Optical sensor for plotter pen verification

An optical sensor for monitoring plotter pen performance by sensing the quality of lines drawn on a medium. An LED emitting green light beam is angularly directed toward an underlying line so as to reflect into an optical sensor which measures the print contrast ratio of a point on the line. Circuit means amplifies and filters the signal generated by the optical sensor.

RELATED APPLICATIONS 
This application is related to U.S. application Ser. No. 07/763,889 filed 
Sep. 20, 1991 in the name of Messrs, Boeller, Halpenny, Tarradas, Rosello 
and Beauchamp under the title "Monitoring and Controlling Quality Of Pen 
Markings On Plotting Media", and U.S. application Ser. No. 07/763,387 
filed Sep. 20 1991 in the name of Messrs Giralt and Beauchamp under the 
title "Pen Qualification and Verification In A Graphics Plotter", both of 
which are commonly owned by the assignee of the present application. 
BACKGROUND OF THE INVENTION 
The invention relates generally to pen plotters, and more specifically to 
monitoring and controlling the quality of pen markings on plotting media. 
A typical pen plotter comprises a pen for producing markings on a medium in 
response to instructions from, for example, a computer. The medium such as 
paper is movable in a first direction along the x-axis and the pen on a 
main carriage is movable in a second direction along the y-axis which is 
perpendicular to the the first direction. Thus, by appropriate control of 
the drives for the pen and paper movement, any desired graphical 
representation can be produced on the medium. The writing system also 
typically comprises a rotatable pen carousel carrying a plurality of pens, 
for example pens of different colors. A pen handling mechanism is provided 
which permits a pen to move from the pen carousel into a position on the 
carriage for plotting on the medium and replaces that pen by another one 
from the carousel, for example when a different color is desired. 
Writing systems such as the above-mentioned pen plotter are typically used 
for producing rather complex graphical representations, for example 
circuit layouts or construction diagrams, which are electronically stored 
in a computer on which they may also have been created by a user. Once the 
plotting of those electronically stored drawings has been initiated by a 
user, the plotting continues automatically and the user only has to take 
the completed drawing from the plotter. In practice, however, the 
completed drawings have not always been satisfactory as certain lines 
which should have been plotted are missing or have bad quality because the 
corresponding pen is clogged, or dried out, or is inking out or is in 
another way not operating properly. As a consequence thereof, the entire 
drawing generally had to be plotted again, in particular if a pen failure 
had already occurred at the beginning of the plotting process. Thus, a 
considerable amount of time is wasted and the efficiency of the plotting 
is decreased. Usually these problems are resolved by perfecting the 
components of the plotter so that they are less prone to failures. 
In pen plotters, for example, measures are taken to avoid drying out or 
clogging of the ben. One of those measures is to seal the pen tips by 
rubber caps while they are in the pen carousel so that they stay moist and 
ready to write. Despite such attempts for perfection of the writing 
components, writing failures may still occur, for example, when the ink 
reservoir of the pen is empty or nearly empty or foreign substances 
interfere with the writing system or when any other unforeseeable faults 
occur. Since the plotting typically is performed with high speed, i.e., 
high relative velocity between the pen and the medium, even small defects 
in a pen lead to a noticeable degradation of the quality of the drawing. 
There have been plotting systems which have a sensor that retraces along 
the entire actual path of a plotted line and then compares such plotted 
line with the computer program. Such a system is very inefficient due to 
the excessive time consumed, and also due to the need for precisely 
focusing the sensor directly on the plotted line. Also, such a system 
fails to take into account the different line thicknesses and varied line 
intensities that result from diverse pen types, colors, plotting speeds 
and pen force, as well as type of media and ambient lighting conditions. 
BRIEF SUMMARY OF THE INVENTION 
It is an object of the invention to solve the aforementioned deficiencies 
of the prior art, and provide a method and apparatus for assuring that a 
high degree of reliability is achieved for nonattended plotting. A related 
object is to minimize the human supervision required for operating the 
plotter. 
Another object is to provide a pen verification system which takes into 
account the different drawing characteristics of all types of pens such as 
such as fiber-tip paper pens, fiber-tip transparency pens, rollerball 
pens, drafting pens for vellum and paper, and drafting pens for polyester 
film. A related object is to provide a pen verification system that 
provides predictable performance for different color pens, different line 
thicknesses, varied pen speeds and pen pressures, and all media types such 
as paper, vellum, polyester and translucent material. 
A further object is to provide adequate optical sensing of pen lines under 
varying ambient light conditions. 
In accordance with the foregoing objects, the invention provides a method 
of monitoring and controlling the quality of pen markings on a plotting 
medium by qualifying each pen based on optically sensing across a sample 
line drawn on an actual medium. During each subsequent plotting task of a 
particular pen which has been qualified, an actual line plot is optically 
sensed across a selected point to make a comparison with the sample line. 
