Transport item detecting arrangement

The disclosure is directed to a detecting arrangement for detecting presence or absence of an item to be transported, for example, a paper sheet or the like, which includes a light emitting element and a light receiving element disposed to confront a transport passage as a sensor for detection of the item to be transported along the passage. The detecting arrangement is characterized in that there are further provided a comparing circuit for comparing a signal responding to an output of the sensor with an integrated signal from an integrating circuit of an output of the sensor, thereby to employ a signal based on the result of the comparison as a signal indicating presence or absence of the item to be transported.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a detecting apparatus for 
detecting presence or absence of an item to be transported, and more 
particularly, to a detecting arrangement for detecting, for example, a 
copy paper wound around a photosensitive member in an electrophotographic 
copying apparatus. The detecting arrangement is adapted to optically 
effect the detection through employment of a sensor including light 
emitting and light receiving elements for detecting the item to be 
transported without contacting said item. 
Generally, as a means for detecting presence or absence of items to be 
transported, there has conventionally been employed for a wide 
application, an arrangement which utilizes microswitches and the like. In 
such an arrangement, an actuating arm for controlling on or off of 
electric contacts in normally disposed on a transport passage of the item 
to be transported so as to detect the item when the actuating arm is moved 
based on the movement of said item. If the item to be transported is of a 
heavy item, the transportation thereof is seldom obstructed by the 
actuator arm disposed in the transport passage, and thus, an accurate 
detection may be expected. 
However, if the item to be transported is of a very light weight item, for 
example, a piece of paper in the form of a sheet, there have been cases 
where the feeding thereof is obstructed by the actuating arm provided on 
the transport passage, thus resulting in paper jamming or the like. In the 
case where the paper sheet is rather thin, and is not sufficiently 
resilient, there is a stronger tendency for the jamming to take place. 
Accordingly, countermeasures have been taken, for example, by changing the 
shape of the actuating arm in various ways so as not to obstruct 
transportation of paper sheets by the actuating arm. 
Meanwhile, there has been available a detecting arrangement which detects 
an item to be transported without any contact therewith. For the detecting 
arrangement of this type, an optical detecting means has been proposed for 
actual application, in which a light emitting element and a light 
receiving element are disposed, for example, with respect to a transport 
passage as a border line, so that light from the light emitting element is 
received by the light receiving element. There is also proposed an 
arrangement in which light emitting and light receiving elements are 
provided at the same face side for receiving light reflected from the item 
to be transported. In other words, detection of the item to be transported 
is effected through utilization of the fact that light is not received by 
the light receiving element when the light from the light emitting element 
is intercepted by said item to be transported. By the above arrangement, 
paper jamming and the like related to the detection may be advantageously 
prevented without any obstruction by the transportation of the item itself 
or regardless of the fact that the item to be transported is heavy or 
light in weight. 
In the above case, a circuit arrangement, for example, as shown in FIG. 1 
has conventionally been employed in general as a circuit for producing 
signals especially in the case of detecting presence or absence of paper 
sheets in a copying apparatus or the like. 
In the circuit of FIG. 1, light from a light emitting element LED1 is 
arranged to be received by a light receiving element PT1 disposed through 
a transport passage. The light emitting element LED1 has its one end 
connected to a power source +Vcc through a resistor R1, and the other end 
thereof connected to a ground. Meanwhile, the light receiving element PT1 
has the collector connected to the power source +Vcc, and the emitter to 
the ground through a resistor R2. While light is received by the light 
receiving element PT1, said element PT1 is conducted, and a voltage Vo at 
a point A of the emitter terminal becomes generally close to the power 
source voltage +Vcc, with the voltage Vo being further supplied to a (-) 
terminal of a comparator IC1 through a resistor R3. To a (+) terminal of 
the comparator IC1, there is applied a fixed voltage Vth 
##EQU1## 
which serves as a reference voltage at a point B where the power source 
voltage +Vcc is divided by resistors R4 and R5. Accordingly, in the 
absence of any paper sheet between the light emitting element LED1 and the 
light receiving element PT1, the fixed voltage Vth becomes lower than the 
voltage at the point A, and therefore, the output of the comparator IC1 
assumes "Low" state. Thus, when a paper sheet is transported between the 
light emitting element LED1 and the light receiving element PT1, said 
element PT1 is turned off, with the voltage Vo at the point A generally 
reaching the ground potential, and the output of the comparator IC1 is 
inverted from "Low" state to "High" state. Accordingly, presence or 
absence of the paper sheet is detected by the state of output of the 
comparator IC1, and the above signal is, for example, fed to a control 
section (not shown). 
