Raster output scanner with process direction spot position control

Abstract: Compensation for process direction motion of a photoreceptor (24) in a Raster Output Scan system is provided by moving the cylinder lens (16) located between the laser source (12) and a rotating spot scanning mechanism, e.g., a polygon (20). The cylinder lens is moved in the process direction in response to error signals generated by a start of scan (SOS) detector (30) and timing marks (34) on the photoreceptor drum surface, or an encoder on the drum shaft. The lens movement is obtained by sending motion error signals to a piezo-electric actuator (18) which increases in length to impart the desired motion to the cylinder lens.

Description: This invention relates generally to a raster output scanning system for producing a high intensity imaging beam which scans across a movable photoconductive member to record electrostatic latent images thereon. In recent years, laser printers have been increasingly utilised to produce output copies from input video data representing original image information. The printer uses a raster output scanner (ROS) to expose the charged portions of the photoconductive member to record the electrostatic latent image thereon. Generally, a raster output scanner has a laser for generating a collimated beam of monochromatic radiation. The laser beam is modulated in conformance with the image information. The modulated beam is reflected through a lens onto a scanning element, typically a rotating polygon having mirrored facets. The light beam is reflected from a facet and thereafter focused to a "spot" on the photosensitive member. The rotation of the polygon causes the spot to scan linearly across the photoconductive member in a fast scan (i.e., line scan) direction. Meanwhile, the photoconductive member is advanced relatively more slowly than the rate of the fast scan in a slow scan direction which is orthogonal to the fast scan direction. In this way, the beam scans the recording medium in a raster scanning pattern. The light beam is intensity-modulated in accordance with an input image serial data stream at a rate such that individual picture elements ("pixels") of the image represented by the data stream are exposed on the photosensitive medium to form a latent image, which is then transferred to an appropriate image receiving medium such as paper. Data in each of the fast and slow scan directions is generally sampled. The sampling rate of the slow scan direction data equates to 300 lines per inch or more in many printing apparatus. It has been shown that errors in the slow scan direction of as small as 1% of the nominal line spacing may be perceived in a half tone or continuous tone image. This implies a need for a high degree of control in positioning the spot in the slow scan direction on the image plane, especially in such applications as multiple beam and multiple ROS colour printers where a plurality of spots are written onto a single photoreceptor. Furthermore, high resolution printing, on the order of 600 spots per inch or higher, demands very accurate spot positioning. Errors of the spot position in the slow scan direction arise from many sources, including polygon and/or photosensitive member motion flaws, facet and/or image plane (e.g., photosensitive medium) surface defects, etc. In the prior art, these errors are most commonly addressed by passive or active in-line optics as disclosed in, for example, U. S. Patent US-A-4,600,837 and U. S. Patent US-A-4,660,094. However, high quality optical elements are required, together with tight control in the positioning of those optics, in order to obtain the requisite very precise mechanical control sufficient to adjust spot position by .02 mm or less, as required in many cases. It is an object of the present invention to enable that disadvantage to be overcome. US-A-4,453,170 discloses an image forming apparatus for recording information on a moving photosensitive member with a light beam from a laser beam generator. The light beam is intensity modulated and caused to scan across the photosensitive member by a rotating polygon mirror. The scanning light beam is passed through a correction lens and reflected onto and from a fixed mirror onto the photosensitive member. Vibration of the light beam is corrected at the fixed mirror by sensor and actuator shown in Figures 10 and 11. JP-A-3,120,509 discloses an image forming apparatus for recording information on a plane to be scanned with a laser beam from a laser reflected from a rotary polygonal mirror. To provide a large amount of displacement of the laser beam on the plane to be scanned,an electromagnetic actuator which displaces a lens is located at the laser or between the laser and the polygonal mirrors. The electromagnetic actuator has a piezoelectric actuator which displaces the laser or the lens in a subscan direction. JP-A-2 163 721 discloses correction of a shift in an image plane position in the subscanning direction by varying the refractive power of a stationary cylindrical lens. According to the present invention, the ROS system includes a cylinder lens in the pre-polygon optics to focus the beam in the slow scan direction onto the polygon facets. The cylinder lens, a relatively light optical component in the pre-polygon optical