If the actual line plot is unsatisfactory, the deficient pen is replaced 
and the plot is restarted from the beginning, or retraced from the last 
good verification, or is stopped to allow the user to select an 
appropriate corrective procedure. 
In another separate and important aspect of the invention, a customized 
optical sensor is provided for monitoring plotter pen performance by 
sensing the quality of lines drawn on a medium. An LED emitting a green 
light beam is angularly directed toward an underlying line so as to 
reflect into an optical sensor which measures the print contrast ratio of 
a point on the line. Circuit means amplifies and filters the signal 
generated by the optical sensor. 
In another separate and important aspect of the invention, a method is 
provided for comparing a test line of a plotter pen with a subsequent plot 
line in order to compare selected points on the plot line with a benchmark 
for that particular pen. Each pen in a plotter carousel is qualified by 
monitoring the print contrast ratio of multiple points taken from vertical 
and horizontal portions of a sample line drawn by the pen. A record is 
maintained to indicate the good or bad qualification status of each pen in 
the carousel, and the appropriate benchmark for each pen is stored for 
future reference. By comparing the print contrast ratio of a selected 
sequence of points on an actual plot line with the benchmark for the pen 
making the plot, the satisfactory function of the pen can be periodically 
verified.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Generally speaking, the invention is incorporated in a pen plotter 
comprising a pen for producing markings such as graphical representations 
on a medium, for example on a sheet of paper. The pen is held on a main 
carriage by a z-axis carriage which can be lowered such that the tip of 
the pen contacts the medium in order to produce markings thereon. The pen 
comprises an ink reservoir containing ink of a certain color. The pen is 
held on the z-axis carriage in a way so that it can easily be replaced by 
another pen, for example if a different color for the graphical 
representation is desired or if the ink reservoir in the pen is empty. The 
replacement of the pen can be done manually by a user, but it is preferred 
to perform the replacement automatically undercomputer control. Several 
replacement pens and additional pens of different colors can be arranged 
in a pen carousel (not shown) from which they can be transported to the 
z-axis carriage and inserted there. For producing two-dimensional plots on 
a sheet of paper, the paper is moved in a first direction (x-direction) 
and the main carriage is moved in a second direction (y-direction) 
orthogonal thereto. The driving of the paper can be accomplished by means 
of a grit wheel and pinch wheel assembly between which the paper is moved. 
By appropriate movement of the paper (either in the positive or negative 
X-direction) and coordinated related movement of the main carriage (either 
in the positive or negative Y-direction), any desired graphical 
representation can be made on the paper sheet. Of course, the invention is 
not limited to any particular type of plotter, but can be utilized in any 
plotter configuration where line vectors are drawn on media by pens. For 
example, another type of plotter which may utilized the invention holds 
the paper stationary while the main carriage is movable in the X- and 
Y-directions so that the pen can be placed on any desired point of a media 
sheet. 
A sensing system intermittently monitors the quality of the pen on the 
media by scanning across a point on a line, with the point overlying a 
white reference strip in the platen and illuminated by light emitted from 
a light source such as an LED emitting a beam of green light. 
The output signals of the sensing system are amplified, filtered and 
converted into digital data. This digital data is then supplied to a 
microprocessor for comparison with the benchmark data stored for that 
particular pen (or type of pen). In response to such comparison, the 
microprocessor provides output signals indicating a good pen or a bad pen. 
Various corrective actions can be taken when a bad pen is detected by the 
sensing system, or that a good pen is malfunctioning. For example, the 
plotter can be pre-programmed so that the bad/malfunctioning pen is 
automatically replaced by another pen, or that user can be warned that a 
writing error has occurred so that he can decide on appropriate action, or 
those marking which do not meet the desired quality can be replotted. 
Referring more specifically to the drawings, FIG. 1 is a block diagram 
showing the circuit interconnections between a microprocessor (not shown) 
and the various functional components of the plotter. The microprocessor 
circuit is not mounted on the pen carriage and is connected to the sensor 
circuit and the carriage circuit through a flexible cable. The mechanical 
components include a pen carriage 20, front panel 22, turret drive 26 for 
a pen storage carousel (not shown), and ID indicia 30 on the pen storage 
carousel to indicate the type of pens available therein for plotting. A 
keyboard driver 34 connects through a VFD/keyboard latch 36 to receive 
inputs from the microprocessor and to provide inputs to a vacuum 
fluorescent display on the front panel 22. The display also receives 
inputs from the microprocessor. 