However, in the conventional circuit of FIG. 1 as described so far, when a 
comparison between the reference voltage Vth and the light detecting 
signal based on the light receiving element PT1 is to be effected, there 
have been cases where malfunctions take place due to increase of dark 
current in the light receiving element PT1 by heat, lowering of light 
emitting and light receiving amounts by the adhesion of paper scraps, dust 
and dirt, etc. onto the corresponding elements, and reduction of S/N ratio 
owing to deterioration with age and the like. 
More specifically, for example, of the dark current for the light receiving 
element PT1 is increased by heat, even when light is intercepted by a 
paper sheet disposed between the both elements LED1 and PT1, the voltage 
Vo at the point A is raised by the dark current so as to be higher than 
the fixed voltage Vth, and the output of the comparator IC1 remains "Low" 
without being inverted to "High", thus resulting in an erroneous 
detection. Meanwhile, by the lowering of the light emitting and light 
receiving amounts due to adhesion of paper scraps, dust and dirt, etc. 
onto the light emitting element LED1 and light receiving element PT1 or 
adhesion of a developing material (particularly, toner) onto said elements 
in the case of an electrophotographic copying apparatus and the like, when 
the sensor output voltage Vo at the point A is reduced to be lower than 
the fixed voltage Vth, the output of the comparator IC1 becomes "High" to 
produce a signal indicating presence of a paper sheet, even if no paper 
sheet is transported, thus also resulting in an erroneous detection. 
Moreover, in the known circuit of FIG. 1, it has been required to effect a 
fine sensitivity adjustment, since the detection for a presence or absence 
of the paper sheet is effected by setting the fixed reference voltage Vth. 
More specifically, owing to the fact that the characteristics of the light 
emitting element LED1 and the light receiving element PT1 tend to be 
largely scattered or deviated, variable resistors are employed for the 
resistors R1 to R5 in FIG. 1 so as to finely adjust the scattering in the 
characteristics of said light emitting and light receiving elements for 
effecting a proper detection. 
Although the conventional circuit of FIG. 1 has been mainly described with 
reference to the arrangement in which the light emitting and light 
receiving elements are separately disposed via the transport passage to 
effect the detection through the item to be transported, the arrangement 
is not limited to the above, but the same circuit construction may be 
applied to the system in which the light emitting and light receiving 
elements are provided at the same face side for receiving the light 
reflected from the item to be transported. 
The detection of paper sheets, etc. to be transported has been effected in 
the manner as described so far, but the paper sheet detection is not 
limited to be effected at the transport passage alone, but may also be 
effected, for example, with respect to a paper sheet undesirably wound 
onto a photosensitive member, i.e. a photoreceptor in the form of, for 
example, a drum or belt, etc. More specifically, in an electrophotographic 
copying apparatus and the like, a paper sheet is caused to closely adhere 
onto the photosensitive surface, for example, of a photoreceptor drum for 
transfer of a toner image formed on said photosensitive surface, onto the 
paper sheet. Therefore, after completion of the transfer, the paper sheet 
is separated from the photoreceptor drum so as to be further fed to a 
fixing section at a subsequent stage, but there are cases where the paper 
sheet undesirably remains closely adhering to the photosensitive surface 
of the photoreceptor drum without being separated therefrom, and is thus 
transported up to the portion of a cleaning device. If the paper sheet 
should be transported to the cleaning device as described above, the 
cleaning device itself or the photosensitive surface of the photoreceptor 
drum may be damaged in some cases, and therefore, it is necessary to 
detect the paper sheet wound onto the photoreceptor drum particularly 
before it reaches the cleaning device so as to stop the feeding of the 
photoreceptor drum. For the detecting means as referred to above, the 
mechanical switch detecting arrangement by the microswitches and the 
optical detecting means including the non-contacting light emitting and 
light receiving elements as described earlier are disposed to confront the 
photoreceptor drum. 
