Image forming apparatus

There is disclosed an image forming apparatus capable of controlling the image forming conditions in different modes according to the magnification of the image to be formed, or according to the process speed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an image forming apparatus for effecting 
image formation on a recording member, such as a copier. 
2. Description of the Prior Art 
There are already known copiers with variable image magnification for 
enlarging or reducing the original image. In such copiers the distribution 
of the light intensity on the photosensitive drum has to be uniform both 
in the one-to-one copying mode and in the copying mode with modified image 
magnification, and such uniform light distribution has been conventionally 
achieved by the use of a light distribution correcting plate, such as a 
slit, positioned in the vicinity of the photosensitive drum. An example of 
such correction for light distribution in the actual size copying, 
reduction copying and enlarged copying is shown in FIGS. 1A, 2B and 1C. 
The original illuminating system is so constructed as to compensate the 
so-called cos.sup.4 .theta. rule of a lens 50 in the real-size copying 
mode. Thus the illuminating system has a distribution of illumination 
intensity on the original document as represented by a curve 100 to obtain 
a uniform intensity distribution on the photosensitive drum as represented 
by 200 after passing the lens 50. However, in case the image magnification 
is modified, the distribution of the illumination intensity becomes uneven 
as represented by 201 or 202, respectively in case of size reduction or 
size enlargement, since the angle of viewing the original document from 
the lens changes in such cases. Such unevenness has been corrected as 
represented by 201' or 202' by the insertion of a slit 301 or 302, 
respectively in case of size reduction or size enlargement, of which forms 
are respectively shown in FIGS. 2A and 2B. 
The use of such slits in the modified size copying mode enables to obtain a 
uniform light intensity distribution on the photosensitive drum, but at 
the same time results in a loss of light amounting even to about 30%, 
because of partial shielding of the light by said slit. Such loss of light 
is compensated by employing a lower process speed in such modified size 
copying mode than, for example equal to 0.7 times of, the process speed in 
the real-size copying mode, and the amount of light at such modified size 
copying mode is rendered adjustable, for example with a slit, to 0.7 times 
of that in the real-size copying mode. Also the use of such lower process 
speed in the modified size copying mode prevents the vibration of image 
which may be caused, in the reduction copying, by a high scanning speed 
(=process speed/image reduction rate). 
Also there is already known a control process for chargers, developing 
device etc. in response to the detection of surface state, for example 
surface potential of the photosensitive member in order to achieve stable 
image reproduction. FIG. 3 shows an example of such control process, 
wherein a three-layered photosensitive drum 500, having an insulating 
layer, a photoconductive layer and a conductive layer in this order from 
the external periphery, is surrounded, in the order same as the direction 
of rotation thereof, by a primary charger 501 for uniformly charging said 
drum, a secondary charger 502 for charge elimination and a whole-surface 
exposure lamp 503. An original document placed on a carriage is 
illuminated by a light source 504 such as a halogen lamp, and the 
reflected light is focused through a lens 506 onto the photosensitive drum 
500 at a position the charge on said drum according to the amount of 
exposure to the original image, thereby forming an electrostatic latent 
image on the drum corresponding to the original image. The electrostatic 
latent image thus formed is entirely exposed to the light from the 
whole-surface exposure lamp 503 to obtain a latent image with improved 
gradation. Thereafter the latent image moves to a developing device and is 
developed with toner by a developing roller 514 to which a bias voltage is 
supplied. A blank exposure lamp 507 constantly illuminates the 
photosensitive drum when the original exposure lamp 504 is not lighted 
while the chargers are in operation, in order to prevent the toner 
deposition in the non-image area. In the vicinity of the photosensitive 
drum 500 and between the developing device and the whole surface exposure 
lamp 503, there is provided a surface potential sensor 505 for measuring 
the surface potential of the drum. The output signal of said sensor is 
amplified and converted into digital signals in a surface potential 
measuring circuit 508, and is then supplied to a potential control circuit 
513 composed for example of a microcomputer for effecting data processing 
according to the measured surface potential. The results of said 
processing are converted into analog signals and are supplied to 
high-voltage generating circuits 509, 510, a developing bias circuit 511 
and an exposure control circuit 512 for respectively controlling the 
voltages supplied to the primary and secondary chargers, the developing 
bias voltage and the voltage to the halogen lamp. The control of the image 
forming conditions in the above-described apparatus is achieved in the 
following manner. 
After the start of power supply, the drum is subjected to a pre-rotation 
step for stabilizing the performance of the photosensitive member. Then 
reference currents I.sub.po, I.sub.so are respectively supplied to the 
primary and secondary chargers 501, 502, and the surface potential sensor 
505 measures the dark potential V.sub.D after the entire illumination with 
the 
After the start of power supply, the drum is subjected to a pre-rotation 
step for stabilizing the performance of the photosensitive member. Then 
reference currents I.sub.po, I.sub.so are respectively supplied to the 
primary and secondary chargers 501, 502, and the surface potential sensor 
505 measures the dark potential V.sub.D after the entire illumination with 
the whole surface exposure lamp 503, and the light potential V.sub.SL 
after the illumination with the blank exposure lamp 507 at the highest 
intensity. Then the primary and secondary currents I.sub.p, I.sub.s are 
corrected so as to bring the light and dark potentials V.sub.SL V.sub.D 
closer to the target values, and such correcting cycle is repeated for 
example four times. 
