Control system for an image detector

A control system is described for converting the output current of a photosensitive image detector to an image location voltage of predetermined first and second levels in response to the image detector receiving high and low levels of light, respectively. The image location voltage is developed by passing the output current from the image detector, as well as a selectively variable control current, through an impedance such that increases and decreases in the amplitude of the control current result in corresponding decreases and increases, respectively, in the level of the image location voltage. To change the level of the image location voltage, the amplitude of the control current is caused to be proportional to the charge on a charge storing element and the latter is rapidly and alternately charged and discharged within predetermined limits to vary the amplitude of the control current, and thus the level of the image location voltage. By so varying the charge on the charge storing element, the image location voltage is caused to have an average value which is substantially equal to the first predetermined level when the image detector receives a relatively high level of light, and an average value which is substantially equal to the second predetermined level when the image detector receives a relatively low level of light.

BACKGROUND OF THE INVENTION 
This invention relates generally to object detectors and particularly to 
detectors for sensing the presence and absence of documents, images or 
microfilm, and the like. 
In the past, document detectors have been provided for sensing the presence 
of a document as it passes between a light source and a photosensitive 
device. The interruption of the light by the document results in a change 
in the light received by the photosensitive device and a corresponding 
change in the flow of current therein. Typically, the change in the flow 
of current through the photosensitive device has been converted to a 
voltage change for developing a pulse indicative of the presence of a 
document. The number of pulses can then be counted to determine the number 
of documents which pass the detector. Similar systems operate by sensing 
the light reflected by a document as it passes along a document path 
adjacent the photosensitive device. 
A problem which has been addressed and solved by a prior document detector 
is that of maintaining the voltage which is derived from the current 
output of the photosensitive device at a known and constant level under 
the condition existing when no document is passing the detector, 
regardless of long term drifts in the output of the light source, changes 
in the characteristics of the photosensitive device, or the like. Thus, 
despite any long term change in the current flowing in the photosensitive 
device, the voltage derived from that current, referred to herein as the 
image location voltage, is held at a known reference level, typically by a 
feedback loop. Accordingly, only the abrupt and substantial change in the 
flow of current in the photosensitive device which occurs when a document 
passes the detector results in a change in the image location voltage and 
only then is an indication generated of the passage of a document past the 
detector. 
Although prior detectors do reliably accommodate long term drifts in the 
system to establish the image location voltage at a constant reference 
level as an indication that no document is being detected, they rely 
merely on a large change in the level of the image location voltage as an 
indication of the presence of a document. Such prior detectors are, 
however, incapable of maintaining the image location voltage at a second 
predetermined reference level in the presence of a document. Consequently, 
the latter level of the image location voltage is subject to change. For 
example, should a document remain in the detector and interrupt the light 
source for an indefinite period of time, the level of the image location 
voltage may change with drifts in the characteristics of the light source 
or the photosensitive device. The resulting indefiniteness in the level of 
the reference voltage under this latter condition is particularly 
disadvantageous in systems such as microfilm readers in which a microfilm 
bearing images is advanced past the light source for generating a pulse 
each time an image interrupts the light source. Frequently, the film may 
be stopped for an indefinite time at a point where the light source is 
interrupted by an image. Under this condition, it is desirable that the 
detector be able to hold the image location voltage at a predetermined 
level as long as the image interrupts the light source, regardless of any 
drifts in the system or gradual changes in ambient light, in order to 
generate a positive indication that an image is being detected. Of course, 
it is also required that the image location voltage be held at another 
predetermined reference level when no image is interrupting the light 
source. Detectors which maintain a constant reference voltage level only 
under one of the two conditions which are frequently encountered in 
microfilm readers, i.e. the indefinite absence or presence of an image, 
are, accordingly, subject to developing an indeterminate output when a 
document or image remains detected for a prolonged duration. 
Accordingly, it is an object of this invention to provide an improved 
control system for an image detector. 