In the mechanical detecting switch arrangement, either the actuating arm of 
the switch is directly detected by the photoreceptor drum or the paper 
sheet closely adhering to the photoreceptor drum is separated by a 
projecting item such as a sharp claw so as to move the actuating arm of 
the switch by the separated paper sheet. Therefore, if the photoreceptor 
drum has a soft surface of OPC (Organic Photoconductor) or PET 
(Polyethylene-terephthalate), there is a great possibility that the 
surface is undesirably damaged by the actuating arm or projecting item 
referred to above. Moreover, there has also been such an inconvenience 
that the copied images are soiled by the developing material particularly 
toner, etc. adhering to the actuating arm, and further, adhering to the 
photosensitive surface from the soiled actuating arm. 
Concerning the above, when the sensor as described earlier with reference 
to FIG. 1 is employed, the drawbacks referred to above may be eliminated. 
However, the sensor arrangement also has the disadvantage that erroneous 
detection may result due to insufficient light emitting and light 
receiving amounts, when the light emitting and light receiving elements 
are soiled by the developing material, etc. Moreover, due to the 
disposition around the photosensitive member, there is a tendency that the 
light receiving element PT1 is particularly heated, with a consequent 
increase of the dark current, thus giving rise to frequent erroneous 
detections. For the prevention of such an inconvenience, it is necessary 
to increase S/N ratio of the sensor output for improvement, but there is a 
limitation to the increase of S/N ratio. 
Furthermore, when the photosensitive member is in an endless configuration, 
the difference in the output signals between the photosensitive member and 
paper sheet may be detected, but if the photosensitive member has end 
portions, grippers, etc. provided at the forward end portions of the 
photosensitive member to hold said member on a drum or the like are output 
in the form of the same detection signal as for the paper sheet, thereby 
also giving rise to a faulty detection. 
SUMMARY OF THE INVENTION 
Accordingly, an essential object of the present invention is to provide an 
improved detecting arrangement of a type employing a sensor including 
light emitting and light receiving elements as means for detecting 
presence or absence of an item to be transported, which is capable of 
effecting a stable detection at all times without regard to deterioration 
of the elements, adhesion of dust and dirt thereto, and increase of dark 
current, etc. 
Another important object of the present invention is to provide a detecting 
arrangement of the above described type which is capable of positively 
detecting a paper sheet undesirably wound onto a photosensitive member 
such as a photoreceptor drum in an electrophotographic copying apparatus 
and the like, without giving any damage to the photosensitive member. 
A further object of the present invention is to provide a detecting 
arrangement of the above described type which is simple in construction 
and accurate in functioning, and can be readily incorporated into 
electrophotographic copying apparatuses and the like at low cost. 
In accomplishing these and other objects, according to one preferred 
embodiment of the present invention, there is provided a transport item 
detecting arrangement which includes a light emitting element and a light 
receiving element disposed to confront a transport passage as a sensor for 
detection of an item to be transported, for example, a paper sheet to be 
fed along said transport passage, and characterized in that there is 
further provided means for comparing a signal responding to output of the 
sensor with a signal from a circuit which outputs integrated signal of the 
output of the sensor, thereby to employ a signal based on result of the 
comparison as a signal indicative of presence or absence of the item to be 
transported. 
By the arrangement according to the present invention as described above, 
an improved detecting arrangement has been advantageously presented.

DETAILED DESCRIPTION OF THE INVENTION 
Before the description of the present invention proceeds, it is to be noted 
that like parts are designated by like reference numerals throughout the 
accompanying drawings. 
Referring now to the drawings, there is shown in FIG. 2 an electrical block 
diagram illustrating a general construction of a circuit for detecting 
presence or absence of an item to be detected according to the present 
invention. 
