GB2333385A

GB2333385A - Variable rate clock

Info

Publication number: GB2333385A
Application number: GB9907144A
Authority: GB
Inventors: Roderick Iain Craig
Original assignee: SYNECTIX Ltd
Current assignee: SYNECTIX Ltd
Priority date: 1995-02-02
Filing date: 1995-02-02
Publication date: 1999-07-21
Anticipated expiration: 2015-02-02
Also published as: GB2297613B; GB2297613A8; GB2297613A; GB2333385B; GB9502030D0; GB9907144D0

Abstract

In a variable rate clock apparatus a reference clock signal provided by a reference clock 70 is divided in a dividing counter 72 by a value stored in a look-up table 62 to obtain an output clock signal. The output of the dividing counter can be scaled by a PLL 74. The reference clock signal can be divided by one of a number of stored integer values to create the desired output clock signal. Alternatively a desired average clock signal can be obtained by supplying a sequence of divider values to the dividing counter. The variable rate clock can be used to control the timing of sampling signals in a computer to plate apparatus to compensate for variations in the scanning speed during a scan.

Description

COMPUTER-TO-PLATE APPARATUS This invention relates to computer-to-plate apparatus and particularly, but not exclusively, to such apparatus incorporating calibration means for allowing calibration of parameters such as the cross-axis correction and the variable rate pixel clock period.

Modern offset lithographic plates may be produced by a computer-to-plate machine, in which a beam of laser radiation exposes an image on a photosensitive plate by using a raster scan of modulated radiation, in much the same way as a television image is built up by a raster scan of a modulated electron beam. Although there is in fact no discrete pixellated structure, it is convenient and conventional to refer to both the computer-generated image and the corresponding image on the lithographic plate, as being made up of pixels.

The image to be printed may be held on a host computer, and may contain text, figures etc. in the required format.

Once the image is in an acceptable final form on the host computer, it is rasterised. This involves the host computer making a virtual image and then reading lines from that image to provide a serialised string of O's and l's . It will be appreciated that the relative size of the pixels with respect to the virtual image is thus dictated in the direction of the scan line by the number of samples taken per line of the virtual image and, in the transverse direction, by the spacing between adjacent raster lines. In turn, the number of pixels/lines and lines/image, and the size of the printing plate determine the resolution of the printed image.

The serialised data representing the virtual image may be compressed using suitable known compression methods, and then stored ready for supply to the computer-to-plate machine.

A typical example of a computer-to-plate machine includes a scanner head which scans a radiation beam, eg. a laser beam, across the photosensitive plate. An optical modulator or switch modulates the radiation beam by switching it on or off. The optical modulator may comprise an acousto-optic modulator which acts by deflecting the laser beam off or onto the required axis.

The scanner head includes a scanner mirror mounted on a torsion wire and which in use is energised electromagnetically so that the mirror oscillates about the torsion wire at a high frequency, typically 200Hz. The scanner head is continuously advanced relative to the photosensitive plate so that an image is built up line-byline on the plate.

The mirror movement is essentially sinusoidal and so the angular displacement and velocity are generally sinusoidal but 90" out-of-phase. Also, the beam velocity along the scan line has a non-linear component owing to the fact that the beam scans along a flat plate rather than a concentric cylindrical surface. This effect is further complicated by the Fe lens which only approximately corrects this, because it is more important that it produces a focal line that is straight in 3D.

Consequently, the combined effect of these nonlinearities means that at the start and end of a scan line a pixel takes longer to expose than in the middle. Thus, the clock signal which controls operation of the optical modulator must be adjusted by varying its period as the beam progresses along the scan line so that each pixel in the exposed photosensitive plate corresponds precisely in terms of size and spacing from neighbouring pixels, with the relevant pixel in the virtual image, and so the image printed by the lithographic plate once exposed and developed is exactly in accordance with the image prepared on the host computer. Accordingly a need exists for a calibration technique which allows a machine to check calibration of the period of the variable rate pixel clock.

We have also developed a cross-axis correction technique whereby a time-varying cross-axis correction component is applied to the scanning beam to compensate for wobble of the scanning mirror, and for the zigzag or helical error experienced when a machine scans in both forward and reverse scans as described in our co-pending Application No.

9502037.6.