It is a more specific object of this invention to provide a control system 
which maintains the image location voltage at a first predetermined 
reference level under the condition of prolonged presence of an image and 
at a second predetermined reference level under the condition of prolonged 
absence of an image, despite long term changes in ambient conditions or 
detector component values.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Broadly stated, the control system described herein converts the current 
generated by a photosensitive device to a voltage, referred to herein as 
the image location voltage, and operates on that voltage so as to maintain 
it at a first or high predetermined voltage level when the photosensitive 
device receives a relatively high level of light and at a second or low 
predetermined voltage level when the photosensitive device receives a 
relatively low level of light. To control the level of the image location 
voltage, the output of a variable current source is coupled to an 
impedance, the latter impedance also receiving the current output of the 
photosensitive device. Accordingly, the level of the image location 
voltage is a function both of the level of current supplied by the 
photosensitive device and the level of current supplied by the current 
source. The current supplied by the current source, and thus also the 
level of the image location voltage, is varied in proportion to the level 
of charge on a charge storing device, such as a capacitor. Means are 
included for detecting when the image location voltage is between the 
first predetermined level and a selected lower level intermediate the 
first and second predetermined levels. When this condition occurs, the 
charge storing element is discharged until the image location voltage 
exceeds the first predetermined level, after which the charge storing 
element is alternately charged and discharged so as to drive the image 
location voltage alternately above and below the first predetermined 
voltage level to maintain the average image location voltage substantially 
equal to the first predetermined voltage level. 
The same detecting means also detects when the image location voltage drops 
below the selected lower level, as when the light incident on the 
photoelectric device is substantially reduced. When this condition occurs, 
the charge storing element is charged until the image location voltage 
drops below the second predetermined level after which the charge storing 
element is alternately discharged and charged so as to drive the image 
location voltage alternately above and below the second predetermined 
voltage level to maintain the average level of the image location voltage 
substantially equal to the second predetermined voltage level. 
The significance of the operation of the control system described herein is 
best explained by a brief discussion of the operation of a prior image 
detector, as shown very schematically in FIG. 1. It is to be understood 
that the phase "image detector" is used herein in a generic sense to 
include document detectors, detectors for sensing images on microfilm, and 
the like. 
Referring now to FIG. 1, there is shown a prior image detector 10 in which 
a light source 12 directs light toward the base of a photosensitive 
transistor 14. A document 16 is shown interrupting the light path between 
the light source 12 and the transistor 14. Under this condition, the 
current developed by the transistor 14 is minimum. When the document 16 is 
conveyed to a location where it no longer interrupts the light from the 
light source 10, the light impinges on the transistor 14, as a result of 
which transistor 14 generates a high level of current. 
In order to convert the current generated by transistor 14 to a voltage, a 
resistor 17 is coupled from the emitter 18 of transistor 14 to ground. The 
voltage at the emitter 18 is the voltage referred to herein as the "image 
location voltage" because its level is indicative of the absence or 
presence of a document between the light source 12 and the transistor 14. 
To fix the image location voltage to a known predetermined level when no 
document is present, the detector 10 includes a resistor 20 coupled 
between the emitter 18 and a positive voltage source, a transistor 22 
having a collector 24 coupled to the junction of the emitter 18 and the 
resistor 20, and a control system 26 connected as shown for selectively 
varying the current generated by the transistor 22 for fixing the image 
location voltage at the desired level. 
A more complete illustration and description of the detector 10 is given in 
U.S. application Ser. No. 644,798, filed Dec. 29, 1975 and now Pat. No. 
4,027,154, and assigned to the assignee of the present application. 
Suffice it to say that the control 26 operates to maintain the image 
location voltage at a predetermined level under the condition when the 
photosensitive transistor is receiving a maximum amount of light. For 
example, the voltage waveform shown in FIG. 2 represents the image 
location voltage prior to, during, and after a document has traversed the 
light path. The portion 28 of the waveform indicates the level of the 
image location voltage prior to a document traversing the light path, the 
portion 30 indicates the voltage level when a document is in the light 
path, and portion 32 indicates the voltage level when the document has 
left the light path. 
By virtue of the operation of the control 26, the level of the image 
location voltage, indicated by portions 28 and 32 of the waveform of FIG. 
