WO1992004661A1 - A method of adjusting graphical scanner units and a scanner unit having adjustment means - Google Patents

Abstract

In connection with graphical drum scanning a very high accuracy of the drum is required and the scanner unit has to be very accurately adjusted. The invention provides for different purely dynamic adjustment methods, which enable a significant reduction of the accuracy requirements and therewith a reduced price of the scanner units.

Description

A method of adjusting graphical scanner units and a scanner unit having adjustment means.

The present invention relates to a graphical scan¬ ner unit e.g. of the type which comprises a drum shaped holder for a graphical film or printing plate mounted on the inside of the drum and a fast rotating spinner mounted centrally in the drum and carrying an inclined mirror or prism for radially reflecting an axially ap¬ plied laser beam from a beam modulator, the spinner being axially displaceable relative to the drum such that by a combined rotation and displacement it can cause the light beam on the working surface at the in¬ side of the drum to describe a screw shaped working line with closely juxtaposed windings. This technique, which is also known with the use of a light beam inwardly against a fast rotating cylinder, is widely applied for production of graphical film or printing plates based on computer read printing patterns, the computer being used for controlling the beam modulator, which will then serve to enable and disable the passage of the light beam, for producing the ^•'pixels*' or image elements, of which the finally exposed image consists, the pixels typically having a size of 10 x 10 micrometers. When the spinner rotates with a speed of 200-400 r.p.s or, re¬ spectively, when the drum rotates at normally 100-200 r.p.s., a working surface can be exposed reasonably rapidly, and compared with other scanning techniques this technique is relatively simple.

However, in the same connection there are various error sources as to the adjustment of the units, and here this will be explained in more detail, at first with specific reference to the type of unit based on a rotating spinner.

For a perfect result the light beam should hit the spinner highly accurately in the axis of rotation there- of, and this axis should be placed equally accurately coinciding with the axis of the drum, just as the drum should be of an accurately circular cylindrical shape. The latter condition implies that the drum will be ex¬ tremely expensive if produced with optimal accuracy; typically the drum diameter may be some 300 mm, and already a deviation of ten micrometers of this diameter will be undesirable, whether appearing in the longi¬ tudinal or the peripheral direction. So far it has been necessary to work with a suitable compromise between accuracy and price of the drum, although with an as¬ sociated recognition of the facts that the result cannot then be as perfect as desirable.

Quite commonly both the spinner and the beam gene¬ rator are mounted on very finely adjustable supports, such that these parts can be adjusted into exactly cen¬ tral and co-axial positions relative to the drum, and the associated adjustments can be effected manually at the beginning of each working operation with the use of various auxiliaries. It is then to be accepted, however, that the perfect initial adjustment will not remain perfect ^*if or when the operational conditions change during the operation itself due to wrynesses of the drum-shape; it will be very difficult to effect inter¬ mediate operational stops for carrying out control measurements and readjustments. Moreover, already at the initial adjustments the problem will exist that it is necessary to chose an adjustment compromise, should the drum at the relevant place be slightly elliptical; the spinner rotates with a highly constant angular velocity, and when the produced pixels are of a constant duration the associated length of the working or exposing line will be dependent of the radius of the working surface, whereby for a weakly varying radius there will easily be produced visible distortions of the exposed printing image. The present invention is based on the recognition that it is possible to complete the discussed type of units in a manner such that it will be relatively easy to achieve a dynamical adjustment of the working ac¬ curacy in both the length and the peripheral directions of the drum, i.e. in such cases in which the drum is not extremely accurately shaped. In turn, this will imply that generally it will be possible to make use of rela¬ tively very cheap drums, yet still with an obtainable, fully acceptable working result.

According to the invention the spinner can be pro¬ vided with an outer, co-axially rotating sensor having at its outer end sensing means for detecting the distance outwardly to the inner drum or working surface and being operatively connected with a control unit affecting both automatically operating positioning means for centering of the spinner unit and means for control¬ ling the duration of the single pixel impulses, all in a manner such that currently there will be effected both an adjustment of the centered position of the spinner unit and an adjustment of the duration of the single pixels in response to the detected irregularities in the desired circular cylindrical shape of the working sur¬ face.

The said co-axially rotating sensor unit may have distance sensing means mounted with noticeable axial spacing, whereby it is possible to concurrently detect the axial direction of the following cylinder section to be used, as well as an adjustment of the spinner to this direction.