In the block diagram of FIG. 2, a sensor 1 includes the light emitting and 
light receiving elements (not shown here) disposed to confront the item to 
be detected as explained with reference to FIG. 1, and produces the signal 
V1 based on the light receiving element. This output V1 is an output 
obtained through amplification of the voltage for the sensor output Vo, 
and is subsequently applied to an integrating circuit 2 which produces an 
integration signal V2. The above output V1 is also fed to the voltage 
dividing circuit 3 so as to be divided into a proper voltage V3. The 
output V2 from the integrating circuit 2 and the output V3 from the 
voltage dividing circuit 3 are applied to a comparator 4 for comparison 
therebetween, and when either of the voltages becomes higher, the output 
VO of the comparator 4 is inverted. More specifically, on the assumption 
that the output V2 is applied to a line (-) terminal of the comparator 4, 
with the output V3 being applied to a (+) terminal thereof, when the item 
to be detected, e.g., a paper sheet is detected by the sensor 1, the 
voltage V1 rapidly falls or rises, and accordingly, the voltage V3 is 
largely varied in the similar manner as in the voltage V1 so as to fall or 
rise rapidly. Meanwhile, the output V2, which is the integration output 
signal of the output V1, varies only gradually even upon rapid variation 
of the output V1, and in this case, if the voltage V1 at the side of the 
(-) terminal becomes higher than the gradually varying voltage V2, the 
output VO of the comparator 4 is inverted to "High", while on the 
contrary, if the voltage V3 at the side of the (+) terminal becomes lower 
than the voltage V2, the output VO of the comparator 4 is inverted to 
"Low". In the above case, the relation between the voltages V2 and V3 are 
such that, for example, in the state where the paper sheet is not detected 
(i.e., when the voltage V1 is not rapidly varying), the voltage V2 is set 
to become slightly larger or smaller than the voltage V3, and therefore, 
the output VO of the comparator 4 is inverted by the inversion of the 
relation between the voltages V2 and V3 at the rising or falling of the 
voltage V1 during the detection of the paper sheet. 
It should be noted here that one of the features of the present invention 
resides in that there is provided no fixed reference voltage for 
comparison with the detection signal V1 as in the conventional 
arrangements. In other words, based on the fact that the comparison 
voltage V2 is varying in correspondence to the sensor output V1, although 
the output V1 is varied (or lowered) through variation of the light 
emitting and light receiving amounts due to soiling of the elements for 
the sensor, the voltage V2 for comparison is also varied (or lowered), 
with the variation of the voltage V3 in the similar manner, and therefore, 
comparison may be effected for a larger range than in the comparison with 
respect to the fixed reference voltage, thus making it possible to effect 
the positive detection of the item to be detected. Thus, although the 
output V1 of the sensor 1 is largely varied for falling or rising upon 
detection of the item to be detected by the sensor 1, the signal V2 
subjected to the integration only gradually varies following the variation 
of the output V1, while the voltage V3 immediately varies to correspond to 
the voltage V1, and the relation between the voltages V3 and V2 is 
inverted at this time. Accordingly, even when the sensor output V1 is 
varied by certain influences (e.g., increase of the dark current, lowering 
of the light emitting and light receiving amounts, etc.), the relation 
becomes V2&gt;V3 or V3&gt;V2 in the state where the paper sheet is not detected 
so as to maintain said relation at all times for a more accurate 
detection. 
Subsequently, with reference to an example of an actual circuit shown in 
FIG. 3, the detecting circuit according to the present invention will be 
described in more detail hereinbelow. It is to be noted that the circuit 
in FIG. 3 represents only one example of the present invention, and that 
the present invention is not limited thereby. It is also to be noted that, 
in FIG. 3, since the circuit arrangement for the sensor output Vo at the 
portion of the sensor 1 surrounded by dotted lines is generally the same 
as that in the conventional circuit of FIG. 1, detailed description 
thereof is abbreviated for brevity. It should be further noted here that, 
by way of example, description will be made on the assumption that the 
light receiving element PT1 and light emitting element LED1 are provided 
on the same face side at part of the transport passage for the paper sheet 
so that the light receiving element PT1 receives the reflection light 
(especially scattering light) from the paper sheet. 
