U.S. patent number 3,679,306 [Application Number 05/108,504] was granted by the patent office on 1972-07-25 for feedback system for controlling image light energy in electrostatic photocopiers.
This patent grant is currently assigned to Pitney-Bowes, Inc.. Invention is credited to Robert Clark Du Bois, Wayne H. Miller.
United States Patent |
3,679,306 |
Du Bois , et al. |
July 25, 1972 |
FEEDBACK SYSTEM FOR CONTROLLING IMAGE LIGHT ENERGY IN ELECTROSTATIC
PHOTOCOPIERS
Abstract
A control system for electrostatic photocopiers monitors the
light level of a reference beam. The beam is projected at the
imaging station from the projection lamp by way of the illuminating
station and the image projection optics to derive a feedback signal
for regulating either the copy sheet feed rate through the imaging
station or the lamp current to compensate for lamp voltage
variations, lamp aging, dirty projection optics, etc.
Inventors: |
Du Bois; Robert Clark
(Fairfield, CT), Miller; Wayne H. (Stamford, CT) |
Assignee: |
Pitney-Bowes, Inc. (Stamford,
CT)
|
Family
ID: |
22322587 |
Appl.
No.: |
05/108,504 |
Filed: |
January 21, 1971 |
Current U.S.
Class: |
355/68; 399/31;
399/159; 355/38; 355/69 |
Current CPC
Class: |
G03B
27/80 (20130101); G03G 15/043 (20130101) |
Current International
Class: |
G03G
15/043 (20060101); G03B 27/80 (20060101); G03b
027/78 () |
Field of
Search: |
;355/68,69,35,38,11,83 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Matthews; Samuel S.
Assistant Examiner: Hayes; Monroe H.
Claims
We claim:
1. In an electrostatic photocopier having a projection lamp for
illuminating a document at an illuminating station and a motor
coupled to feed photoconductive copy paper through an imaging
station where the copy paper is exposed to a light image reflected
from the document and projected by projection optics, a system for
regulating the total image light energy incident on the copy paper
comprising, in combination:
A. lamp control means operative to maintain lamp current at a
predetermined constant value despite fluctuations in line
voltage,
B. sensing means for monitoring the light intensity of a reference
beam reflected from a target illuminated by the lamp and
transmitted through the projection optics,
C. feedback motor control means for varying motor speed and thus
the copy paper feed rate through the imaging station, and
D. means coupling a feedback signal from said sensing means to said
motor control means proportional to the light intensity of said
reference beam,
1. said feedback signal serving to actuate said motor control means
to control motor speed in accordance with reference beam light
intensity.
2. A system as defined in claim 1 and further including stabilizing
means for deriving a signal representative of motor speed for
transmission to said motor control means to compensate for any
tendency of the motor to slow down due to increased load.
Description
BACKGROUND OF THE INVENTION
In one type of office copying machine an image of the indicia on an
original document is projected by means of visible light onto the
surface of charged photoconductive copy paper to form a latent
electrostatic image. The latent image is then developed and fixed
to produce the finished copy. In machines of this type it is
important to insure that the image light energy to which the copy
paper is exposed is of the proper magnitude to yield good copy.
Although copier machines are of course designed to provide the
proper level of image light energy at the copy paper, in practice
several factors can and do interfere with the maintenance of this
light level.
One factor is that over an extended period of use the optical
system of the copier accumulates dirt and other contaminants which
reduce the intensity of image light below the level it should be
for good copy. This of course means that as the copier ages the
copies produced will become progressively more underdeveloped
unless compensation is somehow made for the dirty optics.
Another factor is the variation in lamp voltage which a copier
experiences even under the best of conditions. For example, if the
lamp voltage decreases by 10 percent (e.g., from 115v to 103.5v),
the light output of the lamp will decrease by about 30 percent.
Several methods have been used to overcome the problems of dirty
optics and undervoltage. In some machines a lamp is employed which
has a light output considerably greater than needed under the best
conditions; for example, twice the minimum light output is common.
A mechanical shutter is then used to limit the light transmitted to
the level needed. The disadvantages to such a system in which, in
effect, light energy is thrown away are that the current
requirements and heat load on the copier are far in excess of what
would be needed if a more efficient use of the light could be made.
This increases the cost of both manufacturing and operating the
machine. In addition the necessary high current rating may prevent
the use of such a copier in buildings where the wiring or fuse
systems provide marginal current capacity. Such a copier also
requires manual adjustment of the shutter to correct for any change
in light level; this is difficult where the problem involves
transient or periodic undervoltage. In any event a need for manual
shutter adjustment means that the problem must first be recognized,
normally by the appearance of unusable, wasted copies. These
procedures are costly and result in annoying inconvenience for the
users of copiers.
