Plotter pen up/down control system

Apparatus for accurately raising and lowering the pen and for generating a positional signal for control use in a graphics plotter having a penholding mechanism slidably and rotationally mounted on a beam for longitudinal positional movement along the beam and rotational movement around the beam to affect raising and lowering of a pen held by the penholding mechanism under the control of plotter control logic. There is an actuator rod disposed parallel to the beam for rotational movement about a longitudinal axis. A spring biases the penholding mechanism to the raised position. A ball bearing linkage is provided for smoothly and slidably linking the actuator rod to the penholding mechanism so that as the penholding mechanism is smoothly moved longitudinally along the beam, rotation of the actuator rod will cause corresponding rotation of the penholding mechanism towards the lowered position. Finally, there is powered rotation apparatus connected to the plotter control logic to receive control signals therefrom for accurately rotating the actuator rod in small increments in response to the control signals whereby the penholding mechanism can be rotated to raise and lower a pen held therein accurately in small increments. To provide the feedback signal, there is a transducer operably connected to sense rotation of the actuator rod and develop a signal at an output thereof related to the rotational position of the actuator rod.

BACKGROUND OF THE INVENTION 
The present invention relates to graphics plotters and, more particularly, 
in a graphics plotter having a penholding mechanism upwardly spring biased 
and slidably and rotationally mounted on a beam for longitudinal 
positional movement along the beam and rotational movement around the beam 
to affect raising and lowering of a pen held by the penholding mechanism 
under the control of plotter control logic, to the improvement to provide 
more accurate raising and lowering of the pen comprising, an actuator rod 
disposed parallel to the beam for rotational movement about a longitudinal 
axis; linkgage means for slidably linking the actuator rod to the 
penholding mechanism so that as the penholding mechanism is moved 
longitudinally along the beam rotation of the actuator rod will cause 
corresponding rotation of the penholding mechanism; and, powered rotation 
means connected to the plotter control logic to receive control signals 
therefrom for accurately rotating the actuator rod in small increments in 
response to the control signals whereby the penholding mechanism can be 
rotated to raise and lower a pen held therein accurately in small 
increments; and to the improvement for providing a feedback signal related 
to the rotational position of the pen comprising, transducer means 
operably connected to sense rotation of the actuator rod and develop a 
signal at an output thereof related to the rotational position of the 
actuator rod. 
The essential elements of a graphics plotter are depicted in simplified 
form in FIG. 1. A penholding apparatus 10 holding a pen 12 is moved along 
a beam 14 from side to side as indicated by the arrows 16 over paper 18 to 
create one axis of the drawing while the paper 18 is moved longitudinally 
under the beam 14 to create the other axis. The pen 12 is raised and 
lowered as indicated by the arrows 20 to affect writing and non-writing 
movement of the pen 12. Prior art approaches to pen raising and lowering 
(also referred to as "dropping") fall into two general types as shown in 
FIGS. 2-3 and FIGS. 4-5, respectively. In the prior art technique of FIGS. 
2-3, the penholding apparatus 10 is free to slide longitudinally on the 
beam 14 only. The pen 12 is held by a vertically slidable member 22 and 
biased to the raised position of FIG. 2 by a spring, or the like (not 
shown). A solenoid actuator 24 is connected by an armature linkage 26 to 
the slidable member 22. By applying an electrical current to the solenoid 
actuator 24, the pen 12 is lowered by sliding the member 22 vertically 
downward as shown in FIG. 3. When the electrical current is removed, the 
member 22 springs vertically upward under the bias force, taking the pen 
12 with it. Thus, the mechanism of FIGS. 2-3 affects what is commonly 
referred to as "bang bang" control; that is, the pen 12 is banged between 
the two extremes of raised and lowered by the bias force and solenoid 
actuator 24, respectively. 
The other common prior art approach to pen raising and dropping is shown in 
FIGS. 4 and 5 wherein the penholding apparatus 10 is both slidably and 
rotatably mounted on a cylindrical beam 14'. An actuator rod 28 is 
rotatably mounted on a bar 29 parallel to the beam 14'. The actuator rod 
28 has an actuating ridge 30 extending therefrom and running 
longitudinally along which the penholding apparatus 10 slides as it moves 
along the beam 14'. As with the prior art of FIGS. 2 and 3, the penholding 
apparatus 10 is rotatably biased to the raised position of FIG. 4 and 
bearing against the actuator ridge 30 by means of a spring 31. A solenoid 
actuator 24 is connected by an armature linkage 26 to rotate the actuator 
rod 28 when energized. Thus, as shown in FIG. 5, to drop the pen 12, 
current is applied to the solenoid actuator 24 causing the armature 
linkage 26 to rotate the actuator rod 28 as indicated by the arrow 32. 
This, in turn, causes the actuating ridge 30 to rotate the penholding 
apparatus 10 about the beam 14' against the bias force of the spring 31 as 
indicated by the arrow 34, thus rotating the pen 12 to its dropped 
position. It should be noted that the rotational aspect of the pen 
dropping movement are exaggerated for purposes of the drawing. 
There are instances wherein it would be desirable to have greater control 
over the pen 12 with respect to its raised and lowered position. Solenoid 
operation as in the above-described prior art approaches to pen control 
provides neither movement control nor positional feedback information. 
Wherefore, it is the object of the present invention to provide apparatus 
for continuously controlling the up and down movement of a pen in a 
graphics plotter. 
It is a further object of the present invention to provide positional 
feedback information relative to the up and down position of the pen in a 
graphics plotter enabling more optimum pen control to be exercised by the 
plotter control logic. 
Additional objects and benefits of the present invention will become 
obvious from the description contained hereinafter taken in combination 
with the drawing figures which accompany it. 
