A large-format plotting device is provided for transferring an image to a relatively large sized medium. A data processing device is used to divide the surface of the medium into a plurality of relatively narrow segments. A beam sweeping device, which can include a high energy beam such as a laser beam, is provided for sweeping the beam the width of the segment. The data processing device causes the beam to turn on and off to define a portion of the desired image. A movement mechanism is included to either move the beam generating device or the support for the medium at a desired velocity so that each sweep of the beam will produce a raster line whereby the entire segment will be swept forming a plurality of parallel raster lines covering the entire segment. The movement mechanism also includes placement mechanism for laterally moving the medium support or beam generating device to the next adjacent segment in sequence until the surface of the entire medium has been imaged. Alignment features can be provided so that the corresponding raster lines in each segment can be aligned so that they coincide and so that the beam will not overlap at the contiguous edges of the segments. Sensors can be provided on the beam generating device or on the medium to identify the edges of the desired segments. The medium can also be mounted on a rotating drum whereby the movement mechanism rotates the drum at a desired velocity synchronized with the sweep of the beam generating device. Raster-scan imaging is controlled by the data processing device which simultaneously controls the beam generating device for the sweep and intensity of the beam as well as the operation of the positioning mechanism.

TECHNICAL FIELD 
The present invention relates generally to plotters using raster-imaging 
and more particularly to a novel approach to plotting relatively large 
images with a minimum amount of distortion at the margins. 
BACKGROUND OF THE INVENTION 
FIG. 1 shows the typical configuration of prior art raster-scan or imaging 
plotters. A beam 1, usually a laser beam, is directed at a medium 2, such 
as a sheet of specially treated photosensitive paper or film. The beam 1 
continuously sweeps or scans across the medium 2 as the medium moves in a 
direction perpendicular to the sweeps of the beam to form a series of 
parallel lines spanning the width of the medium. These lines are called 
sweep paths or raster lines and a series of these lines produces a desired 
image. In a typical plotter or printer a rotating multi-sided (polygon) 
mirror 4 is used to direct or sweep the beam 1, although other means can 
be used. The beam 1 is focused by means of a lens 5, normally positioned 
between the mirror 4 and the medium 2. 
Typically, the photo-sensitive medium 2 is mounted on a mounting support 
device 3 such that the medium can be moved in relation to the beam-mirror 
lens assembly 6 in a direction perpendicular to the raster lines the beam 
1 creates on the medium 2. This relative movement can be accomplished by 
moving either the beam-mirror-lens assembly 6 or the mounting device 3. 
The mounting device is often either a flatbed support as discussed in U.S. 
Pat. No. 4,585,938 issued to Newmann et al. or a rotating drum as 
discussed in U.S. Pat. No. 4,691,212 issued to Solcz et al. 
In typical raster-scan plotters, the energy beam 1 is turned on to create a 
mark on the medium 2, and turned off to leave a blank space on the medium 
2. By turning the beam on and off at predetermined points a contrasting or 
visible pattern can be created on the medium 2. FIG. 2 shows a magnified 
detail of a simplified scan. Although typically black, the raster marks 23 
are shown here as white with black borders for clarity. In FIG. 2, the 
beam sweeps from left to right, parallel to the X-axis, turning on 21 and 
turning off 22 at predetermined points to create a black and white pattern 
or visible image. 
In FIG. 2, the turn on points 21 and the turn off points 22 are marked by 
crosses. Because the mark that the beam forms on the medium when the beam 
hits the medium perpendicularly is generally a round dot 26, the raster 
mark 23 formed by the beam has round or semi-circular ends 25. Each scan 
or raster mark 23 is very thin. A large number of sweeps or raster lines 
24 must be made in order to create a visible pattern. Because of this, the 
jagged edges caused by the rounded ends 25 of the scan marks 23 are nearly 
invisible to the naked eye. 
Typically, a raster line 24 is divided into a large number of pixels or 
dots, and the turn-on and turn-off commands for the beam are given 
coordinates for the pixel where the turn-on or turn-off is to be 
performed. The Y-coordinate for each pixel represents on which raster line 
24 the pixel is located. The X-coordinate represents where on the specific 
raster line 24 the pixel is located. Of course, a different coordinate 
system could be used to designate the positions of the pixels. 
The prior art has several problems, including the following. 
Because of the large sweep angle 7 (see FIG. 1) required to sweep the width 
of the medium, the pixels or dots created by the beam toward the margins 8 
of the medium tend to become oval 27. (See FIG. 2.) 
The large sweep angle 7 also requires that large and specially designed 
focusing lens or f-theta lenses 5 be used. Such large special lenses are 
very expensive. 
Because the beam has a longer distance to travel to the edges of the medium 
8 than to the middle 9, the distance the beam travels between the lens and 
the medium necessarily varies from the fixed focal length of the lens, and 
thus the outer pixels tend to become defocussed. 
In addition, because of the long distance the beam needs to travel, 
especially at the edges, any jitter present in the mirror assembly 4 
becomes magnified. 
Further, the prior art systems typically must have large processing and 
memory requirements, because each sweep path 24 is so long each path has 
many turn-on and turn-off commands which greatly increase the number of 
commands that must be processed. 
