Recording apparatus

A recording apparatus in accordance with the invention has an optical system including a laser oscillator for forming an electrostatic latent image of an original on a photosensitive drum, a developing unit for visualizing the image, a tray for stacking transfer sheets, a transfer charger for transferring the image onto a transfer sheet, a fixer for fixing the image on the transfer sheet, a double transfer detector for detecting double transfer of two or more transfer sheets, a ramp detector for detecting a ramp of the transfer sheet, and a central processing unit for inhibiting feeding of the transfer sheet upon sensing of a double transfer or ramp of transfer sheets. When a double transfer or ramp of transfer sheets occurs, the information which may normally be lost may be printed on a transfer sheet in the right order without requiring interruption of the operation of the recording apparatus, so that the throughput of the recording apparatus may not be degraded.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a recording apparatus such as a laser beam 
printer (to be referred to as an LBP for brevity hereinafter) and, more 
specifically, to a recording apparatus which can eliminate dropping of 
pages or disturbanoe of the order of pages when the paper sheets (to be 
referred to as sheets hereinafter) are transferred two or more at a time 
or ramped. 
2. Description of the Prior Art 
Recording apparatus such as LBPs can continuously form images on a 
photosensitive body. Therefore, if the distance between the sheets is 
reduced to a minimum during the conveying, the maximum number of printed 
sheets may be obtained within a given period, so that the printing time 
may be significantly reduced. With such a recording device, even if a 
trouble occurs during the step of conveying the sheets such as jamming, a 
double transfer in which more than one sheet are fed together, a ramp 
sheet transfer (or simply a "ramp") and so on, it is preferable that the 
throughput (of the number of printing sheets per unit time) of the printer 
not be reduced. When jamming occurs, the printer must, in general, be 
immediately stopped and the jammed sheet must be removed. However, when a 
double transfer or ramp occurs, this does not necessarily result in 
jamming. Therefore, if a separate tray is incorporated to receive the 
double-transferred or ramped sheet, the printing operation need not be 
interrupted and be continued. However, if the double-transferred sheet or 
ramped sheet is simply exhausted to the separate tray, the information 
printed on such a double-transferred or ramped sheet inadvertently drops 
out and this dropped piece of information must be supplemented afterwards. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a recording apparatus 
which overcomes certain of the drawbacks of conventional recording 
apparatus and which reduces the decrease in the throughput of the 
apparatus when an abnormal sheet conveying condition occurs that does not 
require interruption of the operation of the apparatus. 
It is another object of the present invention to provide a recording 
apparatus which is capable of automatically supplementing dropped out 
piece of information due to an abnormal sheet conveying condition in such 
a manner that the order of printing out of the information is not 
disturbed. 
The above and other objects of the present invention will become more clear 
from the following description when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1--1 shows the basic construction of an LBP. A laser beam generated by 
a laser oscillator 1 is radiated into a light modulation system which 
consists of a lens 2, an A/O modulation element 3 utilizing known 
acoustic/optical effects, and a lens 4. The lens 2 serves to focus the 
laser beam on the black reflecting surface in the A/O modulation element 
3. The lens 4 serves to convert into parallel beams the light diffracted 
and dispersed by the black reflecting surface. 
If an E/O modulation element utilizing the known electrooptic effects is 
used in place of the A/O modulation element 3, the lenses 2 and 4 may be 
omitted. If the laser oscillator 1 comprises a gas laser which is capable 
of current conversion, the light modulation system as described above may 
be omitted. 
The laser beam radiated on the lens 4 is converted into parallel beams to 
become incident on a beam expander 7 which enlarges the diameter of each 
parallel beam without deflection. 