Then the original exposure lamp 504 is lighted with a reference voltage 
V.sub.HO, and the surface potential sensor 505 measures the potential 
V.sub.L of the latent image formed on the photosensitive drum 
corresponding to a standard white plate. Then the lamp voltage V.sub.H is 
corrected so as to bring said potential V.sub.L closer to zero, and this 
cycle is repeated for example three times. The developing bias voltage is 
obtained by adding a determined voltage to said potential V.sub.L. The 
above-described control allows to bring the photosensitive characteristic, 
for example represented by a full-lined curve in FIG. 4, to an ideal 
characteristic represented by a broken-lined curve. The succeeding copying 
cycle is conducted with thus corrected primary and secondary charging 
currents I.sub.p, I.sub.s and lighting voltage V.sub.H. 
Such control is conducted, also in the modified size copying mode, with the 
optical path and the process speed for the real-size copying mode to 
determine I.sub.p, I.sub.s and V.sub.H in the aforementioned manner, and 
the primary and secondary charging currents in the modified size copying 
mode are obtained by multiplying for example 0.7 with said values I.sub.p, 
I.sub.s if the process speed in the modified size copying mode is 0.7 
times of that in the real-size copying mode. 
Such control process however requires a long time and involves the 
complexity of maintaining required mechanical precision, since, in the 
modified size copying mode, the optical system is at first returned to the 
position for the real-size copying mode and is then brought to the 
position for the modified size copying mode after the potential control in 
the aforementioned manner. 
Also in such process, the image density and the gradation of intermediate 
image tone in the modified size copying mode become different from those 
in the real-size copying mode, since the reciprocity does not stand in 
strict sense. 
In addition, the number of corrections for the charging currents and for 
the lighting voltage is determined in advance, so that the correcting 
operations have to be repeated wastefully even when the charging current 
or the lighting voltage is already at the target value or when the 
correction is no longer possible because of the limitation in the capacity 
of the power supply. 
SUMMARY OF THE INVENTION 
In consideration of the foregoing, an object of the present invention is to 
provide an image forming apparatus capable of constantly providing images 
with satisfactory image quality. 
Another object of the present invention is to provide an image forming 
apparatus capable of controlling the image forming conditions in different 
modes according to the image magnification. 
Still another object of the present invention is to provide an image 
forming apparatus capable of controlling the image forming conditions in 
different modes according to the process speed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now the present invention will be clarified in detail by embodiments 
thereof shown in the attached drawings. 
FIG. 5-1 is a cross-sectional view of a copier embodying the present 
invention, of which structure and function will be explained in the 
following. 
A photosensitive drum 1 is rotated in a direction indicated by the arrow by 
means of an unrepresented motor. An original document placed on an 
original carriage glass 36 is illuminated by a lamp 23 constructed 
integrally with a first scanning mirror 24, and the reflected light is 
scanned by said first scanning mirror 24 and a second scanning mirror 25, 
which are displaced with a speed ratio of 1:1/2 to maintain a constant 
optical path length in front of a lens 30. 
Said reflected light is focused on said drum 1 in an exposure station, 
through a zoom lens 30 and a third mirror 26. 
The drum 1 is in advance uniformly charged, either positively or 
negatively, by a primary charger 3, and an electrostatic latent image is 
formed on said drum by said reflected light. The electrostatic latent 
image formed on the drum 1 is developed as a visible toner image by a 
developing roller 13' in a developing station, and said toner image is 
transferred onto a transfer sheet by means of a transfer charger 4. The 
transfer sheet contained in a cassette 10 is advanced by a feeding roller 
11, and supplied toward the photosensitive drum 1 with an exact timing 
measured by a registering roller 15 in such a manner that the leading end 
of the latent image coincides with that of the transfer sheet at the 
transfer station. In said transfer station corona discharge is applied by 
the transfer charger 4 from the rear side of the transfer sheet, thereby 
electrostatically transferring the toner image onto said transfer sheet. 
Then, a separating charger 5, generating AC corona discharge or DC corona 
discharge of a polarity opposite to that of the transfer charger 4, 
neutralizes the charge on the rear face of the transfer sheet, whereby 
said transfer sheet is separated from the photosensitive drum 1 and is 
transported by a conveyor belt 6. After said separation of the transfer 
sheet, the photosensitive drum 1 is at first subjected to charge 
elimination by charge pre-eliminator 7, and the toner remaining on the 
photosensitive drum 1 is removed by a cleaner 8. On the other hand the 
transfer sheet passes through a fixing station 9 for permanently fixing 
the toner image thereon. 