More recently systems have been devised to automatically adjust the
shutter in response to changing light conditions. These latter
systems however merely remove the element of manual control. In
such an automatic shutter system a lamp having a light output
greater than that needed under best conditions is still required.
Accordingly the problems resulting from high current requirements
and excess heating remain.
Accordingly, representative objects of the present invention are to
provide a feedback control system for a photocopier which
automatically regulates the available light energy to which the
copy paper may be exposed in response to changing conditions,
either by controlling copy feed rate through the imaging station or
lamp current or both; said system being efficient, economical and
reliable in use.
Other objects of the invention will in part be obvious and will in
part appear hereinafter.
The invention accordingly comprises the features of construction,
combinations of elements, and arrangement of parts which will be
exemplified in the constructions hereinafter set forth, and the
scope of the invention will be indicated in the claims.
SUMMARY OF THE INVENTION
The present invention relates to regulating the total light energy
available at the imaging station of an electrostatic photocopier as
a means of compensating for lamp voltage variations and/or dirty
image projection optics, and more particularly to a feedback
control system for performing such regulation.
The feedback control system in general acts to regulate the feed
rate of copy paper through the imaging station and/or lamp current;
the two factors at the imaging station having a direct effect on
the creation of an electrostatic latent image on the copy paper.
The system utilizes a sensor which monitors the light level of a
reference beam transmitted from the copier lamp through the copier
image projection optics to the imaging station. Thus, the reference
light beam experiences the same variations which effect the total
image light energy projected onto the copy paper. The sensor
response, proportional to the reference beam intensity, serves as
the source of a feedback signal which is used to effect
compensation for spurious variations in the total image light
energy available at the imaging station.
Copy feed rate regulation can be used alone or in combination with
lamp voltage regulation, which maintains the lamp current at a
constant value to eliminate fluctuations in its light output due to
variations in line voltage. When lamp voltage regulation is used,
feedback regulation of the feed rate compensates for dirty
projection optics and lamp aging.
Alternatively, feedback control of the lamp excitation can be used
alone in accordance with the invention. This lamp control not only
regulates lamp current to compensate for variations in line
voltage, but also responds to the feedback signal from the sensor
to increase lamp excitation to compensate for the light attenuating
effect of dirty optics.
The control system of the invention permits more efficient
operation of a photocopier since copy feed rate speed and/or lamp
light output may be continuously and automatically maintained at
optimum levels. This eliminates among other things the waste of
light energy experienced with prior art photocopiers. The system of
the invention also eliminates the need and bother of having a
manually adjustable mechanical shutter of the type used in many
prior art photocopiers.
BRIEF DESCRIPTION OF THE DRAWINGS
For a fuller understanding of the nature and objects of the
invention, reference should be had to the following detailed
description taken in connection with the accompanying drawings, in
which:
FIG. 1 is a schematic representation of a feedback motor control
system in accordance with the invention.
FIG. 2 is a schematic representation of the feedback motor control
system of FIG. 1 with the addition of a simple lamp control used to
regulate lamp current.
FIG. 3 is a schematic representation of a feedback lamp control
system in accordance with the invention.
FIG. 4 is a graphical representation of the operation of the system
shown in FIG. 1.
FIG. 5 is a graphical representation of the operation of the system
shown in FIG. 2.
FIG. 6 is a graphical representation of the operation of the system
shown in FIG. 3.
Similar reference characters refer to similar parts throughout the
several views of the drawings.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, a copier is schematically shown in which
copy feed rate is regulated to adjust for varying total light
energy available at the imaging station.
Specifically, light from a light source such as a lamp 10 is
directed to an illuminating or exposure station 11 accommodating an
original document 12 and a target 14 positioned adjacent the
document. The light reflected from document 12 is then directed by
means of a reflector or mirror 16 through an optical projection
system 18 to focus the document borne image to be copied onto the
charged surface of the copy paper 20 at an imaging station 22. In a
copier of this type the light from lamp 10 typically scans the
surface of the original and the reflected image is scanned over the
surface of the copy to create the corresponding latent
electrostatic image. This may be accomplished in several ways. The
original may be held stationary and the light source scanned over
the surface thereof in synchronism with an optical system which
simultaneously scans the surface of a moving sheet of copy paper.
Conversely, the copy paper may be kept stationary while the
original is fed through the copier. In some copiers both the
original and copy paper feed through the copier in synchronism.
Regardless of the type of mechanism used for scanning however, the
total light energy (as for example measured in foot candle-seconds)
to which the copy paper is exposed is a function of the image light
level and the exposure time.
