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Abstract

A high-resolution laser graphics plotter for plotting data on a recording medium as selectively positioned spots (pixels) of variable intensity is disclosed. The intensity of a collimated laser beam is modulated in response to plotting data to produce a modulated light beam. The modulated light beam produces a spot on a light-sensitive film positioned in a flat field image plane. A multi-facet rotating mirror scans the modulated beam across the image plane. A flat field scan lens is positioned between the rotating mirror and the image plane to provide correction for the non-linear velocity of the beam as it is scanned across the image plane, and to provide a constant diffraction limited spot size for each spot plotted. A spot placement means is provided to generate the spot placement signal which cooperates with the plot data to modulate the laser beam. The spot placement means also provides error correction for the facet-to-facet and facet-to-axis errors in the rotating mirror to permit high resolution (500 spots per inch) plotting on wide format film. A micro-stepping motor assembly is provided to advance the film across the image plane. TABLE OF CONTENTS BACKGROUND OF THE INVENTION SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS THE PLOTTER AT THE FUNCTIONAL LEVEL THE SOURCE OF THE FACET-TO-FACET ERROR THE SOURCE OF THE FACET-TO-AXIS ERROR THE TIMING CIRCUITS AT THE FUNCTIONAL LEVEL Compensation for the facet-to-facet error Compensation for the facet-to-axis errors An Alternate Embodiment for Mirror Error Compensation THE VIDEO DATA CIRCUITS THE PLOTTER IN OPERATION THE CLAIMS BACKGROUND OF THE INVENTION The present invention relates to plotting devices which produce plots using selectively positioned spots or dots on a recording surface. More particularly, the invention relates to high-resolution laser plotting (500 spots per inch) on a wide format light-sensitive film in which the selectively positioned spots are plotted in a raster format of consecutive scan lines across the film. Wide format laser plotters which plot, in a raster format of consecutive scan lines, selectively positioned variable intensity spots or dots on light-sensitive film positioned for exposure in a flat field image plane are known in the prior art. These prior-art plotters have not been able to achieve high-resolution (up to 500 dots per inch both along the length and width of the film) plotting for various reasons. Specifically, major sources or errors in the construction of the rotating optical scanner, such as facet-to-facet and facet-to-spin axis errors, as well as problems in accurately controlling the advancement of the flim across the image plane, and achieving high modulation rates through the light beam intensity modulator, have prevented high-resolution plotting on wide format film. Some of these prior-art laser plotting systems have used stepping motors as the primary motivating force for advancing the film across the image plane. For these prior-art plotters, at the completion of a plot scan line, the film and film advancing mechanism was moved by pulsing the stepping motor. At the completion of the required number of incremental steps to advance the film the specified line-to-line separation, the film handling assembly was commanded to stop. Because of high inertial forces, the ability of the stepping motor to start and stop the film handling assembly between the plotting of each scan line was not adequate to achieve accurate positioning of the film for the very small line-to-line spacing required between consecutive scan lines in high-resolution plotting. The prior-art plotters did not have a film handling assembly with a stepping motor of sufficient speed, resolution and torque to overcome the inertial forces in the film handling assembly to produce incremental steps small enough and accurate enough to achieve high-resolution plotting. Plotting on a flat field image plane by scanning a modulated collimated laser beam across the image plane, whether by the prior art plotters or by the present invention, presents two problems. Both of these problems result from the use of a scanning device in which the total scan angle interval (interval subtended by the scanned beam as it is scanned across the image plane) is not small in comparison to the distance from the mirror to the image plane. That is, the radius of the scanned beam (the distance from the reflecting mirror facet to the image plane) is small in comparison of the scan interval. This relationship is necessitated by physical constraints of the plotter assembly itself. As a consequence, for wide format flat field plotting, the radius of the scanned beam varies considerably over the scanned image plane. This variation in beam radius produces the two problems. First, to produce a given spot size at the image plane, the modulated laser beam is focused on the image plane to produce the desired spot size at the center of the scan angle interval. The beam diameter at that point is equal to the desired spot size. However, the