Beam alignment correction element assembly

Abstract: A correction element assembly for correcting misalignment of a beam of radiation with respect to a location on a target, includes a housing having a correction element fixedly mounted therein. A flexure is attached to the housing and at least two drive devices are disposed radially around the housing. Each of the drive devices is operable to apply a force to the housing to thereby deflect the flexure causing the housing to move such that said correction element moves in a cyclic fashion about its beam axis.

Description: Technical Field The present invention relates to beam scanning systems such as those utilized in electronic prepress operations and more particularly to a correction element assembly suitable for use in such scanning systems to correct beam misalignments in scanning imaging surfaces. Background Art Modern electronic prepress operations utilize laser scanning systems to write or record images for subsequent reproduction or to scan a prerecorded image at a predefined resolution rate. Such scanning systems may write or record images or scan prerecorded images on various prepress media including, photo or thermal sensitive paper or polymer films, photo or thermal sensitive coatings or erasable imaging materials mounted onto an image recording surface or photo or thermal sensitive paper, polymer film or aluminum base printing plate materials, all used in electronic image reproduction. Such media are mounted onto a recording surface which may be planar but which is more typically curved and scanned with a recording or scanning beam. The primary components of such a system include a recording surface, usually a drum cylinder and a scan mechanism disposed and movable within the drum cylinder. The system also includes a processor, with an associated storage device, for controlling the scanning mechanism and for scanning a prerecorded image, a photodetector and detector processor. The processor and associated storage device may be housed within the system itself or separate from the system with appropriate interconnection to the system. The processor, in accordance with stored programming instructions, controls the scanning mechanism to write or read images on the plate or other medium mounted to the inner drum cylinder wall by scanning one or more optical beams over the inside circumference of the drum cylinder while the drum cylinder itself remains fixed. The scanning and hence the recording are performed over only a portion of the cylinder inner circumference, typically between 120° and 320° of the circumference of the drum cylinder. The optical beam(s) are typically emitted so as to be parallel with a central axis of the cylinder and are deflected, by for example, a spinning mirror, Hologon or Penta-prism deflector so as to form a single scan line or multiple scan lines which simultaneously impinge upon the recording surface. The deflector is spun or rotated by a motor about an axis of rotation substantially coincident with the central axis of the drum cylinder. To increase the recording speed, the speed of rotation of the beam deflecting device can be increased. To even further increase the recording speed, multiple beam scanning has been previously proposed. One such proposed multiple beam scanner has utilized a spinning dove prism with a single light source, as discussed, for example, in U.S. Patent No. 5,214,528. Using a dove prism beneficially allows the use of a multiple beam source, e.g. a laser diode array, while eliminating the need for multiple beam correction elements and associated hardware. Additionally, for reasons which need not be discussed here, the scan speed of multiple beam systems using a dove prism can exceed that of other types of proposed multi-beam systems. As shown in Fig. 10A, a dove prism 116 is disposed in the optical path between a laser source array 110 and the spin mirror 130. The prism 116 rotates about an axis coincident with the longitudinal axis of the drum cylinder 140 (or an optical axis which becomes coincident) at half the speed of the spin mirror 130 to scan the image surface 135 of the drum 140. Since the spinning of dove prism 116 produces a 2X axial rotation of all light beams passing through the prism, the multiple beams leaving the prism will rotate in lockstep with the rotation of the spin mirror 130, as shown by the rays marked with filled circles in Figure 10B. Accordingly, by passing the multiple light beams through a spinning dove prism, crossing of the multiple scan lines formed by the spin mirror is avoided. Notwithstanding the type of scanning element being utilized, it is of primary importance that the light beam(s) contact the deflector as close as possible to a desired location to ensure that the appropriate scan line(s) are formed on the recording surface and hence the desired image is properly recorded. In the case of a multiple beam system, this includes maintaining the desired spacing or overlapping relationship of the simultaneously scanned beams with respect to each other and the reduction or elimination of any differential scan line bow between successive scan lines. Further, if a light beam is rotated by a rotating dove prism, the beam's rotation and hence the rotation of the prism must be synchronized with the angular position of the rotating deflector to obtain a proper scan of the recording surface and thereby properly record the desired image. Small changes in the phase locking of the two motors, i.e. the prism and deflector motors, can create banding groups, particularly for commonly used laser array geometries. A wobble in the spinning dove prism motor or other types of anomalies will cause a misalignment of the dove prism and can create significant banding artifacts which repeat every two scan passes of the deflector. If such a misalignment exists in multiple beam systems of the dove prism type as have been proposed, the system is restricted to recording during only every other rotation of the spin mirror to obtain high quality results. A