The parallel beams of enlarged diameter then become incident on a 
polyhedral rotary mirror 8. The polyhedral rotary mirror 8 is mounted on a 
shaft which is supported by a bearing of high precision (e.g., an air 
bearing) and is driven by a motor 9 of constant rotational frequency 
(e.g., a hysteresis synchronous motor or DC servomotor). The beams which 
are horizontally swept by the polyhedral rotary miror 8 are focused on a 
photosensitive drum 14 as a spot by an imaging lens 13 having f-.theta. 
characteristics. With a typical imaging lens, relation (1) below is 
satisfied: 
EQU r=f.multidot.tan.theta. (1) 
where .theta. is the angle of incidence of the light beam, r is the 
position on the imaging surface at which the image is formed, and f is the 
focal length of the imaging lens. If the polyhedral rotary mirror 8 is 
rotated at a constant frequency as in the case of this embodiment, the 
angle of incidence of the reflected laser beam (52 in FIG. 2) on the 
imaging lens 13 linearly changes as a function of time. Therefore, the 
displacement velocity of the spot on the photosensitive drum 14 as an 
imaging surface changes nonlinearly and is not fixed. That is, the 
displacement velocity of the spot increases as the angle of incidence 
increases. If an attempt is made to turn on the beams to draw an array of 
spots on the photosensitive drum 14 at a constant pitch, the pitch at both 
ends of the array becomes greater than at the center. 
In order to prevent this problem, the imaging lens 13 is designed to 
satisfy the relation: 
EQU r=f.multidot..theta. (2) 
The imaging lens 13 which satisfies this relation will be referred to as an 
f-.theta. lens. In order to focus the parallel light beams into a spot by 
an imaging lens, the minimum diameter dmin of the spot is given by: 
EQU dmin=f.lambda./A (3) 
where f is the focal length of the imaging lens, .lambda. is the wavelength 
of the light used, and A is the aperture of the imaging lens. A smaller 
dmin is obtained when A is increased if f and .lambda. remain constant. 
The beam expander 7 described above is used to achieve this effect. 
Therefore, the beam expander 7 may be eliminated if the desired dmin is 
obtainable with the beam diameter of the laser oscillator. 
The laser beam (52 in FIG. 2) which is deflected and modulated in this 
manner is radiated on the photosensitive drum 14 to form an electrostatic 
latent image thereon. The formed image is visualized by the 
electrophotographic process, transferred to a paper sheet, and is fixed to 
provide a hard copy. An example of the electrophotographic process which 
may be used in this embodiment is described in Japanese Patent Publication 
No. 23,910/67 of the present applicant. According to this prior art 
technique, the surface of the insulating layer which basically consists of 
an electrically conductive support layer, a photoconductive layer, and an 
insulating layer is uniformly charged negatively or positively by a 
primary corona charger 16 in advance. Charge of opposite polarity to that 
of the corona charge is trapped at the interface between the 
photoconductive layer and the insulating layer or inside the 
photoconductive layer. When the laser beam is radiated on the surface of 
the charged insulating layer while simultaneously radiating a corona 
discharge by an AC corona discharger 17, the entire surface of the 
insulating layer is exposed by an entire exposure lamp 18 to the pattern 
of the surface potential difference which corresponds to the dark-bright 
pattern of the laser beam to thereby form an electrostatic latent image of 
high contrast thereon. Then, the electrostatic latent image is developed 
for visualization by a developer containing as the main component charged 
colorant particles. Thereafter, with a transfer charger 22, the visualized 
image is transferred to the paper sheet which is tightly held onto the 
photosensitive drum 14 by a means to be described later. The transferred 
image is then fixed by a fixing means to be described later to provide an 
electrophotographically printed image. After the transfer, the surface of 
the insulating layer is cleaned by a cleaner 23 to remove the remaining 
charged particles so that the photosensitive drum 14 may be repeatedly 
used. 
The electrophotographic electrostatic latent image forming process as 
described in Japanese Patent Publication No. 19,748/67 of the present 
applicant is another example. According to this technique, the 
photosensitive plate essentially comprises an electroconductive layer, a 
photoconductive layer, and an insulating layer. The surface of the 
insulating layer is uniformly charged negatively or positively by the 
primary corona charge. The charge of a polarity opposite to that of the 
primary corona charge is trapped in the interface between the 
photoconductive layer and the insulating layer or inside the 
photoconductive layer. The AC corona charge is applied to the surface of 
the charged part to attenuate the charge on the surface of the insulating 
layer. Then, the laser beam as information signals is radiated to form an 
electrostatic latent image according to the dark-bright pattern of the 
laser beam on the surface of the insulating layer. The following steps for 
forming the electrostatic latent image are the same as in the case 
described above. 
