Lens and shutter positioning mechanism for variable-magnification copier

Apparatus for repositioning the lens of a variable-magnification electrophotographic copier to change the image-side path length for a different magnification ratio and for concomitantly adjusting the shutter to compensate for the changed image-side path length. Actuation of a stepper motor rotates a pulley to tension a cable coupled to the lens, pulling the lens to the desired position of a linear track. A portion of the cable extends around a pulley having an axially projecting circumferentially extending ramp on one face. A follower engaging the ramp is coupled to slit-forming members adjacent to the photoconductor so that movement of the cable to reposition the lens produces a concomitant adjustment of the width of the slit to equalize exposure of the photoconductor over a substantially continous range of selected magnifications.

FIELD OF THE INVENTION 
Our invention relates to apparatus for repositioning a lens or other 
optical element of a variable-magnification optical system, such as that 
of an electrophotographic copier, to alter the optical path length for 
operation at a different reproduction ratio and for concomitantly 
adjusting the exposure to compensate for the altered optical path length. 
BACKGROUND OF THE INVENTION 
Electrophotographic copiers capable of variable magnification are well 
known in the art. Generally, the optical system of such a copier includes 
a lens for forming a focused image of at least a strip portion of an 
original document on the photoconductor. An electrostatic latent image 
corresponding to that of the original is formed on the photoconductor, 
which has been previously uniformly electrically charged, by moving the 
photoconductor at a uniform velocity through an exposure station while 
simultaneously effecting relative movement between the optical system and 
the original document. This relative movement may be accomplished either 
by moving the document past an optical system consisting of fixed elements 
or by using one or more movable mirrors to scan a stationary original. To 
change the reproduction ratio of such an optical system one must alter 
both the object distance between the lens and the original document and 
the image distance between the lens and the photoconductor. Generally, 
this is accomplished by moving either the lens or an image-side mirror to 
alter the image distance while concomitantly moving an object-side mirror 
to alter the object distance. 
One of the problems inherent in variable-magnification copiers of the prior 
art is that of maintaining the exposure of the photoconductor surface 
constant for various selected magnifications. In general, for a constant 
document illumination the brightness of the optical image on the 
photoconductor varies inversely with the square of the image distance 
between the lens and the photoconductor. This distance in turn varies with 
the selected magnification. Previous systems have compensated for this 
variation in brightness of the optical image by varying the width of a 
transversely extending optical slit, adjacent to the photoconductor, in 
accordance with the selected magnification. Ikeda et al U.S. Pat. No. 
4,125,323 discloses one such system in which an eccentric cam coupled to a 
lens-positioning motor engages a follower coupled to a shutter member 
adjacent to the photoconductor. While this system does provide some 
exposure correction for variations in brightness due to changes in image 
distance, the particular system disclosed is relatively complicated 
mechanically. In addition, the contour of the cam is such that the actual 
compensation can equal the required compensation at only a limited number 
of magnifications. While this may be adequate in a system such as the one 
disclosed, in which only two reproduction ratios are contemplated, it 
would fail to provide accurate exposure correction in a system in which 
the lens position is continuously adjustable to provide continuously 
variable magnification. Any discrepancy between the actual exposure 
correction and the desired correction would be particularly evident in a 
system in which the selected magnification varies between widely spaced 
limits, such as between 0.5 and 1.5. 
SUMMARY OF THE INVENTION 
In general, our invention contemplates a lens and shutter positioning 
mechanism for a variable-magnification copier in which a lens mounted for 
movement on a linear track is coupled by means of a cable to the pulley of 
a stepper motor which is actuated to move the lens to the desired position 
along the track. A portion of the cable extends around a pulley having an 
axially projecting circumferentially extending ramp on one face thereof. A 
follower engaging the ramp is coupled to slit-forming members adjacent to 
the photoconductor so that movement of the cable to reposition the lens 
produces a concomitant adjustment of the width of the slit to equalize 
exposure of the photoconductor over a substantially continuous range of 
selected magnifications. 