Information Disclosure Statement 
The following information is provided in response to the applicant's duty 
to disclose all information which is pertinent to the examination of this 
application. There should be no inference that the applicant has performed 
a search for prior art relevant to this invention. 
U.S. Pat. No. 4,739,416, issued to Marian, discloses an apparatus for 
reproducing a digitized image. The Marian device uses a rotating drum as 
the mounting means, and a plurality of light sources, which do not sweep 
lines in an axial direction with respect to the drum. By rotating the drum 
and slowly moving the light sources in an axial direction, each light 
source scans a circumferential, helical path around a segment of the drum. 
U.S. Pat. No. 4,698,648, issued to Takahashi discloses an apparatus that 
records a plurality of images on a given medium. In one embodiment (see 
FIG. 9), the Takahashi device includes a rotating drum and a 
beam-mirror-lens assembly that can be moved to a number of positions along 
a line parallel to the axis of the drum. This permits recording onto a 
single large piece of film several complete images from each position. 
SUMMARY OF THE INVENTION 
The present invention provides a plotter that can reproduce relatively 
large images with high resolution and little distortion at the edges. The 
present invention includes a beam sweeping arrangement for projecting a 
beam onto a medium, such that the beam repetitively sweeps raster lines 
across one segment of a number of relatively narrow segments formed across 
the medium. The invention also includes movement and placement mechanisms 
for continuously moving the medium in relation to the beam sweeping 
arrangement and in directions transverse and parallel to the raster lines 
eventually forming a full width image across the medium. A system for 
aligning the plurality of segments of the image in the direction generally 
parallel to the raster lines is provided such that there is neither an 
overlap of the segments nor a space between the segments when they are 
plotted.

DESCRIPTION OF SPECIFIC EMBODIMENTS 
FIG. 3A shows one embodiment of the invention. This embodiment has five 
beam sweeping arrangements or devices 6, each of which may be similar to 
the beam-mirror-lens assemblies of the prior art. Of course, a different 
number of beam sweeping devices could be used by the invention. Each beam 
sweeping arrangement sweeps the width of a segment 31 on the film 2. FIG. 
3A shows each sweeping device is able to cover an area larger than the 
segment over which it is located so that the sweep angles 7 overlap each 
other in an area 32. There is actually no need for the beam to be turned 
on to create a mark in a neighboring segment 31. Thus, the neighboring 
segment can be adequately covered by the respective sweeping arrangement 6 
located above it. 
The sweep angles 7 of the device depicted in FIG. 3A are narrow compared to 
that of the typical prior art raster-scan plotter as shown in FIG. 1. In 
the present embodiment, sweep angles of 7.5.degree. and segments two 
inches wide can be used; however, a broad range of angles and widths can 
be used, if desired. The smaller sweep angles 7 of the present invention 
means that the pixel or dot formed by the beam 1 striking the medium 2 is 
more round and less oval at the margins 8 than at the margins of the prior 
art raster-scan plotters. The smaller sweep angles 7 also mean that the 
very large special lenses used in many prior art sweeping arrangements 
(see FIG. 1), are not necessary, and smaller, much less expensive, lenses 
can be used. Because the optical path length does not substantially vary, 
there is less defocussing at the edges than in prior art raster plotters. 
The sweeping device or arrangement can also be located close to the 
mounting device 3 and the medium 2, since each sweeping device 6 has a 
smaller area to cover. Because any number of sweeping devices 6 can be 
used in the present invention, embodiments of the present invention could 
theoretically be built to cover a medium of any size. 
FIG. 3B shows a side view of the plotter depicted in FIG. 3A. A flatbed 
support is used as the mounting device 3 for the medium 2. In this 
particular embodiment, the sweeping devices or arrangements 6 are lined up 
in a row, so that in this side view 5 only one sweeping arrangement 6, the 
one on the far left of FIG. 3A, is visible. In an alternative embodiment, 
the plurality of sweeping arrangements 6 can be staggered. The beam 1 
appears to go straight down in FIG. 3B, although it can be seen to sweep 
back and forth in the view shown in FIG. 3A. In addition to plotting on 
typical medium, such as film or paper, the flatbed device can also be used 
to scan images directly on a printed circuit board or a photo-conductive 
drum which transfers the image to another medium. 
In order to achieve a series of raster lines within a segment that are 
arranged parallel and adjacent to each other (as shown in FIG. 2), it is 
necessary to move the beam sweeping arrangements 6 relative to the medium 
2 in a direction transverse to the raster lines being swept by the beam, 
and at just the right velocity. This relative movement can be accomplished 
for instance, either by moving the sweeping arrangement 6 in a direction 
as indicated by arrow 33, or by moving the flatbed support 3 or rotating a 
drum 3' in a direction indicated by arrows 34 or 35. In order to maintain 
the correct relative velocity, a variety of mechanisms well known in the 
art can be used. 
FIG. 3C shows a side view of an alternative embodiment based on the 
embodiment depicted in FIG. 3A. A drum 3' is used as a mounting device for 
the photosensitive medium 2' instead of a flatbed 3 as shown in FIG. 3B. 