FIG. 5-2 is a perspective view of the zoom lens 30 and related mechanism, 
wherein same components as those in FIG. 5-1 are represented by same 
numbers. A hood member 39 mounted on the zoom lens 30 for avoiding dusts 
and unnecessary light is provided with a correcting plate 37 for 
correcting the amount of light in response to the cos.sup.4 .theta. rule 
of the lens, thereby obtaining uniform illumination intensity on the 
photosensitive drum. A light amount correcting plate 38, for correcting 
the amount of light in case the process speed is changed for the modified 
size copying mode, is provided with vertical slots so as not to interfere 
with the function of the aforementioned cos.sup.4 .theta. rule correcting 
plate 37, wherein the, width x of each slot and the distance y of 
neighboring slots are selected with a ratio of 7:3 to reduce the amount of 
light falling on the photosensitive drum to 70% when said light amount 
correcting plate 38 is inserted. A solenoid 40 for controlling said 
correcting plate 38 rotates, when energized, a shaft 44 through a wire 41 
and a pulley 42, whereby the correcting plate 38 rotates clockwise by ca. 
90.degree. against the biasing force of a returning spring 43. 
FIG. 5-3 is a plan view showing a part of the control panel of the copier 
shown in FIG. 5-1. In said control panel there are provided numeral keys 
51 for setting a desired copy number, up to 99 copies, on a display unit 
57; a clear key C for clearing the display on said display unit 57; a stop 
key 52 for interrupting the operation of the copier at a stage where the 
copy count (number of prepared copies) does not reach the set copy number, 
wherein the actuation of said stop key terminates the operation of the 
copier as soon as a copying cycle already in execution is completed; a 
start key 53 for initiating the copying operation; 7-segment display units 
57, 58 composed for example of light-emitting diodes or of liquid crystal 
display elements for respectively indicating the set copy number and the 
copy count; a lever 54 for selecting the image density; a key 55 for 
selecting an automatic exposure mode to be explained later; and a key 56 
for enabling manual image density selection with said lever 54. The keys 
55, 56 are internally provided with lamps which are lighted when said keys 
are actuated. The manual density selecting mode activated by the key 56 
returns automatically to the automatic exposure mode if the copier is not 
manipulated in excess of 1 minute. There is also provided a modified size 
mode key 59 for selecting the modified size copying mode and for enabling 
the entry of a desired image magnification with the aforementioned numeral 
keys. Said image magnification can be entered in the order of a number 
above the decimal point, then the decimal point, and numbers below the 
decimal point, and thus entered magnification is displayed on a 3-digit 
7-segment display unit 60. 
FIG. 6 is a block diagram showing the control unit of the copier shown in 
FIG. 5, wherein same components as those in FIG. 5 are represented by same 
numbers. In FIG. 6 there are shown a control unit 14 composed of a known 
one-chip microcomputer incorporating read-only memory, random access 
memory etc.; an exposure control circuit 15 for lighting the illuminating 
lamp 23 and adapted to receive a control signal supplied from an output 
port OUT3 of the control unit 14 through a D/A converter 19; a high 
voltage generating circuit 16 for driving the primary charger 3 and 
adapted to receive a control signal supplied from an output port OUT2 of 
the control unit 14 through a D/A converter 20; an amplifier 17 for 
amplifying the output signal from the potential sensor 12 and supplying 
the amplified signal, after conversion into digital signal by an A/D 
converter 21, to an input port IN1 of the control unit 14; a developing 
bias transformer 18 for generating the bias voltage to be supplied to the 
developing roller 13 and adapted to receive a control signal supplied from 
an output port OUT1 of the control unit 14 through a D/A converter 22; a 
control-display device incorporating the control unit as shown in FIG. 5-3 
and connected to the control unit 14; and a drum clock pulse generator 31 
composed of a clock disk 31a rotated in synchronization with the 
photosensitive drum 1 and a photo-interrupter 31b, wherein said clock disk 
is provided with fine slits along the periphery thereof and said photo 
interrupter 31b senses said slits to generate clock pulses which are 
supplied to an interruption port of the control unit 14 for achieving 
various sequence controls. 
In the following there will be given an explanation of the potential 
control employed in the copier of the present embodiment. In the 
pre-rotation step, the photosensitive drum 1 is rotated several turns to 
stabilize the performance of the photosensitive member while the blank 
exposure lamp 2 is continuously lighted and other conditions are 
maintained same as those in the normal copying operation. Then the control 
unit 14 supplies a signal through the D/A converter 20 to the high-voltage 
generating circuit (HVDC) 16 in order to supply a reference current 
I.sub.po to the primary charger 3. The potential sensor 12 detects the 
dark potential V.sub.D of the photosensitive drum 1 and supplies it to the 
control unit 14 through the amplifier 17 and the A/D converter 21. The 
control unit 14 compares the surface potential V.sub.D detected by the 
potential sensor 12 with a target dark potential V.sub.DO and accordingly 
controls he high-voltage generating circuit 16 for regulating the current 
I.sub.p. 
In the present embodiment the target value is selected as 400.+-.20 V, and 
the detection of the surface potential V.sub.D and the control of the 
high-voltage generating circuit 16 are repeated four times at maximum in 
case the surface potential V.sub.D does not fall within the 
above-mentioned target range. 