According to the invention the spinner may also be provided with means for parallel reflection of a light beam to a co-rotating sensor that will receive and de¬ tect the radiation for supervising its direction per¬ pendicularly against the working surface and, respec¬ tively, for effecting an adjustment of the beam genera- tor for a follow-adjustment of the beam direction and centering in response to detected changes of the orien¬ tation of the spinner.

It is sufficient that the rotating sensor part is rotated relatively slowly, e.g. with only a single or two revolutions per second, as this will be sufficient for the detection of the changes in shape or direction that may occur in the drum surface in the area thereof next to be scanned. On that basis current adjustments can be made such that the centering will be correct, as well as a dynamic control of the pixel duration such that the pixel length will be correct during the single revolutions of the spinner even in areas, where the cylinder cross-section as a whole is not exact circular.

The drum bodies, of course, should still be carefully produced, but it is an essential effect of the invention that they will not have to be produced with extreme accuracy for the achievement of good and fully usable results.

The same will apply to the means for axially dis¬ placing the spinner relative to the cylinder. This dis¬ placement should be effected with a very constant velo¬ city, whereby the displacement means, normally a screw spindle, should be made with extreme accuracy require¬ ments. The spindle may well be rotated with a fully constant angular velocity, but if the resulting dis¬ placement speed should be completely even the spindle will have to be produced with extreme accuracy, whereby it will be very expensive. It is a natural part of the invention to also provide for a method of dynamically controlling the displacements in such a manner that it becomes entirely even, even if use is made of a less accurately shaped, but then also far less expensive spindle, as disclosed in more detail below.

The features here discussed have parallels by the type of scanners, in which the sheet to be exposed is mounted on the outside of a rotatable drum and is irra¬ diated via an inclined mirror on an outer slide which is advanced in the length direction of the drum. Paral¬ lels exist even to scanners with planar arranged expos¬ ing surfaces, where a scanner head or a deviation mirror is rectilinearly advanced in parallel with the surface so as to describe working lines cross-wise to the direc¬ tion of the displacements. In all instances it can be important that the partial areas of the exposing surface are located, in an absolute or relative sense, just where they would be expected to be located according to the geometry of the system, and according to the inven¬ tion this can be under steady control, with associated operation of actuators for displacing the partial areas where this is possible, and/or dynamical adjustment of the irradiation for compensating for the undesired ef¬ fect of a displaced location of the partial area as actually irradiated.

Particularly in case of large displacements an additional adjustment will be desirable and possible, namely an adaptation of the focal point of the applied laser beam.

The means used for detection of e.g. the exact configuration of the surface to be exposed should not necessarily be in operation during the scanning itself, as they may well be adapted so as to complete their detection movement during an initial phase of the opera¬ tion, with an associated recording of the produced flow of measuring results in a memory unit, which will there¬ after hold full information about the required adjust¬ ments during one or perhaps more successive working operations.

The invention, which also comprises the correspond¬ ing method of controlling the scanner unit, will now be described in more detail with reference to the drawing, in which Fig. 1 is a schematic length sectional view of a unit according to the invention,

Fig. 2 is an end view thereof,

Fig. 3 is a perspective view of a modified drum support,

Fig. 4 is an end view thereof.

Figs. 5 and 6 are interrelated side and end views of another scanning unit, and

Fig. 7 is a schematical side view illustrating correction means for an operational displacement.

The unit shown in Figs. 1 and 2 comprises in a well known manner a spinner housing 2, in which there is provided a fast rotating spinner 4 having at its front end a centrally arranged mirror or prism 6 adapted to receive a central laser beam 8 from a laser beam genera¬ tor, this beam passing through a controlled focusing unit 10 and a modulator 12, which, controlled by a com¬ puter, operates as a on/off switch unit for the light beam.

In a coaxial manner the spinner housing 2 is sur¬ rounded by a cylinder or drum 16, on the inner side of which can be mounted a film or another light sensitive plate or sheet member 18 for graphical use. This member, which extends along only a part of the inner periphery of the drum, is held tightly against the inner side of the drum, normally by means of a vacuum applied from outside. The spinner housing is axially movable relative to the drum, and the mirror or prism 6 will serve to deflect the light beam 8 radially against the film 18.

When the spinner 4 rotates and at the same time is displaced axially the radial light beam or working beam designated 20 will describe a screw-line on the film 18, and when the pitch of the screw-line corresponds to the thickness of the working line the result will be a co¬ herent exposure of the film surface, with the exposure pattern determined by the computer 14. Normally the drum 16 is axially split as shown at 22 in Fig. 2, and the spinner housing 4 is mounted on a support 24 extending through this split for connection with a non-illustrated carrier slide that is operable to effect the said axial displacement of the spinner relatively to the rigidly mounted drum 16.