four beam system is accordingly only two times faster than single beam system, an eight beam system only four times faster, and so on. The effect of dove prism wobble on system banding can be reduced by a ratio of the beam diameter at the prism to the beam diameter at the spin mirror, e.g. at least a 20X-30X reduction is obtainable for prism beam diameters of ∼1mm or less. Hence, by increasing the ratio, the contribution of prism wobble to system banding can be reduced significantly. Angular alignment sensitivities for scan line bow are improved by the same ratios. A small beam diameter in the prism also reduces the size of the required prism and prism motor. Hence, banding and twinning caused by misalignment of the dove prism can significantly reduce the advantages which are otherwise obtainable from multi-beam, dove prism type scanning systems. U.S. Patent No. 5,097,351 proposes a multi-beam system which utilizes a controlled moveable reflector in lieu of a dove prism and requires each of two laser beams to follow a separate optical path, each having separate focusing and collimating lenses and an acousto-optical modulator (AOM). The controlled reflector is disposed in only one of the optical paths and is driven to rotate one beam in sync with the rotation of the spin deflector. Errors are detected, and corrected by driving the deflector to adjust angular alignment during recording operations. However, the complexity of the system makes implementation difficult, if not impractical. The above referenced related application describes a multi-beam system which utilizes a spinning dove prism through which all of the writing beams are passed. As described in the application misalignments, caused for example by wobble about the rotation axis of the dove prism, are corrected utilizing a correction lens assembly. The correction lens assembly is controlled to move, e.g. in a cyclic fashion, during recording operations to correct detected misalignment errors. In order to correct for such misalignments the correction lens assembly must be capable of moving in the manner described in detail in the above referenced related application. Because the lens may need to move dynamically, i.e. continuously, during writing operations, vibrations may be transferred to other system components or structures. Additionally, it may be beneficial to encode the actual movement of the correction lens to correct beam misalignment and/or to monitor movement of the lens during writing operations to ensure correction of the misalignment. Objectives of the Invention Accordingly the need remains for a correction lens assembly which can be utilized to correct the misalignment in multi-beam scanning systems. Beneficially such an assembly can be driven in a cyclic fashion during writing operations. It would be advantageous if vibrations caused by the movement of the lens within the assembly were isolated from other system components and structures because of the extreme precision required to avoid artifacts in the range of µ inches and sub-arcsecond angular errors. It would also be beneficial if the movement of the lens to correct beam misalignment could be easily encoded and monitored. Additional objects, advantages, novel features of the present invention will become apparent to those skilled in the art from this disclosure, including the following detailed description, as well as by practice of the invention. While the invention is described below with reference to preferred embodiments for electronic prepress applications, it should be understood that the invention is not limited thereto. Those of ordinary skill in the art having access to the teachings herein will recognize additional applications, modifications, and embodiments in other fields, which are within the scope of the invention as disclosed and claimed herein and with respect to which the invention could be of significant utility. Summary of the Invention The present invention is defined in claims 1 and 2. In accordance with a first embodiment of the present invention a correction element assembly for correcting misalignment of a beam(s) of radiation with respect to a location on a target is provided. The assembly includes a housing having a correction element fixedly mounted therein and moveable therewith. The housing is supported from a flexure, which is preferably formed of urethane and encircles the housing. At least two drive devices are disposed radially around the housing, preferably at 90° separations to generate forces to drive the movement of the housing to correct any misalignment in the radiating beam. More particularly, the drives are operable to apply forces to the housing to deflect the flexure to thereby move the housing such that the correction element moves in a cyclic fashion about its beam axis as it exists with the correction element in a stationary position. To drive this movement of the correction element, each drive axis, i.e. the line along which the force generated by each drive device would be applied as a point load, is made substantially perpendicular to and intersects the longitudinal axis of the housing, which in turn is preferably substantially coincident with the beam axis of the correction element when the correction element is in a stationary position. That is to the extent practicable the longitudinal axis of the housing is made coincident with the optical or beam axis, and the respective drive devices are disposed such that the drive axis will be perpendicular to and intersect the longitudinal axis of the housing. In accordance with other aspects of the invention the housing may be formed to have extended portions which protrude through the flexure at locations aligned with the drive axis of each of the drive devices. A distal end of each of the extended portions of the housing are located proximate to but separated from an associated drive device. The drive devices may for example include one or more coils to generate an electro-magnetic force which interacts with the extended portion of the housing or with