path, is adapted to be moved in the plane parallel to the process direction plane, so as to correct the location of the scanned beams at the photoreceptor. The correction may be enabled by providing a phase error feedback circuit for generating error signals which may be sent to a pizeo-electric actuator to provide high frequency control in the process direction of the position of the cylinder lens. More particularly the present invention relates to a laser printer apparatus of the type which forms an image on a surface of a movable photoreceptor in response to an image data signal, comprising: means for moving the photoreceptor in a process direction, a laser source for generating a coherent light beam along an optical path, modulating means for modulating the light beam generated by said light source in response to said image data signal, scanning means for scanning the light beam produced by said light source and modulated by said modulating means onto the surface of said photoreceptor in a raster fashion and scan direction perpendicular to the process direction, characterized by: a cylindrical lens adjustable in the process direction and disposed in the optical path between the laser source and the scanning means; error detection and feedback control circuit for generating an error signal representative of a detected light beam positional error on the surface of the photoreceptor in the process direction at the point that the light beam is incident upon the photoreceptor surface; and actuator means for adjusting the cylindrical lens in the process direction in response to the error signal received from the control circuit to reposition the cylindrical lens and thereby correct the light beam positional error. By way of example only, an embodiment of the invention will be described with reference to the accompanying drawings, in which : FIG. 1 shows a side view of the general optical configuration of a Raster Output Scanner (ROS) system. FIG. 2 shows a top or plan view of the ROS system of FIG. 1. FIG. 3 shows a flow diagram illustrating the control for regulating the position of the cylinder lens shown in FIG. 1. FIGS. 1 and 2 show a side and top schematic view, respectively, of a ROS system. Input video data is transmitted to a self-modulating light source 12, such as a low powered solid state laser diode, which produces a modulated diverging beam of coherent light. The beam is collimated by a spherical collimating lens 14 and enters cylindrical lens 16. Cylindrical lens 16 serves to focus the beam in the slow scan (process) direction. Lens 16 is movable in the process direction by a pizeo-electric actuator 18 in response to motion error signals sent to actuator 18 from error feedback circuit 19 described in further detail below. The beam is next incident upon a rotating polygon 20 having at least one mirrored facet 21. (Other suitable devices for scanning include rotating hologons, rotating diffraction gratings, etc..) As shown most clearly in FIG. 2, the rotation of the mirrored facets causes the beam to be deflected and thereby scanned across a photosensitive image member which in a preferred embodiment is a photoreceptor drum 24. The ROS illuminates the charged portion of drum 24 at a rate of about 400 pixels per inch; e.g. at 400 spi resolution. Returning to FIG. 1, postscan optics system 22 reconfigures the beam reflected by facet 21 to a circular or elliptical cross-section, refocuses the beam to the proper point on the surface of photoreceptor/drum 24, and corrects for scan nonlinearity (f-theta correction). A 1X (or other working magnification) toroidal lens 28 is disposed between the scanning device 20 and the photoreceptor 24 to correct for wobble (scanner motion or facet errors) where appropriate. FIG. 1 is a view of system 10 oriented so that the process direction is parallel to, or in the plane of the page on which the figure is printed, while the fast scan direction is perpendicular to the plane of the page. The opposite is true for FIG. 2. Thus, if polygon 20 is rotated in a clockwise fashion, as shown in FIG. 2, a beam reflected from one of its moving facets will be caused to scan from left to right on the photoreceptive drum 24. By combining this rotation of polygon 20 with rotation of the photoreceptor drum 24 in a clockwise fashion, as shown in FIG. 1, scanning may proceed in a top to bottom, raster manner. Polygon 20 is preferably driven by a motor 23 through a shaft, with the angular velocity of polygon 20 synchronised with the angular velocity of drum 24. It is evident from the above description that cylinder lens 16 controls the location of the spot of the photoreceptor surface in the slow scan process direction. Thus, if the cylinder lens is moved in the process direction, it will move the location of the scanned spot in the same direction and in an amount dependent on system magnification. For example, if the cylinder lens is moved one micron in a vertical direction in FIG. 1, the image at the polygon facet 21 will move one micron in the same direction. Since the polygon facet is imaged at the photoreceptor surface, the spot will move at the photoreceptor an amount dependent on the magnification of the wobble correcting system (lens 28). For a 1X lens 28, the spot would move one micron at the photoreceptor. FIG. 3 