2, is caused to be fixed to the level V.sub.1. The level V.sub.2 is, 
however, not fixed and is subject to change due to changing component 
values, ambient conditions etc. Should a document remain in the light path 
for an indefinite duration, the level V.sub.2 may vary. Consequently, the 
image location voltage may give an indefinite indication of the condition 
of the light path. 
An image detector and a control system therefor which gives a positive 
indication under both possible conditions, i.e., a document (or image) 
present in the light path and a document (or image) absent from the light 
path is shown schematically in FIG. 3. 
As shown, the image detector includes a conventional photosensitive device 
in the form of a transistor 34 which develops a current proportional to 
the level of light incident thereon. The current developed by the 
transistor 34 is converted to an image location voltage V.sub.1 by 
coupling a resistor 36 between the emitter of the transistor 34 and 
ground. 
Controlled variation in the level of the image location voltage V.sub.1 is 
effected by coupling a variable current source, shown as a transistor 38, 
to the junction between the resistor 36 and the emitter of the transistor 
34. At the junction, a resistor 40 is coupled to a 12 volt voltage source. 
Consequently, the current through the impedance comprising the resistors 
36 and 40 and thus the voltage V.sub.1, is a function of the current 
generated by the transistors 34 and 38. 
The circuitry thus far described with reference to FIG. 3 is conventional. 
Most of the remainder of the circuitry of FIG. 3, however, comprises one 
form of a novel control system for controlling the current generated by 
the transistor 38 such that the image location voltage V.sub.1 is fixed 
between a predetermined first or high voltage level and a predetermined 
second or low voltage level. Such control over the conduction of the 
transistor 38 is generally effected by first sensing the level of the 
voltage V.sub.1 to determine whether the transistor 34 is receiving a high 
or a low level of light. Assuming that the level of the voltage V.sub.1 is 
indicative of the transistor 34 receiving a high level of light, the 
conduction of the transistor 38 is rapidly and alternately increased and 
decreased to drive the level of the voltage V.sub.1 alternately above and 
below the first predetermined voltage level to maintain the average level 
of the voltage V.sub.1 substantially equal to the first predetermined 
voltage level. 
In the case where the voltage V.sub.1 is indicative of the transistor 34 
receiving a low level of light, the conduction of the transistor 38 is 
rapidly and alternately decreased and increased to drive the level of the 
image location voltage V.sub.1 alternately below and above the level of 
the second predetermined voltage level to maintain the average level of 
the image location voltage V.sub.1 substantially equal to the second 
predetermined voltage level. 
The level of current generated by the transistor 38 is proportional to the 
level of charge on a capacitor 42 having a terminal 44 coupled to the base 
46 of the transistor 38 through a resistor 48. The other terminal 49 of 
the capacitor is coupled to ground. The junction of the base 46 and the 
resistor 48 is also coupled to ground through a resistor 50 to provide a 
discharge path to ground for the capacitor 42. As is more fully described 
below, the level of charge on the capacitor 42 is alternately raised and 
lowered to change the level of conduction of the transistor 38 and, 
consequently, the level of the image location voltage V.sub.1. 
In the illustrated embodiment, an amplifier 52 receives the image location 
voltage V.sub.1 at a positive input 54 and delivers at its output 56 the 
same voltage V.sub.1 but at a lower impedance level. Hence, the amplifier 
52 operates as a conventional voltage follower. 
A bi-stable element 58 receives the voltage V.sub.1 at a negative input 
terminal 60 for comparing the voltage V.sub.1 with a selected voltage 
received at its positive input 62. As shown, the input 62 is coupled to a 
6 volt source via a resistor 64, a 6 volt source being chosen because, for 
this partiular embodiment, 6 volts is intermediate the first and second 
desired predetermined voltage levels for the image location voltage, i.e., 
8 volts and 4 volts, respectively. in other environments, a selected 
voltage other than 6 volts may be used but the selected voltage will, in 
any case, be intermediate the first and second predetermined voltage 
levels. 
The positive input 62 of the bi-stable element 58 is coupled through a 
resistor 66 to the output terminal 68 of the bi-stable element 58, thereby 
permitting the bi-stable element 58 to act as a Schmitt trigger. 