magnets mounted to the housing. If the electro-magnetic forces are to act on the housing itself, the housing is formed of a ferrite or other material which will react to such forces. The inventive correction element assembly is particularly suitable for use in beam scanning systems which scan curve surfaces, such as those used in electronic prepress operations. Such systems typically utilize a spinning deflector to deflect the writing or recording beam or beams onto the curved imaging surface. As disclosed in the above referenced related application, multi-beam scanning systems will beneficially also use a spinning element upstream of the spinning deflector to avoid scan line cross over on the curved imaging surface. However, misalignment of the beams with respect to the spinning deflector caused, for example, by wobble about the rotation axis of the spinning upstream element, will degrade the quality of the scan. In such cases, the misaligned beam moves in a pattern determined by the construction and wobble of the upstream spin element. To correct this error, the correction element assembly can be driven to move in synchronization with the misaligned beams, and the spinning upstream element, to correct the misalignment. The correction element may be located either upstream or downstream of the spinning element. In accordance with a second embodiment of the invention, four drive devices are configured radially, at 90° angles, around the correction element housing. The drive devices can be configured to apply forces simultaneously or sequentially. It may be beneficial, in certain implementations, for the opposed drive devices to operate as a drive pair in reverse polarity such that one of the devices will repel the housing at the same time the opposed device attracts the housing. This provides double the force on the housing as that which may be otherwise obtainable and helps to correct for non-linearities in the drive forces. According to a still further embodiment of the invention, the movements of the housing can be encoded and monitored using reticules. More particularly, reticules are respectively mounted to the housing and to another portion of the assembly which remains stationary with respect to movement of the housing. A radiation emitter, such as an LED or laser, and a detector, such as a CCD or photodetector, are also mounted on a portion of the assembly which remains stationary with respect to movement of the housing. The emitter emits a reference beam of radiation on a path which passes through the reticules. The detector is located so as to detect light from the reference beam downstream of the reticules. The detector generates signals corresponding to the received light which represent the movement of the housing. It may be advantageous in certain implementations to detect misalignment in the writing beams prior to actually performing writing operations. In such cases the correction element assembly can be operated to correct the misalignment during this prewriting period and signals issued from the detector during this period can be used to encode the movement of the housing which is required to correct the misalignment errors. The encoded signals can be stored and then retrieved and utilized during writing operations to control the movement of the housing and thereby correct the misalignment errors during writing operations. Alternatively or additionally, the reference radiation emitter and detector can be utilized during writing operations to monitor the movement of the housing to ensure that the housing is being properly moved to correct the writing beam misalignments which were detected during a prewriting period. In accordance with a fourth embodiment of the invention, curved piezo elements are utilized to apply the forces required to move the housing in the desired manner. A member with two orthogonal sections, such as an L-shaped member, is utilized in this particular embodiment of the invention. One piezo element is mounted to one section of the member and to the housing such that it will drive the housing in a first direction. Another piezo element is mounted to the other section of the member and to a base such that it will drive the housing in a second direction which is perpendicular to the first drive direction. For example, in the case of an L-shaped member, one piezo element can be attached to an inside surface of the vertical portion of the L and to the housing. This piezo element will drive the housing in the horizontal direction. The other piezo element is attached to the outer surface of the horizontal portion of the L and to a base so as to drive the vertical movement of the housing. By operating the piezo elements simultaneously, omni-directional movement of the housing can be obtained. In accordance with further embodiments of the invention, the transfer of reaction forces necessarily generated by the drive forces, i.e. forces equal but opposite in direction to the drive forces, and vibrations caused by the movement of the housing can be reduced, if not eliminated, by utilizing compliant members and/or balancing masses. The compliant members may be formed of any type of dampening material or could include a spring arrangement, although the use of a block material having appropriate dampening qualities for the particular implementation is preferred. For example, one or more compliant members can be located between each drive device and the support structure which serves as its base to absorb reaction forces generated in opposition to the drive forces. The use of four compliant members to support each drive device appears to be particularly advantageous in avoiding twisting and other undesirable effects on the drive device during movement of the housing. However, good results may also be obtainable with less than four supports but may require the use of compliant members which are mechanically or electro-mechanically tuned so as to be stiff when the housing is in a stationary position and more compliant or soft during the movement of the