Driving method and apparatus for illumination type imaging system

A reciprocating illumination type imaging system is driven by a first drive source and a rotatable photoconductive drum is driven by a second drive source. A time period necessary for the drum to rotate a given angle is measured. The measured time period is compared with a predetermined reference time period to thereby estimate a fluctuation component in the time period of rotation of the drum in accordance with a result of the comparison. A start or buildup target speed during a buildup scanning speed motion of the imaging system and a constant target speed during a constant scanning speed motion are individually preset or determined in accordance with the estimated fluctuation component. The imaging system comprises an optical lens having a given focal length. The first and second drive sources constitute a drive mechanism. The preset buildup target speed and/or the preset constant target speed is compensated in conformity with a deviation of the focal length of the lens from a nominal focal length and a deviation of a mechanical precision of the drive mechanism from a nominal mechanical precision. Further, the speed of the imaging system during a constant scanning speed motion is detected. The preset constant target speed is compensated in accordance with the detected speed of the imaging system.

BACKGROUND OF THE INVENTION 
The present invention generally relates to a driving method and apparatus 
for driving a reciprocating illumination type imaging system in 
synchronism with a moving photoconductive member in electrostatic 
photography. 
A variety of instruments are known in which an illumination type imaging 
system moves along a predetermined path while a member to be imaged by the 
imaging system moves along a path which is different from the path of 
movement of the imaging system. In such a type of instrument, it is a 
prerequisite that the movement of the imaging system and that of the 
member to be imaged by synchronized or timed accurately to each other. 
Taking an electrostatic copying machine, for example, problems heretofore 
pointed out in connection with instruments of the type described will be 
discussed. 
In an electrostatic copying machine, the member to be imaged by the imaging 
system may comprise a rotatable photoconductive drum as well known in the 
art. For timed movements of the imaging system and drum, it has been most 
customary to employ a design wherein power is supplied from a single drive 
source to both the imaging system and the drum via chains or wires and the 
imaging system is driven for reciprocation through a reversible clutch 
positioned in a power transmission line to the imaging system. A problem 
inherent in this design originates from the use of a single motor as the 
drive source and chains or like members as the gearing. When the operation 
speed of the motor is varied significantly or the chain vibrated as a 
result of a fluctuation of a load, the rotating speed of the drum or the 
moving speed of the imaging system undergoes a fluctuation which 
critically deteriorates the resolution of the copier or invites jittering. 
An expedient hitherto proposed to settle this problem employs separate 
drive lines for the drum and imaging systems, respectively. Each of these 
drive lines is associated with a speed control line which is independent 
of the other but supplied with reference signals from a common reference 
oscillator. More specifically, the drum or the imaging system is driven by 
its own phase locked loop which comprises a phase comparator, an 
amplifier, a servo motor and an encoder. Though free from the drawback 
attributable to the use of a single drive source, this expedient requires 
a disproportionate cost since each of the drum drive line and imaging 
system drive line must be provided with an exclusive feedback control. 
The increase in cost may be avoided by another known expedient which 
controls the movement of the imaging system based on the movement of the 
drum. According to this expedient, whereas the drum is driven by a main 
motor, the imaging system is driven through a phase locked loop as in the 
first expedient which is electrically connected with an encoder associated 
with the drum. While such an expedient succeeds in simplifying the 
construction of the control system to reduce the cost, it still entails a 
drawback that a fluctuation of a load is directly reflected by that of the 
main motor, since the main motor drives not only the motor but other 
various sections of the printer in sequence. With this in view, the 
movement of the imaging system must be so controlled as to well follow 
substantial variations in the drum speed. However, the second expedient 
can not offer such a control but, rather, creates a substantial relative 
variation in speed between the drum and the imaging system with the 
resultant degradation to the quality of reproduced images. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a driving method and 
apparatus for driving an illumination type imaging system, thereby 
eliminating all the drawbacks inherent in the prior art apparatuses. 
It is another object of the present invention to provide a driving method 
for driving a reciprocating illumination type imaging system in 
synchronism with a moving photoconductive member in electrostatic 
photography. 
It is another object of the present invention to provide a driving 
apparatus for driving reciprocating illumination type imaging system in 
synchronism with a moving photoconductive member in an electrostatic 
copying machine. 
It is another object of the present invention to provide a generally 
improved driving method and apparatus for an illumination type imaging 
system. 
In accordance with the present invention, a reciprocating imaging system is 
driven by a first drive source and a moving photoconductive member is 
driven by a second drive source. A time period necessary for the 
photoconductive member to move a given distance is measured. The measured 
time period is compared with a predetermined reference period to thereby 
estimate a fluctuation component in the time period of movement of the 
photoconductive member in accordance with a result of the comparison. A 
buildup target speed during a buildup scanning speed motion of the imaging 
system and a constant target speed during a constant scanning speed motion 
of the imaging system are preset or determined in accordance with the 
estimated fluctuation component. 