Accordingly, whenever the voltage V.sub.1 is positive with respect to 6 
volts, (indicative of the transistor 34 receiving a relatively high level 
of light), the bi-stable element 58 will develop at its output terminal 68 
a low level signal indicative of the condition of the voltage V.sub.1. 
Conversely, when the voltage V.sub.1 is lower than 6 volts, the bi-stable 
element 58 will develop at its output terminal 68 a relatively high level 
signal indicative of the condition of the voltage V.sub.1. In addition to 
bi-stable element 58 generating signals at its output terminal 68 
representative of the condition of the voltage V.sub.1 and, therefore, 
also indicative of the relative amount of light received by the transistor 
34, the voltage at the output 68 of the bi-stable element 48 also serves 
to vary the conduction of the transistor 38 in a manner to be hereinafter 
described. Toward the end, the output 68 of the bi-stable element 58 is 
coupled to the capacitor 42 via a resistor 69. 
The voltage V.sub.1 appearing at the output terminal 56 of the amplifier 52 
is coupled to a positive input terminal 70 of a second bi-stable element 
72 via a resistor 74. A negative input terminal 76 of the bi-stable 
element 72 receives a first control voltage, indicated as 8 volts, which 
is equal to the desired first predetermined level of the image location 
voltage. The output 78 of the bi-stable element 72 is coupled to its input 
70 via a resistor 80 for enabling the bi-stable element 72 to operate as a 
Schmitt trigger. Consequently, the bi-stable element 72 delivers at its 
output 78 a relatively high level signal when the voltage V.sub.1 at its 
input 70 exceeds 8 volts and a relatively low level signal when the 
voltage at its input 70 is below 8 volts. 
The output of the bi-stable element 72 is coupled to a switch 82 which is 
responsive to a relatively high level signal at the output terminal 78 of 
the bi-stable element 72 for coupling a charging potential, indicated 
herein as 12 volts, through a resistor 84 to the terminal 44 of the 
capacitor 42. Thus, when the switch 82 is closed by the operation of the 
bi-stable element 72, the capacitor 42 is charged, the conduction of the 
transistor 38 increases, and the level of the image location voltage 
V.sub.1 decreases. 
To ensure that the capacitor 42 is charged by the operation of the switch 
82, the resistor 84 should be much lower in ohmic value than the resistor 
48 so that the capacitor 42 can be charged at a rate faster than its rate 
of discharge through the resistor 48. Further the ohmic value of the 
resistor 69 should be much greater than the ohmic value of resistors 48 or 
84 so that the charging of capacitor 42 by the switch 82 is isolated from 
the discharging effect of the bi-stable element 58. Under these 
conditions, the bi-stable element 58 will attempt to continuously 
discharge the capacitor 42 when the voltage V.sub.1 exceeds 6 volts. 
However, when the voltage V.sub.1 exceeds 8 volts, the bi-stable element 
72 and the switch 82 charge the capacitor when the voltage V.sub.1 exceeds 
8 volts, thus overriding the discharging of the capacitor 42 by the 
bi-stable element 58. Accordingly, the combination of the bi-stable 
element 72 and the switch 82 co-operate with the bi-stable element 58 to 
alternately charge and discharge the capacitor 42 when the voltage V.sub.1 
is at a level which is indicative of a high level of light being received 
by the transistor 34. 
When the voltage V.sub.1 drops below 6 volts as a result of the transistor 
34 receiving less light, the bi-stable element 58 generates a relatively 
high level signal at its output 68 for charging the capacitor 42 through 
the resistor 69. To effect discharging of the capacitor 42 when the 
voltage V.sub.1 is at a low level, a third bi-stable element 86 receives 
the voltage V.sub.1 at its negative input terminal 88 and receives a 
second control voltage at its positive input 90. The second control 
voltage is indicated herein as 4 volts and is coupled to the input 90 of 
the bi-stable element 86 via a resistor 92. By virtue of a resistor 94 
coupling the input 90 of the bi-stable element 86 to its output terminal 
96, the bi-stable element 86 also acts as a Schmitt trigger. As a 
consequence of the connections shown, the bi-stable element 86 develops at 
its output terminal 96 a relatively low level signal when the voltage 
V.sub.1 exceeds 4 volts and develops a relatively high level signal at its 
output 96 when the voltage V.sub.1 is below 4 volts. 