Referring to FIGS. 1--1 and 1-2, a precharge removal charger 15 keeps the 
surface potential of the photosensitive drum 14 constant and uniform. A 
preexposure lamp 16' serves to keep the characteristics of the 
photosensitive layer constant and uniform. The charger 15 and the lamp 16' 
cooperate together to cancel the preceding history of the photosensitive 
drum 14 that is the surface potential, after the cleaner 23, so that a 
stable image may be obtained constantly. 
The present applicant has proposed a method for stabilizing the 
electrostatic latent image as a means to obtain a constantly stable and 
excellent image in the electrophotographic process which is to be adopted 
according to this embodiment. An electrostatic potentiometer 21 measures 
the electrostatic potentials at the bright part of the photosensitive drum 
14, that is, the part scanned and exposed to the laser beam, and at the 
dark part. 
A carrier remover 20 prevents attraction of carriers in the developer in 
the developing unit 19 to the photosensitive drum 14 or mixing of these 
carriers in the cleaner 23. 
FIG. 2 shows the three-dimensional arrangement of the optical system shown 
in FIG. 1--1. The same reference numerals as in FIG. 1 denote the parts 
having the same functions. (This also will be applicable to the figures to 
be described hereinafter unless otherwise indicated.). 
Referring to FIG. 2, a beam detector 51 comprises a small slit and a 
photoelectric transducer element of fast response time (e.g., a p-i-n 
diode). The beam detector 51 detects the scanning initiating position of a 
laser beam 52 which is swept by the rotating mirror 8. According to the 
detection signal generated by the beam detector 51, the timing is 
determined of generation of a modulation control signal for providing 
desired light information on the photosensitive drum 14 in the manner to 
be described below. 
Referring still to FIG. 2, the scanning direction of the laser beam is 
indicated by an arrow 53. 
Conveying operation of the sheet will now be described below with reference 
to FIGS. 1--1 and 1-2. When sheets 31 stacked on a paper feed stage 30 are 
supplied to the conveying path by a paper feed roller 32, any ramp 
occurring during feeding operation is corrected by a pair of register 
rollers 33 and the leading ends of the sheets 31 are also aligned. The 
sheets 31 passing between the register rollers 33 are brought into contact 
with the photosensitive drum 14 through a conveying guide 34 which defines 
part of the conveying path. A double transfer sensor 47 is arranged in the 
conveying path to detect whether or not a double transfer of sheets 31 
occurs. The image formed on the photosensitive drum 14 is transferred onto 
the sheet 31 by the transfer charger 22. The sheet 31, after the transfer, 
is separated from the photosensitive drum 14 by a separating means (not 
shown) to be supplied to a conveyor belt 39. A photosensor 35 is arranged 
in the region of the conveyor belt 39. By the output signal from the 
photosensor 35, jamming in the conveying path from the paper feed roller 
32 to the photosensitive drum 14 is detected. The sheet 31 is further 
conveyed by the conveyor belt 39 to reach a fixer 40. The toner on the 
sheet 31 is melted by the heat and pressure of the fixer 40 and forms a 
toner image. The sheet 31, after fixing, is conveyed by another conveyor 
belt 41. Any jamming occurring in the conveying path between photosensors 
35 and another photosensor 36 is detected by the photosensor 35 and the 
photosensor 36 which are arranged before exhaust rollers 42. A sheet which 
is conveyed normally is stacked on a first output tray 43 through the 
exhaust rollers 42. 
When a double transfer is detected by the double transfer sensor 47 to be 
described later, the sheets are detected by the photosensor 36. Then, a 
paper exhaust switching plate 44 is switched to the position indicated by 
broken lines, and the sheets detected to be abnormally fed are stacked on 
a second output tray 46 through paper exhaust rollers 42 and 45. A 
photosensor 37 is arranged immediately before the paper exhaust rollers 
45. This photosensor 37 detects if the abnormal sheet is conveyed to the 
second output tray 46 without failure. The double transfer sensor 47 
detects a double transfer by the amount of light transmitted through the 
sheet. 