Although we can determine within reasonable limits the waveform of a cross-axis correction to compensate for these effects, and digitally store said waveform, it is still necessary to calibrate each machine by incrementing or decrementing the stored digital values to ensure that the corrected scan line follows a preset datum.

Whilst in theory calibration could be done once during assembly, in practice this is not realistic for a variety of reasons. It wculd make field service more difficult if calibration tools have to be carried to the machine; it means that the machine cannot determine whether it is well set up; it also requires provision of tools and training to field service personnel and it does not allow for gradual drift in the errors.

Accordingly, it is an aim of this invention to provide a machine-to-plate apparatus which is capable of selfcalibration, or at least capable of indicating when manual intervention is required. For this the position of the beam in the focal plane (ie. on the platen) needs to be determined.

Previous proposals to determine the position include placing a beam splitter such as a semi-silvered mirror in the optical path of the beam so that the transmitted component passes to the platen to produce a scan line and the reflected component produces a second scan line on a graticule, behind which is placed a photodetector.

However this does lose laser power, which may mean that a more powerful - and thus more expensive - laser is required, and it is not suitable when the optic system is moving, as then the splitter, graticule and detector must be moved as well. Also, of course, the scan line is calibrated to the graticule, which is physically quite separate from the platen.

In another system, which is sometimes used for telecentric systems, a graticule made from opaque and clear areas is placed in front of a curved mirror coincident with the curved focal plane of the machine. Light is then reflected by the curved mirror back through the optical system, and the reflected beam is extracted and monitored.

This system does not however work with flat-bed machines, because the light beam is not normal to the reflecting surface and so is reflected away from the incident beam, rather than back along the incident optical path and back through the optical system, and so cannot be easily detected within the existing optical system.

We have therefore designed a computer-to-plate machine with a calibration system which does not require the laser beam to be split before reaching the platen, which may be used for both curved bed (telecentric) and flat bed machines, and which does not require production of a curved mirror.

Accordingly, in one aspect, this invention provides a computer-to-plate apparatus, including a platen means, a scanning head mounted above said platen means, said scanning head including means for generating a beam of radiation, directing it down an optical path and scanning it along a scan line over said platen means, a target means disposed in or adjacent said platen means and including thereon reflective datum mark means (eg. a line or series of lines, or a graticule in positive or negative), and detector means offset from said optical path for detecting light reflected from said target means, and incidence determining means responsive to said detector means for determining incidence or non-incidence of said scan line, or a selected part thereof, on said datum mark means.

This arrangement has the advantages that it does not require the laser beam to be split, it does not require laser light to be reflected back up the optical path, it does not require the construction of costly curved reflectors, and furthermore the target line is actually in or adjacent the platen of the machine.

The reflective datum means may comprise white or other reflective lines or marks on a relatively absorbent background, or absorbent lines or marks on a reflective background provided that the reflective areas reflect a portion of any radiation incident thereon back to the detector. In order to reduce the sensitivity required of the detector, and thus its cost, a variety of measures may be implemented. In a particularly preferred apparatus the reflective areas comprise retro-reflective material such as eg. the retro-reflective material used in the manufacture of reflective road signs.

Most such material consists of tiny corner cube reflectors or spherical beads of transparent material located on a transparent membrane and backed by a reflective layer of, eg. aluminium. Such materials have the property that they reflect light back in the direction of incidence, but we make use of the fact that, although the reflection is directional. the reflected beam does spread, and so the detector may detect a strong signal even when offset slightly from the optical path.

The amount of radiation detected may also be increased by incorporating means for collecting radiation and directing it onto the detector. This collecting means may comprise a simple curved reflector.

A particularly preferred form of target means comprises a transparent substrate eg. of glass with generally parallel plane upper and lower surfaces, the upper surface carrying datum means in the form of a target mask and the lower surface carrying a layer of retro-reflective material.

The datum means may comprise an absorbent, nonreflective datum line corresponding to the required scan line, so that the processor may calibrate the cross-axis correction so that the beam does not deviate frorn the required scan line in use. In practice, the computer-toplate machine will usually have a single cross-axis correction line in the middle of the platen, corresponding to the line scanned when the machine is half way through a plate exposure, although the line may be located elsewhere, or more than one calibration line may be included.