illustrates one embodiment of an error detection and feedback control circuit for determining the existence and extent of the rotational error of drum 24 and for controlling the operation of actuator 18 so as to move lens 16 the required distance to correct the error. Referring to FIGS. 2 and 3, FIG. 2 shows a start of scan (SOS) sensor 30 positioned adjacent to the photoreceptor drum. Each time a scanning beam sweeps across the drum surface and passes over sensor 30, a start of scan signal is emitted. The sensor emits signals at a frequency equal to the system slow scan resolution, e.g. 4000Hz for a 10 ips process speed with 400 Ipi slow scan resolution. A set of timing marks, 34, spaced 1/400th of an inch apart, or some other appropriate spacing are located along the side of the drum. These timing marks are sensed by a light source/detector combination and provide the basic timing signals for the synchronisation of the control system. Alternatively, the timing signals can be generated by a shaft encoder mounted on the photoreceptor drum shaft. As the beam scans across the SOS detector 30, it generates timing pulses. The beam is initially aligned and adjusted with some relationship to the occurrences of the drum timing marks. A phase detector detects the phase difference between the signal from the SOS detector 30 and the timing marks on the drum and generates a phase error voltage that represents the phase error and polarity. This signal is fed to pizeoelectric actuator 18 which transmits the induced, mechanical motion to the cylinder lens 16 to make the necessary process direction correction. Actuator 18, in a preferred embodiment, is a Burleigh model PLZ-020. When the cylinder lens is moved in the process direction, perpendicular to the optical axis, a slightly different portion of the collimated beam is selected at each location. By making the diameter of the collimated beam slightly larger than the aperture of the cylinder lens, the cylinder lens will always be the limiting aperture in the system. Since the cylinder lens is moving such a small distance (<.02mm), the effect of selecting different sections of the collimated beam on photoreceptor exposure will be negligible. An error correction arrangement as described above is especially, but not exclusively, intended to provide compensation for photoreceptor motion (vibration) errors in the range of approximately 0 - 150hz.

Claims: Laser printer apparatus (10) of the type which forms an image on a surface of a movable photoreceptor (24) in response to an image data signal, comprising: means for moving the photoreceptor in a process direction, a laser source (12) for generating a coherent light beam along an optical path, modulating means for modulating the light beam generated by said light source in response to said image data signal, scanning means (20) for scanning the light beam produced by said light source and modulated by said modulating means onto the surface of said photoreceptor in a raster fashion and in a scan direction perpendicular to the process direction, characterized by: a cylindrical lens (16) positionally adjustable in the process direction and disposed in the optical path between the laser source and the scanning means; error detection and feedback control circuit (19, 30) for generating an error signal representative of a detected light beam positional error on the surface of the photoreceptor in the process direction at the point that the light beam is incident upon the photoreceptor surface; and actuator means (18) for adjusting the cylindrical lens position in the process direction in response to the error signal received from the control circuit to reposition the cylindrical lens and thereby correct the light beam positional error. Apparatus as claimed in claim 1, wherein the error detection and feedback control circuit includes a start of scan sensor (30), which emits a start of scan signal each time a light beam sweeps across the photoreceptor surface, and means (34) for generating timing signals for the synchronization of the control circuit; and wherein the photoreceptor is a drum rotated about a shaft. Apparatus as claimed in claim 2, wherein the means for generating timing signals is a set of timing marks (34) and a light source/detector for sensing the timing marks and generating the timing signals or a shaft encoder mounted on a shaft of the photoreceptor drum for generating the timing signals. Apparatus as claimed in any of the preceding claims, wherein the actuator means is a piezoelectric actuator; and wherein the lens has an aperture with a predetermined size. Apparatus as claimed in claim 4, wherein the apparatus further includes a collimating lens (14) located in the optical path of the light beam generated by the laser source and positioned between the laser source and cylindrical lens to collimate the light beam into a light beam having a diameter larger than the aperture of the cylindrical lens, so that the repositioning of the cylindrical lens effects the selection of different sections of the collimated beam from said collimating lens.

Inventors: APPEL JAMES J, OSSMAN KENNETH R, APPEL, JAMES J., OSSMAN. KENNETH R.
Applicants: XEROX CORP, XEROX CORPORATION
Application year: 1992
Main classification: H04N1
IPC codes: H04N1, B41J2, G06K15, G02B26