OBJECTS OF THE INVENTION 
One object of our invention is to provide apparatus for positioning the 
lens and shutter of a scanning system that provides accurate exposure 
compensation for variations in image brightness over a wide range of 
selected magnifications. 
Another object of our invention is to provide a system for positioning the 
lens and shutter of an optical scanning system that is mechanically 
simple. 
Other and further objects of our invention will be apparent from the 
following description.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIG. 1, a copier indicated generally by the reference 
numeral 10 incorporating our lens and shutter positioning mechanism 
includes a housing 12, the upper wall of which supports a transparent 
exposure platen 14 for receiving an original document 16. Copier 10 
includes an electrophotographic imaging drum, indicated generally by the 
reference numeral 20, mounted on a shaft 22 for rotation therewith and 
having a photoconductor 24 supported by a conductive substrate 26. Drum 20 
is driven at a substantially uniform velocity in a manner to be described. 
In a manner well known in the art, the drum photoconductor 24 is rotated 
first past a charging station C at which the surface of the photoconductor 
receives a uniform electrostatic charge, then past an exposure station E 
at which the electrostatically charged surface is exposed to an optical 
image of the document 16 on the platen 14 to form an electrostatic latent 
image, then past a developing station D at which a liquid developer 
containing charged toner particles is applied to the latent-image-bearing 
surface to form a developed toner image, and finally to a transfer station 
T at which the developed toner image is transferred from the 
photoconductor 24 to a carrier sheet P. 
The optical scanning system of the copier 10, indicated generally by the 
reference numeral 18, includes a first, or full-rate, scanning carriage 
indicated generally by the reference numeral 28. Full-rate carriage 28 
supports an elongated exposure lamp 30, an elliptical reflector 32 which 
focuses light from the lamp 30 onto a transversely extending linear strip 
of the document 16, and a mirror 34 arranged to receive light reflected 
from the illuminated portion of the document 16. 
A second, or half-rate, scanning carriage indicated generally by the 
reference numeral 36 supports an upper mirror 38 and a lower mirror 40. 
Mirror 34 of the full-rate carriage 28 reflects light from the document 16 
to upper mirror 38 of half-rate carriage 36 along a path segment a 
parallel to the imaging platen 14. Mirror 38 in turn reflects the light 
downwardly onto lower mirror 40, which reflects the light along the 
optical axis b of a lens, indicated generally by the reference numeral 42, 
which is parallel to platen 14 and path segment a. A stationary mirror 44 
disposed on the other side of lens 42 from mirror 40 reflects the light 
downwardly onto an upwardly facing stationary mirror 46. Mirror 46 abuts a 
horizontally extending partition 52 isolating the scanning system 18 from 
the processing portion 54 of the copier 10 disposed therebelow. A 
downwardly facing mirror 48 reflects light from mirror 46 through a 
transversely extending slot 50 in partition 52 onto the portion of the 
photoconductor 24 passing through the exposure station E. Respective 
slit-forming members 144 and 146, to be described below, adjacent to the 
drum 20 in the exposure station E, regulate the exposure of the 
photoconductor 24 to the optical image of the original 16. 
In the case of a one-to-one reproduction ratio, the drum 20 is rotated 
counterclockwise as viewed in FIG. 1 at a predetermined surface speed 
while full-rate scanning carriage 28 is simultaneously moved at the same 
speed from the position shown in solid lines in FIG. 1 to a displaced 
position such as the position 28' shown in phantom lines in the same 
figure to scan a document 16 placed on platen 14. Simultaneously with the 
movement of drum 20 and full-rate carriage 28, half-rate carriage 36 is 
moved in the same direction as full-rate carriage 28, but at half the 
speed, between the position shown in solid lines in FIG. 1 and the 
position 36' shown in phantom lines in the same figure to maintain a 
constant optical path length between document 16 and photoconductor 24. At 
the end of the forward scanning stroke, scanning carriages 28 and 36 are 
moved in the reverse direction to their original positions in preparation 
for another scanning cycle. 
