Abstract:
A print apparatus includes a photoconductor and a mechanical system for moving the photoconductor past a scan line exposure station. The print apparatus includes a signal generator for providing outputs indicative of the movement of the photoconductor. A first comparator produces a first position error signal that is derived from a difference between a reference signal and a position error signal output, such position error signal indicating that the position of the photoconductor differs from a predetermined print position that is determinable with respect to the reference signal. The print apparatus further includes a laser beam scanner and a beam detector for producing a scan position signal. A second comparator is responsive to the scan position signal and the position error signal from the first comparator to produce a beam deflection control signal that is applied to a beam deflector having a mirror attached to a piezoelectric bimorph crystal cantilever element. The beam deflector moves the laser beam in a direction which reduces the beam deflection control signal and position the laser scan at a position closer to a predetermined print position.

Description:
This application is a continuation in part application and claims priority from application Ser. No. 08/621,053 filed Mar. 22, 1996, now U.S. Pat. No. 5,760,817, which is a continuation of application Ser. No. 08/262,405 filed Jun. 20, 1994, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to laser print mechanisms and, more particularly, to a system for reducing &#34;banding&#34; which occurs when print scan lines in a laser printer are subject to positioning errors. 
     BACKGROUND OF THE INVENTION 
     Banding is a term used to define alternate streaks of light and dark areas (or bands) that are produced on an output sheet in a laser printer. These bands are produced as a result of non-constant placement of print scan lines. Dark bands are produced when the scan lines are closer together and lighter bands are produced when the scan lines are farther apart. 
     In a laser printer, pitch variation between scan lines can occur as the result of a number of system errors. The major cause is speed variations in the moving photoconductor. Those speed variations occur due to imperfections in the mechanical parts that comprise the photoconductor drive system. These imperfections may include out-of-round parts, gear inaccuracies, bearing runouts, mechanical coupling errors, etc. Structure vibration states which result from mechanical resonances can also cause relative motion between a laser scanner and the photoconductor and result in a banding affect on output sheets. 
     In general, banding is not apparent to the user when documents contain only text. However, because the resolution available from laser printers has seen substantial improvements, such printers are now employed to print full grey scale graphics images. Banding is clearly visible in such images. 
     When banding is apparent the extent of misalignment may exceed several scan line widths. In a monochrome printer, scan line misalignment is usually less than plus or minus up to one scan line width. For a color printer, banding is aggravated by misregistration of color planes in addition to scan line misalignment. A color printer has several color planes, for example, black, yellow, cyan, and magenta. Misregistration between planes can be several scan line widths. 
     The prior art has attempted to solve the banding problem by specifying more accurate (and more expensive) parts for the laser printer. Vibration problems have been reduced through provision of stiffer structures that are usually heavier and more expensive. Further, increasingly more sophisticated (and expensive) speed controls for drive motors have been implemented so as to assure relatively constant photoconductor speeds. Nevertheless, for laser printers that employ photoconductive belts, such improvements still do not eliminate banding. This is due to the fact that the belts themselves evidence some stretch and positional modification when subjected to impulse loading. A feedback system which attempts to correct for mechanical movements of the photoconductor and/or drive system cannot respond fast enough to totally overcome the banding effect. While high bandwidth servo systems approach a solution to the problem, they are still confronted with the problem of altering speed and/or position of mechanical elements which have both inertia and momentum that tend to negate the immediate correction actions. Such mechanical elements also exhibit a &#34;resiliency&#34; that tends to limit the responsiveness of the system to high frequency correction actions. 
     Accordingly, it is an object of this invention to provide an improved system for reduction of banding in a laser printer. 
     It is a further object of this invention to provide a banding reduction system in a laser printer wherein few additional mechanical parts are required. 
     It is yet another object of this invention to provide a banding reduction system for a laser printer wherein the banding reduction action is not dependent upon a high bandwidth servo system for correcting printer mechanical movements. 