The imaging system comprises an optical lens having a given focal length. 
The first and second drive sources constitute a drive mechanism. The 
preset buildup target speed and/or the preset constant target speed is 
compensated in conformity with a deviation of the focal length of the lens 
from a nominal focal length and a deviation of the mechanical precision of 
the drive mechanism from a nominal mechanical precision. Further, the 
speed of the imaging system during a constant scanning speed motion is 
detected and the preset constant target speed is compensated in accordance 
with the detected speed of the imaging system. 
Other objects, together with the foregoing, are attained in the embodiment 
described in the following description and illustrated in the accompanying 
drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
While the driving method and apparatus for an illumination type imaging 
system of the present invention is susceptible of numerous physical 
embodiments, depending upon the environment and requirements of use, 
substantial numbers of the herein shown and described embodiment have been 
made, tested and used, and all have performed in an eminently satisfactory 
manner. 
Referring to FIG. 1 of the drawings, a prior art driving apparatus 
comprises a reference oscillator 10, counters 12 and 14 for frequency 
conversion (frequency dividers), phase comparators 16 and 18, amplifiers 
20 and 22, servo motors 24 and 26 and encoders 28 and 30. The phase 
comparator 16, amplifier 20, servo motor 24 and encoder 28 constitute a 
phase locked loop PLL.sub.a which is associated with a photoconductive 
drum (not shown). This phase locked loop PLL.sub.a is supplied with an 
output of the counter or frequency divider 12 which is in turn supplied 
with an output of the reference oscillator 10. Likewise, the phase 
comparator 18, amplifier 22, servo motor 26 and encoder 30 constitute a 
second phase locked loop PLL.sub.b which is associated with an imaging 
system. The output of the common oscillator 10 is coupled to the counter 
or frequency divider 14 whose output is connected to the phase locked loop 
PLL.sub.b. With this construction, while a reciprocating mechanism of the 
imaging system is in a constant speed motion, the motor 24 for driving the 
drum and the motor 26 for driving the imaging system can operate in 
synchronism with each other. 
This prior art apparatus shown in FIG. 1 is free from the previously 
discussed drawback originating from the use of a single drive source for 
the drum and imaging system, offering quality reproduction of desired 
images. However, the independent phase locked loops for discrete feedback 
controls are undesirable from the standpoint of economy. 
Referring to FIG. 2, there is shown another prior art driving apparatus 
which is more economical than the apparatus of FIG. 1. In FIG. 2, whereas 
a photoconductive drum 32 is driven by a main motor 34, an imaging system 
is caused into a controlled movement which is subordinate to the movement 
of the drum 32. Associated with the drum 32 is an encoder 36 whose output 
is coupled to a counter or frequency divider 38 whose output is in turn 
coupled to a phase locked loop PLL.sub.b common to the phase locked loop 
PLL.sub.b of FIG. 1. 
The apparatus of FIG. 2 thus simplifies the control system and thereby cuts 
down the cost but still permits the main motor 34 to fluctuate in 
operating speed in accordance with a fluctuation in a load. This is 
because, apart from the drum 32, the main motor 34 drives various 
component sections of the printer in sequence during a copying cycle of 
the copying machine. Thus, the apparatus of FIG. 2 should allow a control 
on the movement of the imaging system to well follow any substantial 
fluctuation in the moving speed of the drum. However, in view of the 
inertia of the imaging system and the large mechanical time constant 
determined by the servo motor 26, such an apparatus cannot be expected to 
control the movement of the imaging system well following a fluctuation in 
the speed of the drum but, rather, it tends to create a significant 
variation in relative speed between the drum and the imaging system. The 
result is the degradation to the quality of reproduced images. 
Referring now to FIG. 3, a driving apparatus embodying the present 
invention is shown which is applied to an electrostatic copying machine by 
way of example. The copying machine includes a photosensitive drum 100 and 
a main motor 102 for driving the drum 100 and other component parts of the 
machine as usual. A light intercepting plate 104 is rigidly connected to 
the drum 100 to cooperate with a photointerrupter 106 which is mounted on 
a stationary frame member of the machine (not shown). The photointerrupter 
106 comprises a light emitting element and a light receiving element. 
There are also shown a control circuit 108, an encoder 110, a servo motor 
112 and an illumination type imaging system 114 which the apparatus of the 
present invention is to drive. 
During rotations of the drum 100, the light intercepting plate 104 rotating 
with the drum 100 intercepts the optical path between the light emitting 
and receiving elements 106 repeatedly at a predetermined angular position 
of the drum. The photointerrupter 106 therefore supplies the control 
circuit 108 with a train of pulses PHP each representing the predetermined 
drum angular position. 