The beam 1 is swept in an axial direction with respect to the drum. In 
order to achieve the relative movement between the sweeping means 6 and 
the medium 2', the drum 3'can be rotated in the direction shown by arrow 
35. Otherwise, the device shown in FIG. 3C is identical to the device 
shown in FIGS. 3A and 3B. 
FIG. 3D shows a rotating drum 3" which is used as the primary imaging 
device. This type of drum which has a photoconductive coating is widely 
used in xerography. In this application, the surface of the rotating drum 
is electrostatically charged with the polarity of the charge forming the 
scanned image. The correctly polarized image attracts toner and transfers 
the toner to the medium such as plain paper to produce the image. A fusing 
roller or other high heat device is then used to fuse the toner and image 
to the paper. The drum 3" is used as an imaging and image transfer device 
rather than as a mounting device for the final medium 2 as shown in FIG. 
3B. The beam is swept in an axial direction with respect to the drum. In 
order to achieve the relative movement between the sweeping means 6 and 
the medium 2, the drum can be rotated in the direction shown by arrow 35. 
In this embodiment the length of the drum can correspond to the 
approximate width of the equal segments of the image. 
FIGS. 4A and 4B show the general arrangement of a preferred embodiment of 
the invention. This embodiment is similar to the embodiment depicted in 
FIG. 3C, except the drum 3, which is rotated by means of a motor 44 and a 
belt 45, is located above or below the sweeping arrangement 6. FIG. 4A is 
simplified for the sake of clarity. The device shown here is divided off 
into six two-inch segments to cover the medium As noted before, any number 
of segments can be used, and segments of various widths can be used if 
required. It is to be understood that a support structure (not shown) is 
necessary to support the drum 3' over or under the sweeping means 6. Also 
not shown is an enclosure which surrounds the device in order to prevent 
dust from settling on the plotter. 
In this preferred embodiment, a single beam sweeping arrangement 6 is used. 
In FIG. 4A, a placement mechanism 41 is used to move the sweeping 
arrangement 6 in a direction generally parallel to the raster lines swept 
by the sweeping arrangement 6. The single sweeping arrangement 6 is 
mounted on top of a movable laser carriage 40. While the laser carriage 40 
is centered and stationary with respect to the first segment 31, and as 
the drum 3' turns, the beam 1 is swept across the first segment 31 of the 
medium 2'. When the segment 31 is completely imaged, the laser carriage 40 
is moved with respect to the next segment 31', which is in turn swept, and 
this process continues until all of the segments are completely swept. 
Preferable, the sweeping arrangement 6 is located at the center of each 
the segments being swept, so that the angle of the beam 1 is as 
perpendicular as possible when striking either margin of the segment. This 
will minimize the amount of distortion at the margins of the segments. The 
laser carriage 40 can be moved into place by a variety of placement 
mechanisms 41, including a simple pulley system or a long rotating screw. 
The preferred embodiment uses a chain and sprocket system with a motor to 
move the laser carriage 40 movable mounted on rails 41', 
FIG. 4B shows a perspective variation of the preferred embodiment of the 
invention. This variation is similar to the embodiment shown in FIG. 3D, 
except the drum 3", which is rotated by a motor 44 and a belt or gear 
assemble 45 is located below the sweeping arrangement 6. In this 
embodiment, the drum has a photoconductive coating which transfers its 
image to the medium 2 as the sweeping arrangement 6 is moved on the laser 
carriage 40. FIG. 4B is simplified for the sake of clarity. The device 
shown here uses six segments to cover the entire medium. As noted before, 
any number of segments can be used. Of course, some sort of well known 
structure (not shown) can be used to support the drum 3", the sweeping 
assembly and the rail assembly in concert with the placement mechanism 
41". Also not shown is an enclosure which surrounds the device in order to 
prevent dust from settling on the plotter and to exclude stray light from 
the photoconductive imaging device. 
In this preferred embodiment, a single beam sweeping device is used. A 
placement mechanism 41" is used to move the medium 2 in a direction 
generally parallel to the raster lines swept by the sweeping means to 
establish the matching position of the image of the next segment. The 
single sweeping arrangement 6 is mounted to a movable laser carriage 40. 
While the laser carriage is centrally positioned above the first segment 
31, and as the drum 3" turns, the beam 1, is swept across width of the 
drum and the image is transferred to the first segment 31 on the medium 2. 
When the segment 31 is completely swept or imaged, the medium 2 is moved 
to the next segment 31' by the medium placement mechanism 41", and this 
process continues until all of the segments are completely swept. As in 
the embodiment of 4A, the center of each segment 31 is preferably located 
under the center of the sweeping arrangement 6 to minimize the amount of 
distortion at the margins of the segment 31. Also, the laser carriage 40 
can move the sweeping arrangement 6 by a variety of movement mechanisms 
40' including a pulley system, or a long rotating screw on a gear and 
track system. The preferred embodiment uses a gear and a track system to 
move the laser carriage 40 which can be mounted on rails. 