However, the succeeding operation is initiated even if the surface 
potential V.sub.D does not fall within the target range after the fourth 
control of the high-voltage generating circuit 16. Also the surface 
potential V.sub.SL is not controlled since it is equal to zero when the 
blank exposure lamp 2 is lighted. 
Then the control unit 14 releases a control signal through the exposure 
control circuit (CVR) 15 and the D/A converter 19 to light the 
illuminating lamp 23 with a standard intensity corresponding to a position 
"5" of the lever 54, whereby the lamp 23 illuminates the standard white 
plate 39 to project a corresponding image onto the photosensitive drum 1. 
The potential sensor 12 detects the corresponding potential V.sub.L and 
supplies it through the amplifier 17 and the A/D converter 21 to the 
control unit 14. The control unit 14 compares the surface potential 
V.sub.L detected by the potential sensor 12 with a target value V.sub.LO 
and accordingly control the exposure control circuit 15 to regulate the 
illuminating lamp 23. 
In the present embodiment the target value V.sub.LO is selected as 
100.+-.15 V, and the detection of the surface potential V.sub.L and the 
control of the exposure control circuit 15 are repeated three times at 
maximum in case the surface potential V.sub.L does not fall within the 
above-mentioned target range. 
However the succeeding operation is initiated even when the surface 
potential V.sub.L does not fall within the target range after the third 
control of the exposure control circuit 15. 
After the completion of copying operations for the set copy number, a 
post-rotation step is conducted to electrostatically clean the 
photosensitive drum 1. 
The above-described control of the dark potential V.sub.D and the light 
potential V.sub.L corresponding to the light reflected by the standard 
white plate 39 allows modification of the photosensitive characteristic, 
as represented by a solid-lined curve in FIG. 7, close to an ideal 
characteristic represented by a broken-lined curve. 
The developing bias voltage for the developing roller 13 is determined by 
adding a determined voltage to the final light potential V.sub.L at the 
exposure control, and is supplied through the D/A converter 22 and the 
developing bias transformer 18. 
Now there will be given an explanation on the automatic exposure mode. The 
copier of the present embodiment is capable of effecting a preliminary 
scanning on the original document to detect the density of the original 
document from the surface potential, and accordingly controlling the 
amount of light from the original exposure lamp thereby reproducing a 
white background even from an original with a colored background. The 
above-described mode is selected except when the key 56 is actuated. The 
reciprocating motion of the optical system for scanning the original 
document is effected by suitably energizing unrepresented forward and 
backward clutches. Now reference is made to FIGS. 6, 8 and 9 for further 
explanation. 
In the present embodiment, an area corresponding to the minimum copiable 
size, for example B5-size, determined in the copier is scanned in the 
aforementioned manner under a determined illumination intensity of the 
illuminating lamp 23, and the corresponding surface potential on the 
photosensitive drum 1 is detected by the potential sensor 12. 
The control unit 14 controls the exposure control circuit 15 to regulate 
the lighting voltage of the illuminating lamp 23, in response to the 
average value, or the minimum value of the surface potential detected in 
the preliminary scanning. 
FIG. 8 shows the relationship, in the automatic exposure control mode, 
between the amount of regulation .DELTA.V.sub.H..sub.AE of the lighting 
voltage of the illuminating lamp 23 and the mean (or minimum) value of the 
surface potential V detected in the preliminary scanning. 
The correction curve shown in FIG. 8 is stored in the control unit 14, and 
the regulation of the lighting voltage of the illuminating lamp 23 along 
said correction curve in response to the minimum (or mean) value of the 
detected surface potential enables to obtain a copy with white background 
from an original with colored background or from a newspaper. 
In FIG. 8, an area A0, for example ranging from 100 V to 130 V, is called 
automatic exposure insensitive area. Such area is provided in order to 
avoid defective image reproduction which may arise if the lighting voltage 
for the illuminating lamp 23 is elevated directly proportional to the 
surface potential, since a potential sensor 12 of insufficient resolving 
power may detect an original containing small characters on white 
background or a blueprint of low density as gray. 
Also an area A1, in excess of 200 V, is called an automatic exposure 
saturation area. In general the background of original documents is not 
black but may be colored to the extent of newspaper, roughly corresponding 
to a surface potential of 200 V. Therefore, a surface potential eventually 
detected as 300 V or 400 V would simply indicate that the potential sensor 
12 is eventually positioned at a solid black area. The above-mentioned 
saturation area is provided since satisfactory image reproduction cannot 
be expected if the lighting voltage of the illuminating lamp 23 is 
elevated in response to such high surface potential. 
The copier of the present embodiment can copy up to A3 size, but the 
preliminary scanning is conducted only in the minimum copiable B5 size, 
because preliminary scanning in such limited area can provide enough 
information on the original document and because preliminary scanning, if 
conducted in an area corresponding to the entire original size, will 
unnecessarily delay the first copying operation. However it is possible 
also to conduct said preliminary scanning in an area corresponding to the 
original size. 