In FIG. 3, the sensor output Vo by the light receiving element PT1 is 
applied to one input terminal (i.e., (+) terminal) of an operational 
amplifier 5 through a resistor R6. The output of the operational amplifier 
5 is subjected to voltage division by resistors R7 and R8 so as to be 
negatively fed back to the other (-) terminal of said amplifier 5. 
Accordingly, the operational amplifier 5 has its output voltage 
represented by 
##EQU2## 
with respect to the input Vo, thus amplifying the sensor output Vo by 
##EQU3## 
times. The amplified output V1 is divided into proper voltages by 
resistors R9 and R10 which constitute the voltage dividing circuit 3, the 
output V3 of which is applied to the (+) terminal of the comparator 4. 
Here, the output V3 is the signal corresponding to the sensor output Vo, 
and varying in immediate response to the variation of the sensor output 
V0. 
On the other hand, the output V1 from the amplifier 5 is applied to the (-) 
terminal of the comparator 4 through resistors R11 and R12. Since a 
capacitor C1 is connected between a junction of the resistors R11 and R12 
and the ground, the integrating circuit 2 is constituted by the resistor 
R11 and the capacitor C1. Therefore, the voltage V2 obtained by 
integrating the output V1, is determined by the time constant t of the 
resistor R11 and the capacitor C11 (t=R11.multidot.C1), and is to be 
gradually varied without immediately responding to the variation of the 
sensor output V1. 
Now, on the assumption that the light emitting element LED1 and the light 
receiving element PT1 are disposed, for example, on the same face so that 
irregular reflection light from a paper sheet which is the item to be 
detected is received by the light receiving element PT1, waveforms when 
the values for the resistors R9, R10, R11 and R12 and the capacitor C1 are 
properly determined, are shown in FIG. 4, in which it is indicated that 
the paper sheet reaches the region to be detected when the relation is 
t=t1, with the leading edge appearing on the light receiving element PT1. 
In the output variation, the output V3 immediately rises in the similar 
manner as in the sensor output Vo, while the output V2 gradually rises as 
shown in FIG. 4. Accordingly, the resistors R9 and R10 are so set that, in 
the stable state of the sensor 1, i.e., in the state where no paper sheet 
is present, the outputs V2 and V3 of the respective circuits based on the 
sensor output Vo are in the relation V2&gt;V3. Therefore, the light reflected 
from the paper sheet is received by the light receiving element PT1 at the 
relation t=t1, and the output V3 varies immediately, while the output V2 
gradually varies according to the time constant t, and the relation 
between the outputs V2 and V3 is inverted when the relation is 
T1.ltoreq.t.ltoreq.t2. thus, the output of the comparator 4 which was 
"Low" up to that time due to the relation V2&gt;V3 is inverted to "High", 
since the relation becomes V2&lt;V3. The state of outputs at the above time 
is shown in FIG. 5. 
Here, if the light emitting amount and light receiving amount of the light 
emitting element LED1 and light receiving element PT1 are reduced by some 
causes, the respective signals V2 and V3 based on the sensor output Vo 
similarly vary as in curves V2' and V3' shown in FIG. 4. However, in the 
stable state, the relation between the signals V2 and V3 remains to be at 
V2'&gt;V3' at all times. Thus, when the leading edge of the paper sheet 
reaches the sensor portion, the signal V3 is instantaneously varied as in 
V3', while the signal V2 is gradually varied as in V2', and therefore, the 
relation becomes V3'&gt;V2' for effecting the paper sheet detection. 
Meanwhile, although the voltage at the point A, i.e., the sensor output Vo, 
rises due to increase of the dark current by heat, etc., the outputs V2 
and V3 are also increased following the rising of the output Vo., and 
thus, the relation V2&gt;V3 is not varied at all in the non-detecting state 
so as to become V2&lt;V3 only during the detecting period for the positive 
detection of the paper sheet. 