FIG. 5 depicts a partially imaged medium 2, and shows how the beam 1 of the 
preferred embodiment covers the medium 2. Starting on the bottom of the 
segment 31 on the left-hand side of the medium 2, the sweeping arrangement 
sweeps a raster line spanning the width of the segment. Because the medium 
is mounted on a constantly rotating drum, when the sweeping arrangement 
sweeps a second line, this second line is adjacent and parallel to the 
first line. The sweeping arrangement sweeps a series of lines in such a 
fashion from the bottom of the segment to the top covering the whole 
segment. Then the placement mechanism moves the next segment of the medium 
into alignment with the sweeping arrangement and drum and the next segment 
is likewise swept from bottom to top. Then the placement mechanism moves 
the next segment of the medium into alignment with the sweeping 
arrangement and drum and the next segment which is likewise swept from 
bottom to top. This is repeated until all of the segments are covered. As 
mentioned previously the beam is not always on as it is swept from side to 
side It is turned on to create a series of dots or marks on the 
photosensitive medium 2. In an alternative embodiment, in which a flatbed 
support is used as a mounting device instead of a drum, a single sweeping 
arrangement can cover the segments in a similar fashion, or alternatively, 
from bottom to top, and then from top to bottom, etc. 
In the preferred embodiment, in which a photoconductive drum is used, a 
single imaging arrangement will preferably image the segments in a similar 
fashion by imaging on the photoconducting coating and then 
electrostatically transferring the image to the medium by the use of toner 
or other appropriate transfer medium. 
In order to properly produce the desired image, it is important to align 
the segments properly. The segments need to be aligned horizontally; that 
is, each segment must abut and match the next segment. There cannot be any 
overlap or space between the segments. The segments must also be aligned 
vertically; that is, the first line of each segment must line up with all 
of the other first lines, the second line of each segment must line up 
with all of the other second lines, etc. 
One way to achieve horizontal alignment is simply to use the placement 
mechanism to move the laser carriage 40 the distance precisely equal to 
the width of a segment each time a new segment is to be started. However, 
it is difficult to retain the tolerances necessary with such a mechanical 
system, and the constant movement of the laser carriage 40 can soon wear 
the movement mechanism 40', especially if it is a simple pulley system, so 
that the very fine tolerances required can not be maintained. Although, in 
the preferred embodiment shown in FIG. 4B, a mechanical system is used as 
the placement mechanism 41" for moving the laser platform to roughly the 
correct location, the mechanics of the placement mechanism are not 
necessarily relied upon to assure that the segments are horizontally 
aligned. 
The preferred embodiment uses the sweeping beam to align the segments 
horizontally. Preferably, a precision machined gauge bar 42, which can be 
seen in FIG. 4A, is used to mark the borders of the segments. This gauge 
bar is made out of reflective material except for a series of 
non-reflective gauge marks 43 that are located at intervals along the 
gauge bar 42. A chrome-plated metal bar with notches cut in its side can 
be used. The distance between each gauge mark 43 is the exact width of 
each segment 31. Preferably, each segment is of the same width. Because 
the gauge bar 42 is firmly affixed to the rotating drum 3', it is much 
less likely to warp than the placement mechanism 41" which is used to move 
the laser carriage 40. Therefore, the gauge bar 42 can maintain the 
necessary close tolerances, indefinitely. 
FIG. 6 shows the system that is used to "read" the gauge marks 43. A pair 
of gauge marks which delineate a segment must be read before plotting the 
segment. A sensor platform 61 is mounted on struts 62 on top of the laser 
carriage 40 so that it is close to the drum 3' or flatbed support 3. FIG. 
7 shows a top view of the sensor platform 61, and shows a slot 66 that the 
beam can sweep back and forth through. The platform 61 has three light 
sensitive sensors, a reference sensor 63 on the bottom of the platform 61, 
and a start-of-segment sensor 64 and an end-of-segment sensor 65 on top of 
the platform 61 on either end of the slot 66. All three sensors generate 
output signals, which are sent to a data processing computer system. 
Reference sensor 63 generates an output signal when it is struck by the 
sweeping beam at the beginning of the sweep. This signal is used as a 
reference. The reference sensor 63 may be partially painted over in order 
to produce a sharply peaked output signal. Start-of-segment sensor 64 
indicates when the beam sweeps across a gauge mark 43, which is 
non-reflective. This sensor can detect when the beam sweeps across the 
gauge mark 43 because it can sense the laser beam reflecting off the gauge 
bar 42 until the beam hits the gauge mark 43, which does not reflect the 
beam back to the sensor. The beam continues its sweep, and eventually the 
end-of-segment sensor 65 senses when the beam strikes the next 
non-reflective gage mark 43. 