     SUMMARY OF THE INVENTION 
     A print apparatus includes a photoconductor and a mechanical system for moving the photoconductor past a scan line exposure station. The print apparatus includes a test signal generator for providing outputs indicative of the movement of the photoconductor. A first comparator produces a first position error signal that is derived from a difference between a reference signal and a position error signal output, such position error signal indicating that the position of the photoconductor differs from a predetermined print position that is determinable with respect to the reference signal. The print apparatus further includes a laser beam scanner and a beam detector for producing a scan position signal. A second comparator is responsive to the scan position signal and the position error signal from the first comparator to produce a beam deflection control signal that is applied to a piezoelectric beam deflector. The deflector moves the laser beam in a direction which reduces the beam deflection control signal and positions the laser scan at a position closer to a predetermined print position. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a laser printer that incorporates a first embodiment of the invention. 
     FIG. 2 is a side sectional view of a galvanometer-operated mirror assembly employed to reposition a laser beam in accordance with a position error signal derived from mechanical drive components within the printer. 
     FIG. 3 is a block diagram of electronic circuits employed in the first embodiment of the invention shown in FIG. 1. 
     FIG. 4 is a perspective view of a laser printer showing a second embodiment of the invention. 
     FIG. 5 is a block diagram of electronic circuits utilized to implement the second embodiment shown in FIG. 4. 
     FIG. 6 is a side sectional view of a piezoelectric beam deflector in one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 illustrates principal components of a laser printer that incorporates the invention hereof. A photoconductor belt 10 passes over an idler roller 12 and a drive roller 14. Drive roller 14 is operated by a drive motor which, in turn, transmits its rotary action through a gear train to drive roller 14. (The drive motor and coupling gear train are not shown.) Photoconductor belt 10 passes over a laser write platen 16 which has an optical sensor 18 mounted at one extremity. A magnetic sense head 20 is positioned at another extremity of laser write platen 16 and senses signals that are generated when a magnetic synch track 22 passes in contact therewith. Magnetic synch track 22 is recorded on a magnetic track along the interior surface of photoconductor belt 10 and provides synchronizing signals to enable positional detection of photoconductor belt 10 in relation to a reference clock frequency. 
     A laser assembly 24 directs a laser beam 26 at a galvano-motor-mirror assembly 28 (hereafter referred to as &#34;galvano&#34;) which reflects laser beam 26 towards a rotating faceted mirror 30. The rotation of faceted mirror 30, in the known manner, scans laser beam 26 along a path between scan extremity lines 32 and 34. The scanned laser beam passes through a lens 36 and is reflected by an elongated mirror 38 to create a raster scan line 40 on photo conductor belt 10 directly over laser write platen 16. To implement this invention, the scanned laser beam is caused to scan past the edge of photoconductor belt 10 and to intercept optical sensor 18 (as shown by dashed scan line 32&#39;). 
     Optical sensor 18 provides an error voltage when it intercepts a laser beam that is offset from the physical center line of sensor 18. While various types of optical sensors may be employed for optical sensor 18, one that is preferred includes two light sensitive semiconductors that are separated by a non-light-responsive area that is positioned directly at the centerline of optical sensor 18. As a result, a change in position of an incident laser beam from the centerline of optical sensor 18 will cause one or the other of the adjacent photodetectors to provide an output voltage, which output voltage is employed as an error potential to enable a positional correction voltage to be generated. 
     Positional correction of the laser beam is achieved by moving galvano 28 to cause beam 26 to reposition itself with respect to rotating faceted mirror 30. In FIG. 2, a sectional view of galvano 28 is illustrated. In essence, galvano 28 is a motor-driven mirror that is repositionable about an axis that is perpendicular to the paper. Mirror 50 is mounted on a wire-wound rotor 52. A flexible mount 54 connects mirror 50/rotor 52 to a magnetic return stator structure 56. A pair of magnets 58 and 60 are positioned on stator structure 56 and about rotor 52. By appropriate energization of rotor structure 52, mirror 50 may be caused to move in the directions shown by arrows 62. As a result, laser beam 26 may be altered in direction in accordance with the positioning of mirror 50 which is, in turn, dependent upon the energizing current applied to wire-wound rotor 52. 
     Turning to FIG. 3, a circuit is shown which enables banding reduction to be accomplished through the use of the structure shown in FIGS. 1 and 2. It will be recalled that banding is the result of pitch errors between succeeding raster scan lines 40 (FIG. 1) which cause a series of succeeding scan lines to be either increasingly closer together or farther apart, as the case may be. 