A second switch 98 receives the output of the bi-stable element 86 and, in 
response to a high level signal being generated at the output terminal 96, 
closes and couples the terminal 44 of the capacitor 42 to ground through a 
resistor 100 whose ohmic value is much less than the ohmic value of the 
resistor 48 in order to quickly discharge the capacitor 42. Thus, the 
combination of the bi-stable element 86 and the switch 98 serve to 
discharge the capacitor 42 when the level of the voltage V.sub.1 is below 
4 volts. As a result, when the voltage V.sub.1 is indicative of the 
transistor 34 receiving a relatively low level of light, the switch 98 and 
the bi-stable element 58 alternately effect discharging and charging of 
the capacitor 42, all of which is more fully described below. 
In operation, assuming that the transistor 34 is receiving a fairly high 
level of light, the voltage V.sub.1 at the output 56 of the amplifier 52 
will be at a relatively high level, 7 volts, for example. Under this 
condition, the bi-stable element 58 will develop at its output 68 a low 
level signal to permit the capacitor 42 to discharge to ground through the 
resistor 48. A portion of the charge on the capacitor 42 will also flow to 
ground through the resistor 69 which couples the capacitor 42 to the 
output 68 of the bi-stable element 58. Consequently, the voltage on the 
base 46 of the resistor 38 is decreased, thereby decreasing its conduction 
and allowing the voltage V.sub.1 to rise. 
When the voltage V.sub.1 rises above 8 volts, the bi-stable element 72 
develops a high level charging signal at its output terminal 78 for 
actuating the switch 82. In response to the high level signal generated by 
the bi-stable element 72, the switch 82 closes and thereby couples the 
capacitor 42 to the plus 12 volts supply through the resistor 84. 
With the capacitor 42 now being charged through the resistor 84, the 
voltage at the base 46 of the transistor 38 increases, thereby increasing 
the conduction of the transistor 38 and lowering the voltage V.sub.1 to 8 
volts or somewhat lower. When the voltage V.sub.1 reaches approximately 8 
volts, the bi-stable element 72 develops a low level signal at its output 
78 for opening the switch 82, thereby ceasing the charging of the 
capacitor 42. Because the voltage V.sub.1 still exceeds the voltage 
present at the input 60 of the bi-stable element 58, its output still 
remains low to permit the capacitor 76 to discharge. Consequently, the 
capacitor 42 now discharges so as to lower the level of conduction of the 
transistor 38 and increase the level of the voltage V.sub.1. 
When the voltage again exceeds the 8 volt control voltge appearing at the 
input 76 of the bi-stable element 72, the bi-stable element 72 will again 
generate a relatively high level signal at its output 78 for again closing 
the switch 82. As a result, the capacitor 42 will again be charged through 
the resistor 84 for increasing the conduction of the transistor 38 and 
lowering the level of the voltage V.sub.1 below 8 volts. The cycle of 
discharging and charging the capacitor 42 continues in the manner 
described above so as to maintain the average level of the voltage V.sub.1 
substantially equal to the first predetermined voltage level, 8 volts in 
this example. 
The effect of the alternate charging and discharging of the capacitor 42 is 
shown in FIGS. 4A and 4B wherein FIG. 4A illustrates the level of light 
received by the transistor 34 and FIG. 4B illustrates the voltage V.sub.1. 
Referring to FIG. 4A, the portion 104 of the illustrated waveform 
represents the condition wherein the transistor 34 is receiving a high 
level of light. The corresponding portion 106 of the waveform shown in 
FIG. 4B indicates how the level of the voltage V.sub.1 is alternately 
decreased and increased by the operation of the bi-stable element 58 and 
the combined operation of the bi-stable element 72 and its switch 82. As 
shown, the average level of the voltage V.sub.1 is maintained at 8 volts 
while the instantaneous value of the voltage V.sub.1 alternately decreases 
and increases about the 8 volt level. Thus, the control system maintains 
the average level of the voltage V.sub.1 at 8 volts despite changes in the 
current of the transistor 34 due to drifts of component values or changes 
in ambient conditions. 