FIG. 3-1 is a block diagram showing the control circuitry of the LBP. A 
central processing unit (to be referred to as a CPU hereinafter) 100 
controls the operation of the LBP according to a program stored in a ROM 
(not shown). The CPU 100 may comprise a known microcomputer including ROMs 
and RAMs, for example, an M6800 available from Motorola. A double transfer 
sensor 101 corresponds to the double transfer sensor 47 shown in FIG. 1. 
The signal from the double transfer sensor 101 is amplified by an 
amplifier 102, digitalized by an A/D converter 103, and is input to the 
CPU 100. A drive signal from the CPU 100 is output to a paper feed roller 
clutch 106 and a paper exhaust switching solenoid 107 through switching 
circuits 104 and 105 to drive the paper feed roller 32 and the paper 
exhaust switching plate 44. The CPU 100 outputs an image write timing 
signal VTS to a page information output circuit 108. 
A video signal VS from the page information output circuit 108 and a 
permission signal PS from the CPU 100 are output to an AND gate 109, the 
output of which is input to an A/O modulator 111 (corresponding to the A/O 
modulation element 3 shown in FIGS. 1--1 and 1-2) through an A/O driver 
110 (corresponding to the A/O driving element 5 shown in FIGS. 1--1 and 
1-2). Then, the A/O modulator 111 is turned on and off according to the 
video signal. 
FIGS. 4 to 9 show the conveying path from the paper feed section to the 
photosensitive drum. Referring to FIG. 4, an image writing position on the 
photosensitive drum 14 is represented by O, the image transfer position is 
represented by T, and the leading end of each sheet 31 at the paper feed 
stage 50 which comes into contact with the paper feed roller 32 is 
represented by P. The sheets are fed in the order of N, N+1, N+2, and so 
on, and the page information on the image is written on the photosensitive 
drum 14 in the order of n, n+1, n+2, and so on. The distance on the 
photosensitive drum 14 from the image writing position O to the image 
transfer position T is represented by L.sub.1, and the conveying distance 
from the leading end P of the sheet 31 to the image transfer position T is 
represented by L.sub.2. 
If L.sub.1 .ltoreq.L.sub.2 as shown in FIG. 4, in order that the leading 
end of the image and the leading end of the sheet coincide at the image 
transfer position T, the paper feeding is performed first and the image 
writing is performed next. 
If L.sub.1 .ltoreq.L.sub.2 as shown in FIG. 5, in order that the leading 
end of the image and the leading end of the sheet coincide at the image 
transfer point T, the image writing is performed first, and the paper 
feeding is performed next. 
The distance between the leading ends of continuously fed sheets may be 
given by L.sub.P +L.sub.G where L.sub.P is the length of the sheet along 
the direction of its movement and L.sub.G is the paper feeding distance 
between the sheets. 
Control operation in the case of a double transfer will be described with 
reference to the position of the double transfer sensor and the page 
information written on the photosensitive drum 14 referring to the flow 
charts shown in FIGS. 6 to 9 and 10 to 13 and the timing charts shown in 
FIGS. 14 to 17. 
As shown in FIG. 6, if a double transfer with the Nth sheet occurs in the 
conveying path which satisfies the relations L.sub.1 .ltoreq.L.sub.2 and 
L.sub.P +L.sub.G .ltoreq.L.sub.S where L.sub.S is the distance between the 
leading end P of the sheet and the double transfer sensor 47, the feeding 
of the (N+1)th sheet is already started as seen from the flow chart shown 
in FIG. 10 and the timing chart shown in FIG. 14. The information on the 
(n+1)th page is already being written on the photosensitive drum 14. 
Therefore, when the Nth sheet is exhausted to the second output tray 46 
and the (N+1)th sheet which is not detected to be double-transferred is 
exhausted to the first output tray 43, the information on the nth page 
drops from the printed sheets. Even if the information corresponding to 
the nth page is printed on the (N+2)th sheet, the order of sheets is 
disturbed and becomes (n-1), (n+1) and n. 