The developing bias voltage in the automatic exposure mode may be 
determined by adding a determined voltage to the final light potential 
V.sub.L at the exposure control in the aforementioned manner, or by adding 
a determined voltage to the minimum (or mean) value of the surface 
potential determined in said preliminary scanning. 
The position of the potential sensor 12 is, as shown in FIG. 9, not at the 
center of the photosensitive drum 1 but is displaced slightly to a 
reference end of the image in order that said sensor is always positioned 
in the image area even in the modified size copying mode. 
In FIG. 9 there are shown a carriage glass capable of receiving and 
exposing for copying an original document of up to A3 size; an area 71 
which may not be exposed to the original image according to the image 
magnification; an area 72 for detection by the potential sensor 12 on the 
original in case of the real-size copying mode; an area 73 for detection 
by the potential sensor 12 on the original in case of the reduction 
copying mode; and a reference end 74 of the original. In such structure 
the distance l from the reference end 70 of the image to the potential 
sensor 12 can be represented as follows: 
EQU 0&lt;l&lt;.gamma..sub.min .times.L.sub.min 
wherein .gamma..sub.min is the minimum image magnification, and L.sub.min 
is the minimum dimension of the original. In FIG. 9 there is provided only 
one potential sensor 12, but it is also possible to provide plural sensors 
in the above-mentioned area. 
FIG. 10 represents the image control process of the present invention, 
wherein the potential control zones are made different according to the 
image magnification and the process speed. 
Since the reciprocity rule does not stand in strict sense in the modified 
size copying mode as explained before, the zones for potential control are 
made different according to the image magnification and the process speed. 
In the present embodiment the illumination intensity is made uniform and 
constant regardless of the image magnification by the correction already 
explained in relation to FIG. 5-2. 
Also the process speed need not be changed for ordinary copying operations 
since the mechanical designing can sufficiently cope with the increased 
scanning speed at the reduction copying mode. The copier of the present 
embodiment is however provided with a manual feed tray 51a, as shown in 
FIG. 5-1, for copying on a manually fed sheet, said manual feed mode being 
activated by a selector switch 52 to be actuated by said manual feed tray 
51a when it is moved to a position 51b. 
In the copier of the present embodiment, therefore, a lower process speed 
is employed in such manual feed mode in order to interrupt the operation 
in time in case of erroneous feeding of the manually inserted sheet and to 
achieve sufficient image fixing in the fixing station 9 even on a thick 
sheet manually inserted. 
FIG. 10A shows an embodiment of the present invention utilizing four 
different potential control zones A, B, C, D according to the image 
magnification and the process speed. The zone A is used for the high 
process speed for normal copying operation with the image magnification 
within a range from 0.63 to 0.95. The zone B is used for the high process 
speed for normal copying operation with the image magnification within a 
range from 0.95 to 1.41. The zone C is used for the low process speed for 
manually inserted or thick sheets with the image magnification in a range 
from 0.63 to 0.95. The zone D is used for the low process speed for 
manually inserted or thick sheet with the image magnification in a range 
from 0.95 to 1.41. 
There may also be employed six potential control zones A-G, as shown in 
FIG. 10B, according to the extent of the reciprocity failure 
FIG. 10C shows another embodiment in which the low process speed, the same 
as that for the manually inserted or thick sheet is also employed in the 
reduction copying mode with image magnification in a range from 0.63 to 
0.95. 
FIG. 10D shows still another embodiment in which the process speed for the 
manually inserted or thick sheets is selected different from the low 
process speed for normal reduction copying mode. In such case the 
potential control zones B and C may be used in common. 
In any case the potential control zones may be divided according to the 
characteristic of latent image formation of the photosensitive member, or, 
the extent of reciprocity failure. 
FIG. 11 shows the target values of the light and dark potentials, the 
initial values of the reference current to the charger 3 and of the 
reference lighting voltage of the illuminating lamp 23, the reference 
current and the reference lighting voltage of the illuminating lamp 23 
corrected in the preceding cycle, and the reference lighting voltage of 
said lamp 23 in the automatic exposure control, in case the potential 
control zones are divided as shown in FIG. 10A. As will be understood from 
FIG. 10A, the target values V.sub.DO and V.sub.LO of the dark and light 
potentials are selected constant in all the potential control zones. On 
the other hand, the initial values I.sub.pl and V.sub.Hl of the reference 
current of the charger 3 and the lighting voltage of the illuminating lamp 
23 at the first potential control are selected different between the zones 
A, B and the zones C, D. The control unit 14 shown in FIG. 6 is provided 
with a memory for storing the reference current I.sub.pn and the reference 
lighting voltage V.sub.Hn corrected in the preceding cycle for each zone, 
and such corrected values I.sub.pn, V.sub.Hn are used as initial values in 
the succeeding cycle. 