depth of focus (the distance from the focused spot over which the focused beam diameter does not vary appreciably, and over which the spot is diffraction limited--the spot plotted has no rings) is small in comparison to the variation in the radius of the scanned beam. The depth o focus varies inversely to the plotting resolution and as the plot resolution increases, the depth of focus decreases. As a result, for high-resolution plotting, the spot size can vary appreciably over the scan angle interval, with the spots increasing in size near the ends of the scan line. Second, for a constant angular rotation in the scanning mirror, the angular velocity of the beam measured at the image plane as the beam is scanned across the film is not constant. The velocity component at the image plane of the scanned beam varies non-linearly over the scan angle interval, with a higher velocity occuring at the two ends of the scan angle interval. This non-linear velocity of the image spot greatly complicates the timing to achieve exact spot placement as required for high-resolution plotting. The prior-art plotters have attempted to correct for these problems, but high-resolution plotting requires spot placement at smaller intervals than those of the prior art, and the prior-art correction means do not account for this angular velocity distortion to a sufficient degree of accuracy to permit high-resolution plotting. Other problems present in the prior-art plotters which have prevented them from achieving high-resolution plotting capabilities result from inaccuracies which are inherent in the construction of the rotating mirror. These inaccuracies are not a limiting factor in the prior-art wide format plotters because they have limited the plotting resolution to resolutions at which these errors are not as significant (approximately 200 dots per inch at best). However, when the plotting resolution increases, these errors become significant. These errors are referred to as the facet-to-facet errors and the facet-to-spin axis (hereinafter facet-to-axis) errors which result in inaccuracies in the start of each scan line and in the scan line-to-scan line (hereinafter line-to-line) spacing, respectively. Ordinarily, each facet of the multi-facet mirror should subtend a given angle measured at the axis of rotation, i.e. each facet should be the same size. Because of manufacturing tolerances, the angle subtended by each facet will vary. This variation is referred to as the facet-to-facet error. Also, the surface of each facet should be perpendicular to a normal to the sping axis vector. Again, because of manufacturing tolerances, the angle of the facet surface to the normal may not be the same for each fact. This variation is referred to as the facet-to-axis error. Finally, wide format prior-art plotters have achieved a linear spot placement on the recording medium by timing the rotation of the scanning mirror to generate a spot placement signal that enables the plot data to modulate the intensity of the laser beam at the appropriate angular position of the mirror facet. This spot placement signal is produced by various methods, such as generating and counting a timing signal synchronized to the rotation of the mirror. This timing signal will have a frequency proportional to the angular velocity of the mirror, and indirectly, to the angular position of the mirror. In this manner, the prior-art devices could achieve reasonably linear placement of the spots adequate for low plotting resolutions. However, because of the extremely close spacing of the dots in high-resolution plotting, a resolution of the angular position of the rotating mirror to a degree of accuracy not achieved by the prior-art plotters is required. Thus, such problems as accurate advancement of the film in very small increments to control the line-to-line spacing; maintaining a constant and diffraction limited spot size across the entire flat field image plane; compensating for the non-linear velocity of the beam as it is swept across the image plane; correcting for manufacturing tolerance errors in the multifacet scanning mirror, which errors appear as facet-to-facet and facet-to-axis errors; and resolving the angular position of the rotating mirror to a degree which permits the accurate placement of the spots have all combined to prevent the prior-art plotter from achieving high-resolution plots on wide format film. SUMMARY OF THE INVENTION In accordance with this invention, there is provided a laser graphics plotter which solves the problems present in the prior-art plotters to achieve high-resolution flat field plots on wide format film (up to 42 inches in width) as selectively positioned spots (pixels) of variable intensity. A collimated light source, such as a laser, generates a beam of light that ultimately will produce the spots on a light-sensitive film positioned for exposure in a flat field image plane. The collimated light beam is applied to a light intensity modulator that responds to the plot data to modulate the intensity of the collimated light beam. An optical