housing. Compliant members may also beneficially be located between each drive device and the housing. These compliant members can serve as a flexible support for the housing while also absorbing and hence isolating vibration forces caused by the movement of the housing which might otherwise be transferred into other parts of the assembly or other system components or structure. Rather than locating compliant members between the housing and the drive devices, it may be preferable to mount the housing independently of the drive devices and directly to the assembly structure which acts as a base. In this case, compliant members can be located between the housing and the assembly structure to support the housing while serving as a flexure and vibration isolator. It may be particularly advantageous in certain implementations to maintain the drive devices stationary with respect to the housing movement. This can be accomplished by fixedly supporting the drive devices from the assembly structure. In accordance with one embodiment of the invention, opposed coils are included in each drive device. That is the respective drive device coils operate to simultaneously apply a drive force in the direction of the housing and a balancing force in the opposite direction. The drive force drives the movement of the correction element housing while the balancing force is balanced by a balancing mass which is supported from the assembly structure with compliant members. Accordingly, the balancing mass will balance at least a portion of the balancing force while the compliant members will absorb any remaining portion of the balancing force. The balancing mass is preferably supported by two or four compliant members. In such an arrangement the housing is preferably also independently supported by compliant members from the assembly structure. Although the housing and balancing mass could be formed of a ferrite or other material which reacts to electro-magnetic forces, preferably magnets are mounted on the housing and balancing mass in alignment with the respective drive devices to interact with the forces generated by the respective coils. Coil-coil drive geometries are also, of course, possible. Brief Description of the Drawings Figure 1A is a perspective view of a lens housing and coil arrangement in accordance with a first embodiment of the present invention. Figure 1B depicts a longitudinal cross-section of the lens housing of Figure 1A. Figure 1C depicts a cross-section looking through the lens housing of Figure 1A. Figure 2 depicts a second embodiment of the lens correction assembly in accordance with the present invention. Figure 3A depicts a lens correction assembly with a reticule mounted thereto in accordance with a third embodiment of the invention. Figure 3B depicts a cross-section of the correction lens assembly depicted in Figure 3A. Figure 3C details one of the reticules shown in the Figure 3A. Figure 3D depicts the other reticule shown in the Figure 3A. Figure 4 depicts a correction lens assembly which includes piezo elements in accordance with a fourth embodiment of the invention. Figure 5A depicts a lens correction assembly which includes vibration and force dampening in accordance with a fifth embodiment of the invention. Figure 5B details the force and vibration dampening components of the embodiment to Figure 5A. Figure 6A depicts another embodiment of the invention which includes force and vibration dampening. Figure 6B details the dampeners of the Figure 6A embodiment. Figure 7A depicts another embodiment of the invention which includes force and vibration dampeners. Figure 7B details the dampening members shown in the Figure 7A embodiment. Figure 8A depicts a correction lens assembly which includes force and vibration dampeners and a balancing mass in accordance with still another embodiment of the invention. Figure 8B details the dampening members and balancing mass of the Figure 8A embodiment. Figure 9A depicts a correction lens assembly similar to that depicted in Figure 8A but with the balancing mass supported by four vibration dampeners in accordance with a further embodiment of the invention. Figure 9B details the dampening members and balancing mass of the Figure 9A embodiment. Figure 10A depicts a conventional multi-beam scanning system. Figure 10B depicts the rotation of beams in relation to the rotating spin mirror of Figure 9A. Description of the Preferred Embodiments Figures 1-8 depict various correction lens assemblies in accordance with the present invention. Turning first to Figure 1A, a perspective view of a two coil lens correction assembly is depicted. The lens 880 is housed in housing 810 which has a longitudinal axis which is substantially coincident with the longitudinal axis of the cylindrical drum. Coils 820a and 820b are positioned at a 90° angle from each other. Figure 1B is a longitudinal cross-section taken through the center of the housing 810. As shown in Figure 1B, the housing 810 is suspended from stationary supports or base 840 by a flexure 850 which encircles the housing 810. The flexure may beneficially be formed of urethane. If desired, the shape and material of the flexure 850 could be modified so long as the modified flexure is capable of suspending the lens housing 810 in a manner which allows the housing to move radially about the optical axis of the correction lens as it exists in a stationary position, with a minimal amount of force being applied. Referring now to Figure 1C, each the coils 820a and 820b is supported in a stationary housing 860a and 860b. As shown, the housing 810 extends or protrudes beyond the flexure 850 at each coil location. The housing 810 can be formed of any material which will react to the electro-magnetic forces generated by the respective coils 820a and 820b. The frequency, amplitude and shape of the pattern that the movement of lens 880 will inscribe is controlled by the wave form of the signals issued from the lens correction assembly controller (not shown) as input signals to the respective