According to the present invention there is mounted, in connection with the spinner housing and exactly coaxially with the spinner, a rotating measuring rotor 26 consisting of a hub portion 28 having outwardly projecting radial arms 30, which at their outer ends carry a cylindrical shell 32 extending along a partial length of the outer drum 16, with its front end portion projecting in front of the spinner housing to a position slightly behind the working beam 20. The measuring rotor is driven at an angular velocity much lower than that of the spinner.

At the respective front and rear ends of the cylinder shell 32 and at the outside thereof there is mounted a pair of distance measuring units 34, e.g. in the form of well known radiation based sensors, which can currently detect the distance to the inner side of the drum or rather to the inner side of the film 18. The sensors 34 are connected to a microprocessor 36, i.e. via contact rings not shown.

At the inner side of the cylinder shell 32, at the front end thereof, there is mounted a light sensor 38 adapted to receive a light beam 40 deflected radially from the spinner 4 in parallel with the working beam 20, this being enabled by the mirror 6 being slightly trans¬ parent, such that the beam 8 is carried further in a weak beam reaching an extra mirror or prism 42, which will then produce the radial beam 40. Thus, this beam will always be exactly parallel with the working beam 20, and it will hit the sensor 38 by each revolution of the spinner 40, subject to the beam 8 being switched on, and it will be unimportant that the sensor 38 by each beam passage has been moved very slightly due to the slow rotation of the measuring rotor 26.

The sensor 38 is not a light intensity gauge, but a beam direction gauge which will detect whether the re¬ ceived beam 40 is exactly perpendicular to the mounting plane of the sensor, respectively whether the received beam is located exactly at the middle of the sensor. By way of example, should the beam 8 be slightly inclined relative to the common axial direction of the considered system the beam 40 will not be deflected entirely per¬ pendicularly from the axis of the system, and at least in certain positions of the measuring rotor this will imply that the beam 40 moves forwardly or rearwardly from the middle of the sensor 38, which can thereby detect the obliqueness of the beam 8. Also the sensor 38 is connected to the microprocessor 36.

The spinner housing 2 is mounted in a non-illustrated support by intermediate of a number of piezo ceramic actuators 44 which at the respective front and rear ends of the spinner housing may be operated to effect a small transverse displacement of these housing ends by means of electrical control signals from the microprocessor 36. Correspondingly, the laser generator 9 may be supported in a similar manner, such that also the centering and beam direction of this unit can be controlled dynamically within an estimated required range. In both cases the required displacements will be so small that they can be effected by the use of simple piezo elements 44.

It will be readily understood that the measuring rotor 26, via the outer sensors 34, will control the centered location of the spinner 4 in the outer drum during the on-going, relatively slow axial movement of the spinner. Should a section of the cylinder change the direction or the location of the center axis this will quite rapidly be sensed by the sensors 34, which will then, via the micro processor 36, affect the elements 44 to adjust the spinner according to the changed circum¬ stances. If or when an associated wry or off-centered incidence of the laser beam 8 occurs this will be de¬ tected by the sensor 38, whereby the micro processor causes the laser generator 9 to change its position for correction of the beam incidence so that this will again be correct for achieving a perpendicular incidence of the working beam 20 on the film 18. By this dynamic control the working beam will not get an opportunity to be displaced noticeably from its expected track along the film surface, whereby a perfect reproduction of the image pattern will be secured. Alternatively, the spin¬ ner position may be adjusted by a corresponding, current adjustment of the guiding means for the spinner housing. During the axial movement of the spinner the length of the beam 8, 20 will change. The focusing unit 10 should focus the laser beam exactly in the working point on the exposure surface, this being controlled auto¬ matically via the computer 36. According to the inven¬ tion it is possible to improve this control based on the additional changes of distance as detected by the sensor 34.

The irregularities that may occur when the drum 16 is not shaped with extreme accuracy may also be due to local deviations from the whole-circular cross-section of the drum, whereby as mentioned the problem will arise that the single pixels will appear with mutually dif¬ ferent lengths, while for a perfect reproduction they should appear with constant length. The dynamic adjust¬ ment possibilities as discussed above will thus only amount to half a solution of the discussed problem, i.e. no complete solution, and it is therefore equally im¬ portant that also this problem be solved.