SUMMARY 
The foregoing objects have been achieved in a graphics plotter having a 
penholding mechanism slidably and rotationally mounted on a beam for 
longitudinal positional movement along the beam and rotational movement 
around the beam to affect raising and lowering of a pen held by the 
penholding mechanism under the control of plotter control logic, by the 
improvement of the present invention to provide more accurate raising and 
lowering of the pen comprising, an actuator rod disposed parallel to the 
beam for rotational movement about a longitudinal axis; spring bias means 
biasing the penholding mechanism towards the pen raised position; linkage 
means for slidably linking the actuator rod to the penholding mechanism so 
that as the penholding mechanism is moved longitudinally along the beam 
rotation of the actuator rod will cause corresponding rotation of the 
penholding mechanism against the bias spring towards the pen lowered 
position; and, powered rotation means connected to the plotter control 
logic to receive control signals therefrom for accurately rotating the 
actuator rod in small increments in response to the control signals 
whereby the penholding mechanism can be rotated to raise and lower a pen 
held therein accurately in small increments. 
In the preferred embodiment, there is also transducer means operably 
connected to sense rotation of the actuator rod and develop a signal at an 
output thereof related to the rotational position of the actuator rod. 
The preferred powered rotation means comprises, a motor operably connected 
to be rotated in a direction and an amount as determined by the plotter 
control logic; a spur gear driven by the stepping motor; a sector gear 
driven by the spur gear; and, a lever arm connecting the sector gear to 
the actuator rod, the lever arm being positioned and of a length to place 
the sector gear along a circle concentric with the actuator rod. 
The preferred transducer means comprises, a light emitting diode and a 
light sensing diode placed in opposed spaced relationship to one another 
to develop a signal at an output of the light sensing diode which is a 
function of the amount of light from the light emitting diode striking it; 
and, optical shutter means disposed between the light emitting diode and 
the light sensing diode and connected to be moved by rotation of the 
actuator rod for changably blocking portions of the light between the 
light emitting diode and the light sensing diode in relationship to 
rotation of the actuator rod.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is a modification to the prior art approach of FIGS. 
4 and 5. Accordingly, where common elements are employed, like numerical 
designations are employed. 
As with the prior art approach of FIGS. 4 and 5, the present invention 
employs a penholding apparatus 10 slidably and rotatably mounted on a 
cylindrical beam 14'. Likewise, as with the embodiment of FIGS. 4 and 5, 
there is an actuator rod 28' mounted for rotation parallel to the beam 14' 
and having a longitudinal actuating ridge 30' extending radially outward 
therefrom. The ridge 30' has a planar surface 33 along one side along 
which a roller bearing 35 can smoothly roll. The roller bearing 35 is 
rotatably mounted on the end of a shaft 37 extending radially outward from 
the penholding apparatus 10. Again, there is the biasing spring 31 which 
biases the penholding apparatus 10 to the raised position. Thus, as the 
actuator rod 28' is rotated in either direction as indicated by the arrows 
38, the penholding apparatus 10 is smoothly rotated correspondingly in the 
opposite direction as indicated by the arrows 40. To affect controlled 
rotation of the actuator rod 28', a driving arm 42 is attached to one end 
of the rod 28' and extends radially outward therefrom to provide a lever 
and extended circumference whereby large movements with small forces can 
be employed to affect small, accurate rotations of the actuator rod 28' 
with high applied force. The outer end of the driving arm terminates in a 
gear segment 44 having equally spaced teeth 46 on the outer periphery 
thereof. A small matching spur gear 48 is operably meshed with the teeth 
46 of gear segment 44 and mounted on the shaft 50 of a motor 52 of a type 
such as those well known in the art (such as a stepping motor, or the 
like) which can be accurately moved in either direction by appropriate 
power application over line 54 from plotter control logic 56 to thereby 
move the gear segment 44 and actuator rod 28' in very small increments 
quite accurately. 
To close the loop and provide positional feedback information, the actuator 
rod 28' has a tab 58 extending from the end adjacent the periphery thereof 
as best seen in FIG. 7. The tab 58 has a longitudinally angled upper 
surface 60 which is employed as an optical shutter mechanism with respect 
to an optical sensor 62 of a type also well known in the art and 
commercially available. As depicted in FIGS. 6 and 7, the optical sensor 
62 comprises two parallel spaced arms 64, 66 containing a light emitting 
diode 68 and a light sensing diode 70, respectively. Sensor 62 develops a 
progressive signal as graphed in FIG. 11 on the output wires 72 as a 
function of the amount of light from the LED 68 striking the light sensing 
diode 70. Thus, by positioning the tab 58 so that the angled upper surface 
60 moves between the LED 68 and the light sensing diode 70 as the actuator 
rod 28' rotates, the tab 58 acts as an optical shutter between the LED 68 
and the light sensing diode 70 in the manner shown in FIGS. 8-10. As a 
consequence, as the actuator rod 28' rotates, the tab 58 is rotated in 
combination therewith and the angled upper surface 60 progressively blocks 
off the path of light, creating an output signal on wires 72 as in FIG. 
11. The angle provides for optimum shuttering action in the presence of 
small rotational movement. Since adjacent the edges the output signal is 
erratic as shown in FIG. 11, it is preferred that movement of the angled 
upper surface 60 be limited to the central portions of the LED 68 and 
light sensing diode 70 such that the straight line portion of the output 
signal, as indicated in the figure, is employed. 
Returning once again to FIG. 6, the output from the optical sensor 62 on 
wires 72 is connected as an input to an analog-to-digital converter 74. 
The thus digitized positional signal is input to the plotter control logic 
56 to close the loop and enable the logic 56 to, in turn, control the 
motor 52 to raise and lower the penholding apparatus 10 in a desired 
manner according to the positional signal being provided by the optical 
sensors 62.