coils. Accordingly, the lens can be driven to move such that the lens is correctly positioned to deflect the beams passing therethrough to thereby offset any misalignment in the beams. The movement of the lens 880 is such that its optical axis traces a pattern corresponding to the misalignment pattern of an upstream rotating element, such as a dove prism, but 180° out of phase therewith, with a sufficient amplitude to deflect the beams into correct alignment with the axis of rotation of for example a rotating spin mirror deflector. In the depicted lens correction assembly, the housing has a weight of less than .5 ounces, the lens weight is less than .15 ounces and the weight of both coils, including the coil housings, is less than .75 ounces. Accordingly, the entire assembly is very light weight. Figure 2 depicts another lens correction assembly arrangement. As shown, four magnetic disks or squares 970a-d are mounted to the housing 910 of the lens 980. The four disks or squares 970a-d are spaced at 90° angles around the periphery of the housing 910. Although the housing 910 is depicted as having four flat outer surfaces, it will be understood that, if desired, the housing outer surface could be of virtually any shape. The housing 910 is suspended by a urethane or other flexure 950 in a manner similar to that described above in connection with Figures 1A-C. Each of the coils 920a-d are housed in a cylindrical ferrite housing 960a-d, respectively, and disposed opposite one of the ferrite disk or squares 970a-d. The coil housings 960a-d are mounted so as to be stationary with respect to the movement of the lens housing 910. The opposed coils 920a-920c and 920b-920d form two coil pairs which are respectively operated so that when one coil of the coil pair is driven to produce a force which attracts its adjacent ferrite disk or square, the other coil of the pair is driven to generate an electro-magnetic force which repels its adjacent ferrite disk or square and vice-versa. Accordingly, the lens correction assembly controller will simultaneously issue signals to the four coils 920a-d to move the optical axis of the lens to correct the misalignment in the beams. Alteratively, if desired, the respectively paired coils could be driven sequentially rather than simultaneously to drive the required movement of the lens 980. Figure 3A depicts a still further lens correction assembly. As shown, Figure 3A provides a four coil arrangement with the coils 1020a-d disposed 90° from each adjacent coil around the lens 1080. The lens 1080 is supported within a housing 1010. The housing includes an extension 1010A which supports a reticule which is utilized to encode the proper motion of the correction lens 1080 to correct for detected misalignment. The housing 1010 further supports magnets 1070a-d which are disposed adjacent to the coils 1020a-d. The gap between each coil and the adjacent magnet is such that the magnet and coil will not come into contact during the movement of the lens. Opposed coils are wired as a coil pair and controlled so as to operate in reverse polarity to double the force available to move the lens housing 1010 in the same manner as described in Figure 2. As shown in Figure 3B, the pair of reticules 1090a and 1090b are mounted to move relative to each other. Reticule 1090a is mounted to the stationary housing. The other reticule 1090b is mounted to move with the lens housing 1010 and hence the lens 1080. A light emitter 1092 and quad detector 1094 are also included in the assembly and mounted to the stationary housing. The reticules 1090a and 1090b are respectively detailed in Figures 3C and 3D, with opaque areas indicated by diagonal lines. The reticules 1090a and 1090b and quad detector 1094 are aligned to the axes of the coil pairs. The detector produces two signals capable of defining the motion of the lens housing 1010 as is well understood in the art. The signals can be processed to encode the movement of the housing 1010 required to correct any beam misalignment as discussed above. During writing operations, the quad detector 1094 may also be utilized to track the movement of the lens housing 1010 to verify proper alignment of the writing beam. Figure 4 depicts still another assembly arrangement for moving the lens housing. The Figure 4 configuration does not require any flexures to support and suspend the lens housing 1110. As depicted, the lens 1180 is supported in the housing 1110. The housing 1110 is supported from L-shaped member 1150 by way of the piezo-electric element 1120a which is fixedly attached at its ends to the support member 1150. A second piezo-electric element 1120b is fixedly attached at its ends to a stationary base surface 1130. Accordingly, the piezo-electric elements can be driven by the lens correction assembly controller (not shown) in the manner described above to move the lens housing 1110 such that the lens 1180 is translated to correct detected misalignment of the beams. As depicted in Figure 4 the lens housing 1110 is affixed to a part of the piezo-electric element 1120a which is positioned to drive the movement of the housing 1110 and lens 1180 in the horizontal or X direction. The support member 1150 is attached to a portion of piezo-electric element 1120b. Accordingly, the piezo-electric element 1120b can be driven to move the support member 1150 along with the lens housing 1110 and lens 1180 in a vertical or Y direction. Connected to the lens housing 1110 is a moving reticule 1090a' and connected to the stationary base surface 1130 is a stationary reticule 1090b'. The reticules 1090a' and 1090b' are substantially similar to reticules 1090a and 1090b described above with reference to Figures 3A-3D and accordingly will not be further described to avoid unnecessary duplication. In the lens correction assemblies described above, the movement of the correction lens could induce vibrations in various other scanning system components and/or the system structure. Although vibrations which are coherent in frequency and phase lock with the beam scanning