FIG. 8 shows representative output signals generated by the sensors 63, 64, 
65 when the gauge bar is directly over the sensor platform. Preferably, 
when the gauge bar is in this position, the beam is turned on full. The 
computer system controlling the beam stores the amount of time elapsed 
between the reference sensor's signal and the start-of segment sensor's 
signal. The computer system also stores the time elapsed between the 
start-of-segment signal pulse and the end-of-segment signal pulse. The 
computer system determines the position of the sweeping arrangement 6 in 
relation to the gauge marks, and thus determines where the segment 31 
should be in relation to the sweeping arrangement 6. When the drum 3' 
rotates so that the gauge bar is no longer over the sensing platform, and 
is scanning the medium 2', as shown in FIG. 9, the computer system uses 
the stored segment information to determine when to begin and end the 
segment 31. The reference sensor 63 is still activated on each sweep even 
when the gauge bar is no longer above the sensor platform. The 
start-of-segment and end-of-segment sensors are not used when the gauge 
bar is not over the sensor platform. The computer system uses the pulse 
from the reference sensor 63 to start the clock running, and starts and 
ends the segment based on the timing information it stored during the 
sweep across the gauge bar. Because the thickness of the film or other 
medium is probably different than the thickness of the gauge bar, and may 
vary from application to application, the thickness of the medium being 
used must be input into the computer system, so that the computer can make 
the necessary calculations in order to accurately adjust to the beginning 
and end of each segment. 
After a segment has been covered, the placement mechanism moves the 
sweeping arrangement a distance approximately equal to the width of a 
segment. The sweeping arrangement then scans the gauge bar again, and 
information regarding the location of the next segment in relation to the 
sweeping arrangement is derived from the sensors. The computer processes 
the readings of the sensors and then the next segment is swept. This 
process continues until all the segments are swept. 
In lieu of a gauge bar and gauge marks, longitudinal alignment can be 
achieved by placing a set of light sensitive sensors directly on the drum 
adjacent to where the medium is mounted, and aligned with the edge of the 
segments. Such an arrangement is depicted in FIG. 10. The sensors 101, 
101', 101" will perform the functions of the start-of-segment and 
end-of-segment sensors, as well as the gauge bar. The reference sensor 63 
will still have to be mounted somewhere on the laser carriage 40, so that 
it is moved with the sweeping arrangement. 
In FIG. 10, the reference sensor 63 is shown mounted on a strut 62; 
however, it may still perform its function located in a different 
position, as long as its position is constant with respect to the beam 
sweeping arrangement. Instead of sensing the beam reflecting off the gauge 
bar, the sensors 101, 101', 101" will sense the beam directly. Each sensor 
can perform the dual function of both a start-of-segment sensor and an 
end-of-segment sensor, depending on whether the segment 31 or 31' is to be 
scanned. Alternatively, different sensors can be used to perform the 
start-of-segment sensor function and the end-of-segment sensor function 
for each segment. When the sensors on the drum are above the sweeping 
arrangement, the signals from these sensors will be transmitted to the 
computer. These sensors can be partially painted over to produce sharper 
or peaked signals. 
The computer measures two time lapses, the time lapse between the signals 
from the reference sensor 63 and the sensor 101' functioning as the 
start-of-segment sensor for the first segment, and the time lapse between 
the signals from the reference sensor 63 and the sensor 101" functioning 
as the end-of-segment sensor for the first segment. For the second 
segment, sensor 101" will act as the start-of-segment sensor while the 
next sensor 101 will act as the end-of-segment sensor. In this manner, 
which is very similar to the gauge bar embodiment, the computer can 
precisely determine the location of the segment to be scanned in relation 
to the sweeping arrangement and thus begin and end each raster line at the 
correct location. If the medium is transparent or translucent, these 
sensors can be located underneath the medium itself. Such a set-up could 
ensure longitudinal alignment throughout the coverage of a segment, 
instead of just at the beginning. This arrangement will also compensate 
for play or wobble in the various mounting devices which can result in 
mis-alignment or variations during the coverage of a segment. 
Of course, if less resolution is required, fewer and wider segments can be 
used. Images of varying resolution can be plotted on a single machine by 
skipping over some sensors; such as, every other sensor, or every second 
or third sensor, etc. Accordingly, the segments will be twice or three 
times as wide. Similarly, this can be done on a plotter using a gauge bar 
as part of its alignment system. If desired, more sensors may be provided 
on top of the sensor platform, and the gauge bar can be tilted so that the 
sensors can be placed to the side of the sweeping beam, instead of on 
either end of the slotted area. 
In the multiple sweeping arrangement embodiment, depicted in FIG. 3A, start 
and end sensors need not be used, because the beam sweeping devices do not 
move in a direction parallel to the raster lines, but instead remain 
stationary above their assigned segments. However, reference sensors can 
be used in order to aid in timing the start of the segment. If desired, 
start and end sensors can be used to ensure alignment. 
When the embodiment shown in FIG. 6 is used, longitudinal alignment is 
achieved by mounting a reference sensor 63 on the laser carriage 40 and 
supporting structure 40" that mounts the sweeping arrangement 6, the 
photosensitive drum 3 and other optical components, as shown in FIG. 13. 
The reference sensor 63 performs the function of the start-of-segment 
sensor as described previously. 