For this purpose, according to the invention, use is made of the already described equipment, namely the sensors 34 for detecting whether in the generally cen¬ tered position of the spinner housing there are any differences in the distance between the sensors 34 and the film 18 during the revolutions of the measuring rotor 26. Between the spinner 4 and the spinner housing 2 detector means 45 are provided for detecting the an¬ gular position of the spinner relative to the housing, and via a wire 46 these means are connected to a control unit 48 for the duration of the pixel pulses. Also the unit 48 is connected with the micro processor 36, and it is achievable hereby that a periodic variation of the distance out to the film 18 as detected by one of the sensors 34 will produce a corresponding change of the pixel length by the beam sweeping of the film, such that the neighbouring pixel lengths will be quite uniform anyway. Typically the pixel length will be increased where the drum radius is being increased and decreased where the drum radius is decreasing.

It will be natural to let the sensor 34 located adjacent the front end of the measuring rotor 26, i.e. nearest to the working beam 20, be responsible for the control of the pixel length.

The invention comprises further adjustment means for alternative or supplementing control of the center¬ ing of the spinner relative to the cylinder 16 and par¬ tial areas thereof, namely means for a controlled mechanical deformation of the cylinder itself depending of variations in the measuring results from the distance sensors 34. Hereby it will be possible to produce the cylinder of a relatively thin and light material, al¬ though it may have to be arranged inside an outer cyl¬ inder, this being illustrated in Fig. 3.

Between the inner cylinder 16 shown in Fig. 3 and the said outer cylinder 50 there is mounted with even and reasonably close distribution a number of piezo electrical distance elements 52 that are individually electrically connected to the micro processor 36. When the sensors 34 detect a non-normal distance to the cyl¬ inder 16 the relevant elements 52 may be actuated to remedy the deviation, the inner cylinder 16 here being sufficiently yieldable for being locally deformed by the action of the elements 52.

The effect of the elements 52 is not bound to be a local deformation of the cylinder 16, as another result may be a pure moving of the entire cylinder cross-section relative to the stronger outer cylinder 50 for achieving a perfect centering.

It is shown in Fig. 3 that the cylinder unit 16, 50 is supported by a support 54, which, optionally, can be slidably arranged on rails 56 for carrying out a desired axial displacement during the scanning, namely if this movement is to be made by the cylinder and not by the spinner. It is also shown in dotted lines that the sup¬ port may have an extended length and that in the surface of the groove 58, in which the cylinder unit is sup¬ ported, holes 60 can be provided. This refers to the cylinder being slidably held in the groove 58 in a still-standing block 54 and to the holes 60 being blow-out holes for compressed air, such that the cyl¬ inder will be carried without friction on an air bear¬ ing.

This generally is an advantageous support system for an element that is desired to be easily displace- able, but in connection with the invention the arrange¬ ment may furthermore be utilized for the desired dynam¬ ical control of the centering of the system. To that end, as shown in Fig. 4, automatically operated control valves 62 may be provided in the single air connections to the holes or nozzles 60, and it has been found pos¬ sible that in controlling these valves from the computer 36 an extremely sensitive adjustment of the location of the cylinder relative to the groove 68 is achievable, i.e. also here it is possible to provide a rapid center¬ ing control responding to signals from the sensors 34.

Such a control could well be effected on a single cylinder 16 according to Fig. 1, but of course the ad¬ justment of the active cylinder part can be made still more accurate when as shown in Figs. 3 and 4 there is used an active cylinder, which can be moved as a whole by means of the controlled air nozzles and additionally can be locally adjusted with the use of the piezo elec¬ trical elements 52.

It has already been mentioned that the position of the spinner can be adjusted by use of the piezo elec¬ trical elements 44, but it should be added that also this unit may alternatively or supplementary adjusted by means of the said controlled air bearings.

In Figs. 5 and 6 another scanner principle is il¬ lustrated, where the exposure surface, e.g. in the form of an offset film 64, is mounted in a well-known manner on the outer side of a drum 66, which, by driving means not shown, is rotated with a speed of e.g. 1000-2000 r.p.m. Along the drum there is provided a guiding rail for a slide 68 carrying an inclined mirror 70, which receives a laser beam 8 and deflects this beam ortho¬ gonally onto the drum surface as a focused working beam 20. The slide is driven along the drum by means of a spindle (not shown) .