The surface potential control is conducted according to the following 
correction formulas: 
(1) Correction of the reference current I.sub.p in response to dark 
potential: 
EQU I.sub.p2 =.alpha.(V.sub.D -V.sub.D1)+I.sub.p1 
EQU I.sub.pn+1 =.alpha.(V.sub.D -V.sub.Dn)+I.sub.pn 
in which I.sub.pn .ltoreq.I.sub.p limit, n.ltoreq.4 
wherein: 
I.sub.pl : initial value of reference current 
I.sub.pn : reference current after control of (n-1) times 
V.sub.D : target value for dark potential 
V.sub.D1 : dark potential corresponding to reference current I.sub.pl 
V.sub.Dn : dark potential corresponding to reference current I.sub.pn, i.e. 
after control of (n-1) times 
I.sub.p limit : limit of the power supply .alpha.: correction coefficient 
(2) Correction of the lighting voltage V.sub.H in response to the light 
potential 
EQU V.sub.H2 =.beta.(V.sub.L -V.sub.L1)+V.sub.H1 
EQU V.sub.Hn+1 =.beta.(V.sub.L -V.sub.Ln)+V.sub.Hn in which V.sub.Hn 
.ltoreq.V.sub.H1 limit,n.ltoreq.4 
wherein: 
V.sub.H1 : initial value of reference lighting voltage for the illuminating 
lamp 23 
V.sub.Hn : reference lighting voltage after controls of (n-1) times 
V.sub.L target value for light potential 
V.sub.L1 : light potential corresponding to the initial value V.sub.H1 of 
the reference lighting voltage 
V.sub.Ln : light potential corresponding to the reference lighting voltage 
V.sub.Hn, i.e. after controls of (n-1) times 
V.sub.H1imit : limit of the lighting power source 
.beta.: correction coefficient 
The reference lighting voltage V.sub.H..sub.AE of the illuminating lamp 23 
in the automatic exposure mode is corrected for each zone in response to 
the reference current I.sub.pn and the reference lighting voltage V.sub.Hn 
corrected in the preceding cycle, according to the following correction 
formulas: 
##EQU1## 
wherein: 
.DELTA.V.sub.H..sub.AE amount of correction of the reference lighting 
voltage of illuminating lamp 23 in automatic exposure control 
V.sub.Ln+1 : final light potential after controls of n times 
V.sub.L.AE : mean (or minimum) potential at the original exposure in the 
automatic exposure scanning 
.gamma.: correction coefficient 
.DELTA.V.sub.H..sub.AElimit : upper limit of amount of correction of the 
reference lighting voltage of the illuminating lamp 23 in the automatic 
exposure control. 
FIG. 12 is a flow chart showing the control flow of the present invention. 
After the start of power supply, a step 1 performs the pre-rotation step 
of the photosensitive drum for electrostatic cleaning of the drum surface. 
Then a step 2 discriminates whether the copy key has been actuated, and, 
if so, a step 3 executes the pre-rotation step in the same manner Then a 
step 4 and succeeding steps execute a control rotation step for obtaining 
a desired surface potential. 
Steps 4 to 6 identify the zone A, B or C according to the entered image 
magnification and the process speed, and, if the zone A is selected, the 
program proceeds to a step 7 to effect the potential control corresponding 
to said control zone A shown in FIG. 11. Also the program proceeds to a 
step 8 or 9 if the zone B or C is respectively selected, or otherwise to a 
step 10, thereby effecting the potential control corresponding to the zone 
B, C or D. 
In the following there will be explained, as an example, the potential 
control in the zone A. At first a step 7-1 identifies if the zone A is 
selected for the first time, and, if so, the program proceeds to a step 
7-2 for reading, from the memory of the control unit 14, the reference 
current I.sub.pl.sup.(A,B) for the charger 3 and the lighting voltage 
V.sub.H1.sup.(A,B) of the illuminating lamp 23, which are common for the 
zones A and B. Then the program proceeds to a step 7-5. On the other hand, 
if the selection of the zone A is not for the first time, the final values 
I.sub.pn, V.sub.Hn at the preceding control are read, from the memory of 
the control unit 14, as the initial values I.sub.pl, V.sub.H1 of the 
reference current of the charger 3 and of the lighting voltage for the 
illuminating lamp 23, and step 7-4 identifies whether the time from the 
previous selection of the zone A is within 1 minute. The program proceeds 
to a step 11 if said time is within 1 minute. If not, the program proceeds 
to steps 7-5, 7-6 for effecting the control to bring the dark potential 
V.sub.D to the target value V.sub.DO and to bring the light potential V to 
the target value V.sub.LO, as will be shown in detail in FIG. 13 
Now there will be explained the procedure of controlling the dark potential 
V.sub.D. At first a step 7-5-1 discriminates whether the initial value 
I.sub.pl of the reference current is below the limit value I.sub.p limit, 
and, if so, a step 7-5-2 is executed to detect, by the sensor 12, the dark 
potential V.sub.Dn generated on the photosensitive drum 1 when said 
reference current I.sub.pl is given to the charger 3 and to supply the 
detected potential to the control unit 14 after amplification in the 
amplifier 17 and digital conversion by the A/D converter 21. Then a step 
7-5-3 discriminates whether said dark potential V.sub.D1 is positioned 
within the target range, and, if not, a step 7-5-4 identifies whether the 
number of controls is within four, and, if so, a step 7-5-5 calculates 
I.sub.p2 according to the aforementioned formula: 
EQU I.sub.pn+1 =.alpha.(V.sub.DO -V.sub.Dn)+I.sub.pn. 