For calibration of the variable pixel rate, the datum means preferably comprise a series of reflective lines or marks spaced at equal intervals along the scan line, so that the pixel rate timing can be adjusted to bring the proper pixel onto its corresponding calibration line.

The target means is preferably securely attached to or integral with the platen, although in some circumstances a removable target means may be used, but this of course requires accurate registration of the target means with the platen.

During a calibration routine, the processor means preferably controls the scanning head to execute a scan along the target means and then determines from the output of said detector means whether the scan parameter of interest deviates from the desideratum.

This may be in terms of transverse error, where the scan line moves off the calibration line, or an error in the period of the variable-rate pixel clock. The pixel clock is used by the processor during normal operation of the machine to clock ON OFF modulation data to the scanning head, and thus defines the pixels along the scan line. Errors in the pixel clock rate mean that the virtual image created by the host machine does not map precisely to the image exposed on the plate, with consequent distortion of the image produced by the printing plate. Each pixel thus corresponds to a time increment of each scan cycle, usually measured with respect to the time of the beam detect pulse. The processor can determine which pixel should correspond to a particular calibration line or mark at a given position along the scan line, and thus the precise time during the scan cycle that the beam should be incident on the calibration line or mark.

By setting a pixel corresponding to a particular line or mark, and monitoring the output of the detector at that time, the processor can determine whether the beam is on a calibration line.

Although the processor may simply indicate whether or not the machine is calibrated, it is preferred for the machine to adjust the scan parameters as necessary to return the machine to a calibrated state.

Thus the machine preferably includes means for adjusting a cross-axis correction means for compensating for cross-axis deviation from the scan lines, and likewise means for adjusting the variable pixel rate.

If the machine already includes cross-axis correction means, for example to compensate for scanning mirror wobble or helical or zigzag error, the processor may modify the pre-determined cross-axis correction data in accordance with the data derived during calibration. Thus, for example, the cross-axis correction data may be stored as a digital signal representing the varying amount of cross-axis correction required for each scan, forward and reverse. The stored digital values may be incremented or decremented as required to compensate for the deviations noted.

For the variable pixel rate, again the machine may have data stored representing the pixel rate variation during each scan, and the processor means preferably increments or decrements the digital values to re-calibrate the machine. so that the scanning beam is at the right place at the right time.

Whilst the invention has been described above, it extends to any inventive combination of the features set out above or in the following description.

The contents of our co-pending British Patent Application No. 9502037.6, filed on even date herewith are incorporated herein by reference. In our co-pending Application we describe a scanner apparatus incorporating a resonant scanning mirror and a cross-axis correction mirror which compensates for cross-axis deviations due, for example, to wobble of the scanning mirror and zigzag movement of the scan beam due to the continuous movement of the scanner apparatus relative to the surface being scanned.

By way of example only, a specific embodiment of computer-to-plate apparatus will now be described with reference to the accompanying drawings, in which: Figure 1 is a schematic view from one end of a computer-to-plate machine in accordance with this invention; Figure 2 is a schematic top plan view of the machine of Figure 1; Figure 3 is a side view of the machine of Figures 1 and 2, taken on arrow I of Figure 1; Figure 4 is a schematic cross-section view through a target in accordance with this invention; Figures 5(a) and 5(b) are examples of a cross-axis correction target and a variable-rate pixel clock calibration target respectively; and Figure 6 is a block diagram illustrating the calibration system of the present invention.

Referring initially to Figures 1 to 3, the computer-toplate machine 10 comprises a scanner head 12 mounted at a fixed height over a platen 14 for transverse movement in the direction A shown in Figure 3. The scanner head 12 is moved by a conventional lead screw driven by a precision stepper motor (neither shown) to allow extremely accurate relative movement.

The scanner head 12 comprises an optically rigid chassis member 16 on one side of which the main lens system or Fe lens 18 is mounted, and on the other side of which a resonant scanning mirror assembly 20 is mounted, with the chassis cut away to allow passage of the laser beam 22. A leg 24 of the chassis member extends downwardly and carries the beam generating/correction subassembly. This comprises a laser 25, an optical isolator 26, an acousto-optic modulator 28 which modulates the laser beam, mirrors (not shown) reflecting the beam 22 through an aperture in the leg 24. The beam passes back up the other side through a telescope 30 which makes the beam 22 parallel, thence via a further mirror 32 to a cross-axis correction mirror 34 and then to the resonant scanning mirror assembly 20. The assembly 20 consists of a scanning mirror 36 mounted on a torsional wire 35, the mirror being driven at its resonant frequency (in this example about 200Hz) by an armature/coil arrangement (not shown). The scanning beam 22 passes through the Fe lens 18 to a corner reflector assembly 36 which consists of two elongate mirrors 38 which fold the beam 22 as seen clearly in Figure 3.

