Registration system for a moving substrate

A registration system is provided for maintaining accurate correspondence between an image on a moving substrate and the action of an operating element with respect to the substrate. A comparison optical device is mounted such that the timing marks pass in correspondence with the comparison optical device when the substrate is moving. A detector senses the intensity of radiation reflected from the substrate through the comparison optical device and generates a firing signal based on the reflected radiation intensity. The firing signal is provided to the operating element to activate it in correspondence to the passage of the timing marks.

RELATED APPLICATION 
U.S. Application Ser. No. 06/906258, filed Sept. 11, 1986, and entitled 
SYSTEM AND METHOD FOR INITIATING START-OF-PLOT IN A PRINTING SYSTEM and 
assigned to the assignee herein. 
BACKGROUND OF THE INVENTION 
1. Technical Field 
The present invention relates to registration systems and, in particular, 
to a system and method for providing accurate registration between a 
printed image and previously printed images on a moving substrate. 
2. Discussion of the Prior Art 
Systems for controlling the relative positions of a moving substrate and an 
element or elements which operate on the moving substrate have been in use 
for some time. Control systems of this type typically use marks or indicia 
which are printed on the substrate material at regular intervals. These 
marks are scanned by a sensor as the substrate is fed past the operating 
element. When a mark passes the sensor in such a manner as to produce a 
signal indicative of an error in registration between the mark and the 
operating element, the sensor, in conjunction with control circuitry, 
generates a correction signal and an adjustment is made in the 
registration. 
As an early example of a control system of this type, U.S. Pat. No. 
2,250,209 issued July 22, 1941 to Shoults et al., discloses a control 
system for a stamp perforating machine. In the Shoults system, a 
perforating element operates on a continuous length of moving postage 
stamps. The Shoults control system includes a photocell which monitors 
marks formed on the length of stamps and produces a corresponding signal. 
A second photocell produces a second signal representative of the position 
of the perforating element. A motor which is responsive to variations in 
the two signals corrects the relative positions of the stamps and the 
perforating element. 
In a later example, U.S. Pat. No. 3,781,490, issued Dec. 25, 1973 to 
Phillips, discloses a reel-to-reel magnetic tape transport system. The 
tape carries a number of laterally spaced data track groups and 
prerecorded reference tracks. A control transducer senses the reference 
tracks and provides an output signal indicative of increments of tape 
movement. This information is used to derive an output signal indicative 
of tape speed, i.e., displacement per unit time. The control transducer 
also provides a signal indicative of the lateral position of the tape. The 
control transducer includes a tension transducer which provides an output 
signal indicative of the tape tension. The lateral tape position signal 
controls the lateral position of a data processing head. The tape speed 
signal and tape tension signal jointly control two reel drive motors to 
maintain a desired tape speed, i.e., a desired incremental tape distance 
per unit time, and a desired tape tension. 
More recently, U.S. Pat. No. 4,569,584, issued Feb. 11, 1986 to St. John et 
al., discloses a color electrographic recording device which produces a 
composite color image on a recording medium. The St. John printer 
transports the recording medium along a predetermined path. A print 
station in the transport path of the medium includes a recording head with 
an electrode which forms an electrostatic latent image on the medium. A 
number of developing stations in the transport path develop the latent 
image into a corresponding visible component image of a respective color. 
The registration system disclosed in the St. John patent utilizes a series 
of solid, spaced-apart tracking marks which are printed on the print 
medium adjacent both of its edges. The tracking marks are printed to have 
a known, constant number of print lines between adjacent marks; the 
constant number of print lines is representative of a given constant 
value. The tracking marks are observed electro-optically as the print 
medium moves through the device. The signals generated by the photosensors 
in conjunction with appropriate electronics are used to determine whether 
the value obtained from the photosensor observation, relating to the 
spacing between adjacent tracking marks, is the same or different from the 
given constant value. Any differences between the observed value and the 
given constant value are processed to form an error sample representative 
of the differences. A number of these error samples are then averaged to 
produce an error correction signal which corresponds to an average of the 
physical longitudinal shrinkage or expansion of the print medium that has 
occurred between the time the device printed the timing marks and the 
later time that the marks are observed. The error correction signal is 
utilized to prevent image misalignment. 
A deficiency of the St. John registration technique lies in the fact that 
the actual correction to registration is not applied at the precise point 
on the medium from which the correction signal was generated. Rather, 
because the correction signal is based on an average of signals taken over 
a length of the medium, the registration correction is applied at a point 
on the medium removed from the physical source of the correction signal. 
