Ink sensing system for vector plotters

An ink sensing system for a vector plotter in which a pen is adapted to be raised and lowered from a drawing media surface along a system axis. An optical detection system is provided which has a focal point on the system axis and below the media surface when the pen is in its lowered position and at the media surface when the pen is in its raised position. The optical detection system includes a source of illumination and a detector system to determine the degree of reflectance from the media surface to indicate the presence or absence of ink on the media surface. In a particular embodiment the optical detection system is moveable and the movement of the optical detection system is coupled to the movement of the pen. Furthermore, the focal point of the optical detection system coincides substantially with the last point of contact of the pen with the media surface when the pen is in the raised position.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to the field of vector plotters and, in particular, 
to an ink sensing system for a vector plotter in which an optical 
detection system determines the degree of reflectance from a drawing media 
surface to indicate the presence or absence of ink on the media surface. 
2. Description of Related Art 
In the field of plotters using ink pens, it is necessary to determine, 
continuously or at intervals, that the ink is being properly laid down on 
the drawing media surface during the plotting process. Such failure can be 
due to lack of ink in the pen or due to obstructions in the pen itself 
which prevents the ink from flowing freely onto the media surface. In the 
former instance, infrared ink-level sensing systems have been proposed to 
monitor the ink level in the pen itself. In the latter instance, sensing 
systems have been proposed, such as those described in U.S. Pat. Nos. 
3,335,287 and 4,435,674, which utilize light-emitting diodes and 
photodiode detectors carried on the drawing head to continuously monitor 
the line being drawn by detecting variations in the intensity of light 
reflected from the drawing media surface due to the presence or absence of 
ink to absorb the light focussed on the surface. These sensing systems 
have, however, several disadvantages. Since the sensors comprise a 
circular array of optical fibers located around the scriber, the optical 
fibers focus light on and scan an annular area of the plotting paper 
surrounding the tip of the pen. Since only a small portion of the focused 
light can be absorbed by ink on the surface of the media, the sensitivity 
to small variations in detected light is critical to the successful 
operation of the system. Furthermore, since the detector system looks at 
an annular area around the tip of the pen, the system is sensitive to what 
has been drawn but cannot detect the current status of the ink flowing 
onto the media surface. Finally, since the systems look at an annular area 
surrounding the pen and operate in a continuous back-looking mode, 
excessive power and expensive and complicated electronics are needed for 
successful operation of such systems. 
Thus, it is a primary object of the present invention to provide an 
improved ink sensing system for a vector plotter. 
It is another object of the present invention to provide an improved ink 
sensing system in which a positive indication is given as to the presence 
or absence of ink on a drawing media surface. 
It is a further object of the present invention to provide an improved ink 
sensing system which can determine the current status of ink flowing onto 
the drawing media surface. 
It is still another object of the present invention to provide an improved 
ink sensing system in which optical and electronic power requirements and 
complexity are minimized. 
SUMMARY OF THE INVENTION 
An ink sensing system for a vector plotter is provided in which a pen is 
adapted to be raised and lowered from a drawing media surface along a 
system axis. The ink sensing system includes an optical detection system 
which has a focal point on the system axis and below the media surface 
when the pen is in its lowered position and a focal point on the system 
axis and at the media surface when the pen is in its raised position. The 
optical detection system includes a source of illumination and a detector 
system to determine the degree of reflectance from the media surface to 
indicate the presence or absence of ink on the media surface. In a 
particular embodiment, the optical detection system is moveable and the 
movement of the optical detection system is coupled to the movement of the 
pen. Furthermore, the focal point of the optical detection system 
coincides substantially with the last point of contact of the pen with the 
media surface when the pen is in the raised position. 
The novel features which are believed to be characteristic of the 
invention, both as to its organization and method of operation, together 
with further objects and advantages thereof, will be better understood 
from the following description in connection with the accompanying 
drawings in which the presently preferred embodiment of the invention is 
illustrated by way of example. It is to be expressly understood, however, 
that the drawings are for purposes of illustration and description only 
and are not intended as a definition of the limits of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1, 2, and 3, a pen 10 is carried by a penholder 12 
over the surface 14 of a drawing media 16. The penholder 12 is adapted, as 
described hereinafter, to move in a vertical direction toward and away 
from surface 14, as indicated by arrow 17, and, alone or in conjunction 
with movement of the drawing media 16, is adapted to cause the pen 10 to 
lay down a line of ink on the surface 14 in two dimensions. A system for 
driving penholder 12 in longitudinal and vertical directions is described 
in U.S. Pat. Nos. 4,777,727, 4,872,023, and 4,935,878, whose teachings are 
incorporated herein by reference. The penholder 12 also carries a 
plurality of optical fibers 18, 20, 22 arranged in an array 24 in the 
bottom portion 26 of the penholder 12 around the point 28 of the pen 10. 