On the contrary, in the case where the comparison is effected by the 
comparator 4 through employment of the fixed reference voltage Vth as in 
the conventional arrangements, even when the voltage V3' rises, this 
voltage V3' does not exceed the reference voltage Vth, thus making it 
impossible to effect the paper sheet detection. In other words, it becomes 
impossible to detect the paper sheet passing through the sensor portion. 
In the arrangement in which the sensor 1 is provided in the transport 
passage for the paper sheet as in the electrophotography and the like, 
there is no possibility that rapid variations (noises) are produced in the 
sensor output Vo unless external light is incident thereupon. However, if 
noises should be produced by some causes, then the output VO of the 
comparator 4 varies from "Low" to "High" for effecting the paper sheet 
detection. 
From the viewpoint of preventing the inconvenience as described above also, 
it is effective to add the circuit as shown in FIG. 6. 
More specifically, in FIG. 6, the output VO of the comparator 4 as in FIG. 
3 is further subjected to the integration by an integrating circuit 
including a resistor R13 and a capacitor C2 to produce an output V5 which 
is applied to one comparison input terminal (i.e., the (+) terminal) of a 
comparator 40, while a fixed reference voltage V6 is fed to the other (-) 
terminal of the comparator 40. The reference voltage V6 is the voltage 
obtained by dividing the power source voltage +Vcc by resistors R14 and 
R15, and is normally set in the relation as V6&gt;V5. Accordingly, the output 
of the comparator 4 remains to be "Low" when the paper sheet detection is 
not effected. In the above state, even when the output VO of the 
comparator 4 instantaneously becomes "High" by the noise signal, since the 
output is subjected to the integration, the output VO becomes "Low" before 
the output V5 becomes higher than V6, whereby the erroneous detection by 
noises may be prevented for making it possible to effect the paper sheet 
detection at a higher accuracy. 
Referring to FIG. 13, there is shown a circuit arrangement in which the 
additional circuit of FIG. 6 is combined with the fundamental circuit of 
FIG. 3. In FIG. 13, a charge erasing lamp 21 is connected to an AC power 
source through a switch SW. More specifically, the switch SW is adapted to 
be turned on based on a control signal fed from a control circuit (not 
shown), for example, of a copying process, so as to drive the charge 
erasing lamp 21, and is arranged to energize the charge erasing lamp 21 
before a leading edge of the image formed on the photosensitive member 
(not shown here) reaches a position where it confronts said lamp 21. For 
the charge erasing lamp 21, there is selected such a lamp as is capable of 
emitting light having a frequency not damaging the photosensitive member 
during erasing of charge therefrom. In other words, the charge erasing 
lamp 21 has been conventionally employed as part of the process for 
erasing charge of the photosensitive member, and according to the present 
invention, the lamp 21 is commonly used as a sensor to prevent any damage 
to the photosensitive member. The light receiving element PT1 
corresponding to the lamp 21 for receiving light reflected from the 
photosensitive member has its collector connected to the power source 
voltage +Vcc, and the emitter thereof to the ground through a parallel 
connection of the resistor R2 and a capacitor C3 (with the resistor R6 in 
FIG. 3 being omitted here). The sensor output Vo at the point A by the 
light receiving element PT1 is applied to the (+) terminal of the 
operational amplifier 5. This operational amplifier 5 has its output V1 
fed back to the (-) terminal thereof through a parallel connection of the 
resistor R7 and a capacitor C4, and the amplification constant thereof is 
determined based on the relation with respect to the resistor R8. In other 
words, the output V1 of the amplifier 5 is in the relation 
##EQU4## 
with respect to the input Vo thereof as described earlier. 
The above features according to the present invention will be described in 
more detail with respect to detection of a paper sheet wound onto a 
photosensitive member, e.g., photoreceptor drum in an electrophotographic 
copying apparatus shown, by way of example, in FIG. 7. 
