a 1-axis scanning device1000in which the mirror1002, such as a micro-mirror, is mounted to torsion bars1004so as to define an axis about which mirror1002oscillates. Actuator1006, which may be a piezo-electric actuator, is coupled to one of the torsion bars1004so that actuator1006may be controlled to modulate torsion bar1004in a direction orthogonal to the axis formed by torsion bars1004.

Similar to the technique described with respect toFIGS. 10 and 11, another technique involves controlling the movement of scanning device104to follow an oscillation pattern as shown inFIG. 13. With scanning device104having the oscillation pattern ofFIG. 13, scan lines provided on photoconductive member120in forward and reverse scan directions that are substantially parallel with each other, as shown inFIG. 14. Though the scan line locations inFIG. 14on photoconductive member120are more idealized than the scan line locations illustrated inFIG. 11, control of scanning device104to follow the oscillation pattern ofFIG. 13would be more complex than the control of scanning device104following the oscillation pattern ofFIG. 10. As a result, a scanning device104controlled with the Lissajous pattern ofFIG. 10may be a more practical implementation.

As mentioned, because the scan units described above substantially reduce or eliminate zig-zag distortion seen in scan units having bidirectional scanning devices, bidirectional scanning devices are more easily implemented in multi-emitter and color applications.FIG. 15illustrates such an application in which a scan unit1310includes light source1312, such as a module having multiple LEDs, which generates at least two light beams LB for reflection by scanning device1320towards one or more photoconductive members1330. The number of photoconductive members1330depends upon the particular application. A single photoconductive member1330may be used in a monochrome imaging device and multiple photoconductive members1330, such as four, may be used in a color imaging device. Scan unit1310may utilize optical components, such as mirrors (circular, parabolic or planar), prisms or the like as described above depending upon the particular scan unit that is selected. Alternatively, for use in a color imaging device, scan unit1310may utilize four scanning devices1320, one for generating scan lines on each photoconductive member1330.

It is understood that the scan units described herein may utilize additional components, such as optical components, that are not shown or illustrated, for controlling or otherwise directing light beams to the photoconductive member(s).

FIG. 16depicts a color image forming device1400which incorporates a scan unit described above. Image forming device1400includes a first toner transfer area1402having four developer units1404that substantially extend from one end of image forming device100to an opposed end thereof. Developer units1404are disposed along an intermediate transfer member (ITM)1406. Each developer unit1404holds a different color toner. The developer units1404may be aligned in order relative to the direction of the ITM1406indicated by the arrows inFIG. 14, with the yellow developer unit1404Y being the most upstream, followed by cyan developer unit1404C, magenta developer unit1404M, and black developer unit1404K being the most downstream along ITM1406.

Each developer unit1404is operably connected to a toner reservoir1408for receiving toner for use in a printing operation. Each toner reservoir1408is controlled to supply toner as needed to its corresponding developer unit1404. Each developer unit1404is associated with a photoconductive member1410that receives toner therefrom during toner development to form a toned image thereon. Each photoconductive member1410is paired with a transfer member1412for use in transferring toner to ITM1406at first transfer area102.

During color image formation, the surface of each photoconductive member110is charged to a specified voltage, such as −800 volts, for example. At least one laser beam LB from a printhead or laser scanning unit (LSU)130is directed to the surface of each photoconductive member1410and discharges those areas it contacts to form a latent image thereon. In one embodiment, areas on the photoconductive member1410illuminated by the laser beam LB are discharged to approximately −100 volts. The developer unit1404then transfers toner to photoconductive member1410to form a toner image thereon. The toner is attracted to the areas of the surface of photoconductive member1410that are discharged by the laser beam LB from scan unit1430. Scan unit1430may incorporate any of the scan units described herein.

ITM1406is disposed adjacent to each of developer unit1404. In this embodiment, ITM1406is formed as an endless belt disposed about a drive roller and other rollers. During image forming operations, ITM1406moves past photoconductive members1410in a clockwise direction as viewed inFIG. 1. One or more of photoconductive members1410applies its toner image in its respective color to ITM1406. For mono-color images, a toner image is applied from a single photoconductive member1410K. For multi-color images, toner images are applied from two or more photoconductive members1410. In one example embodiment, a positive voltage field formed in part by transfer member1412attracts the toner image from the associated photoconductive member1410to the surface of moving ITM1406.

ITM1406rotates and collects the one or more toner images from the one or more developer units1404and then conveys the one or more toner images to a media sheet at a second transfer area1414. Second transfer area114includes a second transfer nip formed between at least one back-up roller1416and a second transfer roller1418.

A fuser assembly1420is disposed downstream of second transfer area1414and receives media sheets with the unfused toner images superposed thereon. In general, fuser assembly1420applies heat and pressure to the media sheets in order to fuse toner thereto. After leaving fuser assembly1420, a media sheet is either deposited into output media area1422or enters duplex media path1424for transport to second transfer area1414for imaging on a second surface of the media sheet.

Image forming device1400is depicted inFIG. 14as a color laser printer in which toner is transferred to a media sheet in a two step operation. Alternatively, image forming device1400may be a color laser printer in which toner is transferred to a media sheet in a single step process—from photoconductive members1410directly to a media sheet. In another alternative example embodiment, image forming device1400may be a monochrome laser printer which utilizes only a single developer unit1404and photoconductive member1410for depositing black toner directly to media sheets. Further, image forming device1400may be part of a multi-function product having, among other things, an image scanner for scanning printed sheets.

Image forming device1400further includes a controller1440and memory1442communicatively coupled thereto. In addition to being coupled to and controlling scan unit1430, controller1440may also be coupled to components and modules in image forming device1400for controlling the same. For instance, controller1440may be coupled to toner reservoirs1408, developer units1404, photoconductive members1410, fuser assembly1420, as well as to motors (not shown) for imparting motion thereto. Further, controller1440is associated with heat control circuitry1444that is coupled to fuser assembly1420to control the generation of heat used to fuse toner to sheets of media. It is understood that controller1440may be implemented as any number of controllers and/or processors for suitably controlling image forming device1400to perform, among other functions, printing operations.

The description of the details of the example embodiments have been described using imaging devices. However, it will be appreciated that the teachings and concepts provided herein may also be applicable to other relatively stationary computing devices deployed in a particular environment.