According to the invention there is mounted on the slide 68 a distance detector 72 at either end of the slide, and two corresponding sensors 74 are placed on a carrier arm 76 in positions spaced 90° from the slide 68 (Fig. 6) . These sensors are connected in the system just as the sensors 44 in Fig. 1, whereby they can control the same kind of corrections as specified above. Thus, also here the drum can be produced less accurately than otherwise. For a good result it is important that the applied means for effecting the axial displacement operate with a highly even speed. Commonly a screw spindle is used, and it can well be rotated with even speed, but it should be worked extremely accurately for translating the rotation into an even displacement of the driven elements. In Fig. 7 the screw spindle is designated 80 and an associated motor 82. On the spindle is mounted a slide, e.g. the slide 68 of Fig. 5, this slide having a nut member 84 in threaded engagement with the spindle.

According to the invention the nut member 84 is axially connected with the slide 68 through a number of piezo electrical actuators 86 connected to a servo con¬ trol unit 88. A laser interferometer 90 is mounted for momentary detection of the moving velocity of the slide 68 by means of a detector beam 92 and for reading in the velocity into the control unit 88. Hereby, in case of a velocity that is too high or too low, the control unit 88 may adjust the speed of the motor 82, but because the reaction time of the motor is relatively long care is taken for arranging a more rapid or instantaneous acti¬ vation of the actuators 86 for displacing the slide in the relevant direction relative to the nut member 84. These movements are controllable such that the adjust¬ ments can be initiated by the first weak signs of ir¬ regularities, whereby these will not develope into larger and noticeable irregularities. It is perfectly possible to achieve a strong attenuation e.g. of vibra¬ tions that would otherwise incur axial displacements of the magnitude 10 micrometers.

Claims

C L A I M S :

1. A method of adjusting graphical scanner units in which an exposure surface is swept by a modulated scan¬ ner light beam generated by a controlled light source, said beam rapidly producing pixels on the exposure sur¬ face along a working line thereon, a mutual displacement in a direction across the working line being effected between the exposure surface and the element emitting light thereagainst, such that the exposure surface is successively irradiated along working lines located immediately side by side, characterized in that a fine adjustment of one or preferably more of the respective relevant operational conditions, e.g. the angle of inci¬ dents of the light beam on the exposure surface or the displacement velocity between the light emitting element and the exposure surface, is effected currently during the operation by correction of the relevant condition based on current or previous . measurements thereof by means of measuring and correction means directly inte¬ grated with the unit.

2. A method according to claim 1, whereby one or more of the following adjustments are made:

A) Detection of the distance between the light source and partial areas of the exposure surface and an associated controlled activation of actuator means a) for adjusting the partial areas of the exposure surface into a correct position, and/or b) for adjusting the focus of the scanner beam on partial areas located in different distances from the light beam, and/or c) for adjusting the pixel duration so as to obtain working lines of uniform length on partial areas located at different distances from the light source, and/or d) for currently adjusting the light source to a correct spacing from the partial areas of the exposure surface, and/or

B) Detection of correct angle of incidents of the scanner beam on the exposure surface and an associated activation of actuator means a) for adjusting the radiation direction of the light source, and/or b) for adjusting the spacial direction of the exposure surface; and/or

C) Detection of the conveyance of the light source relative to the exposure surface and an associated con¬ trolled activation of actuator means for compensation means operable to compensate for occurring speed vari¬ ations, or for adaptation of such variations to require¬ ments as to varying speed detected in connection with A) or B) , respectively.

3. A method according to claim 2, whereby the scan¬ ner unit is driven according to the rotation principle, i.e. with a fast rotating, radiating spinner mounted in the centre-line area of a surrounding, cylindrically arranged exposure surface, or with a relatively lateral¬ ly movable light source arranged outside a less fast rotating, cylindrically arranged exposure surface, char¬ acterized in that the measuring of the distance between the light source and the exposure surface is effected by means of a distance sensor slowly co-rotating with the spinner, or, respectively, by means of an outer, non-rotating device, which, together with the scanner light source, is laterally displaced relative to the drum.

4. A method according to claim 1, whereby the rele¬ vant measurements, e.g. of the positioning of the ex¬ posure surface in the various partial areas thereof, are effected prior to start of the scanning operation, the measuring results being supplied to a memory, from which they are successively read out for effecting the re¬ quired adjustments during the subsequent scanning operation.

5. A scanner system for carrying out the method according to claim 1, characterized in that it is pro¬ vided with means for measuring relevant operational parameters and with associated actuator means for dynamically affecting such adjustment means, by which the relevant operational parameters can be adjusted.

6. A system according to claim 5 and comprising the required means for carrying out the various disclosed methods.