The operation of the scanning system 18 for reproduction ratios other than 
one-to-one is generally similar except that the full-rate and half-rate 
carriages are moved at velocities equal respectively to Vp/m and Vp/2m, 
where Vp is the peripheral velocity of the photoconductor drum 20 and m is 
the selected magnification. Further, if the copy length remains the same, 
the scanning length is changed to L/m, where L is the scanning length for 
a one-to-one reproduction ratio. Thus, for a 1.5:1 reproduction ratio 
(m=1.5), carriages 28 and 36 are driven at two-thirds and one-third the 
peripheral drum velocity, respectively, and are moved through 
displacements respectively equal to two-thirds and one-third the desired 
image length. Although the system for controlling the movement of 
carriages 28 and 36 does not as such form part of the present invention, a 
more detailed description of the scanning system 18 may be found in the 
copending application of applicant Benzion Landa et al, Ser. No. 628,239, 
filed July 6, 1984, entitled "Optical Scanning System for 
Variable-Magnification Copier". 
In general, the object distance p between the lens 42 and the original 
document 16 and the image distance q between the lens and the imaging 
surface 24 are related by the equation 
EQU 1/p+1/q=1/f, (1) 
where f is the focal length of the lens 42. Since the image magnification m 
is given by the equation 
EQU m=q/p, (2) 
we may solve for p and q in terms of m and f, and obtain 
EQU p=(1+1/m)f, (3) 
EQU q=(m+1)f, (4) 
and 
EQU p+q=(m+1).sup.2 f/m. (5) 
From these relations, it follows that for a given reproduction ratio the 
lens 42 must be shifted from its position for one-to-one magnification by 
a distance 
EQU q=(m-1)f. (6) 
The lens 42 is shifted to the right as viewed in FIG. 1 for enlargements 
and to the left for reductions. Further, to obtain the proper total path 
length p+q for a given magnification, the half-rate carriage 36 must be 
shifted to the right, relative to full rate carriage 28, by a distance 
EQU y=(m-1).sup.2 f/2m. (7) 
The required shifting of half-rate carriage 36 is accomplished in the 
manner disclosed in the copending application of applicant Benzion Landa 
et al referred to above, Ser. No. 628,239, filed July 6, 1984. 
Referring now to FIGS. 2 to 4, the lens and shutter positioning mechanism, 
indicated generally by the reference numeral 56, includes a rectangular 
lens holder 58 extending generally transversely of the lens axis and 
formed with a circular opening 60 for receiving the barrel 62 of the lens 
42. Respective screws 68 and 70 secure the lens barrel 62 to lugs 64 and 
66 of lens holder 58. Referring now particularly to FIG. 3, lens holder 58 
is formed with an upper lip 72 and rear and front lower lips 74 and 76 to 
ensure smoothness of movement along a guideway defined by a housing 78 
(not shown in FIG. 2) and the partition 52. A track indicated generally by 
the reference numeral 80, secured to the partition 52, supports lens 42 
for sliding movement along the optical axis b. As shown in FIG. 3, track 
80 is formed with transversely spaced, longitudinally extending rounded 
edge portions 82 and 84, along which lens barrel 62 slides, as well as 
with longitudinally extending lower edges 86 and 88 against which lower 
lips 74 and 76 bear to maintain the lens 42 on the track 80. 