In order to prevent the dropout of the page and disturbance of the order of 
pages, if a double transfer occurs with the Nth sheet, the (N+1)th sheet 
is also exhausted to the second output tray 46 and the information on the 
nth page is printed on the (N+2)th sheet. In this manner, the dropout of 
the page and the disturbance of the order of pages may be prevented. More 
specifically, when the double transfer sensor 101 (47) detects a double 
transfer and the output signal therefrom goes to high level, the 
permission signal PS from the CPU 100 goes to low level, so that the video 
signal VS from the page information output circuit 108 is not supplied to 
the A/O modulator 111 (3). When the photosensor 36 detects the 
double-transferred sheet, the CPU 100 produces a drive signal which turns 
on the paper exhaust switching solenoid 107 through the switching circuit 
105. Then the paper exhaust switching plate 44 is switched to the position 
indicated by the broken line shown in FIG. 1, so that the Nth and (N+1)th 
sheets are exhausted to the second output tray 46. When the paper feed 
roller clutch 106 is turned on and the (N+2)th sheet is fed, the 
permission signal PS goes to high level at a predetermined time to output 
the information on the nth page to the A/O driver 110 (5) through the AND 
gate 109 from the page information output circuit 108. Then, the A/O 
modulator 111 (3) modulates the laser beam to write the information on the 
nth page on the photosensitive drum 14. The sheet to which is transferred 
the image at the image transfer position T is exhausted to the first 
output tray 43. During this operation, the paper exhaust switching 
solenoid 107 is kept off and the paper exhaust switching plate 44 is at 
the position indicated by the solid line shown in FIG. 1. Thereafter, the 
information on the (n+1)th page is transferred to the (N+3)th sheet. 
The operation will now be described with reference to a case wherein a 
double transfer occurs with the Nth sheet in the conveying path shown in 
FIG. 7 which satisfies the relations L.sub.1 .ltoreq.L.sub.2 and L.sub.P 
+L.sub.G &gt;L.sub.S, by referring to the flow chart shown in FIG. 11 and the 
timing chart shown in FIG. 15. At the instant when the double transfer 
sensor 101 (47) detects a double transfer, the information on the nth page 
is being written on the photosensitive drum 14. Therefore, it is necessary 
to output the information on the nth page to the (N+1)th sheet in place of 
the information on the (n+1)th page. At the leading edge of the output 
signal from the double transfer sensor 101 (47), the permission signal PS 
goes to low level so that the video signal VS may not be output to the A/O 
modulator 111 (3). When the paper feed clutch 106 is turned on and the 
(N+1)th sheet is fed, the information on the nth page is written on the 
photosensitive drum 14 at a predetermined time and is transferred to the 
(N+1)th sheet. In the same manner, the information on the (n+1)th page is 
transferred to the (N+2)th sheet, the information on the (n+2)th page is 
transferred to the (N+3)th sheet and so on. When the photosensor 36 
detects a double-transferred sheet after the leading edge of the output 
signal from the double transfer sensor 101 (47), the paper exhaust 
switching solenoid 107 is turned on, and the Nth sheet is exhausted to the 
second output tray 46. 
The model view of the conveying path which satisfies the relation L.sub.1 
&gt;L.sub.2 is shown in FIGS. 8 and 9. In this case, as has been described 
hereinabove, the timing of writing the image on the photosensitive drum is 
earlier than the timing of feeding the sheet. FIG. 8 shows a case wherein 
the relation L.sub.P +L.sub.G .ltoreq.L.sub.S is satisifed. FIG. 12 shows 
the control flow chart for the case shown in FIG. 8, and FIG. 16 shows the 
timing chart when a double transfer occurs at the Nth sheet. In this case, 
when the Nth sheet is double-transferred, the information on the (n+2)th 
page is already being written on the photosensitive drum 14. Furthermore, 
since the (N+1)th sheet is already fed, the Nth and (N+1)th sheets must be 
exhausted to the second output tray 46 so as to prevent the dropout of the 
page and the disturbance of the page order. In order to accomplish this, 
at the leading edge of the double transfer sensor 101 (47), the permission 
signal PS goes to low level in the similar manner as described above, so 
that the video signal VS may not be output to the A/O modulator 111 (3). 