The electrophotographic copying apparatus of FIG. 7 generally includes a 
photosensitive member or photoreceptor 8 in a configuration of a drum 
having a photosensitive surface 8a formed on the other peripheral surface 
thereof, and rotatably provided generally at a central portion of an 
apparatus housing G for rotation in a direction indicated by an arrow f, 
and various processing devices such as a corona charger 9 for 
preliminarily charging the photosensitive surface 8a of the photoreceptor 
drum 8 before exposure, a developing and cleaning device 10 for 
visualizing an electrostatic latent image formed on the photosensitive 
surface 8a after the exposure, into a visible toner image, and a transfer 
device 11 for transferring the toner image onto copy paper sheets, etc., 
all of which are sequentially provided around the photoreceptor drum 8. On 
the upper portion of the apparatus housing G, an original document placing 
platform 6 of a transparent light transmitting material is provided, while 
in position below the platform 6, there is provided an optical system 7 
including a light source 71 for illuminating an original document (not 
particularly shown) placed on the platform 6, mirrors 72, 73, 74 and 75 
for reflecting light image from the original document and a lens assembly 
76 for forming the image of the original document onto the photosensitive 
surface 8a. 
The copy paper sheets are accommodated in a cassette 12 or 13 provided in 
upper and lower stages at the right side in FIG. 7, and are fed, one sheet 
by one sheet, based on the selective driving of a corresponding paper 
feeding roller 14 or 15. The copy paper sheet thus fed is once stopped at 
register rollers 16 so as to be controlled thereby so that a leading edge 
of the toner image formed on the photosensitive surface 8a of the 
photoreceptor drum 8 may be aligned with the leading edge of the paper 
sheet of the transfer device 11. After transfer of the toner image at the 
transfer section, the copy paper sheet is separated from the 
photosensitive surface 8a so as to be fed to fixing rollers 18 through a 
transport belt 17, and then, onto a paper discharge tray 19 along a 
transport passage generally indicated by an arrow 20 in FIG. 7. In the 
above transport passage 20, the sensor 1 described earlier is provided on 
the upper portion of the charge erasing lamp 21 referred to earlier for 
detection of the paper sheets, and also, of a paper sheet undesirably 
wound onto the surface 8a of the photoreceptor drum 8 after completion of 
the transfer. 
Meanwhile, part of the toner not subjected to the transfer still adheres to 
the photosensitive surface 8a after the transfer. In order to remove such 
remaining toner and also, to make the potential on the photosensitive 
surface 8a uniform, a charge eraser 22 and the charge erasing lamp 21 are 
disposed around the photoreceptor drum 8 in positions subsequent to the 
transfer device 11 and prior to the corona charger 9 with respect to the 
rotating direction of the photoreceptor drum 8. By passing through these 
charge erasing means, the toner remaining on the photosensitive surface 8a 
is cleaned when the surface 8a confronts the developing and cleaning 
device 10 in preparation for the subsequent image formation. 
In the copying apparatus having the construction as described so far, for 
the necessity of detecting the paper sheet undesirably wound onto the 
photoreceptor drum 8 before the paper sheet reaches the developing and 
cleaning device 10, the sensor 1 is provided, for example, at the position 
of the charge erasing lamp 21. In FIG. 8, this sensor 1 is provided on a 
casing 23 of the charge erasing lamp 21. More specifically, in this sensor 
1, the light emitting element 101 (LED1) and the light receiving element 
102 (PT1) provided side by side are disposed at an inclination of about 
45.degree. with respect to the direction of a rotational axis of the 
photoreceptor drum 8, whereby light is projected from the light emitting 
element 101 onto the photosensitive surface 8a of the photoreceptor drum 
8, and irregular reflected light therefrom is received by the light 
receiving element 102. In the above case, if the photosensitive surface 8a 
has a face in a state of a mirror face (e.g., OPC (Organic Photoconductor) 
photosensitive member, etc.), most of the light is subjected to a regular 
reflection without being received by the light receiving element 102, with 
the irregular reflected light from the paper sheet being mostly received 
thereby. Therefore, the outputs V2 and V3 of the respective circuits 2 and 
3 assume the state as shown in FIG. 4, thus making it possible to detect 
the paper sheet wound onto the photoreceptor drum 8. 