In use, the scanner head 12 scans the laser beam 22 across the platen 14 as the head 12 is gradually traversed relative to the platen in the perpendicular direction by the lead screw.

In a conventional, uncorrected, system the beam 22 would be subject to two error components, each of which are in the cross-axis direction, ie. perpendicular to the scan line 8. The two error components are the zigzag or helical error and the wobble of the scanning mirror 36 which tends to move slightly about an axis perpendicular to the torsion wire 35 at the beginning and end of each stroke, so that a beam deflected by said mirror 36 tends to describe an ellipse of instead of a single straight line.

The magnitude of these error components may be determined by analysis or calculation. In a particular example, we have found that the deviation due to mirror wobble was equivalent to an optical angle of about lO0rad full scale. The full scale magnitude of the correction for zigzag is the scan line separation which for this example was 1"/2540 (lO m) or 1"/1270 (20cm) (ie. a resolution of 2540 or 1270 lines per inch (2.54cm)), giving optical angles of 16prad or 32prad respectively.

The variation of each correction along the scan line 8 is calculated, and both components are added together to produce a compensation curves representing the angular correction needed to provide a series of substantially straight parallel lines, with the lead screw moving the scan head to give a feed of 1270 or 2540 lines/inch, and taking account of the frequency of the scanning mirror.

The compensation curve is made up of the elliptical correction (maximum swing of + SOprad) and the zigzag or helical correction (+ 8prad or + 1611rad respectively).

The platen 14 has a calibration plate 42 (see Figures 1 and 3) let into the upper surface thereof midway along the platen. Referring to Figure 4, the calibration plate consists of a glass strip 44, the lower surface of which is covered with a layer 46 of retro-reflective material and the upper surface of which carries a mask 48 defining calibration lines or marks 50 for a calibration target. The calibration mask preferably defines the targets shown in Figures 5(a) and 5(b), side by side. The cross-axis calibration target of Figure 5(a) comprises a retroreflective background 52, and a non-reflective straight line 54, 0.7 pixel wide, representing the required scan line. The variable pixel-rate calibration target of Figure 5(b) is a series of slots 56 in the mask, defining equispaced reflective lines transverse to the scan lines, corresponding to equal spaced pixels in the scanned beam.

A photodetector 57 (see Figure 3) is mounted on the scanning head at a position clear of the area swept by the scanning beam, but adjacent the optical path of the machine, facing a curved radiation collector/reflector 58. The collector/reflector 58 is disposed so that, when the beam is incident on the region of the calibration plate corresponding to the calibration line, the beam, or part thereof passes through the mask, and is reflected back in generally the same direction as the direction of incidence.

However the retro-reflective material does cause spreading of the reflected beam to that a significant proportion of the beam falls on the curved radiation collector/reflector 58 and thence passes to the photodetector.

Thus the photodetector detects light when the scanned bean is incident on a retro-reflective portion of the target.

Referring to Figure 6, operation and calibration of the computer-to-plate machine is controlled by a processor 60.

In normal use the processor 60 takes and decompresses data from the host computer 66 to assemble in its memory data for a raster line, arranging that the data is ordered so that it can be read sequentially along the line (or backwards if the scanning beam is writing on the reverse scan).

When the machine is ready for another line of data, the processor pulls successive bytes of data from memory and serialises them. The serialised data is loaded in parallel into a shift register, from which the data is shifted out, and fed serially to the acousto-optic modulator 28. As discussed previously, to compensate for variation in the speed of the bean across the platen, data is clocked to the acousto-optic modulator by a variable-rate pixel clock signal whose period changes along the scan line.

For each scan (forward or reverse), the processor 60 reads data controlling the period of the clock signal as a function of the time elapsed from the beam detect pulse, from a look up table 62. The beam detect pulse is a pulse detected shortly before each scan and is used in conventional timing for scanners. The processor then uses the data for the look up table to construct the variable rate pixel clock signal.