The turret drive 26 receives inputs from the microprocessor through a 
carousel driver 42 to rotate the carousel to a proper position for 
transferring pens back and forth between the carousel and the pen 
carriage. A carousel sensor 44 interacts with the ID indicia 30 to 
identify which carousel slots are empty and which type of pens are in each 
of the other carousel slots. 
A servo integrated circuit 50 receives inputs from the microprocessor and 
also interconnects an X-encoder 52 through an X-drive 54 to an X-motor 56 
in order to monitor the position of the medium in the X-direction and then 
cause the X-motor to move the medium to a new position. In a similar way, 
interconnections are made from a Y-encoder 58 through a Y-drive 60 to a 
Y-motor 62 in order to monitor the position of the pen carriage in the 
Y-direction and then cause the Y-motor to move the pen carriage to a new 
position. 
A fan/roll latch 64 receives inputs from the microprocessor to a fan drive 
66 to operate a fan 68 which provides a vacuum through holes in the platen 
to help hold the media securely at the point where a pen is drawing 
vectors on the media. The latch 64 also provides inputs through a rollfeed 
drive 70 to a media roll 72 for bringing media into position under the pen 
carriage 20 in order to commence a new plot. 
A pen lift 74 is included with the other drives on an analog printed 
circuit assembly 76 and receives inputs from the microprocessor through 
the servo IC 50 and then sends output signals to the pen carriage 20 in 
order to move the pen from a normally raised position to a lowered 
position into contact with the media at the point where it overlies the 
platen. Optical sensor circuits 78 provide input signals to a light source 
on the pen carriage 20 and then process output signals from the optical 
sensor on the pen carriage 20 back to the microprocessor. 
Referring now to FIG. 2, the various interconnecting circuits for actuating 
the pen verification procedures are shown in a block diagram. Since a 
light source in the form of an LED 80 and a optical sensor 82 are directly 
mounted on the pen carriage 20, a pen-lift drive circuit 83 is 
interconnected between the microprocessor 79 and the pen carriage 20 to 
move the carriage into the optimum focal distance above the media. Signals 
from the microprocessor 79 are passed through latch 84 to a digital/analog 
converter 86 which produces an output signal which passes through driver 
88 to the LED 80. The LED transmits a wide beam of light "(see outwardly 
flared arrows schematically extending below LED 80 in FIG. 8)" having a 
color centered in the visual spectrum to a predetermined locale around a 
line vector on the medium, and the sensor 82 measures the intensity of the 
reflected light as the sensor scans across the line (i.e., from one side 
of the line through a point on the line to the other side of the line). 
Typical scans of actual plots are shown in FIG. 13 at 96, 97, and 98, and 
a typical curve of the intensity for a satisfactory pen marking as shown 
in graph 100 of FIG. 14. In order to enhance the reliability of such light 
intensity curves, a customized platen 102 is provided. In addition to the 
usual drawing notch 104 which includes vacuum apertures 106 for holding 
the drawing medium in secure position under a pen tip, the invention 
provides a white reference strip 108 (see FIG. 3) which extends parallel 
to the drawing notch 104 in the Y-direction, with the strip and notch 
having the same predetermined offset 110 along their full length. As shown 
in the detailed sectional views of FIGS. 4 and 5, the white reference 
strip in the presently preferred embodiment is formed by a white tape 
which is recessed below the surface of the platen 102 in a groove 112 so 
as to be positioned along the bottom surface of the groove without the 
tape visibly extending up either side 113 of the groove. Because of 
different light reflective properties of different types of media and 
different room environments, it is necessary to conduct an initialization 
scan at the beginning of each plot (see FIG. 15). By proper movement of 
the carriage 20 in the Y-direction by the Y-drive 60 and coordinated 
movement of the medium 114 in the X-direction by the X-drive 54, the point 
of the line being scanned is positioned directly over the center of the 
white strip 108 (see FIG. 14). 
The structural details of the pen carriage 20 are shown in the exploded 
view of FIG. 6, and the front view of FIG. 8. A main carriage 120 carries 
variously positioned Y-direction tires 122 mounted on bearings 124, and a 
bumper 123, to facilitate the movement along the Y-axis. In order to 
provide movement of the Z-carriage and the pen between a raised position 
to a lowered position "(see the double pointed arrows in FIG. 8)", the 
main carriage also carries a magnetic cup 126 and a Z-direction tire 130 
for engagement with a Z-axis carriage 128. An energized coil 132 mounted 
on an insert 133 in the Z-axis carriage is magnetically pushed away from 
its matching magnetic cup 126 to move the pen into the down position. The 
actual location of the pen (and sensor/LED assembly described in more 
detail below) relative to the underlying media is monitored by an encoder 
scale 134 which moves up and down adjacent to an optical encoder 136. A 
carriage PCB 138 carries the encoder 136 and also provides the circuit 
interconnections through wires 140 to the coil as well as the circuit 
interconnections to the LED 80 and the sensor 82. A stationary inner 
linear bearing 142 engages a matching moving outer bearing sleeve 143, and 
an expansion spring 144 holds the z-carriage in a normally raised 
position. 