For purposes of the present description, it will be assumed that
the original 12 and the copy sheet 20 are fed in synchronism
through the exposure station 11 and imaging station 22,
respectively, by feed rollers 25 driven by a motor 24. Accordingly,
the speed at which motor 24 operates determines the rate at which
the original is scanned and the rate at which the copy paper passes
through the imaging station, and in turn the total amount of image
light energy to which the copy paper is ultimately exposed.
Therefore, by controlling the speed of motor 24 in response to
changing light conditions, it can be assured that the copy paper
will be exposed to the proper amount of light energy at all times
to produce good copies.
As shown in FIG. 1, the system which controls motor speed in the
copier comprises a sensor 26 coupled with a feedback motor control
network 28. Sensor 26, such as a photoresistor, phototransistor,
photocell, etc., is positioned to monitor the light intensity of a
reference beam 15 reflected from target 14 after it has passed
through the optics of the copier. Since target 14 is closely
adjacent to the original document 12, and sensor 26 is in the same
relative position in the optical path as the copy paper 20,
variables affecting the image light level on the copy equally
effect the amount of light reaching sensor 26. Target 14 is
preferably white to match the typical image background.
The operating speed of motor 24 is regulated, as shown in FIG. 1,
by motor control network 28 which in turn is regulated by a
feedback signal derived by sensor 26. Since the feedback signal
from sensor 26 is proportional in amplitude to the amount of
reflected light from target 14, when the light level sensed by
photocell 26 decreases as a result of decreased lamp voltage and/or
dirty optics, the feedback signal amplitude reflects the change and
signals network 28 to reduce the speed of motor 24 accordingly. As
motor 24 slows down, the feed rates of the original and copy paper
are likewise slowed. In this way the light energy (foot
candle-seconds) of a given image to which the copy paper is exposed
can be maintained constant; as the amount of light (foot candles)
at the copy paper decreases, the time during which the copy paper
is exposed (seconds) increases to keep the product thereof--which
is a measure of light energy--the same.
The motor control 28 used in the system of FIG. 1 should be matched
to the type of motor 24 provided in the copier, and a number of
combinations will function satisfactorily. For example, the motor
may be a DC type in which case a silicon-controlled rectifier (SCR)
motor control could be employed. Alternately a series wound AC
motor may be used with a TRIAC solid state motor control. A TRIAC
motor control employs a pair of oppositely poled silicon-controlled
rectifiers and acts to vary motor speed by reducing motor voltage;
this is done by chopping part of the AC sine wave. However, since
series would AC motors are sensitive to changes in load and line
voltage, additional controls should be used to minimize the effect
of these factors.
The preferred combination of motor and control, however, is a
squirrel cage induction motor with a TRIAC control. The TRIAC
control, as in the series wound motor combination, regulates motor
speed by regulating motor voltage. With this type system, sensor
26, a photoresistor, functions as the external control resistor for
the TRIAC circuit. Since the induction motor like the series wound
motor may also be line voltage sensitive, additional voltage
regulation may be used for stability in this combination as
well.
Regardless of the motor-control combination chosen, the sensor 26
used in the system should be carefully matched to the motor. The
matching should provide as nearly a linear relationship as possible
between the sensor output and the motor speed over the ranges of
light and speed which can be expected during the life of the
copier. When the proper sensor is selected for the motor, neutral
density filters can also be used in the copier to adjust the light
on the sensor at the level needed to produce the proper resistance
range for motor control.
Additionally, it is advantageous to further stabilize the copier
motor speed in many motor-control combinations, particularly when
using motors that are load sensitive. Again referring to FIG. 1,
this may be accomplished by using the back emf of motor 24 as a
negative feedback signal which is supplied over lead 29 to the
motor control network 28. Accordingly, if motor 24 slows down under
load, back emf decreases and the negative feedback signal to
control network 28 allows motor voltage to be increased to bring
the motor back up to speed. Alternatively, but less preferably, a
tachometer may be incorporated in the speed control circuitry for
the same purpose.
Referring now to FIG. 4, a pair of graphs illustrate the manner in
which motor speed varies in a system of the type discussed above,
in response to changing light conditions. Graph A depicts the
situation in a copier with clean optics; as lamp current and light
output decrease as a result of undervoltage, motor speed likewise
must decrease to maintain at a minimum acceptable level the total
light energy of a given image projected onto the copy paper. The
minimum acceptable level of light energy (as shown in the drawings)
will typically be determined through use of a test pattern
original. Similarly motor speed increases to compensate for
overvoltage. Graph B depicts the situation where the optics have
become dirty to the extent that there is a 30 percent decrease in
image light normally transmitted to the copy paper for a given
image. In such a situation the entire motor speed curve is shifted
to lower speeds to compensate for the dirty optics. The motor speed
however varies at the lower level in the same manner as in Graph A
to compensate for undervoltage or over-voltage to the lamp.