According to the St. John et al. teaching, an opto-mechanical encoder 
provides a series of pulses where each pulse represents an incremental 
distance of print medium movement. Control circuitry is provided to count 
the number of encoder pulses generated. To discern dimensional changes in 
the longitudinal direction, the control circuitry counts the number of 
encoder pulses occurring between adjacent printed tracking marks. As the 
pulse sensor observes movement of the print medium from one tracking mark 
to the next, the number of pulses observed between the two tracking marks 
will be indicative of either no dimensional change, a shrinkage of the 
print medium, or a stretch or expansion of the print medium. Since, as 
discussed above, there is a given constant value of encoder pulses 
associated with the longitudinal distance between tracking marks when no 
dimensional change has occurred in the longitudinal direction, if the 
print medium has stretched, there will be an increase in the number of 
observed encoder pulses above the constant value between the tracking 
marks. Conversely, if the print medium has shrunk in the longitudinal 
direction, there will be a decrease in the number of observed encoder 
pulses below the constant value between the tracking marks. These pulse 
counts above and below the constant value are termed samples. 
To remove any noise associated with a sample signal, a number of samples 
are averaged and a registration correction is made based on the resulting 
average. This is accomplished mathematically by taking a running average 
over the sample group, i.e., the most current sample is added to the 
sample group and the oldest sample in the sample group is dropped out. 
Thus, the St. John device produces a running average registration 
correction signal utilizing measurements taken over a series of 
consecutive tracking marks. Therefore, the correction in registration 
provided by the St. John device is always "running behind" the physical 
source of the correction signal by the number of tracking mark intervals 
used to generate the correction signal. 
SUMMARY 
The present invention provides a registration system for maintaining 
accurate correspondence between a printed image on a moving substrate and 
the action of an operating element with respect to the printed image. The 
system of the present invention applies a required registration correction 
at the point on the substrate from which an error signal is derived. 
The preferred embodiment of the registration system of the present 
invention includes a series of spaced-apart timing marks formed on the 
substrate. A comparison optical device, which comprises a series of lines 
and spaces formed on a transparent medium, is positioned such that the 
timing marks pass in optical alignment with the comparison optical device 
when the substrate is moving. An optical sensing device is positioned to 
detect light reflected from the substrate through the comparison optical 
device. The sensing device responds to the reflected light by generating a 
signal which is processed by appropriate electronic circuitry and provided 
to the operating element to activate the operating element in 
correspondence to the passage of the timing marks so as to accurately 
register the printing of a new image with respect to previously printed 
images. 
Thus, it is an object of the present invention to provide a real time 
registration system for a moving substrate. 
It is also an object of the present invention to provide accurate 
registration for a multi-color imaging system at high resolution and low 
cost. 
These and other objects and advantages of the present invention will become 
apparent and be appreciated by referring to the following detailed 
description of the invention considered in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
Referring to FIG. 1, electrostatic printer 10 includes supply roller 12 
which feeds print medium substrate 14 past a series of print stations. 
In the embodiment illustrated in FIG. 1, electrostatic printer 10 includes 
four print stations, each including a write head 16, toner roller 18 and 
drying/fixing station 20. In addition, each print station except the first 
includes a pair of sensor assemblies 28 which includes comparison optical 
device 26, reflective sensor 27 and lateral detector 42, for effecting 
print image registraton in accordance with the present invention. 
Each write head 16 includes an array of conductive elements or wire stylii 
arranged in a linear configuration which is well known in the art. The 
stylii of each write head 16 (except the first, which responds to encoder 
125) deposit charges on the surface of moving substrate 14 in a 
predetermined configuration according to instructions provided by an 
associated conventional computer system (not shown) and in response to a 
write head firing signal generated by the registration system of the 
present invention, as described hereinbelow. The substrate 14 then moves 
past toner station 18 where it picks up charged ink particles which are 
fused to the substrate at drying/fixing station 20. 
In the embodiment illustrated in FIG. 1, each print station prints a 
different primary color in registration with any colors printed by 
previous print stations. 
As shown in FIGS. 1 and 1A, a series of equally spaced opaque timing marks 
22 is printed by the first print station at each edge of substrate 14 on 
its print surface in response to signals from encoder 125. Encoder 125 
produces a signal that is related to the movement of substrate 14. This 
signal is delivered to the electronics that control the first write head 
16 of printer 10 so that it may print timing marks 22 and other print 
data. 
Timing marks 22 are printed on substrate 14 such that if timing marks 22 
are printed to be one print line wide, then one print line is skipped 
between adjacent timing marks; if timing marks 22 are printed to be two 
print lines wide, then two print lines are skipped between adjacent timing 
marks 22; and so on. This spacing configuration is referred to as "one-on, 
one-off", or "two-on, two-off", as the case may be. In the preferred 
embodiment of the invention, timing marks 22 are printed in a "one-on, 
three-off" configuration with timing marks 22 on opposite edges of 
substrate 14 being in lateral correspondence. As described below, because 
the actual size of the printed dots is 0.004" whereas the distance between 
four dots is 0.01", this configuration is effectively "two-on, two-off". 