Then the program returns to the step 7-5-1 to repeat the control with thus 
calculated value I.sub.p2. The program proceeds to a step 7-6 after 
repeating the above-described control four times, or if the reference 
current I.sub.pn exceeds the upper limit I.sub.p limit in the step 7-5-1 
or the dark potential V.sub.Dn falls within the target range in the step 
7-5-3. 
After the completion of the control of dark potential V.sub.D in the step 
7-5, the step 7-6 executes the control of the light potential V.sub.L as 
will be explained in the following in relation to FIG. 13. At first a step 
7-6-1 compares the initial value V.sub.H1 of the lighting voltage of the 
illuminating lamp with the upper limit V.sub.H1imit, and, if said initial 
value is below said upper limit, a step 7-6-2 is executed to light the 
lamp 23 with said voltage V.sub.H1 thereby forming the reflected image of 
the standard white plate 39 on the photosensitive drum 1, and to detect 
the surface potential V.sub.L1 with the potential sensor 12. Then a step 
7-6-3 discriminates whether thus detected potential is within the 
tolerance range of the target value V.sub.LO and, if not, a step 7-6-4 
identifies whether the number of controls is within three. As the number 
of controls is within three in this state, a step 7-6-5 is executed to 
calculate V.sub.H2 according to the formula: 
EQU V.sub.Hn+1 =.beta.(V.sub.LO -V.sub.Ln)+V.sub.Hn 
and the program returns to the step 7-6-1 for repeating the same control. 
The program proceeds to a step 7--7 after repeating the above-described 
control three times, or if the lighting voltage V.sub.Hn exceeds the upper 
limit V.sub.H1imit in the step 7-6-1 or if the light potential falls 
within the target range in the step 7-6-3. 
The step 7-7 stores the reference current I.sub.pn and the lighting voltage 
V.sub.Hn, corrected in the steps 7-5 and 7-6, into the memory of the 
control unit 14. 
In case the zone B, C or D is selected in the steps 4 to 6, a similar 
control is executed respectively in a step 8, 9 or 10 according to the 
control values corresponding to thus selected zone. 
Then a step 11 discriminates whether the automatic exposure mode is 
selected, and, if not, the program proceeds to a step 13. 
On the other hand, if the automatic exposure mode is selected, the program 
proceeds to a step 12 to light the illuminating lamp 23 with the lighting 
voltage V.sub.Hn corrected in the above-described control, to scan the 
original to be copied in an area of B5 size for forming a corresponding 
electrostatic latent image on the drum 1, and to detect the potential 
thereof by the potential sensor 12 for determining the mean value 
V.sub.L..sub.AE of said potential. Thus the amount of correction 
V.sub.H..sub.AE for the lighting voltage of the illuminating lamp is 
determined according to the correction formula: 
EQU .DELTA.V.sub.H..sub.AE =.gamma..{V.sub.L..sub.AE -(V.sub.Ln+1 +30)} 
and the lighting voltage of said lamp 23 is obtained by adding said 
correction .DELTA.V.sub.H..sub.AE to the voltage V.sub.Hn. The 
above-described control may also be conducted with the minimum value of 
the potential of the latent image. 
Then a step 13 lights the illuminating lamp 23 with the voltage thus 
corrected and effects the ordinary original scanning, thereby forming an 
electrostatic latent image corresponding to the original document on the 
photosensitive drum 1. Said latent image is then developed with a 
developing bias voltage obtained by adding a determined voltage to the 
mean (or minimum) value of the surface potential determined in the 
preliminary scanning in case of the automatic exposure mode, or otherwise 
with a developing bias voltage obtained by adding a determined voltage to 
the final light potential controlled in the step 7-6. After the completion 
of copying operations of the entered copy number, a step 14 effects a 
post-rotation step of the photosensitive drum 1 for electrostatically 
cleaning the photosensitive member. 
In an apparatus in which the original scanning is carried out during the 
backward motion of the optical system, it is possible to effect said 
preliminary scanning during the forward motion of the optical system and 
to effect the usual scanning during the backward motion. FIG. 14-1 is a 
timing chart showing the function in such case after the copy key is 
actuated for making two copies in the automatic exposure mode. The forward 
and backward motion of the optical system is realized respectively by 
energization of a forward clutch and a backward clutch. 
Upon actuation of the copy key 53 at a time T1, the blank exposure lamp 2, 
pre-charger 7, primary charger 3 etc. are activated and the pre-rotation 
step is started to electrostatically cleaning the surface of the 
photosensitive drum 1. Thereafter the first control rotation is started 
from a time T2, in which the blank exposure lamp 2 is extinguished and the 
dark potential V.sub.D formed on the photosensitive drum is measured by 
the potential sensor 12 to control the reference current for the primary 
charger 3. In the present embodiment there are conducted four measurements 
and four controls, but it is also possible to vary the number of such 
controls according to the unmanipulated time of the apparatus. 