While the control system described heretofore is satisfactory for
many purposes, it can be seen that motor speed will vary both as
lamp current changes and as the optics become dirty. Preferably, a
more stable system can be obtained if the lamp current is also
controlled. In this way motor speed need only be varied as the
optics become dirty. Such a system is shown in FIG. 2. The motor
control circuit of FIG. 2 is identical to that previously described
in conjunction with FIG. 1. The one change in the system of FIG. 2
is that a lamp control 30 is added to regulate the voltage to lamp
10 and maintain it at a constant value. This has the further
advantage of permitting the copier to be current rated for the
current actually drawn by the lamp rather than having to allow for
a possible overvoltage of perhaps 10 percent, as is the case with
motor control alone.
The lamp control 30 (FIG. 3) may be a 115 volt regulation
transformer; however, for most commercial applications such a
transformer is too large and too expensive. More preferably, lamp
control 30 comprises a TRIAC solid state control of the type used
for motor control 28 and discussed hereinabove. Such a TRIAC
control 30 would be used to regulate lamp voltage at, for example,
a constant 90 volts AC when operating in a system fed, as in FIG.
2, by 115 volt line voltage.
Referring now to FIG. 5, the variations in copier motor speed
resulting from changing light conditions are graphically
illustrated for a system in which lamp current is regulated and
where motor speed is controlled by sensor 26. Graph C illustrates
the situation in a copier with clean optics. Since lamp current and
thus light output is held constant and is independent of line
voltage, motor speed is also held constant regardless of changes in
line voltage. Motor speed will accordingly only vary as the copier
optics become dirty; this is illustrated by Graph D which shows the
situation resulting from a 30 percent decrease in image light
normally transmitted to the copy paper. Under such conditions the
motor speed is slowed to a new level by the motor control to keep
the light energy of a given image to which the copy paper is
exposed at an acceptable value. However, motor speed is still held
constant at the lower level.
Another system for compensating for spurious variations in the
amount of light energy reaching the copy paper is shown in FIG. 3.
In this system there is no motor control so motor speed and thus
the operating speed of the copier remains constant. The voltage to
lamp 10 however is regulated in response to variations in the
intensity of the reference beam light sensed by photosensor 26.
This is accomplished by providing a TRIAC solid state lamp control
32 in the lamp circuit as shown in FIG. 3. The feedback signal from
photocell 26 is transmitted to lamp control 32 (rather than to a
motor control as in the previously discussed systems). Lamp control
32 serves to maintain lamp 10 at a constant voltage of, for
example, 90 volts AC despite variations in the normal 115 volt AC
line voltage. However, as the copier optics become dirty with time
and the amount of light for a given image normally transmitted to
copy paper 20 decreases, lamp control 32 in response to the
feedback signal from photocell 26 proportionally increases the lamp
voltage and thus the lamp current. In this way proportionally more
light is generated by lamp 10 to compensate for the dirty
optics.
The functioning of the feedback lamp control system is graphically
illustrated in FIG. 6. Graph E shows the situation when the copier
optics are clean. Since motor speed remains constant, the lamp
current is kept constant and matched to motor speed by the lamp
control to provide an acceptable level of light energy at the copy
paper for a given image. Graph F illustrates the change occurring
when the optics become sufficiently dirty to reduce the light level
reaching the photocell sensor by 30 percent. Since motor speed
still remains constant, lamp current is increased and maintained as
shown at a new value to produce the compensating amount of light
needed.
All of the systems described hereinabove permit the elimination of
the mechanical type shutter used in prior art copiers. The function
of the shutter, however, can be performed in the systems of the
invention by means of a manually adjustable potentiometer placed in
series with the feedback photoresistor in the control circuit for
either the lamp or the motor.
It will be appreciated that the principles of the present invention
may be applied to a variety of processes and apparatus where it is
important to control the total energy to which objects are exposed
in terms of the factor of intensity and exposure time. Thus, other
applications for the present invention are contemplated, such as,
for example, Diazo blueprint machines, automated film duplicating
machines, and automated apparatus for making photographic prints
from film. Moreover, the present invention is not limited to any
particular form of energy to be monitored and controlled, and thus
applications to forms of radiant energy other than light energy are
contemplated.
It will thus be seen that the objects set forth above, among those
made apparent from the preceding description, are efficiently
attained and, since certain changes may be made in the above
constructions without departing from the scope of the invention, it
is intended that all matter contained in the above description or
shown in the accompanying drawings shall be interpreted as
illustrative and not in a limiting sense.
It is also to be understood that the following claims are intended
to cover all of the generic and specific features of the invention
herein described, and all statements of the scope of the invention,
which, as a matter of language, might be said to fall
therebetween.
Having described our invention, what we claim as new and desire to
secure by Letters Patent is:
* * * * *