It should be noted here that, if the photosensitive surface 8a is formed 
into an endless configuration, it becomes possible to clarify a border 
between the leading edge of the paper sheet and the photosensitive surface 
8a as shown in FIG. 4, without any tendency toward erroneous detection. 
However, photosensitive members are not endless at all times, but there 
are some photosensitive members having end portions. For example, in an 
arrangement in which a photosensitive sheet member (master sheet) is 
supported on an aluminum drum, the leading and trailing edges of the 
photosensitive sheet member are held on the aluminum drum, with the 
photosensitive sheet member being held in close contact with the drum 
surface. Therefore, grips or the like for holding the photosensitive sheet 
member on the drum are exposed on the surface of the drum so as to be 
undesirably detected by the sensor 1, the state of which is shown in FIG. 
9. 
In FIG. 9, V3 represents the output based on the sensor 1, and a portion 
"a" of V3 relates to a signal based on the grips for holding the 
photosensitive sheet member, and a portion "b" thereof indicates a signal 
based on the paper sheet detection. As shown in FIG. 9, when a paper sheet 
wound onto the photosensitive sheet member in the drum shape having the 
grips is to be detected, the output VO of the comparator 4 in FIG. 3 
assumes the state as shown in FIG. 10 by the detection signal for the 
grips. In other words, the grip portions of the photosensitive sheet 
member 8a are undesirably detected as the paper sheet. 
However, by applying the output VO of the comparator 4 shown in FIG. 3 
further to the circuit given in FIG. 6, there is no possibility that the 
grip portions are erroneously detected as a paper sheet. More 
specifically, the integrated output V5 assumes a state as illustrated in 
FIG. 11, with the signal of the sharp variation at the grip portions being 
removed by the integrating circuit, and the output V0 of the comparator 40 
is inverted to "High" only when the output V5 exceeds the reference 
voltage V6 for the comparison, thus making it possible to accurately 
detect only the wound paper sheet. The above state of detection is shown 
in FIG. 12. Upon feeding of the output signal of this comparator 40 to the 
control section for effecting the process control of the copying 
apparatus, the control section stops the copying process control, thus 
causing the photosensitive member to stop for shutting down the copying 
operation. 
FIG. 14 is a characteristic diagram showing voltage ratios when various 
paper sheets are detected by the sensor 1 according to the present 
invention in which the light receiving element is inclined. In FIG. 14, it 
is shown that, by disposing the light receiving element PT1 of the sensor 
1 in the inclined state, S/N ratios based on the signals from the 
respective paper sheets may be increased as compared with the signals 
(noises) based on the reflected light from the photosensitive member 8. 
As is clear from the foregoing description, according to the present 
invention, in the paper sheet detecting arrangement in which the sensor, 
including the light emitting and light receiving elements, is disposed on 
the transport passage for detecting the paper sheet fed through the 
passage based on the reflected light from the paper sheet or through 
interception of light by the paper sheet, there is provided the means for 
comparing the signal obtained by integrating the sensor output with the 
signal corresponding to the sensor output signal so as to detect the 
presence or absence of the paper sheet, and therefore, even when the 
sensor output fluctuates, the signal for comparison is varied in response 
thereto, thus making it possible to effect a positive detection, with any 
erroneous detection being prevented. Moreover, by further integrating the 
output of the above comparing means so as to compare this signal with the 
reference signal for effecting the paper sheet detection, a still more 
accurate detection may be expected. Particularly, there is no possibility 
that any erroneous detection may result from noises momentarily taking 
place. Furthermore, owing to the employment of the charge erasing lamp, it 
becomes possible to effect a positive paper sheet detection without any 
damage to the photosensitive member. 
Although the present invention has been fully described by way of example 
with reference to the accompanying drawings, it is to be noted here that 
various changes and modifications will be apparent to those skilled in the 
art. Therefore, unless otherwise such changes and modifications depart 
from the scope of the present invention, they should be construed as 
included therein.