A clamp 90 carried by lens holder 58 receives one end of a cord 92 which is 
tensioned in a manner to be described to move the lens 42 along the track 
80. Cord 92 passes around a first pulley 94, located at the end of track 
80 adjacent the exposure slit 50, and rotatably received by a shaft 96 
carried by partition 52. From pulley 94, cord 92 passes around a 
relatively large-diameter pulley, indicated generally by the reference 
numeral 120, to be described in more detail below. From pulley 120, cord 
92 successively passes around a pulley 116 rotating on a shaft 118 carried 
by partition 52, wraps once around a drive pulley 102, passes around an 
end pulley 98 supported by a shaft carried by partition 52, at the 
opposite end of track 80 from pulley 94, and returns to clamp 90 of lens 
holder 58, to which it is attached through a tension spring 128. A 
bi-directional stepper motor 112, supported on a bracket 114 carried by 
partition 52, drives pulley 102. The shaft 110 of stepper motor 112 
carries a gear 108 which meshes with a gear 106 carried by the shaft 104 
supporting pulley 102. 
A pin 122 carried by partition 52 supports pulley 120 for rotation about a 
vertical axis. A peripheral groove 124 of pulley 120 receives cable 92, 
which extends around an approximately semi-circular portion of pulley 120. 
If desired, to prevent slippage between cable 92 and pulley 120, cable 92 
may wrap around the pulley one or more times or may be secured to the 
pulley 120 at a predetermined point along its periphery. Pulley 120 is 
formed on its upper face with an annular incline or cam surface 126, the 
height of which varies angularly about the pulley 120 in a manner to be 
described. 
Referring now also to FIG. 4, a pair of shutter elements 140 and 142 
extending radially of the axis of the drum 20 have lower portions 144 and 
146 extending circumferentially of the drum surface 24, which regulate the 
width of the optical image formed on the drum surface. Shutter elements 
140 and 142 carry respective lugs 148 and 150 which are rotatably 
supported by pins 152 and 154 carried by a bracket 136 supported by 
partition 52 at one end of slot 50. A corresponding bracket (not shown) 
pivotally supports similar lugs (not shown) formed at the ends of shutter 
elements 140 and 142 adjacent to the rear of the copier 10. The bracket 
136 at the front of the copier 10 also carries a pin 134 pivotally 
supporting an arm 132 formed with a follower 130 engaging the cam surface 
126 of pulley 120. A pin 138 carried by follower arm 132 at the end remote 
from follower 130 bears against lugs 148 and 150 of shutter elements 140 
and 142. Any suitable means (not shown) may be used, if desired, to assist 
gravity in resiliently biasing follower 130 against cam surface 126, as 
well as to bias lugs 148 and 150 against pin 138. 
It will be apparent from the foregoing description that in response to 
rotation of pulley 120 upon actuation of motor 112 to vary the position of 
lens 42, pin 138 bears against lugs 148 and 150 to move slit-forming 
portions 144 and 146 either toward or away from each other, depending on 
the direction of rotation of pulley 120. It will be further apparent that 
the angle subtended by the adjacent edges of slit-forming portions 144 and 
146, relative to the axis of the drum 20, determines the effective 
circumferential extent of the exposure station E, and hence the duration 
of exposure of any one point on the surface 24 of drum 20 to an optical 
image of the original 16. Accordingly, the cam surface 126 is so 
calibrated as to provide an exposure window to the drum 20, the angular 
width of which varies directly with the square of the distance along the 
optical path between the lens 42 and the drum surface 24. In such a 
manner, the exposure of the drum surface 24 to an optical image of the 
original 16 can be corrected for variations in image intensity over a wide 
range of selected magnifications, such as between 0.5 and 1.56. 
The use of pulley 120, with its axially projecting cam surface 126, allows 
the use of a very simple linkage between the lens 42 and the shutter 
elements 140 and 142. As shown in FIG. 2, the cable system for regulating 
the position of the lens 42 is most advantageously disposed within a 
single horizontal plane. On the other hand, the movement of the shutter 
elements adjacent to the surface 24 of drum 20 occurs in a vertical plane, 
parallel to the front or back of the copier 10. Pulley 120, with its cam 
surface 126, effectively converts motion of the cable 92 in the horizontal 
plane to motion of the follower arm 132 in a longitudinal vertical plane, 
without any complicated linkage. 