Various combinations of the light intercepting plate 104 and 
photointerrupter 106 are illustrated in FIGS. 4a to 4c. In FIGS. 4a and 
4b, use is made of one photointerrupter 106 and one light intercepting 
plate 104 while, in FIG. 4c, use is made of two photointerrupters 
106.sub.1 and 106.sub.2 and one light intercepting plate 104. The or each 
photointerrupter 106 produces a train of pulses PHP each corresponding to 
a given angular position of the drum 100 which is dependent on the shape 
of the light interceptor 104 and the position of the photointerrupter 106, 
as the drum 100 is driven for rotation by the main motor 102. It will be 
seen that the number and arrangement of photointerrupters 106 and the 
shape of the light interceptor 104 are suitably selectable depending on 
the specific desired angular position or positions of the drum 100. 
Referring to FIG. 5, there is shown an example of the illumination type 
imaging system 114 which is driven for reciprocation by the servo motor 
112 (FIG. 3). The imaging system 114 includes a wire 202, turn pulleys 204 
and 206, a mirror pulley 208 and a pulley 210. The imaging system 114 also 
comprises a first mirror 212, a second mirror 214, a light intercepting 
plate 216, a first photointerrupter 218 responsive to a home position of 
the system, and a second photointerrupter 220 located at a specific 
distance from the home position photointerrupter 218. The first mirror 
212, light intercepting plate 216 and the like are connected with the wire 
202 by a wire clamp 222. The second mirror 214 is pivotally connected to 
the mirror pulley 208. During exposure, the imaging system 114 causes the 
first mirror 212 to move at a speed corresponding to the peripheral speed 
of the drum 100 and the second mirror 214 at a speed one half the speed of 
the first mirror 212. 
Now, the driving apparatus of the present invention drives the imaging 
system 114 while preventing it from following any fluctuation in the 
actual rotating speed of the drum. A procedure for attaining this purpose 
consists in measuring a time period necessary for the drum 100 to rotate a 
predetermined angular distance, comparing the measured actual time period 
with a given reference time period, estimating a fluctuation component of 
the actual time period for the predetermined angle of rotation which may 
occur when the reciprocating mechanism of the imaging system scans a 
document, presetting on the basis of the estimated fluctuation component a 
target speed for a buildup motion and/or a constant speed motion of the 
reciprocating mechanism, and actuating the servo system so that the 
imaging system may move at the preset target speed. Such a function of the 
driving apparatus will be described with reference also to FIG. 3. 
The pulses PHP produced from the photointerrupter 106 during repeated 
rotations of the drum 100 have a period or duration which depends on the 
rotating speed of the drum 100. Therefore, if the period or duration of 
the pulses PHP produced from the photointerrupter 106 is held in 
correspondence with a given angle of movement of the drum 100, the time 
period necessary for the drum 100 to rotate the given angle can be 
determined by measuring the period or duration of the pulses PHP. As well 
known in the art, the period or duration of the pulses PHP can be readily 
measured by counting pulses the repetition period of which is far shorter 
than that of the pulses PHP. 
In FIG. 3, the control circuit 108 includes an oscillator 116 which 
generates clock pulses P.sub.c having a repetition period which is far 
shorter than that of the output pulses PHP of the photointerrupter 106. 
These clock pulses P.sub.c are coupled to a counter 118 and a presettable 
counter 120. The counter 118 is reset by an output pulse PHP of the 
photointerrupter 106. The number "N" of clock pulses P.sub.c counted by 
the counter 118 for one period of the pulses PHP indicates a time period 
which the drum 100 took to rotate the predetermined angle. 
Suppose that "N.sub.o " clock pulses P.sub.c were counted during one period 
of the pulses PHP while the drum 100 was rotating at a reference or 
standard speed without any fluctuation in the speed, and that a count 
attributable to a fluctuation component of the drum speed is ".DELTA.N". 
Then, the count N of the clock pulses P.sub.c is expressed as: 
##EQU1## 
Where the magnification ratio is denoted by m and the preset speed of the 
imaging system at a magnification ratio m=1 is denoted by V.sub.o, a 
preset speed V of the imaging system is expressed as: 
##EQU2## 
Even though the motor 100 may be operated at a predetermined speed, the 
imaging system 114 is caused to move at a speed different from its 
standard speed (preset speed V.sub.o) if various components in the power 
delivery path thereto involve deviations in mechanical precision. 
Accordingly, the motor rotation speed needs be varied to compensate for a 
fluctuation component in speed which originates from the irregular 
precision distributions of the component parts. As such will be readily 
understood by considering possible deviation of the actual diameter of the 
pulleys shown in FIG. 5 from a nominal diameter or that of the actual 
diameter of the wire 202 from a nominal diameter. 