frequency may not cause problematic vibrations in such components and structure, vibrations at half the scanning frequency could produce twinning artifacts in, for example, an imaging medium mounted to the cylindrical drum on which the writing is to occur. These vibrations could also excite non-coherent natural frequencies in the supporting structure thereby producing banding artifacts. Accordingly, it may be beneficial to utilize counterbalancing reaction masses and/or dampeners rather than the structure which supports other components of the scanning system to balance the forces required to move the lens. In this way, the center of mass of the lens correction assembly will remain unchanged during the movement of the lens and no static or dynamic forces which could deflect or vibrate the system structure and/or other system components will occur. Figures 5A-B through 9A-B depict various lens correction assemblies which reduce vibration in the system support structure by directing the forces required to move the lens to correct beam misalignment into reaction masses or dampening elements instead of against the supporting structure. Hence, the reaction masses and dampeners act as force and vibration reducers. By carefully selecting the geometry and materials utilized, the vector sum of the forces on the system structure can be reduced to very close to, if not to, zero. Referring first to Figures 5A and 5B a lens 1280 is housed within a housing 1210. Four coils 1220a-d, each separated from adjacent coils by 90°, are disposed around the lens housing 1210. The coils 1220a-d are motor/solenoid coils. Four motor/solenoid magnets 1222a-d are fixedly mounted to the housing 1210 such that each magnet is proximate to one of the motor/solenoid coils 1220a-d. Each of the coils 1220a-d is supported from the system support structure 1295 by compliant members 1215a-d and 1217a-d. The compliant members 1215a-d and 1217a-d absorb the forces caused by the lens motion. Accordingly, equivalent forces are not produced on the supporting structure 1295, and vibrations in the supporting structure are reduced, if not eliminated. Compliant members 1215a-d and 1217a-d act as soft springs. Although the assembly shown in Figures 5A-B is a relatively uncomplicated structure, static centering of the lens 1280 may be difficult to maintain unless the compliant members can be selectively made softer or stiffer depending upon whether or not movement of the lens 1280 is being driven. That is, the compliant members 1215a-d and 1217a-d are beneficially stiff during periods when it is desirable for the lens to be statically centered, i.e. when the housing and lens are stationary, and soft only during those periods in which the lens is being driven to move. This may require that the compliant members be mechanically or electro-mechanically tuned. Figures 6A-B provide a configuration of the lens correction assembly which avoids the potential problems in static centering of the configuration depicted in Figures 5A-B. As shown in Figure 6A a lens 1380 is supported in housing 1310. Four coils 1320a-d are disposed around the circumference of the lens housing 1310 with each coil being 90° from adjacent coils. Four magnets 1322a-d are respectively disposed proximate to one of the coils 1320a-d and mounted to the housing 1310. Compliant members 1315a-d and 1317a-d are mounted to the assembly support structure 1395. The coil support members 1321a-d are separated by a space from the support structure 1395 and all elements supported therefrom. Accordingly, the lens moving mechanism is connected to the supporting structure 1395 by compliant members 1315a-d and 1317a-d. These compliant members induce some force in the supporting structure 1395 but by proper selection of the spring constant and/or the mass of the assembly components, the vector sum of the forces exerted on the supporting structure 1395 can be substantially reduced or eliminated. For example, a zero sum force can, in theory, be obtained for all frequencies of lens motion if the frequency of mechanical resonance of the mass and dampener system is equal to the frequency of mechanical resonance of each reaction element, e.g. the coil support subassembly. More particularly, if the ratios of the spring constant to mass are equal, the force on the supporting structure will, in theory, be reduced to zero. This is true whether the coil pairs push and pull the lens simultaneously or only one of each pair of coils is driven at a given time to either push or pull on the lens housing 1310. If the frequencies of the lens motion approaches the mechanical resonance frequency, the mechanical damping "Q factor" of the assembly should also be matched to maximize vibration reduction as will be well understood by the skilled artisan. At high frequencies the frictional couplings, e.g. due to air flow friction, and airborne acoustic coupling between the moving components and the supporting structure 1395 are beneficially also considered to further enhance vibration isolation. Turning now to Figures 7A and B, a configuration similar to that shown in Figures 6A-B is depicted. As can be seen from the drawings, the primary difference between Figures 7A-B and 6A-B is in the shape of the coil supports 1421a-d which are supported by compliant members 1415a-d and 1417a-d. The assembly depicted in Figures 7A-B operates in a manner similar to that described above with respect to Figures 6A-B. However, because compliant members 1415a-d and 1417a-d provide multiple supports for each of the coil support members 1421a-d, better stiffness is provided against twisting forces on the coils 1320a-d and balancing masses, including support member 1421a-d, thereby reducing coil to magnet centering errors. The use of equal masses and equal spring constants for many of the parts may also simplify the damping and coupling match in the configuration depicted in Figures 7A-B as compared with that depicted in 6A-B. Figures 8A-B depict a still further configuration of the lens correction assembly. This