As will be explained in detail below, timing marks 22 are utilized to both 
provide a firing signal to write heads 16 and to detect changes in the 
skew of moving substrate 14. 
As is best shown in FIGS. 2 and 3, the generation of a write head firing 
signal is accomplished through use of timing marks 22 on one edge of the 
substrate 14 in conjunction with comparison optical device 26. 
In the preferred embodiment, comparison optical device 26 has matching 
opaque lines and spaces formed on the upper surface of a transparent 
substrate. Such a device is commonly known as a Ronchi Ruling. The spacing 
configuration of the lines and spaces of comparison optical device 26 
corresponds substantially to the configuration of timing marks 22. In this 
case, there is a correspondence of "two-on, two-off", as modified by the 
print dot size (i.e., as explained above, the printed dot size is larger 
than the pitch of dot spacing and, therefore, the white space between the 
printed marks is narrower.) As shown in FIG. 1, in the preferred 
embodiment, one sensor assembly 28 is mounted upstream to all but the 
first write head 16 near each edge of substrate 14, in virtual contact 
with both substrate 14 and its associated write head 16, such that timing 
marks 22 pass in optical alignment with comparison optical device 26 when 
substrate 14 is moving through printer 10. 
As shown in FIGS. 2 and 3, in the preferred embodiment, reflective sensor 
27, such as a unit manufactured by TRW Corporation and having identifying 
number OPB706A, projects light from LED light source 27a onto substrate 14 
through comparison optical device 26 and senses light reflected from 
substrate 14 through comparison optical device 26 at silicon 
phototransistor 27b. (It should be noted that while use of LED light 
source 27a just described is the preferred embodiment, it is possible, and 
within the scope of the present invention, to use other light sources or 
to merely sense the ambient light contrast resulting from the passage of 
timing marks 22 with respect to comparison optical device 26.) 
When the opaque lines of comparison optical device 26 are in complete 
alignment with timing marks 22 on substrate 14, light is reflected from 
substrate 14 from the spaces between timing marks 22. In this position, 
the output signal from phototransistor 27b is at its maximum. Conversely, 
when the opaque lines of comparison optical device 26 are completely 
out-of-phase with timing marks 22, there is maximum absorption of light by 
timing marks 22 of substrate 14 and the opaque lines of comparison optical 
device 26. In this position, the output signal from phototransistor 27b is 
at its minimum. Thus, the intensity of light reflected from substrate 14 
varies from maximum reflection to minimum reflection in the time it takes 
for timing marks 22 to move from being completely in-phase with the opaque 
lines of comparison optical device 26 to being completely out-of-phase. 
Therefore, the frequency of the output signal generated by phototransistor 
27b corresponds directly to movement of substrate 14, where the cycle 
corresponds to a distance equal to one timing mark cycle. Based on this 
correspondence, the output of phototransistor 27b may be used, in 
conjunction with appropriate circuitry as described below, to provide a 
firing signal to its associated write head 16 in direct correspondence to 
movement of substrate 14 and, therefore, to the image printed thereon. 
As stated above, in the described embodiment, timing marks 22 are printed 
in "one-on, three-off" configuration. This means that there are four print 
lines associated with each timing mark, one for the timing mark 22 itself 
and three for the space following the timing mark 22. Since, however, the 
print dot size is 4 mils diameter and the print lines are spaced 2.5 mils 
apart, the actual timing mark configuration is a series of 4 mil wide 
timing marks 22 separated by 6 mil wide spaces, effectively a "two-on, 
two-off" configuration. Based on this effective configuration, as shown in 
FIG. 3, the output signal of phototransistor 27b is provided to frequency 
multiplier circuitry 29 which increases the frequency of the 
phototransistor 27b output by a factor of four so that the firing signal 
provided to write head 16 activates write head 16 to print on a 
line-by-line basis. 
FIG. 6 shows a conventional phase locked loop (PLL) 33 which provides a 
desired frequency multiplication. PLL 33 includes phase comparator 32, 
voltage controlled oscillator (VCO) 34 and frequency-divide circuit 36. 
PLL 33 drives frequency f.sub.1 to be in phase with frequency f.sub.0, the 
output signal from reflective sensor 27 in conjunction with comparison 
optical device 26 and timing marks 22. This is accomplished by driving VCO 
34 to be N times frequency f.sub.0. The frequency f.sub.2 (where f.sub.2 
=Nf.sub.0) of the signal generated by VCO 34 is modified by 
frequency-divide circuit 36 which outputs frequency f.sub.1. This 
frequency f.sub.1 is driven by phase comparator 32 to be in-phase with 
frequency f.sub.0. Thus, PLL 33 produces firing signal f.sub.2 which is N 
times f.sub.0 and in phase with f.sub.0. As stated above, in this 
embodiment, N equals 4. 