assembly is provided between the collimated light source and the light intensity modulator to focus and reduce the beam diameter into the modulator to permit high modulation rates through the light intensity modulator. The modulted beam which exists the modulator enters a second optical assembly which recollimates and focuses the beam to produce the desired spot size on the image plane. The modulated light beam is applied to a rotating multifacet mirror which scans the modulated light beam across the image plane of the film. Positioned between the rotating mirror and the image plane is a 'flat field scan lens' which provides compensation for the non-linear velocity of the modulated light beam across the image plane, and also maintains a constant spot size for each spot plotted. Responsive to the rotation of the mirror, a spot positioning means generates a spot placement signal which cooperates with the plot data to produce the selectively positioned variable intensity spots of the plot. Included in the spot positioning means is a velocity sensitive tachometer that produces two signals, a timing signal and a reference signal. The timing signal is proportional to the angular velocity of the rotating mirror while the reference signal is a pulse produced once during each revolution of the mirror. The reference pulse is produced when the rotating mirror passes a reference position. To achieve high-resolution plotting, manufacturing errors in the rotating mirror which produce errors in the start of each scan line and errors in the line-to-line spacing must be accounted for. An error correcting means is provided to perform this function. Included in the error correcting means is a frequency multiplier. The frequency multiplier multiplies the timing signal frequency by a predetermined number to obtain a high-speed clock signal that is phase-locked to the timing signal. This high speed clock provides the timing signal necessary to achieve the accuracy in the angular position determination of the rotating mirror to permit accurate high-resolution positioning of the spots. Since the facet-to-facet and facet-to-axis errors vary from facet to facet, a control memory has been provided for storing control parameters which specify each facet's compensation parameters. A facet counter is provided to count each facet as it scans across the film. The contents of the facet counter correspond to one of the control memory addresses. The contents of the facet counter cooperates with a control memory address counter to provide the control memory with an address to output the control parameters which specify the necessary compensation to be applied. To correct for the facet-to-facet errors, the angular position of each facet at which the reflected modulated light beam is positioned on the image plane at the start position of the scan lines must be located. This position is called the start plot position for each facet. To locate the start plot position, two counters, a coarse delay counter and a fine delay counter, are loaded with control parameters and are clocked by the timing signal and the high-speed clock, respectively. The results from the two delay counters is to divide each rotation of the rotating mirror into precisely determined intervals. The intervals correspond to the angular rotation from the start plot position of a facet to the start plot position of the next facet. The reference position signal generated each time the rotating mirror passes the reference point re-synchronizes the rotation of the mirror to the location of the start plot position of the facets. Because the angular rotation from start plot position to the next start plot position can accurately be determined, facet-to-facet manufacturing tolerances can be compensated for. Correction for the facet-to-axis errors is achieved in two different ways. First, a step increment counter responsive to the timing signal and a control parameter produces a predetermined sequence of step increment pulses to a stepping motor to advance the film during a scan of the beam across the film (each step increment causing 10 micro-inches of film movement). Second, an offset voltage specified by a control parameter is applied to the light intensity modulator to deflect the modulated beam in a direction to increase or decrease the line-to-line spacing between consecutive scan lines. Thus, the line-to-line spacing may be controlled by a combination of both of these methods or by either one individually. Additionally, the spot positioning means includes the film advancing means for advancing the light sensitive film across the image plane. This means is comprised of a stepping motor connected to a zero-backlash gear reducer. This combination produces a micro-incremental step in film advancement. This micro advancement in film position is required in high-resolution plotting in order to maintain the very small line-to-line spacing, and to minimize the dynamic response time and disturbances caused by moving a large mass very rapidly.

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First Claim