The transverse alignment problem can be solved in a variety of ways. In the 
preferred embodiment shown in FIG. 4A, a second order servo control loop 
can be used. This entails the use of a drum encoder 48, which reads the 
position of the drum 3', and a motor encoder 47, which reads the velocity 
of the motor 44 rotating the drum 3'. By continuously monitoring both the 
position of the drum 3' and the speed with which it rotates, the segments 
31 can be aligned transversely with great accuracy. 
In the embodiment of FIG. 13, a transverse start sensor 1000 is also 
mounted on the laser carriage assembly to determine the position of the 
first raster line in the segment as the laser carriage assembly moves in 
the imaging direction which is orthagonal to the raster-scan line. The 
transverse start sensor 1000 senses the laser beam 1004 which is reflected 
by mirror 1002, attached to the moving laser carriage assembly, and when 
it is first reflected by mirror 1001 which is attached to the stationary 
plotter assembly P. 
As seen in FIG. 6, a computer, or data processing system, 49 is used to 
receive data from the drum encoder 48, the motor encoder 47 and the light 
sensitive sensors. The computer 49 uses this information, as well as 
information inputted to the computer for describing the image to be 
plotted. In this way the computer controls (1) the rotation of the drum 3' 
by means of the motor 44, (2) the movement of the laser carriage 40 by 
means of the placement mechanism 41, and (3) the sweeping arrangement 6, 
including turning the laser beam on and off, controlling the intensity of 
the beam and regulating the speed of the sweep. The computer 49 must also 
manipulate the input information to be plotted by the device. Because 
there are far fewer turn-on and turn-off commands in each of the short 
raster lines which span the width of the segment and not the width of the 
entire image, the memory and simultaneous processing requirements of the 
computer 49 of the present invention are less than those of the prior art 
devices. 
The image of a printed circuit board is an image that is frequently 
plotted. Printed circuit boards, engineering drawings and other images, 
can often be depicted by a set of lines, called vectors, and a standard 
set of non-linear, non-rectangular icons, such as pixels or dots. For 
plotting such images on a prior art plotter, Gerber plot files are 
typically used. Plot primitives, representing a positively sloped vector, 
a horizontal or vertical vector, and apertures or constructs, are 
typically used. Images consisting mostly of vectors can be readily plotted 
on paper by pen-plotters. The information the pen-plotter needs to plot 
each vector is relatively simple, i.e. coordinates for one end of the line 
to be drawn (X1, Y1), coordinates for the second end of the line to be 
drawn (X2, Y2), and the width of the pen to be used in plotting the vector 
(W). 
Raster-plotters need more detailed information to plot a vector. Because 
raster lines are typically very thin (the dot size in one embodiment of 
the invention can be only 0.0015 inches (38 microns)), many raster lines 
intersect the vector being plotted. Each raster line that intersects the 
vector is represented by a Y-coordinate, and each line has an XSTART 
coordinate and an XSTOP coordinate indicating, respectively, when to turn 
the beam on and when to turn it off. Often, a raster line intersects 
several vectors, and therefore has several XSTARTs and XSTOPs. 
Raster-plotters typically plot vectors as rectangles as shown in FIG. 11a, 
which also shows with dotted lines the rounded ends of the same vector as 
it would be plotted by a pen-plotter. FIG. 11a also shows one of the 
raster marks 23 that would be plotted in order to form the vector. It is 
well known in the art how to convert vector information from pen-plotter 
format (XI, Y1, X2, Y2, W) to raster-plotter format (XSTART, XSTOP, Y). 
There are several ways the regular raster-plotter format can be segmented, 
so that it can be readily plotted on the present invention. In the 
preferred embodiment with a single beam sweeping arrangement, a method for 
converting the information into a segmented format can be used that would 
permit "on-the-fly" rasterization, which means that the final processing 
steps involved in converting the vector information into raster 
information can be performed quickly with relatively low peak processing 
and memory requirements. 
FIG. 11a shows a vector that has not been segmented. This vector is defined 
by its four corners, A, B, C and D. This vector has a positive slope, i.e. 
it rises as it goes from left to right. A vector with a negative slope can 
be treated as a positively sloped vector that is very short and very wide. 
Likewise, a horizontal vector can be a short, wide vertical vector. FIGS. 
11b through 11e show vectors that are intersected once by a segmentation 
line S, which is depicted by the vertical lines. These vectors are divided 
into polygons with three, four and five corners. FIGS. 11i through 11m 
show vectors that are intersected twice by segmentation lines S1, S2. 
These vectors are divided into polygons with three, four, five and six 
corners. Thus, each segmented vector polygon can be defined by six corners 
or less, and a hexagon with points A, B, E, C, D and F as its corners, as 
depicted in FIG. 11n, is used as the basis for describing all of the 
polygons. Note that points F and D have the same X coordinate, points B 
and E have the same X coordinate, side AB is parallel to side DC, side CE 
is parallel to side FA, and the slope of FA and CE is negative inverse of 
the slope of DC and AB. 