Apart from such dimensional irregularity of various parts, the rotation 
speed of the motor has to be compensated in accordance with deviation of 
an actual focal length of a lens in an imaging system from a nominal focal 
length. Compensation in this aspect will be described with reference to 
FIG. 10. 
Referring to FIG. 10, there are shown the drum 100, a document surface 224 
and a lens 226. Various relations discussed hereinafter will hold in a 
movable lens type imaging system. 
First, a condition for an image on the document surface 224 to be desiredly 
formed on the surface of the drum 100 is represented by: 
##EQU3## 
where a denotes the distance between the document surface 224 and the lens 
226, b the distance between the lens 226 and the drum 100, and f the focal 
length of the lens 226. 
Expressing the magnification b/a as k, the distance (a+b) between the 
document surface 224 and the drum 100 is indicated by: 
##EQU4## 
From the relation l:(a+b)=X:b, there holds an equation: 
##EQU5## 
Suppose that the actual focal length f of the lens 226 is different from 
its nominal focal length. Despite such deviation, an image of a selected 
magnification can be formed on the drum 100 only if the distances a and b 
are adjusted; this adjustment does not accompany any change in the 
distance X included in the equation (c). In practice, however, a 
difficulty is experienced in adjusting the distances a and b in conformity 
with the focal length f of the lens 226. A common practice is therefore to 
adjust the distance a between the lens 226 and the document surface 224 
until the focus is taken on the drum 100. The adjustment of the distance a 
requires a change in distance X in accordance with the F number of the 
lens 226 as will be seen from Equation (c). Stated another way, the moving 
or scanning speed of the imaging system must be varied in conformity with 
the deviation of the actual focal length f of the lens 226 from a nominal 
focal length so that a predetermined width of document may be scanned by 
the imaging system. 
Therefore, an initial reference speed v which should be determined as a 
final target speed in the imaging system is expressed as: 
##EQU6## 
where V is a reference scanning speed determined by Equation (2) and 
.DELTA.V.sub.m is an adjustment component in speed for each magnification 
ratio (m indicating a reference magnification ratio). 
In determining the initial reference speed v, there should also be 
considered a possible fluctuation in the rotation speed of the drum 100. 
Since the count N is inversely proportional to the drum speed, the 
reference speed v can be expressed as follows from Equations (1) and (3): 
##EQU7## 
The number N.sub.o of clock pulses in Equation (4) is stored in an 
operational circuit 122 of FIG. 3. The count .DELTA.N on the other hand is 
provided by the operational circuit 122 as .DELTA.N=N-N.sub.o ; the count 
N in the counter 18 is latched by a latch circuit 124 which is supplied 
with pulses PHP as latch pulses, and coupled to the operational circuit 
122. 
A group of switches 126 are mainpulatable to supply the operational circuit 
122 with a numerical value which represents a designated magnification 
ratio m. Also, a group of switches 128 are manipulatable to supply the 
operational circuit 122 with an amount of speed adjustment .DELTA.V.sub.m 
for each time of magnification ratio as a necessary initial setting 
condition. Hence, the operational circuit 122 can easily perform Equation 
(4) to provide the initial reference speed v. 
The switch group 128 for setting the initial condition may be constructed 
as shown in detail in FIG. 6. The switch group 128 in FIG. 6 is made up of 
a plurality of switch arrays S.sub.1, S.sub.2 . . . S.sub.n each 
corresponding to a specific magnification ratio. In each switch array S, a 
switch S.sub.a is used to designate a direction of compensation and a 
plurality of switches S.sub.b, S.sub.b . . . are used for data 
designation. The switch group 128 is supplied with an output signal of the 
operational circuit 122 through lines l.sub.1 which designates a 
magnification ratio. The switch group 128 in turn supplies the operational 
circuit 122 with a compensation code through lines l.sub.2. 
In the illustrated embodiment, the switch arrays S.sub.1, S.sub.2 . . . 
S.sub.n are assumed to have correspondence with the magnification ratio m 
which are "1, 0.82 . . . 0.65", respectively. However, where the copying 
machine can maintain the irregularity of its lens and mechanical 
components unchanged regardless of any variation in the magnification 
ratio, only one switch array will suffice and it is naturally needless to 
supply the switch group 128 with information from the operational circuit 
122 via the line l.sub.1. 
In FIG. 3, a phase locked loop is constituted by a phase comparator 130, a 
loop filter 132, a switch 134, an amplifier 136, the servo motor 112 and 
the encoder 110. This phase locked loop is adapted to control the imaging 
system 114 to move at a constant speed according to a reference signal 
during a scanning stroke of the imaging system. Therefore, the moving 
speed of the imaging system 114 can be varied by varying the repetition 
frequency f.sub.r of reference pulses which are supplied from the 
presettable counter 120 to the phase comparator 130 as a reference signal 
S.sub.fr. 