     The banding reduction control circuit comprises two control loops 70 and 72, of which control loop 70 is a standard motor control servo loop present in prior art laser printers. Control loop 72 implements the invention in conjunction with control loop 70. A clock signal is applied via an input 74 to summers 76 and 77. Ideally, if the mechanical drive system for photoconductor belt 10 was &#34;perfect&#34;, the positioning of each scan line 40 in relation to clock signals applied to input 74 would be precise and repeatable. However, due to the above-described mechanical errors, spacings between successive scan lines 40 or groups of successive scan lines can vary to cause the banding effect. The output from summer 76 is applied through an integrator 78 and an amplifier 80 to motor 82. An encoder 84 provides a signal train on feedback line 87 that is indicative of succeeding instantaneous positions of motor 82. The difference between the feedback signals on line 87 and the input clock signals applied via input 74 provides a motor error voltage which is utilized to correct the speed of motor 82 in the known manner. Control loop 70 acts in a direction to drive the output of summer 76 towards zero. 
     As motor 82 rotates, it operates a gear train 85 which, in turn, causes rotation of drive roller 16 and photoconductor belt 10. The instantaneous position of photoconductor belt 10 is sensed by an output from position sensor 20 which, in this case, is a magnetic head. Those skilled in the art will fully understand that any appropriate positional sensing system can be utilized to provide a pulse train that is synchronized with the movement of photoconductor 10 (e.g., optical, electrical or otherwise). 
     The pulse sequence output from position sensor 20 is fed via an amplifier 86 as an input to summer 77. The output from summer 77 (as integrated by integrator 88) provides a photoconductor belt error signal that includes a component which is directly related to any mechanical positioning errors. A positioning error causes a variation in an expected time difference between pulses from position sensor 20 and clock pulses applied to summer 77. As a result, the output from integrator 88 is a photoconductor position error signal that is applied to summer 90. 
     Summer 90 controls the operation of loop 72, which comprises amplifier 92, galvano 28, laser beam 26 and optical sensor 18. The objective of loop 72 is to adjust galvano 28 to reposition laser beam 26 (as sensed by optical sensor 18) in a direction to reduce the difference between a feedback voltage applied via line 94 to summer 90 and the photoconductor belt error potential as applied to summer 90. More specifically, if the photoconductor belt error signal, by its value, indicates that photoconductor 10 is lagging from where it should properly be positioned, the voltage applied to summer 90 is increased, thereby causing a larger galvano error signal to be applied, via amplifier 92, to galvano 28. Mirror 50 (FIG. 2) is thus rotated to position laser beam 26 to compensate for the positional lag of photoconductor 10. Galvano 28 thus is caused to move scan line 40 in a direction to maintain the spacing from the immediately previous scan line at a constant value. More specifically, galvano 28 deflects laser beam 26 so as to intercept a facet on rotating faceted mirror 30 at a displaced position. That displaced position results in a movement of scan line 40 with respect to laser write platen 16. 
     Because optical sensor 18 only provides an updated correction signal once per scan, laser beam 26 is only repositioned once per scan. As a result, optical sensor 18, as shown in FIG. 3, incorporates a voltage holding circuit that enables the maintenance of the feedback level on line 94. 
     Turning to FIGS. 4 and 5, a further embodiment of the invention is shown which enables a beam position signal to be generated during a full scan and obviates the need for optical sensor 18 to include a voltage holding circuit. In FIG. 4, each element shown therein that is common to an element shown in FIG. 1 is numbered identically. It is to be noted that a beam splitter 100 has been placed in the beam path between galvano 28 and rotating faceted mirror 30. Beam splitter 100 directs beam 26 towards a beam position sensor 102 which is constructed similarly to optical sensor 18. Beam position sensor generates a continuous beam position voltage. The output from beam position sensor 102 is employed in the control loop 72 shown in FIG. 5. Optical sensor 18 is still employed but provides its output directly to summer 90. Beam position sensor 102 replaces optical sensor 18 in control loop 72 and enables a continuous beam position voltage to be fed via line 94 to summer 90. As beam position sensor 102 is now located to provide a continuous beam position feedback potential on line 94, feedback loop 72 need not await for a change in output from optical sensor 18 to provide a correction signal to galvano 28. 