configuration is somewhat more complicated than that depicted in Figures 5-7 but facilitates the use of multiple motor/solenoid coils and magnets for electronic control of the matching or balancing forces. Accordingly, the configuration of Figures 8A-B provides enhanced vibration isolation as compared with the previously described configurations. It will be understood that by adding a vibration sensor and feedback system elimination of virtually any vibrations could be assured. As depicted in Figures 8A and B the coil supports 1521a-d are fixedly mounted to the supporting structure 1595. Balancing masses 1597a-d are supported from the supporting structure 1595 by compliant members 1415a-d. As shown, each of the coil support structures 1521a-d support multiple coils 1520a-d and 1520'a-d. Additionally, the balancing masses further support motor/solenoid magnets 1522'a-d which interact with respective coils 1520'a-d. The coils 1520'a-d can be driven in relation to the driving forces applied by coils 1520a-d on the lens housing 1510 to thereby insure equal but opposite offsetting forces are applied to the balancing masses 1597a-d, thus eliminating the possibility of vibrations in the supporting structures. Figures 9A-B depict yet another configuration of the lens correction assembly. This configuration is similar to that depicted in Figures 8A-B and similar elements are referenced with similar reference numerals. As shown in Figures 9A-B, each of the balancing masses is supported from the supporting structure 1595' by four compliant members 1415a-d and 1417a-d which provide multiple supports for each of the balancing masses. As has been previously described with reference to Figures 7A and B, this support arrangement provides better stiffness against twisting forces on the balancing masses, thereby reducing coil to balancing mass centering errors. It will also be recognized by those skilled in the art that, while the invention has been described above in terms of preferred embodiments it is not limited thereto. Various features and aspects of the above described invention may be used individually or jointly. Further, although the invention has been described in the context of its implementation in a particular environment and for particular applications, e.g. electronic prepress applications, those skilled in the art will recognize that its usefulness is not limited thereto and that the present invention can be beneficially utilized in any number of environments and implementations. Accordingly, the claims set forth below should be construed in view of the scope of the invention as disclosed herein.

Claims: A method of correcting misalignment of a beam of radiation in an optical scanner in which a beam of radiation passes through a rotating optical element which is misaligned and hence causes a periodically changing misalignment of the beam of radiation, the method comprising the following steps: a) passing the beam of radiation through an optical correction element (880) movably supported in a correction element assembly and including an optical axis; characterized by the following further steps: b) driving the optical correction element (880) by use of drive means (820a, 820b) that are mounted to a stationary housing of the correction element assembly nonlinearly in a plane perpendicular to the optical axis of the optical correction element by applying driving forces along two axes perpendicular to the optical axis of the optical correction element; and c) controlling the step of driving the optical correction element such that the misalignment of the beam of radiation is corrected by providing a periodical movement of the optical axis of the optical correction element with respect to the beam axis of the beam of radiation corresponding to the misalignment of the rotating optical element. A scanning system for scanning at least one writing beam over a curved image surface, comprising: a rotating optical element through which the beam passes, whereby said rotating optical element causes a periodically changing misalignment of the at least one writing beam due to its own misalignment, a correction element assembly for correcting said misalignment of the at least one writing beam of radiation propagating along a beam axis of the correction element assembly; an optical correction element (880) movably supported in the correction element assembly, said optical correction element including an optical axis and the correction element assembly being designed such that it is able to move said optical axis with respect to the beam axis in a controlled manner, so that the optical correction element redirects the at least one writing beam; drive means (820a, 820b) controllable to provide driving forces for providing the controlled movement of the optical axis of the optical correction element with respect to the beam axis, characterized in that said drive means (820a, 820b) is mounted to a stationary housing (860a, 860b) of the correction element assembly and is controlled to apply the driving forces along two axes, each of the two axes being substantially perpendicular to the beam axis, and wherein said controlled movement is a periodic movement for correcting said periodically changing misalignment of the at least one writing beam. A scanning system according to claim 2 further comprising: a movable housing (810) having said optical correction element (880) fixedly mounted therein and moveable therewith, and a movable member (850) mounted to said movable housing (810), wherein said drive means deflects said movable member (850), thereby moving said movable housing (810). A scanning system according to claim 3, wherein said movable housing (810) has a longitudinal axis substantially coincident with the optical axis of said optical correction element (880). A scanning system according to one of claims 2 to 4, wherein said at least one driver is configured to apply said force along an axis which is substantially orthogonal to said optical axis. A scanning system according to one of claims 2 to 