When a pen is mounted on the carriage 20, a compression spring 146 forces a 
pawl 148 into locking engagement against the outer pen casing 150 (see 
FIG. 7). 
It is preferable to calibrate each plotter before it is used in order to 
optimize the ability of the sensor to measure the light intensity of a 
plotted line. Accordingly, as best shown in FIG. 8, the pen is removed and 
a paper feeler 152 can then be used to determine the actual distance to a 
sheet of underlying media for this particular plotter. In addition, the 
paper feeler can scan the platen along the entire length of the Y-axis to 
determine variations in this actual distance measurement. Such actual 
distances measured during calibration are recorded in memory so that 
during normal operation the Z-axis carriage can be moved to achieve the 
optimum focal distances as shown in mm in the drawing of FIG. 8. It has 
been determined that the present invention nevertheless operates 
satisfactorily for a photodiode 156 spaced a distance "Si" of 17.2 mm from 
a lens 154 even though the distance "So" of 15.1 mm between the lens and 
the medium may vary plus or minus 1.5 mm. 
The sensor 82 is shown in detail in FIGS. 9-10 and includes a casing 160, a 
chip assembly 162, a cap 164, and a sleeve 166 having a bracket 168 for 
holding the LED 80. The sleeve snaps into position at the bottom of the 
casing 169 and holds the lens in fixed position inside the sleeve at the 
lower end of the casing. The chip assembly includes a photodiode as well 
as a two-stage amplifier, and the lower portion 179 of the chip assembly 
is transparent (such as clear plastic) to allow reflected light to pass 
unimpeded to the photodiode. The cap 164 must fit snugly over the top end 
of the chip assembly to nest into a casing receptacle to prevent any 
extraneous light from passing through to the photodiode. Although both the 
sensor and the LED are shown at an angle with respect to the Z-direction, 
it is possible to have other angular positioning of the sensor, so long as 
the LED preferably emits light at an angle to avoid undue specular glare 
from the media. 
Thus, the invention provides a method and apparatus for using a uniquely 
designed optical sensor that periodically senses the quality of plotted 
lines by scanning across selected points on the lines, and measuring the 
difference in contrast between the actual plotted line and a benchmark 
such as a default value or an actual value obtained when that particular 
pen was qualified. When the optical line sensor detects a pen failure or 
pen deficiency, the plotter corrects the problem by picking a fresh pen 
and retracing the plot from the last good verification point, restarting 
the plot, or stopping and alerting the user that a pen failure has been 
detected. 
FIG. 12 shows in more detail the processing of the output signal from the 
sensor through op-amp 180, resistors 182,183, filter circuits 184,185 and 
A/D converter 186. 
FIG. 11 shows the interconnections between PCB 1 (processor), PCB 2 
(carriage) and PCB 3 (sensor) including the LED input 188, the sensor 
output 190, the encoder circuits 192 and the coil circuits 194. 
In the flow chart of FIG. 14, it is important to first check the media such 
as paper at the actual spot where the sample vectors are to be drawn to be 
sure there are not a ready previous plots or other non-white interference. 
The first percentage measurement at 200 is based on the percentage drop in 
reflected signal intensity from total white to absolute dark (total light 
absorbence). Thus, if five of the proposed six points each show a print 
contrast ratio of less than 8%, the it is ok to proceed to the next stage 
of actually drawing the sample plot as shown at 202. 
In order to establish the PCR threshhold, various procedures can be used. 
In the preferred form of the invention, the average intensity of the PCRs 
for five points is computed, and then so long as the pen plots checked 
during the pen verification procedure have PCRs of not less than 40% of 
such average, then the plots actually tested are deemed satisfactory. If a 
user is using only black pens and does not need high quality plotting, it 
is possible to forego the actual pen qualification procedure and just 
accept that any pen having a print contrast ratio of more than the default 
of 8% on the scale of percentage signal drop from all white to total 
darkness will be deemed a "good" pen. 
With respect to FIG. 17, the details of box 210 are shown in FIG. 18. With 
respect to FIG. 16, the details of box 212 are shown in FIG. 17. All of 
the flow charts are deemed to be self explanatory and show the presently 
preferred embodiment for pen qualification and pen verification as taught 
by the present invention. 
It should be understood that the foregoing description is only illustrative 
of the invention. Various alternatives and modifications will be 
understood and developed by those skilled in the art without departing 
from the spirit of the invention. Accordingly, the present invention is 
intended to include all such alternatives, modifications and variations 
which fall within the scope of the following claims.