The optical fibers 18, 20, 22 join in a fiber optic bundle 32 after 
leaving the bottom portion 26 of the penholder 12 and are coupled to an 
array 34 of light emitting diodes (LEDs) 36, 38 and a photodiode (or a 
photo-transistor) 40. Electrical signals to actuate the LEDs 36, 38 and to 
carry the output of the photodiode 40 are carried by electrical bundle 42. 
As shown in FIGS. 1 and 3, the optical fibers are located within a 
90.degree. arc about the system axis 30 and are tilted off-axis at a 
45.degree. angle. While these angles are chosen mainly for convenience of 
fabrication, the off-axis tilt of the optical fibers is chosen so that the 
focal point 44 of the detection system is below the surface 14 of the 
drawing media when the point 28 of the pen 10 is in contact with the 
surface 14 of the drawing media 16 and is at the surface 14 when the pen 
10 is in a raised position. 
In the operation of a vector plotter, as the pen 10 plots on the surface 14 
of the drawing media 16, the pen 10 lifts from and drops to the surface 14 
at the beginning and end of each vector. Since the pen 10 is carried by 
the penholder 12 which also carries the optical fibers 18, 20, 22, and 
other optical components hereinafter described, all the components of the 
optical system move up and down with the pen 10 as a unit. Thus, each time 
the pen 10 lifts vertically, the focal point 44 of the optical system 
moves vertically through the drawing media 16 at substantially the last 
point of contact of the pen 10 with the surface 14. During the lifting 
movement of the pen 10, the LEDs 36, 38 are pulsed off and on at a 
rapid-duty cycle, the pulse rate being able to be lower if the depth of 
field of the optical system is greater i.e. the pulse rate is inversely 
proportional to the depth of the field of the optical system. The 
photodiode 40 detects the presence of an ink mark from the pen 10 by an 
attenuation to the normal signal produced by light from the LEDs 36, 38 
being diffusely reflected from the normally blank surface 14 of the 
drawing media 16 and detected by the photodiode 40. 
As shown more explicitly in FIGS. 4-6, the pen 10 is carried by penholder 
12 along a beam 46. The penholder 12 and the beam 46 comprise a linear 
motor system and are actuated by plotter control 48. The pen 10 is 
slidable with respect to the penholder 12 and is biased to a raised 
position by a spring, or the like (not shown). A solenoid actuator 50 
electrically coupled to the plotter control 48 causes the pen 10 to slide 
vertically downward to the surface 14 at the beginning of each vector upon 
a current being applied to the solenoid actuator 50 by the plotter control 
48. At the end of each vector, the plotter control 48 removes the current 
from the solenoid actuator 50 causing the bias spring to return the pen 10 
to its original position. Simultaneously with the removal of such current 
by the plotter control 48, the plotter control 48 sends a signal to sensor 
control 52 which activates LEDs 36, 38 and photodiode or phototransistor 
40. Sensor control 52, upon receiving the output of photodiode or 
phototransistor 40, compares such output to a preselected voltage or 
current level in a standard comparator circuit, and if such output exceeds 
such level, indicating a lack of attenuation by the total absence of ink 
on the surface 14, or an insufficient amount of such ink, sensor control 
52 sends a signal to plotter control 48 to cause further plotting to stop. 
At the beginning of each vector, the plotter control 48 sends a signal to 
the sensor control 52 to deactivate the operation of LEDs 36, 38 and 
photodiode 40. 
In a particular embodiment, the LEDs 36, 38 produce light in the green and 
the red portion of the light spectra allowing the detection of the eight 
colors commonly used in vector plotters, namely: black, blue, red, green, 
brown, orange, magenta, and purple. The addition of a third LED producing 
blue light would allow detection of any color. The optical fibers 18, 20, 
22 are 1.00 mm in diameter with an overall 2.2 mm diameter including the 
sheath. The lenses for the optical system are spherical ball lenses, 2.375 
mm in diameter, with the image distance being approximately 9.06 mm and 
the object distance being -12.09 mm, producing an image size of 0.75 mm 
and a magnification of -0.75. While high-quality optics are not required, 
a maximum blur spot of 0.75 mm is preferred. The ball lenses would be 
inserted in the channels in the penholder 12 holding the optical fibers 
18, 20, 22, as indicated by numeral 54 in FIG. 2. While the use of a 
plurality of pulsed LEDs is preferred because of better signal-to-noise 
ratio, a non-pulsed single LED or a white incandescent light source 
carried by a single optical fiber could be used to reduce system costs as 
long as interference from stray light is minimized. In addition, 
aspherical lenses can be used to more precisely focus the light from the 
LEDs. 
While the invention has been described with reference to a particular 
embodiment, it should be understood that the embodiment is merely 
illustrative as there are numerous variations and modifications which may 
be made by those skilled in the art. The optical detection system, for 
example, could be mounted separately from the penholder and the movement 
of its focal point slaved to the movement of the pen by the plotter 
control. Thus, the invention is to be construed as being limited only by 
the spirit and scope of the appended claims.