Referring to FIGS. 2 to 4, we provide an adjustable abutment or limit stop 
160 defining a limit position of lens 42 at one end of the track 80, in 
this case the end adjacent to the slot 50. Limit stop 160 is formed at one 
end of an adjustment arm 156 carried by a pivot 158 secured to partition 
52. A portion of arm 156 extending outwardly through a slot formed in a 
frame portion 168 of the copier 10 is formed with a slot 162 extending 
circumferentially with respect to the axis defined by pivot 158. A screw 
164 extending through slot 162 and threadably received by a bracket 66 
carried by frame portion 168 is normally tightened against arm 156 and 
bracket 166 to immobilize the arm 156 against rotation. Screw 164 may be 
loosened by a serviceman, however, to adjust the position of abutment 160 
relative to track 80. 
Referring now to FIG. 5, the control circuit for regulating the position of 
lens 42 and shutter elements 140 and 142 is indicated generally by the 
reference numeral 170. The circuit 170 includes a magnification selector 
172 of any suitable type known in the art for providing a multiple-channel 
digital signal, shown as being on a single line in FIG. 5 for convenience 
of exposition, indicating the selected magnification m. A presettable 
up-down counter 174 stores the current position of the lens 42 along the 
track 80. The scale factor and offset of the lens position as indicated by 
counter 174 are such that when the lens 42 is at the proper position for a 
selected magnification m, the output of counter 174 is equal to the 
se1=cted magnification. A digital comparator 176 responsive to the outputs 
of magnification selector 172 and counter 174 provides respective outputs 
to AND gates 180 and 182 indicating a lens position to the right or to the 
left of the proper position for the selected magnification m as viewed in 
FIG. 2. Each of AND gates 180 and 182 also receives an input from a pulse 
generator 178. AND gate 180 drives the down input of counter 174 as well 
as one input to stepper motor 112. Likewise, AND gate 182 drives the up 
input to up-down counter 174, as well as the other input to stepper motor 
112. 
If lens 42 is to the left of the proper position for the selected 
magnification ratio m, as viewed in FIG. 2, comparator 176 supplies a 
signal to AND gate 182, causing that gate to supply a pulse input to one 
directional input of stepper motor 112, as well as to the up input of 
position counter 174. This pulse output from AND gate 182 drives stepper 
motor 112 in such a direction as to move lens 2 to the right as viewed in 
FIG. 2. At the same time, the pulse output from AND gate 182 increments 
the position count contained in counter 174. When the position count 
reaches a value corresponding to the selected magnification m, comparator 
176 will terminate the input to gate 182, thereby terminating the pulse 
train to stepper motor 112 and position counter 174. Lens 42 is thus moved 
to the proper position for the selected magnification, which position is 
stored in counter 174. The operation of circuit 170 to move lens 42 to the 
left as viewed in FIG. 2 is similar, except that the down input of 
position counter 174 and the left directional input of stepper motor 112 
are actuated by pulses from AND gate 180. To correct the position of lens 
42 for slippage between cable 92 and motor pulley 102, magnification 
selector 172 may be periodically actuated so as to provide a selected 
magnification signal m sufficiently low (less than 0.5) to drive lens 
holder 58 against abutment 160, inducing slippage between the cable 92 and 
motor pulley 102. Thereafter, position counter 174 may be preset, by way 
of an input on a preset line 184, to a count corresponding to the nominal 
position of the stop 160. 
It will be seen that we have accomplished the objects of our invention. Our 
lens and shutter positioning mechanism, while being simple and 
inexpensive, provides accurate exposure compensation for variations in 
image brightness over a wide range of selected magnifications. 
It will be understood that certain features and subcombinations are of 
utility and may be employed without reference to other features and 
subconbinations. This is contemplated by and is within the scope of our 
claims. It is further obvious that various changes may be made in details 
within the scope of our claims without departing from the spirit of our 
inventnon. It is, therefore, to be understood that our invention is not to 
be limited to the specific details shown and described.