The reference pulses are coupled to the phase comparator 130 to determine 
an initial reference speed v of the imaging system which is indicated by 
Equation (4). The repetition frequency f.sub.r of the reference pulses is 
expressed as: 
##EQU8## 
where .alpha. is a proportional constant. 
Where the reference pulses having the repetition frequency f.sub.r 
indicated by Equation (5) are prepared by dividing at the presettable 
counter 120 the frequency of output clock pulses P.sub.c of the oscillator 
116 whose repetition frequency is f.sub.c, the count or frequency division 
ratio W at the presettable counter 120 is related with the repetition 
frequencies f.sub.r and f.sub.c as: 
##EQU9## 
From Equations (5) and (6), 
##EQU10## 
Since m.DELTA.V.sub.m /V.sub.o &lt;&lt;1, Equation (7) may be rewritten as: 
##EQU11## 
Since .DELTA.N/N.sub.o &lt;&lt;1, 
##EQU12## 
Suppose that the count at the presettable counter 120 is W.sub.o under the 
conditions that the magnification is 1:1, the drum 100 is moving at a 
speed V.sub.o as expected, each lens has a focal length equal to a nominal 
focal length, and each mechanical element has a precision equal to a 
nominal precision. Then, since m in Equation (8) is "1" and .DELTA.N and 
.DELTA.V.sub.m are "0", the count W.sub.o is expressed as: 
##EQU13## 
Accordingly, the count W is expressed as: 
##EQU14## 
Thus, in order that an image may be obtained without any error in 
magnification regardless of a change in drum speed, deviation of lens 
focal length f or deviation of mechanical element precision, the count W 
(frequency division ratio W) of the reference pulses at the presettable 
counter 120 which are coupled to the phase comparator 130 as reference 
signals S.sub.fr must be varied such that their repetition frequency 
f.sub.r has a variable value determined by such a drum speed fluctuation, 
or lens focal length or mechanical precision deviation. 
The presettable counter 120 in FIG. 3 is supplied with a result of 
calculation represented by Equation (10) from the operational circuit 122 
as a numerical value to be preset therein. In the arrangement of FIG. 3, 
every time the presettable counter 120 produces a borrow signal, it is 
preset to a numerical value provided by Equation (10) at the operational 
circuit 122. The calculation according to Equation (10) at the operational 
circuit 122 covers even the fluctuation component .DELTA.N of the drum 
speed and the fluctuation component .DELTA.V.sub.m of the imaging system 
speed attributable to the deviation in the focal length f of the lens for 
each selected magnification ratio m and that in the mechanical precision 
of various parts and elements. It will thus be seen that the numerical 
value W resulting from the calculation at the operational circuit 122 has 
any error attributable to such fluctuation components .DELTA.N and 
.DELTA.V.sub.m compensated for when it is preset in the counter 120. 
In FIG. 3, the presettable counter 120 is shown to comprise a decrement 
type counter in which a numerical value from the operational circuit 122 
is preset every time the counter produces a borrow signal. It will be 
apparent, however, that use may be made of a counter which is reset every 
time the count coincides with a numerical value preset therein. The gist 
of the presettable counter 120 is that it operates with a frequency 
division ratio which is a numerical value supplied from the operational 
circuit 120 and produces a necessary repetition frequency f.sub.r. 
The operational circuit 122 starts the calculation in response to a 
calculation start signal P.sub.s coupled thereto from a sequence 
controller 138 via a line l.sub.3. Of the various numerical values 
necessary for the calculation of Equation (10) at the operational circuit 
122, the magnification ratio m is supplied to the operational circuit 122 
from the previously stated switch group 126; the velocity adjustment 
component .DELTA.V.sub.m for each magnification ratio is supplied to the 
operational circuit 122 from the switch group 128; the count N.sub.o 
corresponding to the reference or standard drum rotation speed and the 
frequency division ratio W.sub.o under the reference or standard operation 
conditions are stored in the operational circuit 122; and the count 
.DELTA.N corresponding to a fluctuation component in the drum speed is 
provided by the operational circuit 122 through a calculation 
(N-N.sub.o)=.DELTA.N, where N is a numerical value supplied from the latch 
circuit 124 and N.sub.o a numerical value stored in the operational 
amplifier 122. 
In the above description, a numerical value fed from the switch group 128 
to the operational circuit 122 indicates a velocity adjustment component 
.DELTA.V.sub.m for each magnification ratio. To promote easy calculation 
at the operational circuit 122, it is desirable that the switch group 128 
is constructed such that the numerical value from the switch group 128 to 
the operational circuit 122 corresponds to m.sup.2 W.sub.o 
/V.sub.o..DELTA.V.sub.m in Equation (10). 