The use of a phase locked loop frequency adjustment, as described above, 
works well in the absence of electrical noise. However, if there are noise 
sources present, then a phase locked loop may have trouble tracking the 
signal from reflective sensor 27. Electrical noise can come from various 
sources. One potential source would be uneven substrate 14 movement past 
the comparison optical device 26. Uneven substrate 14 movement could have 
various causes. Slow changes in movement would result from the diameter of 
the substrate roll being out-of-round. Fast changes could come from gear 
tooth inaccuracies in the substrate 15 drive motor, toner roller 
eccentricities or general machine vibrations. Whereas the PLL circuit 
described above can handle low frequency noise sources, it cannot respond 
adequately to high frequency noise such as the fast changes mentioned 
above. The result is that PLL 33 may go into error and, since output 
f.sub.2 of PLL 33 is the firing signal that controls the printing of each 
line on substrate 14, print registration is compromised. 
Two alternative methods of frequency multiplication may be used to produce 
the required write head firing signals. One alternative is to use a 
"one-on, one-off" timing mark configuration. The output signal frequency 
of reflective sensor 27 would then be one half the print line clock rate, 
but each cycle would have two zero crossings which could be used to create 
a signal that activates write head 16 to print a line at the proper time. 
Unfortunately, in the typical electrostatic printer of the type described 
above, the size of the stylii used and the resulting printed dot size 
cause an erosion of the unprinted space between the every-other-line of 
printed timing marks. For example, as described above, if the pitch of the 
line-to-line printing is 2.5 mils, then in a "one-on, one-off" 
configuration, the 2.5 mil space between timing marks 22 is reduced to 
about 1.0 mil because, while the centers of adjacent timing marks are 5.0 
mils apart, the dots forming the marks are 4.0 mils in diameter. This 
situation significantly reduces the contrast ratio of the resulting 
comparison optical device 26 light signal and makes a "one-on, one-off" 
timing mark configuration less than desirable. A "one-on, one-off" 
configuration also makes initial skew adjustment more difficult. 
It should be noted, however, that if special wires, or even thin shim 
material, are placed in the first print station where timing marks 22 are 
produced, then a "one-on, one-off" configuration could be utilized with a 
resultant simplification of the system. 
In the absence of the ability to use a "one-on, one-off" timing mark 
configuration, the next best configuration is the effective "two-on, 
two-off" configuration described above. However, as discussed above, the 
use of a "two-on, two-off" sequence requires production of a 4.times. 
frequency multiplication and as discussed above, a PLL has problems in 
this application. However, an alternative technique for producing a 
4.times. multiplication is through the use of "quadrature." 
In quadrature, two signals are created that have a 90.degree. phase 
relationship. As described below, through the use of these two signals, a 
4.times. multiplication of the frequency of the output of reflective 
sensor 27 can be effected. This is a direct process that does not involve 
a PLL and, therefore, is not affected by moderate variations in the speed 
of substrate 14. 
To produce quadrature according to the present invention, a comparison 
optical device having two distinct sections is substituted for the 
single-section comparison optical device 26 described above. As shown in 
FIG. 7, the two distinct comparison optical device sections A and B have 
exactly the same line spacing and line width as the previously-described 
comparison optical device 26 and, in fact, are identical gratings. The 
only difference in the gratings is their placement; they are placed in 
series so that their phase relationship is 90.degree. with respect to each 
other. To accommodate the two distinct sections A and B, two reflective 
sensors, one for each section, are required to generate a dual output 
signal. The two output signals are provided to appropriate conventional 
circuitry, shown in FIG. 13B and described below, to produce the required 
frequency multiplication. 
A further improvement of the basic 2-signal quadrature resulting from the 
two-section comparison optical device arrangement described above in 
conjunction with FIG. 7, and the preferred embodiment of the present 
invention, is 4-signal quadrature. Referring to FIG. 8, four separate 
output signals are generated by a comparison optical device having four 
distinct sections 1-4 of the type described above and, in the preferred 
embodiment, being positioned with respect to each other so that their 
sequential phase relationship is 0.degree., 90.degree., 270.degree. and 
180.degree., respectively. The four resulting signals represent sine, 
cosine, -cosine and -sine. These four signals are used in conjunction with 
appropriate conventional circuitry to generate the required frequency 
multiplication so as to produce a write head firing signal that is 
independent of substrate background "whiteness." 
The four distinct comparison optical device sections 1-4 shown 
schematically in FIG. 8 have the same line width and line spacing, but, as 
stated above, each section is phase shifted with respect to the others. 