Since the transfer of the information on the (n+2)th page is not 
necessary, the feeding of the (N+2)th sheet is inhibited. When the 
photosensor 36 senses the double-transferred sheet after the leading edge 
of the output signal from the double transfer sensor 101 (47), the drive 
signal is supplied to the switching circuit 104 from the CPU 100 to switch 
the paper exhaust switching plate 44, so that the Nth and (N+1)th sheets 
are exhausted to the second output tray 46. The control is performed 
thereafter to write the information on the photosensitive drum 14 starting 
from the nth page. More specifically, at the timing at which the 
information on the (n+3)th page is output in the normal sequence, the 
information on the nth page is written on the photosensitive drum 14 and 
the (N+2)th sheet is fed at a predetermined time (feeding time of the 
(N+3)th sheet in the normal sequence). After the transfer of the image on 
the (N+2)th sheet, the sheet is exhausted to the first output tray 43. 
FIG. 9 shows a case wherein the relation L.sub.P +L.sub.G &gt;L.sub.S is 
satisfied. The control flow chart of the Nth sheet is shown in FIG. 13, 
and the timing chart when the Nth sheet is double-transferred is shown in 
FIG. 17. At the time when the Nth sheet is detected to be 
double-transferred, the information on the (n+1)th page is already being 
written on the photosensitive drum 14 but the (N+1)th sheet is not yet 
fed. Therefore, the control is performed so that the feeding operation of 
the (N+1)th sheet is inhibited and the Nth sheet alone is exhausted to the 
second output tray 46. At the leading edge of the output signal from the 
double transfer sensor 101 (47), the permission signal PS goes to low 
level in the manner as described above, so that the video signal VS may 
not be supplied to the A/O modulator 111 (3). The feeding operation on the 
(N+1)th sheet is also inhibited. Then, at the time at which the 
information on the (n+2)th page is written on the photosensitive drum 14 
in the normal sequence, the information on the nth page is written on the 
photosensitive drum 14. At the time at which the (N+2)th sheet is fed in 
the normal sequence, the (N+1)th sheet is fed. The information on the nth 
page is thus transferred to the (N+1)th sheet and the sheet is exhausted 
to the first output tray 43. Thereafter, the information on the (n+1)th 
page is output. When the photosensor 36 senses a double-transferred sheet 
after the detection of the double transfer, the drive signal from the CPU 
100 turns on the paper exhaust solenoid 107, and the Nth sheet is 
exhausted to the second output tray 46. 
The description has been made with reference to the case of a double 
transfer. However, similar control operation is performed when a ramp of 
the sheet is detected. In this case, a ramp sensor 147 is arranged as 
shown in FIG. 1-2 in place of the double-transfer sensor 47 shown in FIG. 
1--1. When two sensors arranged perpendicularly to the direction of paper 
feeding do not sense the sheet within a predetermined period of time, the 
sensors thereby detect a ramp. In FIG. 1-2, the same reference numerals 
denote the same parts as in FIG. 1--1. FIG. 3-2 is a block diagram showing 
the control circuitry of the arrangement shown in FIG. 1-2. In FIG. 3-2, 
the same reference numerals denote the same parts as in FIG. 3-1. A ramp 
sensor 201 corresponds to the ramp sensor 147 shown in FIG. 1-2. The 
signal output from the ramp sensor 201 is amplified by an amplifier 202, 
and amplified signal is supplied to a comparator 203 for comparison and is 
input to the CPU 100. In response to the detection signal, the control as 
in the case of the double transfer indicated in the flow charts shown in 
FIGS. 12 and 13 and the timing charts shown in FIGS. 16 and 17 is 
performed. 
In summary, according to the present invention, unnecessary feeding of the 
sheets is eliminated when a double transfer or a ramp is detected, and the 
dropout of the page or disturbance of the order of pages is prevented. 
Since the image is formed without interruption of the operation of the 
recording apparatus, the decrease in the throughput of the apparatus may 
be reduced to the minimum.