The processor also controls the position of the crossaxis correction mirror using a piezo-electric transducer 68 using a precalculated waveform intended to compensate for cross-axis deviation due to wobble of the main scanner mirror and zigzag movement. This data too is stored in a look up table 64 as a digital values representing the correction amplitude as a function of elapsed time.

During a calibration routine, the processor moves the scanner head directly over a calibration line, eg. the cross-axis correction line. It then loads pixel data setting one or more specific bits and then causes the scanner to scan the beam along the line and monitors the output of the photodetector to determine whether the set bits are detected. For extreme precision, the photodetector signal may be monitored for each pixel but in practice high accuracy may be achieved by obtaining the phase signal during the scan line and the time and thus pixel number where it occurred allowing compensation for any lag in the system.

The preferred calibration line is a single black of width about 0.7 of the pixel's nominal dimension transverse to the scan line, against a reflective background.

If the present processor determines that the scan line deviates from the calibration line at any point, it will update the corresponding value in the look up table 64.

Various schemes may be used, the following being just one example: - Turn on the cross-axis correction system Turn on first pixel of the forward scan line Position the carriage to place that pixel on the target line Until (Calibration is done) For each pixel in turn Turn on just CurrentPixel Adjust value in look up table until CurrentPixel is centred on target line If it is not possible to get pixel centred (no minimum) Move carriage so as to remove bias Restart calibration Note range of acceptable signals for this pixel For each pixel in turn Turn on just CurrentPixel If signal is not acceptable Restart calibration The exact nature of the calibration for the variable rate pixel clock will depend on the mechanism for loading correction data into the pixel clock system. The preferred pattern to use for the target for calibrating the variablerate pixel clock is a form of grating, with a small number (less than 100, almost certainly) of calibration points, an example of target being shown schematically in Figures 5(a) and (b).

The forward and reverse scan each have their own calibration, and it is important that the calibration method does not have a bias that would be different for forward and reverse scans. This argues against calibrating against a dark to light edge, since it would be a light to dark edge for the opposite scan direction, and so the preferred target will probably be a 0.7 pixel wide white line at each calibration point. A suitable procedure for calibrating the variable-rate pixel clock might be as follows: For forward and then reverse scan direction f For each calibration line, from the start of line Repeat f Determine which pixel is actually over the calibration line, using the Locate Straddling Pixel Procedure (see below) If the pixel found to be over the calibration line is not the correct one, then adjust the variable rate pixel clock so as to bring the correct pixel to the calibration line.

Until the correct pixel is over the calibration line ) (next calibration line) ) (next scan direction) Locate Straddling Pixel Procedure f Detect which pixel straddles a calibration line by a binary chop Fill a block of pixels (in the raster) which straddles the one where the calibration line should be, and extending halfway to the calibration lines on either side. Make sure calibration detector senses an on pixel, or the system is broken.

While there is more than one pixel in the block f Clear the pixels in the top half of the block.

If the calibration detector senses an on pixel Then Continue using the top half of the block Else Continue using the bottbm half of the block Endif The one pixel block is the pixel that straddles the calibration line.

One example of variable-rate pixel clock sub-system is shown in Figure 7. The system comprises the look-up table 62, a reference clock 70, a dividing counter 72, which divides the output from the reference clock 70 by the value presented by the look up table 62, and supplies it to a phase locked loop 74 which scales up by a constant multiplier, in this example 10, as well as providing a smoothing effect. The reference clock 70 operates at 40 MHz. To produce a pixel clock rate of 40 MHz the counter 72 would be (repeatedly) loaded with 10, and to obtain a pixel clock rate of 25 MHz, the counter 72 would have to be 16.

Thus, to achieve a general pixel clock rate of x MHz, the counter needs to be loaded with an average value of 400/x.

Thus, an average rate of 22.73 MHz requires an average counter of 17.598. Now, this is achieved by running a sequence of 17 and 18 into the counter, with a slight preponderance of 18 (598 to 402). The mechanism chosen to represent this is an initial counter value, and a chain code which can make the next counter value the same or different (plus or minus) by one.