The sections 1-4 can be described in terms of phase angle where 
360.degree. is the distance from the side of one line in a section to the 
same side of an adjacent line in the same section. Thus, in the preferred 
embodiment section 1 is 0.degree. (sine), section 2 is +90.degree. 
(cosine), section 3 is +270.degree. (-cosine) and section 4 is 
+180.degree. (-sine). The four sections can be arranged in any order, 
however, and still perform in accordance with the present invention. 
Each of the four sections 1-4 shown in FIG. 8 has an individual 
LED/phototransistor pair associated with it. 
According to a preferred embodiment of the invention, rather than the 
LED/phototransistor pair being arranged sequentially as shown in FIGS. 2 
and 3, LED 127a and phototransistor 127b are mounted in a side-by-side 
configuration as shown in FIG. 8A. This side-by-side configuration 
substantially eliminates any "shawdow" effect caused by deeply etched 
lines in the comparison optical device. LED 127a and phototransistor 127b 
can be purchased and mounted separately. LED 127a in the FIG. 8 embodiment 
is a TRW GaAlAs LED, Part No. OP268FA; phototransistor 127b is also a TRW 
product, Part No. OP508FA. LED 127a and phototransistor 127b are shielded 
by a piece of copper 127 which is grounded because of the proximity of 
these components to write head 16. 
Thus, four output signals, representing sine, cosine, -cosine and -sine, 
are generated by the four-section comparison optical device. These four 
signals are provided to appropriate circuitry which generates a single 
firing signal for write head 16. An example of a circuit which can be used 
to generate the firing signal is provided by the combination of the 
circuitry shown in FIGS. 13A and 13B. 
FIG. 13A shows circuitry for generating a sine output signal SINE from the 
signal generated by the first phototransistor 127b of the four-section 
comparison optical device. Identical circuitry is utilized to generate 
similar signals corresponding to the other three comparison optical device 
sections, i.e., cos, -cos and -sine. These four signals (SINE, MSINE, COS 
and MCOS) are then provided to circuitry as shown in FIG. 13B. As shown in 
FIG. 13B, the SINE and MSINE signals are provided to the "plus" and 
"minus" inputs of a difference amplifier. The COS and MCOS signals are 
similarly processed. The two resulting ground centered signals are then 
split and squared-up to form four 45.degree. square wave signals SW1-SW4 
which are delivered as inputs to an exclusive-OR tree. The exclusive-OR 
changes state with each transition of one of the four comparison optical 
device signals, thereby providing four write head FIRING SIGNAL outputs 
with the passage of each timing mark 22. 
By utilizing circuitry of the type just described, any DC offset in the 
signal from the four-section comparison optical device is cancelled. A DC 
offset is always present since the opaque lines of the comparison optical 
device and opaque timing marks 22 are not completey absorbing and there is 
generally some scattered light present. 
The aforedescribed method of using signals representing sine and -sine and 
cosing and -cosine can be used to partially cancel mottling of the 
substrate reflectivity. Since there are four comparison optical device 
sections and four separate reflective sensor pairs, the effect is limited 
mottling artifacts of size comparable to the distance between sine and 
-sine and cosine and -cosine detectors. 
A further improvement over use of a 4-signal quadrature comparison optical 
device uses a single comparison optical device where the distance between 
comparison optical device elements which produce the sine and cosine 
signals effectively disappears. In accordance with this concept, FIG. 9 
shows a comparison optical device wherein instead of having alternate 
clear and opaque (black) stripes, alternating stripes of different colors 
are provided. Thus, both the sine and the cosine signals are generated 
from the same comparison optical device grating. If timing marks 22 are 
black, then as they move over each color on comparison optical device 426, 
the black timing marks 22 back up first one color and then the other. When 
the black timing marks back up one color, color is removed, since the 
light that passes through that color is not reflected. Therefore, as 
substrate 14 is moved, the black timing marks 22 on substrate 14 cause the 
reflected light that passes through colored comparison optical device 426 
to change from color 1 to color 2 and then back again. Two reflective 
sensors 430 and 440 are positioned so that they collect the scattered 
light that reflects from substrate 14 through two-color comparison optical 
device 426. Sensor 430 has a color filter 410 for color 1 and sensor 440 
has a colored filter 420 for color 2. Thus, each sensor generates a 
waveform which is 90.degree. out of phase with the other sensor's 
waveform. This arrangement generates sine and cosine. The advantage of 
this technique is that the two signals come from the same location on the 
substrate and, thus, see the same substrate reflectivity characteristics. 
When the two signals are combined, any variations in the DC level of one 
signal will be the same in the other signal and therefore, the effects of 
substrate 14 mottling will diminish. 