FIG. 11m shows a six-sided polygon labeled as a Type 6. This Type 6 polygon 
is represented by the following twelve word record: 
TABLE 1 
______________________________________ 
Word: 
1 2 3 4 5 6 7 8 9 10 11 12 
______________________________________ 
1 YA XA YB YC YD YE YF DY/ DX/ 
DX DY 
______________________________________ 
The first word contains a code indicating what kind of plot primitive is 
being represented, i.e. "1" being a positively sloped vector, "3" being a 
vertical vector, and "2" being an aperture or construct. The second and 
third words contain the Y and X coordinates of corner A. The fourth 
through eighth words contain the Y coordinates for the remaining corners. 
The ninth through twelfth words contain the slope of the vector and its 
inverse. This record contains all of the information needed to represent 
this type of hexagon, which has two vertical sides in which each side is 
parallel to another side. 
The remaining polygons and plot primitives, can be represented using the 
same six-corner record format: 
TABLE 2 
__________________________________________________________________________ 
Word 
Type 1 2 3 4 5 6 7 8 9 10 11 12 
__________________________________________________________________________ 
3L 1 YA XA YB YC YC YB YA DY/DX 
DX/DY 
3R 1 YA XA YA YC YD YC YD DY/DX 
DX/DY 
4 1 YA XA YB YC YD YB YD DY/DX 
DX/DY 
4L 1 YA XA YB YC YD YB YA DY/DX 
DX/DY 
4R 1 YA XA YB YC YD YC YD DY/DX 
DX/DY 
4LS 1 YA XA YB YC YC YB YF DY/DX 
DX/DY 
4RS 1 YA XA YA YC YD YE YD DY/DX 
DX/DY 
5L 1 YA XA YB YC YD YB YF DY/DX 
DX/DY 
5R 1 YA XA YB YC YD YE YD DY/DX 
DX/DY 
5CR 1 YA XA YB YC YD YC YF DY/DX 
DX/DY 
5CL 1 YA XA YB YC YD YE YA DY/DX 
DX/DY 
C 1 YA XA YB YC YD YC YA DY/DX 
DX/DY 
6 1 YA XA YB YC YD Y YF DY/DX 
DX/DY 
HorV 3 YA XA YB XD 
APERT 
2 YA XA YB YC YD XS XD 
__________________________________________________________________________ 
In the preferred embodiment of the invention the preceding processing 
steps, i.e. (1) converting the vector information from pen-plotter format 
to four-corner format and (2) segmenting the vector information, i.e. 
converting it into six-corner format, can be done by a computing or data 
processing unit equivalent to a "PC-AT" computer. This computing unit also 
sorts the records by the YA values in ascending order. These records are 
then sent to a vector-to-raster-converter (VRC), which computes the 
XSTARTs and XSTOPs. The steps remaining in the vector to raster conversion 
can be performed by the VRC as fast as the image can be plotted, thereby 
permitting "on-the-fly" rasterization. 
When the VRC is computing the XSTARTs and XSTOPs for raster line NA-1, it 
comes upon a record for a vector that starts on line NA-a, the vector's YA 
coordinate equals NA-1. The VRC adds two entries, XSTART and XSTOP, to the 
record. Taking the Type 6 polygon's record for example, XSTART and XSTOP 
would both be equal to XA, the only point of the vector intersecting 
raster line NA. Of course, for a horizontal or vertical vector, XSTART 
would equal XD and XSTOP would equal XA. To modify the record for the next 
raster line NA, the VRC substitutes NA for YA, and according to the slope 
information, DY/DX and DX/DY, subtracts the appropriate amount from XSTART 
and adds the appropriate amount to XSTOP. The record for the Type 6 vector 
polygon will look like this: 
TABLE 3 
__________________________________________________________________________ 
Word 
1 2 3 4 5 6 7 8 9 10 11 12 
13 14 
__________________________________________________________________________ 
1 XA NA YB YC YD YE TF DY/DX 
DX/DY 
XSTART 
XSTOP 
__________________________________________________________________________ 
For the next raster line, one is added to NA, and the appropriate amounts 
added and subtracted to XSTART and XSTOP. This process is repeated until 
the VRC reaches the raster line equal to YF or YB. when YF is reached, 
XSTART remains constant (it has the same X coordinate as the first segment 
line bounding the segment) until YD is reached, at which point appropriate 
amounts based on the slop of the vector are added to XSTART. When YB is 
reached, XSTOP remains constant (it has the same X coordinate as the 
second segment line) until YE is reached, at which point appropriate 
amounts based on the slope of the vector are subtracted from XSTOP. When 
YC is reached, XSTART and XSTOP are once again the same point, and the 
polygon is completed, and the record representing the polygon can be 
dropped from the VRC memory. The other polygon types can be rasterized in 
a similar fashion. 
Horizontal and vertical vectors (FIG. 11h) are quite easy to segment, 
because, when such a vector is segmented, the resulting polygons are also 
horizontal or vertical vectors. Segmented and unsegmented, horizontal and 
vertical vectors can be represented by coordinates XA, YA, YB and XD as 
shown in Table 2 above. 