As mentioned, the phase locked loop produces a smoothing effect, and hence a delay. This means that the counts that determine which pixel is coincident with a calibration bar may (and probably will) have ended somewhat before the calibration bar. However, a first pass to determine which counts contribute to which calibration points is performed. It might be necessary to iterate the process, as the counts that contribute to a calibration point will depend on the pixel coincident with the point; changing one may change the other.

Claims

Claims 1. A variable-rate clock apparatus including: reference clock means for providing a reference clock signal, and a dividing counter for dividing the reference clock signal with a pre-stored value to obtain an output clock signal.
2. A variable rate clock apparatus according to Claim 1, wherein the output of said dividing counter is supplied to a phase lock loop means adapted to scale the output by a preset multiplier value.
3. A variable rate clock apparatus according to Claim 1 or Claim 2, which includes store means for storing a plurality of integer divider values, said apparatus being operable to select one of said divider values to obtain a desired output clock signal.
4. A variable clock rate apparatus according to Claim 3, wherein said apparatus is operable to select two of said divider values and to supply a sequence of said two divider values to provide an output clock signal having a predetermined average clock rate.
5. A variable clock rate apparatus according to Claim 4, including means for assembling a chain code for supplying a sequence of adjacent integer divider values to said divider.
6. A computer-to-plate apparatus, including a platen means, a scanning head mounted above said platen means, said scanning head including means for generating a beam of radiation, directing it down an optical path and scanning it along a scan line over said platen means, a target means disposed in or adjacent said platen means and including thereon reflective datum mark means, and detector means offset from said optical path for detecting light reflected from said target means, and incidence determining means responsive to said detector means for determining incidence or non-incidence of said scan line, or a selected part thereof, on said datum mark means.
7. A computer-to-plate apparatus according to Claim 6, wherein the reflective datum means comprises white or other reflective lines or marks on, or spaced in front of, a relatively absorbent background.
8. A computer-to-plate apparatus according to Claim 6 or Claim 7, wherein said reflective datum means comprises absorbent lines or marks on, or spaced in front of, a reflective background.
9. A computer-to-plate apparatus according to Claim 7 or Claim 8, wherein the reflective areas comprise retroreflective material.
10. A computer-to-plate apparatus according to Claim 9, wherein said retro-reflective material comprises retroreflective elements of transparent material located on a transparent membrane and backed by a reflective layer.
11. A computer-to-plate apparatus according to any of Claims 6 to 10, including means for collecting radiation and directing it onto the detector means.
12. A computer-to-plate apparatus according to Claim 11, wherein the collecting means comprises a curved reflector.
13. A computer-to-plate apparatus according to of Claims 6 to 12, wherein the target means comprises a transparent substrate, eg. of glass, with generally parallel plane upper and lower surfaces, the upper surface carrying datum means in the form of a target mask and the lower surface carrying a layer of retro-reflective material.
14. A computer-to-plate apparatus according to any of Claims 6 to 13, wherein said datum mark means indicates a predetermined scan line, and said apparatus includes crossaxis correction means for applying to said beam a variable cross-axis correction, and processor means responsive to the output of said incidence determining means to control said cross-axis correction means to cause said beam to follow said predetermined scan line.
15. A computer-to-plate apparatus according to Claim 14, wherein said cross-axis correction means includes means for storing predetermined cross-axis correction data, and said processor means is operable to modify said cross-axis correction data in accordance with the output of said incidence determining means.
16. A computer-to-plate apparatus according to any of Claims 6 to 15 which includes a variable-rate pixel clock, and, wherein said datum mark means comprises a series of marks disposed at preset intervals along said scan line, said apparatus further including variable-rate pixel clock correction means for modifying the period of the variable rate pixel clock in response to the output of said incidence determining means.
17. A computer-to-plate apparatus according to Claim 16, wherein said variable-rate pixel clock correction means includes store means for storing predetermined variable-rate correction data, and said processor means is operable to modify said variable-rate correction data in accordance with the output of said incidence determining means.
18. A computer-to-plate apparatus according to Claim 17, wherein said variable-rate pixel clock correction means includes reference clock means, and a dividing counter which divides the output from the reference clock means by a digital value retrieved from said store means.
19. A computer-to-plate apparatus according to Claim 18, wherein the output of said dividing counter is supplied to phase lock loop means which scales up by a preset multiplier value.