The above-described "two-color" comparison optical device concept can be 
extended to quadrature as well. As shown in FIG. 10, for quadrature, four 
stripes of different colors, with a spacing of one half the spacing of 
timing marks 22, are used in a single comparison optical device 526. 
Referring to FIG. 11, in a manner similar to two-color comparison optical 
device 426, four reflective sensors f.sub.1 -f.sub.4 of the type described 
above are positioned so that each senses the light that reflects from 
substrate 14 through four-color comparison optical device 526. Sensor 
s.sub.1 includes a color filter f.sub.1 which passes two colors, C1 and 
C2; sensor s.sub.2 includes a color filter f.sub.2 which passes colors C2 
and C3; sensor s.sub.3 has a filter f.sub.3 which passes colors C3 and C4; 
and sensor s.sub.4 has a filter f.sub.4 which passes colors C4 and C1. As 
substrate 14 moves over four-color comparison optical device 526, black 
timing marks 22 will back up different colors, resulting in 90.degree. 
phased output signals from the four sensor s.sub.1 -s.sub.4. As stated 
above, each of the color stripes C1-C4 are one half the width of the 
respective black timing marks 22. 
For example, as shown in FIG. 11, at time t.sub.0, the black timing mark 
backs up the color C1 and C2 stripes. Therefore, at time t.sub.0, sensor 
s.sub.1 senses low intensity reflection since substantially all light 
passing through the comparison optical device behind those two stripes is 
absorbed by timing mark 22. Similarly, at time t.sub.0, sensor s.sub.2 
senses medium intensity light since low light is reflected through the 
color C2 stripe but high light is reflected through the color C3 stripe, 
which is backed by a "space"; sensor s.sub.3 senses high intensity light 
because both the color C3 stripe and the color C4 stripe are backed by a 
"space"; and sensor s.sub.4 senses medium intensity light because the 
color C4 stripe is backed by a "space" and the color C1 stripe is backed 
by a timing mark 22. 
As substrate 14 moves from time t.sub.0 through time t.sub.x, the detectors 
sensors s.sub.1 -s.sub.4 generate the 90.degree. phased signals shown in 
FIG. 11. These waveforms can be used to generate a write head firing 
signal in the same manner as the signals derived from the four-section 
comparison optical device described above with respect to FIG. 8. The 
advantage of this four-color technique is that the firing signal can be 
derived from a single point on substrate 14. 
Referring back to FIG. 4, the adjustment for skew in moving substrate 14 is 
based upon the signals generated by the two comparison optical devices 26 
associated with each print station. The output of reflective sensor 27 
associated with each comparison optical device 26 is processed by 
circuitry, described below, and provided to a conventional phase sensitive 
detector 38. Phase sensitive detector 38 generates an error signal which 
is based on the phase difference .phi. between the left and right sensor 
27 outputs. The error signal drives a servo motor (not shown) which 
angulates the associated write head 16, to which sensor assemblies 28 are 
attached, to remove the skew error. Alternatively, the error signal could 
be used to steer substrate 14. 
The circuitry for phase sensitive detector 38 is shown in the combination 
of FIGS. 13A-13C. As described above, the circuitry shown in FIGS. 13A and 
13B generates four 45.degree. square wave signals. As shown in FIG. 13B, 
one of these signals, designated E-LEFT, is used as the skew servo signal 
from the comparison optical device located at one side of substrate 14. A 
similar signal, E-RIGHT, is generated from the output of the comparison 
optical device 26 located on the opposite side of substrate 14 utilizing 
the circuitry shown in FIG. 13C (the circuit shown in FIG. 13C is less 
complex than that shown in FIGS. 13A and 13B because it does not include 
elements required to produce the write head firing signal). The two 
signals, E-LEFT and E-RIGHT, are then provided to a conventional phase 
sensitive detector, which based on the phase difference between signals 
E-LEFT and E-RIGHT, generates a correction signal that is applied to a 
servo motor to appropriately adjust the associated write head 16 on which 
is mounted sensor assembly 28. This movement of write head 16 also moves 
one of the comparison optical devices 26 which removes the skew error. 
As shown in FIGS. 1 and 1A, solid lateral tracking lines 24 are formed on 
each side of substrate 14 adjacent timing marks 22. In the illustrated 
embodiment, tracking lines 24 shown inside timing marks 22, but they could 
be located outside timing marks 22 as a matter of choice. Solid tracking 
lines 24 are used in the conventional manner to maintain the correct 
lateral position of write heads 16. As best shown in FIG. 2A, a lateral 
detector 42 comprising split sensors 42a and 42b detects light reflected 
from the substrate 14 and the lateral tracking line 24 as illuminated by 
LEDs 42c to monitor the position of solid tracking lines 24 to maintain 
write heads 16 centered with respect to the previously printed image. In 
the preferred embodiment, balanced detectors which are insensitive to 
paper optical density variations are used. A lens 42d is used to image 
substrate 14 in a 1:1 magnification configuration. 