Apertures or constructs are also not difficult to segment. An aperture 
file, which is typically in ASCII, already contains a set of Y, XSTART and 
XSTOP coordinates for the icon to be plotted. More than one pair of XSTART 
and XSTOP coordinates can be located on each line of the aperture. To plot 
the segmented aperture shown in FIG. 11f, XSTOPs can be inserted where the 
segment line intersects the aperture, and the XSTARTs and XSTOPs to the 
right of the segment line can be ignored. To plot the segmented aperture 
shown in FIG. 11g, the XSTARTs and XSTOPs to the left of the segment line 
can be ignored, (they should have been plotted in the previous segment) 
XSTARTs can be inserted where the segment line intersects the aperture, 
and amount XS, representing the length along the X-axis of that portion of 
the aperture plotted in the previous segment, can be subtracted from the 
remaining XSTART and XSTOP coordinates. 
In one of the embodiments, where the segments are approximately two inches 
wide, there are 2,047 pixels used in a two-inch length of raster line. Of 
course, different numbers of pixels can be used. If the segments vary in 
size, the segments can have different numbers of pixels. Even with the use 
of a precision machined gauge bar the number of pixels may vary slightly. 
As noted above, the beam sweeping across the gauge can measure the size of 
the segment, as shown in FIG. 6. For each raster line, an output memory 
list representing the 2,047 pixels (numbered 0 through 2,046) is created 
by reading the fourteen-word polygon record. For every XSTART coordinate, 
a positive one (+1) is put in the appropriate address. For every XSTOP 
coordinate, a negative one (-1) is put in the appropriate address. If a 
single address has multiple commands, the positive and negative ones are 
added together. For Example: 
______________________________________ 
Pixel #0: 
+1 (representing the XSTART of a first vector 
polygon) 
Pixel #105 
+2 (representing the XSTARTs of two 
overlapping vector polygons) 
Pixel #442: 
-1 (represent the XSTOP of the second vector 
polygon) 
Pixel #516: 
-1 (representing the XSTOP of the first vector 
polygon) 
Pixel #760: 
-1 (representing the XSTOP of the third vector 
polygon) 
Pixel #1,583: 
+1 (representing the XSTART of a fourth 
vector polygon) 
Pixel #2,046: 
-1 (representing the XSTOP of the fourth 
vector polygon) 
______________________________________ 
In this way the plotter can handle overlapping vectors. As the beam sweeps 
from the first pixel to the last, the modulator controlling the beam adds 
the amounts in the pixel memories; when the sum is a positive integer the 
beam is on, when the sum is zero the beam is off. In the above example, 
the beam turns on at Pixel #0, where the sum is one. When Pixel #105 is 
reached, the sum is three, so the beam remains on. The arithmetic sum 
remains greater than zero until Pixel #760, when the sum becomes zero and 
the beam turns off. The beam turns on at Pixel #1,583 and off at Pixel 
#2,046, the end of the raster line. 
This embodiment uses two output memories. At any given time, one output 
memory is read by the beam modulator, while the other receives the output 
from the VRC processor. When the plotting of a raster line is finished, 
the modulator reads the memory with the fresh VRC output, while the other 
memory, which has just been read by the modulator, is used to receive the 
next output from the VRC processor. Because the raster lines being plotted 
are only two inches wide, the memory and peak processing requirements of 
the invention are relatively small. If bigger segments are used, of 
course, the processing and memory requirements will increase. 
FIG. 12 shows, in a simplified manner, the information or data processing 
components of the preferred embodiment of the invention. As noted above, 
some of the data processing can be done by a typical "PC-AT" computer 100 
. At the same time some of the data processing is done on a VRC 102 which 
is a special purpose processor. The remaining data processing is done on 
circuitry located on the plotter platform 104. Not withstanding the above, 
the processing functions can be arranged in different devices. For 
example, all of the data processing can be done on the host computer 100, 
if desired. 
The host computer 100 performs several functions: (1) it converts vector 
information from pen-plotter format to four-corner record format, if 
necessary; (2) it converts four-corner record format to six-corner record 
format, if necessary (of course, the host computer can receive input in 
four-corner or six-corner format, instead of in pen-plotter format); (3) 
it controls the motor based on the data it receives from the motor and 
drum encoders; (4) it controls the placement of the laser carriage; (5) it 
provides system control and drives the VRC; and (6) it provides an 
interface to the plotter user. The VRC 102 converts the six-corner records 
into records that tell the laser modulator when to turn on and off during 
the sweep of a raster line. On the plotter 104 is located electronic 
circuitry for pixel alignment, which basically provides information to the 
beam modulator as to when to begin and end the segment based on the 
signals received from the reference sensor MS, start-of-segment sensor Ml 
and end-of-segment sensor M2. The pixel alignment electronics uses a pixel 
clock to keep track of the length of the offset and the width of the 
segment. (See FIG. 8.) 
Throughout this application it is to be understood that any kind of energy 
beam generator can be used to provide the image scanning arrangement. This 
generator could produce a high energy laser beam, a high intensity light 
beam, a high electro-magnetic energy beam or any other beam which in 
conjunction with the appropriate medium will produce the desired imaging 
results. 
While an improved large format plotter has been shown and described in 
detail in this application, it is to be understood that this invention is 
not to be limited to the exact form disclosed and changes in detail and 
construction of the various embodiments of the invention may be made 
without departing from the spirit thereof.