Assume now that an image interrupts the impingement of light on the 
transistor 34 such that the voltage V.sub.1 falls to a level below 6 
volts. Under this condition, the bi-stable element 58 will generate a 
relatively high level signal at its output terminal 68 for charging the 
capacitor 42 through the resistor 69. Accordingly, the conduction of the 
transistor 38 will increase and the voltage V.sub.1 will decrease. When 
the voltage V.sub.1 decreases below the level of the second control 
voltage at which the input 90 of the bi-stable element 86 is held (4 volts 
in this embodiment), the bi-stable element 86 will generate at its output 
96 a relatively high level discharging signal which actuates the switch 98 
for discharging the capacitor 42 through the switch 98, the resistor 100, 
and thence to ground. Consequently, the level of conduction of the 
transistor 38 decreases, and the level of the voltage V.sub.1 increases 
toward 4 volts. When the voltage V.sub.1 reaches 4 volts or slightly more, 
the bi-stable element 86 turns off, the switch 98 opens, and the capacitor 
42 resumes charging through the resistor 69 by virtue of the high output 
which is still being generated by the bi-stable element 58. 
When the capacitor 42 charges sufficiently to increase the conduction of 
the transistor 38 to the point where the voltage V.sub.1 again decreases 
slightly below 4 volts, the bi-stable element 86 will again turn on and 
generate at its output 96 a relatively high level signal for actuating the 
switch 98 and again discharging the capacitor 42 in order to decrease the 
conduction of the transistor 38 and raise the level of the voltage V.sub.1 
above 4 volts. This cycle continuously repeats while the transistor 34 is 
receiving a relatively low level of light. 
Referring again to FIG. 4, the above described sequence of events is 
illustrated by portion 108 of the waveform shown in FIGS. 4A and portion 
110 of the waveform V.sub.1 shown in FIG. 4B. As indicated, the voltage 
V.sub.1 alternately rises above and decreases below the 4 volt control 
level so that the average value of the voltage V.sub.1 is maintained 
substantially at 4 volts. 
The signal output of the bi-stable element 58 is indicated in FIG. 4C. 
Because the ripples on the voltage V.sub.1 do not cross the 6 volt level 
to which the positive input 62 of the bi-stable element 58 is held, the 
output of the bi-stable element 58 changes only when the current output of 
the transistor 34 changes drastically. Thus, the output of the bi-stable 
element 58 may be coupled to a pulse sensing circuitry for counting the 
number of pulses to give an indication of the number of images which 
interrupt the light to the transistor 34. 
As shown in FIg. 3, the output of the bi-stable element 58 may be coupled 
to a driver transistor 110 having an output terminal 112. FIG. 4D 
illustrates the signal appearing on the output terminal 112. 
The control system described above operates to maintain the image location 
voltage at first and second predetermined levels despite variations in 
component values or ambient conditions. It is particularly well suited for 
use in microfilm readers in which a film bearing images is transported 
along a path between a light source and a photosensitive device. The 
photosensitive device will respond to the differences in light 
transmission between the film itself and the images thereon for developing 
a current indicative of the presence or absence of an image in the light 
path. The control system described above will convert the current from the 
photosensitive device to an image location voltage and hold that voltage 
between first and second predetermined levels. 
Alternately, the microfilm reader may be of the type wherein the light 
received by the photosensitive device is reflected by the film. In the 
latter case, the control system will be substantially the same as 
described. 
Although the invention has been described in terms of a specific 
embodiment, it will be apparent to those skilled in the art that many 
modifications and alterations may be made to the disclosed embodiment 
without departing from the spirit and scope of the invention. Accordingly, 
it is intended that all such modifications and alterations be included 
within the spirit and scope of the invention as defined by the appended 
claims.