FIG. 14A shows circuitry which amplifies the outputs of two split sensors 
42a, 42b on one side of substrate 14 and takes their difference to arrive 
at a signals, A-B LEFT, which is representative of the position of solid 
tracking line 24 on that side of substrate 14 with respect to lateral 
detector 42. A corresponding signal, A-B RIGHT, is generated by lateral 
detector 42 located on the opposite side of substrate 14. Referring to 
FIG. 14C, the two signals A-B LEFT and A-B RIGHT are then processed to 
generate two signals, RAS ABS ERR and RAS RAW DIR, which are, 
respectively, respresentative of the value and the direction of the 
correction required, and are utilized to drive a raster connection servo 
motor to keep the associated write head 16 centered with respect to the 
previously printed image. 
Referring back to FIG. 1, the solid tracking line 24 on each side of 
substrate 14 includes at least one Start-of-Plot mark 124 for each 
individual image plot that is printed. The Start-of-Plot mark 124 works in 
conjunction with the split sensors 42a and 42b. When line 24 passes over 
sensor assembly 28, part of which consists of lateral detector 42, the 
tracking line 24 is imaged into sensors 42a and 42b such that the solid 
line is imaged half on sensor 42a and half on sensor 42b. Sensors 42a and 
42b are mounted optically close together and, in the preferred embodiment, 
form what is known in the art as a split detector, that is, two detectors 
formed by photolithography on the same substrate. A product such as 
Silicon Detector Corporation SD 113-24-21-021 will provide a device with 
two detectors that are each 0.1" long by 0.050" wide and separated by 
0.004". The imaging system should be considered such that the width of the 
line 24 is somewhat less than the distance across the sensors 42a and 42b. 
In the case of SD113-21-21-021, the distance across both sensors is 0.1"; 
the width of line 24 is the preferred embodiment is 0.06". 
In the preferred embodiment, the magnification of the imaging system is 1.0 
and is provided by lens 42d which has a focal length of 8 mm and is placed 
16 mm from substrate 14 and 16 mm from split sensors 42a and 42b. This 
allows 0.020" extra detector width on each side of line 24 to allow for 
system dynamic range. The Start-of-Plot mark 124 is defined as a section 
of line 24 where, for a space interval equal to at least the length of 
sensor 42a or 42b, nominally one half of the width of one side of the line 
24 ceases to be printed and then, for a space interval equal to at least 
the length of split sensor 42a or 42b, the side of the line that ceased to 
be printed returns, coincident with a cession of printing on the other 
half of line 24. 
In the preferred embodiment, Start-of-Plot mark 124 is formed by having the 
entire line 24 move first one half its width to one side and then a 
distance equal to its entire width to the opposite side, and then back 
again to the center. At the end of this space interval the entire width of 
the line 24 returns to its normal configuration and remains constant until 
the next Start-of-Polt mark 124 occurs. Other variations on this theme are 
possible. In general, a variation in reflected light effecting the two 
sensors 42a and 42b differentially and then reversing is the method used. 
Start-of-Plot marks 124 on opposite sides of substrate 14 present mirror 
images of each other. That is, when SOP mark 124 on one side of substrate 
14 moves toward the nearest edge of substrate 14, its corresponding SOP 
mark 124 on the other side of substrate 14 also moves toward its 
respective edge; similarly, when SOP mark 124 on one side of substrate 14 
moves toward the center of substrate 14, its corresponding SOP mark 124 on 
the other side also moves toward the center. This mirror-image SOP mark 
configuration allows the raster correction servo motor to remain in 
operation during passage of SOP marks 124 since the two mirror-image SOP 
marks 124 from which the lateral correction signal is taken effectively 
cancel each other. 
The effect of passage of the Start-of-Plot mark 124 over split sensors 42a 
and 42b is that alternately one and then the other sensor 42a, 42b "sees" 
a much higher light level resulting from an increase in the percent of 
unprinted pattern being imaged on it. This causes a signal 224 to be 
generated as shown in FIG. 12A. As the image of Start-of-Plot mark 124 
starts to pass over sensors 42a and 42b, the outputs of the two sensors 
becomes unbalanced and give rise to signal 224a. When the image of 
Start-of-Plot is positioned so that the image of Start-of-Plot mark 124 is 
at a position 226 in FIG. 12, signal 224 is at its maximum unbalanced 
positive voltage 226. As the image further moves to position 227, signal 
224 switches to its maximum unbalanced negative voltage 227. In doing so, 
it passes rapidly through zero voltage 260 where the image of 
Start-of-Plot mark 124 produces equal signals out from sensors 42a and 
42b. As the image further moves to position 228, sensors 42a and 42 b 
again return to a balanced condition. This signal pattern serves to 
generate a unique indication of Start-of-Plot. 