Numerical values mW.sub.o in Equation (10) may be stored in advance in the 
operational circuit 122. Additionally, numerical value mV.sub.o 
/N.sub.o..DELTA.N may be calculated for all the combinations of all the m 
and .DELTA.N in presumable ranges of the latter, and stored in advance in 
the operational circuit 122. These will prove effective to shorten the 
time period consumed by the operational circuit 122 for the calculation. 
In this case, it will be apparent that the range of .DELTA.N should 
correspond at least to a range of speed fluctuations which are expected to 
occur at the drum 100. While the data to be stored in the operational 
circuit 122 may comprise mW.sub.o /N.sub.o calculated for each 
magnification ratio, as such is undesirable in view of the longer time 
period than that necessary for the previously mentioned mW.sub.o 
/N.sub.o..DELTA.N. 
As described so far, the operational circuit 122 performs the calculation 
of Equation (10) within a short period of time in response to a 
calculation start signal P.sub.s from the sequence controller 138. A 
numerical value indicating a given frequency division ratio W (count W) is 
supplied from the operational circuit 122 to the presettable counter 120. 
The sequence controller 138 is supplied with an operation start signal 
S.sub.t, a signal S.sub.os indicating a size of an original document or 
that of a recording medium as the case may be, an output signal P.sub.h of 
the photointerrupter 218 (FIG. 5) responsive to the home position of the 
imaging system, an output signal P.sub.fb of the encoder 110, an output 
signal S.sub.m of the switch group 126, a pulse train PHP, and a start or 
buildup target signal S.sub.dr or a return target signal S.sub.rt output 
from the operational circuit 122. In response to these signals, the 
sequence controller 138 sequentially controls the individual component 
elements of the control circuit 108. 
In an initial stage of operation, the sequence controller 138 supplies a 
control signal to a switch 140 via a line l.sub.4 to close it so that the 
imaging system is accurately located in its home position. For this 
control, there may be employed a known means which locates the imaging 
system with the home position signal P.sub.h to position the light 
interceptor 216 (FIG. 5) at the center of the home position 
photointerrupter 218. 
The reference numeral 142 in FIG. 3 designates a phase compensation 
circuit. 
When the sequence controller 138 receives an operation start signal S.sub.t 
from the outside and then a pulse PHP from the photointerrupter 106, it 
supplies a switch 146 with a control signal via a line l.sub.5 to close it 
and the switch 140 with a control signal to open it. 
At this moment, the counter 118 counts clock pulses P.sub.c supplied 
thereto from the oscillator 116. The count N of the clock pulses, which 
represents a time period consumed by the drum 100 to rotate a 
predetermined angle, is latched in the latch circuit 124 and coupled 
therefrom to the operational circuit 122. It will be noted that the timing 
to latch the count of the counter 118 in the latch circuit 124 is open to 
choice except for the specific period for which the operational circuit 
122 performs calculation. That is, during operation of the circuit 122, 
latch pulses must be gated to prevent an output of the latch circuit 124 
from being fluctuated. 
Upon the closing of the switch 146, a start signal S.sub.ri output from a 
start or buildup circuit 148 is passed through the switch 146 to the 
amplifier 136 to energize the servo motor 112. An exemplary arrangement of 
the start or buildup circuit 148 is shown in FIG. 7. 
Referring to FIG. 7, the start circuit 148 comprises a digital-to-analog 
converter 150, a subtractor 152, a frequency-to-voltage converter 154 and 
a phase compensation circuit 156. The digital-to-analog converter 150 
processes an input buildup target signal S.sub.dr into an analog target 
speed reference voltage V.sub.rff and supplies this voltage to the 
subtractor 152. The frequency-to-voltage converter 154 supplies the 
subtractor 152 with a voltage V.sub.fb provided by frequency-to-voltage 
conversion of an output signal P.sub.fb of the encoder 110. Then, the 
subtractor 152 produces a signal representing a difference between the two 
voltages V.sub.rff and V.sub.fb. The phase compensation circuit 156 
subjects the output signal of the subtractor 152 to necessary phase 
compensation to prepare a start signal S.sub.ri, which is an output signal 
of the start circuit 148. 
The buildup target signal S.sub.dr coupled to the digital-to-analog 
converter 150 is prepared by the operational circuit 122 and fed to the 
start circuit 148 via the sequence controller 138. The phase compensation 
circuit 156 in the start circuit is constructed as a phase advancing 
circuit or a phase retarding circuit, for example, and so designed as to 
prevent the imaging system from overshooting at a start of its operation. 
The start circuit 148 functions to increase the moving speed of the imaging 
system smoothly up to an approximate target speed. The target speed in the 
start circuit 148 may be preset (1) on the assumption that the drum 100 is 
rotating at a reference or standard speed or (1) on the assumption that 
compensation has been carried out for a fluctuation in drum speed and a 
speed fluctuation attributable to deviations in lens focal distance and 
mechanical precision. 