5, wherein said movable member is a flexure. A scanning system according to claim 6, wherein said flexure (850) is formed of urethane. A scanning system according to one of claims 2 to 7, wherein said at least one driver includes at least two drive devices (820a,b) disposed radially around the movable housing (810) and operable to apply said force. A scanning system according to claim 8, wherein each of said drive devices (820a,b) is separated from each adjacent drive device by 90°, has a drive axis substantially perpendicular to and intersecting a longitudinal axis of said movable housing (810), and applies the force to said movable housing along said drive axis. A scanning system according to claim 8 or 9, wherein said at least two drive devices (820a,b) are four drive devices and said drive devices are configured to simultaneously apply forces to said movable housing (810) to deflect said movable member (850). A scanning system according to claim 10, wherein said four drive devices (920a-d) form two pairs of opposed drive device pairs with the respective opposed drive devices adapted to operate in reverse polarity. A scanning system according to one of claims 2 to 11, wherein said force is a magnetic force and said movable housing (810) is formed of a material which reacts to magnetic forces. A scanning system according to one of claims 2 to 12, further comprising a plurality of magnetic members (970a-d) mounted to said movable housing (810) and configured to react to said force applied by said at least one driver, wherein each said magnetic member has a distal end proximate to but separated from a respective driver. A scanning system according to one of claims 2 to 13, wherein: said at least one driver (1220a-d) applies the force in a first direction to said movable housing (1210) to cause said movable housing to move; and a first compliant member (1215a-d) is disposed between said at least one driver (1220a-d) and a first base (1295) and configured to absorb a reaction force in a second direction opposite the first direction, wherein the reaction force corresponds to the drive force. A scanning system according to claim 14, wherein said first compliant member is one of four compliant members (1215a-d) disposed between said at least one driver (1220a-d) and said first base (1295). A scanning system according to any of the claims 2 to 13, wherein: said optical correction element (1580) further includes a moveable balancing mass (1597a-d); said at least one driver (1520a-d, 1520'a-d) is fixedly supported by a first base (1595) with respect to said movable housing (1510) and said balancing mass (1597a-d) and operable to apply said force in a first direction to said movable housing and a balancing force in a second direction opposite said first direction to said balancing mass; a first compliant member (1415a-d) is disposed between said balancing mass (1597a-d) and a second base, and configured to support said balancing mass and absorb at least a first part of the balancing force; and said balancing mass (1597a-d) is configured to balance at least a second part of the balancing force. A scanning system according to claim 16, further comprising: a first magnet mounted to said movable housing (1510); and a second magnet (1522'a-d) mounted to said balancing mass (1597a-d); wherein said at least one driver (1520a-d, 1520'a-d) includes a first coil for applying the drive force to said movable housing (1510) and a second coil for applying the balancing force to said balancing mass (1597a-d), said drive force is an electro-magnetic drive force and said first magnet reacts to said electro-magnetic drive force to move said movable housing, and said balancing force is a electro-magnetic balancing force and said second magnet reacts to said electro-magnetic balancing force to move said balancing mass. A scanning system according to claim 16 or 17, wherein said first compliant member is one of four compliant members (1415a-d; 1417a-d) disposed between the balancing mass (1597a-d) and said second base. A scanning system according to any of the claims 14 to 18, further comprising a second compliant member disposed between the movable housing and a or said second base and/or a third base and/or said at least one driver, and configured to deflect responsive to the application of the drive force and/or to absorb forces caused by movement of said movable housing. A scanning system according to any of the claims 14 to 19, wherein said first base also forms said second base and/or said third base, and/or said second base also forms said third base. A scanning system according to any of the claims 14 to 20, wherein said first and/or said second compliant member (1215a-d; 1217a-d) is made stiff enough so as to center said optical correction element (1280) with said movable housing (1210) in a stationary position and soft enough so as to deflect and absorb forces with said movable housing in motion. A scanning system according to any of the claims 2 to 5, wherein said movable member (1150) includes a first and a second portion, said at least one driver includes: a first piezo element (1120a) mounted to the first portion of the movable member (1150) and to said movable housing and configured to move said movable housing (1110) in a first direction, and a second piezo element (1120b) mounted to the second portion of the movable member (1150) and to a base (1130), and configured to move said movable housing (1110) in a second direction substantially perpendicular to said first direction. A scanning system according to one of claims 2 to 22, further comprising a reticule (1090ab; 1090a'b') mounted to and moveable with said movable housing (1010; 1110) for encoding movement of said movable housing.

Inventors: BLAKE LAURENCE S, KELLEY HENRY A, LARSEN DAVID B, ROLFE NORMAN, SCHOLTEN FRANK, BLAKE, LAURENCE S., KELLEY, HENRY A., LARSEN, DAVID B., ROLFE, NORMAN, SCHOLTEN, FRANK
Applicants: AGFA CORP, AGFA CORPORATION
Application year: 1997
Main classification: G02B26
IPC codes: G02B27, G02B7, G02B26, B41J2