Subsequently, at a time T3 the second control rotation is initiated, in 
which the illuminating lamp 23 is lighted in combination with the 
reference current controlled in the aforementioned first control rotation, 
thereby illuminating the photosensitive drum 1 with the light reflected by 
the standard white plate 39. The illumination intensity of said 
illuminating lamp 23 is controlled by the measurement of the surface 
potential V.sub.L in this state. The number of controls for the 
illuminating intensity of said lamp may also be rendered variable 
according to the unmanipulated time of the apparatus. In the present 
embodiment said control procedure consists of three measurements and three 
controls. The finally controlled light of said lamp 23 is again reflected 
by the standard white plate 39 and the corresponding surface potential of 
the drum is measured. The developing bias is determined in response to 
said surface potential, as explained before, in case the automatic 
exposure mode is not selected. 
Then the forward clutch is energized at a time T4 to initiate the forward 
motion of the optical system, thereby effecting the preliminary scanning 
of the original document and forming a corresponding electrostatic latent 
image on the photosensitive drum. The potential of said latent image is 
measured by the potential sensor 12 and the developing bias is determined 
from the mean value of said potential. 
Then at a time T5, the backward clutch is energized to initiate the 
backward motion of the optical system, thereby effecting the ordinary 
original scanning and forming an electrostatic latent image on the 
photosensitive drum 1. Said latent image is developed with the developing 
bias determined in the aforementioned manner, and the obtained image is 
transferred onto a first transfer sheet. 
Subsequently, at a time T6, the optical system starts the second scanning 
motion to effect the ordinary scanning of the original document in the 
backward motion of said optical system, thereby achieving the image 
formation in the aforementioned manner. 
At a time T7, the post-rotation step is started to electrostatically 
cleaning the photosensitive drum, which is stopped thereafter. 
In an apparatus in which the ordinary original scanning is carried out 
during the forward motion of the optical system, the preliminary scanning 
can be carried out in an area corresponding to the minimum copiable 
original size, prior to the first scanning for image formation. 
In the foregoing embodiment, the potential control for each zone is carried 
out after the actuation of the copy key, but it is also possible to effect 
such potential control after the start of power supply. FIG. 14-2 is a 
timing chart showing the function in such case, in which the original 
scanning is carried out during the forward motion of the optical system. 
FIG. 14-2 shows a case of making two copies in the automatic exposure 
mode. In this case, the preliminary scanning for detecting the background 
density of the original document is effected prior to the ordinary 
original scanning. Such preliminary scanning need not be effected over the 
entire area of the original document but may be effected over a distance 
equal to a half of the ordinary scanning distance, or over an area 
corresponding to the minimum copiable original size. In the present 
embodiment the preliminary scanning is effected in an area of B5 size. 
After the start of. power supply, controls same as those at T1 to T4 in 
FIG. 14-1 are conducted at timings T1 to T4. Upon actuation of the copy 
key at a timing T5, the forward clutch is energized to start the forward 
motion of the optical system, thereby initiating the preliminary scanning. 
After such scanning motion over a distance corresponding to the B5 size, 
the illuminating lamp 11 and the backward clutch are turned on at a timing 
T6 to reverse the optical system thereby forming an electrostatic latent 
image corresponding to the original image on the photosensitive drum 1. 
The potential sensor 12 measures the potential of said latent image, and 
the mean value of said potential is obtained. In the automatic exposure 
mode, the developing bias is determined in the aforementioned manner. 
The original scanning is repeated twice in the forward motion of the 
optical system at timings T7 to T9, thereby providing two copies. 
Subsequently the post-rotation step is conducted in the same manner as in 
the case of FIG. 14-1. 
The above-described timing controls are achieved by counting the clock 
pulses supplied by the clock pulse generator 31 to the control unit 14. 
Also in the automatic exposure mode, it is possible to employ the minimum 
value or integrated value of the potential instead of the mean value 
thereof. Furthermore it is possible to detect the light reflected from the 
original document with a photosensor. 
Although the potentials V.sub.D and V.sub.L are independently controlled in 
the foregoing embodiment, these two potentials may also be defined as a 
function of the potential V.sub.D, for example V.sub.L =(V.sub.D 
-300V).+-.15V. Such method is advantageous as it is capable of controlling 
the contrast V.sub.D -V.sub.L within a determined range even when the 
potential V.sub.D cannot be sufficiently stabilized. In such case the 
developing bias voltage can also be defined for example by V.sub.B 
=(V.sub.L +50V).+-.15 V, and it is thus rendered possible to mutually 
correlate the contrast and the development level. 
As explained in the foregoing, the present invention, employing different 
control zones according to the image magnification and the process speed, 
enables constant image formation with stable image density and stable 
intermediate tones regardless of the image magnification or the process 
speed. 
Also the waste in control time can be avoided since the correction of the 
charging current and of the lighting voltage of the illuminating lamp is 
terminated after the repetition of predetermined times. 
Furthermore, the correction of image forming conditions in response to the 
condition of the original after the control of the image forming 
conditions in each control zone allows optimum image formation for various 
original documents.