A target speed at a buildup stage may become deviated a great deal from a 
final target speed predetermined for the imaging system, that is, a target 
speed with the fluctuation components in speed compensated for. Under this 
condition, the target speed if determined in the manner (1) would make 
transition from a buildup mode to a constant speed mode non-smooth causing 
the imaging system to take a prolonged period of time to reach the final 
target speed. In other words, the buildup time of the imaging system would 
be prolonged. 
FIG. 8 shows a curve which demonstrates an exemplary variation in the 
moving speed of the imaging system which occurs when the target speed 
differs from a buildup mode to a constant speed mode. 
It will be seen from the above that setting a buildup or start target speed 
in the manner (2) would promote a favorable buildup action of the imaging 
system controlled by the start circuit 148. 
Where a target speed is set in the manner (2), it is only necessary to 
obtain an equation 
##EQU15## 
from Equation (4), prepare at the operational circuit 122 a buildup target 
signal S.sub.dr which is given by multiplying Equation (11) by a constant 
.beta. as 
##EQU16## 
and supply the buildup target signal S.sub.dr to the start circuit 148. 
It will be analogically understood from the foregoing description that the 
target signal S.sub.dr can be readily obtained through Equation (12) at 
the operational circuit 122. Therefore, no detailed description will be 
made in this respect. 
While the imaging system is in a buildup or starting action, the 
operational circuit 122 is supplied with a calculation start signal 
P.sub.s from the sequence controller 138 via the line l.sub.3. Then, the 
operational circuit starts the previously stated calculation and, within a 
predetermined period of time, it completes the calculation and delivers a 
result of the calculation to the presettable counter 120. The presettable 
counter 120 divides the frequency of input clock pulses P.sub.c with a 
ratio designated thereto in the manner described. The output signal 
S.sub.fr of the counter 120 having a given repetition frequency f.sub.r is 
coupled to the phase comparator 130. 
The sequence controller 138 controls the imaging system to move at a target 
speed in either one of two different modes: a phase locked loop mode in 
which the switch 146 is opened by a control output of the sequence 
controller 138 via the line l.sub.5 while the switch 134 is closed via a 
line l.sub.6, and a combined phase locked loop and speed control mode in 
which the switch 134 is closed with the switch 146 kept closed. Either the 
phase locked loop mode or the combined phase locked loop and speed control 
mode may be selected as desired for the control of the imaging system. 
However, the combined phase locked loop and speed control mode is 
advantageous over the phase locked loop mode in promoting stable 
operations of the imaging system. 
In the phase locked loop mode, the phase locked loop made up of the phase 
comparator 130, loop filter 132, switch 134, amplifier 136, servo motor 
112 and encoder 110 functions to cause the frequency of an output signal 
P.sub.fb of the encoder 110 into coincidence with the frequency of the 
reference signal S.sub.fr, which is coupled from the presettable counter 
120 to the phase comparator 130. Thus, the imaging system 114 is allowed 
to travel at a target speed. 
In the combined phase locked loop and speed control mode, a speed control 
is performed by a loop made up of encoder 110, start circuit 148, 
amplifier 136 and servo motor 112 in addition to the control carried out 
in the phase locked loop mode. In this mode, an adder 158 is connected 
between the switch 134 and the amplifier 136 as indicated by a phantom 
line in FIG. 3 so as to be supplied with signals through the switches 134 
and 146. This control mode improves the stability of constant speed 
operation of the system since the phase locked loop is supplied with an 
output signal S.sub.ri of the start circuit 148 which contains a signal 
component provided by frequency-to-voltage conversion of an output 
P.sub.fb of the encoder 110 which is proportional to a speed; the phase 
locked loop being seemingly fed back with a speed. 
While the imaging system travels at a constant speed under the phase locked 
loop control mode or the combined mode, the sequence controller 138 counts 
output pulses P.sub.fb of the encoder 110. As the count at the encoder 110 
coincides with a signal S.sub.os indicating a document size, the sequence 
control, determining that a scan has ended, opens the switch 134 via the 
line l.sub.6 if under the phase locked loop control mode while, if under 
the combined mode, it opens the switch 134 via the line l.sub.6 and the 
switch 146 via the line l.sub.5. Additionally, in either mode, the 
sequence controller 138 closes a switch 160 supplying a control signal 
through a line l.sub.7. 
When the switch 160 is thus closed, the imaging system strokes back to its 
home position actuated by a return circuit 162. 
The return circuit 162 may be constructed in the same way as the start 
circuit 148 as illustrated in FIG. 7. Where the circuitry of FIG. 7 is 
employed to construct the return circuit 162, a return target signal 
S.sub.rt coupled from the sequence controller 138 to the return circuit 
162 will be provided with a characteristic to lower the target speed 
reference voltage V.sub.rff progressively as the imaging system approaches 
the home position. It will be needless to mention that the polarity of the 
output of the return circuit 162 is opposite to that of the start circuit 
148 and, therefore, the return circuit 162 has to be designed taking this 
into account. 