As shown in FIG. 12A, while the Start-of-Plot signal is unique and well 
defined, the zero crossover 260 will have noise on it which comes from 
both electrical noise as well as noise caused by printing errors such as 
flares or dropouts. As such, this signal is not sufficient to clearly 
identify a particular print line. It is, however, well defined with 
respect to one timing mark interval such as those designated 301, 302, 
303, etc. in FIG. 12A. The noise band 260 of Start-of-Plot signal 224 can 
be placed in a unique timing mark interval such as 302 either by 
physically adjusting the sensor assembly 28 or by electrically delaying 
the zero crossing. Since, as described in detail above, the timing mark 
signal 300 is phase locked to the write head firing signal 280, either 
through a phase lock loop or through a quadrature multiplication circuit, 
the zero crossover 260 that uniquely determines a timing mark interval can 
also uniquely determine a particular print line. This is accomplished 
through use of appropriate conventional circuitry of the type shown in 
FIG. 15. 
As shown in FIG. 1A, series of Start-of-Plot marks 124 is used to prepare 
the printer 10 for plotting. This is accomplished by comparing the time 
interval between the Start-of-Plot transition on opposite sides of 
substrate 14. The time difference between the two signals is indicative of 
an initial gross skew error present between the previously printed pattern 
and the write head associated with the sensor detecting the Start-of-Plot 
transitions. The time difference, either positive or negative, is used to 
drive the skew servo to correct for the gross skew error. Once the gross 
error has been corrected, other circuitry is called upon to maintain 
proper fine skew based upon the phase of left and right timing mark 
signals, as described above. 
The circuitry for generating the Start-of-Plot signal is shown in FIGS. 
14A, 14B and 14D. The two signals A-B RIGHT and A-B LEFT which are 
generated as described above with respect to FIG. 14A are provided to the 
circuitry shown in FIG. 14B. Each is filtered and stored in a 
sample-and-hold circuit. The stored A-B levels are used as a reference 
against the current rapidly-varying A-B signal that is the SOP transition. 
The stored value allows correction for variation in the difference between 
the distance between the two tracking lines 24 and the distance betwee the 
two lateral detectors 42. A SOP-DETECT signal is then provided to the 
processor associated with printer 10. 
The Start-of-Plot signal may not be accurate enough to be repeatable to one 
line. However, as shown in FIG. 12A, since timing marks 22 are printed in 
"two-on, two-off" configuration, if the Start-of-Plot signal makes its 
transition within a window composed of four printed lines, then the logic 
circuitry associated with the write head signal processing electronics 
will know that the last line of that four-line window is the first line of 
the plot. 
As stated above, the signal used to generate the line print enable pulse 
for the first, non-registered print station is derived from encoder 25. 
When printer 10 is in the idle mode and a plot request is received, printer 
10 generates a set or series of Start-of-Plot marks 124. This allows write 
heads 16 to align themselves before a plot begins. When split sensors 42a, 
42b associated with a particular write head 16 encounter the 
"Start-of-Plot" set, the zero transition is detected on each side of 
substrate 14 to correct for gross skew error. The time difference between 
the two zero crossings indicates the amount of skew error. Multiple sets 
of Start-of-Plot marks 124 proceeding the actual plot allow more than one 
attempt at producing correct skew. If the machine is plotting and a new 
plot request is received, then the nelot will have a single pair of 
Start-of-Plot marks 124 generated since the machine should already be 
properly skew adjusted. 
It should be noted that variations in the above-described embodiment of the 
present invention are possible. For example, in each of the 
above-described embodiments, the comparison optical device need not be in 
virtual contact with the moving substrate; it can be mounted a distance 
from the substrate. In this case, light directed at and reflected from the 
substrate timing marks could be focused onto the comparison optical device 
by a lens and passed through the comparison optical device to its 
associated photosensors. The results of the four-signal quadrature 
comparison optical device arrangement described above can be similarly 
achieved by placing four comparison optical device sections in a 
prescribed arrangement removed from the substrate and utilizing 
beamsplitting prisms to direct the light reflected from the substrate 
through the sections to associated photosensors. Comparison optical 
devices need not be used at all; detection of the light reflected from the 
substrate timing marks 22 by a properly arranged array of charge coupled 
devices could provide the desired result. 
Therefore, it should be understood that various alternatives to the 
structures described herein may be employed in practicing the present 
invention. It is intended that the following claims define the invention 
and that the structure within the scope of these claims and their 
equivalents be covered thereby.