     The output from optical sensor 18, as applied to summer 90, enables system errors that occur between beam splitter 100 and scan line 40 to be corrected. For instance, the output from optical sensor 18 will cause an error voltage from summer 90 in the event anomalies are present in the facets of rotating faceted mirror 30. Furthermore, optical sensor 18 will also cause an error voltage should a vibrational mode occur in the operating mechanism. Optical sensor 18 thus enables a periodic error input to supplement the error potential derived from beam position sensor 102 and enables a more precise control of laser beam 26. 
     FIG. 6 is a side sectional view of a piezoelectric beam deflector 110. Piezoelectric beam deflector 110, is used in place of galvano 28 shown in FIGS. 1, 3, 4, and 5, in alternate embodiments of the present invention. Beam deflector 110 is of the type generally described as a cantilever deflector and is preferred over alternate deflectors of the type generally employing a voice coil motor piston or a piezoelectric piston. The cantilever type deflector provides greater beam deflection capability over the piezoelectric piston type and provides more economic life cycle costs when compared to deflectors of all other types. 
     Deflector 110 primarily includes support 112 and 114, bimorph crystal element 120, and mirror 124, attached to element 120 by conventional adhesive. Deflector 110 is supported from a chassis portion 111 of a laser printer, for example the laser printer shown in FIGS. 1 or 4. Support blocks 112 and 114 grip crystal element 120 and are held onto chassis 111 by one or more conventional fasteners or adhesives. Movement of crystal element 120 is substantially constrained by support blocks 112 and 114 so that element 120 mechanically operates as a cantilever. 
     The structure and operation of crystal element 120 are of the type generally known as a bimorph, being constructed primarily of two well known ceramic materials 121 and 122. Crystal element 120 is of the type currently marketed for positioning systems and displacement transducers. Wires 126 and 128 apply a predetermined potential difference across materials 121 and 122, for example, causing material 121 to expand along the length of element 120 and the material 122 to contract. Reversing polarity has the opposite effect on each material. Consequently, element 120 bends and so moves mirror 124 to a particular position corresponding to the predetermined potential. 
     Accurate positioning of laser beam 26 is accomplished along the motion direction indicated in FIG. 6. When element 120 is bent to orient mirror 124 at a first position, laser beam 26 follows a facet path across a facet of rotating faceted mirror 30. Movement of photoconductor belt 10 may be insufficient to place a subsequent scan line at a proper distance from a previous scan line, where the distance is measured between the parallel scan lines along the direction of paper movement through the printer. In such a case, an appropriate error signal, for example as provided by amplifier 92, causes element 120 to bend to a second position. The aforementioned error signal remains constant and element 120, therefore, maintains the second position throughout the subsequent scan. Laser beam 26 is deflected by mirror 124 to follow a second facet path across a facet of rotating faceted mirror 30. The second facet path differs from the first facet path in axial position on a facet, where axial refers to a measurement along a line parallel to the axis of rotation of mirror 30. Motion direction indicated by arrows 62, therefore, corresponds to the movement of paper through the printer as opposed to the direction of a scan line across the paper being printed. 
     Because element 120 and mirror 124 together have little mass as compared to the moving portions of galvano 28, shown for example in FIG. 2, faster positioning of laser beam 26 by deflector 110 is accomplished with less power and without bearing surfaces that wear. 
     Deflector 110 is a type of absolute positioning component with electromechanical properties sufficient, in several embodiments, for open loop positioning applications. In an alternate embodiment derived from the block diagram shown in FIG. 3, deflector 110 replaces galvano 28; and, summer 90, cooperation for sensing beam 26, sensor 18, and line 94 are omitted. In another alternate embodiment derived from the block diagram shown in FIG. 5, deflector 110 replaces galvano 28; and, cooperation for sensing beam 26, sensor 102, and line 94 are omitted. Control loop 70 remains for both derivative embodiments. 
     In all of the above described embodiments, repositioning laser scan line 40 in accordance with photoconductor error positioning voltages enables feedback signals to be generated at electronic speed to correct for mechanical anomalies that cause a mispositioning of photoconductor belt 10. As a result, banding is minimized and higher quality graphics images are produced. 
     It should be understood that the foregoing description is only illustrative of the invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the invention. For instance, while a motor structure has been shown for controlling galvano 28, piezoelectric control is equally applicable. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.