As the return stroke of the imaging system 114 proceeds until it neighbors 
the home position, the switch 160 is opened and thd switch 140 is closed 
to establish a stop mode. Then, the imaging system is controlled to stop 
at a position where the light interceptor 216 (FIG. 5) becomes located at 
the center of the photointerrupter 218 (FIG. 5). 
One control cycle of the driving apparatus is completed in this way. It 
will be seen that the operational circuit 122, sequence controller 138 and 
other various functions are readily achievable using a single 
microcomputer (including a random access memory, a read only memory and an 
I/O port). In the foregoing description, a fluctuation in drum speed has 
been described as being estimated by measuring a fluctuation in one 
rotation of the drum immediately before a start of movement of the imaging 
system. Alternatively, the estimation may be made by measuring the time 
period for a plurality of rotations of the drum before a start of movement 
of the imaging system. 
FIG. 9 is a graph demonstrating a possible method for the estimation of a 
drum rotation time during a scanning stroke of the imaging system. In this 
method, the drum rotation time is measured over two successive rotations 
immediately before the imaging system starts a scanning stroke. 
As generally described with reference to FIGS. 4a to 4c, a photointerrupter 
and a light interceptor are utilized as a part of the means for measuring 
a drum rotation time. In the combination shown in FIG. 4b, a time period 
the light interceptor 104 takes to move past the photointerrupter 106 is 
measured; in the combination shown in FIG. 4c, a time period the light 
interceptor 104 moves from one to the other of the two photointerrupters 
106.sub.1 and 106.sub.2 is measured. 
Now, another factor affecting the moving speed of the imaging system is a 
change in the dimensions of the component parts of the reciprocating 
mechanism associated with the imaging system either with the lapse of time 
or with a change in the surrounding conditions such as temperature. For 
instance, in the imaging system with the mechanism shown in FIG. 5, any 
change in the diameters of the pulleys and wire with time or due to a 
change in temperature or the like will naturally entail a change in the 
moving speed of the imaging system though the rotation speed of the servo 
motor 112 may remain constant. 
Turning back to FIG. 5, an example of means for compensating for a 
fluctuation in imaging system speed attributable to such a factor will be 
discussed. In FIG. 5, the time period necessary for the light interceptor 
216 to move from one to the other of the spaced photointerrupters 218 and 
220 is inversely proportional to the moving speed of the imaging system. 
This time period for the travel over the distance between the two 
photointerrupters can be measured by counting output pulses P.sub.fb of 
the encoder 110 which is driven by the servo motor 112. Thus, after the 
assembly of the imaging system in the machine, suppose that the distance 
between the photointerrupters 218 and 220 is adjusted such that "J.sub.o " 
output pulses P.sub.fb of the encoder 110 are counted during a passage of 
the light interceptor 216 between the photointerrupters 218 and 220, while 
maintaining the moving speed of the imaging system at a reference speed. 
Then, if the diameters of the pulleys or that of the wire is changed with 
time or due to a change in temperature to in turn change the count of the 
pulses P.sub.fb from "J.sub.o " to "J", the number of pulses J can be 
expressed as: 
##EQU17## 
where .DELTA.J denotes a fluctuation component of the pulse number. 
Since the servo motor 112 is controlled by the output P.sub.fb of the 
encoder 110 which is driven by the servo motor 112, there must 
additionally be compensated for a fluctuation in the moving speed of the 
imaging system attributable to fluctuation .DELTA.J in pulse number. If 
without this compensation, the scanning or moving speed v.sub.j will be 
expressed as follows when a speed fluctuation entailed by the drum and 
irregularity in lens focal distance and mechanical precision are zero with 
the ratio of magnification change m=1: 
##EQU18## 
Therefore, what is only necessary is to multiply the reference speed by 
(1+.DELTA.J/J.sub.o) and Equation (4) can be rewritten as: 
##EQU19## 
Considering .DELTA.J/J.sub.o &lt;&lt;1, 
##EQU20## 
A procedure for the calculation at the operational circuit 122 according to 
Equation (17) is analogical to that for the calculation thereat according 
to Equation (10) and, therefore, will neither be described herein nor 
shown in FIG. 3. 
The number of pulses J may be measured during a return stroke or a forward 
stroke of the imaging system one cycle ahead. A result of the measurement 
may be stored in a non-volatile memory so that the compensation can be 
performed without any trouble even at the first scan after the turn-on of 
a power switch. 
In summary, it will be seen that the present invention provides a driving 
method and apparatus for an illumination type imaging system and succeeds 
in eliminating all the drawbacks encountered in the prior art. 
Various modifications will become possible for those skilled in the art 
after receiving the teachings of the present disclosure without departing 
from the scope thereof.